Molecular dynamics simulations of conjugated semiconducting molecules
Abstract
In this thesis, we present a study of conformational disorder in conjugated
molecules focussed primarily on molecular dynamics (MD) simulation methods.
Along with quantum chemical approaches, we develop and utilise MD simulation
methods to study the conformational dynamics of polyfluorenes and polythiophenes
and the role of conformational disorder on the optical absorption behaviour observed
in these molecules. We first report a classical force-field parameterisation scheme for
conjugated molecules which defines a density functional theory method of accuracy
comparable to high-order ab-initio calculations. In doing so, we illustrate the role of
increasing conjugated backbone and alkyl side-chain length on inter-monomer dihedral
angle potentials and atomic partial charge distributions. The scheme we develop
forms a minimal route to conjugated force-field parameterisation without substantial
loss of accuracy. We then present a validation of our force-field parameterisation
scheme based on self-consistent measures, such as dihedral angle distributions, and
experimental measures, such as persistence lengths, obtained from MD simulations.
We have subsequently utilised MD simulations to investigate the interplay of solvent
and increasing side-chain lengths, the emergence of conjugation breaks, and
the wormlike chain nature of conjugated oligomers. By utilising MD simulation geometries
as input for quantum chemical calculations, we have investigated the role
of conformational disorder on absorption spectral broadening and the formation of
localised excitations. We conclude that conformational broadening is effectively independent
of backbone length due to a reduction in the effect of individual dihedral
angles with increasing length and also show that excitation localisation occurs as a
result of large dihedral angles and molecular asymmetry.