J. Phys. Chem. 1996, 100, 10787-10792
10787
Exciton Delocalization Length in the B850 Antenna of Rhodobacter sphaeroides To˜ nu Pullerits, Mirianas Chachisvilis, and Villy Sundstro1 m* Department of Chemical Physics, Lund UniVersity, Box 124, 22100 Lund, Sweden ReceiVed: December 7, 1995; In Final Form: April 23, 1996X
The properties of elementary excited states in the B850 band of the peripheral light-harvesting antenna (LH2) of the photosynthetic purple bacterium Rhodobacter sphaeroides has been studied at room temperature by means of femtosecond transient absorption experiments combined with computer simulations. Polarized pumpprobe kinetics have a fast component of 100 and 65 fs for the anisotropic and isotropic decays, respectively. Direct numerical simulations show that for incoherent hopping-like excitation transfer in the B850 ring of 18 Bchl a molecules at room temperature the fast component of the anisotropy decay is 3 times longer than the corresponding component of the isotropic decay, strongly suggesting that delocalized exciton states are involved in the observed dynamics. To estimate the coherence length of the exciton we have measured absorption difference spectra of LH2 from 810 to 880 nm 2 ps after the excitation into the B800 band with 75 fs laser pulses. Exciton calculations where also monomeric doubly excited states are included give a good fit to the experimental spectra for a coherence length of 4 ( 2 bacteriochlorophyll monomers. The relatively big error limits are due to the lack of detailed enough information about the doubly excited state of Bchl a.
1. Introduction The antenna system of photosynthetic purple bacteria consists of two different light-harvesting complexessthe core antenna LH1 surrounding the reaction center and the peripheral LH2 antenna. The LH2 antenna has two distinct spectral bandssB800 and B850. Recently the crystal structure of LH2 from Rhodopseudomonas (Rps.) acidophila was determined to a resolution of 2.5 Å1 revealing a ring of nine Rβ-polypeptide pairs, each containing two bacteriochlorophyll (Bchl) a molecules of B850 and one Bchl a of B800. This is certain to stimulate strong interest in LH2 complexes and hopefully will eventually lead to a deeper understanding of the fundamentals of primary photosynthesis. Intermolecular excitation transfer in photosynthesis is generally described by the Fo¨rster dipole-dipole resonance mechanism.2,3 Measurements of the transfer between spectrally different antenna forms4,5 and energy migration among similar pigment molecules studied in experiments with relatively limited time resolution6,5 have been well described by this incoherent hopping theory. On the other hand also the exciton concept7 has been applied for photosynthetic antenna systems.8,9 In that case the excitation is delocalized over a number of pigment molecules and the dynamics occurs through the relaxation between different exciton states. However, often it is not so straightforward to unambiguously distinguish in experiment the two qualitatively different kinetic processes, incoherent hopping and exciton relaxation. The actual dynamics can in addition be a combination of these limiting cases. Smaller sections of the full system might behave as a small exciton whereas the dynamics on a larger scale may correspond to the hopping-like transfer of this small exciton. For example, recently Bradforth et al.10 found that the excitation transfer in LH1 of Rhodobacter (Rb.) sphaeroides occurs as a hopping of the exciton delocalized over a Bchl a dimer. An argument against the pure incoherent Fo¨rster mechanism is the recent observation of coherent nuclear motions in the antenna complexes of photosynthetic bacteria.11,12 Vibrational coherence is preserved for approximately the same time as the estimated single-step transfer time in these systems. X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(95)03639-2 CCC: $12.00
This implies that one of the main assumptions of the Fo¨rster theory, that vibrational relaxation occurs much faster than the excitation transfer, is not fulfilled. Furthermore, the structural data1 show a densely packed Bchl a system, where exciton interactions are quite strong. In this paper we first briefly summarize the results of anisotropy decay studies of the photosynthetic purple bacterium Rb. sphaeroides. Simulations of the initial dynamics based on an incoherent hopping approach strongly suggest that this dynamics cannot be described as hopping between Bchl a monomers and is to a considerable extent due to exciton effects. Subsequently we estimate the exciton delocalization length by comparing the experimental absorption difference spectra with exciton calculations based on the available structural and spectroscopic data. A preliminary account of this study has been published in the proceedings of the Xth International Photosynthesis Congress.13 2. Experimental Section The excited state dynamics of the studied light-harvesting complex were measured using both one- and two-color pumpprobe techniques; in the one-color experiments, the excitation and probing pulses were generated using a regeneratively modelocked Ti:sapphire laser, pumped by an argon ion laser. The output from this laser was ∼1 W at a repetition rate of 82 MHz, and to prevent buildup of long-lived photoproducts, the pulse repetition rate was reduced to 40 kHz by using an acoustooptical pulse selector. The pulses were further attenuated to an excitation intensity of
