Fluorescence and photoisomerization studies of p-nitrophenyl-substituted ethenylindoles

Abstract

The synthesis, electronic absorption, fluorescence (λ_f, λ_ex, φ_f, τ_f) and photoisomerization (φ_tc, photostationary state composition) properties of 3-(4-nitrophenylethenyl-E)-NH-indole (1), 3-(4-nitrophenylethenyl-E)-N-ethylindole (2) and 3-(4-nitrophenyl ethenyl-E)-N-benzenesulfonylindole (3) in organic solvents of varying polarity are reported. The absorption maximum of these compounds undergoes a moderate red shift with increasing solvent polarity. However, the fluorescence maximum becomes highly red shifted with increasing solvent polarity. Whereas 1 and 2 show broad fluorescence bands, 3 exhibits dual fluorescence. Further, 1 and 2 fluoresce much more efficiently than 3. Correlation of the Stokes shift with solvent polarity parameters such as Δ_f and E_T(30) and excited-state dipole moment indicate a highly polar excited state for 1-3. Time-resolved fluorescence studies show that the fluorescence decays are single- and multi-exponential type, depending on the solvent polarity. Further, 1 and 2 do not show photoisomerization on irradiation. However, 3 is photoactive and shows efficient photoisomerization in non-polar heptane. The sensitivity (ρ) of the photoreaction is determined in various solvent in terms of the Hammett plot, which showed that the excited states involved are electron deficient in nature and consequently stabilized more by an electron sufficient polar solvent and electron donating substituent. These results led us to suggest the existence of three types of excited states, namely the locally excited state, the intramolecular charge-transfer excited state and the conformationally relaxed intramolecular charge-transfer excited state in the photoprocesses of these compounds.