Multiscale modeling of structural disorder and environmental effects on the ground and excited states properties of a conjugated donor-acceptor polymer in the bulk phase

Journal article, 2025

We herein undertook a multiscale approach combining molecular dynamics (MD) simulations of solution-processed polymer bulk with sequential quantum mechanics/molecular mechanics (s-QM/MM) calculations to assess the influence of structural disorder and environmental effects on the electronic structure of conjugated donor-acceptor (D-A) polymers in bulk phase. As a case study, PF5-Y5 polymer bulk formation is modeled via gradual solvent removal under ambient conditions. The electronic structure is analyzed using state-of-the-art electronic structure methods, including optimally tuned range-separated hybrids (OT-DFT), double-hybrid functionals, and the second order algebraic diagrammatic construction (ADC(2)) method as a reference. Environmental effects are accounted for using both implicit and explicit electrostatic embedding models. Our findings reveal that structural disorder at the D-A interfaces reduces frontier orbital overlap and narrows the fundamental gap by localizing the orbitals, primarily due to significant LUMO stabilization on the acceptor unit. This effect enhances the charge-transfer (CT) character of low-lying singlet and triplet states within the OT-DFT approach, while double hybrid methods preserve a more localized nature. Disorder reshapes the energetic gaps between singlet-singlet and singlet-triplet excited states and increases its energetic disorder, with CT-rich states being particularly sensitive. Explicit electrostatic embedding further amplifies CT character and disorder in singlets while preserving triplet localization. These effects contribute to spectral broadening and help explain a shoulder feature in the visible region, linking it to structural disorder and ambient anisotropy alongside CT excitations. The choice of QM method and environment treatment in QM/MM simulations is critical, neglecting anisotropy in the surroundings can influence the excited-state descriptions in D-A materials. This work advances our theoretical understanding of organic photovoltaics by highlighting these interrelated effects.