Origin of the N-methyl and N-phenyl substituent effects on the fluorescence vibronic structures of trans-4-aminostilbene and its derivatives in hexane

The origin of the N-substituent effect on the room temperature fluorescence vibronic structures of trans-4-aminostilbene (1), its disubstituted derivatives (2 and 3), and the fused-ring analogs (4) in hexane has been investigated. The fluorescence spectra of aminostilbenes 1-4 in hexane are structured and the intensity ratio of the 0,1 vs. the 0,0 band decreases upon N-methyl or N-phenyl substitution, indicating that the N-substituents reduce the displacement in the equilibrium configuration (ΔQ_e) between the ground and the excited states. Our data are consistent with a planar fluorescing state for 1-4 in hexane. The N-substituents affect the planarity of the ground state structures as well as the conjugation length allowed for exciton resonance, both of which are in accord with the observed progression in fluorescence vibronic structures. According to our analysis, the conformation effect that is associated with the geometry of the amino group appears to play a major role. The corresponding studies on the solvent effect suggest that the fluorescing state of 1-4 in more polar solvents is also a planar charge-transfer state.