Cobalt (III) complex imaging compositions having improved photographic properties

There is disclosed an image-forming composition and element comprising a cobalt(III) complex, a reducing agent precursor which, in the presence of reduction products of the complex, forms a reducing agent for the complex, and an organic oxidizing agent to improve the image properties of the composition.

FIELD OF THE INVENTION 
This invention relates to an image-forming composition and element 
featuring the reduction of a cobalt(III) complex in response to activating 
radiation to produce an image having an internal gain. 
BACKGROUND OF THE INVENTION 
Considerable effort has been made to develop imaging compositions 
particularly those useful in the graphic arts, which do not require the 
use of silver. The reason, of course, is the high cost of silver. One such 
nonsilver system involves the use of cobalt(III) complexes which are 
reduced as a result of a photolytic reaction to form as reaction products 
cobalt(II) and a released ligand, such as ammonia. Either of these can in 
turn be used to form images, the cobalt(II) being chelated by compounds 
capable of forming tridentate chelates, or the ammonia or other ligand 
being reacted with dye precursors, including diazo-coupler systems, to 
form a dye; or they can be used to bleach out preincorporated dye. Such 
compositions are disclosed in Research Disclosure, Vol 126, October, 1974, 
Publication No. 12617, Part III, published by Industrial Opportunities 
Limited, Homewell, Havant Hampshire PO91EF, United Kingdom. Amplification 
can be achieved by using a reducing agent precursor capable of producing 
internal gain by forming with the aforesaid reduction products a reducing 
agent for the reduction of remaining cobalt(III) complexes. For example, 
certain of said chelating compounds for cobalt(II) form when chelated a 
reducing agent, as described in Research Disclosure, Vol 135, July, 1975, 
Publication No. 13505. Alternatively, o-phthalaldehyde will react with 
ammonia to form a reducing agent, as disclosed in Research Disclosure, Vol 
158, June, 1977, Publication No. 15874. 
All such compositions feature the release of ligands, preferably amines. 
Quite often, however, the ligand release, particularly when amplified by 
the mechanisms noted above, has been discovered so effective in forming 
images that unwanted "image spread" or excessive contrast can occur. As a 
result, these compositions often exhibit very short processing latitude 
over time or temperature, or short exposure latitude such as when 
reproducing halftone dots. Although these processing and exposure latitude 
characteristics are useful in certain cases, particularly when 
photographing line copy, they can be undesirable in other application, 
such as in those reproducing continuous tone images, where extended 
processing and exposure latitude are advantageous. 
Therefore, there has been a need to modify the cobalt(III) complex imaging 
compositions in a manner that will provide an imaging composition and 
element having improved photographic properties. 
It has been known that halogenated methyl-s-triazines will react with 
ammonia, as noted by Schaeffer and Ross, "Chlorination and Bromination of 
Alkyl-s-Triazines", J Organic Chemistry, Vol 29, page 1527 (1964). 
However, there is no suggestion in this article that such a reaction can 
control photographic properties, or indeed that this reaction has any 
relation to recognized image-forming chemistry. 
RELATED APPLICATIONS 
Commonly owned U.S. Application Ser. No. 087,190 entitled "Cobalt(III) 
Complex Imaging Compositions Having Improved Photographic Properties", and 
cofiled with this application, by T DoMinh, discloses and claims the use 
of oxidizing agents with compounds which contain aromatic dialdehydes 
capable of forming reducing agent precursors for cobalt(III) complexes 
containing amine ligands. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is advantageously provided 
an imaging composition and element utilizing cobalt(III) complexes and 
having improved photographic properties which correct the deficiencies 
noted above. 
In a related feature of the invention there is provided such a composition 
and element, which not only use reducing agent precursors to amplify the 
cobalt(III) reduction, but also have increased resistance to thermal fog 
in D.sub.min areas and/or reduced contrast. 
The aforesaid features of the invention arise from the discovery that 
organic oxidizing agents are capable of improving certain photographic 
properties of cobalt(III) complex-containing imaging compositions. More 
specifically, there is provided a light-sensitive image-forming 
composition, comprising in admixture (a) a reducible cobalt(III) complex; 
(b) a reducing agent precursor which forms, in the presence of reduction 
products of the complex, a reducing agent for the cobalt(III) complex; and 
(c) an organic oxidizing agent capable of functioning as an antifoggant in 
said composition. 
The composition of the invention provides an improved imaging process 
comprising the steps of imagewise exposing the above-noted element to 
activating radiation, and developing the image formed. 
Other features of the invention will become apparent upon reference to the 
following Description of the Preferred Embodiments. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
High speed cobalt(III) complex imaging chemistry typically employs a 
reducing agent precursor to amplify the reduction of the cobalt(III) 
complex. Such precursors can be selected from a variety of materials, 
e.g., those which contain .pi.-bonding systems and are capable of forming 
tridentate chelates with cobalt(III), or dye precursors such as 
phthalaldehyde. Each of these, however, can produce photographic effects 
peculiar to their chemistry which in some instances are less than 
satisfactory. To minimize these photographic effects, an organic oxidizing 
agent, discussed hereinafter, is added. 
Cobalt(III) complexes capable of undergoing a reduction reaction to release 
their ligands are fully described in the literature. Any cobalt(III) 
complex containing releasable ligands and which is thermally stable at 
room temperature will function in this invention. Such complexes on 
occasion have been described as being "inert". See, e.g., U.S. Pat. No. 
3,862,842, columns 5 and 6. However, the ability of such complexes to 
remain stable, i.e., retain their original ligands when stored by 
themselves or in a neutral solution at room temperature until a chemically 
or thermally initiated reduction to cobalt(II) takes place, is so 
well-known that the term "inert" will not be applied herein. 
Such cobalt(III) complexes feature a molecule having a cobalt atom or ion 
surrounded by a group of atoms or other molecules which are generically 
referred to as ligands. The cobalt atom or ion in the center of these 
complexes is a Lewis acid, while the ligands are Lewis bases. While it is 
known that cobalt is capable of forming complexes in both its divalent and 
trivalent forms, trivalent cobalt complexes--i.e., cobalt(III) 
complexes--are employed in the practice of this invention because the 
ligands are relatively tenaciously held in these complexes and released 
when the cobalt is reduced to the (II) state. 
