Optical properties of 3-substituted indoles

The optical properties of various donor or acceptorp-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 forp-nitro phenyl substituted ethenyl indoles (mu(e): 18.2-27.1 debye; Delta mu: 9.4-17.8 debye) is observed as compared to other ethenyls (mu(e): 6.6-9.5 debye; Delta mu: 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 thep-phenyl substitution. The optical band gap of ethenyl3without substitution, is decreased uponp-phenyl substitution either with an electron withdrawing (Cl, NO2) or an electron donating (OCH3, OH, NH2) substituent. The compound with a strong electron accepting,p-nitrophenyl ethenyl indole1shows 12 times better NLO response as compared to the reference ethenyl indole3(beta:1: 115 x 10(-30)esu(-1)cm(5),3: 9 x 10(-30)esu(-1)cm(5)). Ethenyls2-6bearing a weak or moderately electron withdrawing or electron accepting substituent, exhibit lower NLO response. The beta of ethenyl is increased with increasing the order of electron withdrawing nature of phenyl ring. Overall, a correlation of beta with the optical band gap, ground state dipole moment, % of charge transfer in the ground and excited state is found.