Department of Chemistry, University of Southern California, Los Angeles California 90089, United States.
Department of Chemistry, University of Texas at Austin, Austin Texas 78712, United States.
Acc Chem Res. 2022 Jun 7;55(11):1561-1572. doi: 10.1021/acs.accounts.2c00044. Epub 2022 May 23.
Symmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environment in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimer's structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer, the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups.By understanding trends for how rates of SBCT and charge recombination depend on a dimer's internal structure and its environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications and can serve as a model for light-induced charge separation in biological systems.
对称破缺电荷转移（SBCT）是一对相同的生色团吸收一个光子并利用其能量将电子从一个生色团转移到另一个生色团，从而打破生色团对的对称性的过程。这种激发态现象在光合生物中被观察到，它使有效的分离电荷的形成成为可能，最终催化生物合成。SBCT 也被提议作为开发光电和光催化系统的一种手段，这些系统以最小的能量损失运行。已知 SBCT 在生物和人工系统中部分是由于发生它的局部环境，这可以移动以稳定不对称的 SBCT 状态。然而，环境自由度如何协同作用于生色团对的空间和结构限制，以决定其通过 SBCT 产生长寿命电荷对的能力，仍然是研究的开放性课题。在本报告中，我们比较了一系列通过不同桥连基团连接的二吡咯二聚体，以了解相连生色团的空间分离和相互取向以及它们的连接体的结构灵活性如何各自影响 SBCT 的效率。在这个材料集合中，我们观察到一个普遍的趋势，即随着二聚体生色团之间的空间分离减小，SBCT 会加速，这与这些单元之间的电子耦合随它们的分离呈指数变化的预期一致。然而，一个关键的观察结果是，发现 SBCT 后电荷复合的速率随着生色团之间的间隔减小而减慢，而不是加快。这源于由于它们的连接体的长度减小，二聚体的结构刚性增加，导致空间排斥增加。这种刚性进一步抑制了对称性已经强制零 HOMO-LUMO 重叠的系统中的电荷复合。此外，对于正向转移，活性扭转被证明增加了 LUMO-LUMO 耦合，从而允许在桥连基团内更快地进行 SBCT。通过了解 SBCT 和电荷复合的速率如何取决于二聚体的内部结构及其环境的趋势，我们确定了用于驱动持续光诱导电荷分离的人工系统的设计准则。这样的系统可以在太阳能技术和光催化应用中找到应用，并可以作为生物系统中光诱导电荷分离的模型。