Preferred cobalt(III) complexes useful in the practice of this invention 
are those having a coordination number of 6. A wide variety of ligands can 
be used with cobalt(III) to form a cobalt(III) complex. The one of choice 
will depend upon whether the image-forming material described hereinafter 
relies upon amines to generate or destroy a dye, or upon the chelation of 
cobalt(II) to form a dye density. In the latter case, amine ligands or 
nonamine ligands can be used, whereas in the former case amine ligands are 
preferred as the source of initiators for the image-forming reaction. 
Useful amine ligands include, e.g., methylamine, ethylamine, ammines, and 
amino acids such as glycinato. As used herein, "ammine" refers to ammonia 
specifically, when functioning as a ligand, whereas "amine" is used to 
indicate the broader class noted above. Highly useful with any of the 
destabilizer materials hereinafter described are the ammine complexes. The 
other amine complexes achieve best results when used with photoreductant 
as destabilizers as described hereinafter. 
The cobalt(III) complexes useful in the practice of this invention can be 
neutral compounds which are entirely free of either anions or cations. As 
used herein, "anion" refers to a charged species which, in the commonly 
understood sense of the term, does not include species which are 
covalently bonded or bonded directly to the cobalt center. The cobalt(III) 
complexes can also include one or more cations and anions as determined by 
the charge-neutralization rule. Useful cations are those which produce 
readily soluble cobalt(III) complexes, such as alkali metals and 
quaternary ammonium cations. 
A wide variety of anions can be used, and the choice depends in part on 
whether or not an amplifier is used which requires that the element be 
free of anions of acids having pKa values greater than about 3.5. For 
example, the anion(s) can be 
(a) Q'.sub.p --C.sub.n H.sub.m --CO.sub.2.sup..crclbar. wherein n is an 
integer of from 0 to 20, m and p are each individually an integer of from 
0 to 41, provided that if n and m are zero, p is zero; and Q' is alkoxy, 
alkyl, thio, hydroxy, carboxamido, sulfonamido, sulfonyl, sulfamyl, 
phosphonate, phosphinate, sulfato, carbonato, carbamato, carbonyl to form 
pyruvate, aryl or substituted aryl, --O--, or an electron-withdrawing 
group such as halogen, azide, cyanate, or thiocyanate; e.g., any 
perfluorocarboxylate or fully halogenated alkyl carboxylate; 
(b) C.sub.n H.sub.m --Q'.sub.p SO.sub.3.sup..crclbar. wherein n, m, p and 
Q' have the same meaning as described above, to form, for example, 
trifluoromethane sulfonate or SO.sub.3.sup..crclbar. ; 
(c) Q.sup.2 Q.sup.3 PO.sub.4.sup..crclbar. wherein Q.sup.2 and Q.sup.3 are 
each independently aryl, alkyl, or substituted aryl or alkyl; 
(d) MQ.sup.4 wherein M is a group VA element other than nitrogen and 
Q.sup.4 is halogen; 
(e) Q.sup.2 --SO.sub.2 N.sup..crclbar. SO.sub.2 Q.sup.3 wherein Q.sup.2 and 
Q.sup.3 are as defined above; and 
(f) 
##STR1## 
wherein Q.sup.5 is the atoms necessary to form an aromatic or heterocyclic 
ring; provided that for each of these anions used with a reducing agent 
precursor that forms a tridentate chelate with cobalt(III), the pKa of the 
corresponding acid is .ltoreq.3.5. As used herein, unless otherwise 
stated, "alkyl" or "alkoxy" refers to a moiety having from 1 to about 10 
carbon atoms, for example, methyl, ethyl, propyl, isopropyl, and the like, 
or methoxy, ethoxy, etc. "Aryl" or "aromatic" refers to a moiety 
containing from 6 to 10 carbon atoms, for example, phenyl or phenylene, 
naphthyl or naphthalene or the like. 
Further details concerning the cobalt(III) complexes are recited in 
Research Disclosure, Vol 126, Publication No. 12617, October, 1974, Part 
III thereof, the details of which are expressly incorporated herein by 
reference. 
The following Table I is a partial list of particularly preferred 
cobalt(III) complexes useful in the invention. 
TABLE I 
______________________________________ 
hexa-ammine cobalt(III) benzilate 
hexa-ammine cobalt(III) thiocyanate 
hexa-ammine cobalt(III) trifluoroacetate 
hexa-ammine cobalt(III) trifluoromethane sulfonate 
hexa-ammine cobalt(III) perfluorobenzoate 
hexa-ammine cobalt(III) heptafluorobutyrate 
chloropenta-ammine cobalt(III) perchlorate 
bromopenta-ammine cobalt(III) perchlorate 
aquopenta-ammine cobalt(III) perchlorate 
bis(methylamine) tetra-ammine cobalt(III) hexa- 
fluorophosphate 
bis(dimethylglyoxime)ethylaquo cobalt(III) 
cobalt(III) acetylacetonate 
tris(2,2'-bipyridyl) cobalt(III) perchlorate 
trinitrotris-ammine cobalt(III) 
penta-ammine carbonate cobalt(III) perchlorate 
tris(glycinato) cobalt(III) 
tris(trimethylenediamine) cobalt(III) trifluoro- 
methanesulfonate 
tris(trimethylenediamine) cobalt(III) tetrafluo- 
roborate 
tris(ethylenediamine) cobalt(III) dimethane sulfon- 
amidate 
bis(ethylenediamine)bisazido cobalt(III) perchlorate 
triethylenetetraaminedichloro cobalt(III) trifluoro- 
acetate 
aquopenta(methylamine) cobalt(III) nitrate 
chloropenta(ethylamine) cobalt(III) pentafluoro- 
butanoate 
trinitrotris(methylamine) cobalt(III) 
tris(ethylenediamine) cobalt(III) trifluoroacetate 
bis(dimethylglyoxme)bispyridine cobalt(III) tri- 
chloroacetate 
.mu.-superoxodecamine cobalt(III) perchlorate 
trans-bis(ethylenediamine)chlorothiocyanato cobalt(III) 
perchlorate 
trans-bis(ethylenediamine)bisazido cobalt(III) thio- 
cyanate 
cis-bis(ethylenediamine)ammineazido cobalt(III) tri- 
fluoroacetate 
tris(ethylenediamine) cobalt(III) benzilate 
trans-bis(ethylenediamine)dichloro cobalt(III) 
perchlorate 
bis(ethylenediamine)dithiocyanato cobalt(III) 
perfluorobenzoate 
triethylenetetraaminedinitro cobalt(III) di- 
chloroacetate 
tris(ethylenediamine) cobalt(III) succinate 
tris(2,2'-bipyridyl) cobalt(III) perchlorate 
bis(dimethylglyoxime)chloropyridine cobalt(III) and -bis(dimethylglyoxime) 
thiocyanatopyridine cobalt(III). 
