Deactivation via ring opening: A quantum chemical study of the excited states of furan and comparison to thiophene

Minimum nuclear arrangements of the ground and low-lying excited electronic states of furan were obtained by means of (time dependent) Kohn-Sham density functional theory. A combined density functional/multi-reference configuration interaction method (DFT/ MRCI) was employed to compute the spectral properties at these points. Multiple minima were found on the first excited singlet (SI) potential energy hypersurface with electronic structures S1, S2, S3 corresponding to the 1(1) A(2) (pi -> 3s-Ryd), 1(1)B(2) (pi -> pi*), and 2(1) A(1) (pi - pi*) states in the vertical absorption spectrum, respectively. In analogy to recently published studies in thiophene [S. Salzmann, M. Kleinschmidt, J. Tatchen, R. Weinkauf, C.M. Marian, Phys. Chem. Chem. Phys. 10 (2008) 3801 a deactivation mechanism for electronically excited furan was detected that involves the opening of tile pentacyclic ring. We found a nearly barrierless relaxation pathway from the Franck-Condon region along a C-O bond-breaking coordinate. Hereby the initially excited (1)B(2) (pi -> pi*) state undergoes a conical intersection with a (1)B(1)(pi -> sigma*) state. The system can return to tile electronic ground state through a second conical intersection of the (1)(pi -> sigma*) state before the minimum of that B, state is reached. (c) 2008 Elsevier B.V. All rights reserved.