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

Coordination Self-Assembly Induced Hot Exciton Fluorescence and Multi-Source Excitation Long Afterglow

Metal-organic complexes with long afterglow luminescence have attracted extensive attention due to potential applications in display, sensing and information security. However, most of the metal-organic complex long afterglow materials reported so far are limited to the use of UV light as the excitation source, and the ambiguity of the structure-activity relationship makes the development of metal-organic complexes extremely limited. Herein, a series of metal-organic complexes with ultralong emission lifetime is constructed by coordination assembly of Zn(II) with three isomers. These complexes can emit afterglow when excited by UV light, blue LED, cell phone flashlight or even near-infrared light (800 nm) under ambient conditions, and the afterglow is also observed at 360 K. More interestingly, the inactivation pathway of the triplet exciton was altered by the strategy of supramolecular self-assembly, which leads to these complexes having hot exciton fluorescence (HEF) emission that is not present in the ligand. The relationship between structure and optical properties is investigated in detail by experiments and theoretical calculations. This work provides guidance for studying the modulation of optical properties by coordination interactions.

Single-Component Coordination Polymers with Excitation Wavelength- and Temperature-Dependent Long Persistent Luminescence toward Multilevel Information Security

Metal-organic hybrid materials with long persistent luminescence (LPL) properties have attracted a lot of attention due to their enormous potential for applications in information encryption, anticounterfeiting, and other correlation fields. However, achieving multimodal luminescence in a single component remains a significant challenge. Herein, we report two two-dimensional LPL coordination polymers: {[Zn2(BA)2(BIMB)2]·2H2O}n (1) and {[Cd(BA)(BIMB)]·3H2O}n (2) (BIMB = 1,3-bis(imidazol-1-yl)benzene; BA = butanedioic acid). Their LPL colors can be adjusted by the excitation wavelength or temperature variation in a single-component coordination polymer, achieving multimode color adjustment from green to orange or blue to yellow. X-ray single-crystal diffraction analysis and theoretical calculations demonstrate that abundant intermolecular interactions, ligand-to-ligand charge transfer (LLCT) transitions, and heavy atom effects of the central metal can realize multicolor afterglow. This work provides a convenient strategy for new pattern multicolor LPL materials and may also inspire new ideas for advanced information encryption technologies.

Excitation-wavelength-dependent persistent luminescence from single-component nonstoichiometric CaGaxO4:Bi for dynamic anti-counterfeiting

Materials capable of dynamic persistent luminescence (PersL) within the visible spectrum are highly sought after for applications in display, biosensing, and information security. However, PersL materials with eye-detectable and excitation-wavelength-dependent characteristics are rarely achieved. Herein, a nonstoichiometric compound CaGaxO4:Bi (x < 2) is present, which demonstrates ultra-long, color-tunable PersL. The persistent emission wavelength can be tuned by varying the excitation wavelength, enabling dynamic color modulation from the green to the orange region within the visible spectrum. Theoretical calculations, in conjunction with experimental observations, are utilized to elucidate the thermodynamic charge transitions of various defect states, thereby providing insights into the relationship between Bi3+ emitters, traps, and multicolored PersL. Furthermore, the utility of color-tunable PersL materials and flexible devices is showcased for use in visual sensing of invisible ultraviolet light, multicolor display, information encryption, and anti-counterfeiting. These discoveries create new opportunities to develop smart photoelectric materials with dynamically controlled PersL for various applications.

Unlocking multi-photon excited luminescence in pyrazolate trinuclear gold clusters for dynamic cell imaging

The family of coinage-metal-based cyclic trinuclear complexes exhibits abundant photophysical properties, promising for diverse applications. However, their utility in biochemistry is often hindered by large particle size and strong hydrophobicity. Meanwhile, the investigation into multi-photon excited luminescence within this family remained undocumented, limiting their potential in bio-imaging. Herein, we unveil the multi-photon excited luminescent properties of pyrazolate-based trinuclear gold(I) clusters, facilitated by excimeric gold(I)···gold(I) interactions, revealing a nonlinear optical phenomenon within this family. Furthermore, to address issues of poor biocompatibility, we employ electrospinning coupled with hydroxypropyl-beta-cyclodextrin as the matrix to fabricate a flexible, durable, transparent, and red emissive film with a photoluminescence quantum yield as high as 88.3%. This strategy not only produces the film with sufficient hydrophilicity and stability, but also achieves the downsizing of trinuclear gold(I) clusters from microscale to nanoscale. Following the instantaneous dissolution of the film in the media, the released trinuclear gold(I) nanoparticles have illuminated cells and bacteria through a real-time, non-toxic, multi-photon bio-imaging approach. This achievement offers a fresh approach for utilizing coinage-metal-based cyclic trinuclear complexes in biochemical fields.