______________________________________ 
The cobalt(III) complexes described above are themselves responsive to UV 
radiation, i.e., radiation of wavelengths less than 350 nm. In addition to 
exposure to such radiation, a destabilizer material can be added which 
causes release of the ligands from the complex upon appropriate exposure. 
Such destabilizers include 4-phenyl catechol, sulfonamidophenols and 
naphthols, cyclic acids such as phthalamic acid, ureas, amine salts, 
morpholine precursors, aminimides, triazoles, thiolate precursors, blocked 
mercaptotetrazoles, cyclic imides, barbituates, polymers containing 
pendant polysulfonamide moieties, and light-responsitive photoactivators 
responsive to wavelengths greater than 350 nm. Further description and 
detailed lists of such destabilizers can be found in "Inhibition of Image 
Formation Utilizing Cobalt(III) Complexes", Research Disclosure, Vol. 184, 
August, 1979, Publication No. 18436, the contents of which are expressly 
incorporated herein by reference. 
Preferred examples of the photoactivators noted above are photoreductant 
destabilizers, and particularly quinone photoreductants. The quinones 
which are particularly useful as photoreductants include ortho- and 
para-benzoquinones and ortho- and para-naphthoquinones, 
phenanthrenequinones and anthraquinones. The quinones may be unsubstituted 
or incorporate any substituent or combination of substituents which do not 
interfere with the conversion of the quinone to the corresponding reducing 
agent. A variety of such substituents are known to the art and include, 
but are not limited to, primary, secondary and tertiary alkyl, alkenyl and 
alkynyl, aryl, alkoxy, aryloxy, alkoxyalkyl, acyloxyalkyl, aryloxyalkyl, 
aroyloxyalkyl, aryloxyalkoxy, alkylcarbonyl, carboxy, primary and 
secondary amino, aminoalkyl, amidoalkyl, anilino, piperindino, 
pyrrolidino, morpholino, nitro, halide and other similar substituents. 
Aryl substituents are preferably phenyl substituents. Alkyl, alkenyl and 
alkynyl substituents, whether present as sole substituents or present in 
combination with other atoms, typically contain about 20 or fewer 
(preferably 6 or fewer) carbon atoms. 
A preferred class of photoreductants is internal hydrogen source quinones, 
that is, quinones incorporating labile hydrogen atoms. These quinones are 
more easily photoreduced than quinones which do not incorporate labile 
hydrogen atoms. 
Further details and a list of useful quinone photoreductants of the type 
described above are set forth in Research Disclosure, Vol 126, October, 
1974, Publication No. 12617, published by Industrial Opportunities 
Limited, Homewell, Havant Hampshire PO91EF, United Kingdom, the contents 
of which are hereby expressly incorporated by reference. Still others 
which can be used include 2-isopropoxy-3-chloro-1,4-naphthoquinone and 
2-isopropoxy-1,4-anthraquinone. 
With respect to the reducing agent precursors which amplify the reduction 
of the cobalt(III) complexes to cause additional release of ligands, 
examples of such reducing agent precursors include compounds which contain 
conjugated .pi.-bonding systems rendering them capable of forming 
tridentate chelates with cobalt(III). Such compounds first form chelates 
with the reduced cobalt(II) formed by the first exposure, and then reduce 
remaining cobalt(III) complexes to form a cobalt(III) chelate, a colored 
species. 
For this class, any compound can be used if it contains a conjugated 
.pi.-bonding system capable of forming a chelate higher than a bidentate. 
Preferred are those forming a tridentate chelate with cobalt(III). As is 
well-appreciated by those skilled in the art, conjugated .pi.-bonding 
systems can readily be formed by combinations of atoms such as carbon, 
nitrogen, oxygen and/or sulfur atoms, and typically include 
double-bond-providing groups such as vinyl, azo, azinyl, imino, 
formimidoyl, carbonyl and/or thiocarbonyl groups, in an arrangement which 
places the double bonds in a conjugated relationship. A variety of such 
compounds are known to the art including nitroso-arols, dithiooxamides, 
formazans, aromatic azo compounds, hydrazones and Schiff bases. 
Preferred nitroso-arol chelating compounds are those defined by the 
formula: 
##STR2## 
wherein Z is the atoms necessary to complete an aromatic nucleus, such as 
a phenyl or naphthyl nucleus. 
Preferred dithiooxamides are those defined by the formula: 
##STR3## 
wherein Z' is a chelate ligand-forming group, for bonding with cobalt 
complexes as described above, and R' is in each instance chosen from Z', 
hydrogen, alkyl, alkaryl, aryl, and aralkyl. 
Preferred formazan compounds are those defined by the formula: wherein 
##STR4## 
wherein R.sup.2, R.sup.3, R.sup.4, and R.sup.5 are independently chosen 
aromatic groups or hydrogen, provided that at least one of R.sup.2 and 
R.sup.3 is an aromatic group and the compound has a more than bidentate 
chelating capability. 
Preferred aromatic azo compounds having the tridentate chelate-forming 
capability have the formula: 
EQU Z.sup.2 --N.dbd.N--Z.sup.3 
wherein Z.sup.2 and Z.sup.3 are independently chosen aromatic groups. 
Preferred hydrazones having the tridentate chelate-forming capability are 
those having the formula: 
EQU Z.sup.4 --CH.dbd.N--NH--Z.sup.5 
wherein Z.sup.4 and Z.sup.5 are also independently chosen aromatic groups. 
