Ground and excited state dipole moments of 1-methylindole from thermochromic shifts in absorption and emission spectra

The permanent dipole moments of 1-methylindole in its ground and lowest excited singlet pi pi*-state have been determined using the method of thermochromic shifts of the absorption and fluorescence spectra and compared to ab initio calculated dipole moments. While ab initio theory predicts an excited state dipole moment, which nearly equals that of the ground state, the thermochromic shifts give an excited state dipole moment, which is about 2 Debye larger than that of the ground state. Perturbation of the excited state through the large electric field strength of the solvent cavity leads to strong mixing with the next excited singlet state, which has a considerably larger dipole moment. This perturbation is analyzed using the method described by Lombardi [John R. Lombardi J. Phys. Chem. A 1998, 102, 2817-2823]. The perturbing state could be identified as L-1(a)-state, which is the second adiabatic state in the gas phase, but the lowest state in a polar surrounding.