Novel Indolenine-Based hemicyanine dyes with extended π-Linker and auxiliary donor as potential Photosensitizers: Synthesis, Photophysical, electrochemical and computational Insights

Three novel hemicyanine dyes, D1-D3, were synthesized successfully based on dimethylaniline (DMA) and TPA donors and a carboxylated indolium vinyl acceptor linked through benzene unit as an extended π-spacer or through PTZ unit as an auxiliary donor to form the D₁-π-A, D₂-π-A & D₂-D₃-A systems, respectively. A multi-step process was conducted to yield the desired donor and acceptor intermediates. These were finally coupled through the Suzuki-Miyaura coupling and Knoevenagel condensation reactions to obtain the desired target dyes. A panchromatic response and a bathochromic shift of the ICT band maxima in the visible region were observed with the lowest band gap energy, E_g = 1.71 eV and λ_{abs., max.} = 590 nm, given by dye D3, were attributed to the presence of the auxiliary donor, PTZ. However, due to the high coplanarity of dye D1 it showed the highest absorptivity value of ε = 36462 M⁻¹cm⁻¹. The HOMO and LUMO levels are within the range suitable for smooth dye regeneration and electron ejection, E_HOMO = -5.37 to −5.18 eV, and E_LUMO = -3.47 to −3.27 eV. The density functional theory (DFT) and time-dependent TD-DFT at the B3LYP/6-31G(d,p) level were conducted to explore the bridged effect on geometric and TD-BHandH/6-31G(d,p) for optoelectronic properties.