Preferred Schiff bases having the tridentate chelate-forming capability are 
those having the formula: 
EQU Z.sup.6 --CH.dbd.N--Z.sup.7 
wherein Z.sup.6 and Z.sup.7 are independently chosen aromatic groups. 
Exemplary preferred chelate-forming compounds are as follows: 
1-(2-pyridyl)-3-phenyl-5-(2,6-dimethylpheyl)formazan, 
1-(2-pyridyl)-3-n-hexyl-5-phenyl-2H-formazan, 
1-(2-pyridyl)-3,5-diphenylformazan, 
1-(benzothiazol-2-yl)-3,5-diphenyl-2H-formazan, 
1-(2-pyridyl)-3-phenyl-5-(4-chlorophenyl)formazan, 
1,1'-di(thiazol-2-yl)-3,3'-diphenylene-5,5'-diphenylformazan; 
1,3-didodecyl-5-di(benzothiazol-2-yl)-formazan, 
1-phenyl-3-(3-chlorophenyl)-5-benzothiazol-2-yl)formazan, 
1,3-dicyano-5-di(benzothiazol-2-yl)formazan, 
1-phenyl-3-propyl-5-(benzothiazol-2-yl)formazan; 
1,3-diphenyl-5-(4,5-dimethylthiazol-2-yl)formazan, 
1-(2-quinolinyl)-3-(3-nitrophenyl)-5-phenylformazan, 
1-(2-pyridyl)-3-(4-cyanophenyl)-5-(2-tolyl)formazan, 
1,3-naphthalene-bis[2-(2-pyridyl)-5-(3,4-dichlorophenyl)formazan], 
1-(2-pyridyl)-5-(4-nitrophenyl)-3-phenylformazan, 
1-(benzothiazol-2-yl)-3,5-di(4-chlorophenyl)formazan, 
1-(benzothiazol-2-yl)-3-(4-iodophenyl)-5-(3-nitrophenyl)formazan, 
1-(benzothiazol-2-yl)-3-(4-cyanophenyl)-5-(2-fluorophenyl)formazan, 
1-(4,5-dimethylthiazol-2-yl)-3-(4-bromophenyl)-5- 
(3-trifluorophenyl)formazan, 
1-benzoxazol-2-yl)-3,5-diphenylformazan, 
1-(benzoxazol-2-yl)-3-phenyl-5-(4-chlorophenyl)-formazan, 
1,3-diphenyl-5-(2-pyridyl)formazan, 
1-(2,5-dimethylphenyl)-3-phenyl-5-(2-pyridyl)formazan, 
N-(2-pyridyl)-dithiooxamide, 
N,N'-di(2-pyridyl)-dithiooxamide, 
N-(2-benzothiazolyl)dithiooxamide, 
N-(2-quinolinyl)-dithiooxamide, 
1-(2-pyridylazo)-2-naphthol, 
1-(2-pyridylazo)resorcinol, 
2-pyridinecarboxaldehyde-2-quinolylhydrazone, 
disodium 1-nitro-2-naphthol-3,6-disulfonate, 
2-nitrosophenol, 
1-nitroso-2-naphthol, 
2-nitroso-1-naphthol, 
1-nitroso-3,6-disulfo-2-naphthol, 
disodium-1-nitroso-2-naphthol-3,6-disulfonate, 
4-nitrosoresorcinol, 
2-nitroso-4-methoxyphenol, 
1-(2-pyridyl)-3-phenyl-3-(2,6-dimethylpheyl)formazan, 
1-(4,5-dimethylthiazol-3-yl)-3-(4-bromophenyl)-5-(3-trifluoromethylphenyl)f 
ormazan, 
1,3-diphenyl-5-(benzothiazol-2-yl)formazan, 
1,3-diphenyl-5-(2-quinolinyl)formazan, 
1-phenylazo-2-phenol, 
1-(2-hydroxyphenylazo)-2-naphthol, 
1-(2-pyridylazo)-2-phenol, 
4-(2-pyridylazo)resorcinol, 
1-(4-nitro-2-thiazolylazo)-2-naphthol, 
1-(2-benzothiazolylazo)-2-naphthol, 
2-pyridinecarboxyaldehyde-2-pyridylhydrazone, 
2-pyridinecarboxyaldehyde-2-benzothiazolyhydrazone, 
2-thiazolcarboxyaldehyde-2-benzoxazolylhydrazone, 
1-(N-2-pyridylformimidoyl)-2-naphthol, 
1-(N-2-thiazolylformimidoyl)-2-naphthol, 
1-(N-2-benzoxazolylformimidoyl)-2-phenol, 
2-(N-2-pyridylformimidoyl)phenol, 
2-(N-2-pyridylimidoyl)pyridine, and 
1-(2-benzoxazolecarboxaldehyde-imino)-2-oxazole. 
1-(2-pyridylazo)-2-naphthol and 1-(2-pyridylazo)resorcinol are the most 
preferred. 
Further details and additional examples are set forth in U.S. Pat. No. 
4,075,019 issued Feb. 21, 1979 to DoMinh, the contents of which are 
expressly incorporated herein by reference. 
As mentioned in the aforesaid DoMinh patent, the chelating compounds are 
preferably used as the reducing agent precursors in coatings which are 
predominantly free of anions of acids having pKa values greater than about 
3.5. 
As described in the aforementioned DoMinh application, another and 
preferred class of reducing agent precursors which amplifies the reduction 
of the cobalt(III) complexes is aromatic dialdehydes. The currently 
preferred species of such dialdehydes is o-phthalaldehyde, hereinafter 
"phthalaldehyde". In such a case, the ligands of the cobalt(III) complex 
are preferably amine ligands. Phthalaldehyde appears to undergo the 
following reaction, in the presence of the released amines, to provide 
amplification in the exposed areas, as well as a dye (B): 
##STR5## 
Further details of the phthalaldehyde reaction are set forth in DoMinh et 
al, "Reactions of Phthalaldehyde with Ammonia and Amines", J Org Chem, Vol 
42, Dec. 23, 1977, p 4217. 
