Charge-transfer photoluminescence of polyoxo-tungstates and -molybdates

Abstract

Luminescence from the oxygen-to-metal (O → M) ligand-to-metal charge-transfer (l.m.c.t.) triplet states for polyoxometalates of tungsten and molybdenum was observed below 100 K. The Stokes shift was large and the time profile of the emission decay approximated two exponential decays at all temperatures. The emission yield for the edge-sharing MO₆ octahedral polyoxometalates is higher than for the corner-sharing ones, due to the localization of the O → M l.m.c.t. triplet excitation energy at the MO₆ octahedra in the lattice. The effect of a hydrogen-bonded network in the lattice on the non-radiative deactivation of the O → M l.m.c.t. excited states is discussed relative to the crystal structures and explained in terms of the increase in the relaxation of the excited states due to the electrostatic dipole–dipole interaction at the luminescent MO₆ octahedron. The intense luminescenc of the Anderson-type polyoxometalate K_5.5H_1.5[SbW₆O₂₄]·6H₂O can be attributed to four factors: the edge-sharing WO₆-SbO₆ octahedral lattice, a large spin-orbit coupling of the tungsten atom, a small contribution by the hydrogen-bond dipole–dipole interaction and the lack of intramolecular energy transfer into the central Sb^vO₆ site. Intramolecular energy transfer of the triplet energy occurred for Na₃H₆[CrMo₆O₂₄]·8H₂O, where the only luminescence is red emission due to the chromium(III) R line.