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

The 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 p-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.