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

Conformable RGB-color-selective narrowband photodiodecomponentsare desirable for retinal prosthesis and vision restoration, but polymersstrongly absorbing only the red (R) color are particularly rare becausered light (R) absorption achieved by push-pull-type low-bandgapcopolymers is often accompanied by higher energy absorption in green/blueregions (G/B), hampering the color selectivity. The push-pullcopolymers can be designed to suppress such high-energy absorption,but in this case, their low-energy absorption tends to be pushed tothe near-IR region, hampering the red light sensitivity. We have thusdefined the red selectivity (RS) of a polymer as the ratio of itsred region absorption (625-800 nm) to its total absorptionin the visible and near-IR regions (400-1000 nm) and proposeda minimally hybridized narrow-wide (rather than push-pull)design rule for RS-enhancing copolymers. Their HOMO/LUMO are localizedin the narrow-bandgap units, their HOMO-1/LUMO+1 are localizedin the other wide-bandgap units, and the hybridization between thetwo units is minimized by a significant twist introduced to the backboneby a molecular design. Herein, utilizing time-dependent density functionaltheory calculations validated on short oligomer models, we refinethese design rules with additional guidelines on the relative energiesof these frontier molecular orbitals and then apply them to designnew narrow-wide polymers for strong red-selective absorption.We propose not only new polymers based on the previously reporteddiketopyrrolopyrrole (DPP) narrow unit coupled with new wide unitsbut 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 & PRIME;]bis[1,2,5]thiadiazole (B2T), coupledwith typical wide units such as methyl thiophene and xylene, respectively.