Synthesis, characterization and spectral properties of new, highly fluorescent, 4-hydroxythiazoles

• Highly fluorescent 2-aryl-4-hydroxy-5-(2′-hydroxyphenyl)-1,3-thiazoles were prepared.
• The presence of 2′-hydroxyphenyl group enhances fluorescence quantum yields.
• Quantum calculations prove existence of the intramolecular hydrogen bond.
• Quantum calculations prove π–π* HOMO–LUMO S0–S1 transition having CT character.

Eight substituted 2-aryl-4-hydroxy-5-(2′-hydroxyphenyl)-1,3-thiazoles have been prepared and their fluorescence properties have been investigated under neutral and alkaline conditions in solutions of various organic solvents. From the comparison with analogous 4-hydroxy-2,5-diphenyl-1,3-thiazole it is clear that both in neutral as well as in the deprotonated state the presence of the 2′-hydroxy group substantially (2–4 times) enhances fluorescence quantum yields (Φ) – most probably due to formation of an intramolecular hydrogen bond. Introduction of an electron withdrawing substituent into the 2-aryl group (2-pyridyl and 4-trifluormethylphenyl derivatives) further enhances Φ up to 0.93 (in dioxane). Upon deprotonation of the 4-hydroxy group a large bathochromic shift of the absorption (Δλmax ≈ 70 nm) as well as emission (Δλem ≈ 110 nm) bands occurs and the Φ-s are typically between 0.3 and 0.7. On the basis of quantum chemical calculations and spectral results, a hydrogen bond interaction between two hydroxyl groups is obvious. The first absorption band of all studied compounds corresponds to π–π* HOMO–LUMO transition possessing a CT character.

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