Computational investigation of the tunability of HOMO- LUMO levels and band gaps in conducting polymers

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Jayasundara, Saumya
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Conducting polymers have received a lot of attention in various fields due to their tunable properties. Density functional theory (DFT) calculations were carried out to investigate the tunable nature of band gaps and HOMO and LUMO levels of polythiophene (PTh) and polypyrrole (PPy) based conjugated polymers. A GGA functional (PBE) incorporating dispersion corrections was used with TZP or plane-wave basis sets and scalar ZORA or pseudopotentials for the relativistic effects. Molecular and periodic calculations were carried out. Two series of conjugated polymers were studied as S is replaced in PTh with O, Se, Te, and N is replaced in PPy with P, As, Sb, in the presence of Li and Cl atomic dopants. Li and Cl atoms facilitate n-type and p-type doping respectively. Narrowing band gaps or HOMO-LUMO gaps were obtained when going from O to Te in PTh analogs and P to Sb in PPy analogs. Further reduction in band gaps was observed upon doping especially in the series of PTh and analogs. The tuning of the HOMO-LUMO gap was also done by replacing the peripheral hydrogen in PTh by different electron-donating and electron-withdrawing groups. As electron-donating groups -CH3, -OCH3 and -N(CH3)2 were employed, and as electron-withdrawing groups -CHO, -COOCH3, -CN and -NO2 were used. Further, S replaced by Se in PTh (polyselenophene) and molecules containing alternating heteroatoms S-Se were also studied with these different peripheral groups. The addition of electron-donating groups results in increased HOMO and LUMO levels and the addition of electron-withdrawing groups results in decreased HOMO and LUMO levels. Planarization of the molecule was observed with electron-donating groups. By changing the heteroatom from S to Se more planarized structures were observed. The structures containing both S and Se as heteroatoms have intermediate properties. Due to the changes in both HOMO and LUMO levels, the values observed for the band gap did not follow a specific trend. This study is useful in various device applications of conjugated polymers where HOMO-LUMO levels and band gaps are of concern.
Computational chemistry, Conducting polymers, DFT, Doping