Steric and substituent control on the photoreversibility of some novel N-alkyl-3,3′-disubstituted-6-nitro-indolospirobenzopyrans: Evaluation using UV spectroscopic studies

A series of novel 6-nitro substituted indolospirobenzopyrans are reported. The effects of substitution on their photoreversibility are investigated by: synthesising derivatives that possess sterically hindering (with regard to the spiropyran-opening ↔ closing) groups contained within the spirocyclic skeleton. Specifically, spirobenzopyrans possessing combinations of various N-alkyl, and/or, simultaneously, 3,3′-geminal methyl and/or cyclohexyl groups are synthesised. Further, these systems are evaluated in three solvents; methanol, acetonitrile and dichloroethane. The thermal decolourisation rates of the gem-dimethyl and 3,3′-cyclohexyl compounds are additionally evaluated at the temperature of 50 °C. The above systems, and changes in physical parameters are extensively evaluated, and the subsequent biasing of the equilibria established using UV spectroscopy. Rates of colourisation, decolourisation and the equilibrium constants are obtained. The overall photochromic ’tuneability’ of these systems is subsequently established.