# Design of Functional Disorder in Charge-Transfer Cocrystals

## TL;DR

This paper investigates how disorder in molecular crystals can be functional, focusing on a charge-transfer cocrystal and its dynamic properties.

## Contribution

The study introduces a new design strategy for functional disorder based on intermolecular interaction enthalpy.

## Key findings

- DMDBS-DDQ shows dynamic disorder above 33 °C, affecting its electrical and thermal properties.
- The material does not follow previous design principles for functional disorder based on size mismatch.
- Interaction enthalpy surfaces are proposed as a better guide for designing dynamic disorder.

## Abstract

In molecular crystals, disorder is often avoided or ignored
as
a defect; however, fundamental electronic phenomena, such as dielectricity
and ferroelectricity, rely on motion present in the solid state for
functionality. In these materials, crystallographic disorder can be
an indicator of utility. Here, we explore the dynamics, electronic
performance, and origin of whole-molecule disorder in a model charge-transfer
(CT) cocrystal formed between 4,6-dimethyldibenzoselenophene (DMDBS)
and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), DMDBS-DDQ, to
investigate the underlying energetics for design guidelines toward
functional disorder in molecular crystals. DMDBS-DDQ was selected
from the Cambridge Structural Database (CSD) for this investigation
because DDQ is disordered over two positions related by a 180°
rotation, coupling the disorder of DDQ to the inversion symmetry of
the lattice. We prepare DMDBS-DDQ via single-crystal-to-single-crystal
desolvation and demonstrate that the electrical performance and anisotropic
thermal expansion behavior of the cocrystal are consistent with in-plane
dynamic disorder above 33 °C (∼306 K). Importantly, we
find that DMDBS-DDQ does not adhere to previous design principles
regarding functional disorder in molecular systems that target size-mismatched
molecular coformers to access high void/cavity space materials that
may favor dynamic disorder. Instead, our findings suggest that design
strategies toward dynamic disorder should be informed by interaction
enthalpy surfaces of short-range intermolecular interactions.