Subpicosecond pump-supercontinuum probe spectroscopy of LH2 photosynthetic antenna proteins at low temperature

Exciton relaxation and energy-transfer processes in the B850 circular aggregate of bacteriochlorophyll a molecules from the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides were studied at temperatures below 18 K. Excitons were selectively excited by a 7 nm spectral bandwidth pump pulse resonant with the inhomogeneously broadened long-wavelength side of the B850 ground-state absorption spectrum (between 860 and 879 nm). The transient spectra were measured over the 786-924 nm spectral range using a white light continuum probe pulse. Characteristic changes of transient spectra were observed over 4 decades of time, from about 10^-13 to about 10^-9 s. The spectral evolution was pump wavelength-dependent, changes being least notable at far-red excitation. A simple model was put forward to interpret the data, assuming that the sample consists of an ensemble of spectrally disorded excitons, each representing a separate B850 ring. It was found that the exciton coupling and diagonal disorder play almost equally important roles in the formation of the spectral and dynamical properties of light excitations in B850 antenna. The main effects of disorder considered were the spectral shifts, splitting of the degenerate exciton levels, and redistribution of the dipole strength of the transitions. Assuming that the contribution of least disturbed excitons is largest near the peak of the ground-state absorption spectrum and greatest near the edge, most of the known spectroscopic properties of LH2 complexes can be well understood, at least qualitatively. Specifically, the rough 100 fs response time was assigned to interexciton level relaxation; the three time constants, ca. 800 fs, ca. 15 ps, and ca. 150 ps, were attributed to exciton energy transfer, most likely, between the B850 rings. The ca. 1 ns decay time is due to the finite exciton lifetime. The circular LH1 antennas likely possess similar properties.