A novel fluorene-based aggregation-induced emission (AIE)-active gold(i) complex with crystallization-induced emission enhancement (CIEE) and reversible mechanochromism characteristics

Tetraphenylethene-based mononuclear aggregation-induced emission (AIE)-active mechanofluorochromism gold(I) complexes with different auxiliary ligands

In this study, a series of tetraphenylethene-containing gold(I) complexes with different auxiliary ligands have been synthesized. These complexes were characterized using a variety of techniques including nuclear magnetic resonance spectroscopy, mass spectrometry, and single crystal X-ray diffraction. Their aggregation-induced emission (AIE) behaviors were investigated through ultraviolet/visible and photoluminescence spectrum analyses, and dynamic light scattering measurements. Meanwhile, their mechanofluorochromic properties were also studied via solid-state photoluminescence spectroscopy. Intriguingly, all these mononuclear gold(I) molecules functionalized by tetraphenylethene group demonstrated AIE phenomena. Furthermore, five gold(I) complexes possessing diverse auxiliary ligands exhibited distinct fluorescence changes in response to mechanical grinding. For luminogens 2-5, their solids showed reversible mechanofluorochromic behaviors triggered by the mutual transformation of crystalline and amorphous states, while for luminogen 1, blue-green-cyan three-color solid fluorescence conversion was realized by sequential mechanical grinding and solvent fumigation. Based on this stimuli-responsive tricolored fluorescence feature of 1, an information encryption system was successfully constructed.

Controlled nanocrystallization of gold nanoclusters within surfactant envelopes: enhancing aggregation-induced emission in solution

The nanocrystallization of functional molecules has been a subject of recent interest in the current development of nanotechnology. Herein, we report the unprecedented synthesis of single nanocrystals of a molecular gold nanocluster in a homogeneous solution by using surfactant-based nano-envelopes. The co-assembling of a Au8 nanocluster carrying lipophilic phosphine ligands with sodium dodecyl sulfate (SDS) in an aqueous solution results in the formation of sphere-shaped amorphous nano-aggregates coated with the surfactant. Upon sonication, the spherical amorphous aggregates are smoothly shape-shifted into discrete rhombic nanocrystals, which can be tracked by TEM and solution XRD. The transformation into single nanocrystals occurs exclusively without further growth or agglomeration, implying that the crystal growth is restricted within the surfactant nano-envelopes. The robust but flexible nature of the wrapped surfactant is likely responsible for the controlled crystallization. We also demonstrate that the amorphous-to-nanocrystalline transition in solution remarkably enhances the photoluminescence emission from the nanocluster, providing a clear example of crystallization-induced emission enhancement. Notably, the obtained nanocrystals showed high stability in solution and retained their shape, size, and PL intensity even after several months, owing to the densely packed surfactant shell. The present surfactant-directed nanocrystallization method may be applicable to other molecular species to contribute to the development of nanocluster science as well as the designed synthesis of nanomaterials.

Role of Bridging Groups in Regulating the Luminescence and Charge Transfer Properties of Thermally Activated Delayed Fluorescence Molecules: A Theoretical Perspective

Organic emitters with a simultaneous combination of aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in great demand due to their excellent comprehensive performances toward efficient organic light-emitting diodes (OLEDs), biomedical imaging, and the telecommunications field. However, the development of efficient AIE-TADF materials remains a substantial challenge. In this work, light-emitting properties of two AIE-TADF molecules with different bridging groups ICz-BP and ICz-DPS are theoretically investigated in the solid state with the combined quantum mechanics/molecular mechanics (QM/MM) method and the thermal vibration correlation function (TVCF) theory. The research indicates that the C═O bridging bond in ICz-BP is more favorable than the S═O bridging bond in ICz-DPS for enhancing the planarity of the acceptor, increasing conjugation, and thereby elevating the transition dipole moment density. Simultaneously, the stacking pattern of ICz-BP in the solid facilitates a reduction in energy gap between S1 and T1 (ΔEST), achieving rapid reverse intersystem crossing rate (kRISC). Furthermore, compared to toluene, the stacking patterns of ICz-BP and ICz-DPS in the solid effectively suppress the out-of-plane wagging vibration of the acceptor, thereby inhibiting the loss of nonradiative energy in the excited state and realizing aggregation-induced emission. Moreover, the charge transport properties of both electrons and holes in ICz-BP are found to be higher than the corresponding rates in ICz-DPS, attributed to the smaller internal reorganization energy of ICz-BP in the solid state. Additionally, the calculations reveal a more balanced charge transport characteristic in ICz-BP, contributing to efficient exciton recombination and emission and ultimately mitigating efficiency roll-off. Based on these computational results, we aim to unveil the relationship between molecular structure and light-emitting properties, aiding in the design and development of efficient AIE-TADF devices.

