Synthesis and spectral properties of novel series of fluorene-based azo dyes containing thiazole scaffold: Studying of the solvent and substituent effect on absorption spectra

•Eight novel fluorene/thiazole-based arylazo dyes were synthesized via efficient hydrazonoyl halides with the carbothioamide precursor.•The absorption spectra of selected arylazo dyes were measured molecules in nine solvent presenting various chemical and physical properties.•Positive solvatochromism observed of the molecules associated with the increasing of solvents polarity. According to the pH test, it was revealed that the major tautomeric forms present in solutions are azo and hydrazo forms, respectively in acid and base medium.•The change of the azo and hydrazo forms contents in various solvents at equilibrium also examined, and the results exhibited that the form hydrazo present the predominant form at solvents with high basicity while this effect is reverse for the form azo.•Solvatochromism have been used as a powerful tool to investigate the physical as well as chemical properties of the molecules. A novel series of new 2-arylazo-thiazole dyes pendant to fluorene moiety was synthesized. The novel azodyes have been accessed via the reaction of hydrazonoyl halides with the 2-(9H-fluoren-9-ylidene)hydrazine-1-carbothioamide in a simple manner using mild reaction conditions. The structures of the synthesized dyes were elucidated by spectroscopic tools and elemental analyses. The UV visible spectra of the newly synthesized arylazo dyes were recorded in nine different solvents in the region of 300–800 nm. Solvent and substituents effect, as well as the influence of the addition of acid and base on the absorption frequencies of arylazo thiazoles were discussed. To achieve deep understanding about interactions between the synthesized dyes and solvent, Kamlet-Taft and Catalan model have been used for the evaluation of the solvatochromic shifts of the absorption maxima in UV–vis spectra of the azo-hydrazo tautomers of the arylazo thiazole dyes under investigation. The results revealed the great influence of solvent polarizability and hydrogen-bond acceptor basicity parameters in the absorption frequency shifts of the selected synthesized arylazo dyes. The azo-hydrazo tautomeric form was found to depend on the substituents, solvent as well as when varying pH.