Characterization of the indacene S0/S1 conical intersection: an MMVB and CASSCF study

A conical intersection between the S0 and S1 states of s-indacene has been located with the molecular mechanics with valence bond (MMVB) and complete active space SCF (CASSCF) methods. In both cases the intersection minimum is close in energy and geometry to the overall minimum on the S1 surface, and can be reached directly along the coordinate which leads from the Franck—Condon geometry to the minimum on S1. The existence of such an accessible S1/S0 surface crossing leading to efficient non-radiative decay can explain the short S1 excited state lifetime and lack of fluorescence for s-indacene. Based on a comparison with X-ray structures for a derivative of s-indacene, we argue that the CASSCF ordering of S0 and S1 states is the correct one, whereas MMVB inverts the two states. However, because the energy differences involved are a few kcal mol-1, a definitive prediction will probably require a multi-reference correlated level of theory with gradient optimization. © 1999 Taylor & Francis Group, LLC.