Photophysics of bis(p-N,N-dimethylamino)benzylidene acetone: Does photoinduced electron transfer occur from a twisted excited state?

Abstract

Photoexcitation of the title compound (DMBA) in polar solvents leads to sequential formation of at least three excited species as detected by laser transient absorption spectroscopy: the lowest Franck–Condon singlet, a twisted singlet (P-state), and the lowest triplet states. Only the initially formed, localized singlet is emissive under our conditions. Interystem crossing to triplet occurs primarily from the twisted P-state in acetonitrile and in methanol. Photo-CIDNP and quantum yield measurements indicate that electron transfer occurs exclusively from the triplet in acetonitrile or methanol to sensitize decomposition of diphenyliodonium cation and to bleach DMBA. Analysis of the photochemistry is complicated by formation of a charge transfer complex of enhanced photolability. Using tris(trichloromethyl)-sym-triazine as the electron acceptor in hydrocarbon solvent, bleaching of DMBA again occurs, somewhat more efficiently than with the 'onium salt in polar solvent. We infer that under these conditions electron transfer occurs from the P-state. Keywords: photochemistry, fluorescence, electron transfer, TICT, chalcone.