The Crystallisation and Excitonic Luminescence of Molecular Crystals

Abstract

The optoelectronic properties of organic molecular crystals are dependent on the electronic properties of both the constituent molecules and the structure that the molecules adopt, which is in turn affected by the charge distribution across the molecule. Tailoring of these properties may then be utilised in the creation of devices such as transistors, LEDs and solar cells. In this thesis, three studies into the crystallisation and optical properties of organic molecular crystals are presented. The first study concerns the growth of the aromatic molecule bis(4-bromophenyl)fumaronitrile via physical vapour transport, which resulted in the crystallisation of a novel polymorph. This structure has weaker red-shifted fluorescence emission relative to the conventional form owing to π-stacking and molecular conformational differences and crystallises on higher temperature substrates, due to its greater thermodynamic stability over the conventional form. The second study elucidates the mystery as to why crystals of the polyaromatic hydrocarbon coronene grown in a laboratory and karpatite, a naturally forming mineral composed of crystalline coronene, fluoresce different colours when exposed to ultraviolet light. The differences are found to be due to the morphologies of each, with the nano-thick crystalline layers of karpatite preventing the formation of excited dimers and therefore resulting in emittance at lower wavelengths. The third study examines the crystallisation propensity of persulfurated coronene, a sulphur-decorated planar organic molecule, which was unable to be crystallised via physical vapour transport and solution-based growth methods, with electron diffraction tomography showing that no crystallinity exists within molecular aggregates down to the nanoscale. Extensive simulations on monomers, dimers and trimers of persulfurated coronene revealed that the homogeneous charge distribution across the molecule results in a strong preference for face-face over edge-face intermolecular interactions, rendering crystal structure formation unfeasible.

Date of Award	23 Jun 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Simon R Hall (Supervisor)