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

Abstract

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 non-empirically 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 non-planar and up to 14 rings for a backbone forced to be completely planar. Interestingly, up to six polarons can co-exist 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.

Impact of morphology on polaron delocalization in a semicrystalline conjugated polymer

“…size of 3.7 BTTT units, that is, over about 15 thiophene rings. Previous DFT calculations on BTTT cations have shown that saturation of polaron delocalization starts at a length of BTTT4, 40 which is very close to the maximum experimental value measured here for the 20 kD M n PBTTT. This implies that in highly ordered PBTTT films positive polarons can reach their intrinsic delocalization limit, which matches that predicted by DFT calculations.…”

“…67,68 This was confirmed in one lately published computational study of polaron delocalization in PBTTT oligomers. 40 Additional uncertainty comes from the fact that exact information on the conformation of the polymer is missing and the solubilizing side chains containing a large number of hydrogen nuclei are usually not considered or truncated in quantum chemical calculations. We will show here that a reliable simulation requires additional knowledge on the structural conformation of the polymer including its side chains.…”

“…As we have recently shown, long-range corrected (LRC) functionals are essential in order to describe polaron delocalization in PBTTT. 40 Here, we have chosen the LC-BLYP level of theory for all DFT calculations. Note that the differences between tuned LRC-functionals are in general small.…”

“…While a wealth of studies focused on a structural analysis in order to explain the high charge carrier mobility in PBTTT, the fundamental phenomenon of charge delocalization along the backbone has to date only been accessed theoretically. 39,40 Very recently, we reported the synthesis of BTTT oligomers to shed light on the evolution of crystalline behavior, morphology, and interaction with the fullerene acceptor as the molecular conjugation length increases. 41 In turn, this series of compounds now enables us to investigate directly the delocalization of charges in PBTTT in a systematic way via electron nuclear double resonance (ENDOR) spectroscopy, without having to rely exclusively on advanced modeling.…”

Triplet Tuning: A Novel Family of Non-Empirical Exchange–Correlation Functionals

“…OT-ωPBEh was constructed to largely reduce the SIE associated with a charge transfer excited state, [9][10][11][12][13][14][15][16][17]34,37,38 and it proves to exhibit the smallest MAE as expected (Tables 1 and S9). Interestingly, arriving at an analogous long-range HF exchange, [222][223][224] TT-ωPBEh shows a comparable predictive power to OT-ωPBEh and PBE0, and a smaller AE than B3LYP for three quarters of To extend our discussion, we also explored the sensitivity of TT-ωPBEh on T 1 to the charge separation extent in the space. [9][10][11][12][13][14][15][16]225 Due to the SIE and the inaccurate adiabatic local density approximation (ALDA) in TDDFT, poor performance is expected for a severe charge transfer T 1 state using TT-ωPBEh.…”