Narrow–Wide Copolymers Designed for Red-Selective Absorption by Time-Dependent Density Functional Theory Calculations

Conformable RGB-color-selective narrowband photodiode components are desirable for retinal prosthesis and vision restoration, but polymers strongly absorbing only the red (R) color are particularly rare because red light (R) absorption achieved by push–pull-type low-bandgap copolymers is often accompanied by higher energy absorption in green/blue regions (G/B), hampering the color selectivity. The push–pull copolymers can be designed to suppress such high-energy absorption, but in this case, their low-energy absorption tends to be pushed to the near-IR region, hampering the red light sensitivity. We have thus defined the red selectivity (RS) of a polymer as the ratio of its red region absorption (625–800 nm) to its total absorption in the visible and near-IR regions (400–1000 nm) and proposed a minimally hybridized narrow–wide (rather than push–pull) design rule for RS-enhancing copolymers. Their HOMO/LUMO are localized in the narrow-bandgap units, their HOMO–1/LUMO+1 are localized in the other wide-bandgap units, and the hybridization between the two units is minimized by a significant twist introduced to the backbone by a molecular design. Herein, utilizing time-dependent density functional theory calculations validated on short oligomer models, we refine these design rules with additional guidelines on the relative energies of these frontier molecular orbitals and then apply them to design new narrow–wide polymers for strong red-selective absorption. We propose not only new polymers based on the previously reported diketopyrrolopyrrole (DPP) narrow unit coupled with new wide units but also new polymers based on the newly found beyond-DPP narrow units, thieno­[3,4-g]­quinoxaline and benzo­[1,2-c;4,5-c′]­bis­[1,2,5]­thiadiazole (B2T), coupled with typical wide units such as methyl thiophene and xylene, respectively.