Absorption and fluorescence spectral properties of donor-acceptor ethenes bearing indole and p-nitrophenyl substituents

Synthesis, absorption and fluorescence properties (λf, λex, Stokes shifts, Φf, τf, correlation plots of Δf and ET(30) with Φf and Stokes shifts, μf, low temperature fluorescence properties and excited state geometry) of 3-(4-nitrophenyl ethenyl-E)-NH-indole (1) and 3-(4-nitrophenyl ethenyl-E)-N-acetyl indole (2) are described. Occurrence of singlet excited state torsional motions and presence of more than one excited species, which are significantly influenced by the solvent polarity, have been observed. It is suggested that while 1 fluoresces predominantly from the conformationally relaxed intramolecular charge transfer (CRICT) state, 2 fluoresces from three different types of excited species, including the locally excited state, intramolecular charge transfer excited state and the CRICT state.