Molecular dynamics simulations of alkyl substituted nanographene crystals^*

* Dedicated to Professor Jean-Pierre Hansen, with deepest appreciation of his outstanding contributions to liquid and soft matter theory.

Abstract

Discotic polyaromatic molecules, similar to nanometric graphene flakes, constitute an interesting class of materials for organic electronic applications. Grafting flexible side chains around the periphery of such molecules enhances their processability and gives rise to diverse behaviours, such as the manifestation of liquid-crystalline character and anisotropic mechanical response. In this work, we examine by means of molecular dynamics simulations the properties of molecular crystals comprised of alkyl-substituted hexa-peri-hexabenzocoronene mesogens. Pristine and mono-substituted systems by hydrogen or iodine atoms are modelled, with variable side chain length. A general structural and mechanical robustness to peripheral substitution is reported, with the mesogens forming tightly packed molecular wires even at elevated temperature and pressure. In their discotic ordering, the molecules present relatively low translational mobility, a beneficial phenomenon for charge transport. A thermotropic dependence of the mechanical response is identified, with the systems behaving differently in their room-temperature crystalline phase and in their liquid-crystalline phase at elevated temperatures. The melting process is also examined, elucidating an initial negative expansion along a high symmetry direction and the existence of a metastable state, before falling into the final liquid-crystalline state.

Keywords:

• nano graphene
• hexabenzocoronene
• liquid crystal
• molecular dynamics

Acknowledgements

Interactions with Professor George Floudas at the University of Ioannina and with Professor Leonidas Tsetseris at NTUA are deeply appreciated.

Notes

* Dedicated to Professor Jean-Pierre Hansen, with deepest appreciation of his outstanding contributions to liquid and soft matter theory.

Additional information

Funding

This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Programme ‘Education and Lifelong Learning’ of the National Strategic Reference Framework (NSRF) – Research Funding Programme: THALIS. Investing in knowledge society through the European social fund [grant number MIS 379436] and by the IKY Fellowships of Excellence for Postgraduate Studies in Greece – Siemens Programme [award number 11126/13a].