Investigation of quantum coherences in photosynthetic light-harvesting complexes via ultrafast single-molecule spectroscopy

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In photosynthesis sunlight is absorbed in multichromophoric pigment-protein complexes or large molecular aggregates that are commonly referred to as light-harvesting complexes. The energy is available as excitation of the electronic states and transfered in a cascade to the photochemical reaction centre, where it is used to create a stable charge-separated state. The energy transfer may occur over distances in the order of 100 nm involving hundreds of individual molecules. The electronically excited states are described by a quantummechanical wavefunction featuring a magnitude and a phase. In the past it became a dogma that the phase relations (coherences) of the wavefunctions get lost on ultrafast timescales due to the interaction of the molecules with their environment. However, during the last years experiments on large ensembles of light-harvesting complexes provided increasing evidence that some coherences can survive for several hundreds of femtoseconds. Yet, the implicit prerequisite for interpreting ensemble experiments is that all individual objects feature the same behaviour, i.e. also with respect to their interaction with the local environment. Recent work on the single complex level confirmed the existence of coherences but revealed as well that intracomplex energy transfer pathways differ from complex to complex. The latter information is, of course, washed out in ensemble averaging.Aim of the current project is to elucidate the role of coherences for the intra- and intercomplex energy transfer in light-harvesting complexes from photosynthetic purple bacteria employing ultrafast single-molecule techniques. Since the type and degree of the coherences depends crucially on the differences of the excitation energies of the molecular building blocks, we will investigate, next to wild type complexes, also mutants that allow for a systematic variation of these energy differences. The results from the ultrafast single-complex experiments will be correlated (for the same individual complex) with the outcome of polarization-resolved fluorescence-excitation and fluorescence emission spectroscopy. In total this will yield detailed knowledge about the character of the electronically excited states with both high temporal and spectral resolution. This allows to figure out differences between the individual complexes and to determine distributions of parameters as for example the dephasing time, which are not accessible from experiments conducted on ensembles of such complexes. We expect that our results will shed light on the heavily debatted issues such as: Are we dealing with electronic or vibronic coherences? Are coherences also preserved and possibly relevant in the intercomplex energy transfer?