DFT study of dithiafulvenyl phenothiazine double donor dyes modulated by π-spacer engineering

Based on the reference dye WD14 (D-D-A), eleven novel D-D-π-A organic sensitizers (DPT1-DPT11) were designed by systematically introducing π spacers with varied fusion patterns and substituents between the dual donors (dithiafulvenyl and phenothiazine) and the cyanoacrylic acid acceptor. DFT/TD-DFT calculations at the M06-2X/6-311G (d, p) level with IEF-PCM methanol solvation revealed that π spacer incorporation narrows the HOMO-LUMO gap, redshifts absorption, and enhances light-harvesting efficiency. Electron-withdrawing substituents on the π spacer reduce the exciton binding energy and electron reorganization energy, while electron-donating groups lower regeneration free energy ΔG_reg. Notably, DPT2 and DPT8 exhibit theoretically favorable properties among the designed dyes, including a redshifted λ_max, near-unity LHE, a thermodynamically sufficient electron injection driving force, a relatively high V_OC, and low λ_total, which collectively suggest their potential as promising candidates for experimental synthesis. All dyes adsorb stably on (TiO₂)₁₆ via bidentate bridging with high adsorption energies, inducing further absorption redshift and interfacial orbital hybridization.