Photochromic Mechanism

The photochromic mechanism of fulgides can be separated into four parts in this discussion chromophore and excited state, E - Z isomerization, photocycliza-tion, and heliochromic reaction. 

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Photochromism Based on Tautomerism. Several substituted anils of saHcylaldehydes are photochromic but only in the crystalline state. The photochromic mechanism involves a proton transfer and geometric isomerization (21). An example of a photochromic anil is /V-sa1icylidene-2-ch1oToani1ine [3172-42-7] C H qCINO. 

The photochromic mechanism for the chromenes is very similar to that for spiropyrans given in Figure 1.1. Under the influence of UV the C-0 bond in the pyran ring is broken to give either the zwitterionic form or, more likely, the cis- and fran.y-quinoidal forms (Figure 1.6). Studies suggest that formation of the cis-quinoidal species occurs in picoseconds, followed by generation of the trans-form in nanoseconds. 

In addition to the photochromic mechanisms discussed in Secs. III-A-D, which encompass a large number of known photochromes, there may be some new types yet to be discovered. If not, the question remaining concerning the mechanisms for many miscellaneous photochromes should yield answers which will fit into the existing framework. 

Kim, K., C. Seo and H. Cheong (2006b). Photochromic mechanism in a-W03 thin films based on Raman spectroscopic studies. Journal of the Korean Physical Society, 48(6), 1657-1660. 

A. Morinaka, T. Yoshida, and N. Funakoshi, Photochromic mechanisms in deposited spiran thin film, Japan. J. Appl. Physics, 26, Suppl. 26-4, 87-90 (1987). 

L. Yu, Y. Ming, and M. Fan, Studies on the photochromic mechanism of l,2,-dimethyl-5-phenyl-3-pyrryl succinic anhydride, Res. Chem. Intermed., 13, 829-838 (1993). 

L. H. Yu, Studies on synthesis, photochromic mechanism and applications of fulgides, Doctoral dissertation, Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing (1994). 

Following their first experiments devoted to 6-nitro-BIPS,17 Takahashi and co-workers conducted a series of nanosecond TR3 studies24-28 on a limited number of photochromic compounds, namely r,3, 3-trimethylspiro[2ff-l-benzo-pyran-2,2 -indoline] (BIPS), 6-nitro-BIPS, and spirobibenzopyran (SBP) (Scheme 4). This work was undertaken to elucidate the electronic distribution of the metastable photomerocyanines (ort/jo-quinoidal or zwitterionic) and the number of stereoisomers involved in the photochromic process. Therefore, the effects of temperature, solvent, laser excitation wavelengths, and isotopic substitutions (I3C and, 5N) on the resonance Raman spectra were intensively studied to clarify the photochromic mechanism in the compounds investigated. 

Light-induced intramolecular proton transfer has long been suspected to play an important role in the mechanism for photochromism in both crystals and solutions. One class of compounds that has been extensively studied is the A -salicylidene anilines. The main evidence for invoking an excited-state proton-transfer step in the photochromic mechanism comes from the observation that the colored species exists as the keto amine tautomer (in the ground state) while the bleached form exists as an enol imine (in the ground state). Replacement of the labile hydrogen with a methyl group prevents the photochemical reaction that leads to the colored species. Picosecond transient spectroscopy was used to study iV-salycylidene-o-toluidine (NST) and the related compound 2-(o-hydroxyphenyl)benzothiazole (HBT). ... 

Ltd., 26 (1976) 17. A.A. Kuz michev, V.V. Sviridov, Some Properties of Photochromic Films based on Ag and Cu Halides, Inorg. Mater., 15 (1979) 1240. b) T. Yoshida, A. Morinaka, N. Fumakoshi, Photochromism of Vacuum Deposited r3 3 -Trimethyl-6-hydroxyspiro [2H-l-benzopyran-2-2 -iodine] Film, J. Chem. Soc. Chem. Commun., 23 (1988) 437. A. Morinaka, T. Yoshida, N. Fumakoshi, Photochromic Mechanism in Vacuum Deposited Spiran Thin Films, Jap. J. Appl. Phys., 26 (suppl. 26-4) (1987) 87. 

Photochromic mechanism implies a transition of chemical compound from a form A into a form B, each of them of a specific absorption spectrum, implying specific colouring of the form as well. The triggering mechanism is usually ultraviolet (UV)-light induced (process of activation). As a result of incident UV light the compound (molecule) changes its colour. 

Figure 2.1 shows the photochromic mechanism of spironaphtoxazine dye at the molecular level. Spironaphtoxazines have been widely studied and researched in the literature (Bamfield, 2001), and they are an interesting class of photochromic dyes for their appUcation potentials to textile fibres (Durasevic et al., 2011 Bouas-Laurent and Durr, 2001). The mechanism according to which the photochromism... 

The photochromic mechanisms of redox-photochromic compounds are usually difficult to determine because they are often multicomponent systems. However, the methylene blue/iron sulfate system is relatively well understood. 

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