Ab initio investigations on the photophysics of indole

Reaction paths and potential-energy profiles for detachment of the hydrogen atom of the NH group in excited singlet states of indole have been investigated using the CIS, CASSCF and CASPT2 ab initio methods. The potential-energy profile of the lowest pi sigma* excited singlet state is found to be essentially repulsive. It crosses the potential-energy functions of the L-1(b) and L-1(a) excited states of pi pi* character as well as those of the ground state. The resulting multiple conical intersections can provide the mechanism for efficient internal conversion to the ground state. The polarities of the excited states are remarkably different, indicating a complex interplay of internal conversion and solvation dynamics of photoexcited indole in polar solvents. (C) 1999 Elsevier Science B.V. All rights reserved.