Aggregation/Crystallization-Induced Emission in Naphthyridine-Based Carbazolyl-Modified Donor-Acceptor Boron Dyes Tunable by Fluorine Atoms

Four donor-acceptor boron difluoride complexes based on the carbazole electron donor and the [1,3,5,2]oxadiazaborinino[3,4-a][1,8]naphthyridine acceptor were designed, synthesized, and systematically spectroscopically investigated in solutions, in dye-doped polymer films, and in the solid states. The dyes exhibit an intense blue to red solid-state emission with photoluminescence quantum yields of up to 59 % in pure dye samples and 86 % in poly(methyl methacrylate) films. All boron complexes show aggregation-induced emission and reversible mechanofluorochromism. The optical properties of these dyes and their solid state luminescence can be tuned by substitution pattern, i. e., the substituents at the naphthyridine unit. Exchange of CH3- for CF3-groups does not only increase the intramolecular charge transfer character, but also provides a crystallization-induced emission enhancement.

Conformational Isomerization Effect on Singlet/Triplet Energy Consumption Process of Thermally Activated Delayed Fluorescence Molecules with Aggregation Induced Emission: A QM/MM Study

Thermally activated delayed fluorescence (TADF) molecules with aggregation-induced emission (AIE) properties hold tremendous potential in biomedical sensing/imaging and telecommunications. In this study, a multiscale method combined with thermal vibration correlation function (TVCF) theory is used to investigate the photophysical properties of the novel TADF molecule CNPy-SPAC in toluene and crystal and amorphous states. In the crystal state, an increase in radiative rates and a decrease in nonradiative rates lead to AIE. Additionally, conformational isomerization effects result in significantly different luminescent efficiencies between the two crystal structures. Furthermore, the isomerization effect allows for the coexistence of three configurations in the amorphous state. Among them, the non-TADF quasi-axial (Qa) configuration may facilitate energy transfer to the TADF-characteristic quasi-equal/quasi-equal-H (Qe/Qe-H) configurations, enhancing AIE. Moreover, the Qa configuration enables rapid electron transport, offering the potential for self-doped devices. Our work elucidates a new mechanism for the isomerization effect in AIE-TADF molecules.

Fluorene-naphthalene Schiff base as a smart pigment in invisible ink with multiple security features for advanced anticounterfeiting and forensic applications

Smart functional materials with captivating optical properties are of immense importance due to their versatile applicability in anticounterfeiting and forensic science. A fluorene-naphthalene Schiff base (FNH) that displays aggregation induced emission, mechanofluorochromism and excitation wavelength dependent fluorescence inherent to the pristine and ground samples is synthesized. Water/solvent-based invisible security inks for flexo/screen printing were formulated using FNH as a smart pigment to check the originality of documents/branded products etc. The prints with good photostability, adherence to substrate and rub resistance are invisible in daylight showcasing multiple non-destructive and destructive techniques to authenticate the document. The inked area on UV dull paper substrate exhibits a weak emission, which is observed by the forger under UVA light. However, the user can validate the authenticity of the document by rubbing the print with hard objects, especially using a metal coin or glass rod to perceive a human eye detectable intensification in the orange fluorescence under the same illumination source. The intensity of the orange fluorescence reverts to the original, which enables the reuse of the security document after originality check. Yet another nondestructive authentication method is to observe a cyan fluorescence from the print and orangish yellow fluorescence from the rubbed printed region when shined with a 270-400 nm light source, whereas a cyanish green fluorescence both from the unrubbed and rubbed regions of the print when illuminated with a visible light source ranging from 420 to 480 nm. An additional verification through a destructive technique is to perceive red and yellow fluorescence of the ink film upon contact with THF and NaOH/KOH, respectively and a penetrating red fluorescence from the rear side of the THF-exposed printed area of the paper. The multi-level security features that cannot be easily replicated by the forger but allows a simple and easy validation process by the user are unique to FNH, used as a single pigment in the inks. Further, the applicability of the ground FNH in forensic science is established to distinctly observe Level I to II details of latent fingerprints.

