Nowadays, conjugated polymers and oligomers are of considerable interest for application in the field of organic electronics. However, the problem of low efficiency of the devices, which is partly caused by difficulties in control of morphology, has yet to be addressed. One way to overcome the problem is to switch from polymers to extended chains of non-covalently bonded shorter molecules. Numerous experimental studies have shown the possibility to obtain such long chains of π-conjugated molecules. However, even small changes in molecular structure significantly affect the conductivity values. Shokri et al. [1] have shown the possibility to achieve high conductivity values as a result of self-assembly of rigid and planar 2,5-dialkoxy-phenylene-thiophene-based oligomers (TBT) with octyl side groups on epitaxial monolayer graphene by scanning tunneling microscopy at a temperature of 77 K.

Present study aims at investigating the self-organization process of the TBT molecules on monolayer graphene at ambient and elevated temperatures, namely 300 – 600 K, via the molecular dynamics simulations [2]. The interactions in the systems under investigation were described by Gromos53a5 force field with partial charges for TBT atoms additionally calculated using the quantum-chemical Hartree-Fock method with basis set 6-31G* and two approaches, Mulliken and RESP. It is shown that the partial charges of atoms and the method of their calculation are of primary importance for the structure of an individual molecule and, as a consequence, for the self-organization of large groups of molecules. Electrostatic interactions mainly affect the orientation of the alkyl side groups with respect to the backbones of the molecules resulting in separation of the backbones and self-assembly of oligomers on graphene. Moreover, large values of the calculated order parameter and density of interdigitation of octyl side groups allowed us to conclude that TBT oligomers tend to adopt a liquid-crystalline type of order on the surface of graphene. Using the Mulliken partial charges, this order is within the investigated temperature range not dependent on temperature, probably due to effectively increased rigidity of the alkyl side groups. As a result, the interactions of rigid side groups with each other and with graphene lead to a liquid-crystalline type of order within such systems even at elevated temperatures.