1. Field of the Invention
This invention relates to tellurium (IV) [hereinafter referred to as Te(IV)] compounds and their use in photographic elements and processes. More particularly, the Te(IV) compounds are useful in various photographic elements that are processed with heat.
2. Description Relative to the Prior Art
It is known to provide an image in an imaging material containing metal compounds. These materials can be useful in what are described as photographic materials for dry processing with heat, heat-developable photographic materials or photothermographic materials. Such heat-developable photographic materials, after imagewise exposure to provide a developable latent image, are heated to provide a developed image without requiring processing solutions.
An image amplification step is an important factor in providing photographic materials, including photothermographic materials, having increased speed. In these materials, a latent image is generally formed by imagewise exposure of photosensitive material to suitable radiation. The resulting invisible or latent image formed is then used to catalyze the reduction of a material in a high oxidation state to a low oxidation state, thereby forming an image. In silver halide photographic materials, for example, exposure of photographic silver halide to light results in formation of silver nuclei, i.e., specks of silver metal, which then can catalyze the further reduction of silver halide in the presence of a reducing agent.
The use of tellurium compounds in imaging is known. For instance, it is known to produce negative tellurium images by disproportionation of tellurium dihalides. The images are formed in the presence of a processing liquid or solution. The liquid aids in the disproportionation reaction. The unexposed dihalides, however, are dark in color, causing poor discrimination between exposed and unexposed areas. Further, the tellurium dihalides are typically unstable in air and undergo light induced decomposition only when moistened with an organic solvent.
Other tellurium imaging materials are known which contain certain Te(IV) compounds wherein the tellurium is bonded directly to a carbon atom. These tellurium materials undergo what can be described as a unit quantum photoreduction to yield a tellurium (0) image. These materials and imaging processes do not involve a catalytic amplification of a latent image. That is, the tellurium (0) formed upon exposure does not catalyze the decomposition of other components or other reaction of the Te(IV) compounds. These processes and imaging materials are inherently photographically slow in speed and limited in usefulness because they do not employ an amplification reaction.
Useful imaging elements and processes using Te(IV) compounds are disclosed in copending commonly assigned U.S. Pat. Nos. 4,144,062 issued Mar. 13, 1979, and 4,152,155 issued May 1, 1979. In these applications, there are described certain imaging compositions comprising (1) a tellurium compound [which can be a Te(IV) compound] as an oxidizing agent, with (2) a reducing agent. This imaging combination can be useful in heat-developable photographic materials containing, for example, photographic silver halide or other sources of developable nuclei. Among the Te(IV) compounds described in those applications are compounds having the formula: X.sub.3 TeR, wherein R is alkyl or aryl, and X is an anion. While these compounds are particularly useful in solution processes, compounds of this general class, where R is alkyl, may be relatively unstable in air. On exposure to air for even a short period, some of these compounds tend to hydrolyze to a white gum. (See G. T. Morgan and O. C. Elvins, J. Chem. Soc., 2627 [1925]). It is readily apparent that this can severely limit the usefulness of these compounds in commercial elements and processes.