CHARGE-TRANSFER PHOTOLUMINESCENCE OF POLYOXO-TUNGSTATES AND POLYOXO-MOLYBDATES

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 1 00 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 0 --> 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 0 --> 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 luminescence of the Anderson-type polyoxometalate K5.5H1.5[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 SbvO6 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.