Synthesis of Thiazoles Bearing Aryl Enamine/Aza-enamine Side Chains: Effect of the π-Conjugated Spacer Structure and Hydrogen Bonding on Photophysical Properties

An efficient synthesis of a new series of di- and trisubstituted thiazoles (TZs) bearing aryl enamine side-chains (ETZs) has been developed and the optical properties of these compounds compared with structural analogues containing the isoelectronic aza-enamine group (ATZs). Spectral characterization demonstrated the difference in the absorption and fluorescence of the ETZs and ATZs in solution and the emergence of fluorescent ETZs in the solid state. The optimized structural geometries and weak intramolecular interactions, electronic characteristics of the ground and excited states, HOMO and LUMO analyses, changes in electron density after S₀ → S_1v excitation and maps of electrostatic potential (MEPs) calculated by means of DFT have allowed us to estimate the particularities of the geometric and electronic structures of the ETZs and ATZs in the ground and excited states. The availability, synthetic simplicity, stability, large Stokes shifts and high sensitivity to the microenvironment (fluorosolvatochromic behaviour) make the obtained TZs a useful platform for the further design and synthesis of new effective compounds for applications in the field of fluorescence imaging.