Torsional Impacts on Quaterthiophene Segments Confined within Peptidic Nanostructures

The co-assembly behavior of peptide-π-peptide and peptide-alkyl-peptide triblock molecules that form one-dimensional (1D) nanostructures under acidic, aqueous environments is dependent on the peptide sequence and the torsional constraints imposed within the nanomaterial volume. Although a hydrophilic tripeptide sequence (Asp-Asp-Asp, DDD-) previously promoted isolation/dilution of minority π-electron components in the matrix of aliphatic peptides, a β-sheet promoting sequence (Asp-Val-Val, DVV-) led to blocks of the two components distributed within larger 1D self-assembled nanostructures. Furthermore, torsional restrictions exerted on the oligoaromatic π-electron unit by the self-assembly process can lead to changes in its conformation (for example, planarity), which has ramifications on its functionality within the peptide matrix. Here, we study this impact on thiophene-based π-electron units with inherently different geometries, viz., relatively planar 2,2':5',2″:5″,2‴-quaterthiophene and 3″,4'-dimethyl-2,2':5',2″:5″,2‴-quaterthiophene, which is twisted at the core bithiophene unit due to the presence of two methyl groups. These peptides were co-assembled at 5 and 20 mol % with peptide- n-decyl-peptide triblock molecules, and the resultant assemblies were studied using UV-vis absorption, photoluminescence, and circular dichroism spectroscopies. We found that torsional restriction in dimethylated quaterthiophene units can impact the stacking behavior of these 1D peptide nanoassemblies and have consequences on their photophysical properties. Additionally, these insights help in the understanding of the dependence of the optoelectronic properties of these materials on both the intrinsic conformation of π-units and the geometric constraints imposed by their immediate local environment under aqueous conditions.