Photosensitive composition containing a transition metal coordination complex cation and a borate anion and photosensitive materials employing the same

A composition including a cationic transition metal coordination complex and a borate anion, wherein said composition is capable of absorbing actinic radiation and producing free radicals which can initiate free radical addition polymerization of a free radical addition polymerizable or crosslinkable monomer is disclosed. The compositions are particularly useful as visible light photoinitiators.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to photosensitive compositions, and more 
particularly, to compositions which include a cationic transition metal 
coordination complex and a borate anion. These inventive compositions are 
capable of absorbing actinic radiation and producing free radicals which 
can initiate polymerization of a free radical addition polymerizable or 
crosslinkable monomer. 
2 Description of the Prior Art 
U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead Corporation describe 
imaging materials and imaging processes in which images are formed through 
exposure-controlled release of an image-forming agent from a microcapsule 
containing a photohardenable composition. The photohardenable composition 
typically includes an ethylenically unsaturated monomer and a 
photoinitiator material which is capable of absorbing actinic radiation 
and producing free radicals to initiate free radical polymerization of the 
ethylenic monomer. To produce an image, the imaging material is image-wise 
exposed to actinic radiation and the microcapsules are subjected to a 
uniform rupturing force. Typically the image-forming agent is a colorless 
color precursor which is image-wise released from the microcapsules to a 
developer sheet whereupon it reacts with a developer material to form a 
visible image. 
U.S. patent application Ser. No. 339,917, filed Jan. 18, 1982 
(corresponding to U.K. Pat. No. 2,113,860), and U.S. Pat. No. 4,576,891 
disclose a full color imaging system wherein three sets of microcapsules 
which are sensitive to different bands of actinic radiation are employed. 
These microcapsules respectively contain cyan, magenta and yellow color 
precursors. The absorption characteristics of the three sets of 
microcapsules in these photosensitive materials must be sufficiently 
different that the cyan-forming capsules can be differentially hardened at 
a predetermined wavelength or over a predetermined wavelength band without 
hardening the magenta or yellowforming capsules and, likewise, 
sufficiently different that the magenta-forming and yellow-forming 
capsules can be selectively hardened upon exposure respectively to second 
and third wavelengths or wavelength bands, without hardening the 
cyan-forming capsules or hardening the other of the yellow-forming or 
magenta-forming capsules. Microcapsules having this characteristic (i.e., 
cyan-, magenta- and yellow-forming capsules which can be selectively 
hardened by exposure at distinct wavelengths without cross-exposure) are 
referred to herein as having "distinctly different sensitivities." 
U.S. Pat. No. 4,772,541, also assigned to The Mead Corporation disclose 
photohardenable compositions including visible light-sensitive 
photoinitiators. The photoinitiators preferably comprise cationic 
dye-borate anion complexes represented by the formula 
##STR1## 
wherein D.sup.+ is a cationic dye, and R.sub.1, R.sub.2, R.sub.3 and 
R.sub.4 are independently selected from the group consisting of alkyl, 
aryl, alkaryl, allyl, aralkyl, alkenyl, alkynyl, alicyclic and saturated 
or unsaturated heterocyclic groups. In practice, the cyan, magenta and 
yellow color-forming microcapsules are respectively sensitive to red (650 
nm), green (550 nm) and blue (450 nm) light, and contain photoinitiators 
which are sensitive to these wavelengths. 
Transition metal coordination complexes capable of photoexcitation have 
been studied in the literature. See, for example, Sutin and Creutz, 
"Properties and Reactivities of the Luminescent Excited States of 
Polypyridine Complexes of Ruthenium (II) and Osmium (II)", Inorganic and 
Organometallic Photochemistry, pp. 1-27, 1978; Flynn and Demas, "Synthesis 
and Luminescence of the Tris(2,2'-bipyridine) iridium (III) Ion", Journal 
of the American Chemical Society, Vol. 96, pp. 1959-1960, 1974; Reitz et 
al., "Interand Intramolecular Excited-State Interactions of 
Surfactantactive Rhenium (I) Photosensitizers", Journal of the American 
Chemical Society, Vol. 110, pp. 5051-5058, 1988; Kober et al., "Synthetic 
Control of Excited States Nonchromophoric Ligand Variations in Polypyridyl 
Complexes of Osmium (II)", Inorganic Chemistry, Vol. 24, pp. 2755-2763, 
1985; Creutz and Sutin, "Electron-Transfer Reactions of Excited States 
Reductive Quenching of the Tris(2,2'-bipyridine) ruthenium (II) 
Luminescence", Inorganic Chemistry, Vol 15, pp. 496-499, 1976. 
SUMMARY OF THE INVENTION 
It has now been discovered that compounds which contain a cationic 
transition metal coordination complex and a borate anion are useful 
photoinitiators of free radical addition reactions The transition metal 
atom forming the coordination complex is preferably a transition metal 
atom having a d.sup.6 orbital configuration. Further, it has also been 
discovered that the initiator works particularly well if one or more of 
the ligands attached to the metal cation contains a pyridinium group and 
is bi-or tri-dentate. 
The mechanism whereby the compound containing the cationic transition metal 
coordination complex and the borate anion absorbs energy and generates 
free radicals is not entirely clear. It is hypothesized that upon exposure 
of the compound to actinic radiation, the metal atom in the coordination 
complex absorbs light and shifts one or more metal-centered electrons to 
the attached ligands. This is known in the art as metal to ligand charge 
transfer (MLCT). After MLCT, the borate anion reacts with the coordination 
complex by a mechanism which is not clear to form a radical which 
initiates free radical addition polymerization or crosslinking of a 
polymerizable or crosslinkable species. The presumed mechanism is 
oxidation of the borate anion which decomposes to form a triaryl borane 
and an alkyl radical. See, Chatterjee et al., "Electron-Transfer Reactions 
in Cyanine Borate Ion Pairs: Photopolymerization Initiators Sensitive to 
Visible Light", Journal of the American Chemical Society, 1988, Vol. 110, 
pp. 2326-2328. 
