Perylenediimides (PDIs) are important molecular building blocks that are being investigated for their applicability in optoelectronic technologies. Covalently linking multiple PDI acceptors at the 2,5,8,11 (headland) positions adjacent to the PDI carbonyl groups is reported to yield higher power conversion efficiencies in photovoltaic cells relative to PDI acceptors linked at the 1,6,7,12 (bay) positions. While the photophysical properties of PDIs linked via the bay positions have been investigated extensively, those linked at the headland positions have received far less attention. We showed previously that symmetry-breaking charge separation (SB-CS) in PDIs hold promise as a strategy for increasing photovoltaic efficiency. Here we use transient absorption and emission spectroscopies to investigate the competition between SB-CS, fluorescence, and internal conversion in three related PDI dimers linked at the headland positions with o-, m-, and p-phenylene moieties: o-PDI₂, m-PDI₂, and p-PDI₂, respectively. It is found that o-PDI₂ supports SB-CS yielding PDI^•+–PDI^•–, which is in equilibrium with the o-PDI₂ first excited state in a polar solvent (CH₂Cl₂) while m-PDI₂ and p-PDI₂ exhibit accelerated internal conversion due to the motion of the linker along with subnanosecond intersystem crossing (ISC). Electronic coupling and structural dynamics are shown to play a significant role, with o-PDI₂ being the only member of the series that exhibits significant through-bond interchromophore coupling. The pronounced o-PDI₂ steric congestion prevents the free internal rotation that leads to rapid deactivation of the excited state in the other dimers.

中文翻译：

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苯桥连苝二亚胺二聚体中的对称破坏电荷分离

苝二酰亚胺 (PDI) 是重要的分子构件，正在研究它们在光电技术中的适用性。据报道，在与 PDI 羰基相邻的 2、5、8、11（岬角）位置共价连接多个 PDI 受体，相对于在 1、6、7、12（湾）位置。虽然通过海湾位置连接的 PDI 的光物理特性已得到广泛研究，但在岬角位置连接的 PDI 的光物理特性却很少受到关注。我们之前表明，PDI 中的对称破坏电荷分离（SB-CS）有望作为提高光伏效率的策略。在这里，我们使用瞬态吸收和发射光谱来研究 SB-CS、荧光、o-、m-和p-亚苯基部分：分别为o - PDI ₂、m - PDI ₂和p - PDI ₂。发现o - PDI ₂支持 SB-CS 产生 PDI ^•+ –PDI ^•–，它与极性溶剂（CH ₂ Cl ₂）中的o -PDI ₂第一激发态处于平衡状态，而m - PDI ₂和p - PDI ₂由于链接器的运动以及亚纳秒系统间交叉 (ISC)，表现出加速的内部转换。电子耦合和结构动力学显示出重要作用，o - PDI ₂是该系列中唯一表现出显着直通键间发色团耦合的成员。明显的o - PDI ₂空间拥挤阻止了自由内部旋转，从而导致其他二聚体中激发态的快速失活。