PHOTOPHYSICS OF INDOLE-DERIVATIVES - EXPERIMENTAL RESOLUTION OF LA AND LB TRANSITIONS AND COMPARISON WITH THEORY

Fluorescence excitation and anisotropy spectra are presented for a set of methyl and methoxyindoles at -50°C in propylene glycol glass. These spectra are interpreted in terms of two overlapping π → π* electronic transitions (La and Lb). Semiempirical molecular orbital calculations are presented to correlate with the observed spectral changes caused by methyl and methoxy substitution. For the parent compound, analysis of excitation and anisotropy spectra, as done by Valeur and Weber, is consistent with the La and Lb zero-point moments being nearly perpendicular and with La being the lowest in energy. Methyl substitution at positions 2 and 3 causes little change in θba (the angle between the La and Lb moments) and further lowers the energy level of La. Methyl substitution at the imino nitrogen or the benzoid ring causes variable photophysical changes, with the La and Lb energy levels being nearly degenerate in some cases. Methoxy substitution on the benzoid ring results in the Lb state being the lowest in energy. The angle θba remains near 90° for substitution at position 5, but is <50° for 4-, 6-, and 7-methoxyindoles. Molecular orbital calculations predict the relative change in the La-Lb energy levels upon methoxy substitution and upon methyl substitution at the 2- and 3-positions. Also the theory predicts the La and Lb oscillators to be nearly parallel (small θba) for 4-, 6-, and 7-methoxyindoles and to be nearly perpendicular (large θba) for 5-methoxyindole. Thus the spectral changes in indole induced by methyl and methoxy substitutions can be adequately explained in terms of the two independent, overlapping transitions. © 1990 American Chemical Society.