Investigation of Random Copolymer Analogues of a Semi-Random Conjugated Polymer Incorporating Thieno[3,4‑b]pyrazine

Semi-random conjugated polymers based on 3-hexylthiophene have been extensively investigated within the past several years as an effective route to broadening absorption without losing desirable properties of poly­(3-hexyl­thiophene) (P3HT). Here, we closely investigate the structural, optical, and electronic differences found in randomized analogues of poly­(3-hexylthiophene–thiophene–2,3-dimethylthieno­[3,4-b]­pyrazine) (P3HTT-TP) semi-random polymers. Proton nuclear Overhauser effect spectroscopy is used to identify linkages within the conjugated backbone, and UV–vis, cyclic voltammetry, and grazing incidence X-ray diffraction are used to determine the influence of the subtle differences in monomer connectivities. Semi-random P3HTT-TP was found to have a stronger intramolecular charge transfer absorption band and higher thermal transitions compared to its randomized analogue. Additionally, the function of the thiophene unit in the semi-random polymers is observed and demonstrated to have key electronic effects as well as maintaining a higher HOMO level, aside from providing needed stoichiometric balance to the acceptor monomer. This work demonstrates the utility of the semi-random polymer approach as a robust tool for incorporating small amount of electron-poor monomers for the purpose of improving the absorption breadth of P3HT-based polymers.