One of the particular advantages of these initiators is the ability to 
select from a large number of cationic transition metal coordination 
complexes which absorb at substantially different wavelengths. The 
absorption characteristics of the initiators are principally determined by 
the absorption of the coordination complex. Thus, by selecting a complex 
which absorbs at 400 nm or greater, the sensitivity of the photosensitive 
material can be extended well into the visible range. 
The initiator compositions of the present invention are useful in any 
photohardenable composition polymerizable by free radical polymerization. 
They are particularly useful in providing full color photosensitive 
materials in which the photohardenable compositions are microencapsulated. 
In these materials, a layer including three sets of microcapsules having 
distinct sensitivity characteristics is provided on a support. Each set of 
microcapsules respectively contains a cyan, magenta, or yellow 
color-forming agent. Because of the extension of the sensitivities well 
into the visible spectrum, the sensitivities of the three photoinitiators 
selected may be sufficiently spaced apart to prevent unwanted 
cross-exposure of different color-forming microcapsules. Photoinitiators 
can be designed for use in the cyan-, magenta-, and yellow-forming 
capsules which are respectively sensitive to red, green and blue light. 
In comparison to the above-described prior art systems utilizing cationic 
dye-borate anion complexes as photoinitiators, the initiators of the 
present invention can enable the use of two or more quenching borate 
anions per cation, thus improving efficiency of photogeneration of free 
radicals. In addition they are more soluble in water and other polar 
solvents and therefore they can be used in higher concentrations in 
compositions containing more polar monomers. 
A principal object of the present invention is to provide a novel 
photoinitiator which includes a cationic complex of a transition metal and 
a borate anion in solution in a polymerizable material. 
In accordance with one embodiment, the present invention is a 
photohardenable composition including a cationic transition metal 
coordination complex and a borate anion, and a free radical addition 
polymerizable or crosslinkable material wherein said composition is 
capable of absorbing actinic radiation and producing free radicals which 
can initiate free radical addition polymerization or crosslinking of the 
free radical addition polymerizable or crosslinkable material 
It is particularly preferred that the transition metal atom have a d.sup.6 
orbital configuration. It is further preferred that the metal atom have 
one or more pyridinium-group containing ligands covalently bonded to it. 
The transition metal coordination complex and the borate anion may be 
present in the composition as separate ions or may form an ion pair. 
A further embodiment of the present invention provides a photosensitive 
material. The material comprises a support having a layer of the 
above-defined photohardenable composition. It is particularly preferred 
that the composition contain an image-forming agent and that the 
composition be maintained as an internal phase which is surrounded by 
microcapsule walls. It is also preferred that the photosensitive material 
be used in an imaging system whereupon it image-wise hardens as a result 
of being exposed to actinic radiation, particularly visible light. 
In still another embodiment, the photosensitive material is useful for 
forming full-color images. When forming full-color images, the support has 
a layer of photosensitive microcapsules including a first set of 
microcapsules having a cyan image-forming agent associated therewith, a 
second set of microcapsules having a magenta image-forming agent 
associated therewith, and a third set of microcapsules having a yellow 
image-forming agent associated therewith, wherein at least one of the 
three sets of microcapsules contains an internal phase which includes the 
above-defined photohardenable composition. 
Accordingly, it is an object of the present invention to provide a 
composition which is capable of initiating free radical polymerization as 
a result of exposure to visible or near-ultraviolet light 
An additional object of the present invention is to provide a free radical 
photoinitiator composition which is more soluble in polar materials. 
These, and other objects will be understood by those skilled in the art as 
reference is made to the detailed description of the preferred embodiment. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While describing the preferred embodiments, certain terminology will be 
utilized for the sake of clarity. It is to be understood that such 
terminology includes not only the recited embodiments, but all technical 
equivalents which perform substantially the same function in substantially 
the same way to obtain the same result 
U.S. Pat. Nos. 4,399,209, 4,440,846, 4,772,530 and 4,772,541 are 
incorporated herein by reference to the extent that reference thereto may 
be necessary to complete this disclosure. 
The inventive compositions include both a cationic transition metal 
coordination complex and a borate anion. The cationic complex and the 
borate anion may be present in a photohardenable composition as individual 
ions (dissociated) or associated as an ion pair. When present as an ion 
pair the initiator can be represented by the general formula (I): 
##STR2## 
where C.sup.n+ is a cationic transition metal coordination complex; 
R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are independently selected from the 
group consisting of alkyl, aryl, alkaryl, allyl, aralkyl, alkenyl, 
alkynyl, alicyclic and saturated or unsaturated heterocyclic groups; and n 
is an integer ranging from 1 to 3. 
The complex C.sup.n+ is preferably represented by the general Formula 
(II): 
EQU [ML.sub.x ].sup.n + (II) 
wherein M represents a central transition metal atom; L represents one or 
more identical or different ligands covalently bonded to the central 
transition metal atom; x is an integer ranging from 2 to 6; and n is an 
integer ranging from 1 to 3. In the preferred embodiment, the complex 
cation has a coordination number of 6, although the complex may have a 
coordination number of 4,5,7 or 8. 
The transition metal atom is located at the center of the coordination 
complex. In the preferred embodiment, the metal atom has a d.sup.6 orbital 
configuration according to the valence bond hybridization method of 
classifying orbital configurations. Examples of useful atoms include 
Re(I), Fe(II), Ru(II), Os(II), Co(III), and Ir(III). Ru(II) has been found 
to be particularly suitable for use in the present invention. Other 
transition metal atoms may be utilized in accordance with the present 
invention as long as they form a coordination complex capable of metal to 
ligand charge transfer when covalently bonded to one or more ligands when 
photoexcited. 
