Synthesis, photophysical properties and theoretical studies of pyrrole-based azoaromatic Zn(II) complexes in mixed aqueous medium

The mononuclear Zn(II) complexes of general formula [Zn(L)(2)] 1have been synthesized in good yields by reacting Zn(OAc)(2)center dot 2H(2)O with HL in 1:2 stoichiometric ratio in methanol. In this work, L represents deprotonated form of 2,5-Bis(4-bromophenylazo)-1H-pyrrole (HL1), 2-(4-Bromophenylazo)-5-(4-dimethylaminophenylazo)1H-pyrrole (HL2) and 2,5-Bis(4-chlorophenylazo)-1H-pyrrole (HL3), respectively. Coordination mode and geometry of the complexes are authenticated by single crystal X-ray study of the representative complex 1a. The ligands exhibit strong binding ability and ratio metric response to Zn2+ ions. The electronic structure and luminescent behavior of both ligands and the complexes were analyzed by DFT and Time-dependent DFT method. The formation of 1:2 (M:L) complexes and their corresponding association constants, K-assoc (1a:7.2 x 10(4)M(-1), 1b: 8.1 x 10(6) and 1c: 1.02 x 10(4)M(-1)) have been estimated with the help of the Benesi-Hildebrand method. The calculated K-assoc values unequivocally suggest that the current ligand frameworks incorporating typical nitrogenous donors are superior complexing agent. Addition of Zn2+ ions to the ligands in a mixed aqueous solution leads to a twofold change viz. a sharp variation of colour from orange to violet which is imperative for naked eye detection as well as a significant enhancement of the fluorescence intensity during coordination. Additionally, we explored the sensing property of the ligands depending on the nature of the substituent. The importance of the present fluorescent probes (HL) is that they incorporate both electron-excess (pyrrole and aromatic rings) and electron-deficient (azo) domains and thereby govern the ligand-centered fluorescence property in a more controlled way.