Shao-Yun Yin

Ultrafast water sensing and thermal imaging by a metal-organic framework with switchable luminescence

A convenient, fast and selective water analysis method is highly desirable in industrial and detection processes. Here a robust microporous Zn-MOF (metal–organic framework, Zn(hpi2cf)(DMF)(H2O)) is assembled from a dual-emissive H2hpi2cf (5-(2-(5-fluoro-2-hydroxyphenyl)-4,5-bis(4-fluorophenyl)-1H-imidazol-1-yl)isophthalic acid) ligand that exhibits characteristic excited state intramolecular proton transfer (ESIPT). This Zn-MOF contains amphipathic micropores (<3 Å) and undergoes extremely facile single-crystal-to-single-crystal transformation driven by reversible removal/uptake of coordinating water molecules simply stimulated by dry gas blowing or gentle heating at 70 °C, manifesting an excellent example of dynamic reversible coordination behaviour. The interconversion between the hydrated and dehydrated phases can turn the ligand ESIPT process on or off, resulting in sensitive two-colour photoluminescence switching over cycles. Therefore, this Zn-MOF represents an excellent PL water-sensing material, showing a fast (on the order of seconds) and highly selective response to water on a molecular level. Furthermore, paper or in situ grown ZnO-based sensing films have been fabricated and applied in humidity sensing (RH<1%), detection of traces of water (<0.05% v/v) in various organic solvents, thermal imaging and as a thermometer.

A naked eye colorimetric sensor for alcohol vapor discrimination and amplified spontaneous emission (ASE) from a highly fluorescent excited-state intramolecular proton transfer (ESIPT) molecule

A highly fluorescent HPI-based excited-state intramolecular proton transfer (ESIPT) molecule is designed and adopted as a naked-eye colorimetric sensor to distinguish methanol, ethanol and isopropanol vapors. Amplified spontaneous emission was also observed for the C1-form single crystal of the molecule attributed to its intrinsic four-level energy states.

Photoluminescence and white-light emission in two series of heteronuclear Pb(II)–Ln(III) complexes

Two series of Pb(II)–Ln(III) heteronuclear coordination complexes are assembled from a tripodal ligand triCB-NTB ((4,4′,4′′-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzoic acid)). In the Pb2LnL2 series, the Ln3+ ion is encapsulated in a highly symmetrical {LnO6} octahedron with an inversion center, and Pb–Ln–Pb clusters are formed by the linkage of carboxyl groups on triCB-NTB ligands to Pb2+ and Ln3+ simultaneously. However, in the PbLn2L2 series, the Ln3+ ion is encapsulated in a distorted {LnO9} polyhedron without an inversion center. Pb2+ ions are coordinated with benzimidazole and apical N atoms on the ligand in isolation, and the carboxyl groups link two Ln3+ ions into a Ln–Ln cluster. This structural variation leads to different photoluminescence behaviour in these two series of complexes. Most importantly, the linkage of the Pb–Ln–Pb clusters causes more perturbation to the excited states of the ligand. Therefore, a more obvious ligand-to-metal charge transfer (LMCT) process is observed in the Pb2LnL2 series, and the energy transfer to the accepting levels of Ln3+ ions becomes more efficient. Furthermore, the combination of LC (ligand-centered) + LMCT + MC (metal-centered) emissions in the Pb2EuL2 complex results in single component white light emission.

Semidirected versus holodirected coordination and single-component white light luminescence in Pb(II) complexes

2-Methyl-8-hydroxyquinoline (HMq) and the tripodal ligands 4,4′,4′′-(2,2′,2′′-nitrilotris (methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzonitrile (triBZ-NTB) and 4,4′,4′′-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzoic acid (H3triCB-NTB) were used individually to assemble a hetero-nuclear, a tetra-nuclear and two mono-nuclear Pb(II) complexes. The Pb(II) coordination centers in two of these complexes were observed to display semidirected coordination with the ligands and counter anions (small solvent molecules), whereas the other two complexes showed holodirected coordination, together leading to varied coordination geometries. The combination of ligand-to-metal charge-transfer (LMCT) and metal-centered (MC) emissions in the semidirected Pb(II) complexes resulted in single-component white light luminescence.

