Spectroscopic and photochemical evaluation of stereochemically biased 3′-substituted spiropyran photoswitches†

Abstract

Three series of spiropyran photoswitches with an auxiliary chiral centre at position 3′ of the indoline unit were synthesized. Using one example, a novel methodology for synthesis of an optically active spiropyran photoswitch with a defined chirality at position 3′ is demonstrated. Furthermore, a new acid-mediated strategy for spiropyran purification affording moderate to excellent yields (up to 96%) is reported herein. Relative diastereomeric ratios of the prepared spiropyrans were evaluated using NMR spectroscopy in five different solvents (syn : anti up to 21 : 79) and their photoswitching properties determined by UV-vis spectroscopy. It was found that substitution at position 8 of the chromene subunit notably accelerates the photoswitching process.