1. Introduction
Trivalent lanthanides are characterized by their luminescent
properties having sharp line like spectra and relatively long
luminescence lifetimes [1–5]. But, due to small absorption coeffi-
cients, lanthanides are poor luminescent species [2–5]. To overcome
this drawback, the technique of ligand sensitized
luminescence is often used [6–13]. It has been established that
in ligand sensitized luminescence firstly ligand absorbs the light
and makes a transition to singlet excited level from where it
decays to long lived triplet level. The energy transfer takes place
via triplet level to the low lying lanthanide emitting levels; as a
result these emitting levels are populated indirectly. Though by
this way lanthanide can be excited efficiently, eventually solvent
plays an important role in deciding the decay of lanthanides
radiatively. Unfortunately, in aqueous medium lanthanide luminescence
is very tender towards the high energy –OH oscillators
[14,15]. The problem of poor quantum yield can be eliminated by
using some synergistic agents such as trioctyl phosphine oxide
(TOPO), organic phosphates etc. In this process water molecules
from the first coordination sphere of lanthanides are replaced by
the synergistic agents and there by quenching caused by –OH
oscillators is reduced [16–18].