Exciton relaxation and transfer in the LH2 antenna network of photosynthetic bacteria

Exciton relaxation and energy-transfer processes in the circular B800-850 light-harvesting (LH2) complex from the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides were studied at 8 K. Excitons were selectively excited in the B850 aggregate of bacteriochlorophyll a molecules by 5 nm spectral bandwidth, 150 fs duration pump pulses tuned over the whole B850 ground-state absorption spectrum (between 820 and 880 nm). The transient absorption spectra were measured over a 140 nm spectral range using a white-light continuum probe pulse. A strong effect of transient spectral hole burning was observed, with the shape of the transient spectrum being pump wavelength dependent. A single, very narrow bleaching line was observed when the sample was pumped in the region between 840 and 850 nm. Spectra using excitation outside of this wavelength region revealed a more complex structure. The time evolution of the transient spectra in the femtosecond and picosecond time range was both pump and probe wavelength dependent, with time-dependent changes being least notable at far-red excitation. A model was put forward to interpret the data, assuming that the sample consists of an ensemble of spectrally disordered excitons, each representing a separate B850 ring. The model takes into account a dimeric association of the bacteriochlorophyll molecules in the B850 ring and necessarily includes, besides nearest-neighbor transition dipole-dipole couplings, also the nonnearest-neighbor couplings. We conclude that all the observations of the present work and many others in the literature can be satisfactorily explained in terms of this spectrally disordered exciton model. Specifically, (i) the pump wavelength dependence of the shape of transient spectra is due to exciton-site selection within the ensemble of disordered excitons, and (ii) the ultrafast spectral dynamics is due to interexciton state relaxation (characteristic time constant 130 fs) and exciton transfer between different B850 complexes (several time constants spanning from a picosecond to a subnanosecond time range). From the comparison with the experiment, the following model parameters emerge: effective nearest-neighbor exciton coupling energy, V = 350 cm(-1); full width at half-maximum of the Gaussian inhomogeneous distribution function of site energies, Gamma(inh) = 600 cm(-1); average decay time-limited homogeneous width of the upper exciton levels, Gamma = 42 cm(-1) and the lowest level, Gamma(0) = 3.5 cm(-1); mean transition energy of the basic heterodimer of the B850 ring, nu(0) 12 420 cm(-1); average exciton delocalization size in the B850 ring, N-coh = 5 bacteriochlorophyll molecules.