PROCESS AND COMPOSITION FOR GENERATING ACID

Abstract: A process for generation of acid uses a medium comprising a first acid-generating component capable of generating a first acid, and a secondary acid generator, this secondary acid generator being capable of thermal decomposition to form a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the first acid. At least part of the medium is exposed to so as to cause formation of the first acid from the first acid-generating component; and the medium is then heated to cause, in the exposed part of the medium, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the secondary acid. The secondary acid generator has a first site bearing a first leaving group and a second site bearing a second leaving group, the second leaving group, in combination with a proton, forming the secondary acid. Because of the use of two leaving groups, it is possible to provide a storage stable secondary acid generator which still produces a strong acid. The invention also provides certain novel phosphate secondary acid generators, and 2,4,6-trisubstituted pyridine dyes which can function as both sensitizing and image dyes in the aforementioned process.

Description: PROCESS AND COMPOSITION FOR GENERATING ACID This invention relates to a process and composition for generating acid. Preferred forms of the present process and composition are useful for generating images. It is known, for example from International Application No. PCT US95/05130, Publication No. WO 95/29068, and the corresponding U.S. Patents Nos. 5,441,850 and 5,631,118, that acid can be generated by exposing to electromagnetic radiation superacid precursors (typically certain diazonium, phosphonium, sulfonium and iodonium salts) capable of decomposing to produce superacids, i.e., acids with a pKa less than about 0. (This superacid may hereinafter be called the "first acid" or the "primary acid" and the radiation-dependent reaction which produces it may be called the "primary reaction".) Preferably, the superacid precursor is mixed with a secondary acid generator, a material capable of thermal decomposition to produce an acid (hereinafter usually called the "secondary acid"), this thermal decomposition being catalyzed by the first acid. After generation of the first acid, the medium containing the superacid precursor and the secondary acid generator is heated, so that the first acid can bring about acid-catalyzed thermal decomposition of the secondary acid generator; in effect, the secondary acid generator acts as an "acid amplifier" which causes the generation of multiple moles of the secondary acid from each mole of first acid produced in the primary reaction, and thus increases the sensitivity of the medium, as compared with a medium relying only upon the generation of the first acid. An image dye which changes color in the presence of the secondary acid may be added to the medium to allow the acid generating process to form an image. In the absence of a spectral sensitizer, the known superacid precursors decompose to produce superacid only upon exposure to wavelengths which the precursors absorb, which are typically in the short ultraviolet region (below about 280 nm). The use of such wavelengths is often inconvenient, not least because special optical systems must be used. Accordingly, in the acid generating process, a sensitizing dye is often used to render the superacid precursor sensitive to longer wavelengths in the near ultra-violet, visible or even infra-red ranges. The specific process described in International Application No. PCT US95/05130 and U.S. Patent No. 5,441,850 (hereinafter for convenience called the '850 process) uses a sensitizing dye having a first form and a second form, the first form having substantially greater absorption in a first wavelength range than the second form. The medium is exposed to actinic radiation in this first wavelength range while at least part of the sensitizing dye is in its first form so that the sensitizing dye decomposes at least part of a superacid precursor, with formation of the first acid. The medium is then heated to produce the secondary acid, which brings about a change in absorption of an image dye and thereby forms an image. Finally, in the non-exposed areas of the medium, the sensitizing dye is converted to its second form, thus removing the absorption in the first wavelength range caused by the first form of the sensitizing dye, and lowering the minimum optical density (Dmin) in this wavelength range. International Application No. PCT US93/10215, Publication No. WO 94/10606, and the corresponding U.S. Patents Nos. 5,286,612 and 5,453,345, describe a somewhat similar process in which acid is generated by exposing a mixture of a superacid precursor and a dye to actinic radiation of a first wavelength, thereby causing decomposition of part of the superacid precursor, with formation of a protonated product derived from the dye; then irradiating the mixture with actinic radiation of a second wavelength, thereby causing decomposition of part of the remaining superacid precursor, with formation of superacid. (For convenience, the type of process disclosed in this application and these patents will hereinafter be called the '612 process.) International Patent Application No. PCT/US93/10224, Publication No. WO 94/10607, and the corresponding U.S. Patents Nos. 5,334,489 and 5,395,736, describe similar processes for the photochemical generation of acid and for imaging using conventional ultra-violet sensitizers; for convenience, the type of process disclosed in this application and these patents will hereinafter be called the'489 process. The entire disclosures of all the aforementioned applications and patents are herein incorporated by reference. The specific preferred secondary acid generators described in the aforementioned patents and applications are esters of squaric and oxalic acids. In these esters, a basic site is protonated by the first acid, and thereafter a leaving group is released from this first site, leaving an acidic proton at the site. For example, in the squaric acid diester of the formula: protonation occurs at one of the oxygen atoms, ultimately resulting in the formation of a hydroxyl group attached to the four-membered ring (the proton of this hydroxyl group is of course strongly acidic in squaric acid derivatives). A secondary acid generator should have high acid sensitivity (i.e., it should readily undergo thermal decomposition in the presence of the first acid), but should also have high thermal stability in the absence of this acid. In the aforementioned squaric and oxalic acid ester secondary acid generators, because the secondary acid-generating reaction involves only a single site, it is difficult to improve the acid sensitivity of the secondary acid generator by chemical modifications without adversely affecting its thermal stability, and vice versa. Moreover, in these secondary acid generators, the secondary acid released is incapable of protonating the secondary acid generator (or, in more strictly accurate thermodynamic terms, the equilibrium proportion of secondary acid generator protonated by the secondary acid is so low as to have a negligible effect on the decomposition of the secondary acid generator). If such protonation of the secondary acid generator by the secondary acid could be made to occur, the thermal decomposition of the secondary acid generator would also be catalyzed by the secondary acid, and thus this thermal decomposition would be autocatalytic. Such autocatalytic thermal decomposition is desirable in practice because the number of moles of secondary acid which can be generated directly from a single mole of first acid is limited (presumably by factors such as, for example, the limited rate of diffusion of secondary acid generator through the polymeric binders which are usually used in imaging media of the aforementioned types) and autocatalytic thermal decomposition can increase the number of moles of secondary acid generated from a single mole of first acid, and thus increase the sensitivity of the imaging medium. The applicant has developed secondary acid generators in which the secondary acid-forming reaction involves two separate sites within the molecule; such secondary acid generators can comprise a first site having a relatively basic "trigger" group which is first protonated by the first acid with a second site bearing a leaving group which forms a strong secondary acid. Some of these secondary acid generators undergo autocatalytic thermal decomposition. Also, a persistent problem in acid-mediated color imaging processes is finding a yellow indicator dye with sufficient photostability. Trisubstituted pyridine dyes bearing at least one N,N-dialkylaminophenyl substituent are among the most satisfactory yellow image dyes available commercially of which the neutral form has sufficient basicity for use in acid-mediated processes. However, the protonated forms of such dyes, which are present in regions of the final image having yellow density, are susceptible to photobleaching, for example when an image in the form of a slide is projected and hence subject to prolonged exposure to a large radiation flux. Moreover, in the '850 process described above, a sensitizing dye is required having a first form and a second form, the first form having substantially greater absorbance in a first radiation range than the second form. In the case of a sensitizer to blue light, the first form of the sensitizing dye (which absorbs blue light) is a yellow dye. After exposure, the film is heated, thus converting the sensitizing dye to its second (colorless) form in the non-exposed areas. In exposed areas, however, the sensitizing dye may remain in its first form. Accordingly, in the '850 process, at least part of the final yellow image may comprise the first form of the sensitizing dye. Therefore, a sensitizing dye for use in the '850 process must, in its first form, not only function as an photosensitizer for the superacid precursor, and be less basic than the secondary acid generator (properties which are required for the efficient photogeneration and amplification of acid), but must also be sufficiently light-fast to be used to form part of the final image. Prior art pyridine indicator dyes in which one substituent is an aryl group bearing an N-aryl-N-alkyl grouping, although more light-fast than the analogous, commercially-available dyes bearing an N,N-dialkylaminophenyl substituent mentioned above, are too basic in their monoprotonated forms to be used as sensitizer dyes in the aforementioned '850 process with typical secondary acid generators. It has now been found that the photostability of trisubstituted pyridine indicator dyes is acceptable if at least one of the substituents is an aryl group bearing an N,N-diarylamino grouping. Furthermore, the protonated forms of the same N,N- diarylamino substituted dyes are useful in the aforementioned '612, '489 and '850 processes for sensitizing superacid precursors to blue visible or similar radiation; in the '850 process, the same dye can be used both as sensitizer and as image dye. Accordingly, this invention provides a medium for generation of acid and comprising a first acid-generating component capable of generating a first acid; and a secondary acid generator capable of thermal decomposition to form a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the first acid. A first form of this medium is characterized in that the secondary acid generator has a first site bearing a first leaving group and a second site bearing a second leaving group, the first leaving group being capable of protonation by the first acid, with expulsion of the first leaving group, to form a cation, followed by: (i) loss of a proton from the cation to form an unstable intermediate, which then fragments with loss of the second leaving group, accompanied by either (a) loss of a second proton; or (b) addition of a proton- containing nucleophile, followed by loss of a proton; or (ii) electrophilic addition of the cation to an unsaturated reagent bearing a proton at the site of addition and a proton-containing nucleophilic grouping at an adjacent site, following which said proton on the reagent is lost and the second leaving group is displaced by the nucleophilic grouping; the second leaving group, in combination with a proton, forming the secondary acid. (For convenience, this medium may hereinafter be called the "two-site medium".) A second form of this medium is characterized in that the thermal decomposition of the secondary acid is also catalyzed by the secondary acid. (For convenience, this medium may hereinafter be called the "autocatalytic medium".) This invention also provides a process for generation of acid using a medium of either of the aforementioned forms, in which, in at least part of the medium, formation of the first acid from the first acid-generating component is caused to occur, and thereafter the medium is heated to cause, in the part of the medium in which formation of the first acid has occurred, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the secondary acid. Certain of the secondary acid generators used in the medium and process of the invention are themselves novel, and the invention extends to these novel compounds per se. Accordingly, this invention provides a secondary acid generator characterized by one of the formulae: wherein Ar is an aryl group, with the aryl groups being the same or different, R 1 is an alkyl or cycloalkyl group, R represents one or more hydrogen atoms or alkyl, aryl, alkoxy or aryloxy groups on the benzene ring, R is an alkyl or cycloalkyl group, and n is 1 or 2. This invention also provides a 2,4,6-trisubstituted pyridine characterized in that at least one of the substituents is a /?αra-(N,N-diaryl- amino)phenyl group, the other two substituents being alkyl, cycloalkyl or aryl groups. This invention also provides an imaging medium comprising an acid generator capable of generating an acid upon exposure to actinic radiation, and an image dye capable of changing color upon exposure to the acid, wherein the image dye comprises one of the aforementioned 2,4,6-trisubstituted pyridines. (For convenience, this medium may hereinafter be called the "trisubstituted pyridine indicator dye medium" of the present invention.) This invention also provides an acid generating medium comprising a phosphonium, sulfonium, diazonium or iodonium salt capable of decomposing to give an acid with a pK a less than about 0, and a sensitizing dye capable of sensitizing the phosphonium, sulfonium, diazonium or iodonium salt to radiation of a wavelength to which it is essentially not sensitive in the absence of the sensitizing dye, wherein the sensitizing dye comprises a salt of one of the aforementioned 2,4,6- trisubstituted pyridines. (For convenience, this medium may hereinafter be called the "trisubstituted pyridine sensitizing dye imaging medium" of the present invention.) Figure 1 shows the reactions occurring during the acid-catalyzed thermal decomposition of a first preferred secondary acid generator during a process of the present invention; Figure 2 shows the reactions occurring during the acid-catalyzed thermal decomposition of a second preferred secondary acid generator during another process of the present invention; Figures 3A and 3B shows the reactions occurring during two alternative pathways for the acid-catalyzed thermal decomposition of a third preferred secondary acid generator during another process of the present invention; Figures 4A-4C show the acid concentrations in the exposed and non- exposed regions of the acid-generating layer during the various steps of a first preferred process of the present invention, which is of the type described in the aforementioned International Application No. PCT/US95/05130; and Figure 5 is a schematic cross-section through an imaging medium of the present invention as it is being passed between a pair of hot rollers during a preferred imaging process of the present invention. As already mentioned, the process of the present invention differs from the '612, '489 and '850 processes, and other processes using secondary acid generators, in that the secondary acid generator used has a first (or "trigger") site bearing a first leaving group and a second site bearing a second leaving group. In the present process, the first leaving group is protonated by the first acid, leading to expulsion of the first leaving group to form a cation. Thereafter loss of a proton from the cation forms an unstable intermediate, which then fragments, with loss of the second leaving group. There also occurs, simultaneously with or after loss of the second leaving group, either the loss of a second proton, or addition of a proton- containing nucleophile, followed by loss of a proton. Alternatively, formation of the cation is followed by electrophilic addition of this cation to an unsaturated reagent bearing a proton at the site of addition and a proton-containing nucleophilic grouping at an adjacent site, following which the proton at the site of addition is lost and the second leaving group is displaced by the nucleophilic grouping. In all cases, the second leaving group, in combination with a proton, forms the secondary acid. Skilled chemists will appreciate that the use of a secondary acid generator having two separate active sites allows an additional degree of freedom in controlling the properties of the secondary acid generator. In particular, the first leaving group can be chosen relatively basic, so as to be readily protonated, while the second leaving group can be chosen to produce a strong secondary acid. For example, the first leaving group can be a hydroxyl group, which is readily protonated and then lost as water (which is essentially non-acidic, at least in the polymeric media in which the present process is typically carried out), while the second leaving group can be a tosyloxy group, which produces ?