(Figs. 1b and 2b) are shown in the same figure. The luminescence
peak positions in Fig. 2a and c are similar, indicating that the
features at 592 and 615 nm are due to the transitions 5
D0–7
F1 and 5
D0–7
F2 respectively. Despite the fact that emission spectra shown
in Fig. 2a and c have similar spectral characteristics (except for
relative intensities of the two peaks, which will be discussed later),
the excitation spectrum for Eu3þ–BA (Fig. 1c) system is entirely
different from that observed for the uncomplexed Eu3þ (Fig. 1a),
pointing out to different absorbing species in these two experiments.
The excitation spectrum shown in Fig. 1c observed for
Eu3þ–BA in MeCN resembles that of Eu3þ–BA in aqueous medium
(Fig. 1b). In earlier luminescence studies [9], it has been shown
that in the aqueous Eu3þ–BA system, the benzoate is the absorber,
which then sensitizes the luminescence of Eu3þ. Therefore it
indicates that in MeCN too, benzoate sensitizes Eu3þ luminescence.
Although the concentration of Eu3þ in Eu3þ–BA complexes
in aqueous and MeCN medium (Fig. 2b and c) is the same, the
large luminescence intensity for later, manifesting that there is a
huge enhancement in the luminescence of Eu3þ–BA in MeCN
compared to the same in aqueous medium.
Table 1 presents the luminescence enhancement of Eu3þ in its
complexes. Enhancement factor (EF), presented in Table 1, was
calculated as the ratio of Eu3þ concentration that gave the same
luminescence intensity in uncomplexed Eu3þ and Eu3þ–BA complex.
The luminescence intensity of the stronger peaks between 592
and 615 nm was used in calculating EF [25]. For example, the EF of
Eu3þ–BA in aqueous in Table 1 is indicated to be 100, which implies
that the Eu3þ concentration in the Eu3þ–BA in aqueous medium
was 100 times smaller than that of uncomplexed Eu3þ, and yet
produced the same Eu3þ luminescence intensity as in uncomplexed.
The enhancement factor for Eu3þ–BA in MeCN medium is 32,000
which is more than two orders high compared to enhancement factor for Eu3þ–BA in water (1 0 0). This huge increase in the
luminescent enhancement cannot be explained just based on the
reduction of non-radiative decay process because MeCN is a poor
coordinating solvent and hence cannot replace the water from the
inner coordination sphere. Hence the enhancement in luminescent
intensity should be due to ligand sensitization only.
The measurement of luminescence lifetimes of Eu3þ ions in its
complexes can serve to determine the extent of non-radiative
processes in these complexes and hence lifetime measurements
were carried out in acetonitrile medium. Table 1 presents the
lifetimes of Eu3þ luminescence in various experiments. The lifetimes
have been measured corresponding to the 5
D0–7
F2 transition
of Eu3þ emission. The luminescence lifetimes of uncomplexed
Eu3þ in water and MeCN are closer, namely 110 and 120 ms,
respectively; which indicate that being a poor coordinating solvent,
MeCN hardly replace water molecules from the coordination
sphere of Eu3þ. The lifetime of Eu3þ–BA in water is 118 ms, which
is comparable to the lifetime of 110 ms for uncomplexed Eu3þ in
aqueous medium. Even though the benzoate complex in water
yields significant enhancement of the Eu3þ luminescence, only a
marginal increase in lifetime upon complexation is observed. The
luminescence lifetime of Eu3þ in the Eu3þ–BA in MeCN medium
was, on the contrary, considerably longer, 900 μs, clearly indicating
strong reduction in the rates of non-radiative processes in the
MeCN medium. The reduction in nonradiative decay channels is
due to removal of water molecules from the inner sphere of Eu3þ
ion, upon complexation