320 ISSN 1229-9197 (print version) ISSN 1875-0052 (electronic version) Fibers and Polymers 2019, Vol.20, No.2, 320-327 Solvent and Substituents Effect on the UV/Vis Absorption Spectra of Novel Acidochromic 2-Aminothiazole Based Disperse Mono Azo Dyes Manali Rajeshirke, Mayuri Kadam, and Nagaiyan Sekar* Department of Dyestuff Technology, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai - 400 019, India (Received April 21, 2018; Revised September 20, 2018; Accepted November 17, 2018) Abstract: The effect of solvent atmosphere on the electronic absorption properties of 2-aminothiazole based monoazo disperse dyes are studied. The synthesized monoazo dyes with different substituents on the para position of phenyl ring led to changes in the absorption properties of these dyes. Further, the positive solvatochromism data was evaluated to get the excited and ground state dipole moment ratio of the dyes. The Kamlet-Taft and Catalan polarity scales were employed wherein the solvent basicity and dipolarity is responsible for the bathochromic shift with increase in solvent polarity. The free amino group in the dyes made it responsive towards protonation by trifluoroacetic acid. The acidochromic behavior of the dyes was studied and D1, D3, and D4 exhibited major color change to longer wavelength with emergence of a new red shifted absorption band at 556 nm, 580 nm, and 540 nm, respectively, in their absorption spectra. Thus with help of simple UV-Vis absorption studies, we have studied the intermolecular interactions of the 2-aminothiazole based mono azo acidochromic probe. Keywords: 2-Aminothiazole, Absorption solvatochromism, Dipole moment ratio, Acidochromism, Linear solvation energy relationship Introduction 2-Aminothiazole core is one of the important heterocycles used largely in disperse dye synthesis, as precursor for the synthesis of biologically active molecules, and also used in spectrophotometric determination of Iron (III) [1-5]. The color in azo dyes originates from their n-π* and/or π-π* absorption bands. 2-Aminothiazole based azo dye has unshared electron pairs on nitrogen and sulfur leading to a bathochromic shift due to easily excitable electrons. Commercial dye, C.I. Disperse Blue 339 is based on 2- amino-5-nitrothiazole and has been produced since 1950 [6- 8]. The 2-aminothiazole dyes show bathochromic effect compared to corresponding benzenoid analogues and show lower water solubility [7]. The mono azo disperse dyes from 2-amino 5-methyl thiazole give bright dark color shades of red, orange, pink with level dyeing and excellent washing, perspiration, rubbing fastness properties but poor to good sublimation fastness on polyester fabric and loss of color under high temperature exhaust dyeing conditions [9]. In same fashion Kamaljit et al. synthesized monoazo disperse dyes using variously substituted 2-aminothiazoles as diazo components with good wash fastness but poor lightfastness which was improved by the incorporation of electron withdrawing substituents [10]. Thus thiazole based azo- derivatives have wide applications in the dyeing of synthetic fibers [7,8,11]. The color of these azo dyes can be perturbed by different substitutions on aniline ring with electron- donating or electron-withdrawing groups. The ultraviolet- visible (UV-Vis) spectroscopy proves as a useful tool to study the electronic absorption properties of dyes and the solvent effect on the properties based on various polarity scales as demonstrated by Kamlet-Taft and Catlan model. The main objective of our research work is synthesis of new monoazo disperse dyes by diazotizing different substituted primary aromatic amines and coupling with 2-(2-aminothiazol- 4-yl) phenol. In this paper, the UV-Vis absorption spectra of synthesized dyes in several microenvironment of varying polarity at room temperature was studied to get an insight on the intermolecular interactions in solution. The Linear Solvation energy relationships using Kamlet-Taft and Catalan parameters were employed to study the effect of solvent parameter which influences the electronic absorption properties of the dye. Also from the absorption solvatochromism, we made an effort to understand the dipole moment ratio (μ e /μ g ) of *Corresponding author: n.sekar@ictmumbai.edu.in DOI 10.1007/s12221-019-8317-1 Scheme 1. Synthesis scheme of Dyes D1, D2, D3, and D4.