Gelatinous lanthanide coordination polymer with aggregation-enhanced antenna effect for ratiometric detection of endogenous alkaline phosphatase

Aggregation-induced emission (AIE) and antenna effect (AE) are two significant behaviors that have attracted increasing attention. However, it is challenging to achieve the synergistic effect of AIE and AE in luminescent materials for more extensive applications. Here, four gelatinous Ln³⁺ coordination polymers (Ln-CPs) are synthesized by self-assembly of ciprofloxacin (CIP), adenosine monophosphate (AMP), and Ln³⁺ ions in aqueous medium. Encouragingly, a remarkable increase in the characteristic fluorescence of Ln³⁺ and a significant decrease in CIP are observed along with increasing concentration of Ln-CPs, which is attributed to the large aggregates formed by self-assembly that strictly constrain the intramolecular motions of antenna ligands, thereby achieving the aggregation-enhanced AE. More meaningfully, Eu-CP not only shows a rice-like morphology at high aggregation state, but also provides an opportunity for the selective detection of alkaline phosphatase (ALP). A new flower-like polymer is formed upon incubating Eu-CP with ALP, accompanied by the fluorescence quenching of Eu³⁺ and recovery of CIP, a ratiometric determination of ALP in the range of 0.1-6.0 U·L^-1 is thus achieved. Additionally, ALP assay in human serum and bioimaging in living cells have been successfully performed. This research opens a new horizon for the fabrication of Ln³⁺-based luminescent materials with promising applications.

Full-Color Emissive D-D-A Carbazole Luminophores: Red-to-NIR Mechano-fluorochromism, Aggregation-Induced Near-Infrared Emission, and Application in Photodynamic Therapy

The preparation of multifunctionalized luminophores with full-color emission based on an identical core skeleton is a significative but challenging research topic. In this work, eight donor-donor-acceptor (D-D-A)-type luminogens based on a central carbazole core bearing a C₆ hydrocarbon chain were designed by using different kinds of donor and acceptor units on the left and right, and synthesized in good yields. These D-D-A carbazole derivatives display deep-blue, sky-blue, cyan, green, yellow-green, yellow, orange and red fluorescence in the solid state, achieving full-color emission covering the whole visible light range under UV light illumination. Notably, the dicyano-functionalized triphenylamine-containing carbazole derivative exhibits rare aggregation-induced near-infrared emission and red-to-near-infrared mechano-fluorochromism with high contrast beyond 100 nm. Furthermore, the red-emissive luminogen can serve as a potential candidate for cell imaging and photodynamic therapy (PDT). This work not only provides reference for the construction of full-color emissive systems but also opens a new avenue to the preparation of multifunctionalized luminophores capable of simultaneous application in near-Infrared mechanical-force sensors and PDT fields.

Carbazole-based aggregation-induced phosphorescent emission-active gold(I) complexes with various phosphorescent mechanochromisms

A series of carbazole-containing gold(I) complexes modified with different substituents were successfully designed and synthesized, and their molecular structures were characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. The aggregation-induced behaviors of these gold(I) complexes were studied by ultraviolet/visible and photoluminescence spectroscopy. Meanwhile, their mechanical force-responsive emissive properties were also investigated via solid-state photoluminescence spectroscopy. Interestingly, all these gold(I)-based luminogenic molecules were capable of exhibiting aggregation-induced phosphorescent emission phenomena. Furthermore, their solids of three gold(I) complexes displayed contrasting mechano-responsive phosphorescence features. More specifically, trifluoromethyl or methoxyl-substituted luminophores 1 and 3 demonstrated mechanochromic behaviors involving blue-shifted phosphorescence changes, and their mechanoluminochromic phenomena were reversible. However, the solid-state phosphorescence of phenyl-substituted luminophor 2 was not sensitive to external mechanical force.