PMMA-copolymerized color tunable and pure white-light emitting Eu3+–Tb3+ containing Ln-metallopolymers

By anchoring lanthanide coordination monomers onto a PMMA polymer backbone via a pre-designed vinylbenzyl-substituted NTB-type ligand, we succeeded in the fabrication of homo or hetero Ln-metallopolymers that can emit tunable and designable multicoloured photoluminescence as well as white light emission. Copolymerization results in the fine-tuning of the energy state of the ligand in the metallopolymer compared with that in the coordination monomer, thereby changing the energy transfer efficiency to Eu3+ and Tb3+ centers in opposite ways. And further designing of hetero-lanthanide metallopolymers leads to additional energy transfer between Eu3+ and Tb3+, facilitating the generation of pure white light emission. In general, the facile and controllable synthesis procedure, versatile and reproducible combination of lanthanides, ligands and polymer backbones, and relatively high white light emitting luminous efficiency (>5%) prove this method to be promising in future lighting WPLED applications.

Binuclear Ru–Ru and Ir–Ru complexes for deep red emission and photocatalytic water reduction

Four butterfly-like binuclear Ru(II)–Ru(II) and Ir(III)–Ru(II) complexes were designed and synthesized via a stepwise method by Ru(II)/Ir(III) metalloligands containing polypyridine (bpy)/phenylpyridine (ppy), phenanthroline (phen) and bibenzimidazole (BiBzIm) moieties. The absorption and photoluminescence of Ru(II)–Ru(II) compounds are dominated by metal-to-ligand charge-transfer (MLCT) transitions from Ru(II) centers to the organic ligand parts, which emit in the deep red region with a wavelength ∼700 nm. While in Ir(III)–Ru(II) complexes, an additional decay channel is opened for the energy transfer from the higher energy level MLCT state of Ir(III)-coordinated units to the lower-energy level MLCT state of Ru(II)-coordinated units, as approved by both experimental and theoretical DFT calculations. Therefore, similar deep red emission profiles originating from Ru(II) units are observed in Ir(III)–Ru(II) systems. These binuclear complexes were further tested as photosensitizers (PSs) to produce H2 in photocatalytic water reduction systems. The highest H2 production efficiency can be obtained in the heteronuclear IrRu(1) system after 80 hours continuous production with a TON value of 1088 based on the amount of IrRu(1) as PS, much higher than the other binunclear complexes and mononuclear counterparts. The results provide a new insight into the designing guidelines for noble metal complexes as emitting centers and photosensitizers in lighting/display materials and devices, as well as photocatalytic water splitting systems.

Post-synthetic exchange (PSE) of UiO-67 frameworks with Ru/Rh half-sandwich units for visible-light-driven H2 evolution and CO2 reduction

Electron-deficient Ru/Rh half-sandwich units are optoelectronical, stereochemical, and redox and catalytically active in nature. Four different species of these units were designed and successfully incorporated into UiO-67 frameworks by post-synthetic exchange (PSE) method and employed for photocatalytic H2 evolution and CO2 reduction. Among them, the PSE-MOF catalysts containing [M–OH2] (M = Rh or Ru) groups show higher photocatalytic activity than those with [M–Cl], providing a new example of the exploitation of H2O molecules as small ancillary ligands for the construction of Ru/Rh half-sandwich units and subsequent incorporation into MOFs as photocatalytic centers. The application of TEOA (triethanolamine) and DMA (N,N-dimethylaniline) as sacrificial agents leads to different catalytic mechanisms and durations. A long-term hydrogen evolution lasting for 174 h without significant decrease in efficiency was achieved in the RhOH2@UiO-DMA system, showing the advantage of PSE-MOFs as highly stable photocatalytic platforms with superior self-protective properties.