-toluenesulfonic acid as the secondary acid, this being a much more powerful acid than the squaric and oxalic acid derivatives produced by the prior art processes discussed above. The two steps of loss of the first leaving group, and loss of the second leaving group, will of course normally proceed at different rates. Although the invention is not limited to any particular relationship between these rates, in general it is desirable that the second of these steps be slower than the first. If the second step is relatively slow, and minor amounts of thermal decomposition of the primary acid generator occur during storage, such thermal decomposition will not necessarily cause generation of secondary acid from the secondary acid generator, and subsequent catalyzed breakdown of more secondary acid generator molecules. By appropriate choice of the first and second leaving groups, the secondary acid can be made sufficiently strong to be capable of protonating the first leaving group of the secondary acid generator. In such cases, the thermal decomposition of the secondary acid generator is catalyzed not only by the first acid but also by the secondary acid itself, i.e., the thermal decomposition is autocatalytic. Thus, the first few molecules of secondary acid generated from a single molecule of the first acid can in turn catalyze the decomposition of additional molecules of secondary acid generator to produce additional secondary acid, thereby providing a "cascade reaction" which increases the number of moles of secondary acid generated from a single mole of first acid, and thereby enhances the sensitivity of the process. In general, preferred first leaving groups for the secondary acid generators are those comprising a nitrogen, phosphorus, sulfur or oxygen atom bonded directly to the first site, this first leaving group having a PK HA+ greater than about -6. Examples of such first leaving groups include hydroxy, alkoxy, amino, alkylamino, acyloxy, aroyloxy, acylamino, aroylamino, carbamate and 2-oxopyridyl groups, and groups of the formula -O-C(=O)-C(=O)-O-R 4 , wherein R 4 is an alkyl or aryl group. Preferred second leaving groups are neutral groups having the PK HA of their conjugate acids not greater than about 2, or cationic groups having the PK HA + of their conjugate acids not greater than about 2. The first and second leaving groups may both be attached to a carbocyclic ring containing from 5 to 8 carbon atoms; this carbocyclic ring may be fused to an alicyclic or aromatic ring. In addition to the aforementioned secondary acid generators of Formulae I and π, a first preferred group of secondary acid generators for use in the present process are 1,2-diol derivatives of the formula: R 5 R 9 H H (ffl) R 6 R 7 R 8 R 10 or of the formula: wherein L 1 is the first leaving group, L 2 is the second leaving group, and R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are each independently a hydrogen atom, an alkyl, cycloalkyl, aralkyl or aryl group, subject to the proviso that any two of R 5 , R 6 , R 7 , R 8 , R 9 and R 10 may, together with the carbon atom(s) to which they are attached, form a carbocyclic ring. Another preferred group of secondary acid generators for use in the present process are 1,3-diol derivatives of the formula: L 1 H L 2 H R 5 - ■ R 9 (vπ) R 6 R 7 R 8 R 10 or of the formula: H L 1 H R 5 - •R 9 (vm) R 6 R 7 R 8 R 10 wherein L 1 is the first leaving group, L 2 is the second leaving group, and R 5 , R 6 , R 7 , R 8 , R 9 and R 1 are each independently a hydrogen atom, an alkyl, cycloalkyl, aralkyl or aryl group, subject to the proviso that any two of R 5 , R 6 , R 7 , R 8 , R 9 and R 10 may, together with the carbon atom(s) to which they are attached, form a carbocyclic ring and that this carbocyclic ring may optionally be substituted with an additional second leaving group L . In Formulae I and π, R and R (if present) are each desirably an alkyl group, and desirably each group Ar is a phenyl, 4-methoxyphenyl, 4-phenylphenyl or 4-(4-phenylphenoxy)phenyl group. Among the compounds of Formulae m, IV, VII and VHI, especially preferred subgroups are those of the formulae: 1 9 1 1 (wherein L and L are as defined above with reference to Formulae HI and IV, R is an aryl or alkyl group; m is 0, 1, 2 or 3; and N is 0 or 1, subject to the proviso that, 1 1 when L is a hydroxyl group, L and L are in the cis orientation to one another); (wherein L and L are as defined above with reference to Formulae HI and IV, R is an aryl or alkyl group; and m is 0, 1, 2 or 3, subject to the proviso that, when L 1 is a 1 hydroxyl group, L and L are in the cis orientation to one another); and (wherein L 1 is the first leaving group, L 2 is the second leaving group and R 19 is a carbonium ion stabilizing group). Preferred compounds of Formula V, VI and LX are those in which L 1 comprises a nitrogen, phosphorus, sulfur or oxygen atom bonded directly to the 1 9 cyclohexane ring, the group L having a PK HA+ greater than about -6; L comprises a halo, sulfonium, sulfonate, sulfate, sulfamate or phosphate ester group, and R 19 (when present) is an alkyl, cycloalkyl, aralkyl or aryl group. An especially preferred subgroup of the compounds of Formulae V and VI are those in which R 11 is an aryl group, optionally substituted with at least one halo, alkyl, nitro, alkoxy or aryloxy substituent, or is an alkyl group containing not more than about 6 carbon atoms; L 1 1 19 is of the formula OR wherein R is a hydrogen atom or an acyl group, or a group of the formula -C(O)-C(=O)-0-R 13 , in which R 13 is an alkyl or aryl group; L 2 is of the formula -OSO 2 R 14 , wherein R 14 is an aryl group, optionally substituted with at least one halo, alkyl, nitro, alkoxy or aryloxy substituent, or is an alkyl group containing not more than about 12 carbon atoms, which may be substituted with an alicyclic group, this alicyclic group optionally bearing an oxo group, or L is a diarylphosphate ester group or an N,N-disubstituted sulfamate group wherein each of the substituents independently is an alkyl or aryl group; and m is 0 or 1. An especially preferred subgroup of the compounds of Formula LX are those in which L 1 is a hydroxy, alkoxy, amino, alkylamino, acyloxy, aroyloxy, acylamino, aroylamino, carbamate and 2-oxopyridyl group or a group of the formula -O-C(=O)-C(=O)-O-R 20 , wherein R 20 is an alkyl or aryl group; and L 2 is a group of the formula -OSO 2 R 21 or -S + (R 2I ) 2 X " (wherein each 91 R is an aryl group, which may be substituted with at least one halo, alkyl, nitro, alkoxy or aryloxy substituent, or is an alkyl group containing not more than about 12 carbon atoms, which may be substituted with an alicyclic group, this alicyclic group optionally bearing an oxo group, subject to the proviso that when more than one 91 91 group R is present, these groups R may be the same or different), and X is an anion, or L is a diarylphosphate ester group or an N,N-disubstituted sulfamate group wherein each of the substituents independently is an alkyl or aryl group. Specific preferred secondary acid generators are those: (a) of Formula V in which R 11 is a phenyl group, L 1 is a hydroxyl group and L 2 is a tosyl group; (b) of Formula V in which R 11 is a -chlorophenyl, /7-methylphenyl, -phenoxyphenyl or /j-methoxyphenyl group, L 1 is a hydroxyl group and L is a methylsulfonyl group; (c) of Formula VI are those in which R 15 is a methyl group, L 1 is a hydroxyl group and L is a /?-methylphenylsulfonyl, ?-n-butylphenylsulfonyl, ^-n-octylphenylsulfonyl group, (7,7-dimethylbicyclo[2.2. l]heptan-2-onyl)methyl- sulfonyl or N,N-dimethylsulfamate group; and 1 9 (d) of Formula LX in which L is a hydroxyl group, L is a tosyloxy group and R 19 is a 3,5-dichlorophenyl, 4-trifluoromethylphenyl, 4- chlorophenyl, 3-chlorophenyl, phenyl or 4-methylphenyl group. Specific preferred secondary acid generators of the above types are: (a) (l-methoxy-l,2,3,4-tetrahydronaphthalen-l-yl)methyl diphenylphosphate (of Formula I); (b) 3-methoxy-3-(4-phenylphenyl)but-l-yl bis(4-phenylphenyl)- phosphate, the compound of Formula LI in which n is 2, R 1 and R 3 are each a methyl group, and each group Ar is a 4-phenylphenyl group; (c) 2-methoxy-2-[4-(4-phenylphenoxy)phenyl]but-l-yl diphenylphosphate, the compound of Formula LI in which n is 1, R 1 is a methyl group, R 3 is an ethyl group, the group Ar attached to the same carbon atom as the group R 3 is a 4- (4-phenylphenoxy)phenyl group, and the other two groups Ar are phenyl groups; (d) 2-methoxy-2-[4-(4-methoxyphenyl)phenyl]but-l-yl diphenyl- 1 " phosphate, the compound of Formula LI in which n is 1, R is a methyl group, R is an ethyl group, the group Ar attached to the same carbon atom as the group R 3 is a 4- (4-methoxyphenyl)phenyl group, and the other two groups Ar are phenyl groups. Other specific preferred secondary acid generators are described in the Examples below. As will be apparent to skilled chemists, the compounds of Formula V and VI exist in optical isomers, since the carbon atom bearing the groups R 11 (or R ) and L is an asymmetric center, as is the carbon atom bearing the group L . Accordingly, the compounds of Formulae V and VI exist in two diastereomeric forms. As already noted, when L 1 is a hydroxyl group (and in some cases when L 1 is 1 9 an amino group), the groups L and L should be in the cis orientation to one another; it does not matter which enantiomer, or mixture of enantiomers is used. General methods for the synthesis of each of the aforementioned preferred groups of secondary acid generators are known to skilled organic chemists, and examples of such syntheses are given in the Examples below. Accordingly, the synthesis of these preferred groups of secondary acid generators is well within the skill of trained organic synthetic chemists. Figures 1, 2 and 3 of the accompanying drawings illustrate the reactions occurring during the acid-catalyzed thermal decomposition of three preferred secondary acid generators in the present process. In Figure 1, a 1-R n -1- hydroxy-2-tosyloxycyclohexane secondary acid generator (A) (which is of Formula 1 9 V with L being a hydroxyl group, L being a tosyloxy group, m being 1 and n being 0) undergoes protonation of the hydroxyl first leaving group, followed by loss of this group in the form of water to give a carbocation (B). This carbocation (B) undergoes loss of a proton to form an unstable intermediate (C), in which the dotted bonds indicate a double bond between either the 1 -carbon of the ring and the group R n , or the 1- and 6-carbons of the ring. The unstable intermediate (C) then loses a tosyloxy anion to form a carbocation (D), which then adds a proton-containing nucleophile (designated " NuH") and subsequently loses a proton to form a final l-R n -6-Nu- cyclohex-1-ene (E). Alternatively (not shown in Figure 1), the carbocation (D) may lose a further proton to form a cyclohexa-l,3-diene product. In either case, the tosyloxy anion, together with one of the protons lost during the reactions, forms a strong secondary acid, namely -toluenesulfonic acid, which is sufficiently strong to protonate a further molecule of the secondary acid generator (A), so that the thermal decomposition reaction is autocatalytic. Figure 2 shows reactions corresponding to those in Figure 1 where the secondary acid generator is a norbornyl derivative (F) of Formula LX with L 1 being a hydroxyl group and L 2 being a tosyloxy group. As in Figure 1, the first step of the thermal decomposition reaction is protonation of the hydroxyl group, followed by loss of this group as water with formation of a carbocation (G). This carbocation (G) loses a proton, with formation of a 2,3-double bond to give an unstable intermediate (H). The intermediate (H) then loses a tosyloxy anion to give a carbocation (I), in which the dashed bonds denote a non-classical three-center bond. Finally, the carbocation (I) adds a proton-containing nucleophile and subsequently loses a proton to form a final 2-R 19 -7-Nu-norbornyl derivative (J). This sequence of reactions occurs for most nucleophiles NuH. However, if the nucleophile NuH is water (i.e., the nucleophilic group Nu is a hydroxyl group), the 7-hydroxyl derivative (K) first formed is itself unstable and undergoes fragmentation, via a carbocation (L), so that the final product is a 3-R 19 - cyclohex-3-ene aldehyde (M). Figures 3A and 3B shows the reactions occurring during two alternative pathways for the thermal decomposition of a preferred phosphate ester secondary acid generator. For ease of comprehension, the formulae shown in Figure 3 are simplified; in Figure 3, "MeO" denotes a methoxyl group, "Ar" denotes a 4-(4- phenylphenoxy)phenyl group and denotes a grouping of the formula: -© The full formula of this phosphate secondary acid generator is given at Formula (X) below. As shown in Figure 3A, the first two steps in the decomposition of the phosphate ester secondary acid generator (N) are protonation of the methoxyl group on the compound (N) to give a cation (O), which then loses methanol to give an carbocation intermediate (P). It is at this point that the two pathways for the decomposition diverge, as indicated by the two arrows extending from the intermediate (P) in Figure 3A. The carbocation intermediate (P) may either lose a proton to give an unsaturated intermediate (Q), in which the dotted bonds have the same significance as in Figure 1, or may electrophilically add to an unsaturated reagent; the latter possibility is discussed below with reference to Figure 3B. It will be seen that the intermediate (Q) resembles the corresponding intermediate (C) shown in Figure 1. However, the next stage of the process differs significantly from those previously described with reference to Figures 1 and 2. The intermediate (Q) reacts with a nucleophile (resorcinol is shown in Figure 3A); the exact steps involved are not clear, but the overall result is expulsion of the phosphate ester grouping as a phosphoric acid (the secondary acid), and the formation of a cyclic product (R). Alternatively, as shown in Figure 3B, the carbocation intermediate (P) may effect electrophilic addition to an unsaturated reagent (resorcinol is shown in Figure 3B) bearing a proton at the site of the addition and also bearing a proton- containing nucleophilic grouping at an adjacent site (which need not necessarily be α to the addition site). In the case of resorcinol, the addition takes place at the 4-position, which is ortho to one hydroxyl group and para to the other, and hence highly activated for the electrophilic addition, while the ortho hydroxyl group serves as the proton-containing nucleophilic grouping. The electrophilic addition is of course accompanied by loss of the original 4-proton of the resorcinol (omitted from Figure 3B for the sake of simplicity), and the addition species produced is designated (S). As indicated in Formula (S) in Figure 3B, the final step of this pathway is a cyclization reaction resulting from nucleophilic attack of a lone pair from the "ortho" hydroxyl group on the carbon atom α to the addition site and expulsion of the phosphate ester grouping as a phosphoric acid (the secondary acid), and the formation of the same cyclic product (R) as obtained by the alternative pathway shown in Figure 3 A. As shown in Figures 3A and 3B, the specific secondary acid generator (N) illustrated can undergo decomposition by either pathway. However, there are certain secondary acid generators which, it appears, must undergo the electrophilic addition route of Figure 3B, since there is no proton available to permit the elimination reaction to form the unsaturated compound corresponding to (Q) in Figure 3 A. For example, the secondary acid generator of the formula: clearly cannot undergo the elimination reaction corresponding to (P) - (Q) in Figure 3A after loss of methanol from the carbon atom between the benzene ring and the -C(CH 3 ) 2 - grouping. In other cases, although the elimination reaction is at least theoretically possible, little secondary acid formation is observed in the absence of the appropriate nucleophilic reagent, so that, in the presence of this reagent, the reaction must be proceeding predominantly via the electrophilic addition/cyclization route of Figure 3B. The use of secondary acid generators which proceed wholly or predominantly via the electrophilic addition/cyclization route of Figure 3B is generally advantageous in that such secondary acid generators will not generate acid in the absence of the nucleophilic reagent. As discussed in more detail, in practice imaging media of the present invention typically use two separate layers, an acid- generating layer containing the acid-generating component and the secondary acid generator, and a color-forming layer containing the leuco form of an image dye which changes color in the presence of the secondary acid; the components of the two layers are intermixed by heating the medium after imagewise exposure. In such two-layer media, the nucleophilic reagent required for the secondary acid generator to generate the secondary acid can be placed in the color-forming layer. Accordingly, even if trace amounts of first acid are generated in the acid-generating layer prior to the exposure (for example, because of exposure of the medium to minor amounts of radiation during handling, or because of slight thermal stability of the acid-generating component or the secondary acid generator), these trace amounts of acid will not be amplified by the secondary acid generator and no significant amount of acid will be present in the medium prior to imaging, thus reducing the minimum optical density (Dm ) in the final image. In effect, the electrophilic addition/cyclization type secondary acid generator and the nucleophilic reagent form a two-component secondary acid generator system, which is only "assembled", and thus able to effect acid amplification, after imaging has taken place. Thus, this type of secondary acid generator tends to enhanced the thermal stability of the medium during storage, while still permitting high amplification factors to be achieved (i.e., a large number of moles of secondary acid to be generated for each mole of primary acid). The secondary acid generators used in the process of the present invention may have multiple second leaving groups. In certain structures, the departure of a "primary" second leaving group adjacent the first leaving group may cause a change in structure of the secondary acid generator (for example, the formation of a C=C double bond) which destabilizes a "secondary" second leaving group more remote from the first leaving group, thereby triggering departure of this secondary second leaving group, with formation of a second molecule of the secondary acid. Obviously, the departure of the secondary second leaving group may cause a similar change in structure further along the molecule, thereby destabilizing a "tertiary" second leaving group, with formation of a third molecule of the secondary acid. The use of secondary acid generators containing multiple second leaving groups may be advantageous because protonation of only one first site on the secondary acid generator is required to cause departure of multiple second leaving groups, thereby increasing the number of protons generated from each protonation and increasing the "amplification factor" (i.e., the number of moles of secondary acid generated from each mole of first acid) of the secondary acid generator. As already mentioned, the second step in the decomposition of the secondary acid generator may involve, in addition to the loss of the second leaving group, either loss of a second proton or addition of proton-containing nucleophile to the secondary acid generator, followed by loss of a proton. Where the second leaving group comprises a phosphate ester, it is desirable to have a nucleophile present with or adjacent the secondary acid generator so that the second step can proceed by the latter route; appropriate nucleophiles include phenols (especially resorcinol), thiophenols, thiols and phosphines. The nucleophile may be provided either in the same layer as the secondary acid generator or be introduced later, normally by diffusion from an adjacent layer; later introduction is preferred since there is less risk of unintentional acid generation during storage of the medium. The generation of the first acid in the present two-site process may be effected by any means capable of generating an acid from an acid-generating component, for example thermal decomposition of the acid-generating component, or contact with a reagent which decomposes the acid-generating component to produce the first acid. However, in general it is preferred that the generation of the first acid be effected by the action of electromagnetic radiation on the first acid- generating component. Preferred radiation sensitive acid-generating components include phosphonium, sulfonium, diazonium and iodonium salts capable of decomposing to give a first acid with a pK a less than about 0. Especially preferred superacid precursors are diaryliodonium salts, specifically (4-octyloxyphenyl)- phenyliodonium hexafluorophosphate and hexafiuoroantimonate, bis(n-dodecyl- phenyl)iodonium hexafiuoroantimonate and (4-(2-hydroxytetradecan-l-yloxy)- phenyl)phenyl iodonium hexafiuoroantimonate. Whether or not 