REFLECTANCE SPECTROSCOPY FOR THE CHARACTERIZATION OF PHOTOCHROMISM IN THE CRYSTALLINE STATE

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Abstract
Abstract— The Kubelka‐Munk theory for diffuse reflectance has been applied to a quantitative study of photochromism in the crystalline state. For three systems investigated it was found possible to assign first order rate constants to the thermal relaxation process and estimate the pre‐exponential factor A and the activation energy Ea in Arrhenius equation. For the fading of the red photocolored form, Λmax=490 mμ, of benzaldehyde phenylhydrazone A = 1.4×108 min‐1 and Ea= 15.7 kcal mole‐1. For the fading of the blue photocolored form, Λmax=590 mμ, of 2–(2,4‐dinitrobenzyl)pyridine A=5×1014 min‐1Ea=23.3 kcal mole‐1, Cinnamaldehyde semicarbazone showing ‘reversed phototropy’ has a photoactivated state, Λmax=400 mμ, which in dark is transformed into a strongly absorbing yellow species, Λmax= 430 mμ with A =14 × 1010 min‐1 and Ea= 18.7 kcal mole‐1.

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