Intramolecular energy transfer in polyoxometaloeuropate lattices and their application to a.c. electroluminescence device

Abstract

Crystal Structures of four polyoxometaloeuropates, Na 9[EuW 10O 36].32H 2O (1), K 15H 3[Eu 3(H 2O) 3(SbW 9O 33)(W 5O 18) 3].25.5H 2O(2), [NH 4] 12H 2[Eu 4 (H 2O) 16 (MoO 4)(Mo 7O 24) 4].13H 2O(3),and Eu 2(H 2O) 12[Mo 8O 27].6H 2O (4) are investigated to understand the relaxation process of the oxygen-to-metal (O→M) charge-transfer excitation energy in the polyoxometalate lattice. The temperature dependence of the intramolecular energy transfer from the O→M LMCT states to Eu 3+ in the polyoxometaloeuropate lattices indicates that the M-O-M and Eu-O-M bond angles of about 150° allow the hopping of d 1 electron among MO 6 octahedra and to EuO 8 (or EuO 9) site to be the deactivation of the O→M LMCT state due to the deactivated recombination between the electron and hole in the lattice. Co-ordination of aqua ligands to Eu 3+ leads to a decrease in lifetime of the emitting state of 5d 0, due to the vibronic coupling of the 5d 0 state with the vibrational states of high frequency OH oscillators. Dispersion-typed electroluminescence (EL) cell is constructed with a highly photoluminescent [EuW 10O 36] 9− system. With a.c. excitation to the divice consisting of the doublet structure of emissive [EuW 10O 36] 9− and insulating Mylar film layers, the [EuW 10O 36] 9− layer exhibits EL which matches the photoluminescence spectrum of the solid. A multicolor display in combination with other polyoxometalolanthanoates is possible.