Process of amplifying image in image recording layer by releasing reactant from image forming layer containing cobalt(III)complex

#This invention is directed to an image-forming element and to a process for its use in which a complex of cobalt(III) and a chelating compound can be formed to achieve imaging by a reaction sequence exhibiting a gain capability. In a specific form this invention is directed to an image-forming element and process which employs a photoactivator to initiate formation of the chelating compound containing cobalt(III)complex. In a further aspect this invention is concerned with such a photographic element and process capable of forming a photographic image in either a photographic element or layer in which the chelating compound containing cobalt(III)complex is formed or in a separate image recording element or layer.

#Classically, photographic elements have incorporated silver halide as a radiation-sensitive material. Upon exposure and processing the silver is reduced to its metallic form to produce an image. Processing, with its successive aqueous baths, has become increasingly objectionable to users desiring more immediate availability of a photographic image. Despite the processing required, silver halide photography has remained popular, since it offers a number of distinct advantages. For example, although silver halide is itself photoresponsive only to blue and lower wavelength radiation, spectral sensitizers have been found which, without directly chemically interacting, are capable of transferring higher wavelength radiation energy to silver halide to render it panchromatic. Additionally, silver halide photography is attractive because of its comparatively high speed. Frequently, silver halide is referred to as exhibiting internal amplification--i.e., the number of silver atoms reduced in imaging is a large multiple of the number of photons received.

#A variety of nonsilver photographic systems have been considered by those skilled in the art. Typically these systems have been chosen to minimize photographic processing and to provide usable photographic images with less delay than in silver halide photography. Characteristically, these systems require at least one processing step to either print or fix the photographic image. For example, ammonia or heat processing has been widely used in diazo imaging systems. While advantageously simple in terms of processing, these systems have, nevertheless, exhibited significant disadvantages. For example, many nonsilver systems are suitable for producing only negative images (or only positive images). Further, these systems have been quite slow, since they have generally lacked the internal amplification capability of silver halide. Many systems have also suffered from diminishing image-background contrast with the passage of time.

#This invention can be better understood by reference to the following detailed description considered in conjunction with the drawings, in which

#FIG. 1 is a schematic diagram of a radiation-sensitive element according to this invention;

#FIG. 2 is a schematic diagram of the radiation-sensitive element in combination with an original image bearing element receiving a reflex exposure;

#The cobalt(III)complexes employed in the practice of this invention are those which feature a molecule having a cobalt atom or ion surrounded by a group of atoms, ions 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, since the ligands are tenaciously held in these complexes as compared to corresponding cobalt(II)complexes. Preferred cobalt(III)complexes are those which are inert. Inert complexes are defined as those which, when a test sample thereof is dissolved at 0.1 molar concentration at 20.degree. C in an inert solvent solution also containing a 0.1 molar concentration of a tagged uncoordinated ligand of the same species as the coordinated ligand, exhibit essentially no exchange of uncoordinated and coordinated ligands for at least one minute, and preferably for at least several hours, such as up to five hours or more. This test is advantageously conducted under the conditions existing within the radiation-sensitive elements of this invention. Many cobalt(III)complexes show essentially no change of uncoordinated or coordinated ligands for several days. The definition of inert complexes, and the method of measuring ligand exchange using radioactive isotopes to tag ligands are well known in the art. See, for example, Taube, Chem. Rev., Vol. 50, p. 69 (1952) and Basolo and Pearson, Mechanisms of Inorganic Reactions, A Study of Metal Complexes and Solutions, 2nd Edition, 1967, published by John Wiley and Sons, page 141. Further details on measurement of ligand exchange appear in articles by Adamson et al, J. Am. Chem., Vol 73, p. 4789 (1951).

#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 cobalt(III)complexes. Nearly all Lewis bases (i.e., substances having an unshared pair of electrons) can be ligands in cobalt(III)complexes. Some typical useful ligands include halides (e.g., chloride, bromide, fluoride), nitrate, nitrite, superoxide, water, amines (e.g., ethylenediamine, n-propylene diamine, diethylenetriamine, triethylenetetraamine, diaminodiacetate, ethylenediaminetetraacetic acid, etc.), ammine, azide, glyoximes, thiocyanate, cyanide, carbonate, and similar ligands, including those referred to on page 44 of Basolo et al, supra. It is also contemplated to employ cobalt(III)complexes incorporating as ligands Schiff bases, such as those disclosed in German OLS Pat. Nos. 2,052,197 and 2,052,198.

#The cobalt(III)complex useful in the practice of this invention can be neutral compounds which are entirely free of either anions or cations. 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 solubilizable cobalt(III)complexes, such as alkali and quaternary ammonium cations.