National Technical University of Athens
School of Chemical Engineering
Department of Materials Science and Engineering
Computational Materials Science and Engineering Group (Co.M.S.E.)

On the 17th of September 2017, Prof. Doros N. Theodorou was awarded with the European Materials Medal, in recognition of development of new statistical mechanics-based molecular and multiscale simulation methods for the calculation of thermodynamic, structural, interfacial, and permeability properties of polymeric materials, and zeolites.

Sincerest congratulations to our Master thesis student Dora Argyropoulou on winning a postgraduate studies scholarship of the Bodossaki Foundation.

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Discotic liquid crystals

Discotic polyaromatic molecules with flexible chains grafted around their periphery constitute an interesting class of material for organic electronics applications[REF]. These molecules tend to self-organize into columnar arrangements, with the π-π interaction along the columnar direction being responsible for charge transfer phenomena and the entropic behavior of the side chains rendering the materials soluble, thus allowing greater processability in comparison to their solid-state ungrafted counterparts.

Our studies focus on - but are not limited to - to a series of planar polyaromatic cores of hexagonal, trigonal and tetragonal symmetry such as the hexa-peri-hexabenzocoronene (HBC) C42H18[REF], the superphenalene C96H30 and the C132H48 molecules. In order to investigate the soft, liquid-crystalline phases of molecular crystals made of the aforementioned mesogens, a plethora of peripheral side chains is examined, such as linear or branched aliphatic chains comprised of purely sp3 carbon atoms, complex chains containing phenyl and ether groups, and semi- or fully-fluorinated aliphatic side chains.

Figure 1: A HBC-C12H25 molecule forming neatly stacked molecular wires that aggregate to periodic molecular crystals.

Molecular dynamics simulations are employed in order to investigate structural, thermodynamic, mechanical and dynamical properties of molecular crystals comprised of various discotic mesogens. Calculations are carried out with the LAMMPS[REF] package in the canonical, isothermal-isobaric and isothermal-isostress statistical ensembles via the application of suitable thermostat and barostat algorithms in order to equilibrate initial configurations and accumulate trajectories over sufficient time intervals for the determination of various properties. Intramolecular and intermolecular interactions are modeled by means of empirical force fields taken from the literature[REF][REF][REF][REF]. Bonded interactions are quantified utilizing harmonic bond and angle and proper dihedral angle terms. Non-bonded interactions, namely the electrostatic and van der Waals interactions, are modeled through the Coulomb and Lennard-Jones potentials respectively, with the partial charges needed for the Coulomb potential extracted from Mulliken population analyses based on a semi-empirical tight-binding Hamiltonian using the MOPAC package[REF]. All systems are examined in their bulk phase, which is approximated using full periodic boundary conditions and appropriate mesh-based techniques and/or tail corrections associated with non-bonded interaction calculations.

Figure 2: Structural and mechanical characterization of various HBC-based systems in the crystalline (Cr) and liquid crystalline (LC) phases.

The major scope of our analyses is to examine the effect of the chemical composition and geometry of the basis molecule on the properties of the molecular crystals at given temperature and pressure conditions. A detailed knowledge of the relationships between composition of the discotic molecule, phase behavior, and structural and dynamical properties of the condensed phases formed is crucial for the manufacture of efficient materials for organic electronic applications. This is because structural characteristics, such as the core-to-core distance and stacking defects, alongside with temporal phenomena, such as intracolumnar molecular displacements, vibrations and collective columnar motions can alter the electronic properties of the systems dramatically. Structural and dynamical properties calculations are complemented by thermodynamic and mechanical theoretical characterization, leading to a full survey of material properties for every mesogen, providing insight for intelligent materials design.

Relevant publications

[1] Ziogos, O.G.; Theodorou, D.N. "Molecular Dynamics Simulations of Alkyl Substituted Nanographene Crystals" Mol. Phys. 2015, 113, 2776-2790.

Relevant projects

[1] EU META-ASSEMBLY, Self-Assembly and Dynamics in Metastable States. From Molecular and Supramolecular to Mesoscopic Systems.