Photochromism of Acetyl‐Cyclophanochromene: Intriguing Stabilization of Photogenerated Colored o‐Quinonoid Intermediates

Abstract

AbstractThe photochromism of a chromene annulated on [2.2]paracyclophane, Ac‐CPC, was investigated. Whereas the parent chromene 2,2‐diphenylbenzopyran shows photoinduced coloration only at very low temperatures, acetyl‐cyclophanochromene Ac‐CPC was found to exhibit room‐temperature photochromism. With no extended conjugation due to absence of any substituents in two cofacially‐oriented aryl rings, the through‐space delocalization of π‐electrons, i.e., the phane effect, manifests nicely leading to the stabilization of the photogenerated colored o‐quinonoid intermediates. The interaction between cofacial aryl rings is supported by the distance between them, as revealed by X‐ray crystal structure analysis. Additionally, electrophile···nucleophile interactions are surmised to be important for intriguingly longer persistence of the photogenerated intermediates of Ac‐CPC relative to the lifetime of the photogenerated intermediate generated from the parent cyclophanochromene, which is devoid of an electron‐withdrawing acetyl group.