Optical properties of 3-substituted indoles.

The optical properties of various donor or acceptor -phenyl substituted ethenyl indoles were studied in solvents of varying polarity using absorption, fluorescence and TDDFT methods. Ethenyl indole exhibits non-linear optical properties (NLO) in a substituent dependent manner. Compound with a strong electron-attracting substituent, shows large NLO properties with charge transfer behavior, whereas ethenyls with moderate electron withdrawing or electron donating substituent exhibit lower NLO properties with non polar excited state. A highly dipolar excited state for -nitro phenyl substituted ethenyl indoles ( : 18.2-27.1 debye; Δ: 9.4-17.8 debye) is observed as compared to other ethenyls ( : 6.6-9.5 debye; Δ: 4.2-6.2 debye). From TDDFT study, it is shown that the HOMO-LUMO energy of ethenyl is increased with increasing the electron donating ability of the -phenyl substitution. The optical band gap of ethenyl 3 without substitution, is decreased upon -phenyl substitution either with an electron withdrawing (Cl, NO) or an electron donating (OCH, OH, NH) substituent. The compound with a strong electron accepting, -nitrophenyl ethenyl indole 1 shows 12 times better NLO response as compared to the reference ethenyl indole 3 (: 1: 115 × 10 esu cm, 3: 9 × 10 esu cm). Ethenyls 2-6 bearing a weak or moderately electron withdrawing or electron accepting substituent, exhibit lower NLO response. The of ethenyl is increased with increasing the order of electron withdrawing nature of phenyl ring. Overall, a correlation of with the optical band gap, ground state dipole moment, % of charge transfer in the ground and excited state is found.