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

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) 380] a deactivation mechanism for electronically excited furan was detected that involves the opening of the 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 1B2 (π → π∗) state undergoes a conical intersection with a 1B1 (π → σ∗) state. The system can return to the electronic ground state through a second conical intersection of the 1(π → σ∗) state before the minimum of that B1 state is reached.