Exciton Delocalization and Initial Dephasing Dynamics of Purple Bacterial LH2

Abstract

Ultrafast four-wave mixing spectroscopies are employed to study exciton dynamics associated with the B800 and B850 transitions of LH2 from Rhodobacter sphaeroides. Bacteriochlorophyll a , the constituent chromophore of the B800 and B850 aggregates, is studied as a monomer in solution for comparison. Frequency-resolved pump-probe spectra measured across the B800 and B850 bands establish that at zero delay the transition dipole moment of B850 is substantially larger than that of B800, indicating an initial coherence size of ∼13 chromophores in the (18-member) B850 aggregate. Novel frequency-resolved stimulated photon echo measurements show that intermolecular interactions in the B850 ring reduce the coupling of this band's electronic transition to nuclear motion. In contrast, linear electron-nuclear coupling is comparable in the bacteriochlorophyll a monomer and B800, where exciton coupling is weak. Photon echo peak shift data are consistent with these observations. The initial localization dynamics of the B850 exciton are resolved with transient grating and pump-probe magic angle measurements. These data show that the enhanced transition dipole moment of B850 at the moment of excitation contracts significantly with a time constant of ∼50 fs (for transient grating) due to exciton dephasing resulting in localization. Pump-probe anisotropy measurements reveal substantial transition dipole moment orientational relaxation with nearly the same time constant. These experimental data will be useful for the development of a rigorous theoretical picture of ultrafast exciton dynamics in LH2. B850's large transition dipole moment for absorption may play an important role in the biological function of LH2, as it would enhance the energy transfer rate between B800 and B850.

Single-Molecule Pump-Probe Detection Resolves Ultrafast Pathways in Individual and Coupled Quantum Systems

“…Likewise the er -f correlation displays two distinct clusters: electronically coupled and uncoupled. The longer redistribution times measured for the coupled system are a direct indication of a reduced coupling between the delocalized electronic transition and the nuclear motion upon excitonic interaction of two chromophores [21][22][23]. This reduction is one of the fingerprints for coherent Coulombic coupling in dye aggregates, now for the first time observed on a single bichromophoric unit.…”

Exciton Delocalization in the B850 Light-Harvesting Complex: Comparison of Different Measures

“…The faster decay of L ks and L s is due to the above-mentioned interference problem in the first case and the weak population dependence of the latter. Recent experiments on B850 with high time resolution (∼20 fs) 25 also suggest a large exciton size at early time and exciton localization on the time scale of a few hundred femtoseconds.…”

“…Experimentally, L d has been estimated from pump-probe spectra peaksplitting 17 (the energetic difference between the excited-state absorption and the ground-state bleaching peaks), the absorbing dipole strength, 25,32,54 and superradiance measurements. 18,[53][54][55] Theoretically, several tools have been proposed and used to estimate this property.…”

“…13 Contrary to that, the 18 BChl molecules of the B850 ring are densely packed with center-to-center distances of about 9 Å, and strong intermolecular interaction leads to a considerable delocalization of the excitations. Various experimental and theoretical techniques have been employed to characterize the exciton, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] and the resulting values of L d range from a few pigment molecules [15][16][17][18][19] to the entire ring. 31,32 In this paper we will show that the large variation in L d obtained by different authors is the result of different definitions of the exciton delocalization, as well as different experimental probes for exciton coherence.…”

Coherent Electronic and Nuclear Dynamics for Charge Transfer in 1-Ethyl-4-(carbomethoxy)pyridinium Iodide

“…Similar anisotropy decays have been observed for systems of electronically coupled chromophores. 46,[89][90][91][92][93][94] However, ECMPI is not an excitonic system and the absorption spectrum of its radical does not indicate the presence of multiple transitions in the 400 nm region. 34 This leads us to conclude that nuclear relaxation is the origin of the sub-picosecond change in the anisotropy.…”