Structure-fluorescence relationships in 2-ary1-5-(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 (Phi). Introduction of an electron withdrawing groups into the 2-aryl group further decreases Phi. Upon deprotonation of the 4-hydroxy group a large bathochromic shift of the absorption (Delta lambda(A)=100-125 nm) as well as emission (Delta lambda(F)=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 pi-pi* 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.