Piontkowski Zachary, McCamant David W
Department of Chemistry , University of Rochester , Rochester , New York 14627 , United States.
J Am Chem Soc. 2018 Sep 5;140(35):11046-11057. doi: 10.1021/jacs.8b05463. Epub 2018 Aug 23.
Donor-π-acceptor complexes for solar energy conversion are commonly composed of a chomophore donor and a semiconductor nanoparticle acceptor separated by a π bridge. The electronic coupling between the donor and acceptor is known to be large when the π systems of the donor and bridge are coplanar. However, the accessibility of highly coplanar geometries in the excited state is not well understood. In this work, we clarify the relationship between the bridge structure and excited-state donor-bridge coplanarization by comparing rhodamine sensitizers with either phenylene (O-Ph) or thiophene (O-Th) bridge units. Using a variety of optical spectroscopic and computational techniques, we model the S excited-state potential surfaces of O-Ph and O-Th along the dihedral coordinate of donor-bridge coplanarization, τ. We find that O-Th accesses a nearly coplanar (τ = 8°) global minimum geometry in S where significant intramolecular charge transfer (ICT) character is developed. The S coplanar geometry is populated in <10 ps and is stable for ca. 1 ns. Importantly, O-Ph is sterically hindered from rotation along τ and therefore remains at its initial S equilibrium geometry far from coplanarity (τ = 56°). Our results demonstrate that donor-bridge dye sensitizers utilizing thiophene bridges should facilitate strong donor-acceptor coupling by an ultrafast and stabilizing coplanarization mechanism in S. The coplanarization will result in strong donor-acceptor coupling, potentially increasing the electron transfer efficiency. These findings provide further explanation for the success of thiophene as a bridge unit and can be used to guide the informed design of new molecular sensitizers.
用于太阳能转换的供体-π-受体复合物通常由一个发色团供体和一个通过π桥隔开的半导体纳米颗粒受体组成。已知当供体和桥的π体系共面时，供体和受体之间的电子耦合很大。然而，对于激发态下高度共面几何结构的可达性还没有很好的理解。在这项工作中，我们通过比较具有亚苯基（O-Ph）或噻吩（O-Th）桥单元的罗丹明敏化剂，阐明了桥结构与激发态供体-桥共面化之间的关系。使用各种光谱学和计算技术，我们沿着供体-桥共面化的二面角坐标τ对O-Ph和O-Th的S激发态势能面进行建模。我们发现O-Th在S态中达到了一个近乎共面（τ = 8°）的全局最小几何结构，其中发展出了显著的分子内电荷转移（ICT）特征。S态的共面几何结构在<10 ps内形成，并在约1 ns内保持稳定。重要的是，O-Ph在沿着τ旋转时受到空间位阻的阻碍，因此保持在其初始的S平衡几何结构，远离共面（τ = 56°）。我们的结果表明，利用噻吩桥的供体-桥染料敏化剂应通过S态中超快且稳定的共面化机制促进强的供体-受体耦合。这种共面化将导致强的供体-受体耦合，可能提高电子转移效率。这些发现为噻吩作为桥单元的成功提供了进一步的解释，并可用于指导新型分子敏化剂的合理设计。
