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New fluorescent thiazoles were designed and synthesized based on a 3-aryl-2-(thiazol-2-yl)acrylonitrile core. Three synthetic approaches were developed to introduce specific combinations of substituents at the 2-, 4- and 5-thiazole positions. The obtained thiazolyl-2-acrylonitriles exhibited a wide range of fluorescent colours (from green to red), long wavelength maxima and intensity depending on the combination of the substituents located at rings A, B and C. The expanded photophysical investigation established the best substituent combinations to increase their emission. Absorption and emission were studied in solvents with different polarities, as well as in DMSO-water and dioxane-water mixtures. The thiazoles showed multifunctional properties and exhibited good emission in the solid phase and in suspension (aggregation induced enhancement emission/AIEE effect). Photophysical investigations revealed a large Stokes shift, significant positive solvatochromism, and the tunability of the colour and intensity. Sharp strengthening of the emission intensity of the thiazoles was observed upon stimulation with some acid (H2SO4 and BF3·OEt2) in solvents and in the solid phase (HCl). State-of-the-art quantum mechanical calculations were performed to interpret the experimental findings. Biological experiments revealed the good penetration of the thiazoles into living cells and the accumulation both in lysosomes and, to a lesser extent, near membranes.

To enlarge the highly active fluorophore family of small molecular heterocycles, we present an effective strategy for the synthesis of novel 2-aryl-1,2,3-triazol-4-carboxylic acids (ATAs). The behavior of ATAs in different solvents showed that these new 1,2,3-triazole derivatives had bright blue fluorescence with excellent quantum yields, large Stokes shifts, and prolonged fluorescence lifetimes. Their photophysical and physicochemical properties showed extra sensitivity to both structural changes and the microenvironment, usually accompanied by marked changes in quantum yield. The responses of the ATAs to varying pH were different depending on the structure of the amino group at the triazole ring C5 atom. pKa values of 7.65–8.08 were obtained for ATAs bearing a tert-amino group. In contrast, the pKa values of ATAs bearing secondary alkylamines were in the range of 3.20–3.52. Quantum mechanical methods were employed to understand the underlying transitions and explain the obtained experimental data. Among the synthesized compounds, aggregation-induced emission enhancement (AIEE) behavior was observed, with potential applications as sensors for monitoring and controlling pH. The reversibility of ATAs between acid and alkaline pH values, as well as their toxicity, supports their real-time use in biological research.