Probing Charge Delocalization in Solid State Polychromophoric Cation Radicals Using X‑ray Crystallography and DFT Calculations

Assessing
the charge delocalization in polychromophoric assemblies
is a critical step toward designing novel charge transfer materials.
Triptycene-based materials are particularly attractive, owing to their
unique packing arrangement in the solid state. Here, we systematically
probe, both experimentally (with X-ray crystallography) and theoretically
(using Density Functional Theory, DFT), the extent of cationic charge
(i.e., hole) delocalization in a set of triptycene derivatives with
one, two, and three electron-rich 1,2-dimethoxybenzenoid (veratrole)
rings. We demonstrate that the amount of charge at each veratrole
can be deduced from experiment by analysis of the oxidation-induced
bond length changes in comparison with a model compound containing
one veratrole ring as a reference. In contrast, DFT calculations provide
not only oxidation-induced structural reorganization, but also the
charge distribution with the aid of natural population analysis. A
comparative analysis shows that both experiment and theory are of
equal efficacy in quantifying the extent of hole distribution in polychromophoric
cation radicals, despite issues of packing, solvent molecules, and
counterions that are present in the crystals. Therefore, combining
X-ray crystallographic data with insight from DFT calculations can
provide a detailed understanding of the hole distribution in polychromophoric
cation radicals, in turn allowing an informed design of the next-generation
charge-transport materials based on triptycene and other polychromophoric
scaffolds.

评估多发色团组装体中的电荷离域，是设计新型电荷传输材料的关键环节。基于三蝶烯的材料因其固态下独特的堆积排列方式而极具研究价值。本研究通过实验（采用X射线晶体学（X-ray crystallography））与理论（使用密度泛函理论（Density Functional Theory, DFT））两种手段，系统探究了一组分别带有1个、2个及3个富电子1,2-二甲氧基苯（藜芦醚（veratrole））环的三蝶烯衍生物中，阳离子电荷（即空穴）的离域程度。研究表明，相较于以单藜芦醚环为参照的模型化合物，通过分析氧化诱导的键长变化，可从实验数据中推导得到每个藜芦醚上的电荷占比。与之相对，密度泛函理论计算不仅能得到氧化诱导的结构重构结果，还可借助自然布居分析（natural population analysis）获取电荷分布信息。对比分析结果显示，尽管晶体中存在堆积效应、溶剂分子与反离子等因素，实验与理论两种手段在量化多发色团阳离子自由基的空穴分布程度上效果相当。因此，将X射线晶体学数据与密度泛函理论计算的分析结果相结合，可实现对多发色团阳离子自由基空穴分布的精细化阐释，进而为基于三蝶烯及其他多发色团骨架的下一代电荷传输材料的理性设计提供依据。