Vibronic analysis of the 1Lb ↔ S0 transitions of indole and 3-methylindole: The influence of anharmonic, Duschinsky, and Herzberg–Teller contributions

Abstract

The spectral similarities and differences between indole and 3-methylindole were investigated with density functional theory (DFT) and its time-dependent extension (TD-DFT). The geometries, energies, dipole moments, oscillator strengths, and vibrational frequencies of the ground and two lowest excited states (1Lb and 1La) were calculated and compared with experimental data and other theoretical values. The vibrationally resolved 1Lb↔S0 electronic spectra were simulated within the Franck–Condon approximation and included the Herzberg–Teller (HT) and Duschinsky effects. The HT and Duschinsky effects played important roles in weak dipole-allowed electronic transitions, leading to full and rich spectral profiles. Furthermore, the anharmonic correction was also taken into account in the simulations. The simulated spectra of the 1Lb↔S0 transitions of indole and 3-methylindole are in good agreement with the experimental findings, which indicated that the present theoretical method is a useful tool to simulate the optical spectra of indole-like organic molecules.