Sensitizing dyes hole trapping

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. 

In photography, a technique called supersensitization with supersensitizer (SS) is used to improve the quantum yield of spectral sensitization. The SS dye molecules co-adsorbed on the AgBr grains in much smaller concentration than the sensitizing dye improve the sensitization. The shortening of the fluorescence lifetimes by addition of SS is observed and this phenomenon is explained by the hole-trapping mechanism [10,11]. 

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

Figure 52. Hole-trapping mechanism for supersensitization. The super-sensitizer, SS, transfers an electron to the excited state of the dye to form the radical anion of the dye, before electron transfer occurs to the silver halide. The radical anion subsequently injects the electron to the CB [207].

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. 

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. 

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. 

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.

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