Kim Taeyeon, Feng Yuanning, O'Connor James P, Stoddart J Fraser, Young Ryan M, Wasielewski Michael R
Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
J Am Chem Soc. 2023 Apr 5. doi: 10.1021/jacs.2c13576.
Designing and controlling charge transfer (CT) pathways in organic semiconductors are important for solar energy applications. To be useful, a photogenerated, Coulombically bound CT exciton must further separate into free charge carriers; direct observations of the detailed CT relaxation pathways, however, are lacking. Here, photoinduced CT and relaxation dynamics in three host-guest complexes, where a perylene () electron donor guest is incorporated into two symmetric and one asymmetric extended viologen cyclophane acceptor hosts, are presented. The central ring in the extended viologen is either -phenylene () or electron-rich 2,5-dimethoxy--phenylene (), resulting in two symmetric cyclophanes with unsubstituted or methoxy-substituted central rings, and , respectively, and an asymmetric cyclophane with one of the central viologen rings being methoxylated . Upon photoexcitation, the asymmetric host-guest ⊃ complex exhibits directional CT toward the energetically unfavorable methoxylated side due to structural restrictions that facilitate strong interactions between the donor and the side. The CT state relaxation pathways are probed using ultrafast optical spectroscopy by focusing on coherent vibronic wavepackets, which are used to identify CT relaxations along charge localization and vibronic decoherence coordinates. Specific low- and high-frequency nuclear motions are direct indicators of a delocalized CT state and the degree of CT character. Our results show that the CT pathway can be controlled by subtle chemical modifications of the acceptor host in addition to illustrating how coherent vibronic wavepackets can be used to probe the nature and time evolution of the CT states.
设计和控制有机半导体中的电荷转移（CT）途径对于太阳能应用至关重要。为了发挥作用，光生的、库仑束缚的CT激子必须进一步分离成自由电荷载流子；然而，缺乏对详细CT弛豫途径的直接观测。本文展示了三种主客体复合物中的光致CT和弛豫动力学，其中苝（）电子供体客体被纳入两个对称和一个不对称的扩展紫精环番受体主体中。扩展紫精中的中心环要么是-亚苯基（），要么是富电子的2,5-二甲氧基-亚苯基（），分别产生两个中心环未取代或甲氧基取代的对称环番和，以及一个中心紫精环之一被甲氧基化的不对称环番。光激发后，不对称主客体⊃复合物由于结构限制而表现出向能量不利的甲氧基化侧的定向CT，这种结构限制促进了供体与侧之间的强相互作用。通过聚焦于相干振动态波包，利用超快光学光谱探测CT态弛豫途径，相干振动态波包用于识别沿电荷定位和振动态退相干坐标的CT弛豫。特定的低频和高频核运动是离域CT态和CT特征程度的直接指标。我们的结果表明，除了说明如何利用相干振动态波包探测CT态的性质和时间演化外，CT途径还可以通过受体主体的细微化学修饰来控制。
