Steric and electronically biasing substituent effects on the Photoreversibility of novel, 3′-, 5′- and 3-substituted indolospirobenzopyrans. Thermal evaluation using 1H NMR spectroscopy and Overhauser enhancement studies

Several, novel, 3′-, 5- and 3-substituted indolospirobenzopyrans were prepared and their reversible thermochromic equilibria were investigated by synthesising derivatives with appropriately placed “electronically” modifying substituents in both the indole and benzopyran rings as well as by synthesising structural variants that contained sterically hindering (with regard to spiropyran-opening ↔ closing) groups, within the spirocyclic ring-system. In addition, structural combinations of the above, involving the syntheses of sterically restricted and electronically biasing substituents were prepared and their effects upon the spiropyran ring-opening ↔ closing process investigated. The influence of these steric and electronic substituent effects on the relative quantities of the open- and closed-forms was established using 1H NMR at six temperature intervals (298, 320, 340, 360, 380 and 410 K). Further elucidation of the stereochemical conformation in these systems was undertaken using nuclear Overhauser enhancement (nOe) studies.