Reversible Acid-Base Long Persistent Luminescence Switch Based on Amino-Functionalized Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) with long persistent luminescence (LPL) have attracted extensive research attention from researchers due to their potential applications in information encryption, anticounterfeiting technology, and security logic. In contrast to short-lived fluorescent materials, LPL materials offer a visible response that can be easily distinguished by the naked eye, thereby facilitating a much clearer visualization. However, there are few reports on functional LPL MOF materials as probes. In this article, two amino-functional LPL MOFs (VB4-2D and VB4-1D) were synthesized. They both exhibited adjustable fluorescence and phosphorescence from blue to green and from cyan to green, respectively. Notably, the MOFs emitted bright and adjustable LPL upon the removal of the different radiation sources. The basic amino functional groups in the MOFs exhibited acid and ammonia sensitivity, and fluorescence and phosphorescence emission intensities can be burst and restored in two atmospheres, respectively, which can be cycled multiple times. Furthermore, LPL intensity undergoes switching between two different conditions as well, which can be visually discerned by the naked eye, enabling visual sensing of volatiles by LPL. This combination of photoluminescence and the visual LPL switching behavior of acids and bases in functional MOFs may provide an effective avenue for stimulus response, anticounterfeiting, and encryption applications.

One/Two-Photon-Excited ESIPT-Attributed Coordination Polymers with Wide Temperature Range and Color-Tunable Long Persistent Luminescence.

The multiple metastable excited states provided by excited-state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature-dependent and color-tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D-π-A structure to obtain both one/two-photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM-106) from ESIPT ligand to achieve the above goals. By comparing LIFM-106 with the counterparts, we established a temperature-regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM-106 shows outstanding one/two-photon excited LPL performance with wide temperature range (100-380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti-counterfeiting systems.

Switching from Oxygen Quenching Resistance to Linear Response by Smart Luminescent Iridium(III)-Based Metal-Organic Frameworks.

In this study, we utilize a photo-active Ir-metalloligand, Ir(C^N)2(L) (C^N = 2-(2,4-difluorophenyl) pyridine, L = [2,2'-bipyridine]-5,5'-dicarboxylic acid), to assemble with CdX2 under hydrothermal conditions, yielding highly emissive crystals of two-dimensional metal-organic frameworks (2D MOFs) (named Ir-Cd2X2, X = Cl, Br). The Ir-Cd2X2 MOFs exhibit µs-level phosphorescence lifetimes and more than 55% quantum yield (QY) at room temperature because of the rigid framework connected by Cd2X2 clusters. By immersing Ir-Cd2X2 in water solution for 5 min, a new MOF (Ir-Cd) was obtained, which is given a structure with hydrolyzed Cd-nodes by complete removal of halogen bridges as elucidated by single-crystal diffraction. Although the phosphorescence emission of pristine CdX2 MOFs exhibits oxygen quenching resistance, the converted Ir-Cd MOF possesses sensitively oxygen-responsive 3MLCT properties, showing a KSV value as high as 14.5 with strictly linear relation (R2 = 0.995). This work differs from the traditional method for improving oxygen-sensing metrics by enhancing QY and phosphorescence lifetime in Ir complexes, while also demonstrating that the transformation in the surrounding coordination environment on adjacent metal centers can also constitute key factors for improved photoluminescence stability or responsive properties in Ir-based heteronuclear MOFs, providing clues for the development of either oxygen quenching blockers or sensors suitable for different occasions.

Thermally Activated Fluorescence vs Long Persistent Luminescence in ESIPT-Attributed Coordination Polymer.

Excited-state intramolecular proton transfer (ESIPT) molecules demonstrating specific enol-keto tautomerism and the related photoluminescence (PL) switch have wide applications in displaying, sensing, imaging, lasing, etc. However, an ESIPT-attributed coordination polymer showing alternative PL between thermally activated fluorescence (TAF) and long persistent luminescence (LPL) has never been explored. Herein, we report the assembly of a dynamic Cd(II) coordination polymer (LIFM-101) from the ESIPT-type ligand, HPI2C (5-(2-(2-hydroxyphenyl)-4,5-diphenyl-1H-imidazol-1-yl)isophthalic acid). For the first time, TAF and/or color-tuned LPL can be achieved by controlling the temperature under the guidance of ESIPT excited states. Noteworthily, the twisted structure of the HPI2C ligand in LIFM-101 achieves an effective mixture of the higher-energy excited states, leading to ISC (intersystem crossing)/RISC (reverse intersystem crossing) energy transfer between the high-lying keto-triplet state (Tn(K*)) and the first singlet state (S1(K*)). Meanwhile, experimental and theoretical results manifest the occurrence probability and relevance among RISC, ISC, and internal conversion (IC) in this unique ESIPT-attributed coordination polymer, leading to the unprecedented TAF/LPL switching mechanism, and paving the way for the future design and application of advanced optical materials.