ESIPT-Modulated Emission of Lanthanide Complexes: Different Energy-Transfer Pathways and Multiple Responses

Two series of isostructural lanthanide coordination complexes, namely, LIFM-42(Ln) (Ln=Eu, Tb, Gd, in which LIFM stands for the Lehn Institute of Functional Materials) and LIFM-43(Ln) (Ln=Er, Yb), were synthesized through the self-assembly of an excited-state intramolecular proton transfer (ESIPT) ligand, 5-[2-(5-fluoro-2-hydroxyphenyl)-4,5-bis(4-fluorophenyl)-1H-imidazol-1-yl]isophthalic acid (H2 hpi2cf), with different lanthanide ions. In the coordination structures linked by the ligands and oxo-bridged LnIII2 clusters (for LIFM-42(Ln) series) or isolated LnIII ions (for LIFM-43(Ln) series), the ESIPT ligand can serve as both the host and antenna for protecting and sensitizing the photoluminescence (PL) of LnIII ions. Meanwhile, the -OH⋅⋅⋅N active sites on the ligands are vacant, which provides availability to systematically explore the PL behavior of Ln complexes with ESIPT interference. Based on the accepting levels of different lanthanide ions, energy transfer can occur from the T1 (K*) or T1 (E*) (K*=excited keto form, E*=excited enol form) excited states of the ligand. Furthermore, the sensitized lanthanide luminescence in both visible and near-infrared regions, as well as the remaining K* emission of the ligand, can be modulated by the ESIPT responsiveness to different solvents, anions, and temperature.

Semiconductive Amine-Functionalized Co(II)-MOF for Visible-Light-Driven Hydrogen Evolution and CO2 Reduction

A Co-MOF, [Co3(HL)2·4DMF·4H2O] was simply synthesized through a one-pot solvothermal method. With the semiconductor nature, its band gap was determined to be 2.95 eV by the Kubelka-Munk method. It is the first trinuclear Co-MOF employed for photocatalytic hydrogen evolution and CO2 reduction with cobalt-oxygen clusters as catalytic nodes. Hydrogen evolution experiments indicated the activity was related to the photosensitizer, TEOA, solvents, and size of catalyst. After optimization, the best activity of H2 production was 1102 μmol/(g h) when catalyst was ground and then soaked in photosensitizer solution before photoreaction. To display the integrated design of Co-MOF, we used no additional photosensitizer and cocatalyst in the CO2 reduction system. When -NH2 was used for light absorption and a Co-O cluster was used as catalyst, Co-MOF exhibited an activity of 456.0 μmol/(g h). The photocatalytic mechanisms for hydrogen evolution and CO2 reduction were also proposed.

Single-Phase White-Light-Emitting and Photoluminescent Color-Tuning Coordination Assemblies

Metal-organic complexes assembled from coordinative interactions are known to be able to display a wide range of photoluminescent behaviors benefiting from an extensive number of metal ions, organic linkers, and inclusion guests, depending on the multifaceted nature of their chemical structures and photophysical properties. In the past two decades, the white-light-emitting (WLE) and photoluminescent color-tuning (PLCT) materials based on the single-phase metal-organic coordination assemblies have merited particular attention and gained substantial advances. In this review, we give an overview of recent progress in this field, placing emphasis on the WLE and PLCT properties realized in the single-phase materials, which covers the origin, generation, and manipulation of different types of photoluminescence (PL) derived from ligand-centered (LC), metal/cluster-centered (MC or CC), excimer/exciplex-based (EX), metal-to-ligand or ligand-to-metal charge-transfer-based (MLCT or LMCT), or guest-included emissions. The coordination assemblies in this topic can be generally classified into three categories [(1) mono/homometallic coordination assemblies based on main group (s,p-block), transition (d-block), or lanthanide (f-block) metal centers, (2) s/p-f-, d-f-, or f-f-type heterometallic coordination assemblies, and (3) guest-included coordination assemblies] for which WLE and PLCT properties can be achieved by virtue of either a wide-band/overlapped emission covering the whole visible spectrum from a single emitting center or a combination of complementary color emissions from multiple emitting centers/origins. Some state-of-the-art assembly methods and successful design models relevant to the above three categories are elaborated to demonstrate how to achieve efficient and controllable white-light emission in a single-phase material through a tunable PL approach. Potential applications in the fields of lighting and displaying, sensing and detecting, and barcoding and patterning are surveyed, and at the end, possible prospects and challenges for future development along this line are proposed.