3.2.3. Composition of the complex
The lifetime (τ) can be correlated with number of water
molecules (NH2O) present in inner sphere of Eu3þ ion by the
following formula [2]:
NH2Oð70:5Þ ¼ 1:05=τ0:70 ð1Þ
The lifetime in Eu3þ–BA complex in MeCN is 900 μs and using
the above formula, number of water molecules coordinated to
Eu3þ in the above complex is calculated to be almost zero. It
indicates expulsion of all water molecules from the inner coordination
sphere of Eu3þ. Therefore, assuming nine coordination
number of Eu3þ in ML3 complex, six MeCN molecules are
coordinated to Eu3þ ion and hence composition of the formed
complex is proposed as Eu(BA)3(CH3CN)6.
As 1% water was present in our experimental solutions, a
preferential solvation of Eu3þ ion by water molecules over MeCN
can be expected as MeCN is a poor coordinating medium. Therefore
in ML3 type complex as suggested by UV–vis and luminescence
spectroscopy in MeCN, six water molecules can be expected in the
inner sphere of Eu3þ. According to above formalism (Eq. 1) lifetime of Eu3þ in Eu3þ–BA would be nearly 150 ms. It is surprising to see
the luminescence lifetime of 900 ms in the ML3 type complex of
Eu3þ–BA. It indicates that water molecules in the inner sphere of
Eu3þ–BA are getting replaced as more and more benzoate ions are
introduced into the system. It appears that apart from complexation,
benzoate ions make water to be removed from the inner
sphere of Eu3þ–BA and make use of these eliminated water
molecules for the solvation of excess benzoate ions. Since water is
more polar than MeCN, strong solvation of benzoate ions can be
expected by water molecule than MeCN. Thus it can be summarized
that though Eu3þ has a strong affinity towards water molecules, a
strong interaction with benzoate ion forces water molecules to
come out from inner sphere and a less polar solvent, acetonirile,
occupies the position in the inner coordinated sphere of Eu3þ.