Two-dimensional electronic spectroscopy of the B800–B820 light-harvesting complex

Abstract

Emerging nonlinear optical spectroscopies enable deeper insight into the intricate world of interactions and dynamics of complex molecular systems. 2D electronic spectroscopy appears to be especially well suited for studying multichromophoric complexes such as light-harvesting complexes of photosynthetic organisms as it allows direct observation of couplings between the pigments and charts dynamics of energy flow on a 2D frequency map. Here, we demonstrate that a single 2D experiment combined with self-consistent theoretical modeling can determine spectroscopic parameters dictating excitation energy dynamics in the bacterial B800-B820 light-harvesting complex, which contains 27 bacteriochlorophyll molecules. Ultrafast sub-50-fs dynamics dominated by coherent intraband processes and population transfer dynamics on a picosecond time scale were measured and modeled with one consistent set of parameters. Theoretical 2D spectra were calculated by using a Frenkel exciton model and modified Fö rster͞ Redfield theory for the calculation of dynamics. They match the main features of experimental spectra at all population times well, implying that the energy level structure and transition dipole strengths are modeled correctly in addition to the energy transfer dynamics of the system. photosynthetic complexes ͉ excitons ͉ multichromophoric systems ͉ ultrafast spectroscopy T wo-dimensional optical experiments constitute a promising addition to the field of ultrafast spectroscopy. The success of 2D IR spectroscopy in adapting techniques from multidimensional NMR is steadily expanding into the visible range, where coherent couplings between electronic transitions, frequencydependent excitation transfer processes, and chromophoreenvironment interactions in complex molecular systems can be investigated with femtosecond time resolution. As demonstrated first by Jonas and coworkers (1-3) for visible-range laser excitation pulses, Fourier analysis of the signal electric field in a phase-controlled, four-wave mixing experiment yields 2D frequency maps representing the full (within the laser pulse spectral window) third-order optical response of the system. 2D electronic experiments performed thus far are based on heterodyne detection of a three-pulse photon echo signal, which is separated from other nonlinear signals by phase matching in a noncollinear beam geometry (1, 2, 4-6), or fluorescence detection after phase cycling using pulse-shaping techniques, as performed by Tian et al. (7). Brixner et al. (4,5) developed a particularly robust experimental setup combining inherent phase stability, phase matching, and heterodyne detection by spectral interferometry, and they later demonstrated that the method was well suited to the study of multichromophoric pigment-protein complexes (8).Multiple third-order nonlinear signals interfere to give the overall three-pulse photon echo signal, and thus quantitative analysis is essential for disentangling contributions to 2D spectra and identifying the source of spectral features. Simulation o...

Ultrafast energy transfer with competing channels: Non-equilibrium Förster and Modified Redfield theories

“…In molecular complexes, simple predictions of the Förster theory often fail [14][15][16], but generalizations of the original ideas to interactions between the whole molecular complexes is straightforward [14,17,18]. Similarly, Redfield equations adapted for molecular aggregates proved to be an extremely versatile tool [1,2,19].…”

Coherence and population dynamics of chlorophyll excitations in FCP complex: Two-dimensional spectroscopy study

“…The first and the most recognized results of 2D ES demonstrating complex behavior of the excitation evolution were delivered for the photosynthetic FennaMatthews-Olson (FMO) complex from purple bacteria 13 . Since then, 2D ES has been successfully applied for studies of a number of different systems; 2D ES es-sentially helped to establish or update existing kinetic schemes of light-harvesting antennae and reaction centers of plants [14][15][16][17] and bacteria [18][19][20] . The 2D ES was also performed on the whole photosystem I complex demonstrating signatures of fast energy transfer 21 .…”

Probing dynamics of complex molecular systems with ultrafast 2D IR vibrational echo spectroscopy

“…4,5 Photon echoes have been widely used to study electronic excited state dynamics in systems such as low temperature crystals [6][7][8][9][10] and glasses, [11][12][13][14] proteins, [15][16][17] and photosynthetic chromophore clusters. 18,19 The advent of short pulse infrared (IR) sources made it possible to perform ''vibrational echo'' experiments on the vibrations of condensed mater systems such as liquids, glasses and proteins beginning in the early 1990's. [20][21][22][23][24] These first one-dimensional two-pulse vibrational echo experiments were conducted with an IR free electron laser.…”