Deactivation via ring opening: A quantum chemical study of the excited states of furan and comparison to thiophene Nemanja Gavrilov a,b , Susanne Salzmann a , Christel M. Marian a, * a Institute of Theoretical and Computational Chemistry, Heinrich Heine University Du ¨ sseldorf, Universita ¨ tsstraße 1, Geb. 26.32, 40225 Du ¨ sseldorf, Germany b Faculty of Chemistry, University of Belgrade, Studenski trg 12, P.O. Box 137, 11000 Belgrade, Serbia Received 29 November 2007; accepted 31 January 2008 This work is dedicated to Hans Lischka on the occasion of his 65th birthday. His achievements in the development of advanced multiconfiguration interaction methods and their application to electronically excited molecules have greatly contributed to the understanding of photochemistry and photophysics. Abstract Minimum nuclear arrangements of the ground and low-lying excited electronic states of furan were obtained by means of (time depen- dent) 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 (S 1 ) potential energy hypersurface with electronic structures S1, S2, S3 corresponding to the 1 1 A 2 (p ? 3s-Ryd), 1 1 B 2 (p ? p * ), and 2 1 A 1 (p ? p * ) 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) 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 coor- dinate. Hereby the initially excited 1 B 2 (p ? p * ) state undergoes a conical intersection with a 1 B 1 (p ? r * ) state. The system can return to the electronic ground state through a second conical intersection of the 1 (p ? r * ) state before the minimum of that B 1 state is reached. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Electronic structure calculations; Heterocycles; Photochemistry; Quantum chemistry; Ring opening 1. Introduction The vertical absorption spectra of the heterocyclic com- pounds pyrrole, furan, and thiophene have been well stud- ied, both experimentally and theoretically [1–16]. Much less is known about the adiabatic excitation energies [17–21]. In addition to Rydberg excitations and triplet states, these systems possess low-lying valence singlet states, some of which are short-lived with lifetimes of the order of femto- seconds [22]. Some years ago, a relaxation pathway of photo-excited pyrrole was detected in which an N–H bond-breaking dis- sociative 1 ðp ! r  NH Þ state undergoes conical intersections with the primarily excited 1 (p ? p * ) state as well as with the electronic ground state, thus leading to ultrafast decay of the excited state population [23]. Out-of-plane deforma- tions of the pyrrole ring which couple the 1 (p ? p * ) and 1 ðp ! r  NH Þ potential energy hypersurfaces, can also effec- tuate a direct conical intersection between the primarily excited 1 (p ? p * ) state and the electronic ground state.[24] Interestingly, Barbatti et al. [24] observed a significant elongation of one of the CN bonds at the latter conical intersection. While the decay mechanism involving the 1 ðp ! r  NH Þ state is not available in furan and thiophene, 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.01.060 * Corresponding author. Fax: +49 211 81 13466. E-mail address: christel.marian@uni-duesseldorf.de (C.M. Marian). www.elsevier.com/locate/chemphys Available online at www.sciencedirect.com Chemical Physics xxx (2008) xxx–xxx ARTICLE IN PRESS Please cite this article in press as: N. Gavrilov et al., Chem. Phys. (2008), doi:10.1016/j.chemphys.2008.01.060