Highly fluorescent carbon nitride oligomer with aggregation-induced emission characteristic for plastic staining

Polymeric carbon nitride often displays weak photoluminescence in solid state due to the aggregation-caused quenching effect. Herein, highly fluorescent carbon nitride oligomer (CNO) with aggregation-induced emission (AIE) characteristic was prepared via one-step solid-phase thermal condensation of 2,4-diamino-6-phenyl-1,3,5-triazine (DPT) at 350 °C. CNO is mainly composed of DPT dimer connected by rotatable imine groups, and exhibits weak fluorescence in the dispersed state and strong blue-green emission in the aggregated state and solid state. Density functional theory calculations indicate that the restriction of phenyl and triazine ring twisting motions is the main origin of the AIE phenomenon of CNO. Finally, CNO was preliminarily applied for fluorescent staining of plastic pellets. This work not only provides a solid-state strategy to synthesize fluorescent material with AIE characteristic but also extends the application of polymeric carbon nitride.

Highly Emissive Multipurpose Organoplatinum(II) Metallacycles with Contrasting Mechanoresponsive Features

The development of supramolecular coordination complexes (SCCs) with a bright aggregate state or mechanical-stimuli-responsive luminescence is very significant and challenging. Herein, we report the synthesis of three different supramolecular platinum(II) metallacycles via coordination-driven self-assembly of a diplatinum(II) acceptor and organic donors with a triphenylamine, carbazole, or tetraphenylethylene moiety. The triphenylamine-modified SCC exhibits aggregation-induced emission enhancement (AIEE) but no mechanofluorochromism. The carbazole and tetraphenylethylene-based SCCs exhibit changes in aggregate fluorescence and also exhibit reversible mechanofluorochromism. This work not only reports three rare metallacycles with AIEE, aggregate fluorescence change, or mechanofluorochromic nature but also explores their potential applications in cell imaging and solid-state lighting.

Tetraphenylethene-Modified Colorimetric and Fluorescent Chemosensor for Hg2+ With Aggregation-Induced Emission Enhancement, Solvatochromic, and Mechanochromic Fluorescence Features

A tetraphenylethene (TPE)-modified rhodanine derivative was successfully designed and prepared, and this luminophor showed intramolecular charge transfer nature from the TPE unit to the rhodanine-3-acetic acid unit. Interestingly, this luminogen not only exhibited typical aggregation-induced emission enhancement (AIEE) behavior but also showed good cell imaging performance. Remarkably, this AIEE-active TPE-containing rhodanine derivative possessed noticeable solvatochromic fluorescence effect involving multiple fluorescent colors of green, yellow-green, yellow, orange, and red. Meanwhile, this fluorescigenic compound displayed reversible mechanochromic fluorescence behavior based on the mutual transformation of between stable crystalline and metastable amorphous states. On the other hand, this multifunctional fluorophor could selectively and sensitively detect Hg2+ in an acetonitrile solution. Furthermore, this chemosensor could also be used to detect Hg2+ on test paper strips.

Photophysical Properties of Cyclometalated Platinum(II) Diphosphine Compounds in the Solid State and in PMMA Films

Platinum(II) compounds were synthesized with both chelate cyclometalated ligands and chelate diphosphine ligands. The cyclometalated ligands include phenylpyridine and a benzothiophene-containing ligand. The three new benzothiophene compounds were characterized by nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectrometry (HR-MS), and photophysical measurements. In the case of one compound, L1-DPPM, the structure was determined by single crystal X-ray diffraction. The structural coherence of the noncrystalline emissive solid state was measured by X-ray total scattering real space pair distribution function analysis. Quantum yield values of all of the platinum compounds measured in the solid state and in PMMA films were much greater than in solution.

