ORIGINAL ARTICLE Chaitannya W. Ghanavatkar 1 & Virendra R. Mishra 1 & Suryapratap Sharma 1 & Elizabeth Mathew 2 & Subramaniyan Chitrambalam 2 & Isaac H. Joe 2 & Sekar Nagaiyan Nethi 1 Received: 8 October 2019 /Accepted: 20 January 2020 /Published online: 5 February 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract Herein, we report the hydroxybenzazole (HBX) containing azo dyes for “linear and non-linear optical” (NLO) applications. These bi-heterocyclic dyes have HBX scaffold (decorated with ESIPT core) and connected to another thiazole moietiy through azo bond. In DMF and DMSO, dyes are “emissive in yellow-red region” and “large Stokes shift” in the range of 62–121 nm were observed. “Nonlinear absorptive coefficient” (β), “nonlinear refractive index” (ƞ 2 ), “third order non-linear optical susceptibility” (χ 3 ) in DMSO, ethanol and methanol were calculated using simple and effective “Z-scan technique” having “Nd: YAG laser” at 532 nm wavelength. 4.46 × 10 -13 (e.s.u.) was the highest (χ 3 ) was observed in DMSO among all the dyes. Optical Limiting (OL) values are in the range of 7.61–19.06 J cm -2 in solvents. Thermo Gravimetric Analysis (TGA) supports that, these compounds are useful for numerous high-temperature practices in the construction of electronic as well as optical devices. Band gap was calculated by CV as well as by DFT in acetonitrile. The same trend was observed when these HOMO-LUMO gaps were correlated in between CV and DFT. To gain more insights into structural parameters, molecular geometries were optimized at “B3LYP-6-311 + G (d,p)” level of theory. Further, “Molecular Electrostatic Potential” (MEP), “Frontier Molecular Orbitals” (FMO) were presented using “Density Functional Theory (DFT)”. Global hybrid functional (B3LYP, BHandHLYP) and range separated hybrid functionals (RSH) i.e. CAM-B3LYP, ωB97, ωB97X, and ωB97XD were used to calculate linear and NLO properties. Keywords HBX . Benzothiazole . Z scan . NLO properties . Optical limiting . Density functional theory (DFT) Introduction Organic molecules with nonlinear optical (NLO) properties are beneficial than inorganic NLO materials (eg. lithium Niobate) because of simplicity-alternatives in synthesis, low cut-off wavelengths, tiny response time, resonable laser dam- age thresholds, large optical nonlinearity, decent solubility and compatibility with polymer matrix. Therefore the research in this area is in the forefront [1–3]. The bulk NLO properties originate from the “molecular hyperpolarizability” and “di- pole moment” of the molecule [4, 5]. The molecular structural aspects of a high performing NLOphoric molecule lead to a better understanding of the origin of NLO responses in an organic molecule [6]. Investigations at a molecular level with the help of “quantum mechanical computations” have given useful guidelines in the design of organic “NLOphoric mate- rials”. Asymmetric spreading of the electrons within a conju- gated push-pull molecular system leads to an NLO response. Such a conceptual clarity has been “qualitatively and quanti- tatively” evolved in Oudar’ s two-state model, which is usually important as the design tool at the conceptual level in Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10895-020-02493-3) contains supplementary material, which is available to authorized users. * Sekar Nagaiyan Nethi nethi.sekar@gmail.com; n.sekar@ictmumbai.edu.in 1 Department of Dyestuff Technology, Institute of Chemical Technology, Matunga (E), Mumbai, Maharashtra PIN: 400 019, India 2 Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram, Kerala 695015, India Journal of Fluorescence (2020) 30:335–346 https://doi.org/10.1007/s10895-020-02493-3 Red Emitting Hydroxybenzazole (HBX) Based Azo Dyes: Linear and Non Linear Optical Properties, Optical Limiting, Z Scan Analysis with DFT Assessments