Novel semisquaraines based on 2, 3, 3, 8-tetramethyl-3H-pyrrolo [2, 3-f] quinoline: Synthesis, photophysical properties, AIE, viscosity sensitivity and DFT study

A novel molecular design strategy for the synthesis of three A-D-A type 2, 3, 3, 8-tetramethyl-3H-pyrrolo [2, 3-f] quinoline-based semisquaraines, denoted as HSQ 1, HSQ 2 and HSQ 3 were developed to investigate the effect of additional pyridine acceptor on photophysical properties of these dyes. These dyes contain pyridine, indolenine and squaryl moiety as terminal acceptor, central donor, and terminal acceptor respectively. It has been found that the pyridine ring which acts as an acceptor and increases pi-conjugation were exhibited red shifted absorption (20-23 nm) and emission (32-42 nm) as compared to indolenine based parent semisquaraines, SSQ1-3. These dyes displayed Stokes shift in the range of 24-59 nm in non-polar to polar solvents respectively. Moreover, these dyes showed a bright yellow to orange solid state fluorescence (F-max = 548-585 nm). The synthesized dyes displayed charge transfer characteristics and positive solvatochromism which were elucidated by its oscillator strength, transition dipole moments and the solvent polarity plots viz. Lippert-Mattaga, McRae and Weller's plots. These semisquaraine derivatives displayed typical aggregation-induced emission enhancement (AIEE) activity (9-22 fold increase in emission intensity). More interestingly, such dyes with improved photophysical properties exhibited pronounced viscosity sensitive enhanced emission (33-43 folds increase in emission intensity) in methanol: glycerol system. The spectroscopic results of these dyes were correlated theoretically with the help of density functional theory (DFT).