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.