Department of Chemistry, Purdue University , West Lafayette, Indiana 47907, United States.
School of Chemistry, University of Glasgow , University Avenue, Glasgow, G12 8QQ, United Kingdom.
J Am Chem Soc. 2017 May 31;139(21):7287-7293. doi: 10.1021/jacs.7b01550. Epub 2017 May 18.
Long-range exciton transport is a key challenge in achieving efficient solar energy harvesting in both organic solar cells and photosynthetic systems. Self-assembled molecular aggregates provide the potential for attaining long-range exciton transport through strong intermolecular coupling. However, there currently lacks an experimental tool to directly characterize exciton transport in space and in time to elucidate mechanisms. Here we report a direct visualization of exciton diffusion in tubular molecular aggregates by transient absorption microscopy with ∼200 fs time resolution and ∼50 nm spatial precision. These direct measurements provide exciton diffusion constants of 3-6 cm s for the tubular molecular aggregates, which are 3-5 times higher than a theoretical lower bound obtained by assuming incoherent hopping. These results suggest that coherent effects play a role, despite the fact that exciton states near the band bottom crucial for transport are only weakly delocalized (over <10 molecules). The methods presented here establish a direct approach for unraveling the mechanisms and main parameters underlying exciton transport in large molecular assemblies.
长程激子输运是实现有机太阳能电池和光合作用系统高效太阳能收集的关键挑战。自组装分子聚集体通过强分子间耦合提供了实现长程激子输运的潜力。然而，目前缺乏一种实验工具来直接在空间和时间上对激子输运进行表征，以阐明其机制。在这里，我们通过具有 200 fs 时间分辨率和 50 nm 空间精度的瞬态吸收显微镜，直接观察到管状分子聚集体中的激子扩散。这些直接测量提供了管状分子聚集体的激子扩散常数为 3-6 cm s，比假设非相干跳跃得到的理论下限高 3-5 倍。这些结果表明，尽管对于输运至关重要的带底激子态仅弱离域（<10 个分子），但相干效应仍在起作用。这里提出的方法为揭示大分子组装中激子输运的机制和主要参数提供了一种直接的方法。