Dual-Stimuli-Responsive Modulation Organic Afterglow Based on N─H Proton Migration Mechanism

Organic afterglow materials have significant applications in information security and flexible electronic devices with unique optical properties. It is vital but challenging to develop organic afterglow materials possessing controlled output with multi-stimuli-responsive capacity. Herein, dimethyl terephthalate (DTT) is introduced as a strong proton acceptor. The migration direction of N─H protons on two compounds Hs can be regulated by altering the excitation wavelength (Ex) or amine stimulation, thereby achieving dual-stimuli-responsive afterglow emission. When the Ex is below 300 nm, protons migrate to S1-2 DTT, where strong interactions induce phosphorescent emission of Hs, resulting in afterglow behavior. Conversely, when the Ex is above 300 nm, protons interact with the S0 DTT weakly and the afterglow disappears. In view of amine-based compounds with higher proton accepting capabilities, it can snatch proton from S1-2 DTT and redirect the proton flow toward amine, effectively suppressing the afterglow but obtaining a new redshifted fluorescence emission with Δλ over 200 nm due to the high polarity of amine. Moreover, it is successfully demonstrated that the applications of dual-stimuli-responsive organic afterglow materials in information encryption based on the systematic excitation-wavelength-dependent (Ex-De) behavior and amine selectivity detection.

Multifunctional Chiral d10-Metal Coordination Polymers: Tunable Photoluminescence and Efficient Second-Harmonic Generation with Circular Dichroic Response

A series of homochiral coordination polymers (HCPs), [M2(SIAP)2(bpy)2] [M(S)] and [M2(RIAP)2(bpy)2] [M(R)] (M = Zn or Cd, SIAP or RIAP = (S,S)- or (R,R)- 2,2'-(isophthaloylbis(azanediyl))di-propionic acid, bpy = 4,4'-bipyridine), is successfully synthesized through solvothermal reactions, self-assembling d10 metal cations, chiral dicarboxylic ligands, and π-conjugated bipyridyl ligands. The HCPs crystallize in the extremely rare triclinic chiral space group, P1, and present 3D framework structures attributed to the strong intermolecular interactions, such as hydrogen bonds and π-π stacking. Due to the unique crystal structures, the title compounds reveal efficient photoluminescence emission across a broad visible range, with significant brightness and color tuning by varying the excitation wavelength. Moreover, they exhibit efficient phase-matched second-harmonic generation (SHG) with very high laser-induced damage thresholds, essential for high-power nonlinear optical (NLO) applications. Intriguingly, the title compounds exhibit a measurable contrast in the SHG response under right- and left-handed circularly polarized excitation, thereby providing a unique case of SHG circular dichroism from the chiral centers of SIAP2- or RIAP2- ligand packed in the noncentrosymmetric environment. These exceptional attributes position these HCPs as promising candidates for multifunctional materials, with potential applications ranging from NLO devices to tailored luminescent systems with polarization control.

Anionic Hydrogen-Bonded Frameworks Showing Tautomerism and Colorful Luminescence for the Ultrasensitive Detection of Acetone

Tautomers coexisting in an equilibrium system have significant potential for regulating luminescent properties because of their structural differences. However, separating and stabilizing tautomers at room temperature is a considerable challenge. In this study, it is found that hydrogen-bonded organic frameworks (HOFs) composed of Br- anions can effectively separate and stabilize two proton-transfer tautomers of triarylformamidinium bromide: namely, the nitrogen cation (BA-N) and carbon cation (BA-C). The BA-N crystal consisting of a dense anionic HOF and parallelly aligned organic cations exhibits green thermally activated delayed fluorescence and red room-temperature phosphorescence (RTP). The BA-C crystal contains acetone molecules that induce an antiparallel arrangement of the organic cations to form a loose HOF, producing blue prompt fluorescence and green RTP. Interestingly, switching of the HOFs between BA-N and BA-C can be achieved through the uptake and release of acetone, thereby dynamically adjusting multiple luminescent properties. Consequently, the HOF crystals can be used for the highly sensitive and specific sensing of acetone with a detection limit of 66.74 ppm. This study not only stabilizes tautomeric luminescent materials at room temperature, but also provides a new method for constructing smart HOFs with a sensitive response to a stimulus.

