The effect of substituents on the stability of the central four-member ring of unsymmetrical squaraine derivatives

Synthesis of A2-D2-A1-D1 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 (A²-D²-A¹-D¹) 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, ¹H, and ¹³C NMR spectroscopy. This novel unsymmetrical squaraine dye shows a strong absorption with high molar absorptivity (˃10⁵ L mol^-1 cm^-1) 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_eg). 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_T (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, A²-D²-A¹-D¹ type dye (USQ-1) showed an improved photo-stability compared to D²-A¹-D¹ 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).