At least one of the ligands is selected so that upon exposure of the 
complex to actinic radiation, electrons are transferred from the metal 
atom to the ligands. This is preferably accomplished by selecting a ligand 
which possesses one or more unoccupied low-lying pi orbitals capable of 
accepting the transferred electrons according to the molecular orbital 
theory In particular, bidentate ligands (ligands which bond at two sites 
of the center metal atom) and tridentate ligands (ligands which bond at 
three sites of the center metal atom) have proved to be successful, and 
bidentate and tridendate ligands containing one or more heterocyclic 
groups having one or more nitrogen atoms are especially preferred. 
Examples of ligands which are capable of bonding with the transition metal 
atom to produce a photosensitive transition metal coordination complex 
include pyridine (pyr) and substituted pyridines, 2,2'-bipyridine (bipy), 
4,4'-dimethyl-2,2'-bipyridine (Me.sub.2 bipy), 1,10-phenanthroline (phen), 
3,4,7,8-tetramethyl-1,10-phenanthroline (Me.sub.4 phen), 
2,2',2"-terpyridine (terpy), 5,6-dimethyl-1,10-phenanthroline 
(5,6-(CH.sub.3).sub.2 phen), 5-methyl-1,10-phenanthroline 
(5-(CH.sub.3)phen), 5-chloro-1,10-phenanthroline (5-Cl(phen)), 
5-nitro-1,10-phenanthroline (5-NO.sub.2 phen), 
4,7-dimethyl-1,10-phenanthroline (4,7-(CH.sub.3).sub.2 phen), and 
2,4,6-tri(2-pyridyl-s-triazine)(TPTZ) and substituted derivatives thereof. 
The transition metal coordination complexes suited for use in the present 
invention may be commercially obtained or synthesized. Examples of 
photosensitive transition metal coordination complexes include 
Co(bipy).sub.3.sup.2+, Ru(terpy).sub.2.sup.2+, Ru(Me.sub.2 
bipy)(bipy).sub.2.sup.2+, Ru(Me.sub.2 bipy).sub.3.sup.2+, 
Ru(phen).sub.3.sup.2+, Fe(Me.sub.2 bipy).sub.3.sup.2+, 
Ru(bipy).sub.3.sup.2+, Ru(phen)(bipy).sub.2.sup.2+ and Ir(Me.sub.2 
bipy).sub.2 Cl.sub.2.sup.+. The following complexes, while not having been 
tested, are also believed to be useful: Ru(5,6 (CH.sub.2).sub.2 
phen).sub.3.sup.2+, Ru(5-(CH.sub.3)phen).sub.3.sup.2 +, 
Ru(5-Cl(phen)).sub.3.sup.2+, Ru(5-NO.sub.2 phen).sub.3.sup.2+, Os(Me.sub.2 
bipy).sub.3.sup.2+, Os(bipy).sub.3.sup.2+, Os(5,6-(CH.sub.3).sub.2 
phen).sub.3.sup.2+, Os(5-Cl(phen)).sub.3.sup.2+, 
Os(5-(CH.sub.3)phen).sub.3.sup.2+, Os(phen).sub.3.sup.2+, 
Ru(4,7-(CH.sub.3).sub.2 phen).sub.3.sup.2+, Ru(TPTZ).sub.3.sup.2+, 
Ir(bipy).sub.3.sup.3+, Re[(bipy)(CO).sub.3 NC(CH.sub.2).sub.n CH.sub.3 
].sup.+ (n=0-17), Zn(bipy).sup.2+, Zn(bipy).sub.3.sup.2+, 
Os(terpy).sub.2.sup.2+, Os(Me.sub.4 phen).sub.2 
(cis-bis(1,2-diphenylphosphino)-ethylene).sup.2+, Os(phen).sub.2 
(MeCN).sub.2.sup.2+, Os(phen).sub.2 
(dimethylphenylphosphine).sub.2.sup.2+, Os(bipy).sub.2 
(bis(diphenylphosphino)methane).sup.2+, Os(phen).sub.2 
(cis-bis(1,2-diphenylphosphino)ethylene).sup.2+, 
Os(bipy)(o-phenylenebis(dimethylarsine).sub.2.sup.2+, Os(bipy).sub.2 
(DMSO).sub.2.sup.2+, 
Os(bipy)(cis-bis(1,2-diphenylphosphino)ethylene).sup.2+. The transition 
metal coordination complexes are characterized by being capable of 
transferring an electron from the central metal atom to the attached 
ligands when exposed to actinic radiation. It is particularly preferred 
that the electron transfer be initiated when the complex cation is exposed 
to visible light. However, depending on the absorption sensitivity of the 
complex cation, other sources of actinic radiation, such as ultraviolet 
light may be selected. 
The borate anion is designed such that the borate radical generated upon 
exposure to light and after electron transfer to the cation readily 
dissociates with the formation of a radical as follows: 
EQU BR.sub.4 .fwdarw.BR.sub.3 +R. 
For example, particularly preferred anions are triphenylbutylborate and 
trianisylbutylborate anions because they readily dissociate to 
triphenylborane or trianisylborane and a butyl radical. On the other hand 
tetrabutylborate anion does not work well presumably because the 
tetrabutylborate radical is not stable and it readily accepts an electron 
back from the dye in a back electron transfer and does not dissociate 
efficiently. Likewise, tetraphenylborate anion is very poor because the 
phenyl radical is not easily formed. 
Preferably, at least one but not more than three of R.sup.1, R.sup.2, 
R.sup.3, and R.sup.4 is an alkyl group. Each of R.sup.1, R.sup.2, R.sup.3, 
and R.sup.4 can contain up to 20 carbon atoms, and they typically contain 
1 to 7 carbon atoms More preferably R.sup.1 -R.sup.4 are a combination of 
alkyl group(s) and aryl group(s) or aralkyl group(s) and still more 
preferably a combination of three aryl groups and one alkyl group. For 
example, it has been discovered that (tris(p-t-butyl phenyl))butyl borate 
and (tris phenyl) hexyl borate can successfully complex with the 
transition metal coordination complex cation to form a free radical 
photoinitiator. 
