The photoexcitation in the oxygen-to-metal (M = Nb or W) charge
transfer (O→M lmct) bands of two X-ray
crystallographically characterized polyoxometalloeuropates,
Na7H19{[Eu3O(OH)3(H2O)3]2Al2(Nb6O19)5}·47H2O
and
[Eu(H2O)8]3K2H3[(GeTi3W9O37)2O3]·13H2O,
induced Eu3+ emission due to the
5D0→7FJ (J
= 0−4)
transition with a single-exponential decay. In the latter compound
an additional 5D1→7FJ
transition with a
weak intensity (with the relative intensity of about 9% of the
5D0→7FJ emission at
4.2 K) was observed. No
observation of the
5D1→7FJ emission in
the former compound was ascribed to the
5D1−5D0
cross-relaxation:
Eu3+(5D1) +
Eu3+(7F0) →
Eu3+(5D0) +
Eu3+(7F3). Both
nonradiative deactivation of the 5D0 state
and
intramolecular energy transfer from the O→M lmct states to
Eu3+ are discussed together with the
luminescence
properties of four other structurally characterized
polyoxotungsto(or molybdo)europates,
Na9[Eu(W5O18)2]·32H2O,
K15H3[Eu3(H2O)3(SbW9O33)(W5O18)3]·25.5H2O,
[NH4]12H2[Eu4(H2O)16(MoO4)(Mo7O24)4]·13H2O,
and
[Eu2(H2O)12][Mo8O27]·6H2O,
on the basis of differences in the number of aqua and hydroxo ligands
at the
first coordination sphere of the Eu3+ site, the
Eu···Eu distance in the molecule, and the structure of
the
polyoxometalate ligands among six compounds. A plot of the
deviation of the reciprocal 5D0 lifetime
from
that of
Na9[Eu(W5O18)2]·32H2O
containing an anhydrous Eu3+ site versus total number of
aqua and hydroxo
ligands coordinating Eu3+ indicates a good linearity
irrespective of the coordination geometry, if the mean
distance between Eu and aqua or hydroxo oxygen atoms is less than 2.5
Å. The kinetic analysis of the
luminescence reveals that the highly symmetrical polyoxometalate ligand
favors the effective nonradiative
deactivation of the O→M lmct excitation energy due to a small
disparity between the O→M lmct excited and
ground states and that the energy transfer into the
5D0 and 5D1 states in
the polyoxometaloeuropates occurs
via the O→M lmct triplet states.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.