Molecular Modeling of Crystalline Alkylthiophene Oligomers and Polymers

Abstract

We present the results of a thorough molecular modeling study of several alkylthiophene-based oligomers and polymers. In particular, we consider two polymers whose limit-ordered crystal structures have been recently reported by our group, on the basis of powder X-ray data analysis: poly(3-(S)-2-methylbutylthiophene) (P3MBT) and form I' of poly(3-butylthiophene) (P3BT). We first describe the development of a series general purpose force fields for the simulation of these and related systems. The force fields incorporate the results of ab initio calculations of the bond torsion energies of selected oligomers and differ in the set of atomic charges used to represent the electrostatic interactions. We then present the results of an extensive validation of these force fields, by means of molecular mechanics (MM) energy minimizations and molecular dynamics (MD) simulations of the crystal structures of these oligomers and polymers. While our "best" force field does not outperform the others on each of the investigated systems, it provides a balanced description of their overall structure and energetics. Finally, our MM minimizations and MD simulations confirm that the reported crystal structures of P3MBT and P3BT are stable and correspond to well-defined energetic minima. The room-temperature MD simulations reveal a certain degree of side-chain disorder, even in our virtually defect-free polymer crystal models.

Materials for organic photovoltaics: insights from detailed structural models and molecular simulations

“…Finally, these structures were subjected to further molecular mechanics and molecular dynamics simulations using our own force field, which had been developed and thoroughly tested on several thiophene oligomers. [12] This allowed us to rule out some candidate structures, whose computed X-ray spectra were compatible with the experimental ones, but which were not viable from an energetic point of view. The final results are given in Figures 2 and 3, respectively for form I' [9] and form II [10].…”

“…The latter includes both ab initio electronic structure calculations and molecular dynamics (MD) simulations. [12] 3 Structure and modelling of P3BT…”

“…[21] Film discontinuity, related to spherulitic morphology, poor stacking and small crystallite dimensions along the chain direction all suggest that crystallization of P3AT's in form II will not be beneficial for OPV applications. The MD simulations of the polymers and the oligomers [12] demonstrate that, even in the absence of chain folding, thermal motions can produce significant disordering of the side chains. This is evident in the non-interdigitated form I' (Figure 2), and will certainly be even greater in the more common form I.…”

“…We will then review our studies of poly(3-butylthiophene) (P3BT) and of [6,6]phenyl-C 61 -butyric acid methyl ester (PCBM). [9][10][11][12] Their molecular structures are displayed in Figure 1, together with a simplified scheme of an organic solar cell (see below). Our aim will be to demonstrate the synergistic roles played by experimental and computational methods in this field.…”

Chemical engineering in the electronics industry: progress towards the rational design of organic semiconductor heterojunctions

“…From a computational viewpoint, the challenges are different, but equally difficult. For computation, intermolecular and intramolecular interactions govern the eventual outcome, but expressing these forces is an ongoing challenge [45]. A second major challenge is to leverage and scale this knowledge to move from the electronic structure or molecular level simulations to experimentally relevant length-scale and timescale.…”