Ultrafast Carotenoid Band Shifts: Experiment and Theory ² J. L. Herek, # M. Wendling, | Z. He, ‡ T. Polı ´vka, ‡ G. Garcia-Asua, § R. J. Cogdell, ⊥ C. N. Hunter, § R. van Grondelle, | V. Sundstro 1 m, ‡ and T. Pullerits* ,‡ Department of Chemical Physics, Lund UniVersity, P.O. Box 124, S-22100 Lund, Sweden, Krebs Institute and Robert Hill Institute for Photosynthesis, Department of Molecular Biology and Biotechnology, UniVersity of Sheffield, Western Bank, Sheffield S10 2TN, U. K., DiVision of Physics and Astronomy, Faculty of Sciences, Vrije UniVersiteit, de Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, IBLS, UniVersity of Glasgow, Glasgow G12 8QQ, U. K., and FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands ReceiVed: February 4, 2004; In Final Form: May 14, 2004 The ultrafast carotenoid band shift upon excitation of nearby bacteriochlorophyll molecules was studied in three different light harvesting complexes from purple bacteria. The results were analyzed in terms of changes in local electric field of the carotenoids. Time dependent density functional theory calculations based on known and model structures led to good agreement with experimental results, strongly suggesting that the mutual orientation of the pigment molecules rather than the type of the carotenoid molecules determines the extent of the ultrafast band shift. We further estimate that the protein induced local field nearby carotenoid molecule is about 4 or 6 MV/cm, depending on the orientation of the change of the electrical dipole in the carotenoid upon optical transition. Introduction Carotenoids (Car) have a wide range of roles in nature. In photosynthetic antenna systems they have two major func- tions: photoprotection and light harvesting. 1,2 In some antennas they also serve a structural role. 3 Electronic level structure and ultrafast photophysical processes in Car molecules have become a hot topic in photosynthesis research. This interest has been largely stimulated by availability of high quality structural information with Car molecules clearly resolved. 4,5 One of these structures, the peripheral light harvesting complex (LH2) of purple bacteria consists of two concentric rings of BChl molecules named B800 and B850 according to their characteristic Q y absorption maxima at 800 and 850 nm. The BChls of B800 are well-separated from each other and from B850s and thereby have mainly monomeric spectroscopic properties. Contrary to that, the B850 ring forms a rather densely packed excitonically coupled aggregate with partially delocalized excited states. The Car molecules in LH2 “snake” between the two BChl rings. Excitation absorbed in the B800 ring is transferred to the B850 ring with a time constant of 0.7 ps at room temperature. 6,7 The transfer slows down at low temper- atures. 8 It has been argued that the Car molecule can be involved in facilitating the B800 to B850 transfer, 9 whereas in more recent models the coupling of the B800 with nearly resonant B850 exciton levels, which have negligible transition dipole moment, seems to lead to a quantitative agreement between theory and experiment. 10,11 Also the generalized master equation approach has been recently successfully applied. 12 Electronic states of Car molecules are usually assigned in terms of the C 2h point symmetry group. Even though the Car molecules’ C 2h symmetry is not perfect, the assignment enables understanding of many photophysical observations. 2 The main absorption of Car in the visible spectral region is due to the transition from the ground-state S 0 (1A g ) to the second excited- state S 2 (1B u ). The lowest singlet excited-state S 1 (2A g ) has the same parity as the ground state and therefore has negligible one- photon absorption. The Pariser, Parr, and Pople (PPP) Hamil- tonian of the polyene π-electron system gives rise to yet another approximate symmetry, a so-called Pariser alternancy sym- metry, 13 leading to ( labels on top of the point symmetry notation. Based on these considerations a dark state identified in resonance Raman excitation profiles of spheroidene located between S 1 and S 2 states was assigned as a 1B u state. 14 Interestingly, neither time dependent density functional theory (TDDFT) 15 nor complete active space (CAS) 16 calculations have found such a state. Recently a Car singlet state S* was assigned as a distinct precursor of the ultrafast singlet-to-triplet conversion process found in some antenna complexes. 17 The identity of that state is still not entirely clear. In most photosynthetic systems antenna Car molecules efficiently transfer excitation energy to chlorophyll or BChl molecules. The transfer occurs from both the S 2 and S 1 states. 18,19 The transfer from S 2 is well understood in terms of the Fo ¨rster mechanism. 20 Since S 1 does not have any transition dipole moment, the transfer mechanism from that state has been heavily debated. 2 Already Nagae et al. concluded that the quadrupole-dipole interaction is much larger than exchange interaction for that transfer step. 21 Transition quadrupole can be seen as a first refinement of the transition dipole in calculating the Coulomb coupling. A so-called transition density cube method 22 is in this context exact as long as the wave functions, which are used for constructing the transition densities, are correct. Recent transition density cube calculations based on ² Part of the special issue “Gerald Small Festschrift”. * Corresponding author. ‡ Lund University. § University of Sheffield. | Vrije Universiteit. ⊥ University of Glasgow. # FOM-Institute for Atomic and Molecular Physics. 10398 J. Phys. Chem. B 2004, 108, 10398-10403 10.1021/jp040094p CCC: $27.50 © 2004 American Chemical Society Published on Web 06/02/2004