Origin of the Aggregation-Induced Phosphorescence of Platinum(II) Complexes: The Role of Metal-Metal Interactions on Emission Decay in the Crystalline State

Discerning the origins of the phosphorescent aggregation-induced emission (AIE) from Pt(II) complexes is crucial for developing the broader range of photo-functional materials. Over the past few decades, several mechanisms of phosphorescent AIE have been proposed, however, not have been directly elucidated. Herein, we describe phosphorescence and deactivation processes of four class of AIE active Pt(II) complexes in the crystalline state based on experimental and theoretical investigation. These complexes show metal-to-ligand and/or metal-metal-to-ligand charge transfer emission in crystalline state with different heat resistance against thermal emission quenching. The calculated energy profiles including the minimum energy crossing point between S₀ and T₁ states were consistent with the heat resistant properties, which provided the mechanism for AIE expression. Furthermore, we have clarified the role of metal-metal interaction in AIE by comparing two computational models.

Osmaindenes: Synthesis and Reversible Mechanochromism Characteristics

A series of novel osmaindenes 1 - 6 bearing different substituents (CF 3 , H, I, Br, OCH 3 , N(Ph) 2 ) has been synthesized by nucleophilic reaction of water with the corresponding aromatic osmanaphthalyne complexes. All osmaindenes 1 - 6 have been characterized by elemental analysis (EA) and nuclear magnetic resonance (NMR) spectroscopy, although the low solubilities of 3 and 4 precluded the accumulation of their 13 C NMR spectra. Osmaindenes 2 , 3 and 5 have also been characterized by single-crystal X-ray diffraction analysis. Subsequently, through solid-state fluorescence spectroscopy, mechanochromic studies, and powder X-ray diffraction (XRD) analysis, we found that osmaindenes 1 - 6 fluoresce at wavelengths in the range 500-800 nm, while also displaying reversible mechanochromic properties. The solid-state fluorescence emission of 1 after grinding extends into the near-infrared region. This research provides new insight into the design and synthesis of metallic materials with excellent mechanochromic properties.

Persistent room-temperature phosphorescence or high-contrast phosphorescent mechanochromism: polymorphism-dependent different emission characteristics from a single gold(I) complex

Luminophores with persistent room-temperature phosphorescence (p-RTP) or effective phosphorescent mechanochromism features have significant potential applications in the field of optoelectronic materials. Until now, p-RTP and remarkable phosphorescent mechanochromism phenomena have been observed in some luminescent molecules with different molecular structures. However, separately realizing p-RTP and high-contrast phosphorescent mechanochromism in different polymorphs from a single luminophore is still a valuable and challenging topic. In this work, two polymorphs 1B and 1YG of a new gold(i) complex with blue and yellow-green luminescence, respectively, are reported. Interestingly, 1B exhibits high-contrast phosphorescent mechanochromic behavior, while 1YG exhibits a persistent room-temperature phosphorescence effect. This is the first example of simultaneously obtaining double-purpose crystalline materials with a high-contrast phosphorescent mechanochromism or persistent room-temperature phosphorescence feature from a single luminophore.

Mechanoluminescence and aggregation-enhanced emission (AEE) of an In-MOF based on a 9,9'-diphenyl-9H-fluorene tetraacid linker

A water-stable In-MOF, constructed based on a conformationally-flexible tetraacid linker, i.e., 2,7-bis(3,5-dicarboxyphenyl)-9,9'-diphenyl-9H-fluorene, i.e., H4DPF, is shown to exhibit a significantly enhanced solid-state fluorescence quantum yield (φf) of 23% in comparison with that of the linker (φfca. 4%) as a consequence of rigidification of the latter by metalation. Application of external stimulus in the form of grinding of the In-MOF leads to a drastic enhancement by 29%, φf from 23 to 52%. Solid-state absorption and emission spectra show that the absorption in the region of 368-550 nm gets diminished with a concomitant change in the emission maximum with a blue shift upon grinding. Fluorescence enhancement with grinding is correlated with a gradual reduction in the size of the particles, as established by SEM analysis. MOF particle aggregation has been invoked to account for the observed fluorescence enhancement in addition to a subtle conformational change in the structure of the linker upon grinding. Intriguingly, the ground MOF particles exhibit aggregation behaviour in the DMF-water solvent system with the emission further increasing up to 75% for the increase in the water fraction (fw) from 0 to 60%; hydrophobic aggregation of particles evidently leads to a change in the conformation of the linker and particle aggregation-enhanced emission (AEE). De-aggregation of particles ensues for fw = 70-90%, as reflected by a gradual decrease in the emission intensity. It is shown that the suspension of ground In-MOF particles in water permits sensing of metal ions, in particular Al3+ ions, by fluorescence quenching with detection at a sub-ppb level. The observed results comprise first demonstration of both mechanoluminescence and AEE of MOF particles.

