Positional isomers of heterocyclic azo dyes: Investigation of NLO properties by Z-scan and correlative DFT studies

Abstract

We report four azo dyes and their positional isomers having a benzothiazole-pyridone core. Selected heterocyclic emissive azo dyes in this work are analysed for their non-linear optical (NLO) properties. Non-linear absorptive coefficient (β), non-linear refractive index (ƞ2), third-order NLO susceptibility (χ3), and optical limiting (OL) in different solvents are analysed and promising results are obtained using Z-scan technique. Laser damage threshold (LDT) and thermogravimetric analysis (TGA) supports that these dyes can be a good constructive unit in laser assisted devices and several high-temperature NLO processes respectively. Molecular electrostatic potential (MEP), Frontier molecular orbitals (FMOs) are also presented using B3LYP/6–311+ G (d,p) level of theory. Global hybrid functional (GH) and Range separated hybrid functional (RSH) i.e. B3LYP, BHandHLYP and CAM-B3LYP, ωB97, ωB97X, ωB97X-D respectively used for the calculation of NLO parameters. Limits of hyperpolarizabilities and intrinsic hyperpolarizabilities are calculated and all the hyperpolarizabilities of the molecules are within estimated limits. HOMO-LUMO gap have an inverse correlation with the experimental non-linear optical susceptibility (χ3). Inverse or direct correlation with hyperpolarizability with different DFT descriptors such as electrophilicity index (ω), hyperhardness is also presented.