3.2.2. Luminescence spectroscopy
Time resolved luminescence spectroscopy is used to obtain
stability constant of Eu3þ complexes [36,37]. The sensitivity of the 5
D0–7
F2 transition can be utilized for studying complexation
behavior of Eu3þ as the nature of environment of the Eu3þ site
makes this hypersensitive transition, as described earlier. The
luminescence spectra of different solutions having Eu3þ concentration
of 1 105 M with different metal to ligand ratio are
shown in Fig. 5 after normalizing to the intensity of 592 nm peak.
It can be seen from this figure that intensity of 615 nm peak
increases as the BA concentration is increased from 1 105 to
4 104 M. This is due to the fact that complexation with
benzoate ion changes the site geometry of Eu3þ ion, making
transition corresponding to 615 nm peak to be more intense.
Fig. 6a shows the ratio of peak areas of 615–592 nm as a function
of BA concentration. The luminescence lifetimes measured for all these solutions are also plotted in Fig. 6b. It can be seen from this
figure that both lifetime and asymmetry ratio increase gradually
with increasing BA concentration and remain nearly constant
at and above BA concentration of 3 104 M. The value of R
increases from 3.7 to 13.2 and lifetime increases from 190
to 900 μs as BA concentration is changed from 1 105 to
4 104 M. The lifetime and asymmetry ratio data obtained from
titration were used in the HypSpec 2009 computation program and
stability constants (log β) were obtained for ML3 type complex.
The values of log β calculated are 12.9470.08 and 13.2070.05
respectively. Averages of these values (13.07) are found to be in
good agreement with the log β value (12.5570.25) obtained by
the UV–vis method (Table 2) and strongly support the formation of
ML3 type complex.
Eu3þ forms 1:1 and 1:2 complexes in aqueous medium with
benzoic acid with stability constant of 1.8470.02 and 2.9270.04,
respectively [28]. In aqueous medium the limited stoichiometry of
Eu3þ–BA up to 1:2 might be due to competition of ligand with
water molecules towards metal ion, whereas in a poor coordinating
solvent such as MeCN, more free ligands are made available
towards Eu3þ ion by less solvating compared to water. Therefore
there is a possibility of forming high stoichiometry complex in
MeCN compared to aqueous medium which hampers the interaction
of metal ion to ligand.