Each of these two classes of reducing agent precursors has photographic 
effects that can be improved. The conjugated .pi.-bonding compounds which 
form tridentate chelates with cobalt(III) tend to fog thermally at 
D.sub.min (minimum density) areas. However, if phthalaldehyde is the 
reducing-agent precursor, the problem is not thermal fog, but high 
contrast values. Surprisingly, it has been found that organic oxidizing 
agents are useful in dealing with the differing problems of both classes 
of reducing-agent precursors. 
Preferred organic oxidizing agents are those which meet the following test: 
when 1 to 2 mg are added to a 2 g solution mixture of about 0.19 g of a 
binder such as a polyaldehyde, 0.03 mmoles of a cobalt(III) complex, and 
0.04 mmoles of a reducing agent precursor in 1.8 g of a suitable solvent 
or solvent mixture, and coated and dried, and subsequently heated 
unexposed, face up on a 125.degree. C. hot block, the length of time 
required to fog the sample is greater than the same sample prepared 
without any organic oxidizing agent. 
Useful oxidizing agents that are capable of functioning as antifoggants can 
be selected from the following: 
##STR6## 
wherein R.sup.6 and R.sup.7 are the same or different and each is 
CX.sub.3, H, or CH.sub.3 ; and Z.sup.8 is the atoms necessary to complete 
one or more aromatic rings containing one or more hetero atoms, such as 
pyridyl, benzimidazolyl, benzothiazolyl, thiazolyl and quinolinyl; and X 
is halogen such as bromine and chlorine; 
##STR7## 
wherein Z.sup.8 and X are as defined above; 
##STR8## 
wherein Z.sup.9 is the number of atoms necessary to complete an aryl ring, 
such as phenyl; and X is as defined above; 
EQU R.sup.8 --SO.sub.2 --CBr.sub.3 (IV) 
wherein R.sup.8 represents oxazole, benzoxazole, thiazole, benzothiazole, 
phenyl, tolyl, benzyl, or the group (R.sup.9 --.sub.3 C--CH.sub.2 --.sub.n 
wherein n is an integer of from 0 to 4; R.sup.9 is H or X, and X is as 
defined above; 
##STR9## 
wherein R.sup.10 is hydrogen or methyl, J is hydrogen or X, and X is as 
defined above; 
(IV) iodoso-substituted benzenes, such as iodosobenzene diacetate and those 
having the structure 
##STR10## 
wherein R.sup.11 and R.sup.12 are each individually hydrogen or a straight 
chain or branched alkyl having from 1 to about 12 carbon atoms, examples 
of which are o-iodosobenzoic acid, methyl o-iodosobenzoate, octyl 
o-iodosobenzoate, 2-iodoso-4-methylbenzoic acid, and methyl 
2-iodoso-4-methylbenzoate; 
##STR11## 
wherein Z.sup.10 is the number of atoms necessary to complete a ring 
together with --CO--NBr--CO to which Z.sup.10 is bonded, for example 
##STR12## 
etc., wherein R.sup.13 has the same significance as R.sup.12, examples 
including N-bromosuccinic acid imide and N-bromophthalic acid imide; 
EQU R.sup.14 --CONHBr (VIII) 
wherein R.sup.14 represents a straight chain or branched alkyl having from 
1 to about 21 carbon atoms, examples of which include N-bromoacetamide, 
and N-bromostearic acid amide; 
##STR13## 
wherein X is as defined above, examples of which include 
tetrachlorophthalic anhydride and tetrabromophthalic anhydride; 
##STR14## 
wherein X is as defined above and X.sup.1 is cyano or X, of which examples 
include 3,4,5,6-tetrachloro-1,2-benzoquinone; 
2,3,5,6-tetrabromo-1,4-benzoquinone; 
2,3-dichloro-5,6-dicyano-1,4-benzoquinone, and 
2,3-dibromo-5,6-dicyano-1,4-benzophenone; 
##STR15## 
wherein R.sup.11, R.sup.12 and X are as defined above, examples of which 
include tetrachlorophthalic acid; tetrachlorophthalic acid, monomethyl 
ester; tetrachlorophthalic acid, diethyl ester; and tetrachlorophthalic 
acid,dioctyl ester; and 
##STR16## 
wherein R.sup.10 and R.sup.11 are as defined above, and m and n are each 
individually 0, 1, or 2 (joined to the same carbon atom), an example of 
which is 2,2,6,6-tetramethyl-4-oxa-piperidino oxy. 
A preferred form of the oxidizing agent of class (I) is 
##STR17## 
wherein R.sup.10 and R.sup.11 are the same or different and are each H, 
methyl or CBr.sub.3. Thus, the currently preferring oxidizing agent is 
2,4-bis(tribromomethyl)-6-methyl-s-triazine. 
Although the exact mechanism by which these agents improve the photographic 
properties is not completely understood, it is believed it is one of 
oxidation. For example, in the case of phthalaldehyde as the 
reducing-agent precursor providing an amplification of the reduction of 
cobalt(III), and of 2,4-bis(tribromomethyl)-6-methyl-s-triazine as the 
oxidizing agent, it is believed the reaction proceeds as follows: 
##STR18## 
Further details of reactions such as 4) above are described by F Schaeffer 
et al, J Org Chem, Vol 29, p 1527 (1964). 
In the case of compounds capable of forming tridentate chelates as the 
reducing-agent precursor, the oxidizing agents appear to function as 
antifoggants. 
Yet another class of materials which function as reducing-agent precursors 
to provide an amplified reduction of the cobalt(III) complexes, and which 
are useful as such in this invention, are blocked dye precursors. 
"Dye precursor" means any compound capable of being oxidized to a form 
which is either itself the desired dye or which is capable of combining 
with another compound in the element, such as a color coupler, without 
further processing, to form the desired dye. Thus, preferred examples of 
such dye precursors include leuco dyes which already contain a color 
coupler as part of the compound and color-developing agents. Known 
color-developing agents include primary aromatic amines, such as 
p-phenylenediamines, p-aminophenols and sulfonamido anilines. 
"Blocked dye precursor" means a dye precursor to which a group or radical 
is attached so as to interfere with the ability of the dye precursor to be 
oxidized. In the case of coupler-containing dye precursors, such as leuco 
dyes, the blocking group is preferably a carbonyl which has displaced the 
hydroxyl hydrogen of a phenol moiety or is attached to a conjugated 
nitrogen atom which links the coupler to the remainder of the dye. In the 
case of color-developing agents, such as p-phenylenediamines not yet 
coupled, the block mechanism can be by the protonation of one or both of 
the two amine groups which, when deprotonated, forms an amine group 
capable of being oxidized in a redox reaction with the cobalt complex, or 
by acetylation. 
