No abstract

We compare the aggregation properties of two model compounds, differing in their stiffness, in solution by a combined spectroscopic and computational approach.

The performance of semiconducting polymers strongly depends on their intra- and intermolecular electronic interactions. Therefore, the morphology and particularly crystallinity and crystal structure play a crucial role in enabling a sufficient overlap between the orbitals of neighboring polymers. A new solution-based in situ polymerization for the fabrication of native polythiophene thin films is presented, which exploits the film formation process to influence the polymer crystal structure in the resulting thin films. The synthesis of the insoluble polythiophene is based on an oxidative reaction in which the oxidizing agent, iron(III) p -toluenesulfonate (FeTos), initially oxidizes the monomers to enable the polymer chain growth and secondly the final polymers, thereby chemically doping the polythiophene. To exploit the fact that the doped polythiophene has a different crystal packing structure compared to the undoped polythiophene, we investigate the structural effect of this inherent doping process by varying the amounts of FeTos in the reaction mixture, creating polythiophene thin films with different degrees of doping. The structural investigation performed by means of grazing incidence wide-angle X-ray scattering (GIWAXS) suggests that the strongly doped polymer chains aggregate in a π-stacked manner in the film formation process. Moreover, this π-stacking can be maintained after the removal of the dopant molecules. GIWAXS measurements, molecular dynamics simulations, and spectroscopic analysis suggest the presence of polythiophene in a novel and stable crystal structure with an enhanced intermolecular interaction.

We investigated the aggregation behavior of the donor-acceptor molecules p-DTS(FBTTh) (T1) and p-SIDT(FBTTh) (H1) in MTHF solutions. Using optical spectroscopy, we found that T1 forms aggregates in solution while H1 aggregates only when processed as a thin film, but not in solution. Free energy molecular dynamics (MD) simulations based on force fields derived from quantum-mechanical density functional theory fully reproduce this difference. Our simulations reveal that this difference is not due to the lengthy carbon side chains. Rather, the molecular symmetry of T1 allows for an aggregated state in which the central donor units are spatially well-separated while a similar configuration is sterically impossible for H1. As a consequence, any aggregation of H1 necessarily involves aggregation of the central donors which requires, as a first step, stripping the central donor of its protective MTHF solvation shell. This unfavorable process leads to a significant kinetic hindrance for aggregation and explains the strongly differing aggregation behavior of T1/H1 in MTHF despite their otherwise similar structures.