Although considerable effort has been dedicated to the controlled synthesis of nanoparticles with classical inorganic structures, there are few reports on the formation of nanoscale materials based on supramolecular compounds such as transition metal coordination polymers. Here we describe the synthesis of crystalline nanoparticles of three different molecule-based magnetic materials, cobalt hexacyanoferrate, cobalt pentacyanonitrosylferrate, and chromium hexacyanochromate, by coprecipitation reactions involving mixtures of water-in-oil microemulsions. The cobalt-containing nanoparticles are regular in shape and size and have dimensions between 12 and 22 nm depending on the concentration of the reactants trapped within the water droplets. At sufficiently high particle concentrations, superlattice structures are formed by solvent evaporation. Growth of the nanoparticles occurs by interdroplet aggregation of primary clusters that are nucleated in the confined spaces of the microemulsion reaction field.

Triadic photoanodes have been prepared based on nanoporous films of the metal oxides ZrO2, TiO2 and SnO2, sensitizer [Ru(bpy)2(dpbpy)] 2+ (P2) and polyoxometalate water oxidation catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] 10-(1), and investigated for their potential utility in water-splitting dye-sensitized photoelectrochemical cells. Transient visible and mid-IR absorption spectroscopic studies were carried out to investigate the charge separation dynamics of these systems, indicating that the electron transfer from photoexcited P2 to TiO2 and SnO2 is still the main excited state quenching pathway in the presence of 1. Furthermore, the accelerated recovery of the P2 ground state bleach in the presence of 1 results from ultrafast (nanosecond) electron transfer from catalyst to oxidized dye. Catalyst loading appears to depend largely on the point of zero charge of the supporting oxide and as such is significantly lower on SnO2 than on TiO2: nonetheless, the rate of recovery of the ground state bleach is similar in both TiO2-P2-1 and SnO2-P2-1 films. Spectral evidence for the formation of long-lived charged separated states is provided by the observation of signals persisting beyond 0.5 μs which are attributed to Stark effect induced change of the P2 spectrum and/or formation of oxidized 1. Photoelectrochemical measurements on TiO2-P2 and TiO2-P2-1 photoanodes under visible light irradiation indicate a ca. 100% photocurrent enhancement in the presence of 1, suggesting light-driven water oxidation by the TiO2-P2-1 system with an internal quantum efficiency of ca. 0.2%. The fast formation and long lifetime of the photo-oxidized catalyst sug-

The effect of thermal fluctuations on the dynamics of a gapped quantum magnet is studied using inelastic neutron scattering on copper nitrate, a model material for the spin-1/2, one-dimensional (1D) bond alternating Heisenberg chain. A large, highly deuterated, single-crystal sample of copper nitrate is produced using a solution growth method and measurements are made using the high-resolution backscattering spectrometer OSIRIS at the ISIS Facility. Theoretical calculations and numerical analysis are combined to interpret the physical origin of the thermal effects observed in the magnetic spectra. The primary observations are (1) a thermally induced central peak due to intraband scattering, which is similar to Villain scattering familiar from soliton systems in 1D, and (2) the one-magnon quasiparticle pole is seen to develop with temperature into an asymmetric continuum of scattering. We relate this asymmetric line broadening to a thermal strongly correlated state caused by hard-core constraints and quasiparticle interactions. These findings are a counter example to recent assertions of the universality of line broadening in 1D systems and are applicable to a broad range of quantum systems.

The viable production of solar fuels requires a visible-light absorbing unit, a H2O (or CO2) reduction catalyst (WRC) and a water oxidation catalyst (WOC) that work in tandem to split water or reduce CO2 with H2O rapidly, selectively and for long periods of time. Most catalysts and photosensitizers developed to date for these triadic systems are oxidatively, thermally and/or hydrolytically unstable. Polyoxometalates (POMs) constitute a huge class of complexes with extensively tunable properties that are oxidatively, thermally and (over wide and adjustable pH ranges) hydrolytically stable. POMs are some of the fastest and most stable WOCs to date. This Microreview updates the very active POM WOC field, reports the first POM WRCs and initial selfassembling metal oxide semiconductor-photosensitizer-POM catalyst triad photoanodes. The complexities of investigating these POM systems, including but not limited to the study of POM-hydrated metal ion-metal oxide speciation processes, are outlined. The achievements and challenges in POM WOC, WRC and triad research are outlined. IntroductionMeasurements and models make it ever more certain that the planet will face a serious energy shortage as the availability of economically accessible fossil fuels fails to keep pace with global energy needs. [1] Data and analysis also indicate that the environmental change caused by fossil fuel combustion will become increasingly problematic. Although green and alternative energy sources are rapidly becoming more available and less expensive, the net consumption of environmentally worrisome fossil fuel is not dropping significantly. Increases in both global population and average global standard of living paint a less-thanrosy picture for our energy future. [1b, 1g, 2] Solar remains the most likely source of sustainable energy for the medium and longer-term future. The other renewable sources of energy, with the arguable exception of biofuels provided the energy production efficiency (photosynthesis and other efficiencies) can be significantly increased, will not likely be sufficient to power the planet. In addition, high density energy will be needed in enormous quantities moving forward; electricity and other sources of energy will not provide sufficient energy density for our major transportation needs (ships, aircraft). Unlike the production of solar electricity, which is a now a rapidly maturing technical area and a major and growing market sector, production of solar fuel is in its infancy.The principal reactions for the generation of solar fuel are H2O splitting to produce H2 and O2 (eq. 1) and H2O splitting coupled to CO2 reduction (eq. 2). Technology is needed so both these processes can be driven by terrestrial sunlight and proceed with high rates and selectivity to the desired products. A factor in the slow rates observed for H2O oxidation by many systems is that it is a four-electron, four-proton process, hence the need for a catalyst that can facilitate the multiple proton-coupled electron transfer (PCET) processes with lo...

