Structure-fluorescence relationships in 2-aryl-5-(2′-aminophenyl)-4-hydroxy-1,3-thiazoles

Abstract

Five substituted 2-aryl-4-hydroxy-5-(2′-aminophenyl)-1,3-thiazoles have been studied for their fluorescence properties under neutral and alkaline conditions in solutions of various organic solvents. From comparison with the analogous 2-aryl-4-hydroxy-5-(2′-hydroxyphenyl)-1,3-thiazoles it is clear that both in neutral as well as in the deprotonated state the presence of the 2′-amino group lowers the fluorescence quantum yields (Φ). Introduction of an electron withdrawing groups into the 2-aryl group further decreases Φ. Upon deprotonation of the 4-hydroxy group a large bathochromic shift of the absorption (ΔλA = 100–125 nm) as well as emission (ΔλF = 100–120 nm) bands occurs. An intramolecular hydrogen bond between the amino and hydroxyl groups is obvious. The first absorption band of all studied compounds corresponds to a π-π* HOMO-LUMO transition with a CT character. Thiazoles 3b-f and their deprotonated forms 3b–-f– display fluorescence properties with large Stokes shift (ca 9000 cm−1 and 6000 cm−1, respectively) and moderate quantum yields (0.11–0.41), both are connected with the presence of inter and intramolecular hydrogen bonds.