Development of a quantum chemical descriptor expressing aromatic/quinoidal character for designing narrow-bandgap π-conjugated polymers and construction of a bandgap prediction model

Abstract

A new quantum chemical descriptor, the quinoid stabilization energy (QSE), is established for the computational design of narrow-bandgap polymers. QSE was constructed based on the energy change of homodesmotic reactions of a dimethylated monomer with oligoacetylene. Density functional theory (DFT) calculations revealed a relationship between QSE and bandgap of polymers. According to the relationships obtained for 268 homopolymers and 179 alternate copolymers selected from many different families, narrow-bandgap polymers can be designed with QSE = 0, which indicates the intermediate state between aromatic and quinoid forms. Copolymers having QSE = 0 can be achieved by combining a quinoidal monomer with an aromatic one. The main advantage of this approach of designing narrow-bandgap polymers is that it requires only information of the monomers and their linking site. The reason why the polymers show a narrow bandgap around QSE = 0 was shown to be related to level crossing of the aromatic and quinoid type orbitals. In order to rationally design ultra-narrow bandgap polymers considering aromatic-quinoidal and donor-acceptor characters, the bandgap prediction model was constructed by machine-learning methods using QSE and the difference of LUMO of acceptor and HOMO of donor as descriptors.