Recent Progress on Supramolecular Luminescent Assemblies Based on Aurophilic Interactions in Solution

The development of supramolecular systems showing aurophilic interactions in solution is gaining much attention in the last years. This is due to the intriguing photophysical properties of gold(I) complexes, which usually confer to these supramolecular assemblies interesting luminescent properties, as well as the possibility of morphological modulation, through fine tuning of inter- and intramolecular aurophilic interactions, in synergy with the formation of other supramolecular contacts. In this work, an overview of the advances made in this area since 2015 is presented. A large variety of systems showing different spectroscopical and structural topologies has been reported. Moreover, these supramolecular assemblies have proven to be useful in a wide range of applications.

An antipyrine based fluorescent probe for distinct detection of Al3+ and Zn2+ and its AIEE behaviour

A simple antipyrine based fluorescent probe, 4-[(2-hydroxy-3-methoxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (OVAP), has been successfully synthesized using a one-step condensation method. It exhibits dual sensing properties toward Al3+ and Zn2+ in the presence of other relevant metal ions and also displays novel aggregation induced emission enhancement (AIEE) characteristics in its aggregated/solid state. Aggregated OVAP microstructures with interesting morphologies have been synthesized using SDS as a morphology directing agent. Morphologies of the particles are characterized using optical microscopy. Photophysical properties of the as-synthesized OVAP hydrosol are studied using UV-Vis absorption, steady state and time resolved fluorescence spectroscopy. The 'turn on' luminescence property of OVAP is used for the selective detection of trace amounts of Al3+ and Zn2+ and a significant turn on fluorescence enhancement over ∼100-fold is triggered via chelation-enhanced fluorescence (CHEF) through complex formation. The 1 : 1 stoichiometry of each sensor metal ion complex is observed from Job's plot based on UV-Vis absorption titration. The LODs for Al3+ and Zn2+ are found to be 1.05 nM and 2.35 nM, respectively. Notably, the sensor, OVAP, is further demonstrated using a molecular INHIBIT logic gate.

Studies of cyanomethylcarbamoyl-bridged anthracene and pyrene fluorophores

Cyanomethylcarbamoyl-bridged anthracene and pyrene derivatives were prepared as functional fluorophores with diverse structural, electronic, and fluorescence properties.

Multi-color mechanochromic luminescence of three polymorphic crystals of a donor–acceptor-type benzothiadiazole derivative

The three polymorphic crystals of a donor–acceptor dye exhibited different luminescence colors, which changed in response to mechanical grinding.

Highly efficient and air-stable Eu(II)-containing azacryptates ready for organic light-emitting diodes

Divalent europium 5d-4f transition has aroused great attention in many fields, in a way of doping Eu²⁺ ions into inorganic solids. However, molecular Eu²⁺ complexes with 5d-4f transition are thought to be too air-unstable to explore their applications. In this work, we synthesized four Eu²⁺-containing azacryptates EuX₂-N_n (X = Br, I, n = 4, 8) and systematically studied the photophysical properties in crystalline samples and solutions. Intriguingly, the EuX₂-N₈ complexes exhibit near-unity photoluminescence quantum yield, good air-/thermal-stability and mechanochromic property (X = I). Furthermore, we proved the application of Eu²⁺ complexes in organic light-emitting diodes (OLEDs) with high efficiency and luminance. The optimized device employing EuI₂-N₈ as emitter has the best performance as the maximum luminance, current efficiency, and external quantum efficiency up to 25470 cd m⁻², 62.4 cd A⁻¹, and 17.7%, respectively. Our work deepens the understanding of structure-property relationship in molecular Eu²⁺ complexes and could inspire further research on application in OLEDs.

