Accounting for Thermal Effects in the Charge-Transfer Properties of Dipolar Organic Dyes for Dye-Sensitized Solar Cells

Metal-free donor–π–acceptor (D–π–A) dyes are attractive, low-cost sensitizers for dye-sensitized solar cells (DSSC), and computational screening allows rapid exploration of this vast chemical space. However, most screening protocols characterize each candidate from a single low-energy conformer, neglecting the conformational flexibility that these dyes retain at typical operating temperatures. Here we present a thermally aware screening framework that combines CREST conformational sampling with CENSO ensemble refinement (r2SCAN-3c) and Boltzmann-weighted TD-DFT (CAM-B3LYP) descriptors to evaluate 150 D–π–A architectures built from five donors, ten π-bridges, and three acceptors. A composite Z-score ranking of five charge-transfer and photovoltaic descriptors selected the ten most promising sensitizers, which were further examined across three functionals (B3LYP, CAM-B3LYP, ωB97XD), charge-transfer kinetics, and explicit anchoring onto a (TiO2)14 anatase cluster. Boltzmann-averaged and single-conformer descriptors differed most strongly for dyes in which the lowest-energy conformers had very low oscillator strengths, making the ensemble properties sensitive to higher-energy, more emissive conformers. For D4-B1-A1, for example, the difference in the idealized short-circuit photocurrent descriptor JSC−max approached 100%, while the average deviation across the dataset was 18%. A similarly pronounced difference was observed for the charge-transfer rate kCT for the same sensitizer, with the trend persisting across two additional functionals. These results suggest that conformational effects can become particularly important when torsional motions involving the donor or acceptor groups substantially alter the electronic character of the D–π–A sensitizer. Thus, for architectures with appreciable donor–bridge or bridge–acceptor torsional flexibility, ensemble averaging may provide a more representative description than relying on a single low-energy conformer. Anchoring calculations further showed that dyes bearing the dicyanomethylene-rhodanine acceptor adsorb only weakly onto TiO2, despite favorable electronic descriptors. These results establish conformational averaging as a useful component of computational DSSC screening for structurally flexible sensitizers.