Is a Single Conformer Sufficient to Describe the Reorganization Energy of Amorphous Organic Transport Materials?

The reorganization energy (lambda), which quantifies the structural rearrangement of a molecule when accommodating a charge, is a key parameter in the evaluation of charge mobility in molecular solids. However, it is unclear how lambda is influenced by conformational isomerism, which is diverse in amorphous solids. Here, we examine the conformational space of a family of model amorphous organic hole transport materials (HTMs), derived from triphenylamine in a core-arm template, and probe the effect of conformational complexity on lambda. We observe an extreme dependence of lambda on the conformer geometry of sterically congested HTMs, which to the best of our knowledge has not been described previously. These results serve as a cautionary tale that, while extracting the reorganization energy from a single molecular conformer optimized in the gas phase may be appropriate for rigid and sterically unencumbered structures, this approach is not suitable for many state-of-the-art HTMs that contain multiple bulky substituents.