Explaining Level Inversion of the La and Lb States of Indole and Indole Derivatives in Polar Solvents

Quantum chemical methods are used to study the solvent effects on the spectra of indole and a series of methyl-substituted indoles. We focus on the low-lying L-a and L-b states and study their interplay. We find that the solvent mainly affects emission from the L-a state, by stabilizing its energy in its excited-state geometry. The stabilization of the L-a state increases with increasing solvent polarity, which accounts for the large fluorescence shift observed in indoles and leads to an inversion in the nature of the lowest emitting state, from L-b in vacuum to L-a in water. To the best of our knowledge, this is the first theoretical evidence for level inversion done for a series of indoles. The underlying mechanism of level inversion is analyzed in detail. The usual interpretation of level inversion in terms of their static dipole moment is criticized and an improved predictive measurement is suggested.