Though divalent-europium-based complexes are promising materials for next-generation light-emitting devices, their poor air stability limits their applicability. Here, the authors report the design of air stable divalent-europium-based complexes for efficient organic light-emitting diodes.

Functionalized Copper Nanoclusters-Based Fluorescent Probe with Aggregation-Induced Emission Property for Selective Detection of Sulfide Ions in Food Additives

In this paper, a novel and facile synthetic method of 3-mercaptopropionic acid functionalized copper nanoclusters with aggregation-induced emission (AIE) induced by Cu²⁺ (Cu²⁺@MPA-Cu NCs) was developed by a one-pot reaction as a fluorescent probe for the detection of sulfide ion (S^2-). The prepared Cu²⁺@MPA-Cu NCs behaved as aggregated clusters and had strong pink fluorescence under 365 nm UV light with excellent fluorescence emission at 610 nm. The quantum yield increased from 0.56% to 4.8% before and after Cu²⁺ added. The presence of S^2- would strongly bind to Cu²⁺, which caused the structure of the aggregated Cu²⁺@MPA-Cu NCs to be destroyed and then the fluorescence quenched. On the basis of this principle, a fluorescent probe was constructed for the detection of S^2- with a very good linearity in the range 0-600 μM (R² = 0.9843) and a detection limit of 26.3 nM. Finally, the nanohybrids were successfully demonstrated for the application in the selective detection of S^2- in food additives. This study essentially paved a new avenue for effectively developing an easy sensor platform for S^2- measurements in food additives.

Seeing luminescence appear as crystals crumble. Isolation and subsequent self-association of individual [(C6H11NC)2Au]+ ions in crystals

Non-luminescent, isostructural crystals of [(C6H11NC)2Au](EF6)·C6H6 (E = As, Sb) lose benzene upon standing in air to produce green luminescent (E = As) or blue luminescent (E = Sb) powders. Previous studies have shown that the two-coordinate cation, [(C6H11NC)2Au]+, self-associates to form luminescent crystals that contain linear or nearly linear chains of cations and display unusual polymorphic, vapochromic, and/or thermochromic properties. Here, we report the formation of non-luminescent crystalline salts in which individual [(C6H11NC)2Au]+ ions are isolated from one another. In [(C6H11NC)2Au](BArF24) ((BArF24)- is tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) each cation is surrounded by two anions that prohibit any close approach of the gold ions. Crystallization of [(C6H11NC)2Au](EF6) (E = As or Sb, but not P) from benzene solution produces colorless, non-emissive crystals of the solvates [(C6H11NC)2Au](EF6)·C6H6. These two solvates are isostructural and contain columns in which cations and benzene molecules alternate. With the benzene molecules separating the cations, the shortest distances between gold ions are 6.936(2) Å for E = As and 6.9717(19) Å for E = Sb. Upon removal from the mother liquor, these crystals crack due to the loss of benzene from the crystal and form luminescent powders. Crystals of [(C6H11NC)2Au](SbF6)·C6H6 that powder out form a pale yellow powder with a blue luminescence with emission spectra and powder X-ray diffraction data that show that the previously characterized [(C6H11NC)2Au](SbF6) is formed. In the process, the distances between the gold(i) ions decrease to ∼3 Å and half of the cyclohexyl groups move from an axial orientation to an equatorial one. Remarkably, when crystals of [(C6H11NC)2Au](AsF6)·C6H6 stand in air, they lose benzene and are converted into the yellow, green-luminescent polymorph of [(C6H11NC)2Au](AsF6) rather than the colorless, blue-luminescent polymorph. Paradoxically, the yellow, green-luminescent powder that forms as well as authentic crystals of the yellow, green-luminescent polymorph of [(C6H11NC)2Au](AsF6) are sensitive to benzene vapor and are converted by exposure to benzene vapor into the colorless, blue-luminescent polymorph.

