Design of Novel Iminocoumarins for D‑π‑A System DSSCs: A (TD)DFT Study

Dye-sensitized solar cells (DSSCs) are third-generation photovoltaic devices in which photogenerated electrons are injected from an excited dye into a mesoporous TiO₂ film. The molecular design of the dye therefore plays a central role in tuning the light absorption and charge-transfer efficiency. In this work, we investigate the photovoltaic potential of a new series of 3-benzo-[d]-[1,3]-thiazole-substituted iminocoumarin dyes by comparing their properties with the benchmark coumarin dye NKX-2677. Using DFT and TD-DFT calculations, we evaluated Boltzmann-weighted conformer distributions, absorption/emission profiles, π-electron delocalization, orbital overlap, and the feasibility of electron injection and dye regeneration. The incorporation of 2,2'-bithiophene and benzo-[d]-[1,2,5]-diathiazole π-linkers was found to be essential for narrowing the HOMO-LUMO gap and enhancing molecular planarity. Among the designed sensitizers, A3, C3, and D3 display the most favorable spectral properties, with tunable absorption profiles modulated by substitution at the 7-coumarin position. Their light-harvesting efficiencies (LHE), reported here as a spectral proxy derived from the oscillator strength (f), highlight their potential as DSSC photosensitizers. Specifically, A3 shows λ_max = 427 nm, f = 0.971, LHE = 89.3%; C3 exhibits λ_max = 520 nm, f = 0.999, LHE = 90.0%; and D3 presents λ_max = 457 nm, f = 1.248, LHE = 94.4%. These λ_max/f/LHE data indicate strong photon-absorption characteristics and suggest competitive charge-separation behavior relative to that of the reference dye. Furthermore, the visible-light transition dipole moment (TDM) emerges as a robust descriptor for guiding the rational in-silico design of future DSSC sensitizers.