Sensitizing dyes desensitization

Sensitizers as well as desensitizers form a reversal oxidoreduction system with silver halides, according to both pH and pAg of the photographic emulsion. But besides the specific influence of the emulsion, the efficiency of a sensitizing dye depends on many other factors such as its adsorption, its spectral absorption, the energetic transfer yield, the dye aggregate to the silver halide, and finally on its desensitizing property in... 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

A good sensitizing dye does not interfere with other system properties. Sensitizing dyes can sometimes influence the intrinsic response of a chemically sensitized emulsion, leading to desensitization or additional sensitization. The dye can also interfere with development rate, increase or decrease unwanted fog density, and remain as unwanted stain in the film after processing. The dye should have adequate solubihty for addition to the emulsion, but should not wander between layers in the final coating. 

Chemical and spectral sensitization by dyes is a powerful method of increasing the photosensitivity. As a rule the dye plays a double role - the photogeneration sensitizer and desensitizer due to the change of the recombination and trapping processes. Various types of dyes with donor or acceptor properties were used for such purpose. 

The most important reason for the laige number of technical applications of polymethine dyes is their relatively low electron-transition energies and their highly intense and narrow spectral bands. Indeed, polymethines display strong light absorption and emission, between 300 and 1600 nm. In the 1990s, these dyes are mainly used as photographic sensitizers and desensitizers (11,90), as laser dyes (12,13,91), as probes of membrane potentials (14), and in other applications where the theoretical aspects of polymethines are useful. 

Many sensitizing dyes, including dyes used in commercial emulsions, can decrease the sensitivity for radiation absorbed by the silver halide. This desensitization extends to the region of spectral absorption by the dyes, and thereby competes with the spectral sensitizing action. 

The earlier values for Er and Eqx are retained in the subsequent discussion of the sensitizing and desensitizing properties of dyes to conform to the earlier literature. The general conclusions would not be changed by using Lenhart s values, except for the quantitative correlation with activation energies of sensitization referred to in Section X. 

The pronounced dependence of the crossover potential on chemical sensitization, and particularly on the gaseous environment, shows that the crossover does not represent a division between dyes that can cause the appearance of conduction electrons in the silver halide and dyes that cannot. Instead, it represents an energy level determined by a kinetic balance between the formation and loss of photoelectrons and/or silver in the silver halide, a kinetic balance between sensitization and desensitization (259,265). One cancels the other and the net formation of latent image is zero. The actions of oxy-gen/moisture and of mobile holes are important sources of desensitization. 

Figure 49. Some possible photoinduced electron transfer processes involving dyes adsorbed to the surface of crystalline silver halide. CB and VB refer to the conduction and valence bands of the silver halide, AgX, and HOMO and LUMO refer to the highest occupied and lowest unoccupied molecular orbitals of the sensitizing dye. a) Electron injection from the excited state of the dye b) hole injection from the excited state of the dye c) electron transfer to the valence band after excitation of the silver halide d) desensitization by an adsorbed dye.

Figures 49(c) and 49(d) show two other important processes which can occur when the silver halide is excited directly in the presence of adsorbed dyes. In these cases an electron is transferred from the VB to the CB upon excitation, and the holes in the VB may be filled by electron transfer from the HOMO of the adsorbed dye. The product of this process in Figure 49(c) is the same as that from the electron-injection dye sensitization in Figure 49(a), i.e., a dye radical cation and a conduction band electron which may be trapped and contribute to latent image formation. Illustrated in Figure 49(d) is the consequence of excitation of silver halide in the presence of a dye in which the energy of the LUMO is lower than that of the CB. In this case, direct excitation of the silver halide results in a conduction band electron which can be transferred to the LUMO of the dye. Subsequent electron transfer of an electron from the HOMO of what would then be a dye radical anion results in effective deactivation of the band-gap excitation, and overall reduced photographic sensitivity of the silver halide toward direct excitation due to the presence of the dye. This process is known as dye desensitization.

Analysis of light-induced ESR signals of sensitizing dyes on the surface of AgBr microcrystals in photographic emulsions revealed that positive holes trapped by the dyes were responsible for the ESR signals and desensitization caused by the dyes. It was demonstrated from the ESR and sensitometric measurements that positive holes trapped by the dyes could react in several minutes with latent images on the surface of the microcrystals. 

