Supramolecular metal-organic frameworks that display high homogeneous and heterogeneous photocatalytic activity for H2 production

Abstract

Self-assembly has a unique presence when it comes to creating complicated, ordered supramolecular architectures from simple components under mild conditions. Here, we describe a self-assembly strategy for the generation of the first homogeneous supramolecular metal-organic framework (SMOF-1) in water at room temperature from a hexaarmed [Ru(bpy)3](2+)-based precursor and cucurbit[8]uril (CB[8]). The solution-phase periodicity of this cubic transition metal-cored supramolecular organic framework (MSOF) is confirmed by small-angle X-ray scattering and diffraction experiments, which, as supported by TEM imaging, is commensurate with the periodicity in the solid state. We further demonstrate that SMOF-1 adsorbs anionic Wells-Dawson-type polyoxometalates (WD-POMs) in a one-cage-one-guest manner to give WD-POM@SMOF-1 hybrid assemblies. Upon visible-light (500 nm) irradiation, such hybrids enable fast multi-electron injection from photosensitive [Ru(bpy)3](2+) units to redox-active WD-POM units, leading to efficient hydrogen production in aqueous media and in organic media. The demonstrated strategy opens the door for the development of new classes of liquid-phase and solid-phase ordered porous materials.

Figure 1. Compounds used in this study.

The structures of compounds 1–3 and CB[8].

Figure 2. Self-assembly of 3D cubic SMOF-1 and WD-POM loading.

Formation of SMOF-1 and WD-POM@SMOF-1.The space-filling structural models were obtained using Materials Studio 7.0. H, white; C, light grey; N, blue; O, red; Ru, cyan; WD-POM ([P₂W₁₈O₆₂]⁶⁻), purple polyhedron.

Figure 3. SAXS and X-ray diffraction profiles of 3D SMOF-1.

(a) Solution-phase synchrotron SAXS ([1]=3.0 mM) in water. a.u., arbitrary unit. (b) Solution-phase synchrotron X-ray diffraction ([1]=3.0 mM) in water. (c) Solid-phase SAXS. (d) Solid-phase X-ray diffraction. (e) Solution-phase synchrotron SAXS of the aqueous solution of WD-POM-encapsulated SMOF-1. [1]=3.0 mM, [WD-POM]=0.2 mM. (f) Solid-phase X-ray diffraction of WD-POM@SMOF-1. The sample was obtained by slow evaporation of the aqueous solution. [1]=3.0 mM, [WD-POM]=0.2 mM. (g) 2D solid-phase synchrotron X-ray scattering of SMOF-1. The peak values in a–f were attributed by choosing the position that was highest above the straight line defined by the two saddle points of the broad peak.

Figure 4. TEM images of the solid samples.

(a) TEM images of SMOF-1. Scale bar, 10 μm. Inset: SAED pattern showing the reciprocal lattice observed for the {100} facet, which showed foursquare order. Scale bar, 2 nm. (b,c) High-resolution cryo-TEM images of SMOF-1 from different facets showing different lattice spacings. (b) Scale bar, 20 nm. (c) Scale bar, 10 nm. (d) HR-TEM image of solid WD-POM@SMOF-1 from the {100} direction. The sample was obtained by slowly evaporating the aqueous solution ([1]=0.3 mM, [WD-POM]/[1]=0.067). Scale bar, 100 nm. (e) High-resolution TEM image of solid WD-POM@SMOF-1. Scale bar, 20 nm. (f) HAADF-STEM image of solid WD-POM@SMOF-1. Scale bar, 20 nm.

Figure 5. Fluorescence quenching and photo-driven hydrogen production.

(a) Normalized fluorescence (λ=640 nm, λ_ex=500 nm) quenching of SMOF-1 ([1]=0.02 mM) by WD-POM in water. Inset: quenched fluorescence spectra. [−]/[+]=0–1.74. [−] and [+] represent the total charge molar amount. (b) TONs of the solution of WD-POM@SMOF-1 ([1]=0.3 mM]) after irradiating for 12 h. [WD-POM]/[1]=0.0067–2. (c) TONs of the solution of WD-POM@SMOF-1 and H₂ production amount after irradiating for 7 h. [1]/[WD-POM]=15, [1]=0.03 to 6 mM. (d) Time-dependent TON and TOF of the solution of WD-POM@SMOF-1. [1]=0.3 mM, [WD-POM]=0.02 mM. Methanol was used as the sacrificial electron donor.