We describe a series of nine new complex salts in which electron-rich Ru II or Fe II centers are connected via π-conjugated bridges to six electron-accepting N-methyl-/N-arylpyridinium groups. This work builds upon our previous preliminary studies (Coe, B.

In an effort to develop robust molecular sensitizers for solar fuel production, we examine the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compound in this context. (4a), finding the longest lived charge transfer excited state so far observed in a POM and elucidating the electronic structures and excited state dynamics of these compounds at an unprecedented level. All species exhibit a biexponential decay in which early dynamic processes with time constants in the fs domain yield longer lived excited states which decay with time constants in the ps to ns domain. The initially formed states of 1a and 3a are considered to result from metal-topolyoxometalate charge transfer (MPCT) from Co II to W, while the longer-lived excited state of 1a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1a) is far longer-lived ( = 420 ps in H2O;  = 1700 ps in MeCN) than that of 3a ( = 1.3 ps), where the single Co II atom is located in a pseudo-octahedral addendum site. Short-lived states are observed for the two Co III containing heteropolyanions 2a ( = 4.4 ps) and 4a ( = 6.3 ps) and assigned solely to OCo III charge transfer. The dramatically extended lifetime for 1a vs 3a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.

Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS β values for chromophores with resonance electron donors are significant (up to 139 × 10 esu, ∼5 times greater than that of the DAS cation), but systems with no donor, or the -NO acceptor show no activity, in some cases, despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β/N (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (β/N vs λ) for planar organic systems. However, β values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue, N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms the involvement of the POM in the electronic transitions, whether donor groups are present or not, but Stark spectroscopy indicates that, in their absence, the transitions have little dipolar character (hence, NLO inactive), consistent with DFT-calculated frontier orbitals, which extend over both POM and organic group. Stark and DFT also suggest that β is enhanced in the short compounds because the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. With extended π-systems, this effect does not increase CT distances, relative to a -NO acceptor, so β values do not exceed that of DMNPEA. Overall, our results show that (i) the organoimido-POM unit is an efficient acceptor for second-order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation.

In this article, we present a detailed study of structure−activity relationships in diquaternized 2,2′-bipyridyl (diquat) derivatives. Sixteen new chromophores have been synthesized, with variations in the amino electron donor substituents, π-conjugated bridge, and alkyl diquaternizing unit. Our aim is to combine very large, two-dimensional (2D) quadratic nonlinear optical (NLO) responses with reversible redox chemistry. The chromophores have been characterized as their PF6 − salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Their visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all show two reversible diquat-based reductions. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering with an 800 nm laser, and Stark spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β0. The directly and indirectly derived β values are large and increase with the extent of π-conjugation and electron donor strength. Extending the quaternizing alkyl linkage always increases the ICT energy and decreases the E 1/2 values for diquat reduction, but a compensating increase in the ICT intensity prevents significant decreases in Stark-based β0 responses. Nine single-crystal X-ray structures have also been obtained. Time-dependent density functional theory clarifies the molecular electronic/optical properties, and finite field calculations agree with polarized HRS data in that the NLO responses of the disubstituted species are dominated by ‘off-diagonal’ β zyy components. The most significant findings of these studies are: (i) β0 values as much as 6 times that of the chromophore in the technologically important material (E)-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate; (ii) reversible electrochemistry that offers potential for redox-switching of optical properties over multiple states; (iii) strongly 2D NLO responses that may be exploited for novel practical applications; (iv) a new polar material, suitable for bulk NLO behavior.