Analysis of quasielastic neutron scattering (QENS) data of discotic systems using different molecular dynamics (MD) models

Discotic molecules are composed of an aromatic core surrounded by aliphatic chains. These molecules are of importance because they can form columns in which the p orbitals of neighbouring molecules overlap leading to conductivity along the column. These materials find applications in molecular electronics and recently-with record quantum efficiencies-in photo voltaics. Because the correlation time of the electron (or hole) hopping is in the picosecond region, molecular dynamics on this timescale is of central importance. We have recently shown that these dynamics, which is easily measured by quasielastic neutron scattering (QENS), can be understood with a rather simple "short single-column" model via an MD simulation that reproduces the measured QENS spectra. Before progressing to the larger technologically important systems we must understand any fortuitous error cancellations that may cause the simple model to reproduce the experimental signal so well. By taking a very simple discotic, hexamethyltriphenylene (HMT), we are able to compare QENS data with three types of models: simple column, cluster and periodic. It transpires that the cluster model cannot properly accommodate inter column interactions, and a fairly modest periodic model overcomes this problem and the tendency for un-physical harmonic modes along the column. (C) 2004 Elsevier B.V. All rights reserved.