Representative examples of alkyl groups represented by R.sup.1 -R.sup.4 are 
methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, stearyl, etc. The 
alkyl groups may be substituted, for example, by one or more halogen, 
cyano, acyloxy, acyl, alkoxy or hydroxy groups. 
Representative examples of aryl groups represented by R.sup.1 -R.sup.4 
include phenyl, naphthyl and substituted aryl groups such as anisyl and 
alkaryl such as methylphenyl, dimethylphenyl, etc. Representative examples 
of aralkyl groups represented by R.sup.1 -R.sup.4 groups include benzyl. 
Representative alicyclic groups include cyclobutyl, cyclopentyl, and 
cyclohexyl groups. Examples of an alkynyl group are propynyl and ethynyl, 
and examples of alkenyl groups include a vinyl group. 
As a general rule, useful transition metal coordination complex 
cation-borate anion complexes must be identified empirically, however, 
potentially useful cationic and borate anion combinations can be 
identified by reference to the Weller equation (Rehm, D. and Weller, A., 
Isr. J Chem. (1970), 8, 259-271), which can be simplified as follows. 
EQU .DELTA.G=E.sub.ox -E.sub.red -E.sub.h .nu. 
where .DELTA.G is the change in the Gibbs free energy, E.sub.ox is the 
oxidation potential of the borate anion BR.sub.4.sup.-, E.sub.red is the 
reduction potential of the complex and E.sub.h .nu. is the energy of light 
used to excite the cation Useful complexes will have a negative free 
energy change. Similarly, the difference between the reduction potential 
of the cation and the oxidation potential of the borate must be negative 
for the complex to be dark stable, i.e., Eox-Ered&gt;0. 
As indicated, the Weller equation is a simplification and it does not 
absolutely predict whether a complex will be useful in the present 
invention or not. There are a number of other factors which will influence 
this determination. One such factor is the effect of the monomer on the 
complex. It is also known that if the Weller equation produces too 
negative a value, deviations from the equation are possible. Furthermore, 
the Weller equation only predicts electron transfer, it does not predict 
whether a particular complex is an efficient initiator of polymerization. 
The equation is a useful first approximation. 
The initiators of the present invention can be prepared by mixing together 
two solutions, the first containing a salt of the transition metal 
coordination complex (e.g., a halogen salt dissolved in water), and the 
second containing the borate anion, typically as a sodium or ammonium salt 
of the borate anion dissolved in an organic solvent, and isolating the 
resultant product. The resultant material containing the cation-anion pair 
of the present invention is water or oil soluble and is particularly 
useful as a free radical photoinitiator. 
The most typical examples of a free radical addition polymerizable or 
crosslinkable compound useful in the present invention is an ethylenically 
unsaturated compound and, more specifically, a polyethylenically 
unsaturated compound. These compounds include both monomers having one or 
more ethylenically unsaturated groups, such as vinyl or allyl groups, and 
polymers having terminal or pendant ethylenic unsaturation. Such compounds 
are well known in the art and include acrylic and methacrylic esters of 
polyhydric alcohols such as trimethylolpropane, pentaerythritol, and the 
like; and acrylate or methacrylate terminated epoxy resins, acrylate or 
methacrylate terminated polyesters, etc. Representative examples include 
ethylene glycol diacrylate, ethylene glycol dimethacrylate, 
trimethylolpropane triacrylate (TMPTA), pentaerythritol tetraacrylate, 
pentaerythritol tetramethacrylate, dipentaerythritol hydroxypentacrylate 
(DPHPA), hexanediol-1, 6-dimethacrylate, and diethyleneglycol 
dimethacrylate. 
The transition metal coordination complex cation-borate anion initiator is 
usually used in an amount up to about 25 percent by weight based on the 
weight of the photopolymerizable or crosslinkable species in the 
photohardenable composition. More typically, the transition metal 
coordination complex cation-borate anion initiator is used in an amount of 
about 0.1 to 10 percent by weight. 
While the transition metal coordination complex cation-borate anion 
initiator can be used alone as the initiator, film speeds may be quite 
slow and oxygen inhibition may occur. It has been found that it is 
preferable to use the initiator in combination with an autoxidizer and/or 
other additive materials. An autoxidizer is a compound which is capable of 
consuming oxygen in a free radical chain process. 
Examples of useful autoxidizers are N,N-dialkylanilines. Examples of 
preferred N,N-dialkylanilines are dialkylanilines substituted in one or 
more of the ortho-, meta-, or para- positions by the following groups: 
methyl, ethyl, isopropyl, t-butyl, 3,4-tetramethylene, phenyl, 
trifluoromethyl, acetyl, ethoxycarbonyl, carboxy, carboxylate, 
trimethylsilymethyl, trimethylsilyl, triethylsilyl, trimethylgermanyl, 
triethylgermanyl, trimethylstannyl, triethylstannyl, n-butoxy, 
N-pentyloxy, phenoxy, hydroxy, acetyl-oxy, methylthio, ethylthio, 
isopropylthio, thio-(mercapto-), acetylthio, fluoro, chloro, bromo and 
iodo. 
Representative examples of N,N-dialkylanilines useful in the present 
invention are 4-cyano-N,N-dimethylaniline, 4-acetyl-N,N-dimethylaniline, 
4-bromo-N,N-dimethylaniline, ethyl 4-(N,N-dimethylamino) benzoate, 
3-chloro-N,N-dimethylaniline, 4-chloro-N,N-dimethylaniline, 
3-ethoxy-N,N-dimethylaniline, 4-fluoro-N,N-dimethylaniline, 
4-methyl-N,N-dimethylaniline, 4-ethoxy-N,N-dimethylaniline, 
N,N-dimethylthioanicidine, 4-amino- N,N-dimethylaniline, 
3-hydroxy-N,N-dimethylaniline, N,N,N',N'-tetramethyl-1,4-dianiline, 
4-acetamido-N,N-dimethylaniline, etc. 
