Influence of Triptycene Annulation on the Photochromism of Diphenylnaphthopyrans: Entropic Control of Thermal Reversion

Abstract

Regioisomeric naphthopyrans annulated with triptycene, i. e., Prox-NP and Dist-NP, display divergent photochromic behaviors. While steady-state photolysis of Dist-NP led to a very labile colored intermediate that is not observable at room temperature, Prox-NP yielded a remarkably stable species characterized by X-ray crystallography as the TT isomer of o-quinonoid intermediate (Prox-NPQ) with t1/2 ca. 0.18 years at 298 K. The kinetic analysis of thermal reversion reveals that the bleaching of Prox-NPQ is entropically controlled; the steric effect due to the rigid triptycene scaffold renders Prox-NP a highly constrained system such that the photogenerated colored o-quinonoid form is more entropically relaxed. This constitutes the first instance of an entropically-controlled thermal reversion for the celebrated class of photochromic naphthopyrans. Based on the response of Prox-NP and its colored intermediate Prox-NPQ to different stimuli, namely, light, heat, and acid, the molecular system can be likened to a logic gate with the ‘INHIBIT’ function.