Reaction field calculation of the spectral shifts of indole

Abstract : The excitation energy of indole has been investigated at geometries relevant to both the absorption and emission spectra using first-order configuration interaction (FOCI) methods. The calculation focused on the shift in the relative energy between the two excited states, L sub B and L sub A, and the excitation energies with geometry. A reaction field (RF) calculation also examined the shift in excitation energies when the molecule is dissolved in water. The excited states are found to be separated in vacuo by about 4,500 /cm at both the ground state and an approximation to the equilibrium L sub B geometry, M, and by 2,500/cm at the optimized geometry of both the first triplet excited state, T, and the first excited singlet state represented by a singles-only configuration interaction (S1). In water, the excited states are essentially degenerate and axed at the ground-state geometry. However, at the excited-states geometries, M and T, the L sub A state is shifted lower in energy than the L sub B state due to the large L sub A excited-state RF. At the S1 geometry, the electronic behavior of the two excited states is different from that at the other geometries. An approximation to the L sub A state is now the lower state both in vacuo and in the presence of the RF. There is a large calculated solvent fluorescent red shift in water at any geometry as well substantial Franck-Condon (FC) shift due to the steeper energy surface in the ground state compared to the excited states. The calculated energy shifts, dipole moments, and relative transition dipoles are in qualitative accord with experiment. However, the calculated fluorescent reaction dipoles are substantially larger than estimated in earlier studies. (jg)