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 ΔGreg. Notably, DPT2 and DPT8 exhibit theoretically favorable properties among the designed dyes, including a redshifted λmax, near-unity LHE, a thermodynamically sufficient electron injection driving force, a relatively high VOC, and low λtotal, 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.