Preferred N,N-dialkylanilines are substituted with an alkyl group in the 
ortho-position and include 2,6-diisopropyl-N,N-dimethylaniline, 
2,6-diethyl-N,N-dimethyl-aniline, N,N,2,4,6-pentamethylaniline (PMA) and 
p-t-butyl-N,N-dimethylaniline. 
It may also be desirable to utilize an additional material to improve the 
photosensitive properties of the initiator system. Examples of these 
compounds include acylthiohydroxamates, 2-mercaptobenzothiazole, 
6-ethoxy-2-mercaptobenzothiazole, 2-mercaptobenzoxazole and 
phenylmercaptotetrazole. Disulfides of the above listed thiol compounds 
are also useful compounds. 
The autoxidizers and/or additive compounds are preferably used in the 
present invention in concentrations of about 0.1 to 10 percent by weight. 
The photohardenable compositions of the present invention can be coated 
upon a support in a conventional manner and used as a photoresist or in 
photolithography to form a polymer image; or they can be encapsulated as 
described in U.S. Pat. Nos. 4,399,209 and 4,440,846 and used to control 
the release of an image-forming agent. The latter processes typically 
involve image-wise exposing the photosensitive material to actinic 
radiation and subjecting the layer of microcapsules to a uniform rupturing 
force such as pressure, abrasion, or ultrasonic energy. 
Several processes can be used to form color images as explained in U.K. 
Pat. No. 2,113,860 and U.S. Pat. No. 4,772,541. If microcapsules 
containing cyan, magenta and yellow image-forming agents are sensitive to 
red, green and blue light, images can be formed by direct transmission or 
reflection imaging or by image processing. Image processing may involve 
forming color separations (color-seps) corresponding to the red, green and 
blue component images and sequentially exposing the photosensitive 
material to three distinct banks of radiation hereinafter designated 
.lambda.-1, .lambda.-2, and .lambda.-3 sources through each color 
separation. Otherwise, it may involve electronic processing in which the 
image or subject to be recorded is viewed through a Dunn or matrix camera 
and the output from the camera electronically drives three exposure 
sources corresponding to .lambda.-1, .lambda.-2, and .lambda.-3. 
While the discussion herein relates to forming 3-color full color images, 
4-color images are also possible. For example, microcapsules containing 
cyan, magenta, yellow, and black image-forming agents can be provided 
which have distinct sensitivities at four wavelengths, e.g., .lambda.-1, 
.lambda.-2, .lambda.-3, and .lambda.-4. 
In accordance with the invention, at least one set of the microcapsules in 
a full color system contains a composition including a cationic transition 
metal coordination complex and a borate anion. The other sets also may 
include similar types of photoinitiators, or they may contain conventional 
photoinitiators. 
In accordance with the preferred embodiments of the invention, a full color 
imaging system is provided in which three sets of microcapsules containing 
cyan, magenta and yellow image-forming agents are sensitive to red, green, 
and blue light respectively. The photosensitive composition in at least 
one and possibly all three sets of microcapsules are sensitized by 
photoinitiators containing a transition metal coordination complex cation 
and a borate anion. For optimum color balance, the microcapsules are 
sensitive (.lambda.max) at about 450 nm, 550 nm, and 650 nm, respectively 
Such a system is useful with visible light sources in direct transmission 
or reflection imaging. Such a material is useful in making copies of 
full-color originals or contact or projected prints of color photographic 
slides. They are also useful in electronic imaging using lasers or pencil 
light sources of appropriate wavelengths. 
The photohardenable compositions of the present invention can be 
encapsulated in various wall formers using techniques known in the area of 
carbonless paper including coacervation, interfacial polymerization, 
polymerization of one or more monomers in an oil, as well as various 
melting, dispersing, and cooling methods. To achieve maximum 
sensitivities, it is important that an encapsulation technique be used 
which provides high quality capsules which are responsive to changes in 
the internal phase viscosity in terms of their ability to rupture. Because 
the borate tends to be acid sensitive, encapsulation procedures conducted 
at higher pH (e.g., greater than about 6) are preferred. 
Oil and water soluble materials have been encapsulated in hydrophilic 
wall-forming materials such as gelatin-type materials (see U.S. Pat. Nos. 
2,730,456 and 2,800,457 to Green et al.) including gum arabic, polyvinyl 
alcohol, carboxy-methylcellulose; resorcinol-formaldehyde wall formers 
(see U.S. Pat. No. 3,755,190 to Hart, et al); isocyanate wall-formers (see 
U.S Pat. No. 3,914,511 to Vassiliades); isocyanate-polyol wall-formers 
(see U.S. Pat. No. 3,796,669 to Kiritani et al); urea-formaldehyde 
wall-formers, particularly urea-resorcinol-formaldehyde in which 
oleophilicity is enhanced by the addition of resorcinol (see U.S. Pat. 
Nos. 4,001,140; 4,087,376 and 4,089,802 to Foris et al); and 
melamine-formaldehyde resin and hydroxypropyl cellulose (see commonly 
assigned U.S. Pat No. 4,025,455 to Shackle). Because the inventive 
photoinitiator compositions are water soluble, the artisan may have 
greater selectivity in choosing an encapsulation procedure and in choosing 
microcapsule walls. 
Urea-resorcinol-formaldehyde and melamine-formaldehyde capsules with low 
oxygen permeability are preferred. In some cases to reduce oxygen 
permeability it is desirable to form a double walled capsule by conducting 
encapsulation in two stages. 
A capsule size should be selected which minimizes light attenuation. The 
mean diameter of the capsules used in this invention typically ranges from 
approximately 1 to 25 microns. As a general rule, image resolution 
improves as the capsule size decreases. If the capsules become too small, 
they may disappear in the pores or the fiber of the substrate. These very 
small capsules may therefore be screened from exposure by the substrate. 
