PHOTOCYCLIZATION OF N,N'-DIPHENYL-1-HYDROXY-9,10-ANTHRAQUINONE DIIMINES VIA EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER (ESIPT)

Photolysis of 1,5-dihydroxy- and 1-hydroxy-substituted N,N'-diphenyl-9 ,10-anthraquinone diimines (1a and 1b) produces acridine-condensed rin g systems due to oxidative cyclization. In contrast, anthraquinone dii mines which bear no hydroxy group on the C1-position show no photoreac tivity, providing chemical evidence for participation of excited-state intramolecular proton transfer (ESIPT) in the photocyclization. The g round-state of 1a includes only a small fraction (ca. 4%) of the keto enamine form in fast equilibrium with the enol imine form, as proved b y temperature dependent H-1 and C-13 NMR spectroscopy. An X-ray crysta l analysis of 1a reveals that the molecule exists in the crystalline s tate as the enol imine form with a butterfly conformation of the anthr aquinone ring. The involvement of ESIPT in photoexcited la has been re vealed by means of sub-microsecond time-resolved IR spectroscopy. Upon irradiation of 1a an increase of the ground-state keto form (B) is de tected and its lifetime is deduced to be 1.3 ms. Another transient spe cies (A) with a lifetime of 80 mu s is also observed, which is assumed to be the intramolecular cycloadduct as a precursor of the photoprodu ct.