Optoelectronic Properties of High Triplet σ-π-Conjugated Poly[(biphenyl group IV-A atom (C, Si, Ge, Sn)] Backbones

A series of σ-π-conjugated polymers composed of biphenyl and X atom as backbone repeat unit (where X is the group IV-A atom: carbon, silicon, germanium, or tin) grafted with two alkoxy-substituted biphenyls at the X atom as side chains are synthesized and their optoelectronic properties are studied systematically. We choose biphenyl rather than alkyl as the side chain because its frontier molecular orbital distributions are close to those of our previously reported σ-π-conjugated polymer grafted with transport moieties. The present σ-π polymers with various X atoms show significant differences in triplet energy (E_T) ranging from 2.58 to 2.83 eV with the sequence Ge > Si > C > Sn and in charge mobilities from 10^-9 to 10^-7 cm²/(V s) with the sequence Si > Ge > Sn > C, indicating that the properties of the σ-π polymers are largely affected by their X atoms. The Ge- and Sn-based σ-π-conjugated polymers show the highest and lowest E_T values, respectively, due to their different levels of π-electron delocalization caused by size effects and (d-p)π orbital interaction. For their charge transport properties, the Si-based conjugated σ-π polymer gives the highest hole and electron mobilities due to the stronger σ-π conjugation and shorter Si-C bond length between the attached carbon atom in biphenyl and Si. On the contrary, the C-based σ-π-conjugated polymer gives the lowest charge mobilities due to a lack of d orbital in the C atom leading to a poor σ-π conjugation characteristic. These σ-π polymers with different E_T levels and charge transport properties show a significant effect on their electroluminescence characteristics. Among them, the Ge-based σ-π-conjugated polymer when used as host shows the best device performance due to its higher E_T and reasonable charge mobility. Such findings of different optoelectronic properties of these σ-π-conjugated polymers provide useful guidelines for the selection of backbone for designing σ-π-conjugated polymer host grafted with charge transport moieties.