Polyoxometalates for Molecular Devices: Antitumor Activity and Luminescence

Abstract

For the purpose of constituting a molecular device system based on the polyoxometalates, as well as a nonmolecular metal-oxide-based device system, we have worked energy and electron transfer processes induced by oxygen (O)-to-metal (M) charge transfer (O → M LMCT) band photoexcitation and antitumoral and antiviral activities of certain polyoxometalates. This paper describes both antitu-moral activity of polyoxomolybdates and luminescence involving in the energy transfer processes in the polyoxometalate lattice. [NH3Pri]6[Mo7O24].3H20 has been found to represent a potent antitumor activity in vivo against Meth-A sarcoma, MM-46 adenocarcinoma, and MX-1 human breast, OAT human lung, and CO-4 human colon cancer xenografts. The MovO5(OH) site in the Mo7O24 framework exhibits a strong toxicity in contrast to the d°-configurated MoVIO6 site but the antitutmoral potency of the former is similar to the latter. In conjunction with the fact that Mo7O24]6- interacts with flavin mononucleotide (FMN) to yield a 1:1 complex which gives a redox potential 0.1 V more positive than FMN, the antitumor activity can be explained by the repeated redox cycles of [Mo7O24]6- in tumor cells, which would inhibit the ATP generation coupled with the electron transfer from NADH to coenzyme Q in/on the mitochondria. The intramolecular energy transfer from the O → M LMCT excited states to Eu3+ in the polyoxo-metaloeuropate lattices such as Na9[EuW10O36].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 is investigated with a help of crystal structures. The M-O-M and Eu-O-M bond angels of about 150° provided the hopping of d1 electron among MO6 octahedra and to EuO8 (or EuO9) site with a resultant increase in the recombination between the electron and hole in the lattice. The coordination of aqua ligands to Eu3+ decreases liftime of the 5Do emitting state, due to the vibronic coupling with the vibrational states of high frequency OH oscillators. A dispersion-typed electroluminescence (EL) cell based on [EuW10O36]9- is exemplified. With a.c. excitation to the cell the [EuW10O36]9- layer exhibits EL which matches the photoluminescence spectrum of the solid. Broad luminescence from the O → M LMCT triplet states for polyoxometalates is also found below 100K. The localization of the O → M LMCT triplet excitation energy at the MO6 octahedra results in an increase of the emission yield. The luminescence properties of Anderson-type polyoxometalates of K5.5Hu1.5[SbW6O24].6H2O and Na3H6[CrMO6O24]-8H2O are discussed.