Intramolecular charge transfer processes in donor-acceptor substituted vinyltetrahydropyrenes

Abstract

Two novel donor-acceptor-substituted vinyltetrahydropyrene derivatives, 2-N,N-dimethylamino-7-(1-carbethoxyvinyl)-4,5,9,10-tetrahydropyrene, 1, and 2-N,N-dimethylamino-7-(1,1-dicyanovinyl)-4,5,9,10-tetrahydropyrene, 2, were synthesized and their photophysical properties investigated in solvents of different polarities. Our studies revealed the existence of intramolecular charge transfer excited states in these molecules. For both compounds the fluorescence maxima exhibited solvent polarity-dependent red shifts. These were quantitatively analysed by the Lippert-Mataga and Liptay equations to obtain the excited state dipole moments. Our results indicated that in the case of 1, emission takes place from a planar ¹CT state in all non-protic solvents. In the case of 2, the nature of the excited state depends on the solvent. A fast relaxation to a triplet state is proposed in cyclohexane. The emitting state in medium polar solvents is a planar ¹CT state. In highly polar solvents a twisted ¹CT state is invoked to explain the low fluorescence quantum yield. For both compounds CT nature of the emitting states were further confirmed by studies in acidic medium. The ground and excited state pK_a values for 1 and 2 were determined using absorption and emission spectral changes observed in the presence of protic acids.