Genesis of the Chirality of Polythiophene Aggregates from Classical Molecular Dynamics

Conjugated polymers are widely used in electronic devices whose performance is highly dependent on their electronic properties. Properties and performance are influenced by the supramolecular organization of the chromophores constituting the polymer backbone, and identifying structure-property relationships is crucial to design better devices. Polythiophenes (PTs) are among the most-studied polymers, and their ordered aggregation in solution is often induced by the introduction of chiral side chains. This allows using chiroptical spectroscopy, such as electronic circular dichroism, to monitor aggregation, but since experimental atomistic demonstrations of aggregate arrangements are lacking, computational models have been developed to identify the most likely structure. In this work, we simulate the aggregation of chiral polythiophenes by using classical molecular dynamics. Standard simulations show the formation of aggregates with herringbone arrangements, typical of polycyclic aromatic hydrocarbon crystals and also known for P3HT, the most used polythiophene, rather than the chiral H aggregates proposed in the literature. We exploited Hamiltonian Replica Exchange simulations to explore a larger portion of the free energy surface in the search for chiral conformations that would explain the experimentally observed chiroptical response. We identified cisoid and transoid helical structures with intramolecular chirality, without the formation of pi-stacked helical aggregates commonly suggested, which resulted unstable. Given past studies of the spectroscopic response of these structures, we assign the genesis of the chirality of polythiophene aggregates to intramolecular arrangements of polymer chains, rather than to interactions among different chains organized in a chiral fashion.