Solvent effect on the conical intersection of 4-cyclopentadienylidene-1,4-dihydropyridine (CPDHP) S. Alfalah a , O. Deeb a , S. Zilberg b , Y. Haas b * a Faculty of Pharmacy, Al-Quds University, P.O. Box 20002, Jerusalem, Palestine b Institute of Chemistry, The Farkas Center for Light Induced Processes, The Hebrew University of Jerusalem, Jerusalem, Israel article info Article history: Received 19 March 2008 In final form 14 May 2008 Available online 20 May 2008 abstract A conical intersection that exists between the S 0 and S 1 surfaces of the title molecule in the gas phase and in non-polar solvents is shown to be eliminated in polar solvents. Potential surfaces are calculated using the high level CASSCF method along the two coordinates predicted to lift the degeneracy according to the Longuet-Higgins loop method. The larger gap between the two surfaces in polar solvents results from the stabilization of a zwitterion transition state. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Fluorescence experiments on (1-butyl-4-(1H-inden-1-ylidene)- 1,4-dihydropyridine (BIDP, Scheme 1) have revealed that the fluo- rescence quantum yield of the S 1 state is of the order of 20–60% in various cryogenic glasses but becomes very small in fluid solutions [1]. Moreover, the fluid solution yield is about five times larger in a polar solvent (acetonitrile, MeCN) than in a non-polar one (meth- ylcyclohexane, MCH). The rapid nonradiative transitions in fluid solutions were assigned to internal conversion in both solvent clas- ses, as intersystem crossing is much slower and no net reaction is observed [2]. These results are in agreement with predictions made for the closely related (in terms of electronic structure) but simpler molecule 4-cyclopentadienylidene-1,4-dihydropyridine (CPDHP) for which a S 1 /S 0 conical intersection was recently proposed [3] based on a method developed to locate conical intersections be- tween the ground state potential surface and the first electroni- cally excited states of polyatomic molecules [4]. The method is an extension of the Longuet-Higgins sign-change theorem [5] and uses reaction coordinates of elementary reactions as the starting point of the analysis. The structure of the molecule is shown in Scheme 1. The gas phase isomerization around the double bond connect- ing the two rings can be carried out along two reaction coordi- nates: in one the transition state is of biradical nature, as in ethylene; the transition state is of C 2V (A 2 ) symmetry and will be referred to as the BRTS. The second transition state is ionic: in it an electron is transferred from the pyridine ring to the cyclopenta- diene one; this transition state also belongs to the C 2V symmetry group, transforms as A 1 and will be referred to as the zwitterion transition state (ZWTS). A phase inverting Longuet-Higgins loop can be constructed for this system by connecting the reactant and product in a closed loop via the two transition states. Therefore, a conical intersection is located within the loop. This situation was discussed in previous publications and is not re- peated here. The loop is shown schematically in Figure S1 in the Supplementary Material, where the two transition states are also schematically sketched. In this Letter, we report a quantum chemical study on CPDHP, in which the potential surface along the coordinates lifting the degen- eracy was calculated. The results show that a conical intersection is found in the gas phase and in MCH, but disappears in the more po- lar solvents. The gap in MeCN is calculated to be about 15 kcal/mol; the experimentally observed fast decay in the latter indicates that although the S 1 and S 0 are not touching, a fairly efficient funnel persists with this relatively small energy gap. Solvent effects on conical intersections were discussed by sev- eral authors [6–12]; however, to our knowledge this is the first sys- tematic calculation of the effect of solvent polarity on the energy of conical intersections that analyzes both polar and non-polar solvents. 2. Method and computational details In order to locate the conical intersection, critical points on the ground state potential must be located; these include the reactant, product and two transition states [3,13]. Calculations were carried out using the CASSCF method [14]; the active space was con- structed from orbitals suitable for both transition states, in which the two rings are perpendicular to each other (C 2V geometry), Fig. 1. This figure shows that the C 2 –C 8 bond lies on the Z-axis, the pyridine ring lies in the YZ-plane and the cyclopentadienyl ring lies in the XZ-plane. Since the CI is also expected to be of v2 C sym- metry, this choice facilitated the computation. 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.05.024 * Corresponding author. Fax: +972 25618033. E-mail address: yehuda@chem.ch.huji.ac.il (Y. Haas). Chemical Physics Letters 459 (2008) 100–104 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett