ARTICLES Stairway to the Conical Intersection: A Computational Study of the Retinal Isomerization Robert Send Institut fu ¨r Physikalische Chemie, UniVersita ¨t Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany Dage Sundholm* Department of Chemistry, P.O. Box 55 (A.I. Virtanens plats 1), FI-00014 UniVersity of Helsinki, Finland ReceiVed: May 21, 2007; In Final Form: July 9, 2007 The potential-energy surface of the first excited state of the 11-cis-retinal protonated Schiff base (PSB11) chromophore has been studied at the density functional theory (DFT) level using the time-dependent perturbation theory approach (TDDFT) in combination with Becke’s three-parameter hybrid functional (B3LYP). The potential-energy curves for torsion motions around single and double bonds of the first excited state have also been studied at the coupled-cluster approximate singles and doubles (CC2) level. The corresponding potential-energy curves for the ground state have been calculated at the B3LYP DFT and second-order Møller- Plesset (MP2) levels. The TDDFT study suggests that the electronic excitation initiates a turn of the -ionone ring around the C 6 sC 7 bond. The torsion is propagating along the retinyl chain toward the cis to trans isomerization center at the C 11 dC 12 double bond. The torsion twist of the C 10 sC 11 single bond leads to a significant reduction in the deexcitation energy indicating that a conical intersection is being reached by an almost barrierless rotation around the C 10 sC 11 single bond. The energy released when passing the conical intersection can assist the subsequent cis to trans isomerization of the C 11 dC 12 double bond. The CC2 calculations also show that the torsion barrier for the twist of the retinyl C 10 sC 11 single bond adjacent to the isomerization center almost vanishes for the excited state. Because of the reduced torsion barriers of the single bonds, the retinyl chain can easily deform in the excited state. Thus, the CC2 and TDDFT calculations suggest similar reaction pathways on the potential-energy surface of the excited state leading toward the conical intersection and resulting in a cis to trans isomerization of the retinal chromophore. According to the CC2 calculations the cis to trans isomerization mechanism does not involve any significant torsion motion of the -ionone ring. I. Introduction The photochemical isomerization reaction of retinal chro- mophores takes place in the femtosecond regime. 1,2 Experi- mental information about the evolution of the excitation wave packet and the structural dynamics must therefore be obtained by spectroscopical studies using femtosecond laser pulses. Femtosecond vibrational spectroscopy provides structural in- formation about the molecule as a function of time, that is, it gives an understanding about the reaction pathway. For pho- toreactions, Raman spectroscopy is a very powerful tool since the electronic excitation initiates vibrational dynamics in the vicinity of the reaction center of the photoabsorbing chro- mophore where the reaction occurs, without significantly affecting the surrounding protein and the less photoactive parts of the chromophore. Mathies et al. have developed a femto- second-stimulated Raman spectroscopy (FSRS) method and applied it in studies of the photoreaction dynamics of retinal chromophores in the visual pigment rhodopsin. 3-5 Atkinson et al. used in their retinal studies a different femtosecond vibra- tional spectroscopy technique denoted coherent anti-Stokes Raman scattering (CARS). 6,7 Femtosecond time resolution of the photoreaction dynamics can also be deduced from Fourier transformed optical absorption (FTOA) spectra. 8 By using FTOA spectroscopy, Akiyama et al. suggested three main reaction steps in the isomerization mechanism of rhodopsin and bacteriorhodopsin. 9,10 They found that the initial step is completely temperature and isotopomer independent and that it occurs within 20 fs. 8-10 Thus, it can most likely be assigned to Franck-Condon (FC) relaxation. The dynamics of the second phase of the reaction is only slightly dependent on isotope substitutions at the C 11 dC 12 double bond where the cis to trans isomerization of rhodopsin takes place, whereas the third reaction phase indeed involves the isomer- ization center. 10 The FTOA measurements yielded qualitative information about the reaction steps in the femtosecond regime. However, no details about the changes in the molecular structure in the three reaction steps could be obtained from the FTOA experiments. Atkinson et al. found in their picosecond time-resolved coherent anti-Stokes Raman spectroscopy PTR/CARS studies on bacteriorhodopsin pigments that the trans to cis isomerization * To whom correspondence should be addressed. E-mail: sundholm@ chem.helsinki.fi. Fax: +358-9-19150169. Phone: +358-9-19150176. 8766 J. Phys. Chem. A 2007, 111, 8766-8773 10.1021/jp073908l CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007