Solvent Effects on the Fluorescent States of Indole Derivatives‐dipole Moments

Electronic spectra of indoles are very sensitive to solvents. These solvent effects are particularly pronounced in the case of the fluorescence spectra of indoles. In order to elucidate the solvent band shifts during the relaxation of the excited states, particularly in polar solvents, the dipole moments of the excited singlet states have been estimated from the data of solvent‐dependent Stokes shifts. In addition to indole, indole‐5‐carboxylic acid, indole‐3‐carboxylic acid, indole‐2‐carboxylic acid, 5‐cyano‐ and 5‐bromo‐indoles have been investigated. All indoles showed a substantial increase in dipole moment upon excitation to the emitting state. These results are generally consistent with the Pariser‐Parr‐Pople SCF MO calculations.