Dihydroazulene/Vinylheptafulvene Photochromism: A Model for One-Way Photochemistry via a Conical Intersection Martial Boggio-Pasqua, Michael J. Bearpark, Patricia A. Hunt, and Michael A. Robb* Contribution from the Department of Chemistry, King’s College London, Strand, London WC2R 2LS, U.K. Received May 9, 2001 Abstract: Dihydroazulene (DHA)/vinylheptafulvene (VHF) photochromism has been investigated by studying the isomerization of 1,2,3,8a,9-pentahydrocyclopent[a]azulene-9,9-dicarbonitrile through complete active space-self consistent field calculations on the ground (S0) and first excited (S1) states of smaller model compounds. In each case, the S1 reaction coordinate is characterized by a transition structure for adiabatic ring opening, connecting a DHA-like intermediate to a much more stable VHF-like structure. This VHF-like structure is not a real S1 minimum but a crossing (i.e., a conical intersection) between the excited- and ground-state potential energy surfaces. The existence of such a crossing is consistent with the lifetime of ∼600 fs recently measured for the DHA-like intermediate on S1 (Ern, J.; Petermann, M.; Mrozek, T.; Daub, J.; Kuldova, K.; Kryschi, C. Chem. Phys. 2000, 259, 331-337). The shape of the crossing is also crucial; it not only explains the fact that the quantum yield approaches 1.0 for the forward DHA f VHF reaction, but also the lack of any fluorescence or photochemical back-reaction from VHF. These findings are supported by ab initio direct dynamics calculations. This work suggests that calculating and understanding the topology of excited-state potential energy surfaces will be useful in designing photochromic molecules. Introduction Organic compounds with photochromic properties are of considerable interest at present, because of their potential applications for data storage and processing 1a,b and molecular switching. 1c Many have now been characterized, 2 including the dihydroazulene/vinylheptafulvene (DHA/VHF) couple, 3-5 for example, 1, Scheme 1. 5 This type of system has been intensively studied because it is an example of “one-way” photochromism; the photochemical rearrangement of DHA to VHF cannot be reversed by VHF absorption at a different frequency, but only by heat. Using femtosecond-resolved transient absorption spectroscopy, VHF formation has been detected 5 within 1.2 ps of the initial excitation of DHA 1, and the speed of this reaction has been attributed to a conical intersection 6 between the ground and first excited states as shown in Figure 1. 5 In this paper, we confirm the existence of an intersection, but show that the shape of the surrounding potential energy surfaces is central to understanding the high quantum yield for the forward DHA f VHF reaction and the lack of any fluorescence or photochemical back-reaction from VHF. Despite the fact that any plot of energy against a single “reaction coordinate” will be misleading in some respect, we have also produced a revised Figure 1 (Figure 2) which is consistent with both the experiments 5 and the calculations we describe below. Several features of the excited-state potential energy surfaces of DHA and VHF can be inferred from experiment. Steady- state and time-resolved measurements on a series of DHA derivatives were carried out in 1993. 4 (We often use the generic labels “DHA” and “VHF” in what follows, as we are interested in features of the potential energy surfaces common to all such compounds.) These DHA derivatives were fluorescent: weakly (1) (a) Irie, M. Chem. ReV. 2000, 100, 1685-1716. (b) Yokoyama, Y. Chem. ReV. 2000, 100, 1717-1739. (c) Mrozek, T.; Daub, J.; Ajayaghosh, A. Optoelectronic Molecular Switches Based on Dihydroazulene-Vinylhep- tafulVene (DHA-VHF); Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001; pp 63-106. (2) Du ¨ rr, H., Bouas-Laurent, H., Eds. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 1990. (3) Daub, J.; Knochel, T.; Mannschreck, A. Angew. Chem., Int. Ed. Engl. 1984, 23, 960-961. (4) Gorner, H.; Fischer, C.; Gierisch, S.; Daub, J. J. Phys. Chem. 1993, 97, 4110-4117. (5) Ern, J.; Petermann, M.; Mrozek, T.; Daub, J.; Kuldova, K.; Kryschi, C. Chem. Phys. 2000, 259, 331-337. (6) (a) Teller, E. J. Phys. Chem. 1937, 41, 109. (b) Teller, E. Isr. J. Chem. 1969, 7, 227-235. (c) Herzberg, G. The Electronic Spectra of Polyatomic Molecules; Van Nostrand: Princeton, 1966; p 442. (d) Salem, L. Electrons in Chemical Reactions: First Principles; Wiley: New York, 1982; pp 148- 153. (e) Yarkony, D. R. Acc. Chem. Res. 1998, 31, 511-518. (f) Robb, M. A.; Garavelli, M.; Olivucci, M.; Bernardi, F. ReV. Comput. Chem. 2000, 15, 87-146. Scheme 1 Published on Web 01/24/2002 1456 VOL. 124, NO. 7, 2002 9 J. AM. CHEM. SOC. 10.1021/ja0161655 CCC: $22.00 © 2002 American Chemical Society