Blocked dye precursors are preferably selected for use with cobalt(III) 
complexes containing amine ligands, as defined above. Exposure to 
activating radiation causes the formation of a free amine, and this amine 
unblocks the dye precursor. The dye precursor is then capable of 
undergoing a redox reaction with remaining cobalt(III) complexes to 
produce more free amine, etc. The oxidized form of the dye precursor can 
itself be a dye, as in the case of leuco dyes which are converted to the 
dye form by the redox reaction; or, alternatively, the blocked dye 
precursor can be a protonated color developer, the unprotonated form of 
which, when oxidized, can combine with a color coupler which is either 
preincorporated into the composition or is added during development. 
Blocked leuco dyes particularly useful in this embodiment of the invention 
have the structures: 
##STR19## 
wherein COUP is a photographic color-forming coupler linked to said 
nitrogen atom through a carbon atom at the coupling position, such as, for 
example, a phenolic coupler, a pyrazolone coupler, a pyrazolotriazole 
coupler, couplers having open-chain active methylene groups and the like, 
and soluble couplers which have solubilizing groups attached thereto to 
provide a diffusible coupler, and the like; 
Ar is an aromatic group containing from about 6 to about 20 carbon atoms, 
including substituted and unsubstituted phenylene and naphthylene groups, 
and the like, and is preferably a phenylene group which is preferably 
substituted with halogen atoms or groups containing halogen atoms in the 
ortho and/or meta position of the ring; 
X.sup.1 can be an amino group, including substituted amines, or preferably 
is a hydroxyl group or the radical --O--R.sup.13, wherein R.sup.13 is a 
carbonyl-containing group such as a group of the formula: 
##STR20## 
R.sup.14 being a group containing 1 to about 12 carbon atoms which can be 
an alkyl group or substituted equivalents thereof such as haloalkyl 
alkoxy, aminoalkyl and the like; or an aryl group or substituted 
equivalents thereof such as halo-aryl, alkylaryl, aryloxy and the like; 
R.sup.12 is a hydrogen atom or the same substituent as R.sup.13, provided 
that at least one of R.sup.12 and R.sup.13 is a carbonyl-containing group; 
and X.sup.2 is 
##STR21## 
and R.sup.15, R.sup.16 and R.sup.17 are alkyl groups or alkylsulfonyl 
groups, such as sulfonamidoalkyl, preferably having from 1 to about 10 
carbon atoms. 
Preferably, R.sup.14 is an alkyl group having 1 to about 4 carbon atoms. 
The group defined as Ar above is preferably the residue of an aromatic 
color-developing agent such as an aminophenol, a phenylenediamine and the 
like and, of course, including the various substituents on the aromatic 
group which are known in the art for the respective color-developing 
agent. In one preferred embodiment where Ar is the mnucleus of an 
aminophenol developing agent, it can contain the same substituents as 
disclosed, for example, on the aminophenol developing agents disclosed by 
Bush et al, U.S. Pat. No. 3,791,827 issued Feb. 12, 1974. Further details 
on coupler definitions are found in U.S. Pat. No. 3,620,747 issued Nov. 
16, 1971, and in the aforesaid Bush patent. 
Additional details and lists of specific examples of such blocked dye 
precursors are given in Research Disclosure, Vol 152, Pub 15246, December, 
1976, the contents of which are expressly incorporated herein by 
reference. 
The blocked dye precursors described above can also be used in admixture 
with the .pi.-bonding, chelate-forming compounds first described above as 
examples of reducing-agent precursors. 
Certain materials can be added as optional ingredients. For example, if the 
composition is to be coated as a film on a support, as opposed to being 
sprayed into filter paper, a binder is desirable. Any binder compatible 
with cobalt(III) complexes can be used, for example, the binders listed in 
the aforesaid Publication No 12617 of Research Disclosure, especially 
paragraph I(D), the details of which are expressly incorporated herein by 
reference. Typical of such binders are acetates, cellulose compounds, 
vinyl polymers, polyacrylates and polyesters. In addition, in those 
embodiments relying upon phthalaldehyde as the dye-forming material and/or 
as the reducing-agent precursor, the binder can be selected to maximize 
the maximum neutral densities produced during exposure and development. 
Highly preferred examples of such binders include certain 
polysulfonamides, for example, 
poly(ethylene-co-1,4-cyclohexylenedimethylene-1-methyl-2,4-benzenedisulfon 
amide), poly(ethylene-co-hexamethylene-1-methyl-2,4-benzenedisulfonamide) 
and poly(methacrylonitrile). 
Yet other optional ingredients include additional materials for forming a 
detectable product in the imagewise-exposed areas beyond the optically 
dense cobalt(III) chelate or the oligomer dye (B) described above. A 
preferred form of such additional discriminating materials is one which 
will form a polymer and preferably an inkable polymer such as can be used 
to provide a lithographic printing plate. Particularly useful polymers are 
polyaldehydes capable of being crosslinked by amines to form a 
photohardened layer. Most preferred examples of such polyaldehydes are 
those described in Research Disclosure, Vol 181, Publication No 18183 
(May, 1979), the details of which are expressly incorporated herein by 
reference, e.g., a polymer having recurring units with the structure: 
##STR22## 
Still another, optional discriminating material is an amine-responsive 
image-recording layer of the type described in the aforesaid Research 
Disclosure, Publication No 13505, Paragraph V(K). 
If the image-forming composition is to be coated on a support to form an 
element, any of the supports listed in the aforesaid Research Disclosure, 
Publications 12617 or 13505, can be used, e.g., poly(ethylene 
terephthalate) film. 
The coating solvent selected will, of course, depend upon the makeup of the 
composition, including the binder, if any. Typical preferred solvents 
which can be used alone or in combination are lower alkanols, such as 
methanol, ethanol, isopropanol, t-butanol and the like; ketones, such as 
methylethyl ketone, acetone and the like; water; ethers, such as 
tetrahydrofuran, and the like; acetonitrile; dimethyl sulfoxide and 
dimethylformamide. 
