Analysis of pH Dependent Luminescence of an Europium Complex

Abstract

The pH dependence of the photoluminescence of an europium complex in aqueous solution was measured and investigated. The fluorescence emission spectra showed pH sensitivity. The analysis of the relative intensity ratio (R) of D0→F2 to D0→F1 transition and Judd-Ofelt experimental intensity parameters Ω2 indicated that the local environment around the Eu and the asymmetry of the Eu complex have been influenced by the pH value of the aqueous solution. Introduction The design of compounds that facilitate sensing is an active area of research [1]. Lanthanides possess intrinsic luminescence that originates from f-f electron transitions in the 4fn shell of the [Xe]5s25p6 configuration and offer unique properties for optical sensing. The unique luminescence properties of Eu(III) complexes, such as long luminescence lifetimes, sharp and intense emission bands, make them especially useful for the design of luminescent sensing materials[2~4]. In the fields of chemistry, biochemistry and environmental science, the detection and quantification of pH value is very important. In this work, the pH dependence of the photoluminescence of an europium complex in solution was measured and investigated by Judd-Ofelt theory. The result will provide useful information for designing lanthanide complex-based luminescent sensors. Experimental The molecular structure of the europium complex with cyclen derivative is shown in Fig.1 It was synthesized according to the procedure reported by Gunnlaugsson et al [5].Elemental analysis calculated for the europium complex (EuC35H51N10O7F3S): C,43.57%; H,5.33%; N,14.52%; S,3.32%; found: C, 43.49%; H, 5.29%; N, 14.61%; S,3.30%. Fig. 1 Molecular structure of europium complex with cyclen derivative Elemental analyses were performed using a Perkin-Elmer 2400CHN elemental analyzer. The fluorescence emission spectra of the europium complex were recorded on a Shimadzu RF-5301PC spectrofluorophotometer. The pH dependence of the Eu(III) emission was evaluated in H2O in the presence of 0.1 M tetramethylammonium perchlorate to maintain constant ionic strength. International Conference on Material Science and Application (ICMSA 2015) © 2015. The authors Published by Atlantis Press 683 Results and Discussion PH Dependence of Fluorescence Emission Spectra of Eu Complex in Solution The pH dependence of fluorescence emission spectra of Eu complex in solution are shown in Fig.2. 580 600 620 640 660 680 700 720 740 0.0 0.2 0.4 0.6 0.8 1.0 pH=8.0 pH=7.5 pH=7.0 pH=6.5 pH=6.0 pH=5.5 Wavelength(nm) No rm al ize d In te ns ity Fig.2 pH dependence of fluorescence emission spectra of Eu complex in solution The emission spectrum was recorded form 550 nm to 750 nm under the excitation at 266nm. It can be found five emission peaks corresponding to D0→F0,1,2,3,4 can be clearly distinguished for Eu complex in solution. The presence of only one D0→F0 line indicates that the Eu ion occupies only a single site and a single chemical environment exists around it [6]. It can be found in Fig.2 that the emission intensity decreased with the increase in pH. Through an intramolecular ligand to rare earth ion energy transfer process, the metastable state of the rare earth ion can be populated by pumping ligand absorption bands, which are much stronger than those of the rare earth. In this case, the population of the Eu(III) excited state (5D0) is achieved by the ligands antenna effect. Both the amide and the nitrogens of the phen ligand are sensitive to protonation, thus the emission intensity decreased significantly with the increase in pH. Relative Intensity Ratio The D0→F1 transition is usually used as a reference because it is allowed by magnetic dipole and its intensity is independent of the environment. Because the variation of the relative intensity ratio (R) of 5D0→7F2 to 5D0→7F1 transition is very sensitive to the structural change in the vicinity of Eu3+ ions, it can be used to reflect the local structure and asymmetry in the vicinity of europium ions [7]. Fig.3 shows the R ratio values of Eu complex in solution with different pH. 5.5 6.0 6.5 7.0 7.5 8.0 1.0 1.1 1.2 1.3 1.4 1.5