They may also fail to rupture when exposed to pressure or other rupturing 
means. In view of these problems, it has been determined that a preferred 
mean capsule diameter range is from approximately 3 to 10 microns. 
Technically, however, the capsules can range in size up to the point where 
they become visible to the human eye. 
An open phase system may also be used in accordance with the invention 
instead of an encapsulated one. This can be done by dispersing what would 
otherwise be the capsule contents throughout the coating on the substrate 
as discrete droplets. Suitable coatings for this embodiment include 
polymer binders whose viscosity has been adjusted to match the dispersion 
required in the coating. Suitable binders are gelatin, polyvinyl alcohol, 
polyacrylamide, and acrylic lattices. Whenever reference is made to 
"capsules" and "encapsulation" without reference to a discretre capsule 
wall in this specification or the appended claims, those terms are 
intended to include the alternative of an open phase system. 
The photosensitive material of the present invention can be used to control 
the interaction of various image-forming agents. 
In one embodiment of the present invention the capsules may contain a 
benign visible dye in the internal phase in which case images are formed 
by contacting the exposed imaging material under pressure with a plain 
paper or a paper treated to enhance its affinity for the visible dye. A 
benign dye is a colored dye which does not interfere with the imaging 
photochemistry, for example, by relaxing the excited state of the 
initiator or detrimentally absorbing or attenuating the exposure 
radiation. 
In preferred embodiment of the invention, images are formed through the 
reaction of a pair of chromogenic materials such as a color precursor and 
a color developer, either of which may be encapsulated with the 
photohardenable composition and function as the image-forming agent. In 
general, these materials include colorless electron donating type 
compounds and are well known in the art. Representative examples of such 
color formers include substantially colorless compounds having in their 
partial skeleton a lactone, a lactam, a sulfone, a spiropyran, an ester or 
an amido structure such as triarylmethane compounds, bisphenylmethane 
compounds, xanthene compounds, fluorans, thiazine compounds, spiropyran 
compounds and the like. Crystal Violet Lactone, Copikem X, IV and XI and 
commercially available cyan, magenta and yellow colorless color-forming 
agents are often used. The color formers can be used alone or in 
combination. 
The developer materials conventionally employed in carbonless paper 
technology are also useful in the present invention. Illustrative examples 
are clay minerals such as acid clay, active clay, attapulgite, etc.; 
organic acids such as tannic acid, gallic acid, propyl gallate, etc.; acid 
polymers such as phenol-formaldehyde reins, phenol acetylene condensation 
resins, condensates between an organic carboxylic acid having at lease one 
hydroxy group and formaldehyde, etc.; metal salts or aromatic carboxylic 
acids such as zinc salicylate, tin salicylate, zinc 2-hydroxy naphthoate, 
zinc 3,5 di-tert butyl salicylate, zinc 3,5-di(.alpha.-methylbenzyl) 
salicylate, oil soluble metal salts or phenol-formaldehyde novolak resins 
(e.g., see U.S. Pat. Nos. 3,672,935; 3,732,120 and 3,737,410) such as zinc 
modified oil soluble phenol-formaldehyde resin as disclosed in U.S. Pat. 
No. 3,732,120, zinc carbonate etc. and mixtures thereof. Particularly 
preferred developers are described in U.S. application Ser. No. 073,036, 
filed July 14, 1987. 
As indicated in U.S. Pat. Nos. 4,399,209 and 4,440,846, the developer may 
be present on the photosensitive sheet (providing a so-called 
self-contained system) or on a separate developer sheet. 
In self-contained systems, the developer may be provided in a single layer 
underlying the microcapsules as disclosed in U.S. Pat. No. 4,440,846. 
Alternatively, the color former and the color developer may be 
individually encapsulated in photosensitive capsules and upon exposure 
both capsule sets image-wise rupture releasing color former and developer 
which mix to form the image. Alternatively, the developer can be 
encapsulated in non-photosensitive capsules such that upon processing all 
developer capsules rupture and release developer but the color former 
containing capsules rupture in only the unexposed or underexposed area 
which are the only areas where the color former and developer mix. Still 
another alternative is to encapsulate the developer in photosensitive 
capsules and the color former in non-photosensitive capsules 
The present invention is not necessarily limited to embodiments where the 
image-forming agent is present in the internal phase. Rather, this agent 
may be present in the capsule wall of a discrete capsule or in the binder 
of an open phase system or in a binder or coating used in combination with 
discrete capsules or an open phase system designed such that the 
image-wise ruptured capsules release a solvent for the image-forming 
agent. Embodiments are also envisioned in which a dye or chromogenic 
material is fixed in a capsule wall or binder and is released by 
interaction with the internal phase upon rupturing the capsules. 
The most common substrate for this invention is a synthetic film and 
preferably a metallized film. 
The photoinitiators of the present invention may have additional practical 
uses other than in imaging systems. Some of these uses include, but are 
not limited to, use in radiation-curable inks, use in an adhesive for 
laminating transparent or translucent materials together, use in magnetic 
recording compositions, use in dental adhesives and dental compositions, 
use in producing holograms by photopolymer holography, use in forming 
three dimensional models from monomer solutions, and use in underwater 
coatings. 
Synthesis Example 1 illustrates the preparation of an inventive composition 
containing a transition metal coordination complex cation and a borate 
anion.

