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.