Photoreactivity of a Push-Pull Merocyanine in Static Electric Fields: A Three-State Model of Isomerization Reactions Involving Conical Intersections X. F. Xu, A. Kahan, S. Zilberg,* and Y. Haas* Institute of Chemistry and the Farkas Center for Light Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem, Israel ReceiVed: May 3, 2009; ReVised Manuscript ReceiVed: July 10, 2009 The photochemistry of a prototype push-pull merocyanine is discussed using a simple three-state model. As a derivative of butadiene, the model focuses on two isomerization reactions around the two double bonds of the butadiene backbone. As a molecule substituted by an electron donor and electron acceptor at opposite ends, its structure as well as its photochemistry are expected to be strongly affected by the environment. In polar solvents, a zwitterion transition state for each of the isomerization reactions is stabilized, and its energy is on the same order as that of the biradical one; this leads to the symmetry allowed crossing (S 0 /S 1 conical intersection). It is shown that applying an external electric field or varying the solvent polarity changes the relative energies of the different transition states as well as that of the conical intersection, and thus different photochemical products can be obtained. In particular, the very existence of conical intersections is found to depend on these external parameters. This work provides a theoretical foundation for ideas expressed by Squillacote et al. (J. Am. Chem. Soc. 2004, 126, 1940) concerning the electrostatic control of photochemical reactions. I. Introduction Merocyanines are an interesting class of compounds because of their remarkable solvatochromic behavior, 1-7 photochemical isomerization, 8-14 and solvent-dependent hyperpolarizability. 15-23 These molecules, which may be considered to be butadiene derivatives substituted by donor and acceptor groups at the opposite ends, are referred to as push-pull molecules. Their unique electronic structure is generally represented at the π level in terms of a resonance hybrid of two VB forms, quinonoid and zwitterion; the attractive properties of the merocyanines are traditionally understood in terms of these VB forms. Which of these structures dominates the ground state depends on the nature of the donor and acceptor positioned at the two opposite ends of the molecule and on the surrounding environments and leads to large changes in molecular properties. Applied electric fields as well as solvents have been found to affect the relative weights of the two VB structures in hybrid resonance of merocyanines in the ground state through polarization; 15 this was demonstrated, for instance, in the case of Brooker’s merocyanine (stilbazolium betaine). 24 In this article, we are interested in the photochemical properties of these molecules and report our results on the prototype molecule I, in which the methyl group attached to the nitrogen atom in Brooker’s merocyanine is substituted by a hydrogen atom. It is referred to as MRCN. Scheme 1 shows the two main VB forms of this molecule and a VB presentation of the ground state wave function Ψ g , considered to be the in- phase combination of the two VB forms. The excited state (Ψ e ) conjugate to this state is formed by the out-of-phase combination of the same two VB forms. It is referred to as the twin state. 25 As suggested by ref 26, the relative contribution of the two forms may be derived from experimentally observable parameters using the following equation where μ g and μ e are the dipole moments in the ground state and excited state, respectively, and μ eg is the transition dipole moment between the two states. When c 2 ) 0.5, the quinonoid and Zwitterion forms have equal weights in resonance hybrid; for c 2 < 0.5, the quinonoid form dominates the ground state; for c 2 > 0.5, the Zwitterion form dominates the ground state. Squillacote 27 presented evidence of competition between two conical intersections through the electrostatic control of the * Corresponding authors. SCHEME 1: Quinonoid (Q), Zwitterion (Z), and Biradical (B) Structures for MRCN Showing the Notation Used to Name Bonds a a In the quinonoid form, bonds b4 and b6 are essentially double bonds, whereas in the zwitterion form, they are single. The in-phase and out-of-phase combinations of the Q and Z VB structures form the twin states: the ground state (Ψ g ) and one of the excited states (Ψ e ). The c 2 parameter quantifies the contributions of the Z form in the ground state: c 2 ) 0.5 means the Q and Z forms have equal weights in resonance hybrid; for c 2 < 0.5, the Q form dominates the ground state; for c 2 > 0.5, the Z form dominates the ground state. Another excited state (Ψ e′ ) is a pure biradical state. c 2 ) 1 2 {1 - (μ e - μ g )[4μ eg 2 + (μ e - μ g ) 2 ] -1/2 } (1) J. Phys. Chem. A XXXX, xxx, 000 A 10.1021/jp904097k CCC: $40.75  XXXX American Chemical Society Downloaded by HEBREW UNIV on August 14, 2009 Published on August 14, 2009 on http://pubs.acs.org | doi: 10.1021/jp904097k