ContentsI. Introduction 307 A. Historical Background 307 B. Scope and Limitations 308 II. Photochromism and Electrochromism of Metal Oxides 309 A. Photoinduced Coloration of Metal Oxides Due to Impurity Effects 309 B. Electrochromism 310 1. WO 3 310 2. Other Metal Oxides 310 III. Photochromism of Polyoxometalate Solids 311 A. Alkylammonium Polyoxomolybdates 311 B. Other Polyoxometalates 314 IV. Electrochromism of Polyoxometalates 314 A. H 3 PW 12 O 40 314 B. K 0.33 WO 3.165 and Peroxopolytungstic Acids 314 V. Photoluminescence and Intramolecular Energy Transfer in Polyoxometalate Solids 315 A. Polyoxometalloeuropates 315 1. Structural Feature 315 2. Intramolecular Energy Transfer from the OfM LMCT States to Eu 3+ 316 B. OfM LMCT Triplet Emission 317 VI. Photoinduced Formation of Heteropoly Blues 318 A. Polyoxomolybdates 318 B. Polyoxotungstates 319 C. Polyoxovanadates and Self-Assembly Encapsulation 321 VII. Conclusions 322 VIII. Acknowledgments 323 IX. References 323

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Na9[EuW10O36]·32H2O, obtained from aqueous solutions, crystallizes in space group Cc with a = 11.484(5), b = 23.020(7), c = 23.581(6) Å, β = 91.71(3)°, V = 6231(3) Å3, Mr = 3349.4, Dx = 3.57 Mg m−3, and Z = 4. A full-matrix least-squares refinement finally yields R = 0.070 and Rw = 0.086 for 6473 independent reflections. The local symmetry around Eu3+ for the [EuW10O36]9− anion is approximately square antiprismatic. Photoexcitation into the oxygen-to-tungsten charge transfer bands of a solid sample leads to red emission due to f–f transitions within Eu3+, as a result of an intramolecular energy transfer from the W5O18 group to the Eu atom. The spectral transitions of photoluminescence recorded at 4.2 K can be interpreted based on the assumption of a C4v point group for Eu3+, which is consistent with the co-ordination geometry for the Eu atom.

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.

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In our continuous work on the enhancement of the antibacterial activity of beta-lactam antibiotics against the cells of methicillin-resistant Staphylococcus aureus (MRSA) strains by Keggin-structural polyoxotungstates and their lacunary species, Wells-Dawson, double-Keggin, and Keggin-sandwich polyoxotungstates are also found to be synergistic but highly cytotoxic. The coexistence of polylysine or protamine sulphate decreased the synergistic potency of the polyoxotungstates, due to their electrostatic interaction with negatively charged polyoxotungstates. Inductively coupled plasma atomic emission spectrometry (ICP) analysis of the polyoxotungstate-treated cells indicated that the polyoxotungstates uptaken in the cell are preferentially located at the membrane fraction with intact composition. The polyoxotungstates depressed not only the production of PBP2', but also the production of beta-lactamase which hydrolyzes beta-lactam antibiotics on the membrane. This leads to the synergistic effect of polyoxotungstates against the MRSA cells in the coexistence of beta-lactam antibiotics which have high affinities to PBPs 1-4. MRSA cells which were modified to be susceptible to beta-lactam antibiotics during incubation in the presence of polyoxotungstates recovered their resistance to beta-lactam antibiotics when they were subcultured in the absence of the polyoxotungstate.

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