Department of Chemistry, Purdue University , West Lafayette, Indiana 47907, United States.
Acc Chem Res. 2017 Jul 18;50(7):1725-1733. doi: 10.1021/acs.accounts.7b00155. Epub 2017 Jul 5.
Long-range transport of Frenkel excitons is crucial for achieving efficient molecular-based solar energy harvesting. Understanding of exciton transport mechanisms is important for designing materials for solar energy applications. One major bottleneck in unraveling of exciton transport mechanisms is the lack of direct measurements to provide information in both spatial and temporal domains, imposed by the combination of fast energy transfer (typically ≤1 ps) and short exciton diffusion lengths (typically ≤100 nm). This challenge requires developing experimental tools to directly characterize excitation energy transport, and thus facilitate the elucidation of mechanisms. To address this challenge, we have employed ultrafast transient absorption microscopy (TAM) as a means to directly image exciton transport with ∼200 fs time resolution and ∼50 nm spatial precision. By mapping population in spatial and temporal domains, such approach has unraveled otherwise obscured information and provided important parameters for testing exciton transport models. In this Account, we discuss the recent progress in imaging Frenkel exciton migration in molecular crystals and aggregates by ultrafast microscopy. First, we establish the validity of the TAM methods by imaging singlet and triplet exciton transport in a series of polyacene single crystals that undergo singlet fission. A new singlet-mediated triplet transport pathway has been revealed by TAM, resulting from the equilibrium between triplet and singlet exciton populations. Such enhancement of triplet exciton transport enables triplet excitons to migrate as singlet excitons and leads to orders of magnitude faster apparent triplet exciton diffusion rate in the picosecond and nanosecond time scales, favorable for solar cell applications. Next we discuss how information obtained by ultrafast microscopy can evaluate coherent effects in exciton transport. We use tubular molecular aggregates that could support large exciton delocalization sizes as a model system. The initial experiments measure exciton diffusion constants of 3-6 cm s, 3-5 times higher than the incoherent limit predicted by theory, suggesting that coherent effects play a role. In summary, combining ultrafast spectroscopic methods with microscopic techniques provides a direct approach for obtaining important parameters to unravel the underlying exciton transport mechanisms in molecular solids. We discuss future directions to bridge the gap in understanding of fundamental energy transfer theories to include coherent and incoherent effects. We are still in the infancy of ultrafast microscopy, and the vast potential is not limited to the systems discussed in this Account.
长程弗伦克尔激子输运对于实现高效的分子基太阳能收集至关重要。理解激子输运机制对于设计太阳能应用材料非常重要。揭示激子输运机制的一个主要瓶颈是缺乏直接测量，由于快速能量转移（通常≤1 ps）和短激子扩散长度（通常≤100 nm）的结合，无法在时空域中提供信息。这一挑战需要开发实验工具来直接表征激发能输运，从而促进对机制的阐明。为了解决这一挑战，我们采用超快瞬态吸收显微镜（TAM）作为一种手段，以 200 fs 的时间分辨率和 50nm 的空间精度直接成像激子输运。通过在时空域中绘制种群图，这种方法揭示了原本被掩盖的信息，并为测试激子输运模型提供了重要参数。在本述评中，我们讨论了通过超快显微镜对分子晶体和聚集体中的弗伦克尔激子迁移进行成像的最新进展。首先，我们通过一系列经历单重态裂变的聚并烯单晶中的单重态和三重态激子输运来验证 TAM 方法的有效性。TAM 揭示了一种新的三重态激子通过单重态介导的输运途径，这是由于三重态和单重态激子种群之间的平衡。这种三重态激子输运的增强使三重态激子能够像单重态激子一样迁移，并导致在皮秒和纳秒时间尺度上明显的三重态激子扩散率提高了几个数量级，有利于太阳能电池应用。接下来，我们讨论了超快显微镜获得的信息如何评估激子输运中的相干效应。我们使用管状分子聚集体作为模型体系，该体系可以支持大的激子离域化尺寸。初始实验测量得到的激子扩散常数为 3-6cm/s，比理论预测的非相干极限高 3-5 倍，这表明相干效应起作用。总之，将超快光谱方法与显微镜技术相结合，为获得重要参数提供了一种直接方法，这些参数可用于揭示分子固体中基本激子输运机制的内在机制。我们讨论了弥合对基本能量转移理论理解的差距的未来方向，以包括相干和非相干效应。我们仍处于超快显微镜的初期阶段，其巨大潜力不仅限于本述评中讨论的系统。