SYNTHESIS EXAMPLE 1 
1.0 gram of Ru(bipy).sub.3 Cl.sub.2 were dissolved in 50 ml of deionized 
water. 1.59 grams of the tetramethyl ammonium salt of 
tris(p-t-butylphenyl) butyl borate was dissolved in 55.5 ml of ethyl 
acetate. The two solutions were mixed together in a beaker, and stirred at 
60 r.p.m. at room temperature for 30 minutes. A large amount of red 
colored liquid was deposited on the beaker walls. After stirring, the 
contents of the beaker were transferred to a separatory funnel where the 
contents separated into a pale green colored aqueous phase and a 
reddish-yellow colored organic phase. The aqueous phase was removed and 
discarded. The liquid from the organic phase was evaporated on a rotary 
evaporator to yield a reddish colored oil. Both this oil, as well as the 
reddish oil deposited on the beaker walls were washed with hexane to yield 
2.0098 grams of a red solid. The percent yield was nearly quantitative. 
EXAMPLE 1 
The red solid produced in Synthesis Example 1 (0.1 gram) and 25 grams of 
trimethylolpropane triacrylate (TMPTA) were mixed together and heated at 
60.degree. C. for 30 minutes to form a yellow solution. Most of the solid 
dissolved in the TMPTA. Drops of the yellow solution were placed on glass 
microscope slides and the slides were exposed to light from one F15-Cool 
White Fluorescent tube at a distance of 4 inches. After 16 seconds, the 
drops solidified and thereby cemented the glass slides together. This 
indicates that the composition of Synthesis Example 1 effectively 
initiated polymerization of the TMPTA by generating free radicals as a 
result of exposure to visible light. .lambda.Max of the composition is 450 
nm. 
EXAMPLE 2 
2,6-Diisopropyl-N,N-dimethylaniline, a known autoxidizer(0.5 grams) was 
added to the solution of Example 1. A few drops of the solution were 
placed between two glass slides and the experiment of Example 1 was 
repeated. The slides were cemented in 6 seconds. 
EXAMPLE 3 
0.5 grams of the commerically available chloride salt of 
Co(bipy).sub.3.sup.+2 was dissolved in 75 to 150 ml of deionized water. A 
stoichiometric amount of the tetramethyl ammonium salt of tris(p-t-butyl 
phenyl)butyl borate was dissolved in 75 to 150 ml of ethyl acetate. The 
two solutions were poured together and stirred for 30 minutes at room 
temperature. The mixture was poured into a separatory funnel and shaken. 
The organic layer was recovered and dried with magnesium sulfate. The 
ethyl acetate was removed by rotary evaporation. The resultant metal 
borate salt was collected in 90-95% yield. 
EXAMPLE 4 
1.0 gram of (cis-dichlorobis(2,2'-bipyridine) ruthenium(II) hydrate and a 
1.1 molar excess of 4,4'-dimethyl-2,2'-bipyridine were placed into 150 
grams of reagent grade alcohol. The mixture was heated at reflux for 20 
hours to turn the solution a transparent reddish-yellow color. The solvent 
was removed by rotary evaporation and the resultant solid was washed 
several times with hexane to obtain a quantitative yield of a chloride 
salt of a cationic transition metal coordination complex. 0.5 grams of the 
salt were mixed with a stoichiometric amount of the tetramethyl ammonium 
salt of tris(p-t-butyl phenyl)butyl borate using the method of Example 3 
to form a photoinitiator composition. The resultant composition was 
collected in 90-95% yield. 
EXAMPLE 5 
1.0 gram of Ruthenium(III) chloride hydrate and a 3.1 molar excess of 
4,4'-dimethyl-2,2'-bipyridine were placed into 150 grams of reagent grade 
ethanol and the solution was refluxed for three days to turn the solution 
a transparent reddish-yellow color. The solution was treated as in Example 
4 to produce a chloride salt of a cationic transition metal coordination 
complex 0.5 grams of the salt were mixed with a stoichometric amount of 
the tetramethyl ammonium salt of tris(p-t-butyl phenyl) butyl borate using 
the method of Example 3 to form a photoinitiator composition. The 
composition was collected in 90-95% yield. 
EXAMPLE 6 
0.5 grams of the commercially available chloride salt of 
Ru(terpy).sub.2.sup.+2 were mixed with a stoichiometric amount of the 
tetramethyl ammonium salt of tris(p-t-butyl phenyl) butyl borate using the 
procedure of Example 3 to form a photoinitiator composition. The 
composition was collected in 90-95% yield. 
EXAMPLE 7 
0.5 grams of the commercially available chloride salt of 
Ru(phen).sub.3.sup.+2 were mixed with a stoichiometric amount of the 
tetramethyl ammonium salt of tris(p-t-butyl phenyl) butyl borate using the 
procedure of Example 3 to form a photoinitiator composition. The resultant 
composition was collected in 90-95% yield. 
EXAMPLE 8 
0.5 grams of the commercially available chloride salt of Fe(Me.sub.2 
bipy).sub.3.sup.+2 were mixed with a stoichiometric amount of the 
tetramethyl ammonium salt of tris(p-t-butyl phenyl) butyl borate using the 
procedure of Example 3 to form a photoinitiator composition. The 
composition was collected in 90-95% yield. 
EXAMPLE 9 
1.0 gram of cis-dichlorobis (2,2'-bipyridine)ruthenium(II) hydrate and a 
1.1 molar excess of 1,10-phenanthroline were reacted according to the 
procedure set forth in Example 4 to form a chloride salt of a cationic 
transition metal coordination complex. 0.5 grams of the salt were mixed 
with a stoichiometric amount of the tetramethyl ammonium salt of 
tris(p-t-butyl phenyl) butyl borate using the method of Example 3 to form 
a photoinitiator composition. The resultant composition was collected in 
90-95% yield. 
EXAMPLE 10 
1.0 gram of cis-dichlorobis-(2,2'-bipyridine)ruthenium(II) hydrate and a 
1.1 molar excess of 5-chloro-1,10-phenanthroline were reacted according to 
the procedure set forth in Example 4 to form a chloride salt of a cationic 
transition metal coordination complex. 0.5 grams of the salt were mixed 
with a stoichiometric amount of the tetramethyl ammonium salt of 
tris(p-t-butyl phenyl) butyl borate using the method of Example 3 to form 
a photoinitiator compostion. The resultant composition was collected in 
90-95% yield. 
