Photoprocesses in Spirooxazines and Their Merocyanines

Abstract

The photocoloration of four spirooxazines upon excitation of the closed form and the subsequent relaxation were studied in various solvents at different temperatures. The relaxation kinetics contain a minor shorter-lived component and a major longer-lived one. Both are activated processes leading back to the ring-closed form; the activation energy is typically 70 kJ mol -1 . The activation parameters are used as a basis to examine the photomerocyanine form(s), where weak fluorescence appears in glassy media at −196 °C. Excitation (at 530 nm) of the photomerocyanine in solvents of different polarity at room and lower temperatures leads to reversible bleaching in the visible range and weak absorption in the UV. The lifetime at 25 °C is in the 0.5−5 μs range and the activation energy is 21−29 kJ mol -1 . These oxygen-insensitive changes are attributed to cis−trans photoisomerization. The mechanism of photochromism and relaxation and the photoprocesses in spirooxazine-derived merocyanines are discussed.