Genesis of the\nChirality of Polythiophene Aggregates\nfrom Classical Molecular Dynamics

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