Bandgap engineering of novel tetracyanoquinodimethane & polycyclic aromatic hydrocarbon based co-crystals

Abstract

Single crystal organic semiconducting materials offer a laboratory-like environment
to investigate the intrinsic relationship between the structure and electronic properties
of novel, engineered, binary supramolecular charge transfer complexes. The
near-perfect structure and low periodicity of imperfections in such materials allow for
gold-standard crystallographic analytical techniques to quantify these dependencies,
thereby improving our understanding of the complex relationship between a material’s
physical properties and its crystalline structure.
The uniquely weak binding interactions between charge transfer constituents in the
lattice, in contrast to high-energy covalent and ionic analogues, allow for the direct
observation of how various reversible non-covalent intermolecular interactions—such
as hydrogen bonding, dispersion forces, π−π interactions, halogen bonding, and other
electrostatic effects—can directly alter the crystal structure and thus the electronic properties.
By experimentally investigating such diverse systems, critical structure-electronic
relationships can be identified, which may be used in the future to tailor these complex
systems for specific applications.
This thesis aims to explore how small, elemental changes to a single acceptor molecule
can have far-reaching consequences for the structure and electronic properties of the
novel charge transfer complex formed between triphenylene and the fluorinated derivatives
of tetracyanoquinodimethane (TCNQ). To this end, three original single crystal
structures synthesized via low-temperature solution-based methods are elucidated using
nanobeam precession-assisted 3D electron diffractometry. Their atomic structures
are refined through ab initio calculations of ground-state energy and are probed via
ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopy, infrared (IR) spectroscopy,
scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning
calorimetry (DSC), and various quantum chemical calculations. The analysis focuses on
comparing the estimated degree of charge transfer across the three systems to define
structure-property relationships.

Date of Award	13 May 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Simon R Hall (Supervisor)