Heterogeneity and Relaxation Dynamics of the Photoexcited Retinal Schiff Base Cation in Solution Goran Zgrablic ´, † Stefan Haacke, ‡ and Majed Chergui* ,§ Ecole Polytechnique Fe ´de ´rale de Lausanne, Laboratoire de Spectroscopie Ultrarapide, ISIC, Faculte ´ des Sciences de Base, BSP, CH-1015 Lausanne-Dorigny, Switzerland, Sincrotrone Trieste Elettra, S.S. 14 km 163.5 in Area Science Park, 34012 BasoVizza, Trieste, Italy, and Institut de Physique et Chimie des Mate ´riaux de Strasbourg, UMR 7504 CNRS-ULP, 67034 Strasbourg Ce ´dex, France ReceiVed: August 30, 2008; ReVised Manuscript ReceiVed: December 17, 2008 We present steady-state and broadband femtosecond fluorescence spectra of the protonated Schiff base of retinal in various protic and aprotic solvents, as a function of the excitation wavelength. A detailed spectral decomposition of the time-resolved fluorescence spectra allows us to isolate three spectral components: (i) the vibrationally relaxed S 1 fluorescence, (ii) a vibrationally hot S 1 fluorescence, and (iii) a higher-lying emission that undergoes spectral evolution on a time scale of 300-400 fs, which we assign to S 2 fluorescence. The vibrationally “cold” S 1 fluorescence exhibits three decay components upon 400 nm excitation (except in acetonitrile, which has two), but only two of them upon 570 nm excitation. These components clearly demonstrate the heterogeneity of the S 1 state in the sense that emission stems from several shallow potential surface minima. We discuss a connection between these decay channels and reactive and nonreactive excited- state paths on the basis of their solvent-dependent population and of previous high-performance liquid chromatography studies. There is no clear trend of the fluorescence decay times with solvent properties. Rather, a solvent effect manifests itself in acetonitrile, in that the number of fluorescence decay channels is smaller and that the quenching of the hot fluorescence seems more efficient. This effect has to do with the population pathways leading to the fluorescent states. These observations stress the heterogeneity of excited retinal Schiff base, influencing the decay but also the population channels. They also reinforce the claim that steric effects play an important role in the dynamics of the protein. Introduction All-trans retinal is the photosensitive chromophore in many bacterial forms of retinal proteins such as bacteriorhodopsin (bR) and sensory rhodopsin II found in halobacterium salinarum. 1 These proteins are important, relatively small model systems for trans-membrane proton/ion pumping and for bacterial photosensors involved in phototaxis. Ultrafast trans-cis pho- toisomerization of the retinal chromophore is an important premise of the protein function, and it has thus extensively been studied by femtosecond (fs) pump-probe and fluorescence spectroscopy. 2-9 The protein environment plays a central role in the efficiency and selectivity of isomerization. Indeed, in bR the isomerization occurs selectively around the C 13 dC 14 double bond, at a yield of 65%, whereas in solution the yield is low (<20% in most solvents) and the isomerization is highly nonselective. 10-12 A 12-18 D dipole moment change occurs in retinal upon vertical excitation, 13-15 and even larger values are observed in bacteriorhodopsin. 16,17 It has been suggested that the sudden polarization of the protein pocket induces an ultrafast dielectric response of the environment 18-20 and may drive the isomeriza- tion dynamics, whereas others have suggested that the “catalytic” action of the protein is determined by steric effects, among others, the presence of charged residues (Arg82, Asp85, and Asp212) in the vicinity of retinal. 21-25 The study of the ultrafast events in the retinylidene chro- mophore (protonated Schiff base of retinal, PSBR) has long been used as the benchmark for the understanding of environment effects, by comparison with the protein and the solvents. Such studies have been carried out using ultrafast transient absorption spectroscopy in the visible, near-infrared, 26,27 and mid-IR. 28 Ultrafast fluorescence offers the advantage of connecting the excited-state and the ground-state surfaces only, thus rendering the observations simpler, in principle. We recently reported a study of the ultrafast fluorescence of the retinylidene chro- mophore, with a resolution of e120 fs, as a function of solvent properties, such as viscosity, dielectric constant, and heat conductivity, in order to identify the environment effects that play a role in the dynamics, and to disentangle intramolecular from intermolecular contributions. 29 The spectral evolution of the fluorescence of all-trans PSBRs was recorded in protic (methanol, MeOH; 1-octanol, OctOH; and 2-propanol, ProOH) and nonprotic (cyclohexane, cHex; and dichloromethane, DCM) solvents. The fluorescence exhibited a triexponential decay with time constants ranging from 0.5 to 6 ps. The main observation was that these features were literally independent of solvent despite the more than 1 order of magnitude difference in dielectric constant and/or viscosity between the various solvents. This lack of sensitivity to the properties of the environment stands in clear contrast to the very different ultrafast kinetics detected between solvents and the protein environment. It is also confirmed by the observation of the vibrational coherences of the torsional mode of retinal, whose frequency was also found * Corresponding author. E-mail: majed.chergui@epfl.ch; phone: +41 21 6930457. † Sincrotrone Trieste Elettra. ‡ Institut de Physique et Chimie des Mate ´riaux de Strasbourg. § Ecole Polytechnique Fe ´de ´rale de Lausanne. J. Phys. Chem. B 2009, 113, 4384–4393 4384 10.1021/jp8077216 CCC: $40.75  2009 American Chemical Society Published on Web 02/27/2009 Downloaded by CILEA CONSORTIA ITALY on July 15, 2009 Published on February 27, 2009 on http://pubs.acs.org | doi: 10.1021/jp8077216