Reto Born, Walter Fischer, Dominik Heger, Bogdan Tokarczyk, and Jakob Wirz, "Photochromism of Phenoxynaphthacenequinones: Diabatic or Adiabatic Phenyl Group Transfer?", Photochem. Photobiol. Sci. 2007, 6, 552 - 559.
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 (tau ca. 2 ns) and spiro-bridged biradical intermediates (ca. 6 ns). The long-lived (microseconds) 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 pi,pi* singlet state of the -quinones.