First reported evidence that solvent polarity induces an 1Lb↔1La inversion in the indole chromophore

The solvatochromism of indole in 26 pure solvents, 21 binary solvent mixtures, and the gas phase was determined. The description of the solvatochromism of indole in the light of the SPP, SB, and SA scales allowed us to derive the first reported evidence that the inversion in its first two excited electronic states is caused by an increase in the general solvent effect. Based on the results, indole only emits from its 1La state in solvents with SPP>0.8 and from its 1Lb state in solvents with SPP<0.8.