An ab initio study of the low-lying 1A′ electronic states of indene

The electronic absorption spectrum of indene in the region of the lowest-lying valence pi-->pi* singlet excited states was studied using multi-configurational second-order perturbation theory through the CASPT2 formalism. Four (1)A' states were found to be the most important for describing the excitation spectrum up to 6.8 eV. Their transition energies (oscillator strengths in parentheses) were computed to be: 4.46 eV (0.0004), 5.02 eV (0.175), 6.14 eV (0.221), and 6.36 eV (0.434). These states can be labelled and described as the four Platt states of indene: L-1(a), L-1(b), B-1(b), and B-1(a), respectively. This description of the spectrum explains the experimental observations better than previous theoretical investigations. (C) 1999 Elsevier Science B.V. All rights reserved.