A flexible ligand and halogen engineering enable one phosphor-based full-color persistent luminescence in hybrid perovskitoids

Color-tunable room temperature phosphorescent (RTP) materials have raised wide interest due to their potential application in the fields of encryption and anti-counterfeiting. Herein, a series of CdX2-organic hybrid perovskitoids, (H-apim)CdX3 and (apim)CdX2 (denoted as CdX-apim1 and CdX-apim2, apim = 1-(3-aminopropyl)imidazole, X = Cl, Br), were synthesized using apim with both rigid and flexible groups as ligands, which exhibit naked-eye detectable RTP with different durations and colors (from cyan to red) by virtue of different halogen atoms, coordination modes and the coplanar configuration of flexible groups. Interestingly, CdCl-apim1 and CdX-apim2 both exhibit excitation wavelength-dependent RTP properties, which can be attributed to the multiple excitation of imidazole/apim, the diverse interactions with halogen atoms, and aggregated state of imidazoles. Structural analysis and theoretical calculations confirm that the aminopropyl groups in CdCl-apim1 do not participate in luminescence, while those in CdCl-apim2 are involved in luminescence including both metal/halogen to ligand charge transfer and twisted intramolecular charge transfer. Furthermore, we demonstrate that these perovskitoids can be applied in multi-step anti-counterfeiting, information encryption and smart ink fields. This work not only develops a new type of perovskitoid with full-color persistent luminescence, but also provides new insight into the effect of flexible ligands and halogen engineering on the wide-range modulation of RTP properties.

Transformational Modulation of Fluorescence to Room Temperature Phosphorescence in Metal-Organic Frameworks with Flexible C-S-C Bonds

Photoluminescent metal-organic frameworks (MOFs) have been a subject of considerable interest for many years. However, the regulation of excited states of MOFs at the single crystal level remains restricted due to a lack of control methods. The singlet-triplet emissive property can be significantly influenced by crystal conformational distortions. This review introduces an intelligent responsive MOF material, denoted as LIFM-SHL-3a, characterized by flexible C-S-C bonds. LIFM-SHL-3a integrates elastic structural dynamics with fluorescence and room temperature phosphorescence (RTP) modulation under heating conditions. The deformable carbon-sulfur bond essentially propels the distortion of molecular conformation and alters the stacking mode, as illustrated by single-crystal-to-single-crystal transformation detection. The deformation of flexible C-S-C bonds leads to different noncovalent interactions in the crystal system, thereby achieving modulation of the fluorescence (F) and RTP bands. In the final state structure, the ratio of fluorescence is 66.7%, and the ratio of RTP is 32.6%. This stands as a successful demonstration of modulating F/RTP within the dynamic MOF, unlocking potential applications in optical sensing and beyond. Especially, a PL thermometer with a relative sensitivity of 0.096-0.104%·K-1 in the range of 300-380 K and a H2S probe with a remarkably low LOD of 125.80 nM can be obtained using this responsive MOF material of LIFM-SHL-3a.

Iodine Adsorption-Desorption-Induced Structural Transformation and Improved Ag+ Turn-On Luminescent Sensing Performance of a Nonporous Eu(III) Metal-Organic Framework

Posttreatment of pristine metal-organic frameworks (MOFs) with suitable vapor may be an effective way to regulate their structures and properties but has been less explored. Herein, we report an interesting example in which a crystalline nonporous Eu(III)-MOF was transferred to a porous amorphous MOF (aMOF) via iodine vapor adsorption-desorption posttreatment, and the resulting aMOF showed improved turn-on sensing properties with respect to Ag+ ions. The crystalline Eu-MOF, namely, Eu-IPDA, was assembled from Eu(III) and 4,4'-{4-[4-(1H-imidazol-1-yl)phenyl]pyridine-2,6-diyl}dibenzoic acid (H2IPDA) and exhibited a two-dimensional (2D) coordination network based on one-dimensional secondary building blocks. The close packing of the 2D networks gives rise to a three-dimensional supramolecular framework without any significant pores. Interestingly, the nonporous Eu-IPDA could absorb iodine molecules when Eu-IPDA crystals were placed in iodine vapor at 85 °C, and the adsorption capacity was 1.90 g/g, which is comparable to those of many MOFs with large BET surfaces. The adsorption of iodine is attributed to the strong interactions among the iodine molecule, the carboxy group, and the N-containing group and leads to the amorphization of the framework. After immersion of the iodine-loaded Eu-IPDA in EtOH, approximately 89.7% of the iodine was removed, resulting in a porous amorphous MOF, denoted as a-Eu-IPDA. In addition, the remaining iodine in the a-Eu-IPDA framework causes strong luminescent quenching in the fluorescence emission region of the Eu(III) center when compared with that in Eu-IPDA. The luminescence intensity of a-Eu-IPDA in water suspensions was significantly enhanced when Ag+ ions were added, with a detection limit of 4.76 × 10-6 M, which is 1000 times that of pristine Eu-IPDA. It also showed strong anti-interference ability over many common competitive metal ions and has the potential to sense Ag+ in natural water bodies and traditional Chinese medicine preparations. A mechanistic study showed that the interactions between Ag+ and the absorbed iodine, the carboxylate group, and the N atoms all contribute to the sensing performance of a-Eu-IPDA.