BTD-4

Two new porphyrin-polyoxometalate hybrids, namely, [(C4H9)4N]10H2[{COHNC(CH2O)3P2V3W15O59}2C44N4H28]·CH3CN 1, bearing two covalent-bonded Wells-Dawson-type polyoxometalates (POMs), and [(C4H9)4N)]5H[COHNC(CH2O)3P2V3W15O59{C44H29N4}]·CH3CN 2, bearing one covalent-bonded POM, have been synthesized and thoroughly characterized by means of elemental analysis, powder XRD, FT-IR, 1H (31P, 51V) NMR, MALDI-TOF-MS, UV-vis spectra, and cyclic voltammetry measurement. Experimental results demonstrate that while all the compounds show remarkable third-order optical nonlinearities, the hybrids 1 and 2 are superior to their corresponding porphyrin precursors (molecular second hyperpolarizability γ = 8.0 × 10-28 esu for 54-N-N'(1,3-tetrahydroxy-2-(dihydroxymethyl)propan-4-diyl)benz-diamide,10,15,20-triphenyl porphyrin that is the precursor for the hybrid 1, γ = 2.6 × 10-28 esu for 54-N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)benz-amide,10,15,20-triphenyl porphyrin that is the precursor for the hybrid 2), and the hybrid 1 (γ = 12.9 × 10-28 esu) is superior to the hybrid 2 (γ = 12.2 × 10-28 esu) reflecting more POM moieties covalently bonded to the porphyrin moiety exerting more significant influence on the third-order optical nonlinearities. Meanwhile, attachment of POMs on the porphyrin results in significant fluorescence quenching (fluorescence intensity is decreased 97% for the hybrid 1 and 80% for the hybrid 2 with respect to that of their corresponding porphyrin precursors) indicating strong electron transfer from porphyrin moiety to the polyoxometalate moiety. Lower transition energy, small energy difference between singlet and triplet excited states, and faster intersystem crossing (ISC) process of the hybrids are favorable to enhance the NLO responses of hybrids 1 and 2 resulting from the facile electron transfer from the porphyrin moiety to the Dawson POM moiety when the hybrids are subjected to laser irradiation, which is thought to be responsible to the superior of the hybrid 1 to hybrid 2 and the superior of the hybrids to their corresponding porphyrin precursors as well.

Polyoxometalate (POM)-based organic-inorganic hybrid materials possess versatile properties and applications; however, the ratios of organic cations to POM anions still remain to be solved. In this work, 14 POM-based organic-inorganic hybrid materials were synthesized by the precipitation, hydrothermal, and solvent-evaporation methods. These hybrid materials consisted of a wide range of quaternary ammonium and imidazolium cations with different alkyl chains and different Keggin-type heteropolyanions [i.e., phosphotungstic ([PW12O40]3-), phosphomolybdic ([PMo12O40]3-), silicotungstic ([SiW12O40]4-), and silicomolybdic ([SiMo12O40]4-) anions]. Their compositions and structures were characterized complementarily by elemental analysis, powder X-ray diffraction, single-crystal X-ray diffraction, and Fourier transform infrared spectroscopy. The actual ratios of organic cations to heteropolyanions of [PW12O40]3-, [PMo12O40]3-, [SiW12O40]4-, and [SiMo12O40]4- were found to always be 3:1, 3:1, 4:1, and 4:1, respectively, independent of the organic cations, synthesis methods, and reaction parameters. This finding demonstrates that the organic cations completely substituted the protons of the heteropolyacid precursors in the hybrid materials, which thus hardly possessed Brønsted acidity probed by the pyridine adsorption and cellulose hydrolysis reaction. Such complete substitution of the protons arose apparently from the strong noncovalent interactions between the organic cations and heteropolyanions (such as electrostatic and C-H···O interactions) in the POM-based hybrid materials.

Herein, we report on a three-component supramolecular hybrid system built from specific recognition processes involving a Dawson-type polyoxometalate (POM), [P2W18O62]6-, a cationic electron-rich cluster [Ta6Br12(H2O)6]2+, and γ-cyclodextrin (γ-CD). Such materials have been investigated using a bottom-up approach by studying the specific interactions between γ-CD and both types of inorganic units. Their ability to interact has been investigated in the solid state by single-crystal X-ray diffraction (XRD) and in solution using multinuclear NMR methods (including DOSY, EXSY, and COSY), electrospray ionization mass and UV-vis spectroscopies, electrochemistry, and isothermal titration calorimetry experiments. Single-crystal XRD analysis reveals that POM:γ-CD constitutes a highly versatile system which gives aggregates with 1:1, 1:2, and 1:3 stoichiometry. Surprisingly, these arrangements exhibit a common feature wherein the γ-CD moiety interacts with the Dawson-type POMs through its primary face. We present also the first structural model involving an octahedral-type metallic cluster with γ-CD. XRD study reveals that the cationic [Ta6Br12(H2O)6]2+ ion is closely embedded within two γ-CD units to give a supramolecular ditopic cation, suitable to be used as a linker within extended structure. Solution study demonstrates clearly that pre-associations exist in solution, for which binding constants and thermodynamic parameters have been determined, giving preliminary arguments about the chaotropic nature of the inorganic ions. Finally, both building blocks, i.e., the ditopic supramolecular cation {[Ta6Br12(H2O)6]@2CD}2+ and the Dawson-type anion, react together to give a three-component, well-ordered hybrid material derived either as a supramolecular hydrogel or single crystals. The solid-state structure shows an unprecedented helicoidal tubular chain resulting from the periodic alternation of POM and supramolecular cation, featuring short hydrogen-bonding contacts between the electron-poor POM and electron-rich cluster. The 1D tubular ionic polymer observed in the single crystals should make it possible to understand the long-range ordering observed within the hydrogel hybrid material. The supramolecular chemical complementarities between the γ-CD-based ditopic cation and POM open a wide scope for the design of hybrid materials that accumulate synergistic functionalities.