Charge Delocalization in Oligomers of Poly(2,5-bis(3-alkyl­thiophene-2-yl)­thieno­[3,2‑b]thiophene) (PBTTT)

We investigate theoretically charge delocalization in radical cations, i.e., positive polarons, formed on oligomer chains of poly­(2,5-bis­(3-alkyl­thiophene-2-yl)­thieno­[3,2-b]­thiophene) (PBTTT). We use nonempirically tuned range-separated density functionals (TRS-DFT), including LC-ωPBE, LC-BLYP, and ωB97XD. We consider the evolution with oligomer length of the molecular geometric and electronic structures, optical absorption features, and spin densities. The TRS-DFT results indicate that a positive polaron can delocalize ideally over some 10 thiophene rings when the backbone is nonplanar and up to 14 rings for a backbone forced to be completely planar. Interestingly, up to six polarons can coexist side-by-side in a hexamer (which contains 24 thiophene rings), which is consistent with the highest degrees of doping (oxidation) experimentally achievable in polythiophene derivatives.