The proportions of the nonbinder reactants forming the composition and/or 
the imaging element can vary widely, depending upon which materials are 
being used. Because, in any event, cobalt(III) complex is present, the 
molar amounts are expressed per mole of complex. The amount of the 
oxidizing agent which is to be added depends in part upon the desired 
photographic effect. It also depends in part on the species used as the 
reducing-agent precursor. In those cases in which the reducing-agent 
precursor is a chelating compound capable of forming a tridentate chelate 
with cobalt(III), the amount can be between 0.1 mole per mole of 
cobalt(III) complex, and about 10 moles per mole. If the reducing-agent 
precursor is a dye precursor such as phthalaldehyde, the amount can be 
between about 1 mole per mole of cobalt(III) complex and about 15 moles 
per mole. 
A convenient range of coating coverage of cobalt(III) complex is between 
about 5 and about 50 mg/dm.sup.2. 
Typically, solutions are coated onto the support by such means as whirler 
coating, brushing, doctor-blade coating, hopper coating and the like. 
Thereafter, the solvent is evaporated. Other exemplary coating procedures 
are set forth in the Product Licensing Index, Vol 92, December, 1971, 
Publication No 9232, at page 109, published by Industrial Opportunities 
Limited, Homewell, Havant Hampshire P091EF, United Kingdom. Addenda such 
as coating aids and plasticizers can be incorporated into the coating 
composition. 
In certain instances, an overcoat for the radiation-sensitive layer of the 
element can supply improved handling characteristics and can help to 
retain otherwise volatile components. Useful examples include gelatin 
overcoats crosslinked with an agent, such as a 5 weight percent aqueous 
solution of hexamethoxymethyl melamine and various copolymers. 
The image-forming composition described above, preferably as a coated 
element, is exposed imagewise to a suitable light source, for example, an 
IBM Microcopier IID, and the development of the image is completed in a 
rapid manner by heating the element to a temperature of between about 
90.degree. and about 160.degree. C., for a time of between about 2 and 
about 30 seconds. In such a heating process, the oxidizing agents of the 
invention serve, at least when phthalaldehyde is the reducing-agent 
precursor, to increase the available exposure and processing latitude of 
the element. One convenient measure of such exposure latitudes is the 
contrast control available to the composition. To the extent the 
composition has a reduced contrast, the greater is the latitude in 
exposure which is available over usual density values. The preferred 
oxidizing agent of the invention demonstrates a marked reduction in the 
contrast which would result if the oxidizing agent were not included. 
Another technique for measuring the effect of the oxidizing agent on 
exposure latitude is by determining the exposure range which will 
reproduce an integrated density of halftone dots to a value which is 0.75 
to 1.25 times that of the actual value of the original dot images. In 
other words, the oxidizing agents reduce the "image spread". The preferred 
oxidizing agent of the invention can provide such a halftone-dot 
reproduction over at least 0.3 log E exposure when phthalaldehyde is the 
reducing-agent precursor. When exposure latitude is measured by this 
technique, the amount of oxidizing agent which is required is generally 
less than is required for preferred contrast control.

The following examples are included for a further understanding of the 
invention. 
EXAMPLES 1-6 
To demonstrate the antifoggant properties of the oxidizing agents when used 
with, e.g., a reducing agent precursor capable of forming a tridentate 
chelate with cobalt(III), the following coating solutions were prepared. 
(w/w means weight per weight of solvent. 
______________________________________ 
Solution 1: 
acetone 20 g 
tris(trimethylenediamine)cobalt(III) 
180 mg 
trifluoromethanesulfonate 
tris(trimethylenediamine)cobalt(III) 
150 mg 
tetrafluoroborate 
Solution 2: (Polyaldehyde solution) 
20% (w/w) of poly(o-formylphenylvinyl- 
10 g 
benzyl ether) in tetrahydrofuran 
20% (w/w) of poly(o-formylphenylvinyl- 
10 g 
benzyl ether) in cyclopentanone 
1-(2-pyridylazo)-2-naphthol (PAN) 
200 mg 
(reducing agent precursor) 
2-isopropoxy-1,4-naphthoquinone 
400 mg 
______________________________________ 
To 1 g of Solution 1 were added 1-2 mg of the oxidizing agents listed in 
Table II as antifoggants. After dissolution, 1 g of Solution (2) was then 
added and stirred. The solution was then coated with a 100-micron doctor 
blade on subbed poly(ethylene terephthalate) support and dried 
sequentially for 1 min at 21.degree. C., 1 min at 66.degree. C., and 1 min 
at 100.degree. C. 
Strips of each coating were then evaluated in terms of: 
(1) fog time (expressed as seconds required for a visible green color of 
dye to appear from the chelating of cobalt with the PAN in an unexposed 
coating with thermal processing, face up, on a 125.degree. C. hot block). 
(2) speed (expressed as the number of 0.3 log E steps of green dye visible 
after a 1/2-sec exposure through a 1.0 neutral density filter and a 0.3 
log E step tablet in an IBM Microcopier IID exposing apparatus and 
processing, as above, to a point 1 sec short of fog). 
TABLE II 
______________________________________ 
Ex- Fog 
ample Antifoggant Time Speed Dmax 
______________________________________ 
control 
none 6 sec 4 0.28 
1 2-(tribromomethyl)-quin- 
21 sec 3 0.20 
line 
2 2-(tribromomethyl)-quin- 
22 sec 2 0.17 
oxaline 
3 2-tribromomethyl-sulfonyl 
25 sec 3 0.26 
benzothiazole 
4 2,4-bis(tribromomethyl)- 
30 sec 2 0.14 
6-methyl-s-triazine 
5 chlorotriphenylmethane 
35 sec 2 0.37 
6 bis(tribromomethyl)- 
&gt;40 sec 1 0.08 
sulfone 
______________________________________ 
Thus, Examples 1-6 demonstrated antifoggant properties by drastically 
increasing the heating time necessary to fog the composition, compared 
with the control which lacked the oxidizing agent. The small loss in speed 
is an expected adjunct of the antifoggant property. 
