pH dependent sensitization of europium in a hydrogen bonded three-dimensional metal–organic compound with (4966)2(4462)3 topology: luminescence titration and time-resolved studies

Abstract

A rare-earth-based metal–organic compound (MOC), [Y₂(pydc)₆(H₂pip)₃]·20H₂O (pydc = 2,6-pyridine dicarboxylate, H₂pip = diprotonated piperazine), (1), was prepared by a hydrothermal technique using 2,6-pyridine dicarboxylic acid, piperazine, and Y(III) ions. Applying the same procedure, an isomorphous compound of europium-doped rare-earth-based MOC, [Y_1.6Eu_0.4(pydc)₆(H₂pip)₃]·20H₂O (1a) was prepared. Single-crystal X-ray diffraction analysis of 1 indicated a three-dimensional hydrogen bonded binodal structure with (4⁹6⁶)₂(4⁴6²)₃ topology. Upon excitation at 280 nm, 1a showed an intense red emission, which resulted from the efficient energy flow from the excited pydc ligand to the Eu center. The efficiency of energy flow has been tuned by changing the pH of the medium. With the increase in pH, sensitization becomes more and more efficient. Compound 1a remains stable in the wide pH range 2–11, and the Eu-centered luminescence turn-on showed a linear increase from pH 2 to 7. The steady-state pH-dependent luminescence response and the lifetime decay analysis of both the ligand and metal-centered emission were performed to understand the mechanism of sensitization and its pH dependence. The stepwise deprotonation of the carboxylate oxygen was found to be responsible for the linear increase of luminescence intensity. A huge increase of the luminescence intensity at pH 11 was observed due to the breaking of the Eu–oxygen bond, which facilitated the sensitization of the Eu center via the Eu–N bond with the pydc ligand and that probably provided highly favorable geometry for excited state energy flow.