Synthesis of A-D-A-D type red-emitting unsymmetrical squaraine dye: Influence of additional pyridine moiety on photophysical, electrochemical, photo and thermal stability.

We have synthesized a π-conjugated acceptor-donor-acceptor-donor (A-D-A-D) type of unsymmetrical squaraine dye, signified by USQ-1 based on 1-ethyl-2,3,3,8-tetramethyl-2,3-dihydro-1H-pyrrolo[3,2-h] quinoline moiety to improve the absorption as well as emission spectra to the red-emitting region. The influence of pyridine as a subsidiary electron acceptor unit on various properties in USQ-1 dye was compared with their close relative dye (USQ-2). The synthesized dyes were examined by FT-IR, HR-MS, H, and C NMR spectroscopy. This novel unsymmetrical squaraine dye shows a strong absorption with high molar absorptivity (˃10 L mol cm) in the range of 676-661 nm from non-polar to polar solvents. The intramolecular charge transfer (ICT) process from donor to the acceptor moiety in a non-polar to polar solvent has been investigated with the help of oscillator strengths (f), as well as transition dipole moments (μ). These dyes showed a negative solvatochromic shift from non-polar to polar solvent. The quantitative analysis of charge transfer from the ground to the excited state of these squaraine dyes was calculated by Reichardt's E (30) scale plot. The comparative electrochemical, as well as the thermal study of these unsymmetrical squaraines, were studied with the aid of cyclic voltammetry and thermogravimetric analysis (TGA), respectively. Additionally, A-D-A-D type dye (USQ-1) showed an improved photo-stability compared to D-A-D type dye (USQ-2). The computational (theoretical) study of these dyes was supported with the assistance of density functional theory (DFT) together with time-dependent density functional theory (TD-DFT).