Photocyclization of N,N′-diphenyl-1-hydroxy-9,10-anthraquinone diimines via excited-state intramolecular proton transfer (ESIPT)

Abstract

Photolysis of 1,5-dihydroxy- and 1-hydroxy-substituted N,N′-diphenyl-9,10-anthraquinone diimines (1a and 1b) produces acridine-condensed ring systems due to oxidative cyclization. In contrast, anthraquinone diimines which bear no hydroxy group on the C1-position show no photoreactivity, providing chemical evidence for participation of excited-state intramolecular proton transfer (ESIPT) in the photocyclization. The ground-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 by temperature dependent ¹H and ¹³C NMR spectroscopy. An X-ray crystal analysis of 1a reveals that the molecule exists in the crystalline state as the enol imine form with a butterfly conformation of the anthraquinone ring. The involvement of ESIPT in photoexcited 1a has been revealed by means of sub-microsecond time-resolved IR spectroscopy. Upon irradiation of 1a an increase of the ground-state keto form (B) is detected and its lifetime is deduced to be 1.3 ms. Another transient species (A) with a lifetime of 80 µs is also observed, which is assumed to be the intramolecular cycloadduct as a precursor of the photoproduct.