Viscosity sensitive semisquaraines based on 1, 1, 2-trimethyl-1H-benzo[e]indole: Photophysical properties, intramolecular charge transfer, solvatochromism, electrochemical and DFT study

Abstract

1, 1, 2-Trimethyl-1H-benzo[e]indole based D-π-A type semisquaraines denoted as, SSK 1–3 were synthesized to investigate their primary photophysical properties, intramolecular charge transfer, solvatochromism, electrochemical properties and viscosity dependent enhanced emission. These semisquaraines contain 1, 1, 2-trimethyl-1H-benzo[e]indole and squaryl ring as donor and acceptor respectively. These dyes displayed absorption between 437 and 444 nm and emission in the range of 469–493 nm. Stokes shift of the synthesized compounds were in the range of 32–54 nm. The SSK 1–3 exhibited intense yellow-orange colored solid state fluorescence (Fmax = 520–549 nm). These dyes exhibited intramolecular charge transfer characteristics which were confirmed from their oscillator strengths, transition dipole moments, ground and excited state dipole moments and generalized Mulliken-Hash analysis (GMH). Moreover, the positive emission solvatochromic behaviour (Δλem = 21–24 nm) of these dyes was quantitatively analyzed from different solvent polarity plots. More interestingly, these D-π-A type FMRs displayed viscosity sensitive emission enhancement (23 to 32 fold increase in fluorescence intensity) in 99% methanol:glycerol system at the visible region of electromagnetic spectrum (450–650 nm). The molecular structures of SSK 1–3 were correlated with photophysical properties using density functional theory (DFT) and theoretical absorption spectra in eight different solvents were obtained from time dependent density functional theory (TD-DFT).