Effect of the length of alkyl side chains in the electronic structure of conjugated polymers

Abstract

Computational modeling studies of conjugated polymers have been shown to present many challenges. Dne such challenge is to find ways to reduce the computational cost for these studies without compromising the quality of the results. An approach longly used in the literature for this purpose is replacing long alkyl side chains (with six or more carbons) with a methyl group. This work reports on a theoretical study conducted with the conjugated polymer poly(3-hexylthiophene), which contains a hexyl side chain attached to the monomer, to verify the influence of the size of the alkyl side chain on its electronic structure. The results indicated that, for polymers containing long alkyl side chains, replacement with a propyl group offered full saturation of all properties under review, showing it to be a good approach.

Theoretical evaluation of chemical substitutions along the main chain of poly(3‐hexylthienylene‐vinylene) for solar cell applications

“…In general, the more a substituent withdraws charge from the polymer system (represented by positive σ ), the more stable the HOMO and LUMO orbitals become. The opposite effect, that is, releasing charge to the polymer system (negative σ ), destabilizes the HOMO and LUMO orbitals;, this trend was already observed in our previous study of P3HT …”

“…19 Theoretically, we obtained corresponding values of −4.51, −2.34, 2.17 and 1.84 eV, presenting deviations of approximately 8, 21, 11 and −6% relative to the experimental values, respectively (see Table S3 of supporting information for more details). It is already known that using the DFT/B3LYP methodology in studies of polymers presents the best results when compared with the experimental ones, wherein deviations of around 10% are observed for the electronic properties; 14,34 however, it is also verified that with this methodology the estimates for E LUMO can reach deviations of up to 1 eV in relation to experiment. 14,34 In general, we observe here that the deviations found for the electronic and optical properties of P3HTV are within those expected for the methodology adopted.…”

“…It is already known that using the DFT/B3LYP methodology in studies of polymers presents the best results when compared with the experimental ones, wherein deviations of around 10% are observed for the electronic properties; 14,34 however, it is also verified that with this methodology the estimates for E LUMO can reach deviations of up to 1 eV in relation to experiment. 14,34 In general, we observe here that the deviations found for the electronic and optical properties of P3HTV are within those expected for the methodology adopted. First, we will analyse the results obtained for the individual substitutions at the T or V positions of P3HTV ( Fig.…”

“…As in our preliminary calculations we obtained smaller rotations than indicated by the work of Zade and Bendikov, to use completely planar structures in this study is in fact a realistic approach. We also replace the hexyl side chains of the thiophene rings by methyl groups, an approximation that has proved before to be feasible because the electronic and optical properties remained practically unchanged –. The data obtained from PM6 are presented in Table S2 of the supporting information.…”

“…We also replace the hexyl side chains of the thiophene rings by methyl groups, an approximation that has proved before to be feasible because the electronic and optical properties remained practically unchanged. [34][35][36][37] The data obtained from PM6 are presented in Table S2 of the supporting information.…”