'onium salts are employed as the acid-generating component, it may often be desired to generate the first acid using radiation of a wavelength to which the first acid-generating component is not inherently sensitive, and for this purpose to include within the medium a sensitizing dye which sensitizes the first acid-generating component to the radiation used. Similarly, the present two-site is not limited to any particular use of the secondary acid, and this acid may be used in various ways, for example by triggering an acid-dependent chemical reaction, such as a cationic polymerization (and hence the present process may be useful in the production of photolithographic masks). However, one preferred use for the present process is the formation of images, and for this purpose the medium desirably contains an acid-sensitive material capable of undergoing a color change in the presence of the secondary acid, so that the secondary acid produced causes the color change in the acid-sensitive material, thereby forming the image. Preferred imaging processes of the present invention include those similar to the aforementioned '489, '612 and '850 processes, especially the last two, and for fuller details of preferred components and process steps (including the use of a cosensitizer in conjunction with the sensitizing dye and "fixing" of the medium by destroying unchanged first acid-generating component remaining after the medium has been exposed to radiation) the reader is referred to the aforementioned International Applications and U.S. patents describing these processes. To illustrate the chemical reactions which may take place during an imaging process of the present invention, a preferred process of this type using an acid-generating layer comprising an iodonium salt, a sensitizing dye, a cosensitizer, and a secondary acid generator, and a color change layer comprising an indicator dye and a fixing reagent, will now be described, with reference to Table 1 below and Figures 4A-4C of the accompanying drawings. Table 1 Table 1 and Figures 4A-4C of the accompanying drawings show the changes in acid concentration in exposed and non-exposed areas of the acid- generating layer used at various stages during the imaging process. The last section of Table 1, headed "AFTER FLXLNG," shows the composition of the combined acid- generating and color-change layers after the components thereof have become intermixed. The imaging medium initially contains the sensitizing dye in its first form, which is effective to sensitize the iodonium salt used to radiation. Both the exposed and non-exposed areas comprise a quantity (shown in Table 1 as 1 mole for simplicity; all references to moles concerning Table 1 refer to moles per unit area of the imaging medium, and are only by way of illustration, since the proportions of the various components may vary widely) of the sensitizing dye, a larger molar quantity of the iodonium salt (7 moles are shown in Table 1) and a still larger molar quantity (41 moles are shown in Table 1) of a secondary acid generator, together with 14 moles of a cosensitizer. The imaging medium is imagewise irradiated with radiation of a wavelength absorbed by the sensitizing dye, preferably visible radiation. For ease of illustration, it is assumed that exposed areas receive sufficient radiation to decompose 1 mole of the iodonium salt, thus producing a corresponding amount of first acid. This acid immediately protonates 1 mole of the secondary acid generator, which is arranged to be the most basic species present in the acid-generating layer. Thus, after the exposure, as shown in Table 1, the exposed areas of the acid- generating layer contain 1 mole of protonated secondary acid generator, 40 moles of unprotonated secondary acid generator, 6 moles of unchanged iodonium salt and 13 moles of cosensitizer; 1 mole of the cosensitizer is consumed during the decomposition of the 1 mole of iodonium salt, and the decomposition products from this 1 mole of cosensitizer are ignored in Table 1 for simplicity. The unexposed areas are of course unchanged by the exposure. This situation is illustrated in Figure 4A, which shows the 1 mole of acid present in the exposed area BC and the absence of acid in the unexposed areas AB and CD. The imaging medium is next heated. In the exposed area BC, the first acid, which has already protonated 1 mole of the secondary acid generator, catalyzes the decomposition of this secondary acid generator (and the secondary acid thus produced catalyzes the decomposition of additional secondary acid generator), thus producing a large quantity of the secondary acid (41 moles are shown by way of example in Table 1 , which assumes complete decomposition of the secondary acid generator; Figure 4B is not strictly to scale). However, in the non-exposed areas AB and CD, no such primary acid is present, so that essentially no decomposition of the secondary acid generator occurs and essentially no secondary acid is generated. Depending upon the specific secondary acid generator and heating conditions employed, some uncatalyzed thermal decomposition of the secondary acid generator may take place in the non-exposed areas, and in order to illustrate that such uncatalyzed thermal decomposition does not affect the result of the present process, Table 1 and Figure 4B assume that 1 mole of secondary acid is generated in the non- exposed areas. In practice, substantially less secondary acid would be expected in these areas. In the final step of the process, as discussed in more detail below, the components of the acid-generating and color change layers become intermixed. Table 1 assumes that the color-change layer contains 40 moles of an indicator image dye and 10 moles of a base, which serves to deprotonate the sensitizing dye to its second form. Table 1 further assumes that the color-change layer contains at least enough fixing reagent to ensure that all remaining iodonium salt is destroyed; residual fixing agent is ignored in Table 1 for clarity. In the exposed areas, the fixing reagent decomposes all remaining iodonium salt, with the generation of a further 6 moles of first acid. Thus, each unit area of the medium now contains 48 moles of acid (the 41 moles of secondary acid and 1 mole of first acid present after the heating step, together with the further 6 moles of first acid generated by decomposition of the iodonium salt). Of this acid, 10 moles is consumed by protonating the 10 moles of base, and the remaining 38 moles are used to protonate 38 moles of image dye, thus producing color in the exposed areas, and leaving 2 moles of unprotonated image dye in these areas. Since there is no surplus base present, the sensitizing dye is not deprotonated and remains in its first form. This situation is represented in Figure 4C, which shows the large amount of acid present after fixing. Note that although the sensitizing dye is still is its first form, the destruction of all remaining iodonium salt ensures that the exposed areas of the medium are no longer photosensitive. In contrast, in the non-exposed areas, only 8 moles of acid are generated per unit area of the medium (the 1 mole of secondary acid from the heating step plus the 7 moles of first acid generated by destruction of the 7 moles of iodonium salt by the fixing reagent). All 8 moles of acid are used to protonate 8 moles of the base. A further mole of base is used to deprotonate the sensitizing dye, thus converting it to its second form and removing the contribution of the first form of the sensitizing dye to the color of the medium, and thus minimizing the optical density (D mιn ) in the non-exposed areas. Furthermore, 1 mole of free, unprotonated base still remains, thus ensuring that should any small amount of secondary acid be generated from the remaining secondary acid generator (for example, by exposure of the medium to elevated temperatures during transportation or storage), this small amount of secondary acid will immediately be neutralized by the base, and will thus not change the D mm of the non-exposed areas. From the foregoing description, it will be seen that, in the exposed areas, the first acid catalyzes the breakdown of the secondary acid generator, and, because the secondary acid generator used in this example produces a secondary acid which can protonate the first site of the secondary acid generator, the resultant secondary acid catalyzes the breakdown of additional secondary acid generator, so that the final quantity of secondary acid present is substantially larger than the quantity of first acid produced directly by the imagewise radiation acting on the superacid precursor, although of course the secondary acid is typically a weaker acid than the first acid. This "chemical amplification" of the first acid by the autocatalytic secondary acid generator increases the number of moles of acid generated per einstein of radiation absorbed, and thus increases the contrast of the image produced by the present process as compared with simple generation of first acid by a superacid precursor, and as compared with an '850 type process using a non-autocatalytic secondary acid generator. In practice, it has been found that, under proper conditions, at least 20, and in some cases 100 or more, moles of secondary acid can be liberated for each mole of first acid present in the exposed areas following the imagewise irradiation. From Table 1 and the related description above, it will be seen that, after an '850 medium, which uses a sensitizing dye which is converted to its second form by deprotonation with a base, has been imaged and fixed, in both the exposed and non-exposed areas the sensitizing dye has been returned to its unprotonated form. This is always the case in the non-exposed areas, and is also the case in the exposed areas if the image dye is substantially more basic than the sensitizing dye. If this is not so, in the exposed areas the sensitizing dye will remain protonated and the absorption in the first wavelength range is a combination of that due to the protonated image dye and that due to the protonated sensitizing dye. In such cases, the sensitizing dye should be chosen so that the presence of its protonated form in the D max areas does not cause objectionable effects on the image. This is especially important in color media having a plurality of different acid-generating layers and color-change layers since if, for example, the protonated form of the sensitizing dye used in the acid-generating layer associated with the magenta color-change layer has a yellow color, crosstalk will result between the magenta and yellow components of the image. To reduce or eliminate such objectionable effects, it is desirable that the protonated form of the sensitizing dye have a color similar to that of the colored form of the associated image dye. Sometimes it may be possible to use the same (or a chemically similar) dye as both the sensitizing dye and the image dye. Among the dyes which can be used in this manner are the 2,4,6- trisubstituted pyridine dyes of the invention; as discussed above and also below with reference to Figure 5, a negative-working '850 type or similar imaging medium can employ a salt of a 2,4,6-trisubstituted pyridine as a blue sensitizing dye in an acid- generating layer, and the free base form of the same 2,4,6-trisubstituted pyridine as a yellow image dye in the color-forming layer associated with this acid-generating layer. When the components of these two layers become mixed during the development of the medium, in exposed areas (i.e., in areas in which the acid- generating layer was exposed to blue radiation during imaging) both the sensitizing dye and the image dye are present in their protonated forms in the final image. If the sensitizing dye and image dye were different, one would have to allow for the effect of the sensitizing dye on the color of the image dye. However, by making the sensitizing dye identical to the protonated form of the image dye, only a single yellow colored species is present in the final image, and no "correction" of the color is required. This invention extends to the 2,4,6-trisubstituted pyridines in both their free base forms and their salts; it will be appreciated that when these 2,4,6- trisubstituted pyridines are used as yellow indicator image dyes in an acid-mediated imaging process, the dyes will remain in their free base, essentially colorless, form in areas of the image having no yellow density, while being converted to a yellow salt form in areas of the image having maximum yellow density. Similarly, when used as blue sensitizing dyes for 'onium salts, the dyes need to be in their yellow salt form during the actual sensitization, but may if desired be converted to their essentially colorless free base form later to avoid unwanted yellow color in the final image. It should be stressed that the trisubstituted pyridine indicator dyes of the present invention can be used in any known type of imaging medium which uses an indicator dye, and that this medium does not necessarily make use of the secondary acid generators of the present invention, or indeed any secondary acid generator whatsoever. Similarly, the trisubstituted pyridine dyes can be used in any known type of imaging medium using superacid precursors, whether or not that process makes use of any secondary acid generator or image dye; for example, the salts of the 2,4,6-trisubstituted pyridines can be used as sensitizing dyes in a photoresist. In the 2,4,6-trisubstituted pyridines of the invention, preferably the diarylaminophenyl group is a diphenylaminophenyl group, and desirably this group is in the 4-position on the pyridine ring. Also, it is preferred that the substituents other than the diphenylaminophenyl group be aryl groups, desirably unsubstituted or alkoxy-substituted phenyl groups; in particularly preferred compounds of this type, the other two substituents are each independently a phenyl group, a 2-alkoxyphenyl group, a 4-alkoxyphenyl group, a 2,4-dialkoxyphenyl group, a 2,6-dialkoxyphenyl group or a 2,4,6-trialkoxyphenyl group, wherein each of the alkoxy groups comprises not more than about 6 carbon atoms. A preferred group of 2,4,6-triphenylpyridines of the invention are those of the formula: 1 9 "X wherein one of R , R and R is an N,N-diphenylaminophenyl group, and the other 1 9 " two of R , R and R are each independently a phenyl group, unsubstituted or substituted with from one to three alkoxy or etherifred hydroxy groups, or an alkyl group. Specific preferred compounds falling within this formula are those in which: 1 9 (a) R is a methyl group, R is a 4-(N,N-diphenylamino)phenyl group and R 3 is a 2,4-dimethoxyphenyl group; 1 9 (b) R is a 4-(N,N-diphenylamino)phenyl group, and each of R and R is a 2,4-dimethoxyphenyl group; 9 • 1 (c) R is a 4-(N,N-diphenylamino)phenyl group, and each of R and R is a 4-methoxyphenyl, 2,4-dimethoxyphenyl, 2,4- diethoxyphenyl, 2,4-dipropoxyphenyl, 2,4-dibutoxyphenyl, 2,5- dimethoxyphenyl, 3,5-dimethoxyphenyl or 3,4-dimethoxyphenyl group; (d) R 1 is a phenyl group, R 2 is a 4-(N,N-diphenylamino)phenyl group and R is a 2,4-dimethoxyphenyl group; or (e) R 2 is a 4-(N,N-diphenylamino)phenyl group, and each of R 1 and R 3 is a -C 6 H 4 -O-CH 2 -C(=O)-N(CH 2 CH 2 OH) 2 grouping. When used in the trisubstituted pyridine indicator dye medium of the invention, the aforementioned trisubstituted pyridines produce yellow dyes with greater photo- stability than similar dyes which lack the diarylaminophenyl substituent on one of the phenyl groups. Most triphenylpyridines are soluble in non-polar solvents but almost insoluble in polar solvents such as alkanols, and increased alkanol solubility is desirable since it is often advantageous to form imaging media by coating at least some of the layers from alkanols. It has been found that the alkanol solubility of triphenylpyridines can be greatly increased by providing, on one or both or the phenyl groups which does not bear the diarylamino substituent, at least one etherified hydroxy grouping, with the ether portion of this grouping bearing at least one free hydroxyl group. Preferably, the ether portion includes a bis(hydroxy- alkyl)amino grouping; one specific preferred compound of this type is that mentioned in paragraph (e) above, in which R 2 is a 4-(N,N-diphenylamino)phenyl group, and each of R 1 and R 3 is a/?-C 6 H 4 -O-CH 2 -C(=O)-N(CH 2 CH 2 OH) 2 grouping. Such bis(hydroxyalkyl)amino groupings are especially advantageous in improving alkanol solubility. As discussed in more detail below, methods for the synthesis of triphenylpyridines are known in the art, and illustrations of several preferred synthetic methods are given in the Examples below. Accordingly, skilled organic chemists will readily be able to carry out the synthesis of any desired trisubstituted pyridine used in the imaging media of the present invention. Besides the aforementioned 2,4,6-trisubstituted pyridines, preferred indicator sensitizing dyes for use in the '850 process include fluoran dyes, phthalide dyes, xanthene dyes, acridine dyes, and dyes of the formulae: or wherein: α and δ are each a hydrogen atom or an organic group in which a carbon atom is directly bonded to the ring carrying the groups α and δ, or α and δ together comprise the atoms necessary to complete a substituted or unsubstituted benzene ring; β and γ are each a hydrogen atom or an organic group in which a carbon atom is directly bonded to the ring carrying the groups β and γ, or β and γ together comprise the atoms necessary to complete a substituted or unsubstituted benzene ring; ε is a oxygen, sulfur or selenium atom, or is an N-R a group, in which R a is a hydrogen atom, an alkyl group containing from about 1 to about 20 carbon atoms, or an aryl group; ζ is an anion; η is a CR C R group, a CR C =CR group, an oxygen or sulfur atom, or an N-R b group; θ is an oxygen, sulfur or selenium atom, or is an N-R a group; R a is a hydrogen atom, an alkyl group containing from about 1 to about 20 carbon atoms, and optionally bearing a protonated heteroatom substituent, or an aryl group; R b is an alkyl group containing from about 1 to about 20 carbon atoms, or an aryl group; R c and R are each independently a hydrogen atom, an alkyl group containing from about 1 to about 20 carbon atoms or an aryl group; n is 0, 1, 2 or 3; and Ar is an aryl or heterocyclic group. The groups α, β, γ and δ may be, for example: a. an alkyl group, for example an isopropyl, sec-butyl, tert-butyl, 2-ethyl-2-methylbutyl or 2,2-dimethylbutyl group; b. an alkenyl group, for example a vinyl group; c. an alkynyl group, for example an ethyne group; d. a cycloalkyl group, for example a cyclohexyl group; e. a cycloalkenyl group, for example a cyclohexenyl group; f. a polycyclic saturated hydrocarbon group, for example a decalinyl or adamantyl group; g. a polycyclic, ethylenically unsaturated hydrocarbon group, for example a 6,6-dimethylbicyclo[3.1.1]hept-2-en-2-yl or bicyclo[2.2.1]hept-2-en-5-yl group; h. an aryl group, for example a phenyl ring; or i. any of the foregoing substituents substituted with aryl, halo, cyano, amino or oxo groups, or containing ether, amine or urethane linkages. Sensitizing dyes suitable for use in this type of process in which the conversion of the sensitizing dye to its second form is effected by deprotonation (hereinafter called for convenience "the deprotonation process") are the fluoran, phthalide, xanthene and acridine dyes, and those of Formula XLLI above in which ε is an N-R a grouping and R a is a hydrogen atom, and those of Formula XLV above in which θ is an N-R a grouping, in which R a is a hydrogen atom or an alkyl