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

在光合作用中，阳光被多色色素-蛋白质复合体或通常被称为捕光复合体的大分子聚集体吸收。能量可以作为电子态的激发，以级联的形式传递到光化学反应中心，在那里它被用来创建稳定的电荷分离状态。能量转移可能发生在100纳米量级的距离内，涉及数百个单独的分子。电子激发态由一个具有大小和相位的量子力学波函数来描述。在过去，由于分子与环境的相互作用，波函数的相位关系(相干性)在超快时间尺度上会丢失，这已经成为一种教条。然而，在过去的几年里，对大型捕光复合体的实验提供了越来越多的证据，表明一些相干可以持续数百飞秒。然而，解释总体实验的隐含前提是所有单独的物体具有相同的行为，即也就是关于它们与当地环境的相互作用。最近关于单一复合体水平的工作证实了相干的存在，但也揭示了复合体内的能量传递途径因复合体而不同。当然，后一种信息是在整体平均中被淘汰的。当前项目的目的是利用超快单分子技术来阐明相干在光合作用紫色细菌的捕光复合体中对于复合体内和复合体间能量转移的作用。由于相干的类型和程度关键取决于分子构件的激发能的差异，我们将在野生型复合体旁边研究允许这些能量差异的系统变化的突变体。超快单一络合物实验的结果将与偏振分辨荧光激发和荧光发射光谱的结果相关联(对于相同的单个络合物)。总而言之，这将产生关于具有高时间和光谱分辨率的电子激发态特征的详细知识。这允许计算出各个复合体之间的差异，并确定参数的分布，例如退相时间，这些参数的分布不能从对这种复合体的整体进行的实验中获得。我们预计，我们的结果将阐明激烈争论的问题，例如：我们正在处理的是电子相干还是振动相干？连贯性是否也被保存，并可能与复合体间的能量转移有关？

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?