Photochromism of phenoxynaphthacenequinones: diabatic or adiabatic phenyl group transfer?

Abstract

The photochromic reactions of 6-phenyloxy-5,12-naphthacenequinone (1) and of the 6,11-diphenyloxy derivative 2 were investigated by subpicosecond pump–probe, photoacoustic, and emission spectroscopies, and by nanosecond laser flash photolysis (LFP). The transformation of the trans-quinones 1 and 2 to their ana-isomers proceeds via short-lived triplet states of 1 and 2 (τca. 2 ns) and spiro-bridged biradical intermediates (ca. 6 ns). The long-lived (µs) ana-triplets that are observed by LFP of 1 and 2 are formed (predominantly) by reexcitation of the biradicals and ana-quinones, which appear during the laser pulse. The reverse reaction, ana → trans, proceeds exclusively from the lowest π,π* singlet state of the ana-quinones.