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Calculation of Orbital Energies for Electrochromic Poly(3,4-propylenedioxythiophene)s with Different Side Chains
Kenji Kanazawa, Younghoon Kim, Haijin Shin, Byeonggwan Kim, Minsu Han and Eunkyoung Kim

Orbital energies relevant to the electrochromic conversion were calculated for poly(3,4-propylenedioxythiophene)s (PRs) substituted with different side chains by density functional theory (DFT) calculation to elucidate their color switching and optical memory in electrochromic conversion. The orbital energies of HOMO (EHOMO), LUMO (ELUMO), and band gap (Eg) of PRs were calculated for finite-length oligomers up to the hexamer by the DFT calculation and then determined for infinite-length polymers using an extrapolation scheme using the 6-311G(d) basis set. Interestingly the calculated HOMO energies of PRs were dependent on the electronic nature of the side-chains, while experimental energies reflected both electronic and volume of the side chain of PRs. The volume of hexamer was well correlated to the gap between the calculated against experimental EHOMO, with a slope of 0.045 eV/nm3, indicating that the calculated EHOMO could be accurately correlated to the experimental EHOMO when considering the volume of the side chain. The orbital energies of the PR series were correlated to the EC properties such as color of an electrochromic display (ECD). In particular, ECPs with a low HOMO energy gave a long CM at the bias free. These results provide insights into the EC properties of polymers in terms of their structure–property relationships, tunability of orbital energies by sidechain of PRs, and prediction of optical memory at colored state.

Keywords: Electrochromism, electrochromic polymer, DFT calculation, Color memory, polythiophene, HOMO-LUMO energy level

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