Université Paris Diderot , Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75013 Paris, France.
J Phys Chem A. 2012 Aug 2;116(30):7880-91. doi: 10.1021/jp3043158. Epub 2012 Jul 24.
In regard to semirigid donor-spacer-acceptor (D-S-A) dyads devised for photoinduced charge separation and built from an unsaturated spacer, there exists a strategy of design referred to as "geometrical decoupling" that consists in introducing an inner-S twist angle approaching 90° to minimize adverse D/A mutual electronic influence. The present work aims at gaining further insights into the actual impact of the use of bulky substituents (R) of the alkyl type on the electronic structure of spacers (S) of the oligo-p-phenylene type, which can be critical in the functioning of derived dyads. To this end, a series of 12 novel expanded pyridiniums (EPs), regarded as model S-A assemblies, was synthesized and its structural, electronic, and photophysical properties were investigated at both experimental and theoretical levels. These EPs result from the combination of 4 types of pyridinium-based acceptor moieties with the three following types of S subunits connected at position 4 of the pyridinum core: xylyl (X), xylyl-phenyl (XP), and xylyl-tolyl (XT). From comparison of collected data with those already reported for eight other EPs based on the same A components but linked to S fragments of two other types (i.e., phenyl, P, and biphenyl, PP), the following quantitative order in regard to the pivotal S-centered HOMO energy perturbation was derived (sorted by increasing destabilization): P < X ≪ PP ≈< XP ≈< XT. This indicates that spacers (S) are primarily distinguished on the basis of their mono- or biaryl composition and secondarily by their number of methyl substituents (R). The electron-donating inductive contribution of methyl substituents (HOMO destabilization) more than counterbalances the effect of conjugation disruption (HOMO stabilization). This "compensation effect" suggests that mildly electron-withdrawing hindering groups are better suited for "geometrical decoupling", given that high-energy S-centered occupied MOs can assist charge recombination within D-S-A dyads.
关于为光诱导电荷分离而设计的半刚性给体-间隔基-受体（D-S-A）二聚体，并由不饱和间隔基构建，存在一种称为“几何去耦”的设计策略，该策略包括引入接近 90°的内-S 扭转角，以最小化不利的 D/A 相互电子影响。本工作旨在进一步了解使用烷基类型的大体积取代基（R）对聚对亚苯基型间隔基（S）的电子结构的实际影响，这在衍生二聚体的功能中可能是关键的。为此，合成了一系列 12 种新型扩展吡啶鎓（EP），将其视为 S-A 组装体的模型，并在实验和理论水平上研究了其结构、电子和光物理性质。这些 EP 是由 4 种基于吡啶鎓的受体部分与吡啶鎓核心的 4 位连接的以下三种 S 亚基组合而成：二甲苯基（X）、二甲苯基-苯基（XP）和二甲苯基-甲苯基（XT）。通过将收集到的数据与已报道的另外 8 种基于相同 A 组分但与另外两种类型（即苯基，P 和联苯基，PP）的 S 片段连接的其他 8 种 EP 的数据进行比较，得出了以下关于关键 S 中心 HOMO 能量扰动的定量顺序（按增加的去稳定化排序）：P < X ≪ PP ≈< XP ≈< XT。这表明间隔基（S）主要基于其单芳基或双芳基组成来区分，其次是其甲基取代基（R）的数量。甲基取代基的供电子诱导贡献（HOMO 去稳定化）超过了共轭破坏的影响（HOMO 稳定化）。这种“补偿效应”表明，温和的吸电子阻碍基团更适合“几何去耦”，因为高能 S 中心占据 MO 可以辅助 D-S-A 二聚体中的电荷复合。