group bearing a protonated heteroatom, for example a hydroxyl group. The remaining dyes of Formulae XLLI and XLV are suitable for use in this type of process in which the conversion of the sensitizing dye to its second form is effected by a nucleophile (hereinafter called for convenience "the nucleophile process"). Specific pyridinium dyes of Formula XLLI which have been found useful are the hexafiuoroantimonate salts of the protonated forms of: 2,4,6-tris(4-methoxyphenyl)pyridine; 2,6-bis(4-methoxyphenyl)-4-(2-thienyl)pyridine; 2,6-bis(4-methoxyphenyl)-4-(2-(4-bromophenyl)pyridine; 2,6-bis(4-methoxyphenyl)-4-(2-naphthyl)pyridine; 2,4-bis(4-methoxyphenyl)-6-(2-naphthyl)pyridine; 2,4,6-tris(2,4,6-trimethoxyphenyl)pyridine; and 2,6-bis(4-methoxyphenyl)-4-(2-( 1 ,4-dimethoxy)naphthyl)pyridine; 2,4,6-tris(2,4-dimethoxyphenyl)pyridine; and 4-(9-ethylcarbazol-3-yl)-2,6-bis(4-methoxyphenyl)pyridine. A useful quinolinium dye is the protonated form of 2-[2-[2,4 bis[octyloxy]phenyl]ethen-l-yl]quinoline (the unprotonated form of this dye is available from Yamada Chemical Co., Kyoto, Japan), while a useful xanthene dye is the protonated form of 3',6'-bis[N-[2-chlorophenyl]-N-methylamino]spiro[2-butyl- l,l-dioxo[l,2-benzisothiazole-3(3H),9'-(9H)xanthene]] (which may be prepared as described in U. S. Patent No. 4,345,017). Other dyes useful in the deprotonation process are 2-[2-[9- ethylcarbazol-3-yl]ethen-l-yl]-l-[2-hydroxyethyl]-3,3-dimethyl-3H-indolium hexafiuoroantimonate; and 5-bromo-2-[2-[9-ethylcarbazol-3-yl]ethen- 1 -yl]- 1 -[2-hydroxy- ethyl]-3,3-dimethyl-3H-indoiium hexafiuoroantimonate. Specific preferred sensitizing dyes which have been found useful in the nucleophile process include: 1 -methyl-2-[2-[2,4-bis[octyloxy]phenyl]ethen- 1 -yljquinolinium hexafiuoroantimonate; 1 -methyl-2-[2-[4-diphenylaminophenyl]ethen- 1 -yl]quinolinium hexafiuoroantimonate; l,3,3-trimethyl-2-[2-[9-phenylcarbazol-3-yl]ethen-l-yl]-3H-indolium hexafiuoroantimonate; l,3,3-trimethyl-2-[2-[9-ethylcarbazol-3-yl]ethen-l-yl]-3H-indolium hexafiuoroantimonate; 2,6-di-t-butyl-4-(2-(9-phenylcarbazol-3-yl)ethen-l-yl)pyrylium hexafiuoroantimonate; and 6-(but-2-oxy)-2-( 1 , 1 -dimethyleth- 1 -yl)-4-(2-(9-ρhenylcarbazol-3- yl)ethen- 1 -yl)benz[b]pyrylium hexafiuoroantimonate. Also, the prior art describes various combinations of nucleophiles and dyes which can be used in the nucleophile process; see, for example, U.S. Patents Nos. 5,258,274 and 5,314,795 (although note that in the present process the imaging medium may contain the nucleophile itself rather than a nucleophile-generating species as in these patents, since the nucleophile can be kept in a layer or phase separate from the acid-generating layer until the final heating step when the nucleophile converts the sensitizing dye to its second form). The nucleophile used in the present process may be a neutral molecule, for example a primary or secondary amine, a stabilized carbanion, for example a carbanion derived from a malonate ester or a nitroalkane, or a charged nucleophile, for example a thiolate. The anion of the dye should be chosen with care having regard to the first acid which will be generated during the imaging process. For example, it is inadvisable to use iodide, or another anion derived from a weak acid, as the anion of the dye, since the presence of such an anion in the acid-generating layer during imaging will cause the first acid to protonate the anion, thus leading to the formation of HI, or an acid which is similarly weak in a polymeric medium of low dielectric constant (such as those typically used in the imaging media). Such a weak acid cannot effectively protonate the secondary acid generator, and thus does not initiate the acid amplification process. Conveniently, the anion of the sensitizing dye is chosen to be the same as that of the superacid precursor; thus, for example, when the preferred diphenyliodonium hexafiuoroantimonate is used, the anion is conveniently hexafiuoroantimonate. Methods for the preparation of the aforementioned pyridinium dyes are described in the literature. In the '850 process, it is desirable that the layer or phase containing the sensitizing dye also comprise a cosensitizer. This cosensitizer has an oxidation potential lower than that of the sensitizing dye and reacts with a combination of the excited state of the sensitizing dye and the superacid precursor so as to return the dye to its ground state and transfer an electron to the superacid precursor, thereby bringing about decomposition of the superacid precursor with formation of superacid. The presence of a cosensitizer capable of acting in this manner greatly improves the quantum efficiency of the reaction between the photoexcited sensitizing dye and the superacid precursor (i.e., the quantum efficiency of superacid generation and thus the sensitivity of the imaging medium). The cosensitizer must have an oxidation potential lower than that of the sensitizing dye, but not so low that it reacts thermally with the superacid precursor; in practice, the cosensitizer should have an oxidation potential of about 700 to about 1100 mV relative to a saturated calomel electrode. Desirably, following the electron transfer to the superacid precursor, the cosensitizer should decompose quickly to prevent reverse electron transfer from the superacid precursor. The rate of decomposition of any proposed cosensitizer can be estimated by cyclic voltammetry; desirably, this rate of decomposition should be such that no reversible potential can be measured at a scanning rate below about 1 V sec "1 . Finally, the cosensitizer must not be more basic than the secondary acid generator, so that it does not interfere with the acid-catalyzed thermal decomposition of the secondary acid generator. Preferred cosensitizers include triarylamines (for example, triphenyl- amine) and hydroquinones. Preferred triarylamine cosensitizers are triphenylamines in which at least one of the phenyl rings bears a para hydroxyalkyl, alkoxyalkyl or α-alkenyl group, and those in which at least one of the phenyl rings bears a meta substituent selected from the group consisting of alkoxy, aryloxy, arylamino, alkyl and aryl groups. In the latter case, the ring bearing the meta substituent should either have no para substituent, or should have a para hydroxyalkyl, alkoxyalkyl or α-alkenyl substituent. A preferred embodiment of the invention will now be described, though by way of illustration only, with reference to Figure 5 of the accompanying drawings, which shows a schematic cross-section through a full color imaging medium (generally designated 10) of the present invention as the image therein is being fixed by being passed between a pair of hot rollers 12. The imaging medium 10 comprises a support 14 formed from a plastic film. Typically the support 14 will comprise a polyethylene terephthalate film 3 to 10 mils (76 to 254 mμ) in thickness, and its upper surface (in Figure 2) may be treated with a sub-coat, such as are well-known to those skilled in the preparation of imaging media, to improve adhesion of the other layers to the support. On the support 14 is disposed a first acid-generating layer 16 comprising: (a) an iodonium salt, namely [4-[2-hydroxytetradecan-l-yloxy]- phenyl]phenyliodonium hexafiuoroantimonate; (b) a nucleophilically-bleachable sensitizing dye of the formula: This sensitizing dye sensitizes the iodonium salt to red visible radiation; (c) a cosensitizer, namely tris(/w-methylphenyl)amine; (d) a secondary acid generator, which undergoes thermal decomposition to form a secondary acid; this secondary acid generator is of the formula: (e) a fixing agent, namely a hydroquinone; and (f) a polystyrene binder. On the opposed side of the acid-generating layer 16 from the support 14 is disposed a first color-change layer 18 comprising: (a) a first image dye, of the formula: which changes from colorless to cyan in the presence of an acid; (b) a phenolic nucleophile, namely l,3-dihydroxy-4-dodecyl- benzene (c) an amine, namely N,N'-bis(3-aminopropyl)piperazine; and (d) a binder, namely poly(2-hydroxypropyl methacrylate). The binders used in the acid-generating layer 16 and the color-change layer 18 both have a glass transition temperature substantially above room temperature. Superposed on the first color-change layer 18 is an acid-impermeable layer 20, which serves to prevent acid generated in the second acid-generating layer 22 (see below) during imaging penetrating to the first color-change layer 18. Superposed on the acid-impermeable layer 20 is a second acid-generating layer 22, which contains the same iodonium salt, secondary acid generator, cosensitizer and binder as the first acid-generating layer 16. However, the second acid-generating layer 22 contains, as a nucleophilically-bleachable sensitizing dye: which sensitizes the iodonium salt to green visible radiation. Superposed on the second acid-generating layer 22 is a second color- change layer 24 which is identical to the first color-change layer, except that the image dye previously described is replaced by a second image dye, of the formula: (available from Hilton Davis Co., 2235 Langdon Farm Road, Cincinnati, Ohio 45237 under the tradename "Copikem 35"), which changes from colorless to magenta in the presence of an acid. The next layer of the imaging medium is a second acid-impermeable interlayer 26, identical to the layer 20. Superposed on the acid-impermeable layer 26 is a third acid-generating layer 28, which contains the same iodonium salt, secondary acid generator, cosensitizer and binder as the first and second acid-generating layers 16 and 22 respectively. However, this third acid-generating layer 28 contains an indicator sensitizing dye of the formula: which sensitizes the iodonium salt to blue visible radiation. Superposed on the third acid-generating layer 28 is a third color-change layer 30 which is identical to the first color-change layer, except that the image dye is the free base form of the sensitizing dye used in the third acid-generating layer; this image dye changes from colorless to yellow in the presence of an acid. Finally, the imaging medium 10 comprises an abrasion-resistant topcoat 32. The imaging medium 10 is exposed by writing on selected areas of the medium with three radiation sources having wavelengths in the red, green and blue visible regions respectively. The red radiation, which carries the cyan channel of the desired image, images the first acid-generating layer 16, the green radiation, which carries the magenta channel, images the second acid-generating layer 22 and the blue radiation, which carries the yellow channel, images the third acid-generating layer 28. Thus, as described above with reference to Figures 4A-4C, since the sensitizing dyes in the three acid-generating layers 16, 22 and 28 are present in their first (i.e., strongly absorbing) form, latent images in acid are formed in the acid- generating layers 16, 22 and 28. The imaging medium 10 is passed between the heated rollers 12; the heat applied by these rollers causes the acid present in the exposed areas of the acid- generating layers 16, 22 and 28 to cause catalytic breakdown of the secondary acid generator therein, thus causing formation of a quantity of secondary acid substantially greater than the quantity of acid generated by the imagewise exposures. The secondary acid thus produced also catalyzes the thermal decomposition of further secondary acid generator, thus further increasing the amount of secondary acid present. The heat applied by the heated rollers 12 also raises the acid- generating layers 16, 22 and 28 and the color-change layers 18, 24 and 30 above their glass transition temperatures, thus causing the components present in each acid- generating layer to intermix with the components present in its associated color- change layer. Accordingly, the three associated pairs of acid-generating and color- change layers are "developed" and fixed as described above with reference to Table 1; i.e., the fixing reagent decomposes the remaining iodonium salt and the base neutralizes the acid produced by this decomposition. In the exposed areas, the secondary acid produced in the acid-generating layer effects the color change of the image dye in the associated color-change layer, thereby forming cyan, magenta and yellow images in the layers 18, 24 and 30 respectively. In the non-exposed areas, excess base remains and the image dye remains uncolored. The acid-impermeable interlayers 20 and 26 prevent the acids generated in the second and third acid- generating layers 22 and 28 respectively migrating to the first and second color- change layers 18 and 24 respectively, thus preventing crosstalk among the three images. The mixing of the components present in each bilayer also causes the base present in each of the color-change layers to deprotonate and/or nucleophilically attack the original forms of the sensitizing dyes present in the non-exposed areas of its associated acid-generating layer, thus removing the visible absorption due to the first forms of the sensitizing dyes, and reducing the D m j n of the images to a low level. The following Examples are now given, though by way of illustration only, to show details of preferred reagents, conditions and techniques for use in the process and medium of the present invention. Examples 1-11 : Preparation of two-site secondary acid generators Example 1 : Preparation of r2R*.7S*l-2-hvdroxy-2-(4-methylDhenyl)bicvclor2.2.11- heptan-7-yl p-toluenesulfonate This Example illustrates the preparation of the secondary acid generator of Formula LX in which L 1 is a hydroxyl group, L 2 is a (4-methylphenyl)- sulfonyl group and R 19 is a 4-methylphenyl group. The starting material used is anti- 7-norbornenol (7-hydroxybicyclo[2.2.1]hept-2-ene); this starting material may be prepared by the process described in Story, J. Org. Chem., 26, 287 (1961). Part A : Preparation of bicvclor2.2.11hept-2-en-α/ϊt -7-yl benzoate A 500 mL, three-necked, round-bottomed flask, equipped with a magnetic stirrer and nitrogen gas inlet, was charged with dry pyridine (150 mL), αnt -7-norbornenol (10.0 g, 0.091 mole) and benzoyl chloride (19.1 g, 0.136 mole). The resultant reaction mixture was stirred overnight under nitrogen at ambient temperature. Saturated sodium bicarbonate solution (500 mL) was then added, and the resultant mixture extracted twice with 300 mL aliquots of diethyl ether. The ether extracts were combined, washed with distilled water and dried over anhydrous sodium sulfate, then the ether was removed on a rotary evaporator. The pale yellow solid residue thus produced was recrystallized from methanol to give the desired product as a white crystalline solid, melting point 46-47°C, yield 16.5 g, 84% based upon the norbornenol starting material. The product was characterized as follows: Proton Nuclear Magnetic Resonance (NMR) (in deuterochloroform): δ 1.07 (2H, multiplet), 1.83 (2H, multiplet), 2.83 (2H, singlet), 4.53 (1H, singlet), 6.01 (2H, doublet), 7.37 (2H, multiplet), 7.48 (1H, doublet) and 7.94 (2H, doublet) Carbon-13 NMR (in deuterochloroform): δ 21.92, 43.64, 82.87, 128.37, 129.50, 130.50, 132.93, 134.00 and 165.95 FAB Mass spectrum M + Na + : 237 Part B : Preparation of 2-oxobicyclor2.2.nheptan-α»t/-7-yl benzoate This compound corresponds to Formula LX with L being a benzoyloxy group and L 1 and R 19 together forming an oxo group. A 3 L, three-necked, round-bottomed flask, equipped with a mech- anical stirrer, a thermometer, a condenser and a nitrogen gas inlet, was dried in an oven, assembled while still hot and cooled while passing nitrogen therethrough. The flask was then charged with bicyclo[2.2.1]hept-2-en-α«t -7-yl benzoate (30 g, 0.14 mole, prepared in Part A above), and anhydrous diethyl ether (500 mL) and the resultant ether solution, maintained under a nitrogen atmosphere, was cooled to 0-2°C with an ice water bath, and the thermometer was replaced by a rubber septum. Borane-tetrahydrofuran (THF) complex (280 mL of a 1.0 M solution in THF, 0.28 mole) was added through the rubber septum using a syringe over a period of 20 minutes. The resultant reaction mixture was stirred for 90 minutes at 0-2°C under a nitrogen atmosphere and then ice water (250 mL) was added cautiously to decompose excess borane. Water was added to the flask and then the reaction mixture was extracted twice with 250 mL aliquots of diethyl ether. The ether extracts were combined and dried over anhydrous sodium sulfate, then the ether was removed on a rotary evaporator. To remove trace amounts of water azeotropically from the residue thus produced, toluene was added to the residue and then removed under reduced pressure. The resulting solid white residue was dried in a vacuum desiccator over phosphorus pentoxide for two hours and then dissolved in dry dichloromethane (1.5 L) in a 3 L, three-necked, round-bottomed flask, equipped with a mechanical stirrer, a thermometer and a condenser with a nitrogen gas inlet; the flask had been dried in an oven, assembled while still hot and cooled while passing nitrogen therethrough. Pyridinium chlorochromate (189 g, 0.88 mole) was added to the flask and the resultant reaction mixture stirred under a nitrogen atmosphere for 3 hours under reflux. The reaction mixture was then cooled, saturated brine (300 mL) was added and the resultant mixture was extracted once with dichloromethane (300 mL). The dichloromethane phase was separated from the aqueous phase, and the dichloromethane solvent removed on a rotary evaporator to give a dark brown residue, which was purified by flash column chromatography on silica gel, using a 1 : 1 v/v diethyl ether/hexane mixture as eluant, to give the desired product as a white crystalline solid, melting point 66-68°C, yield 25.3 g, 79% based upon the benzoate starting material. The product was characterized as follows: Proton NMR (in deuterochloroform): δ 1.49-1.61 (2H, multiplet), 2.03-2.23 (4H, multiplet), 2.80 (2H, doublet), 5.15 (1H, singlet), 7.41 (2H, multiplet), 7.52 (1H, doublet) and 7.95 (2H, doublet) Carbon-13 NMR (in deuterochloroform): δ 21.66, 25.01, 38.62, 40.11, 53.21, 78.02, 128.52, 129.60, 129.75, 133.36, 165.73 and 212.39 FAB Mass spectrum M + 1 : 231 Part C : Preparation of r2R"'.7S*l-2-r4-methylphenvnbicvclo- r2.2.11heptane-2.7-diol This compound corresponds to Formula LX with L 1 and L 2 each being a hydroxyl group and R 19 being a 4-methylphenyl group. A 100 mL, three-necked, round-bottomed flask, equipped with a mechanical stirrer, a condenser and a nitrogen gas inlet, was dried in an oven, assembled while still hot and cooled while passing nitrogen therethrough. The flask was then charged with anhydrous diethyl ether (15 mL) and 2-oxobicyclo- [2.2.1]heptan-anti-7-yl benzoate (2.0 g, 8.7 mmole, prepared in Part B above). To the resultant ether solution, -tolylmagnesium bromide (39 mL of a 1.0 M solution in diethyl ether, 39 mmole) was added over a period of 15 minutes, with stirring, using a syringe inserted through a rubber septum, then the resultant mixture was stirred for 2 hours under a nitrogen atmosphere at ambient temperature. Saturated brine (15 mL) was then added cautiously, and the resultant mixture extracted twice with 20 mL aliquots of diethyl ether. The ether extracts were combined, washed with distilled water and dried over anhydrous sodium sulfate, then the ether was removed on a rotary evaporator. The pale yellow solid residue thus produced was purified by flash column chromatography on silica gel, using a 1 : 1 v/v diethyl ether/ hexane mixture as eluant, to give the desired product as a white crystalline solid, melting point 130-131°C, yield 1.8 g, 