ESIPT clubbed azo dyes as deep red emitting fluorescent molecular rotors: Photophysical properties, pH study, viscosity sensitivity, and DFT studies

Abstract

Monoazo colorants with separate inbuilt ESIPT core were selected to examine the influence of azo group para to the hydroxyl group of ESIPT core and their effect on their spectral properties. They displayed absorption maxima in the visible region around 480–493 nm. 6a exhibited a broad peak along with a bathochromic shift of 40/90 nm in the solvents DMF and DMSO due to the existence of tautomeric equilibrium. They are sensitive to basic pH with a bathochromic shift of 30 nm. They are emissive in the deep red region (565–640 nm) accompanied by a Stokes shift of 62–121 nm in DMF and DMSO. More interestingly, they showed viscosity sensitive enhancement in emission in DMF: PEG 400 system in the deep red region. 6(a–c) exhibit higher quantum yield in viscous solvent i.e. 99% PEG 400 compared to that of DMF. 6a and 6c show viscosity sensitive (x) value of 0.522 and 0.513 respectively. The enhancement of emission intensity in the viscous solvent is due to charge transfer which was elucidated by polarity plots i.e. “Lippert-Mataga”, “Mac-Rae”, “Weller's” and “Rettig's” plots. DFT and TD- DFT methods were used to correlate the experimental findings.