Novel A-D-A-D-A type NIR absorbing symmetrical squaraines based on 2, 3, 3, 8-tetramethyl-3H-pyrrolo [3, 2-h] quinoline: Synthesis, photophysical, electrochemical, thermal properties and photostability study.

Two novel acceptor-donor-acceptor-donor-acceptor (A-D-A-D-A) type pi-conjugated symmetrical squaraine dyes, denoted by PQSQ 1 and PQSQ 2 based on 2, 3, 3, 8-tetramethyl -3H-pyrrolo [3,2-h] quinoline were successfully synthesized for the first time to arrive absorption and emission at NIR region. These dyes comprise indolenines as electron donor units, squaryl ring as a central electron acceptor and pyridines as terminal electron acceptor units. The relationship between molecular structures and photophysical properties of these dyes was studied in comparison with their parent compounds (ISQ and N-Et ISQ). These novel squaraine dyes displayed an intense absorption within the range 671 to 692 nm in polar to non- polar solvents respectively with good molar extinction coefficients (˃10 Lmol cm). Compared to their parent squaraines, both dyes showed red-shifted absorption (33-44 nm) as well as emission (38-59 nm) due to the electron-accepting ability of the ancillary pyridine acceptors and extended pi-conjugation. These dyes exhibited negative solvatochromism and Reichardt's ET (30) scale was applied to propose a quantitative relationship of the relative stability of ground and excited-state of these squaraines with solvent polarity. The electrochemical and computational properties of these symmetrical squaraines were investigated with the help of cyclic voltammetry and density functional theory (DFT). Moreover, PQSQ 1-2 exhibited high thermal and photo-stability. These A-D-A-D-A type dyes showed improved photostabilities compared to their parent D-A-D type dyes.