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

在光合作用中，太阳光被多发色团色素-蛋白质复合物或大分子聚集体吸收，这些复合物通常被称为捕光复合物。能量可以作为电子态的激发，并以级联方式转移到光化学反应中心，在那里它被用来产生稳定的电荷分离态。能量转移可以在100 nm量级的距离上发生，涉及数百个单独的分子。电子激发态由具有幅度和相位的量子力学波函数描述。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）在超快时间尺度上丢失成为一种教条。然而，在过去的几年里，对大型集光复合物的实验提供了越来越多的证据，证明一些相干光可以存活数百飞秒。然而，解释集合实验的隐含先决条件是，所有单个对象都具有相同的行为，即它们与当地环境的相互作用。最近的工作在单一的复杂水平上证实了cohesivity的存在，但也揭示了复杂的能量传递途径不同的复杂。后者的信息，当然，洗出合奏averaging.Aim当前项目的目的是阐明cohesion的作用的内部和intercomplex的能量转移在光捕获复合物从光合紫色细菌采用超快单分子技术。由于类型和程度的cohesiveness取决于至关重要的分子结构单元的激发能的差异，我们将调查，旁边的野生型复合物，也突变体，允许这些能量差异的系统性变化。超快单复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（对于相同的单个复合物）。总之，这将产生详细的知识与高的时间和光谱分辨率的电子激发态的字符。这允许找出各个复合物之间的差异，并确定参数的分布，例如失相时间，这是无法从对这种复合物的集合进行的实验中获得的。我们希望我们的研究结果将揭示严重争论的问题，如：我们处理电子或振动相干？内聚性是否也被保留下来，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?