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.

Multiresponsive UV-One-Photon Absorption, Near-Infrared-Two-Photon Absorption, and X/γ-Photoelectric Absorption Luminescence in One [Cu4I4] Compound.

A new Cu4I4-cluster-based compound is constructed to show multifaceted photoluminescent attributes: (1) ultraviolet (UV)-excited thermo-, mechano-, and rigido-chromic phosphorescence by the OPA (one-photon absorption) pathway, due to the interchanging emissions from cluster-centered (3CC) and halide-to-ligand charge-transfer (3XLCT) excited triplet states, (2) the ability to convert X/γ-ray and near-infrared (NIR) radiation to visible-light emission, in which the heavy Cu4I4 cores serve as the efficient X/γ-PEA (photoelectric absorption) or NIR-TPA (two-photon absorption) trapper and convertor to photons in the visible spectrum from the same emissive triplet states as those produced by UV excitation. This all-in-one compound affords a highly integrated nanolab for understanding and exploiting a wide range of photophysical phenomena simultaneously and is further fabricated into fiber-coupled long-range, in situ cryogenic thermometer and poly(methyl methacrylate) (PMMA)-embedded monolith gel, providing access to advanced applications in multifunctional optical materials and devices.

Pressure-Induced Multiphoton Excited Fluorochromic Metal-Organic Frameworks for Improving MPEF Properties.

In multiphoton excited fluorescence (MPEF), high-energy upconversion emission is obtained from low-energy excitation by absorbance of two or more photons simultaneously. In a pressure-induced fluorochromic process, the emission energy is switched by outer pressure stimuli. Now, five metal-organic frameworks containing the same ligand with simultaneous multiphoton absorption and pressure-induced fluorochromic attributes were studied. One-, two-, and three-photon excited fluorescence (1/2/3PEF) can be achieved in the frameworks, which exhibit pressure-induced blue-to-yellow fluorochromism. The performances are closely dependent with the topologies, flexibilities, and packing states of the frameworks and chromophores therein. The multiphoton upconversion performance can be intensified by pressure-related structural contraction. Over ten-fold increment in the 2PA active cross-section up to 2217 GM is achieved in pressed LIFM-114 compared with the 210 GM for pristine sample at 780 nm.

A Metal-Organic Supramolecular Box as a Universal Reservoir of UV, WL, and NIR Light for Long-Persistent Luminescence.

Long persistent luminescence (LPL) materials have a unique photophysical mechanism to store light radiation energy for subsequent release. However, in comparison to the common UV source, white-light (WL) and near-infrared (NIR) excited LPL is scarce. Herein we report a metal-organic supramolecular box based on a D-π-A-type ligand. Owing to the integrated one-photon absorption (OPA) and two-photon absorption (TPA) attributes of the ligand, the heavy-atom effect of the metal center, as well as π-stacking and J-aggregation states in the supramolecular assembly, LPL can be triggered by all wavebands from the UV to the NIR region. This novel designed supramolecular kit to afford LPL by both OPA and TPA pathways provides potential applications in anti-counterfeiting, camouflaging, decorating, and displaying, among others.

White-Light Emission from Dual-Way Photon Energy Conversion in a Dye-Encapsulated Metal-Organic Framework.

The design of white-light phosphors is attractive in solid-state lighting (SSL) and related fields. A new strategy in obtaining white light emission (WLE) from dual-way photon energy conversion in a series of dye@MOF (LIFM-WZ-6) systems is presented. Besides the traditional UV-excited one-photon absorption (OPA) pathway, white-light modulation can also be gained from the combination of NIR-excited green and red emissions of MOF backbone and encapsulated dyes via two-photon absorption (TPA) pathway. As a result, down-conversion OPA white light was obtained for RhB+ @LIFM-WZ-6 (0.1 wt %), BR-2+ @LIFM-WZ-6 (2 wt %), and APFG+ @LIFM-WZ-6 (0.1 wt %) samples under 365 nm excitation. RhB+ @LIFM-WZ-6 (0.05 wt %), BR-2+ @LIFM-WZ-6 (1 wt %) and APFG+ @LIFM-WZ-6 (0.05 wt %) exhibit up-conversion TPA white light under the excitation of 800, 790, and 730 nm, respectively. This new WLE generation strategy combines different photon energy conversion mechanisms together.

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 LnIII 2 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.

Epitaxial Growth of Hetero-Ln-MOF Hierarchical Single Crystals for Domain- and Orientation-Controlled Multicolor Luminescence 3D Coding Capability.