As will be apparent, the principal interest in this ring system is as a source of carbocyanine dyes. In the last few years over 40 patents have been listed in Chemical Abstracts describing their use as sensitizers and desensitizers of direct positive photographic emulsions, A further dozen or so patents claim their use as sensitizers for electrophotographic recording materials. Other uses for the ring system are as fungicides and as initiators of photochemical polymerization of vinyl monomers. ... 

Because of local variations in the relevant energy levels, sensitizing dyes could also desensitize when their excited energy level is below the CB of the AgX, causing them to behave as an electron trap (Fig. 2c) this would possibly allow the electron to combine with oxygen and prevent it from taking part in the formation of metallic Ag. Also, a dye may trap a hole (Fig. 2d) by transferring an electron... 

FIGURE 2 Energy levels of AgX and of color-sensitizing dyes, showing sensitization by (a) electron- and (b) energy-transfer, and desensitization by (c) electron trapping and (d) hole trapping, e represents electron pathways. 

Tani, T. Honda, K. Kikuchi, S. Studies on spectral sensitization and desensitization in photography. XIII. Discussions on spectral sensitization of dyes from electronic energy levels, photographic halfwave potentials, and excitation energies. Kogyo Kagaku Zasshi 1968, 71, 42-47 Chem. Abstr. 1968, 68, 118488. 

Based on correlations between energy level positions and electrochemical redox potentials, it has been established that polymethine dyes with reduction potentials less than —1.0 V (vs SCE) can provide good spectral sensitization (95). On the other hand, dyes with oxidation potentials lower than +0.2 V are strong desensitizers. 

Improvement of spectral sensitization can he accomplished by dye combinations. The effect has been found to often be greater than the predicted additive sensitivity increase. This phenomenon is called snpersensiti7ation, which is applied most effectively to polymethine aggregates. The opposite phenomenon, a decrease of sensitivity, is known as desensitization. The main reasons for desensitization are the results of relative electron level positions as well as the secondary processes of the photoelectrons. 

It has been assumed that desensitization by spectral sensitizers occurs to an equal degree in the regions of inherent absorption by the silver halide and absorption by the dye, but data on the effects of oxygen and moisture in these two absorption regions show that the degree of desensitization can be different under some conditions. The effect of the environment on the relative quantum efficiency of some dyes can depend on whether the dye is in the monomeric or aggregated state (266). This is illustrated in Table A. 

The extent to which desensitization can occur by recombination of a photoelectron with a hole trapped by a dye is not known. This mode is theoretically possible, but it is also possible that hole trapping could increase sensitivity if the probability of reaction of a mobile hole with a trapped electron or silver atom is greater than the probability of loss of a mobile electron to a trapped hole or if the cross-section for recombination of an electron with a dye-trapped hole is less than with an intrinsically trapped hole. Sensitization by hole trapping dyes has been demonstrated by Leubner (269) who showed that, independently of Er, dyes with Eqx less than 0.5 V can chemically sensitize emulsions with about.constant efficiency. 

The discovery of the sensitizing properties of dyes that formerly had been considered as strictly desensitizing dyes, and the high efficiency with which some of these dyes can spectrally sensitize under proper conditions, requires reconsideration or extension of earlier theories of spectral sensitization. Unless previous estimates of the position of the singlet excitation level of these dyes relative to the conduction band of the silver halide are in serious error, these dyes should not be able to photoinject electrons directly into the conduction band from the level. The clue to their sensitizing action, I think, is contained in an explanation offered by Sturmer, Gaugh, and Bruschi for the "abnormal" spectral sensitization obtained with one of the desensitizing dyes used in their study of the effect of chemical sensitization on the efficiency of spectral sensitization (273). They wrote that... 

The "desensitizing" dyes that become good spectral sensitizers under the vacuum-hydrogen conditions and have values less negative than the crossover potential in air all have oxidation potentials in the range for photohole injection into the valence band. 

For Class 1 dyes, sensitization occurs by transfer of the electron from the state of the excited dye to the conduction band of the silver halide. The hole created by the transfer remains in the dye molecule. If the hole is not too deeply trapped, it may eventually escape into the valence band with the aid of thermal energy. These dyes provide few, if any, electron traps and desensitization by oxygen/moisture in their presence would equal that for the undyed emulsion. 

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