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.