95% based upon the norcamphor starting material. The product was characterized as follows: Proton NMR (in deuterochloroform): δ 1.48-1.52 (4H, multiplet), 1.80-1.84 (2H, multiplet), 2.07-2.20 (3H, multiplet), 2.28 (3H, singlet), 2.51 (1H, singlet), 4.02 (1H, singlet), 7.10 (2H, doublet) and 7.34 (2H, doublet) Carbon-13 NMR (in deuterodimethyl sulfoxide): δ 18.96, 20.50, 26.09, 41.29, 43.89, 52.04, 75.67, 76.40, 125.76, 128.25, 134.93 and 147.35 FAB Mass spectrum M + Na + : 241. Part D : Preparation of r2R* S*1-2-hydroxy-2-(4-methylphenyl> bicyclo[ " 2.2.1 " lheptan-7-yl p-toluenesulfonate A 50 mL round-bottomed flask, equipped with a drying tube, was charged with dry pyridine (5 mL), [2R*,7S*]-2-(4-methylphenyι)bicyclo[2.2.1]- heptane-2,7-diol (0.8 g, 3.7 mmole, prepared in Part C above) and -toluenesulfonyl chloride (0.74 g, 3.88 mmole), and the resultant reaction mixture allowed to stand for 36 hours at ambient temperature. Saturated sodium bicarbonate solution (25 mL) was then added, and the resultant mixture extracted twice with 25 mL aliquots of diethyl ether. The ether extracts were combined, washed with distilled water and dried over anhydrous sodium sulfate, then the ether was removed on a rotary evaporator. The pale yellow solid residue thus produced was purified by flash column chromatography on silica gel, using a 1 : 1 v/v diethyl ether/hexane mixture as eluant, then recrystallized from diethyl ether/pentane to give the desired product as a white crystalline solid, yield 1.0 g, 74% based upon the diol starting material. This product decomposed at 68°C before melting. The product was characterized as follows: Proton NMR (in deuterochloroform): δ 1.48-1.55 (2H, multiplet), 1.72-1.76 (2H, multiplet), 2.04-2.16 (4H, multiplet), 2.29 (3H, singlet), 2.39 (3H, singlet), 2.61 (1H, singlet), 4.48 (1H, singlet), 7.07 (2H, doublet), 7.19 (2H, doublet), 7.22 (2H, doublet) and 7.70 (2H, doublet) Carbon-13 NMR (in deuterochloroform): δ 18.80, 20.92, 21.02, 26.06, 40.72, 43.98, 49.43, 75.90, 85.91, 125.31, 127.76, 129.15, 129.55, 129.90, 133.72, 136.88 and 144.86 FAB Mass spectrum M + Na + : 395. Example 2 : Preparation of riS,2S R,5S1-2-hvdroxy-2 6-trimethylbicvclo[ " 3.1.1 " l- heptan-3-yl p-toluenesulfonate This Example illustrates the preparation of the secondary acid 1 1 1 9 generator of Formula V in which R is a methyl group, L is a hydroxyl group, L is a/7-methylphenylsulfonyl group, m is 1 and N is 1. This secondary acid generator was prepared by reacting pinanediol with >-toluenesulfonyl chloride in pyridine solution at ambient temperature for five days, as described in J. Org. Chem., 36, 412 (1971). Recrystallization from hexane containing a small amount of ethyl acetate gave the pure compound (melting point 76°C, in agreement with the aforementioned paper) in 60% yield based upon the pinanediol starting material. The structure of the product was confirmed by mass, proton NMR and 13 C NMR spectroscopy. Example 3 : Preparation of riS*,2R*l-l-hydroxy-l-methyl- 2.3.4-tetrahydro- naphthalene-2-yl (±)- 10-camphorsulfonate This Example illustrates the preparation of the secondary acid generator of Formula VI in which R is a methyl group, L is a hydroxyl group, L is a (7,7-dimethylbicyclo[2.2.1]heptan-2-onyl)methylsulfonyl group and m is 1. Part A : Preparation of 3,4-dihvdro-l-methylnaphthalene α-Tetralone was reacted with methylmagnesium iodide as described in J. Org. Chem., 26, 4165 (1961), without isolation of the intermediate 1-methyl- 1-tetralol, to give the desired product, after distillation, in 68% yield as a colorless oil, boiling point 55-57°C at 0.8 mm Hg. Part B : Preparation of riS !t '.2R*l-l-methyl- 2,3,4-tetrahvdro- naphthalene- 1 ,2-diol Oxidation of the 3,4-dihydro-l-methylnaphthalene prepared in Part A above with trimethylamine N-oxide dihydrate in the presence of pyridine and a catalytic amount of osmium tetroxide in t-butanol/water under reflux, followed by recrystallization from ethyl acetate/hexane, as described in J. Indian Chem. Soc, 59, 1139 (1987) gave l-methyltetralin-cw-l,2-diol, melting point 75°C, in 80% yield. 1 "X The structure of the product was confirmed by mass, proton NMR and C NMR spectroscopy. The product had an R f of 0.5 upon thin layer chromatography on silica gel with 2% methanol/dichloromethane as eluant. Part C : Preparation of riS*.2R*1-l-hvdroxy-l-methyl-1.2,3.4- tetrahydronaphthalene-2-yl (±)- 10-camphorsuIfonate [1S*,2R*]-1 -Methyl- l,2,3,4-tetrahydronaphthalene-l,2-diol (6.6 g, 37 mmoles, prepared in Part B above) and 4-dimethylaminopyridine (0.1 g) were dissolved in pyridine (60 mL) and the resultant solution mixed with (±)-10- camphorsulfonyl chloride (10.8 g, 43 mmoles). The temperature of the resultant reaction mixture rose rapidly from 25°C to 45°C, with subsequent precipitation of pyridine hydrochloride. The reaction mixture was allowed to cool to ambient temperature and then stirred at this temperature for 16 hours. The pyridine solvent was then removed on a rotary evaporator and the resultant semi-solid, amber-colored residue was treated with 50 mL of water and 200 mL of diethyl ether. The ether phase was separated and washed with 0.1 N hydrochloric acid until the aqueous phase remained acidic, then further washed with dilute sodium bicarbonate solution and with water. The ether extract was then dried over anhydrous sodium sulfate, and concentrated after decantation from the desiccant. Upon addition of petroleum ether (35-60°C fraction) to the ether extract, crystallization occurred and the desired product was obtained as an off-white solid, melting point approximately 105°C (sintering at 90°C), yield 7.5 g, 52% based upon the diol starting material. The product had an R f of 0.4 upon thin layer chromatography on silica gel with 1% methanol/dichloromethane as eluant. The mass spectrum in the presence of sodium iodide showed the expected parent peak at 415 (392 + 23). The proton and carbon- 13 NMR spectra were found to be complex because of the presence of two diastereoisomers in the product. Using the CH-multiplet corresponding to the tosylate methyl group at δ = 5 as a reference point, the integrations of signals in the NMR spectra were found to be consistent with the presence of 28 protons, as expected for the empirical formula (C 2 ιH 28 OsS). Two sulfoester carbons at 84.5 and 85.2 ppm, and two tertiary C-OH carbons at 71.06 and 71.15 ppm, were also consistent with the expected mixture of diastereoisomers. Example 4 : Preparation of riS*,2R*1-l-hvdroxy-l-methyl-L2,3,4-tetrahydro- naphthalene-2-yl N,N-dimethylsulfamate This example illustrates the preparation of a secondary acid generator of Formula VI in which R is a methyl group, L is a hydroxyl group, L is a N,N- dimethylaminosulfonyl group and m is 1. To a solution of [1 S*,2R*]-1 -methyl- 1,2,3,4-tetrahydronaphthalene- 1,2-diol (350 mg, 2.0 mmole, prepared in Example 3, Part B above) in dimethyl- formamide (2.5 mL) was added at 0°C 60 mg (2.5 mmole) of sodium hydride. The resultant mixture was stirred at 20°C for 45 minutes, then cooled to 0°C. A solution of N,N-dimethylsulfamoyl chloride (320 mg, 2.22 mmole) in DMF (1 mL) was added dropwise over a period of 10 minutes and the resultant reaction mixture allowed to stir at 0°C for 30 minutes, then at 20°C overnight. The reaction mixture was then quenched into cold water (65 mL) containing acetic acid (3 drops) and the resultant mixture extracted twice with 10 mL aliquots of dichloromethane. The organic extracts were combined, washed with water (50 mL) and evaporated to give a pale brown oil, which was chromatographed on silica gel, eluting successively with hexanes containing 20% and 25% ethyl acetate, to give the pure desired product as a colorless oil weighing 255 mg (45% yield), and exhibiting the expected mass spectrum and proton and carbon NMR spectra. Example 5 : Preparation of riS*,2S*1-l-hydroxy-l-(4-methoxyphenyl cvclohex-2-yl p-toluenesulfonate This Example illustrates the preparation of the secondary acid generator of Formula V in which R 1 is a /?-methoxyphenyl group, L 1 is a hydroxyl group, L is a/?-methylphenylsulfonyl group, m is 1 and N is 0. Part A : Preparation of riS*.2S*1-l-(4-methoxyphenv0cvclohexane- L2-diol This preparation is based upon the method described in Davies et al., Tetrahedron, 75, 751 (1962). /7-Methoxyphenylmagnesium bromide was prepared by adding magnesium ribbon (5.9 g, 0.242 mole) to /?-bromoanisole (27.6 g, 0.147 mole) dissolved in anhydrous ether (250 mL). After 2 hours refluxing, the resultant reagent was cooled to 0-5°C and adipoin (2-hydroxycyclohexanone dimer, 10.3 g, 0.045 mole) was added slowly through a solid addition funnel, 50 mL of additional ether being used to rinse adhering solid from the neck of the flask into the reaction mixture. The reaction mixture was then refluxed for 1 hour, allowed to cool to ambient temperature and stirred overnight at this temperature. Saturated aqueous ammonium chloride solution (50 mL) was added, the solids which precipitated were filtered off, and the ether phase of the filtrate separated from the aqueous phase. The ether phase was then washed once with water (50 mL), dried over anhydrous sodium sulfate and concentrated under reduced pressure to give the crude desired product as an off- white solid (9.5 g). Recrystallization from ether/pentane gave a first crop (5.65 g) of crystals of pure product, and a second crop (2.90 g) was obtained from the mother liquor; the product had melting point 102-103°C and the total yield was 8.55 g, 43% based upon the adipoin starting material. Both crops of crystals gave identical 1 "X proton and C NMR spectra and the FAB mass spectrum in the presence of sodium iodide showed the expected parent peak at 246 (M + 1 + Na + ). Part B : Preparation of riS*,2S*1-l-hvdroxy-l-(4-rnethoxyphenyD- cyclohex-2-yl p-toluenesulfonate [lS*,2S*]-l-(4-methoxyphenyl)cyclohexane-l,2-diol (2.02 g, 9.1 mmole, prepared in Part A above) was dissolved in pyridine (25 mL) and cooled in ice bath, then / toluenesulfonyl chloride (2.71 g, 14.3 mmole) was added in portions. The resultant orange solution was allowed to warm to ambient temperature over a period of about 1 hour, stirred at ambient temperature overnight, and then added slowly to ice-water with stirring. The resultant gray solids were filtered and dried to yield the desired crude product (1.073 g). Recrystallization from benzene/petroleum ether gave the purified product as pale straw-colored plates (0.88 g, 0.23 mmole, 25% yield based upon the diol starting material). Treatment of a saturated benzene solution of this purified product with charcoal and recrystallization by addition of petroleum ether to the benzene solution resulted in a product in the form of white plates. The structure of the product was confirmed by mass, proton NMR and 13 C NMR spectroscopy; the FAB mass spectrum showed a parent peak at 399 (M + Na + ). Example 6 : Preparation of 3-hydroxy-3-phenyl-cvclohex-l-yl diphenylphosphate This Example illustrates the preparation of a secondary acid generator 7 1 9 of Formula Vffl in which R is a phenyl group, L is a hydroxyl group, L is a diphenylphosphate group, R 6 , R 8 and R 10 are each a hydrogen atom, and R 5 and R 9 are each a methylene group, the carbon atoms of R 5 and R 9 being bonded to one another, so that R 5 and R 9 , together with the four intervening carbon atoms, form a six-membered ring. Part A : Preparation of 3-hvdroxycvclohexanone A 3 L, three-necked, round-bottomed flask was charged with 1,3-cyclohexanediol (42.5 g, 0.366 mole) and acetone (1.5 L). This mixture was stirred until dissolution was complete, then the resultant solution was cooled to below 5°C in an ice bath. The solution was then vigorously stirred while Jones reagent (92 mL, prepared by dissolving 14 g of chromium tri oxide in 100 mL of water, then adding 12.2 mL of concentrated sulfuric acid with stirring) was added dropwise over a period of 1 hour, while maintaining the temperature below 5°C. The resultant mixture was stirred for an additional hour at room temperature, then concentrated. The concentrated product was diluted with diethyl ether (1 L) and stirred for an additional 12 hours over excess powdered sodium carbonate. The resultant mixture was filtered and chromatographed on silica gel, eluting with diethyl ether. The eluate was concentrated and distilled, and the distilled product was collected as a clear oil (12.7 g, 30.6% yield) boiling at 115°C under 2.5 mm Hg pressure. Carbon-13 NMR (in deuterochloroform): δ 210.9, 69.6, 50.3, 40.9, 32.6 and 20.7 Part B : Preparation of 1 -phenyl- 1, 3 -dihydroxycyclohexane Phenyl magnesium bromide (43.8 mL of a 1M solution in ether) was added dropwise at room temperature to a stirred solution of 3-hydroxycyclohex- anone (5.0 g, 0.044 mole) in 300 mL diethyl ether. The reaction mixture was stirred at room temperature for one hour, then refluxed for two hours, and finally stirred overnight at room temperature. Saturated aqueous ammonium chloride solution (60 mL) was added dropwise to the reaction mixture with constant stirring. The resulting suspension was filtered and the filtrate was washed with brine (100 mL). The organic layer was dried over sodium sulfate, filtered, and concentrated to give a crude product, which was triturated with pentane, then recrystallized from dichloromethane/pentane (1:2 v/v) to give the desired product (4.3 g, 51% yield) as a white crystalline solid. Carbon-13 NMR (in deuterochloroform): δ 148.4, 128.3, 126.8, 124.4, 74.5, 67.9, 42.9, 38.5, 32.2 and 16.2. Part C : Preparation of 3-hvdroxy-3-phenylcvclohex-l-yl diphenylphosphate 1 -Phenyl- 1, 3 -dihydroxycyclohexane (192 mg, 1.0 mmole, prepared In Part B above) was dissolved in dry pyridine (2 mL) in a 10 mL round-bottomed flask. The resultant solution was stirred and cooled in an ice bath, then diphenylphosphochloridate (269 mg, 1.1 mmole) was added, followed by 4- dimethylaminopyridine (10 mg). The resultant solution was stirred in the ice bath for 10 minutes during which time a precipitate formed. The ice bath was then removed and the reaction mixture stirred at room temperature overnight. The reaction mixture was then poured into a stirred aqueous buffer of pH 3 (100 mL) and the resultant mixture adjusted to pH 3 by the addition of approximately 1.5 mL of concentrated hydrochloric acid. The mixture was then extracted twice with 30 mL aliquots of diethyl ether, and the ether extracts were combined and washed with brine (50 mL), dried over sodium sulfate, filtered, and concentrated to produce a crude product as a thick yellow oil. The crude product was chromatographed on silica gel (eluting with dichloromethane containing a proportion of ethyl acetate increasing from 2 to 5 percent during the elution). The product containing fractions of the eluate were combined, evaporated, and stored under vacuum overnight to give the desired product (197 mg, 46% yield) as a thick colorless oil. Carbon-13 NMR (in deuterochloroform): δ 150.52, 150.43, 146.84, 129.92, 128.37, 127.09, 125.53, 124.82, 120.18, 120.12, 120.05, 77.97, 77.89, 72.85, 43.00, 42.94, 37.94, 37.94, 31.06, 31.00 and 16.84 Example 7 : Preparation of π-methoxy-l,2.3,4-tetrahvdronaphthalen-l-v0methyl diphenylphosphate This Example illustrates the preparation of a secondary acid generator of Formula I in which R 1 is a methyl group, R 2 is hydrogen, and each group Ar is a phenyl group. Part A : Preparation of sprror3.4-dihydro-l(2H)naphthalene, 2'-oxiranel A three-necked, 200-mL, round-bottomed flask equipped with an overhead stirrer, an addition funnel, and a reflux condenser was charged with trimethylsulfonium bromide (4.7 g, 30 mmole), acetonitrile (30 mL), potassium hydroxide (7.9 g, 140 mmole) and water (0.1 mL). With stirring, the reaction mixture was heated by means of an oil bath (maintained at 60°C) for five minutes before the addition of a solution of α-tetralone (2.8 g, 19.2 mmole) in acetonitrile (6 mL). The oil bath temperature was maintained at 60°C while the reaction was monitored by thin layer chromatography (TLC), using a 4:1 v/v hexanes: diethyl ether mixture as eluant. After 3 hours, the reaction mixture was cooled to room temperature, diluted with ether (150 mL), and filtered to remove insoluble salts. These salts were thoroughly washed with ether to ensure complete recovery of the desired product. The ether was then removed and the resulting residue was dissolved in hexanes and extracted with water (3 x 25 mL aliquots). The organic layer was then dried over anhydrous magnesium sulfate, and filtered to remove the drying agent; the hexanes were then removed to yield a viscous oil, which was used without further purification in Part B below. Part B : Preparation of l-hydroxymethyl-l-methoxy-1,2,3,4- tetrahydronaphthalene The crude product prepared in Part A above was dissolved in dry methanol (15 mL) in an oven-dried, nitrogen-flushed reaction vessel equipped with a magnetic stirring bar and a rubber septum cap. The reaction vessel was purged with nitrogen, and cooled in an ice/water bath. A solution of trifluoroacetic acid in methanol (2.0 mL of a 0.2 N solution) was added to the stirred solution; following this addition, TLC (using the same eluant as in Part A) indicated complete consumption of the starting material in 5-10 minutes. After volatiles had been removed by evaporation, the resulting crude oil was dissolved in hexanes/ether and extracted with water until neutral to pH paper. The crude oil was next purified by column chromatography on silica gel using hexanes/ether as eluant. Following the elution of a minor component, the desired product was obtained after concentration as a viscous oil (2.5 g, 68% over 2 steps). Proton NMR (in deuterochloroform): δ 7.1-7.4 (4H, multiplet), 3.6 (2H, multiplet), 3.1 (3H, singlet), 2.7 (3H, multiplet), and 1.8-2.3 (4H, multiplet). Carbon-13 NMR (in deuterochloroform): δ 139.60, 136.21, 128.84, 127.52, 127.18, 126.04, 79.19, 69.83, 50.13, 29.76, 25.83, 20.95. LR (KBr): 3500, 3050, 2900, 1500, 1190, 1120 and 755 cm 1 . Part C : Preparation of (1-methoxy- 1,2,3, 4-tetrahydronaphthalen- 1 - vDmethyl diphenylphosphate A dry, nitrogen-flushed reaction vessel was charged with dry pyridine (10 mL) and l-hydroxymethyl-l-methoxy-l,2,3,4-tetrahydronaphthalene (500 mg) prepared in Part B above. The solution was cooled using an ice/water bath, and diphenyl chlorophosphate (1.1 g, 4.1 mmole) was added directly to the reaction vessel. With stirring overnight, a thick white precipitate formed. TLC (using a 9: 1 v/v dichloromethane/diethyl ether mixture as eluant) indicated complete consumption of the starting material and the formation of a new material which ran near the solvent front. The reaction mixture was poured into cold water (25 mL) and extracted with ether (3 x 20 mL aliquots). The organic layers were combined and extracted with dilute (pH ~ 3-4) aqueous hydrochloric acid (3 x 10 mL aliquots), and then with water (3 x 20 mL aliquots). The organic layer was dried over anhydrous magnesium sulfate, filtered, and concentrated. Final purification by column chromatography on silica gel using hexanes/ether as eluant yielded the desired secondary acid generator as an oil (800 mg, 72% yield). Proton NMR (in deuterochloroform): δ 7.1-7.5 (14H, multiplet), 4.37 (2H, doublet), 3.1 (3H, singlet), 2.7 (2H, multiplet), 1.8-2.3 (4H, multiplet). Carbon-13 NMR (in deuterochloroform): δ 150.7, 139.4, 134.7, 129.7, 129.1, 128.0, 127.2, 126.2, 125.3, 120.2, 77.2, 73.8, 50.5, 29.6, 26.7, 20.5. Example 8 : Preparation of 3-methoxy-3-(4-phenylphenyl)but-l-yl bis(4-phenyl- phenvDphosphate This Example illustrates the preparation of a secondary acid generator 1 "X of Formula LI in which n is 2, R