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 (mu s) 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 ana-quinones.
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