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

High Resolution Image Download MS PowerPoint Slide Dye-sensitized solar cells (DSSCs) are third-generation photovoltaic devices in which photogenerated electrons are injected from an excited dye into a mesoporous TiO 2 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.