and R are each a methyl group, and each group Ar is a 4-phenylphenyl group. Part A : Preparation of 1 -methyl- l-(4-phenylphenyl)oxetane Trimethylsulfoxonium iodide (8.80 g, 40 mmole) was suspended in t-butanol (80 mL) in a 250 mL round-bottomed flask. The resultant suspension was stirred at 50°C and a warm solution of potassium t-butoxide (4.5 g, 40 mmole) in t-butanol (50 mL) was added. The resultant mixture was stirred at 50°C for 30 minutes, then a warm solution of 4-acetylbiphenyl (2.0 g, 10 mmole) in t-butanol (25 mL) was added dropwise with constant stirring. The mixture thus formed was stirred at 50°C for 18 hours, then cooled to room temperature and poured into stirred water (1 L). The resulting mixture was extracted with diethyl ether (3 x 75 mL aliquots). The ether extracts were combined and washed with brine (75 mL), dried over anhydrous sodium sulfate, filtered, and concentrated to give the crude desired product (2.3 g, 100% yield) as a yellow oil. Carbon-13 NMR (in deuterochloroform): δ 147.24, 140.96, 139.70, 128.79, 127.24, 127.14, 127.06, 124.17, 86.61, 64.64, 35.67, 30.68. Part B : Preparation of 3-methoxy-3-(4-phenylphenvπbutan-l-ol 1 -Methyl- l-(4-phenylphenyl)oxetane (2.2 g, 9.8 mmole, prepared in Part A above) was dissolved in methanol (100 mL) in a 500 mL round-bottomed flask. The resultant solution was stirred and trifluoroacetic acid (3 drops) was added. The solution was stirred under nitrogen for three hours, then poured into stirred water (500 mL). The resultant mixture was extracted with diethyl ether (3 x 200 mL aliquots). The ether extracts were combined and washed with brine (100 mL), dried over anhydrous sodium sulfate, filtered, and concentrated to yield the desired crude product. The crude product was purified by column chromatography on silica gel (eluting with dichloromethane to 10% diethyl ether in dichloromethane). The product-containing fractions were combined, evaporated, and placed under vacuum overnight to give the desired product (1.6 g, 64% yield) as a white crystalline solid. A sample recrystallized from hexane melted at 45.5-47°C. Carbon-13 NMR (in deuterochloroform): δ 143.43, 140.66, 139.97, 128.81, 127.34, 127.09, 127.06, 126.41, 80.74, 59.77, 50.61, 45.42, 22.59. Part C : Preparation of 3-methoxy-3-(4-phenylphenyl)but-l-yl bis(4-phenylphenyl)phosphate 3-Methoxy-3-(4-phenylphenyl)butan-l-ol (256 mg, 1 mmole, prepared in Part B above) and triethylamine (110 mg, 1.09 mmole) in acetonitrile (1 mL) were added dropwise to a solution of phosphorus oxychloride (168 mg, 1.1 mmole) in acetonitrile (3 mL) at room temperature. The resultant reaction mixture was stirred for 2 hours, during which time a white precipitate formed. A solution of 4-phenylphenol (340 mg, 2 mmole) and triethylamine (220 mg, 2.18 mmole) in acetonitrile (2 mL) was then added dropwise, and the reaction mixture was heated at reflux for 2 hours, then cooled and diluted with dichloromethane. The organic layer was washed sequentially with 1M sulfuric acid and 1M aqueous sodium hydroxide. (A slowly-separating emulsion formed during the hydroxide wash.) The organic layer was separated, dried over anhydrous sodium sulfate, and evaporated to give a crude product, which was purified by medium-pressure chromatography on silica gel, with 40-60%) ether/hexanes as eluant, to give the desired product as an oil, which was induced to crystallize from hexanes with ether trituration. (Alternatively, the product could be crystallized from isopropanol.) The yield of crystalline material (melting point. 67-71°C) was 311 mg (49%). The structure of this compound was confirmed by proton NMR spectroscopy. Example 9 : Preparation of 3-methoxy-3-(4-phenyrphenv0but-l-yl p-toluene- sulfonate To a solution of 3-methoxy-3-(4-phenylphenyl)butan-l-ol (2.04 g, 8.0 mmole, prepared in Example 8, Part B above) and triethylamine (1.51 g, 15 mmole) in acetone (10 mL) at 0° was added in one portion a solution of -toluenesulfonyl chloride (2.85 g, 15 mmole) in acetone (4 mL). The resultant suspension was warmed to 20°C and stirred at that temperature for 14 hours, then diluted with diethyl ether (40 mL). The resultant precipitate (triethylamine hydrochloride) was removed by filtration and the filtrate was evaporated to dryness to give a crude product, which was purified by column chromatography on silica gel, eluting with a 1:1 v/v hexanes: dichloromethane mixture, to give the desired product (2.87 g, 87% yield) of colorless solid. Less pure fractions were combined to provide an additional 0.147 g of colorless solid which was crystallized from hexanes (10 mL), furnishing colorless fine matted needles weighing 0.070 g, melting point 69-70.5°C. Example 10 : Preparation of 2-methoxy-2-r4-(4-phenylphenoxy)phenyl1but-l-yI diphenylphosphate This Example illustrates the preparation of a secondary acid generator 1 " of Formula LI in which n is 1, R is a methyl group, R is an ethyl group, the group Ar attached to the same carbon atom as the group R 3 is a 4-(4-phenylphenoxy)phenyl group, and the other two groups Ar are phenyl groups. Part A: Preparation of 4-(4-phenylphenoxy)propiophenone A 100 mL, three-necked, round-bottomed flask equipped with a magnetic stirrer, a Dean-Stark trap, and a nitrogen inlet was charged with 4-phenylphenol (1.70 g, 10 mmole), 4'-fluoropropiophenone (1.52 g, 10 mmole), anhydrous potassium carbonate (1.60 g, 11.6 mmole), N,N-dimethylacetamide (20 mL), and toluene (25 mL). Under a nitrogen atmosphere and with stirring, the resultant mixture was heated at 150°C for 12 hours. Most of the toluene plus some water was removed through the Dean-Stark trap during the first few hours. The reaction mixture was then poured into water (300 mL), and the precipitated solid was collected by filtration and washed with water to give a crude product, which was recrystallized from hexanes to give the desired product as white crystals (2.21 g, 73% yield, melting point 124-126°C). Proton NMR (in deuterochloroform): δ 7.96 (2H, doublet), 7.58 (4H, double doublet), 7.44 (2H, triplet), 7.38 (1H, triplet), 7.12 (2H, doublet), 7.04 (2H, doublet), 2.99 (2H, quartet), and 1.22 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 199.50, 161.68, 155.09, 140.29, 137.64, 131.76, 130.28, 128.87, 128.72, 127.31, 126.99, 120.34, 117.46, 31.61, and 8.38. Part B: Preparation of 1 -ethyl- l-r4-(4-phenylphenoxy)phenyl1oxirane A 100 mL, three-necked, round-bottomed flask equipped with a magnetic stirrer and a nitrogen inlet was charged with trimethylsulfonium bromide (2.5 g, 15.9 mmole), sodium methoxide (0.9 g, 16.7 mmole), and acetonitrile (15 mL). The mixture was stirred under nitrogen for 30 minutes, and then 4-(4-phenyl- phenoxy)propiophenone (2.0 g, 6.6 mmole, prepared in Part A above) in acetonitrile (25 mL) was added. After stirring for five hours at 60°C, the mixture was cooled to room temperature and concentrated. The residue was extracted with water/ dichloromethane, and the combined organic layers were dried and concentrated to yield the desired product (2.1 g, 100% yield, melting point 62-64°C). Proton NMR (in deuterochloroform): δ 7.58 (4H, multiplet), 7.43 (1H, triplet), 7.36 (4H, multiplet), 7.04 (2H, doublet), 7.02 (2H, doublet), 3.00 (1H, doublet), 2.79 (1H, doublet), 2.19 (1H, multiplet), 1.82 (1H, multiplet), and 0.98 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 156.69, 156.47, 140.52, 136.41, 134.99, 128.80, 128.46, 127.59, 127.07, 126.92, 119.11, 118.71, 60.74, 55.37, 28.39, and 9.10. Part C: Preparation of 2-methoxy-2-r4-(4-phenylphenoxy)phenyl " |- butan-1-ol To a 200 mL beaker containing 1 -ethyl- l-[4-(4-phenylphenoxy)- phenyl] oxirane (2.1 g, 6.6 mmole, prepared in Part B above) and methanol (100 mL) was added trifluoroacetic acid (0.5 mL). The resultant mixture was stirred for ten minutes, by which time the epoxide had dissolved completely, and TLC analysis (using a 1:1 v/v hexanes/ether mixture as eluant) indicated completion of the reaction. The resultant solution was concentrated to yield a solid product (2.2 g, 95% yield) which was used in Part D below without further purification. Proton NMR (in deuterochloroform): δ 7.59 (4H, multiplet), 7.43 (1H, triplet), 7.35 (4H, multiplet), 7.05 (2H, doublet), 7.03 (2H, doublet), 3.84 (2H, double doublet), 3.56 (1H, singlet), 3.20 (3H, singlet), 1.92 (2H, multiplet), and 0.82 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 156.55, 156.49, 140.51, 136.56, 135.75, 128.83, 128.50, 128.10, 127.12, 126.94, 119.32, 118.53, 81.43, 64.50, 49.92, 26.79, and 7.66. Part D: Preparation of 2-methoxy-2-r4-(4-phenylphenoxy)phenyllbut- 1 -yl diphenylphosphate A 50-mL round-bottomed flask was charged with 2-methoxy-2-[4-(4- phenylphenoxy)phenyl]butan-l-ol (2.2 g, 6.3 mmole, prepared in Part C above) and pyridine (10 mL). Diphenyl chlorophosphate (4.0 mL, 19.3 mmole) was added while stirring, and the resultant mixture was allowed to stand for 16 hours at room temperature. Water was then added and the mixture was extracted twice with dichloromethane. The organic layers were combined, dried and concentrated, and the resultant residue was crystallized from ether/hexanes. The white crystalline compound produced was then further purified by column chromatography on silica gel, using dichloromethane as eluant, followed by recrystallization from ether/ hexanes to yield the pure desired product (2.36 g, 64% yield, melting point 58-60°C). Proton NMR (in deuterochloroform): δ 7.0-7.6 (23H, multiplet), 4.48 (2H, multiplet), 3.15 (3H, singlet), 1.95 (2H, multiplet), and 0.86 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 156.52, 156.50, 140.50, 136.51, 135.45, 129.76, 128.82, 128.48, 128.21, 127.10, 126.92, 125.36, 120.10, 120.03, 119.25, 118.43, 79.76, 70.17, 50.13, 26.03, and 7.16. Example 11 : Preparation of 2-methoxy-2-r4-(4-methoxyphenyl)phenvnbut-l-yl diphenylphosphate This Example illustrates the preparation of a secondary acid generator 1 "X of Formula LI in which n is 1, R is a methyl group, R is an ethyl group, the group Ar attached to the same carbon atom as the group R is a 4-(4- methoxyphenyl)phenyl group, and the other two groups Ar are phenyl groups. Part A: Preparation of 4-(4-methoxyphenyl propiophenone An oven-dried, nitrogen-swept, 200-mL reaction vessel equipped with a magnetic stir bar, a pressure-equalizing addition funnel, a thermometer, and a rubber septum cap was charged with 4-methoxybiphenyl (5.0 g, 27.1 mmole), dichloromethane (30 mL), and anhydrous aluminum chloride (4.2 g, 31.5 mmole). The addition funnel was charged with propionyl chloride (2.7 g, 29.2 mmole) in dichloromethane (5 mL). The reaction vessel was cooled in an ice/salt bath to bring the flask temperature to about 0°C while maintaining an atmosphere of dry nitrogen in the vessel. The acid chloride solution was the added dropwise over a period of 20 minutes maintaining a reaction temperature of about 5°C. During this time the initial dark brown solution turned greenish in color. After the addition had been completed, the reaction mixture was allowed to warm to room temperature, stirred for an additional 1.5 hours, and then carefully added to a hydrochloric acid/ice mixture. The resultant mixture was allowed to stand overnight, then extracted with dichloromethane (4 x 25 mL aliquots). The organic layers were combined and extracted with water until the water was neutral to pH paper. The organic layer was then dried over anhydrous magnesium sulfate, filtered, and concentrated to yield an off-white crystalline solid. Recrystallization from ethanol provided the desired product (4.42 g, 68% yield, melting point 146-147°C). Proton NMR (in deuterochloroform): δ 8.05 (2H, doublet), 7.60 (4H, double doublet), 7.00 (2H, doublet), 3.85 ( 3H, singlet), 3.03 (2H, quartet), and 1.28 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 200.45, 159.87, 145.13, 135.04, 132.32, 128.62, 128.36, 126.61, 114.40, 55.39, 31.80, and 8.36. Part B: Preparation of 1 -ethyl- l-r4-(4-methoxyphenvDphenyl " l oxirane 4-(4-Methoxyphenyl)propiophenone (2.0 g, 8.3 mmole, prepared in Part A above) was heated at 60°C with trimethylsulfonium bromide (2.0 g, 12.7 mmole), and potassium hydroxide (4.5 g, 80 mmole) in acetonitrile (20 mL) with a trace of water (2 drops from a pipette). Initially the reaction mixture turned yellow in color. This color faded with time and the reaction mixture was nearly colorless after three hours, when the reaction was complete. The reaction mixture was cooled to room temperature, diluted with ethyl ether, and filtered to remove insoluble matter. The filtrate was concentrated, and the resulting residue was dissolved in hexanes and extracted with water (3 x 25 mL aliquots). The hexane layer was then dried over anhydrous magnesium sulfate, filtered, and concentrated to give the product (1.82 g, 86% yield) as a white crystalline solid which was used without further purification in Part C below. Proton NMR (in deuterochloroform): δ 7.5 (4H, doublet), 7.4 (2H, doublet), 7.0 (2H, doublet), 3.9 ( 3H, singlet), 2.9 (2H, double doublet), 2.0 (2H, doublet of multiplets), and 1.0 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 159.18, 139.91, 138.43, 133.28, 128.09, 126.61, 126.45, 114.23, 60.85, 55.52, 55.36, 28.25, and 9.11. Part C: Preparation of 2-methoxy-2-r4-(4-methoxyphenyr)phenyl " l- butan-1-ol 1 -Ethyl- l-[4-(4-methoxyphenyl)phenyl]oxirane (1.8 g, 7.1 mmole, prepared in Part B above) was dissolved in dry dichloromethane (5 mL) in an oven- dried, nitrogen-flushed reaction vessel equipped with a magnetic stir bar and a rubber septum cap. To the resultant solution was added dry methanol (20 mL), and the reaction mixture was then cooled in an ice/water bath. Next, trifluoroacetic acid in methanol (2.0 mL of a 0.2 N solution) was added and the solution was stirred overnight allowing the solution to warm to room temperature. After concentration of the solution, a white crystalline solid was obtained which was recrystallized from hexanes to give the product (1.2 g, 59% yield, melting point 108-109°C). Proton NMR (in deuterochloroform): δ 7.54 (4H, double doublet), 7.38 (2H, doublet), 6.95 (2H, doublet), 3.88 (2H, doublet), 3.84 (3H, singlet), 3.18 (3H, singlet), 1.94 (2H, multiplet), 1.78 (1H, triplet), and 0.83 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 159.18, 139.71, 139.63, 133.19, 128.06, 127.02, 126.59, 114.23, 81.50, 64.62, 55.35, 50.00, 26.76, and 7.65. Part D: Preparation of 2-methoxy-2-r4-(4-methoxyphenyl)phenyllbut- 1-yl diphenylphosphate A 50-mL round-bottomed flask was charged with 2-methoxy-2-[4-(4- methoxyphenyl)phenyl]butan-l-ol (500 mg, 1.75 mmole, prepared in Part C above) and pyridine (5 mL). Diphenyl chlorophosphate (1.0 mL, 4.82 mmole) was added while stirring, and the resultant mixture was allowed to stand for 16 hours at room temperature. Water was then added and the mixture was extracted twice with dichloromethane. The organic layers were combined, dried and concentrated, and the resultant residue was purified by column chromatography on silica gel, using dichloromethane as eluant, followed by crystallization from ether/hexanes to give the pure desired product (581 mg, 64% yield , melting point 57-59°C). Proton NMR (in deuterochloroform): δ 7.0-7.6 (18H, multiplet), 4.48 (2H, multiplet), 3.85 (3H, singlet), 3.15 (3H, singlet), 1.95 (2H, double multiplet), and 0.86 (3H, triplet). Carbon-13 NMR (in deuterochloroform): δ 159.22, 150.44, 139.81, 138.97, 133.12, 129.71, 128.05, 127.12, 126.49, 125.30, 120.05, 114.25, 79.94, 70.07, 55.36, 50.21, 26.13, and 7.17. Examples 12-18 : Preparation of trisubstituted pyridine indicator/ sensitizing dyes Example 12 : Preparation of 2,6-ftz ' sf4-methoxyphenyl)-4-(4-N,N-diphenylamino)- phenylpyridine Part A: Preparation of 4-diphenylamino-4'-methoxychalcone A mixture of 4-N,N-diphenylaminobenzaldehyde (3.35g, 12 mmole), /j-methoxyacetophenone (1.84 g, 12 mmole) and potassium hydroxide (1.0 mL of a 45 percent aqueous solution) in ethanol (34 mL) was warmed gently to about 40°C for 10 minutes under nitrogen with stirring, and the resultant mixture was then left to stand at 20°C overnight. The resulting slurry was filtered, and the filter cake was washed with cold ethanol and dried overnight to provide 4.42 g (92% yield) of the desired product as fine, brilliant yellow needles. Part B: Preparation of 4'-methoxyphenacylpyridinium bromide Pyridine (2.0 mL, 26 mmole) was added to a solution of 4'-methoxy- α-bromoacetophenone (4.58 g, 20 mmole) in acetone (60 mL), and the resultant mixture was stirred at 20°C for two days, then diluted with diethyl ether (50 mL), cooled to 0°C and filtered. The filter cake was washed with ether (30 mL) and dried under vacuum to provide the desired product as colorless prisms (5.97 g, 97% yield). Part C: Preparation of 2.6-b^( ' 4-methoxyphenyl)-4-(4-N,N-diphenyl- amino)phenylpyridine A mixture of 4-diphenylamino-4'-methoxychalcone (4.05 g, 10 mmole, prepared in Part A above), 4'-methoxyphenacyipyridinium bromide (3.08 g, 10 mmole, prepared in Part B above) and ammonium acetate (15 g) in acetic acid (30 mL) was stirred under reflux for 20 hours, then poured into water (400 mL). The resulting solution was made basic with sodium hydroxide, then extracted with dichloromethane (100 mL); the organic layer was separated, washed with water (400 mL) and evaporated to give a brittle, tan foam. This foam was chromatographed on basic alumina, eluting with methylene chloride containing, successively, 60%, 50%, 40% and 0% hexanes. The purest fractions, as determined by TLC, were combined and evaporated to give a very pale yellow oil, which was crystallized from a mixture of ethyl acetate (10 mL) and hexanes (40 mL) to give the desired product as fine, matted, colorless needles (3.52 g, 65.8 % yield). Example 13 : Preparation of 2-(2,4-dimethoxyphenyl -4-(4-N,N-diphenylamino)-6- f2,5-dimethoxyphenvDpyridine Part A: Preparation of 2,5-dimethoxyphenacylpyridinium bromide Pyridine (3.0 mL, 39 mmole) was added to a solution of 2,5-dimethoxyphenacyl bromide (5.18 g, 20 mmole) in acetone (50 mL), and the resultant mixture was stirred at 20°C for five hours, then left to stand at 5°C for 64 hours, diluted with diethyl ether (80 mL) at the same temperature and filtered. The filter cake was washed with ether (20 mL) and dried under vacuum to provide the crude product as colorless needles (7.27 g, 107% crude yield). Part B : Preparation of 2-(2,4-dimethoxyphenyl)-4-(4-KN-diphenyl- amino)-6-(2,5-dimethoxyphenvDpyridine A mixture of 2,4-dimethoxy-4'-diphenylaminochalcone (1.305 g, 3 mmole, prepared in a manner analogous to that of Example 12, Part A above), 2,5-dimethoxyphenacylpyridinium bromide (1.014g, 3.0 mmole, prepared in Part A of this Example) and ammonium acetate (5.4 g) in acetic acid (10 mL) was stirred under reflux for 3 hours, then cooled to 20°C and poured into water (200 mL). The resulting suspension was extracted with dichloromethane (2 25 mL aliquots); the organic layer was separated and evaporated, and the resultant residue chromatographed on silica gel, eluting with dichloromethane. The purest fractions, as determined by TLC, were combined and evaporated to give the desired product as a pale yellow brittle foam (446 mg, 25 % yield). The product showed an absorption maximum (λ max ) of 342 nm in dichloromethane; when protonated with methanesulfonic acid λ max was 452 nm, with a molar absorption ε of 34,800. Example 14 : Preparation of 2-(2,4-dimethoxyphenvD-4-f4-N,N-diphenylamino -6- methylpyridine Part A: Preparation of 2,4-dimethoxy-4'-(N,N-diphenylamino)- chalcone To a suspension of 4-N,N-diphenylaminobenzaldehyde (2.73g, 10 mmole) and 2,4-dimethoxyacetophenone (1.80 g, 10 mmole) in ethanol (40 mL) was added at 20°C a solution of potassium hydroxide (3 drops of a 45 percent aqueous solution dissolved in 3 mL of ethanol). The reaction mixture thus formed was stirred at 20°C for 30 hours, then warmed to 55°C for 1 hour, and allowed to cool to 20°C over a period of 2 hours; at the end of this time, a bright yellow solid had been deposited. The resulting slurry was cooled to 5°C and filtered, and the filter cake was washed with cold methanol and dried overnight to provide the desired product as bright, fluorescent needles (2.943 g, 68% yield). Part B : Preparation of 2-(2.4-dimethoxyphenvπ-4-(4-N,N-diphenyl- amino)-6-methylpyridine A mixture of 2,4-dimethoxy-4'-(N,N-diphenylamino)chalcone (1.305 g, 3.0 mmole, prepared in Part A above), N-acetonylpyridinium bromide (0.515 g, 3 mmole) and ammonium acetate (5.2 g) in acetic acid (10 mL) was refluxed for 4.5 hours, then left to stand at 20°C overnight and poured into water (150 mL). The resulting suspension was extracted with dichloromethane (2 25 mL aliquots); the organic fractions were combined and evaporated, and the resultant residue chromatographed on silica gel, eluting with 2% acetone in dichloromethane, to give the desired product as a colorless brittle foam (744 mg, 52% yield). The product had λ ma 344 nm in dichloromethane; when protonated with methanesulfonic acid λ max was 442 nm, with a molar absorption ε of 33,100. Example 15 : Preparation of 2,6-6z5f2,4-dipropoxyphenyl)-4-(4-N,N-diphenyl- amino)phenylpyridine /?