EXAMPLES 7-9 
The procedure of Example 1 was repeated, except that the polyaldehyde used 
was 1.4 g of a 15% (w/w) solution of polyvinyl butyral available under the 
trademark BUTVAR from Monsanto, in acetone, and the antifoggants were 
those shown in Table III. The two coating solutions were modified as 
follows: 
______________________________________ 
Solution A: 
acetone 2 g 
tris(trimethylenediamine)cobalt(III) 
60 mg 
trifluoromethanesulfonate 
tris(trimethylenediamine)cobalt(III) 
50 mg 
tetrafluoroborate 
Solution B: 
acetone 2 g 
1-(2-pyridylazo)-2-naphthol (as in Ex. 1) 
33 mg 
2-isopropoxy-1,4-naphthoquinone 
133 mg 
______________________________________ 
These solutions were placed in a refrigerator and used as soon as possible 
after preparation. A coating dope was prepared by mixing 0.3 g of A and 
0.3 g of B with the polyvinyl butyral. An antifoggant in the amount of 1-2 
mg was added to the mixture as described in the table. The solution was 
then coated with a 100-micron doctor blade on subbed poly(ethylene 
terephthalate) and dried sequentially at 21.degree. C. for 1 min and 
60.degree. C. for 4 min. 
The time for the appearance of fog when held face up on a 125.degree. C. 
hot block was measured. The film was exposed for about 1/2 sec in an IBM 
Microcopier IID exposing apparatus through an 0.3 log E step tablet and 
1.0 neutral density filter. The film sample was then processed on the 
125.degree. C. hot block as described earlier. The red diffuse Dmax and 
the number of 0.3 log E steps (speed) developed were measured. 
TABLE III 
______________________________________ 
Fog 
Example 
Antifoggant (1-2 mg) 
Time Speed Dmax 
______________________________________ 
control 
none 7 sec 7 0.31 
##STR23## 10 sec 6 0.29 
8 
##STR24## 15 sec 5 0.32 
9 
##STR25## 30 sec 5 0.32 
______________________________________ 
**Prepared according to Journal of Organic Chemistry, Vol 29, p 1527 1964 
 
Examples 7 and 8 used the same antifoggants as were used in Examples 2 and 
5, respectively, and although the fogging times were comparatively 
reduced, they were still improved compared with the faster fogging time 
which existed for the control. 
EXAMPLES 10-14 
Example The procedure of was repeated, using the antifoggants of the 
following Table IV. 
TABLE IV 
______________________________________ 
Speed 
Fog (# of 0.3 
Example Antifoggant Time log E Steps 
D.sub.max 
______________________________________ 
control none 11 sec. 6 0.30 
10 2,2,6,6-tetra- 
30 sec. 6 0.31 
methyl-4-oxo- 
piperidino oxy 
11 iodosobenzene 
20 sec. 4 0.26 
diacetate 
12 tetrachloro- 25 sec. 6 0.27 
phthalic an- 
hydride 
13 tetrachloro-o- 
32 sec. 4 0.22 
benzoquinone 
14 N-bromosuccin- 
&gt;50 sec. 3 0.13 
imide 
______________________________________ 
EXAMPLES 15-21 
To demonstrate contrast control using 
2,4-bis(tribromomethyl)-6-methyl-s-triazine as the oxidizing agent, the 
dopes listed below were coated at approximately 100-micron wet thickness 
on subbed poly(ethylene terephthalate) film support on a 32.degree. C. hot 
block, held there for 1 min, and then heated for 5 additional min at 
60.degree. C. Where overcoated, a 4.3% aqueous solution of 
poly(acrylamide-co-N-vinyl-2-pyrrolidinone-co-2-acetoacetoxyethylmethacryl 
ate), hereinafter AVPA, (50:45:5 monomer weight ratios) was coated in the 
same manner. 
______________________________________ 
phthalaldehyde 0.320 g 
hexa-amminecobalt(III) trifluoroacetate 
0.200 g 
2-isopropoxy-1,4-naphthoquinone 
0.0108 g 
poly(ethylene-co-1,4-cyclohexylene- 
1.90 g 
dimethylene-1-methyl-2,4-benzene- 
disulfonamide) 
2,4-bis(tribromomethyl)-6-methyl-s- 
see 
traizine Table IV 
dimethyl polyoxyalkylene ether 
0.040 g 
copolymer available under the trade- 
mark "SF-1066 Surfactant" from General 
Electric 
acetone 7.6 g 
______________________________________ 
The sensitometry of the elements was determined from transparencies 
prepared by contact-exposing the elements for 8 sec through a 0.3 log E 
silver step tablet in an IBM Microcopier IID exposing device (with a 
400-watt, medium-pressure mercury arc lamp). The image was developed by 
contacting the back of the film for 5 sec to a 140.degree. C. hot block. 
Neutral densities of the black negative-working images were determined, 
contrast (.gamma.) was measured as the slope of the straight-line portion 
of the curve, and toe speed was measured as the number of visible steps. 
The results, both with and without an AVPA overcoat, are tabulated in 
Table IV. 
TABLE IV 
__________________________________________________________________________ 
No Overcoat 
AVPA Overcoat 
mg mmoles 
Toe Toe 
Example 
Triazine 
Triazine 
Speed 
D.sub.max.sup. N 
.gamma. 
Speed 
D.sub.max.sup. N 
.gamma. 
__________________________________________________________________________ 
control 
0 0 6 2.49 
2.9 
6 2.49 
3.0 
15 0.239 
0.00040 
6 2.30 
2.6 
6 2.42 
3.0 
16 2.39 0.0040 
6 2.21 
2.4 
6 2.32 
2.6 
17 2.98 0.0050 
6 2.19 
2.4 
6 2.29 
2.6 
18 5.97 0.010 
6 2.04 
2.0 
6 2.13 
2.3 
19 11.9 0.020 
6 1.79 
1.9 
6 1.83 
1.9 
20 17.9 0.030 
6 1.50 
1.0 
6 1.50 
1.15 
21 23.9 0.040 
6 0.94 
0.58 
6 1.08 
0.73 
__________________________________________________________________________ 
As shown in the above table, as the concentration of the triazine is 
increased, with or without an overcoat, the contrast of the element is 
decreased without loss of toe speed. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention.