Computational Characterisation of Organic Molecules for Electronic Applications and an Experimental Study of Cocrystals for Electronic Devices

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Abstract

A range of small molecules of interest for use in organic semiconductor devices were studied computationally. Trends in geometry, absorption spectra, molecular orbitals, electrostatic potentials, reorganisation energies were studied. Results suggest that, as with acenes, the performance of non-linear cata-condensed polyaromatic hydrocarbons improves as number of fused benzene rings increases. The torsion in these molecules did not appear to have a large impact on the conjugation across the core and little effect on the absorption spectra, although it did affect the reorganisation energies on which charge mobilities depend. Computational studies of mobilities of anthradithiophene molecules were broadly able to reproduce trends seen experimentally and emphasised the importance of crystal morphology. Experimental work was also carried out to search for cocrystals between anthradithiophene derivatives. Many examples were found with some mixtures forming different cocrystals at different mixture ratios. These results were rationalised by a computational study that showed molecules which had a similar binding energy were more likely to be able to form cocrystals. Cocrystal devices were fabricated and 3 out of 7 showed a larger mobility than devices made out of its constituent materials alone. The best of these had a mobility 65% higher than a device made out of the constituent material with the largest mobility.An energy decomposition analysis was carried out on a novel thallophilic system, a complex of thallium with a neutral β-triketimine ligand which was found to form dimers with close Tl-Tl interactions. Calculations show the electrostatic interaction to be repulsive for the dimer with no counter ions, but attractive when 3,5-bistrifluoromethylphenyl borate counter ions are included. This suggests the metallophilic interaction is counter ion-mediated, requiring the anions to provide favourable electrostatics, even in the case of spatially diffuse and distant counter ions such used here.To enable the studies described here software was written for simulating absorption spectra. An implementation into the Gaussian Suite of programs of an energy decomposition scheme and its extension to include an empirical dispersion correction was also carried out.The work carried out here has been published in two papers:(1) An orthogonal C–H borylation – cross-coupling strategy for the preparation of tetrasubstituted ``A₂B₂''-chrysene derivatives with tuneable photophysical properties, K. W. J. Heard, J. J Morrison, L. Weston, C. H. Lo, L. Pirvu, J. Raftery, M. S. Little, J. J. W. McDouall, S. G. Yeates and P. Quayle, Chem. Commun., 2015, 51, 6115.(2) Thallophilic Tl(I)–Tl(I) contacts mediated by Tl-aryl interactions. A computational study, L. Weston, B. T. Pownall, F. S. Mair and J. J. W. McDouall, Dalton Trans., 2016, 45, 8433.

Keyword(s)

anthradithiophene; chrysene; co-crystal; cocrystal; energy decomposition analysis; organic semiconductor; organic thin film transistor; otft; polymorph; x-ray powder diffraction

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