The purpose of this article is to provide a critical review of recent advances in mechanistic understanding of the physics and chemistry of the development process in photothermographic imaging media. Accordingly, it is confined to grayscale imaging media based on silver halide–silver carboxylate formulations. The chemistries of infectious development systems and color photothermographic media are not considered, despite their practical and commercial importance. The literature search on which this review is based was restricted to journal articles and conference proceedings papers; the patent literature was excluded. Distinct differences can be defined between the processes occurring in solvent-based and in water-based media, with consequences for their imaging characteristics.

One route to improving the photographic properties of PTG imaging materials and further the basic understanding of their photophysics is to better quantify the quantum yield of sensitization. A method was developed to estimate the absolute quantum yield of sensitization for IR-sensitive PTG imaging materials. The method utilizes three techniques: sensitometry, photobleach, and Monte Carlo simulation. Sensitometry and Monte Carlo simulation are well-known techniques. The photobleach technique is an optical technique developed to measure the absorptance of the dye adsorbed to the silver halide grains. The absorptance spectrum, by itself, is not sufficient because the absorption peak for dye adsorbed to silver halide and the absorption peak of dye not adsorbed to silver halide are not sufficiently shifted in position to allow us to resolve them. The photobleach technique is based on the discovery that the dye effectively adsorbed to the silver halide photobleaches much more readily. Using this distinction, the absorptance of the dye adsorbed effectively to the silver halide grains was extracted. The mean number of absorbed photons required to form a latent image was estimated to be 35 ± 7 for the PTG samples studied. In addition, practical applications of these techniques are discussed.

The quantitative assessment of silver development efficiency and covering power in silver based imaging elements requires an accurate determination of the coverage of silver metal (Ag(0)) formed as a result of post-exposure image development. The thermographic and photothermographic imaging processes do not incorporate a silver source removal-fixing step. Therefore, it has been necessary to develop an analytical technique capable of providing a quantitative determination of the Ag(0) coverage in the presence of silver salts and complexes. An X-ray diffraction method has been developed that is based on the correlation between silver metal diffraction peak area intensity and silver metal coverage present in the final image area. It has been demonstrated that this methodology may be used for the assessment of the development efficiency, covering power, and quantum efficiency in photothermographic, silver behenate based, imaging elements. The use of X-ray diffraction techniques has been extended to characterization of crystallinity, morphology, and crystallite size of micro- and nano-silver behenate particulates.

The fundamental imaging element in photothermographic media is nanosized silver particles, formed from the thermally induced reduction of silver ions. In these films, such as the Kodak DryView™ laser imaging system media, the developed silver imaging elements contain a mixture of dendritic and filamentary silver. Using a temperature series (105–140°C) arrested development study, the latter was found to be ribbon-like, crystalline, and dispersed with microtwins. Dendritic silver, however, was seen to develop at a higher temperature and reached its maximum size at 122°C, the condition normally used in the thermal processing of Kodak DryView™ films. Its morphology resembled a cluster containing numerous loosely packed silver nanoparticles. At higher temperatures, it gradually collapsed and condensed into a smaller polycrystalline aggregate. With increasing temperature, its degree of structural order gradually increased and was the highest at 140°C. From these results we show how the dendritic silver contributes to the light absorption properties of the image area, and we lay the groundwork for providing better understanding of the silver particle formation that will lead to new and improved imaging materials.

The fundamental building blocks of light absorption in photothermographic imaging systems are the nanosized silver particles formed from the thermally induced reduction of silver ions. The silver particles tend to be filaments or clustered agglomerations of nanospheres (dendrites). While the silver particle size is well known to be important for light absorption properties, it is shown here that the physical proximity between particles is also a critical controlling factor in obtaining a neutral black image tone. It is further shown that the ideal form of metallic silver in any thermally developed black and white imaging system comprises dispersed, tight (physically separated by 0–30 Å) clusters of 10–30 polydisperse, spherical silver nanoparticles, each having diameters in the 5–30 nm range. The minimum cluster aggregation size should be in the 50–200 nm diameter range, depending on shape. This understanding provides a target for silver efficiency, tone, and optical density for photothermographic media constructions.

The effects of intramolecular hydrogen bonding on the electron transfer properties of a series of bisphenol derivatives, as compared with those of monophenol derivatives in which a hydroxy group of the bisphenols is replaced with a methoxy group, have been investigated in relation to their utility as photothermographic developers. The oxidation of bisphenol derivatives with one-electron oxidants occurs to give the radical cation, followed by deprotonation, to produce the phenoxyl radical. Both the radical cations and phenoxyl radicals have been detected by laser flash photolysis measurements. Rates of hydrogen transfer reactions from a series of bisphenol derivatives to cumylperoxyl radicals have also been determined by monitoring the decay of the ESR signal of cumylperoxyl radical produced by photoirradiation of an oxygen saturated propionitrile solution of cumene and di-t-butylperoxide in the presence of the bisphenol derivatives. Intramolecular hydrogen bonding plays an important role in determining the overall oxidation reactivity in the two-electron oxidation process of bisphenol derivatives by decreasing the one-electron oxidation potentials and also by facilitating the deprotonation step from the bisphenol radical cation to produce a phenoxyl radical.

In situ investigation of metallic silver formation, resulting from the thermal decomposition of silver myristate and from the thermally induced reduction of silver myristate incorporated into a photothermographic imaging construction, shows that the metallic silver formed is different in the two systems. The thermal decomposition of this silver carboxylate produces larger metallic silver crystallites than the thermally induced reduction process, which can be attributed to the silver growing within the constraints of the solid state crystal lattice in the former, compared to the transport of silver ions to the development sites in the latter. The conversion of silver ions to metallic silver in the decomposition process occurs only after the silver carboxylate crystallites pass through several solid state phases.

Photothermographic (PTG) imaging materials have become an indispensable application of silver technology that capitalizes on the capability of silver to undergo reduction to form a black and white image. Metallic silver, the light absorbing component of these imaging materials, has a unique morphology that provides good optical density and image tone. Understanding the silver chemistry portion of the metallic image formation process is important toward improving the photographic response of these materials. We have continued our investigation of various aspects of the silver coordination chemistry and report the solid state structures of several new silver complexes, utilizing ligands present in the PTG formulation, as well as model structures. Depending on the complexing agent and silver counterion, various silver complexes can be isolated and characterized. This report illustrates the silver-ligand interactions of silver compounds based on ortho-dicarboxylic acids, as well as the role ligand functionality could play in the imaging reactions based on these compounds.

New technologies will need to be discovered to satisfy consumer film imaging needs for rapid image access and available-everywhere film photofinishing. Fortunately, the film technologies that enable rapid image processing are also the same technologies that form the basis for relatively inexpensive, self-operated, highly distributed photofinishing systems for consumers. Central to such concepts are camera speed films that can be thermally processed for very short times (less than 10–20 seconds). Photothermographic (PTG) technologies offer the possibility of circumventing the major obstacles associated with current conventional photofinishing that relies entirely on wet chemistries. The authors have examined the potential of new PTG technologies to provide the photographic sensitivity and raw stock stability required for a consumer color film. We have concluded that it might be possible to meet such requirements with newly discovered technologies.