Photochromism of novel chromenes constrained to be part of [2.2]paracyclophane: remarkable ‘phane’ effects on the colored o-quinonoid intermediates

Abstract

The photochemistry of rationally designed chromenes that are constrained to be part of [2.2]paracyclophane, i.e., CP-H and CP-OMe, was investigated to examine the effect of through-space delocalization in the cyclophane core (phane effect) on the photochromic behavior. In contrast to the parent chromene, i.e., 2,2-diphenylbenzopyran CH, for which the photoinduced coloration is not observable at room temperature, the cyclophanochromenes CP-H and CP-OMe lend themselves to readily observable photochromism. The photogenerated o-quinonoid intermediates that are responsible for the observed color revert slowly to permit kinetic monitoring of their decays. The difference in the kinetic rate constants for reversion of the photogenerated o-quinonoid intermediates derived from CP-H and CP-OMe attests to remarkable through-space delocalization. To the best of our knowledge, the observed results constitute the first examples of the influence of phane effects on the stabilization or otherwise of the reactive intermediates.