Studies Photocoloration, Photodecoloration, Fatigue and Photodegradation

Mechanistic Studies Photocoloration, Photodecoloration, Fatigue, and Photodegradation 

The various techniques of Raman spectroscopy have recently been applied to the spiropyrans, and this is discussed more extensively in Chapter 8 in Volume 2. BIPS and its 6-nitro-, 6-nitro-8-methoxy-, and 6-nitro-5,7-methoxy derivatives were studied by surface-enhanced (SERS) and resonance-enhanced Raman spectroscopy in neutral, acidic, and basic solutions. The complex results are too numerous to summarize here, but two unusual ones are that even in acid solutions the tested compounds gave the SERS spectra of the unprotonated open forms, and that an aged solid sample of 6-nitroBIPS gave results very different from a fresh sample. This was attributed to the existence of two enantiomers of the spiro form, which are slowly interconvertible in the solid state and which have greatly different photocoloration quantum yields.167 

A similar SERS study was carried out on indolinospironaphthopyran, its aza analog spironaphthoxazine, and the model compounds, 1,2,3,3-tetramethylindoline and 3,3-dimethylnaphthopyran. The orientation of the adsorbed spiro compounds and their indoline and chromene portions on the silver surface was determined. A goal of the work was to develop methods for using the high sensitivity and selectivity of SERS to study the photochromies and their fatigue products in polymer matrices.168 

Nanosecond time-resolved resonance Raman and absorption spectra of 6-nitroBIPS in deoxygenated cyclohexane were obtained in the 20-100-ns time region, extending an earlier study169 in the 200-2000-ns time region. A sequence of three transients was observed (1) the lowest triplet state 7i, X 435 nm, of the spiro form (2) the open merocyanine form, X 580 nm, formed from the triplet (1) and (3) a dimer of the merocyanine, X 540 nm.170 

DABCO increased three- to ninefold the relative amount of 3,3-dimethyloxindole, which is proposed to arise from the reaction between the open form and triplet oxygen. The quencher also considerably increased the fatigue resistance time, the time in minutes necessary to reduce the value of the initial absorbance by half. For the BIPS compound, the increase varied up to nearly tenfold, depending upon the DABCO concentration. 

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