Multi-Mode Color-Tunable Long Persistent Luminescence in Single-Component Coordination Polymers.

Materials with tunable long persistent luminescence (LPL) properties have wide applications in security signs, anti-counterfeiting, data encrypting, and other fields. However, the majority of reported tunable LPL materials are pure organic molecules or polymers. Herein, a series of metal-organic coordination polymers displaying color-tunable LPL were synthesized by the self-assembly of HTzPTpy ligand with different cadmium halides (X=Cl, Br, and I). In the solid state, their LPL emission colors can be tuned by the time-evolution, as well as excitation and temperature variation, realizing multi-mode dynamic color tuning from green to yellow or green to red, and are the first such examples in single-component coordination polymer materials. Single-crystal X-ray diffraction analysis and theoretical calculations reveal that the modification of LPL is due to the balanced action from single molecule and aggregate triplet excited states caused by an external heavy-atom effect. The results show that the rational introduction of different halide anions into coordination polymers can realize multi-color LPL.

One-/Two-Photon Excited Cell Membrane Imaging and Tracking by a Photoactive Nanocage.

Cell membrane imaging by predesigned molecular and supramolecular photoluminescence probes is of great importance in understanding the nano-biointeractions for potential applications in cellular tracking, drug delivery, cancer diagnosis, and treatment. Herein, we report an effective strategy for cell membrane imaging in both living cell and tissue levels on the basis of a multifunctional nanocage (MOC-16) integrating one-/two-photon active phosphorescence, high charges, balanced hydrophobicity/lipophilicity, and proton sensitivity attributes. The intrinsic optical characters, including strong one-/two-photon excitation and pH-dependent red emission, make MOC-16 powerful optical probes for membrane imaging in living cell and tissue levels under both visible and near-infrared irradiations. Meanwhile, the highly positive charges of +28 endow MOC-16 with adequate water solubility and deprotonation ability while still maintaining its hydrophobicity, thus enabling balanced hydrophobic-lipophilic interactions at the nano-biointerface to facilitate a pH-dependent membrane absorption within the biological pH range of 5.3-7.4. However, the low-charged RuL3 metalloligand or polyethylene glycol (PEG)-modified MOC-16PEG with less hydrophobicity cannot offer enough nano-biointeractions for cell membrane tracking. These findings advance the fundamental understanding of nano-biointerface interactions of MOCs with cell membranes and provide further guidance in their biological applications.

Enhanced Long Persistent Luminescence by Multifold Interpenetration in Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) with long persistent luminescence (LPL) have attracted widespread attention due to potential applications in displays, anticounterfeiting, and so on. However, MOFs often have large pore size, which restricts the formation of efficient inter- and intramolecular interactions to realize LPL. Herein, a new approach to achieving LPL in MOFs by multifold interpenetration of discrete frameworks is reported. By comparison between threefold- and twofold-interpenetrating MOFs, it was found that the former, which have higher multiplicity and denser frameworks, can be endowed with enhanced inter- and intramolecular interactions, and thus enhanced LPL is obtained. Meanwhile, metal-cluster and heavy-halogen effects could also cause variations in LPL duration and color.

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.

Acidity and Cd2+ fluorescent sensing and selective CO2 adsorption by a water-stable Eu-MOF.

A new Eu-MOF was designed from an amino-functionalized ligand and Eu(iii) ions under solvothermal conditions. It is a highly porous, water-stable, and luminescent material, exhibiting pH sensing in the acidic range of pH = 7-3 with selective detection for Cd2+ by an enhanced fluorescence of ∼23-fold against a series of metal ions. Gas adsorption performance shows that Eu-MOF exhibits a high CO2/N2 (15/85) selectivity of 109.4 at 273 K and 1 atm. This performance was superior to that of most reported Ln-MOFs owing to its appropriate pore size, dipole-dipole interaction and donor-acceptor Lewis affinities.

Lead-Based Metal-Organic Framework with Stable Lithium Anodic Performance.

A microporous Pb-based metal-organic framework (MOF) [Pb(4,4'-ocppy)2]·7H2O (Pb-MOF) has been constructed from 4-(4-carboxyphenyl)pyridine N-oxide and Pb(NO3)2. Structural analysis reveals that the Pb-MOF possesses three-dimensional framework with a one-dimensional rhombic channel. When tested as a lithium-ion battery anode, a reversible lithium storage capacity of 489 mAh g-1 was maintained after 500 cycles at 100 mA g-1 as well as excellent cycling stability. The superior electrochemical performance may be derived from the sustenance of the Pb-MOF framework and compositional features of the organic moiety.