An ab initio study of the low-lying (1)A ' electronic states of indene

The electronic absorption spectrum of indene in the region of the lowest-ly ing valence pi-->pi* singlet excited states was studied using multi-configu rational second-order perturbation theory through the CASPT2 formalism. Fou r (1)A' states were found to be the most important for describing the excit ation spectrum up to 6.8 eV. Their transition energies (oscillator strength s 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 descr ibed 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 Else vier Science B.V. All rights reserved.