Core-shell or striped heteroatomic lanthanide metal-organic framework hierarchical single crystals were obtained by liquid-phase anisotropic epitaxial growth, maintaining identical periodic organization while simultaneously exhibiting spatially segregated structure. Different types of domain and orientation-controlled multicolor photophysical models are presented, which show either visually distinguishable or visible/near infrared (NIR) emissive colors. This provides a new bottom-up strategy toward the design of hierarchical molecular systems, offering high-throughput and multiplexed luminescence color tunability and readability. The unique capability of combining spectroscopic coding with 3D (three-dimensional) microscale spatial coding is established, providing potential applications in anti-counterfeiting, color barcoding, and other types of integrated and miniaturized optoelectronic materials and devices.

Dimension Increase via Hierarchical Hydrogen Bonding from Simple Pincer-like Mononuclear complexes.

A tetradentate symmetric ligand bearing both coordination and hydrogen bonding sites, N(1),N(3)-bis(1-(1H-benzimidazol-2-yl)-ethylidene)propane-1,3-diamine (H2bbepd) was utilized to synthesize a series of transition metal complexes, namely [Co(H2bbepd)(H(2)O)2]·2ClO(4) (1), [Cu(H2bbepd)(OTs(-))]·OTs(-) (2),[Cu(bbepd)(CH(3)OH)] (3), [Cd(H(2)bbepd)(NO3)2]·CH(3)OH (4), [Cd(H(2)bbepd)(CH(3)OH)Cl]·Cl (5), and [Cd(bbepd)(CH(3)OH)2] (6). These complexes show similar discrete pincer-like coordination units, possessing different arrangements of hydrogen bonding donor and acceptor sites. With or without the aid of uncoordinated anions and solvent molecules, such mononuclear units have been effectively involved in the construction of hierarchical hydrogen bonding assemblies (successively via level I and level II), leading to discrete binuclear ring (complex 2), one-dimensional chain or ribbon (complexes 3, 4 and 6) and two-dimensional layer (complexes 1 and 5) aggregates.

Tailoring exciton and excimer emission in an exfoliated ultrathin 2D metal-organic framework.

Two-dimensional (2D) metal-organic frameworks have exhibited a range of fascinating attributes, of interest to numerous fields. Here, a calcium-based metal-organic framework with a 2D layered structure has been designed. Dual emissions relating to intralayer excimers and interlayer trapped excitons are produced, showing excitation-dependent shifting tendency, characteristic of a low dimensional semiconductor nature. Furthermore, the layer stacking by weak van der Waals forces among dynamically coordinated DMF molecules enables exfoliation and morphology transformation, which can be achieved by ultrasound in different ratios of DMF/H2O solvents, or grinding under appropriate humidity conditions, leading to nano samples including ultrathin nanosheets with single or few coordination layers. The cutting down of layer numbers engenders suppression of interlayer exciton-related emission, resulting in modulation of the overall emitting color and optical memory states. This provides a rare prototypical model with switchable dual-channel emissions based on 2D-MOFs, in which the interlayer excitation channel can be reversibly tuned on/off by top-down exfoliation and morphology transformation.

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.

Near-infrared (NIR) emitting Nd/Yb(III) complexes sensitized by MLCT states of Ru(II)/Ir(III) metalloligands in the visible light region.

Four Ru(II)/Ir(III) metalloligands have been designed and synthesized from polypyridine and bibenzimidazole (BiBzIm) organic ligands, which show strong visible light absorption via metal-to-ligand charge transfer (MLCT) transitions. Nd/Yb(III) complexes were further assembled from these Ru(II)/Ir(III) metalloligands, and Ln(III)-centered NIR emissions can be efficiently sensitized by (3)MLCT states of the metalloligands in the visible-light region. The energy transfer rates for the complexes are generally in the order Nd > Yb, which is due to the better matching between (3)MLCT states of Ru(II)/Ir(III) metalloligands and densely distributed excited states of Nd(III) ions. Long decayed lifetimes on a µs scale and high quantum yields up to 1% are obtained in these lanthanide complexes, suggesting that the Ru(II)/Ir(III) metalloligands can serve as a good visible light harvesting antenna to efficiently sensitize Ln(III)-based NIR luminescence.

Direct white-light and a dual-channel barcode module from Pr(III)-MOF crystals.

Direct white-light emission and further a dual-channel readable barcode module in both visible and NIR region was established by single-component homo-metallic Pr(iii)-MOF crystals for the first time.