EXAMPLE 11 
1.0 gram of Iridium (III) chloride and a 2.1 molar excess of 
2,2'-bipyridine were refluxed in 250 grams of reagent grade ethanol for 
three days. During this time the solution changed from black to pale 
yellow and the solvent was removed by rotary evaporation to yield a yellow 
solid. 0.5 grams of the yellow solid were mixed with an equimolar amount 
of the tetramethyl ammonium salt of tris(p-t-butyl phenyl) butyl borate 
using the method of Example 3 to form a photoinitiator composition. The 
resultant composition was collected in 90-95% yield. 
EXAMPLE 12 
0.5 grams of the commercially available chloride salt of 
Ru(bipy).sub.3.sup.+2 were mixed with a stoichiometric amount of the 
tetramethyl ammonium salt of tris-phenyl hexyl borate using the procedure 
of Example 3 to form a photoinitiator composition. The composition was 
collected in 90-95% yield. 
EXAMPLE 13 
0.5 grams of the chloride salt produced according to Example 10 were mixed 
with an equimolar amount of the tetramethyl ammonium salt of tris-phenyl 
hexyl borate using the procedure of Example 3 to form a photoinitiator 
composition. The composition was collected in 90-95% yield. 
Transition metal coordination complex cation-borate anion photoinitiators 
synthesized by the above described examples are shown in the following 
table with their .lambda.max. The abbreviation "borate-1" represents 
(tris(p-t-butyl phenyl))butyl borate and the abbreviation "borate-2" 
represents (trisphenyl)hexyl borate. 
TABLE 1 
______________________________________ 
Example No. 
Structure .lambda.max (TMPTA) 
______________________________________ 
3 Co(bipy).sub.3 (borate-1).sub.2 
&lt;350 nm 
4 Ru(Me.sub.2 bipy)(bipy).sub.2 (borate-1).sub.2 
458 nm 
5 Ru(Me.sub.2 bipy).sub.3 (borate-1).sub.2 
464 nm 
6 Ru(terpy).sub.2 (borate-1).sub.2 
480 nm 
7 Ru(phen).sub.3 (borate-1).sub.2 
420 nm 
8 Fe(Me.sub.2 bipy).sub.3 (borate-1).sub.2 
532 nm 
9 Ru(bipy).sub.3 (borate-1).sub.2 
454 nm 
10 Ru(Cl-phen)(bipy).sub.2 (borate-1).sub.2 
452 nm 
11 (Ir(Me.sub.2 bipy).sub.2 Cl.sub.2)(borate-1) 
336 nm 
12 Ru(bipy).sub.3 (borate-2).sub.2 
454 nm 
13 (Ir(Me.sub.2 bipy).sub.2 Cl.sub.2)(borate-2) 
338 nm 
______________________________________ 
The photoinitiator compositions of Examples 3-13 were tested using the 
following procedure. 0.1 grams of the initiator were added to 25 grams of 
trimethylolpropane triacrylate (TMPTA) by heating the TMPTA to 60.degree. 
C. while stirring. One drop of the resultant composition was placed 
between two glass microscope slides and the slides were exposed to either 
one cool white fluoroscent tube (GE F15T8-CW) or one black light 
fluoroscent tube (GE F15T8-BLB) at a distance of 10 cm. The exposure times 
required for first notable polymerization (FNP) and complete slide 
immobilization (CSI) are set forth in Table 2. As an additional experiment 
0.25 grams of 2,6-diisopropyl-N,N-dimethyl aniline (DIDMA), a known 
autoxidizer, were added to the TMPTA/photoinitiator composition prior to 
application to the microscope slides. The times of first notable 
polymerization and complete slide immobilization for these samples are 
also shown in Table 2. 
TABLE 2 
______________________________________ 
Cool White Exposure 
Black Light Exposure 
Sample FNP (sec) CSI (sec) FNP (sec) 
CSI (sec) 
______________________________________ 
Example 3 
&gt;120 &gt;120 &gt;120 &gt;120 
Example 3 + 
64 67 37 39 
DIDMA 
Example 4 
17 20 &gt;60 &gt;60 
Example 4 + 
3 4 6 9 
DIDMA 
Example 5 
16 19 23 27 
Example 5 + 
2 3 4 8 
DIDMA 
Example 6 
&gt;60 &gt;60 &gt;60 &gt;60 
Example 6 + 
23 25 41 46 
DIDMA 
Example 7 
&gt;60 &gt;60 &gt;60 &gt;60 
Example 7 + 
6 7 18 22 
DIDMA 
Example 8 
&gt;120 &gt;120 &gt;120 &gt;120 
Example 8 + 
64 67 &gt;120 &gt;120 
DIDMA 
Example 9 
7 10 11 16 
Example 9 + 
3 5 7 12 
DIDMA 
Example 10 
6 8 11 15 
Example 10 + 
3 5 5 8 
DIDMA 
Example 11 
&gt;120 &gt;120 53 56 
Example 11 + 
49 54 16 18 
DIDMA 
Example 12 
25 30 80 85 
Example 12 + 
14 18 21 30 
DIDMA 
Example 13 
&gt;120 &gt;120 &gt;120 &gt;120 
Example 13 + 
57 67 14 22 
DIDMA 
______________________________________ 
The initiator compositions of the present invention achieve a great number 
of advantages. First, because of their sensitivity to visible light, they 
can be successfully used as visible light initiators. By changing the 
ligands attached to the center metal atom, the solubility characteristics 
of the initiator, as well as the absorption sensitivity, can be altered 
and controlled. Further, by changing the center metal atom, the absorbance 
may be altered to successfully cooperate with the light source selected 
for photopolymerization. 
Having described the invention in detail and by reference to preferred 
embodiments thereof, it will be apparent that modifications and variations 
are possible without departing from the scope of the invention defined in 
the appended claims.