Excited-State Dipole Moments of Indoles Using Solvatochromic Shift Methods: An Experimental and Theoretical Study

The electronic absorption and fluorescence spectra of some indoles were recorded at room temperature in solvents of different polarities. In order to elucidate the solvent band shifts during relaxation of the excited states, the dipole moments of the excited singlet states were estimated from solvent-dependent Stokes shift data using a solvatochromic method based on a microscopic solvent polarity parameter (ETN). All indoles show a substantial increase in the dipole moment upon excitation to the emitting state. These results are generally consistent with the Austin model 1 (AM1) calculations, and are found to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts with the microscopic solvent polarity parameter (ETN) is superior to that obtained using bulk solvent polarity functions in almost all the systems studied here.