Photochromism of a novel class of spiroindolines: 6-aroyl-3,5-diarylspiro[cyclohexa-2,4-diene-1,2'-indolines]

The photochromism of a new class of spiro compounds (spiro[cyclohexadieneindolines) has been studied by UV and visible stationary photolysis and laser flash photolyis (lambda = 266, 353, and 532 nm). In analogy to spiropyrans, isomerization to ring-opened compounds (merocyanines) and formation of spiro compound triplets has been found. Visible light illumination of the red merocyanines regenerates the original spiro compound, Since in liquid solutions decomposition reactions compete with the recyclization, under these conditions only three switching cycles could be verified. In the case of spiroindolines embedded in a polyethylene matrix after five cycles, no decomposition has been observed. The photoisomerization process has been found to proceed via the first excited singlet states of the spiro compounds and merocyanines, respectively. Quantum yields of all reactions and spectra and extinction coefficients of the spiro[cyclohexadieneindolines] and of their isomerization products (merocycanines) are given.