Photoinduced bleaching

Hattori, X, Shibata, X, Onodera, S., and Kaino, X Fabrication of refractive index grating into azo-dye-containing polymer films by irreversible photoinduced bleaching. /.. 4pp/. Phys. 2000, 87, pp. 3240-3244. 

C. Photoinduced Bleaching of the Infrared Absorption Bands and Its Recovery as Observed... 

Fig. 15. Photoinduced bleaching ofthe infrared absorption band of an all-/r ns-/3-carotene single crystal recorded at 11 K and excited at 575 nm.

Fig. 16. Single-exponential recovery of the photoinduced bleaching of a single crystal of all-rmns-iS-carotene recorded at 77 K. The crystal was excited at 575 nm and the recovery was probed at 1300 nm.

The questions set forth here are answered as follows Eirst, broad and anisotropic absorption bands where found to gradually increase from the near- to mid-infrared region of all-trans-p-carotenc single crystals. Second, the stretched-exponentiai and power-law recoveries of the photoinduced bleaching of the infrared absorption bands may correspond to the recombination of the solitonlike excitations in all-trans-P-carotene single crystals. 

Figure 3. Temperature dependence of the photoinduced electronic absorption at - 0.13 eV, the photoinduced bleaching at 598 cm-i, and the photoinduced IRAV mode at 520 cm-i.

Photoinduced changes in transmission AT are of two kinds, photoinduced absorption (PA) and photoinduced bleaching (PB). Photoinduced absorption is associated with transitions from initial states when their occupation by electrons is enhanced by the illumination or, alternatively, with transitions whose final states are emptied by the illumination. Photoinduced bleaching is associated with initial states occupied in the dark that the illumination empties or with final states filled by the illumination. We will see that because of this variety of... 

Fig. 22.6 Photomodulation spectrum of rra j-(CH).t at 210 K showing the photoinduced ab.sorption (PA) and photoinduced bleaching (PB) bands, respectively, associated with excitations. (From Ref. 18, with permission.)...

A number of CP properties are directly related to function of devices such as light-emitting diodes (LEDs). These include Photoinduced Absorption (PIA) and the related Photoinduced Bleaching (PIB), Electroluminescence (EL), Photoluminescence (PL), and Fluorescence. 

Clark and Lothian (30) reported an energy of activation of 0.06 ev. for the photoinduced reaction and 0.83 ev. for the bleaching reaction for the crystalline photochrome. The dark reaction, at 25°C., requires about 10 hr. for complete reversion. Although, at room temperature, solutions of 2-(2, 4 -dinitrobenzyl)pyridine do not appear photochromic, Hardwick et al. (31) observed that in low temperature (—30°C.) solutions the same color changes could be induced as in the crystals. 

The thermal bleaching rate of the photoinduced form of 5-methyl-1,4-naphtha-quinone depended on the solvent polarity (Table 7.13)." It was assumed" that an... 

The substituents in the anthraquinone ring affected the rate constant of thermal bleaching of derivatives of 1-methylphenoxyanthraquinone (Table 7.1).17,18,26 The substituents that increased the Tt-electron density on the hydrogen atom of the methide group decreased the lifetime of the photoinduced form (Table 7.1).18,26 The introduction of methoxy and piperidine substituents stabilized the photoinduced form (Table 7.1). 

The extremely rapid rate of formation of bathorhodopsin as compared to isomerization rates observed in model protonated Schiff bases (a factor of 103) suggested the idea that electron transfer between an amino acid residue (e.g., tyrosine or tryptophan) in the protein and the chromophore may catalyse isomerization. Thus, a photoinduced electron transfer leading to a radical anion chromophore, instead of complete cis-trans isomerization, was considered as a plausible alternate mechanism for the primary event [200], This mechanism, however, is difficult to reconcile with the known photoreversibility but thermal irreversibility of the bleaching process. Thermal irreversibility of the light-induced electron transfer would require geometrical separation of donor and acceptor moieties which would then not allow photoreversibility [201]. 

Another similar example, also used in polymerization initiation, is the case of the iodonium salts of Rose Bengal (RB2-) [224, 225]. Methylene chloride solutions of these salts bleach in a few seconds in room light through an electron transfer photoinduced from the excited RB2- to the iodonium cation the resulting phenyl radicals were reported to initiate polymerization of acrylate. 

The chain termination reactions are performed most effectively by resonant optical excitation of the DR and AC intermediates (optical bleaching) as shown in Figs. 6 and 14. The DR and AC reaction centres may be indirectly excited via energy transfer from the monomer matrix. The chain termination reactions are, therefore, in competition with the photoinitiation and photoinduced chain propagation reactions. 

It is apparent in Figure 13.14b that the absolute level of chirality decreases for each cycle, but heating the film above isotropization and then cooling to room temperature restores the initial level that can be attained. Thus, the apparent decrease in photoinduced chirality is due to the increased homeotropic alignment of the azobenzene chromophores, not to a bleaching effect, and it is thus reversible. 

The authors attribute the bleaching of this charge-transfer transition within the 20-ns duration of a laser pulse to a very rapid photoinduced electron injection in the conduction band of Ti02. Another example of an electron transfer through a bridging ligand of a ternary surface complex will be given in Section 4. 

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