-N,N-diphenylaminobenzaldehyde (15.31 g, 56 mmole), 2,4-dipro- poxyacetophenone (18.86 g, 84.1 mmole) and ammonium acetate (108 g, 1.4 mole, 25 equivalents) were dissolved in acetic acid (200 mL) in a three-necked round- bottom flask equipped with an overhead stirrer. The reaction mixture was heated to reflux, and the progress of the reaction followed by analyzing samples by thin layer chromatography, eluting with 1% methanol in dichloromethane; this analysis indicated that the reaction was complete after 1.5 hours refluxing at 127°C. Accordingly, after this time the reaction mixture was poured into a separatory funnel containing water (400 mL) and the resultant aqueous suspension extracted with dichloromethane (200 mL). The organic layer was separated and then added to water (400 mL) with vigorous stirring. Aqueous potassium hydroxide was added to the aqueous phase to give a pH of about 8, whereupon the organic phase changed color from orange to light yellow. The organic phase was separated, dried over sodium sulfate, the sodium sulfate filtered off, and the organic solution concentrated on a rotary evaporator to give an oil. This oil was crystallized from warm t-butyl methyl ether, then from 18% ethyl acetate in methanol, to give the pure desired product as pale yellow, fine matted needles (8.55 g, 14.9% yield). The product had λ max 446 nm in dichloromethane when protonated with methanesulfonic acid, with ε = 35,060. Example 16 : Preparation of 2,6-to(2,4-dimethoxyphenvP-4-(4-diphenylamino- phenvDpyridine. and the hexafiuoroantimonate salt thereof Part A: Preparation of 2,4-dimethoxyphenacylpyridinium bromide Pyridine (8.0 mL, 104 mmole) was added to a solution of 2,4-dimethoxyphenacyl bromide (8 g, 31 mmole) in acetone (110 mL), and the resultant mixture was stirred at 20°C for 24 hours, then diluted with diethyl ether (140 mL) and filtered. The filter cake was washed with ether and dried under vacuum to provide the crude product as very pale tan plates (9.52 g, 91% yield). Part B: Preparation of 2.6-b/s(2,4-dimethoxyphenvD-4-(4-diphenyl- aminophenvOpyridine A mixture of 2,4-dimethoxy-4'-diphenylaminochalcone (2.18 g, 5 mmole, prepared in Example 14, Part A above), 2,4-dimethoxyphenacylpyridinium bromide (2.20 g, 5.07 mmole, prepared in Part A above) and ammonium acetate (5 g) in acetic acid (10 mL) was stirred under reflux for 3.5 hours, then cooled to 20°C and added to a mixture of water (200 mL) and dichloromethane (50 mL). The resulting mixture was neutralized to pH 7 with sodium hydroxide, and the organic layer was separated, washed with water (150 mL) and evaporated to give a brown, semisolid oil. This oil was taken up in dichloromethane and filtered to remove a pale yellow solid, which proved to be the desired product. The filtrate was chromatographed on basic alumina, eluting with methylene chloride, and the purest fractions, as determined by TLC, were combined and evaporated to 40 mL. The previously separated solid was added to the concentrated solution, and the resultant solution was evaporated, gradually replacing the dichloromethane with methanol. The resultant slurry was cooled to 10°C and filtered, and the filter cake was washed with cold methanol and dried under vacuum to give the desired product as fine, matted, pale yellow needles (475 mg, 16 % yield). Part C: Preparation of hexafiuoroantimonate 2,6-όw(2,4-Dimethoxyphenyl)-4-(4-N,N-diphenylaminophenyl)- pyridine (80 mg, prepared in Part B above) was dissolved in dichloromethane (3 mL) and washed with a solution of hexafluoroantimonic acid (0.6 mL of 68% aqueous acid, diluted to 5 mL with water), and again washed with the same concentration of acid (1.5 mL). The resulting organic layer was washed with water (10 mL) and evaporated to 3 mL, then diluted with methyl t-butyl ether (MTBE, 6 mL). The solid which precipitated was collected by filtration, washed with MTBE and dried under vacuum to give the desired product as bright yellow, fine, matted needles (106 mg, melting point 138-142°C). The product had λ max 444 nm in dichloromethane, ε = 31,150; when protonated with methanesulfonic acid, ε = 31,430. Example 17 : Preparation of 2.6-b/,y(2,4-dimethoxyphenyl)-4-(4-diphenylamino- phenvDpyridine This Example illustrates a preparation of the same triphenylpyridine as in Example 16, but using a "one-pot" procedure which does not require isolation of intermediates. A mixture of 2,4-dimethoxyacetophenone (3.60 g, 20 mmole), -diphenyaminobenzaldehyde (2.73 g, 10 mmole), sodium -nitrobenzenesulfonate (2.25 g, 10 mmole) and ammonium acetate (15 g) in acetic acid (35 mL) was stirred under reflux for 2.5 hours, then poured into ice-water (350 mL) to give a red-orange precipitate. The resultant slurry was stirred, with the addition of sufficient potassium hydroxide solution to raise the pH to about 8, and the resultant mixture was filtered. The orange-yellow filter cake was washed with water (100 mL), taken up in dichloromethane, and the resultant solution was separated from the aqueous phase and filtered through a silica gel pad to remove a low R f colored impurity. Further elution of the silica gel pad with dichloromethane, and then with 2% methanol in dichloromethane, followed by evaporation of the combined filtrates to approximately 25 mL, resulted in precipitation of a solid and formation of a slurry. This slurry was diluted with methanol (70 mL) to produce a precipitate, which was isolated by filtration to produce the crude desired product as a yellow solid (2.95 g, 50% yield). This product was recrystallized by dissolving it in dichloromethane (45 mL) and adding methanol (55 mL) to give a final product as a pale yellow solid (2.51 g). The product had λ max 342 nm in dichloromethane, ε = 31,540; when protonated with methanesulfonic acid λ max was 446 nm, ε = 34,170. Example 18 : Preparation of 4-r4-fN,N-diphenylamino)phenvn-2,6-b 5'r4-(2-oxo-2- (N,N-bis(2-hvdroxyethyl))ethoxyphenyl)lpyridine The Example illustrates the preparation of the triphenylamine of Formula Al above, in which R is a 4-(N,N-diphenylamino)phenyl group, and each of R 1 and R 3 is a /?-C 6 H 4 -O-CH 2 -C(=O)-N(CH 2 CH 2 OH) 2 grouping. Part A : Preparation of 4-r4-(N,N-diphenylamino)phenyl]-2,6-6 ^(4- hydroxyphenvOpyridine hydrochloride A mixture of 4-hydroxyacetophenone (9.0 g, 66 mmole), 4-diphenyl- aminobenzaldehyde (10.0 g, 36 mmole) and ammonium acetate (30 g) in acetic acid (100 mL) was stirred under reflux for 2.5 hours, then cooled to 20°C and poured into water (1.3 L). The resultant precipitate was collected by filtration, washed with water (200 mL) and taken up in diethyl ether (350 mL). The organic phase which resulted was separated from the accompanying aqueous phase, dried over sodium sulfate and magnesium sulfate, and filtered to give a dark red solution, through which gaseous hydrogen chloride was bubbled. A dark yellow precipitate thus formed was collected by filtration, washed with diethyl ether (30 mL) and dried under vacuum to give the desired dark yellow hydrochloride salt (12.95 g, 36% yield) in a crude form, which was used in Part B below without further purification. Part B : Preparation of 4-[ " 4-(N,N-diphenylamino)phenyll-2,6-b/ , r4- (ethylcarboxy methoxyphenyllpyridine hydrochloride The crude hydrochloride salt (10.0 g, 184 mmole, prepared in Part A above) was dissolved in dimethylformamide (100 mL). To this solution was added freshly-ground potassium carbonate (35 g), then ethyl bromoacetate (5.94 g, 356 mmole). The resultant reaction mixture was stirred at 20°C for four hours, then filtered to remove inorganic material, and the filter cake was washed with dimethylformamide (30 mL). The filtrate was poured into cold water (1.4 L) containing acetic acid (40 mL) to give a precipitate, which was extracted with diethyl ether (2 250 L aliquots), and the resultant ether solution was washed with water and then with brine. The washed solution was dried over sodium sulfate and evaporated to an oil, which was chromatographed on silica gel (eluting with 25% ethyl acetate in hexanes) to provide a pale yellow glass (3.922 g, 31.4% yield), which slowly solidified. Part C : Preparation of 4-r4-(TSf.N-diphenylamino)phenyl1-2,6-6 4- (2-oxo-2-(N,N-bis(2-hvdroxyethvD)ethoxyphenvD1pyridine A solution of the glass prepared in Part B above (2.33 g, 3.43 mmole) and diethanolamine (4.66 g) was stirred at 115°C under a slow stream of nitrogen for four hours, then left at 20°C under nitrogen overnight. The resultant thick solution was diluted with methanol (5 mL) and dichloromethane (10 mL), then chromato- graphed on silica gel (eluting with dichloromethane containing successively 10%, 15% and 20% of methanol) to give the crude desired product (2.523 g, 92.3% yield) as a pale yellow glass containing diethanolamine as the only impurity observable by NMR spectroscopy. This crude product (2.35 g) was recrystallized as follows. The product was taken up in hot acetone (20 mL), cooled to 20°C and seeded to initiate precipitation. After one hour, the resultant slurry was diluted with diethyl ether (50 mL) and left to stand for two days at 5°C, then filtered to isolate the solid. The filter cake was washed with diethyl ether (10 mL) and dried under vacuum to provide the purified desired product as colorless fine irregular prisms (1.934 g, 82% recovery based upon the crude product). Examples 19-20 : Imaging and Photostability Example 19 : Imaging This Example illustrates the effectiveness of various secondary acid generators in the two-site process of the present invention. Two coating fluids were prepared as follows: Fluid A: A superacid precursor ([4-[2-hydroxytetradecan-l-yloxy]phenyl]- phenyl iodonium hexafiuoroantimonate, 30 mg), an indicator sensitizing dye (2,4,6-trø(2,4-dimethoxyphenyl)pyridinium hexafiuoroantimonate, prepared as described in U.S. Patent No. 5,441,850, 30 mg) and a hydroquinone fixing agent (2,5-bw(2,4,4-trimethylpent-2-yl)-l,4-dihydroxybenzene, 15 mg) were dissolved in a solution of polystyrene (average molecular weight 45,000, available from Aldrich Chemical Company, Milwaukee, Wisconsin, United States of America, 3.0 g of a 10 percent w/w solution in 2-butanone). The acid amplifier being tested (9.0 mg) was dissolved in the resultant solution (300 mg) to form Coating Fluid A. Fluid B: The aforementioned Copikem 35 magenta indicator image dye (2.5 g) and an amine base (l-(3-aminoprop-l-yl)imidazole, 0.275 g) were added to a solution of an acrylate binder (Acrysol B-82, supplied by Rohm and Haas, Philadelphia, Pennsylvania 19104, United States of America, 7.5 g in 40 g of ethyl acetate) and the resultant solution was added to a solution of poly(vinyl alcohol) (Vinol 540, supplied by Air Products Corporation, Allentown Pennsylvania, United States of America, 28.6 g of a 7 percent solution in water). Water (55 g) was added and the mixture was then sonicated, after which evaporation of ethyl acetate gave Coating Fluid B. To prepare an imaging medium, Coating Fluid A was coated on to a reflective poly(ethylene terephthalate) base (Melinex reflective film base, supplied by ICI Americas, Inc., Wilmington, Delaware, United States of America) using a #8 coating rod, and allowed to dry to form an acid-generating layer. Coating Fluid B was then coated over the acid-generating layer using a #6 coating rod, and allowed to dry to form a color-change layer. The imaging medium thus formed was exposed to ultraviolet radiation (10 units) from a nuArc 26- IK Mercury Exposure System (supplied by nuArc Company, Inc., 6200 West Howard Street, Nile Illinois 60648, United States of America) through a step wedge (Stouffer T4105). The films were then heated at 60°C for 30 seconds and thereafter at 120°C for 30 seconds. Green reflection optical densities of the exposed films were measured using an X-Rite 310 photographic densitiometer (supplied by X-Rite, Inc., Grandville Michigan, United States of America) equipped with the appropriate filter (Status A). The secondary acid generator was also tested for thermal stability as follows. The secondary acid generator (5 mg) and the aforementioned Copikem 35 image dye (8.5 mg) were dissolved in 250 mg of a 10% solution of polystyrene (average molecular weight 280,000, available from Aldrich Chemical Company, Milwaukee, Wisconsin, United States of America) in 2-butanone to form a coating fluid which was coated onto the aforementioned Melinex film base using a #16 wire- wound coating rod. The resultant film was laminated to a piece of transparent film base coated with a release layer (a silicone coating on poly(ethylene terephthalate) base of 0.5 mil thickness, available from Release International, 1400 Harvester Road, West Chicago, Illinois 60185, United States of America) at 120°C and approximately 1 ft/minute (approximately 5 mm/sec). The purpose of this lamination was to prevent evaporation of the secondary acid generator during prolonged heating at elevated temperatures. Portions of the laminated structure were heated for various times at 120°C. Any acid generated by thermal decomposition of the secondary acid generator caused a color change in the indicator dye which was proportional to the amount of acid generated. This color change was measured using the aforementioned X-Rite 310 photographic densitometer. The density increase (green) measured after 8 minutes storage at 120°C is shown in Table 2 below. The secondary acid generators tested by this procedure included a control oxalate secondary acid generator, 3-(4-benzyloxybenzyloxyoxalyloxy)but-l- yl 4-benzyloxybenzyl oxalate, of the formula: those produced in Examples 1, 2, 3 and 5 above, and related compounds of the Formulae XI and XLI below. The results are shown in Table 2 below. Table 2 *These compounds deviate slightly from Formula XH, being substituted on the 3- or 3- and 5-carbon atoms of the phenyl group. From Table 2 it will be seen that some of the compounds of the present invention (e.g., the compounds of Examples 1 and 5) are significantly more sensitive than the control (9.89 and 9.73 cm 2 /mJ, respectively, compared with 2.24 cm /mJ for the control). Variation of the substituent (X in Table 2) on a phenyl group adjacent the hydroxyl trigger changes the sensitivity of the secondary acid generator. Example 20 : Imaging This Example illustrates the improved photostability of the trisubstituted pyridine yellow dyes of the present invention, as compared with similar dyes lacking a diarylamino grouping. To test the photostability of the dyes, the dye, toluenesulfonic acid (to protonate the dye to its colored form) and polystyrene (molecular weight approximately 45,000) was dissolved in methyl ethyl ketone and coated on to a reflective ρoly(ethylene terephthalate) film (Melinex 329, available commercially from ICI Americas, Inc., Wilmington, Delaware, United States of America). The quantities of dye, acid and polystyrene in the solution were chosen to provide a coating thickness of approximately 2 μm, with a dye concentration of approximately 0.3 mmoles per square meter, and an acid concentration of approximately 0.4 mmoles per square meter; these proportions allowed the dye to be fully protonated, thereby ensuring that the film achieved the maximum optical density possible for the dye concentration used. Since preliminary observations indicated that the dyes retained color better when protected from oxygen and ultraviolet radiation, and since such protection is readily incorporated into commercial media, to provide a realistic test the media were protected from oxygen and ultraviolet radiation. The oxygen barrier used comprised 1 μm poly(vinyl alcohol) layers coated on both the back surface (the surface remote from the dye-containing coating) and over the dye- containing layer, except in the cases marked with an asterisk in Table 3 below, in which the back surface coating was omitted. The ultraviolet protection used was a 4 mil (101 μm) poly(ethylene terephthalate) film incorporating an ultra-violet absorber (P4C1A film available commercially from E.I. Du Pont de Nemours, Wilmington, Delaware, United States of America), placed over the dye-containing coating. The protected coated films were placed under fluorescent lights having an intensity of 2000 foot candles (20,000 lux) for a period of at least 7 days. Table 3 below shows the percentage of optical density retained after the exposure (determined from D ret aιned = (Df,nai/D ιn , tιa ι)*100, where D retaιned is the percentage of retained density, Dfj na ι is the optical density after exposure, and D ιmtιa ι is the optical density before exposure. Table 3 also shows the wavelength of maximum absorption (λ ma ) of the dyes. This wavelength and the width of the corresponding peak (denoted "λ Peak range") were determined by coating the dyes in a similar manner to that used for the photostability tests, but using a transparent film base, and measuring the variation of absorption with wavelength using a conventional visible light spectrometer. The dyes used in these tests were as follows: Dye A (Control): The dye of Formula Al above in which R 1 is a phenyl group, 9 " R is a -dimethylaminophenyl group, and R is an o-octyloxyphenyl group; Dye B (Control): The dye of Formula Al in which R 1 and R 3 are each a p- methoxyphenyl group, and R is a ?-dimethylaminophenyl group; Dye C: The dye prepared in Example 12 above; Dye D: The dye prepared in Example 14 above; Dye E: The dye prepared in Example 17 above; Dye F: The dye prepared in Example 15 above; Dye G: The dye of Formula Al in which R 1 and R 3 are each a 2,4- dibutoxyphenyl group, and R is a/?-diphenylaminophenyl group; Dye H: The dye of Formula Al in which R 1 and R 3 are each a 3,5- dimethoxyphenyl group, and R is a ?-diphenylaminophenyl group; Dye I: The dye of Formula Al in which R 1 and R 3 are each a 3,4- dimethoxyphenyl group, and R is a 7-diphenylaminophenyl group; 1 9 Dye J: The dye of Formula Al above in which R is a phenyl group, R is a ^-diphenylaminophenyl group, and R is a 2,4-dimethoxyphenyl group; Dye K: The dye prepared in Example 18 above. Table 3 From the data in Table 3 above, it will be seen that Dyes C-J, the diarylaminophenyl dyes of the present invention, displayed much better photostability than the dialkylaminophenyl dyes.

