Journal of Photochemistry and Photobiology A: Chemistry 162 (2004) 213–223 Theoretical and spectroscopic studies of the photochemistry of 3-(4-dimethylaminophenyl)-7-methoxy-cyclohepta-1,3,5-triene V.A. Kharlanov a , W. Abraham a,∗ , U. Pischel a,b a Institute of Chemistry, Humboldt University of Berlin, Brook-Taylor-Street 2, Berlin D-12489, Germany b REQUIMTE/Department of Chemistry, University of Porto, Rua Campo Alegre, 4169-007 Porto, Portugal Received 4 January 2003; received in revised form 1 June 2003; accepted 29 July 2003 Abstract In this paper the photophysics and photochemistry of the title compound has been examined by laser, flash lamp and steady state methods. In protic solvents, photoheterolysis is able to compete with a rapid 1,7-hydrogen shift reaction to give the substituted tropylium methoxide, with the lifetime of the tropylium ion strongly depending on the nature of the alcohol. Hydrogen bonds between the solvent and the methoxy group of the cycloheptatriene derivative favor bond breakage and the solvation of the methoxide ion by protic solvents increases the lifetime of the free ions. Ab initio calculations suggest that the photoheterolysis in the gas phase takes places via a consecutive photohomolysis under formation of a radical pair, which then is undergoing an intra-pair electron transfer to form the tropylium and the methoxide ions. The 1,7-hydrogen shift reaction leads exclusively to the production of 5-(4-dimethylaminophenyl)-1-methoxy-cyclohepta-1,3,5-triene. The fluorescence of the title compound exhibits a very large Stokes shift indicating considerable geometrical changes in the relaxed excited state and according to semi-empirical and ab initio calculations, the seven-membered ring is planar in the fluorescing state. © 2004 Elsevier B.V. All rights reserved. Keywords: Cycloheptatrienes; Sigmatropic hydrogen shift; Photoheterolysis; Tropylium ion; Ab initio calculation; Fluorescence 1. Introduction Cyclohepta-1,3,5-triene (CHT) belongs to a group of com- pounds suitable for studying the course of photoinitiated pericyclic reactions such as the 1,7-hydrogen shift reaction and the electrocyclization [1]. Because the hydrogen shift reaction of the parent com- pound is degenerate, the photoproducts of substituted cy- cloheptatrienes are more easily studied, in particular aryl substituents attached to the seven-membered ring offer some advantages such as selective excitation of reactants and fluorescence [1]. According to ab initio calculations, [2] the 1,7-hydrogen shift of CHT proceeds by passing two conical intersections (S 2 –S 1 and S 1 –S 0 ) in the time scale of about 50 fs. Sub- stituents at the 1-position of CHT control the direction of the hydrogen shift, for example, electron acceptors such as the cyano group direct into the two-substituted CHT; elec- tron donors such as the methoxy group control the forma- tion of the seven-substituted derivative. These results are due ∗ Corresponding author. Fax: +49-30-2093-6940. E-mail address: abraham@chemie.hu-berlin.de (W. Abraham). to the presence of barriers on the excited state surface; the energy required to overcome these barriers depends on the type of substituent at position-1 [2]. Another important re- sult of the quantum mechanical calculations, confirmed by experiments, is the strong change in the conformation of the seven-membered ring. Whilst the boat-like conformation exists in the ground state, during the photoreaction, the for- mation of a more or less flat ring becomes a prerequisite of the hydrogen shift [2]. The strong influence of substituents on the direction and the selectivity of both the hydrogen shift reaction and the electrocyclization has also been observed with phenyl cy- cloheptatrienes bearing different substituents in the phenyl ring, [3,4] in particular, the 4-dimethylaminophenyl group allows the photoreactions to be reversible and selective. 3- (4-Dimethylaminophenyl)-7-methoxy-cyclohepta-1,3,5-tri- ene (compound 1) bears two electron donating groups, the methoxy group is attached directly at the CHT-ring and the dimethylamino group can work only through the -system of the phenyl ring. The photochemistry of com- pound 1 differs from all aryl substituted cycloheptatrienes investigated due to the occurrence of an additional pho- toreaction; in methanol solution a photoheterolysis leads 1010-6030/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/S1010-6030(03)00387-3