Orbital Engineering: Photoactivation of an Organofunctionalized Polyoxotungstate

Abstract

Tungsten-based polyoxometalates( POMs) have been employeda sU V-driven photo-catalysts for ar ange of organic transformations. Their photoactivityi sd ependent on electronic transitionsb etween frontier orbitals and thus manipulation of orbital energy levels provides ap romising meanso fe xtending their utility into the visible regime. Herein, an organic-inorganic hybrid polyoxometalate, K 6 [P 2 W 17 O 57 (PO 5 H 5 C 7 ) 2 ]·6C 4 H 9 NO, was foundt o exhibite nhanced redox behaviour andp hotochemistry compared to its purely inorganicc ounterparts. Hybridization with electron-withdrawingm oieties was shownt o tune the frontier orbitale nergy levels and reduce the HOMO-LUMO gap, leading to direct visible-lightp hotoactivationof the hybrid and establishing as imple, cheap and effective approacht ot he generation of visible-lightactivated hybrid nanomaterials.Systemst hat harness energy from visible light are among the most prized targets in modern materials design.[1] Ap romising group of photoactive materials, polyoxometalates (POMs), are discrete early transition-metal-oxide clusters that exhibite xcellent stability,s olubility in both aqueous and organic mediaa nd aw ide range of potentialc ompositions and structures.[2] These nanoscopic clusters are characterized in large part by their rich redox chemistry,l eadingt op otentiala pplicationsi nm ulti-electron transfer and charge-storaget echnologies.[3] Most importantly,t heir capacity to form highly active photo-excited states (oxo-centred radicals) upon excitation of the O!Ml igand-tometal charge transfer (LMCT) band [4] has led to sustainedi nterest in their use as both heterogeneous and homogeneous photocatalysts for ar ange of transformations.[5] In particular, tungstate-based POMs have been successfully employed in ar ange of UV-light-driven photooxidationr eactions. [4a, 6] In their native state, POMs typically exhibit negligible visible light absorption, with the broad LMCT band located almostentirely within in the UV region of the spectrum.P OMs have thus had limited application in visible-light-driven catalysis and while the absorption profiles of molybdate-and vanadatebased POMs often tail into the near-visible/blue range, [4, 7] they do not generally exhibit the stability or easeo ff unctionalization seen in tungstate-baseda nalogues.Mizuno and co-workersr ecently reported two examples of polyoxotungstate photoactivation in systemsb ased on the lacunary POM [g-SiW 10 O 36 ] 8À ,i nw hich charget ransferf rom secondary components (Ce III or bound substrates) facilitated photo-reduction of the POMcorea nd allowedo xidative catalysis on the appended moieties.[8] Another approach to the visible-lighta ctivation of POMs is through functionalization with (typically, preciousm etal-based) visible-light-activec hromophores or 'photosensitizers' (PS), throughe ither supramolecular or covalenta pproaches.[9] Subsequentp hoto-excitation can lead to PS!POMc harge transfer.[10] Indeed, to the best of our knowledge,a ll previous reports of polyoxot...

Redox-active organic–inorganic hybrid polyoxometalate micelles

“…400 mV towards more positive potentials compared to the equivalent process in {P2W18}. With regard to our previous findings, 26 and those of others, 27 this can be attributed to the electron-withdrawing effects of the phosphonate groups. Consequently, the LUMO is energetically lowered and the first redox process is shifted to a more positive potential.…”

“…26 To expand on this new concept, and probe the potential to translate the redox properties of molecular building units to supramolecular assemblies, we developed a redox-active hybrid POM with long aliphatic chains, and studied its solution state self-assembly behaviour and physicochemical properties ( Figure 1). A Wells-Dawson-type-polyoxotungstate was selected as the inorganic surfactant head group due to its rich redoxchemistry and stability.…”

“…26 The chemical composition of H61 was confirmed by elemental analysis (EA and ICP), ESI-MS, 1 H-and 31 P-NMR, FTIR and TGA (see SI).…”

“…These two bands are characteristic POM-based intervalence charge transfer (IVCT) bands that correspond to the two-electron-reduced species ( Figure 2D). 26 To probe the electrochemical stability of the micellar structures, absorption spectra were collected for the fresh (oxidised) sample in the absence of an applied potential, and spectroelectrochemical analysis (to 0.61 V vs. NHE) was performed to monitor the reoxidation of the electrochemically reduced sample. The fresh sample showed no peaks in the range of 600 to 900nm, while those visible in the reduced sample rapidly disappeared as the POMs were re-oxidised.…”

Polyoxometalates: Tailoring metal oxides in molecular dimension toward energy applications

“…Thermal treatment of [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10− leads to the formation of CoWO 4 ‐based oxygen‐evolving catalyst with enhanced performance ascribed to the existence of oxygen vacancies . Orbital engineering of K 6 [P 2 W 17 O 57 (PO 5 H 5 C 7 ) 2 ]·6C 4 H 9 NO is performed to enhance the photocatalytic properties . The interaction between W 12 O 40 ‐based polyoxotungstate and TiO 2 is studied, suggesting the electron transfer from TiO 2 to the polyoxometalate leads to the efficient redox process for photocatalytic properties, and the electron transfer efficiencies follow the order: H 2 W 12 O 40 6− < SiW 12 O 40 4− < PW 12 O 40 3− .…”