Abstract
Electronic properties of organic charge transfer complexes can fundamentally be broken down to two factors: the initial choice of electronic donor and acceptor molecule, and the final packing arrangement of such crystals. Given the ability of small organic molecules to express polymorphism in structure, the potential of these organic materials to adopt different structures is of paramount interest in terms of organic electronics.As such, this thesis aims to explore how crystal structures may be controlled in terms of growth parameters and choice of donor molecule in order to enhance charge transport in organic electronics based on the electron acceptor TCNQ. In chapter 3, three novel complexes based on orthocetamol are solved by microcrystal electron diffraction, displaying structures in which orthocetamol is able to form rigid networks of donor and acceptor molecules linked by weak hydrogen bonding between successive planes. Implementing a change in growth technique from solution to vapour processing, two polymorphs of orthocetamol-TCNQ are generated where such a change in growth parameters preserves hydrogen bonding yet shifts symmetry from non-centrosymmetric to centrosymmetric geometries.
While the generation of rigid chains of donor and acceptor molecules are observed in growth of orthocetamol-based complexes, chapter 4 explores how crystal structure is altered as ring position is altered using regioisomers of orthocetamol. Such a change in ring position reduces the interaction of hydrogen bonding in these networks; while orthocetamol-TCNQ structures possess four molecules in each unit cell creating chains linked by hydrogen bonds, structures based on regioisomers instead possess only one donor and acceptor in each unit cell where symmetry is purely translational. Although the orientation of chains in these structures may be controlled by growth technique, the degree of charge transfer as probed by spectroscopic methods remains low in all orthocetamol-based complexes.
Chapter 5 displays a novel complex of benzo[a]pyrene as a donor molecule in conjunction with TCNQ, solved by single crystal X-ray Diffraction. Where structures based on regioisomers of orthocetamol are categorised according to weak hydrogen bonding in successive chains, the benzo[a]pyrene-TCNQ complex instead displays disordered packing of molecules, dominated by π − π interactions within successive planes. These structures adopt a larger degree of charge transfer when compared to orthocetamol-based complexes, with ρ ≈ 0.2.
| Date of Award | 6 Dec 2022 |
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| Original language | English |
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| Supervisor | Simon R Hall (Supervisor) & Stephen M Hayden (Supervisor) |