Claims: CLAIMS 1. A medium for generation of acid and comprising: a first acid-generating component capable of generating a first acid; and a secondary acid generator capable of thermal decomposition to form a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the first acid, characterized in that the secondary acid generator has a first site bearing a first leaving group and a second site bearing a second leaving group, the first leaving group being capable of protonation by the first acid, with expulsion of the first leaving group, to form a cation, followed by: (i) loss of a proton from the cation to form an unstable intermediate, which then fragments with loss of the second leaving group, accompanied by either (a) loss of a second proton; or (b) addition of a proton-containing nucleophile, followed by loss of a proton; or (ii) electrophilic addition of the cation to an unsaturated reagent bearing a proton at the site of addition and a proton-containing nucleophilic grouping at an adjacent site, following which said proton on the reagent is lost and the second leaving group is displaced by the nucleophilic grouping; the second leaving group, in combination with a proton, forming the secondary acid. 2. A medium according to claim 1 characterized in that the secondary acid is capable of protonating the first leaving group of the secondary acid generator so that the acid-catalyzed thermal decomposition of the secondary acid generator is autocatalytic. 3. A medium according to either of the preceding claims characterized in that the first leaving group comprises a nitrogen, phosphorus, sulfur or oxygen atom bonded directly to the first site, the first leaving group having a PK RA + greater than about -6. 4. A medium according to any one of the preceding claims characterized in that the second leaving group is a neutral group having the PK HA of its conjugate acid not greater than about 2, or is a cationic group having the PK HA+ of its conjugate acid not greater than about 2. 5. A medium according to any one of the preceding claims characterized in that the first and second leaving groups are both attached to a carbocyclic ring containing from 5 to 8 carbon atoms. 6. A medium according to any one of the preceding claims characterized in that the secondary acid generator is of one of the following formulae: (wherein Ar is an aryl group, with the two aryl groups being the same or different, R 1 is an alkyl or cycloalkyl group, and R 2 represents one or more hydrogen atoms or alkyl, aryl, alkoxy or aryloxy groups substituted on the benzene ring); (wherein Ar is an aryl group, with the three aryl groups being the same or different, 1 " R is an alkyl or cycloalkyl group, R is an alkyl or cycloalkyl group, and n is 1 or 2); R 5 R 9 H H (in) R 6 R 7 R 8 ►10 (where, in Formulae ffl and IV, L 1 is the first leaving group, L 2 is the second leaving group, and R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are each independently a hydrogen atom, an alkyl, cycloalkyl, aralkyl or aryl group, subject to the proviso that any two of R , R , R 7 , R 8 , R 9 and R 10 may, together with the carbon atom(s) to which they are attached, form a carbocyclic ring); L 1 H L 2 H R 5 - ■ R 9 (VH) and R 6 R 7 R 8 R 10 H H R 5 - ■ R 9 (vm) R 6 R 7 R 8 R 10 (where, in Formulae VH and VIE, L 1 is the first leaving group, L 2 is the second leaving group, and R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are each independently a hydrogen atom, an alkyl, cycloalkyl, aralkyl or aryl group, subject to the proviso that any two of R 5 , R 6 , R 7 , R 8 , R 9 and R 10 may, together with the carbon atom(s) to which they are attached, form a carbocyclic ring and that this carbocyclic ring may optionally be substituted with an additional second leaving group L 2 ). 7. A medium according to claim 6 characterized in that the secondary acid generator is of Formula I or H and that R and R (if present) are each an alkyl group, and each group Ar independently is a phenyl, 4-methoxyphenyl, 4-phenylphenyl or 4-(4-phenylphenoxy)phenyl group. 8. A medium according to claim 6 characterized in that the secondary acid generator is of one of Formulae HT, TV, Vπ and VLU and also conforms to one of the following formulae: 1 9 1 1 (wherein L and L are as defined in claim 6, R is an aryl or alkyl group; m is 0, 1, 2 or 3; and N is 0 or 1, subject to the proviso that, when L 1 is a hydroxyl group, L 1 and L are in the cis orientation to one another); 1 9 1 *! (wherein L and L are as defined in claim 6, R is an aryl or alkyl group; and m is 1 1 9 0, 1, 2 or 3, subject to the proviso that, when L is a hydroxyl group, L and L are in the cis orientation to one another); and L 1 (wherein L 1 is the first leaving group, L 2 is the second leaving group and R 19 is a carbonium ion stabilizing group). 9. A medium according to claim 8 characterized in that: L 1 comprises a nitrogen, phosphorus, sulfur or oxygen atom bonded directly to the cyclohexane ring, the group L 1 having a PK HA+ greater than about -6; L comprises a halo, sulfonium, sulfonate, sulfate, sulfamate or phosphate ester group; and R 19 (if present) is an alkyl, cycloalkyl, aralkyl or aryl group. 10. A medium according to claim 9 characterized in that the secondary acid generator is of Formula V or VI and in that: R 11 is an aryl group, optionally substituted with at least one halo, alkyl, nitro, alkoxy or aryloxy substituent, or is an alkyl group containing not more than about 6 carbon atoms; L 1 is of the formula OR 12 wherein R 12 is a hydrogen atom or an acyl group, or a group of the formula -C(=O)-C(=O)-O-R 13 , in which R 13 is an alkyl or aryl group; L 2 is of the formula -OSO R 14 , wherein R 14 is an aryl group, optionally substituted with at least one halo, alkyl, nitro, alkoxy or aryloxy substituent, or is an alkyl group containing not more than about 12 carbon atoms, which may be substituted with an alicyclic group, this alicyclic group optionally bearing an oxo group, or L 2 is a diarylphosphate ester group or an N,N-disubstituted sulfamate group wherein each of the substituents independently is an alkyl or aryl group; and m is 0 or 1. 11. A medium according to claim 9 characterized in that the secondary acid generator is of Formula LX and in that: L 1 is a hydroxy, alkoxy, amino, alkylamino, acyloxy, aroyloxy, acylamino, aroylamino, carbamate and 2-oxopyridyl group or a group of the formula -O-C(=O)-C(=O)-O-R 20 , wherein R 20 is an alkyl or aryl group; and L 2 is a group of the formula -OSO 2 R 21 or -S + (R 21 ) 2 X " (wherein each R 21 is an aryl group, which may be substituted with at least one halo, alkyl, nitro, alkoxy or aryloxy substituent, or is an alkyl group containing not more than about 12 carbon atoms, which may be substituted with an alicyclic group, this alicyclic group optionally bearing an oxo group, subject to the proviso that when more than one 91 91 • group R is present, these groups R may be the same or different), and X is an anion, or L is a diarylphosphate ester group or an N,N-disubstituted sulfamate group wherein each of the substituents independently is an alkyl or aryl group. 12. A medium according to claim 8 characterized in that: (a) the secondary acid generator is of Formula V and R 11 is a 1 9 phenyl group, L is a hydroxyl group and L is a tosyl group; (b) the secondary acid generator is of Formula V and R 11 is a ?- chlorophenyl, 7-methylphenyl, jp-phenoxyphenyl or />-methoxyphenyl group, L 1 is a hydroxyl group and L is a methylsulfonyl group; (c) the secondary acid generator is of Formula VI and R 15 is a 1 9 methyl group, L is a hydroxyl group and L is a -methylphenylsulfonyl, p-n- butylphenylsulfonyl, /?-«-octylphenylsulfonyl, (7,7-dimethylbicyclo[2.2. l]heptan-2- onyl)methylsulfonyl or N,N-dimethylsulfamate group; or 1 9 (d) of Formula LX in which L is a hydroxyl group, L is a tosyloxy group and R 19 is a 3,5-dichlorophenyl, 4-trifluoromethylphenyl, 4- chlorophenyl, 3 -chlorophenyl, phenyl or 4-methylphenyl group. 13. A medium according to claim 6 characterized in that the secondary acid generator is: (a) ( 1 -methoxy- 1 ,2,3 ,4-tetrahydronaphthalen- 1 -yl)methyl diphenylphosphate; (b) 3-methoxy-3-(4-phenylphenyl)but-l-yl bis(4-phenylphenyl)- phosphate; (c) 2-methoxy-2-[4-(4-phenylphenoxy)phenyl]but-l-yl diphenylphosphate; (d) 2-methoxy-2-[4-(4-rnethoxyphenyl)phenyl]but-l-yl diphenyl- phosphate; (e) [2R*,7S*]-2-hydroxy-2-(3,5-dichlorophenyl)bicyclo[2.2.1]- heptan-7-yl ?-toluenesulfonate; (f) [2R*,7S *]-2-hydroxy-2-(4-trifluoromethylphenyl)bicyclo- [2.2.1 ]heptan-7-yl jP-toluenesulfonate; (g) [2R*,7S*]-2-hydroxy-2-(4-chlorophenyl)bicyclo[2.2.1]- heptan-7-yl /?-toluenesulfonate; (h) [2R*,7S*]-2-hydroxy-2-(3-chlorophenyl)bicyclo[2.2.1 ]- heptan-7-yl -toluenesulfonate; (i) [2R*,7S*]-2-hydroxy-2-phenylbicyclo[2.2.1]heptan-7-yl/?- toluenesulfonate; or (j) [2R*,7S*]-2-hydroxy-2-(4-methylphenyl)bicyclo[2.2.1]- heptan-7-yl -toluenesulfonate. 14. A medium according to any one of the preceding claims characterized in that the second leaving group comprises a phosphate ester, and there is present with the secondary acid generator an unsaturated reagent as defined in paragraph (ii) of claim 1. 15. A medium according to claim 14 characterized in that the unsaturated reagent is a phenol, thiophenol, thiol or a phosphine. 16. A medium according to any one of the preceding claims characterized in that there is present in admixture with the sensitizing dye a cosensitizer which: (a) has an oxidation potential of from about 700 to about 1100 mV relative to a standard calomel electrode; and (b) is not more basic than the secondary acid generator. 17. A medium according to claim 16 characterized in that the cosensitizer comprises a triarylamine or a hydroquinone. 18. A medium according to any one of the preceding claims characterized by comprising a sensitizing dye of one of the formulae: and wherein: α and δ are each a hydrogen atom or an organic group in which a carbon atom is directly bonded to the ring carrying the groups α and δ, or α and δ together comprise the atoms necessary to complete a substituted or unsubstituted benzene ring; β and γ are each a hydrogen atom or an organic group in which a carbon atom is directly bonded to the ring carrying the groups β and γ, or β and γ together comprise the atoms necessary to complete a substituted or unsubstituted benzene ring; ε is a oxygen, sulfur or selenium atom, or is an N-R a group, in which R a is a hydrogen atom, an alkyl group containing from about 1 to about 20 carbon atoms, or an aryl group; ζ is an anion; η is a CR c R d group, a CR c =CR d group, an oxygen or sulfur atom, or an N-R b group; θ is an oxygen, sulfur or selenium atom, or is an N-R a group; R a is a hydrogen atom, an alkyl group containing from about 1 to about 20 carbon atoms, and optionally bearing a protonated heteroatom substituent, or an aryl group; R b is an alkyl group containing from about 1 to about 20 carbon atoms, or an aryl group; R c and R are each independently a hydrogen atom, an alkyl group containing from about 1 to about 20 carbon atoms or an aryl group; n is O, 1, 2 or 3; and Ar is an aryl or heterocyclyl group. 19. A medium for generation of acid and comprising: a first acid-generating component, the first acid-generating component being capable of generating a first acid; and a secondary acid generator capable of thermal decomposition to form a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the first acid, characterized in that the thermal decomposition of the secondary acid generator is also catalyzed by the secondary acid. 20. A process for generation of acid and comprising: providing a medium comprising a first acid-generating component capable of generating a first acid, and a secondary acid generator, this secondary acid generator being capable of thermal decomposition to form a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the first acid; in at least part of the medium, causing formation of the first acid from the first acid-generating component; and thereafter heating the medium to cause, in the part of the medium in which formation of the first acid has occurred, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the secondary acid, characterized in that the medium is a medium according to any one of the preceding claims. 21. A secondary acid generator characterized by one of the formulae: wherein Ar is an aryl group, with the aryl groups being the same or different, R is an alkyl or cycloalkyl group,R represents one or more hydrogen atoms or alkyl, aryl, alkoxy or aryloxy groups on the benzene ring, R 3 is an alkyl or cycloalkyl group, and n is 1 or 2. 22. A 2,4,6-trisubstituted pyridine characterized in that at least one of the substituents is a /?αrø-(N,N-diarylamino)phenyl group, the other two substituents being alkyl, cycloalkyl or aryl groups. 23. An imaging medium comprising an acid generator capable of generating an acid upon exposure to actinic radiation, and an image dye capable of changing color upon exposure to the acid, characterized in that the image dye comprises a trisubstituted pyridine according to claim 22. 24. An acid generating medium comprising a phosphonium, sulfonium, diazonium or iodonium salt capable of decomposing to give an acid with a pK a less than about 0, and a sensitizing dye capable of sensitizing the phosphonium, sulfonium, diazonium or iodonium salt to radiation of a wavelength to which it is essentially not sensitive in the absence of the sensitizing dye, characterized in that the sensitizing dye comprises a salt of a 2,4,6-trisubstituted pyridine according to claim 22.

Inventors: BOGGS ROGER A, GRASSHOFF JURGEN M, KOLB ERIC S, MARSHALL JOHN L, MINNS RICHARD A, RAMOS SOCORRO M, STROUD STEPHEN G, TELFER STEPHEN J, YANG HAIXIN, BOGGS, ROGER, A., GRASSHOFF, JURGEN, M., KOLB, ERIC, S., MARSHALL, JOHN, L., MINNS, RICHARD, A., RAMOS, SOCORRO, M., STROUD, STEPHEN, G., TELFER, STEPHEN, J., YANG, HAIXIN
Applicants: BOGGS ROGER A, GRASSHOFF JURGEN M, KOLB ERIC S, MARSHALL JOHN L, MINNS RICHARD A, POLAROID CORP, RAMOS SOCORRO M, STROUD STEPHEN G, TELFER STEPHEN J, YANG HAIXIN, BOGGS, ROGER, A., GRASSHOFF, JURGEN, M., KOLB, ERIC, S., MARSHALL, JOHN, L., MINNS, RICHARD, A., POLAROID CORPORATION, RAMOS, SOCORRO, M., STROUD, STEPHEN, G., TELFER, STEPHEN, J., YANG, HAIXIN
Application year: 1997
Main classification: G03C5
IPC codes: B41M5, G03C1, G03F7