New barbituric acid derivatives for data encryption and decryption based on the mechanochromic fluorescence effect

Two barbituric acid derivatives CB-Ph and CB-Me were synthesized, both of which show a strong aggregation induced emission (AIE) effect. It was found that these two compounds show almost the same absorption and emission spectra. As a result, they show the same yellow color in daylight and the same yellow-green color under UV at 365 nm. Upon grinding, CB-Ph exhibits superior mechanochromic fluorescence (MCF) properties, with its fluorescence color from yellow-green (555 nm) to brown (580 nm) and its emission intensity decreases by 93%. However, neither the fluorescence peak wavelength nor the intensity of CB-Me shows observable changes after being ground, indicating no MCF effect. The X-ray diffraction (XRD) data indicate that the CB-Ph powder changes from the crystalline to amorphous state after being ground, whereas CB-Me remains in its crystalline state. Molecular dynamics simulation (MDS) shows that CB-Ph takes a 3D conformation due to three phenyl groups on the periphery of the molecule, which hinders the molecules from aggregating closely and results in the change of the microstructure by external force stimuli. However, CB-Me aggregated more closely due to its better planarity and hydrophobic n-hexane group, which make it difficult to change the microstructure upon being ground. Based on the unique optical properties of CB-Ph and CB-Me, a new data encryption-decryption technology is developed. CB-Ph is used as cryptographic ink due to its excellent MCF effect upon grinding. CB-Me is used as a camouflage material as it shows the same color as CB-Ph in the daylight and under UV-365 nm; furthermore, its colors remain unchanged after being ground.

Benzosiloles with Crystallization-Induced Emission Enhancement of Electrochemiluminescence: Synthesis, Electrochemistry, and Crystallography

Crystallization-induced emission enhancement (CIEE) was demonstrated for the first time for electrochemilunimescence (ECL) with two new benzosiloles. Compared with their solution, the films of the two benzosiloles gave CIEE of 24 and 16 times. The mechanism of the CIEE-ECL was examined by spooling ECL spectroscopy, X-ray crystal structure analysis, photoluminescence, and DFT calculations. This CIEE-ECL system is a complement to the well-established aggregation-induced emission enhancement (AIEE) systems. Unique intermolecular interactions are noted in the crystalline chromophore. The first heterogeneous ECL system is established for organic compounds with highly hydrophobic properties.

Stimuli-Sensitive Aggregation-Induced Emission of Organogelators Containing Mesogenic Au(I) Complexes

As the luminescence from conventional organic luminophores is typically quenched in constrained environments, the aggregation-induced emission (AIE) phenomenon is of interest for the development of materials that exhibit strong luminescence in condensed phases. Herein, new bismesogenic Au complexes were developed as organogelators and their photophysical properties, including their AIE characteristics, were investigated in organogels and crystals. The crystals of the gold complexes exhibited room-temperature phosphorescence with relatively high quantum yields. Moreover, the gold complexes also showed photoluminescence in the organogels and we demonstrated that the reversible switching of the luminescence intensity was induced by the sol-gel phase transition. The intense photoluminescence in the crystal and gel was induced by the restricted internal motion of the luminophore in the molecular aggregates. However, in the sol, the network structure of the organogel was destroyed and the nonradiative deactivation of the excited states was enhanced. As a result, we can conclude that the switching of the luminescence intensity was induced by changes in the aggregated structures of the molecules. The developed Au-complex-based gelators are excellent candidates for the realization of stimuli-responsive soft and smart luminescent materials.

Effect of Slip-Stack Self-Assembly on Aggregation-Induced Emission and Solid-State Luminescence in 1,3-Diarylpropynones

Co-facial stacking can result in aggregation-caused quenching (ACQ) in conjugated organic luminogens. This study provides an attractive 'slip-stack' self-assembly approach which can eliminate the occurrence of ACQ. The obtained results from steady-state and time-resolved optical studies, along with X-ray diffraction and computational studies demonstrate aggregation-induced emission enhancement (AIEE) of a donor-π-acceptor based 1,3-diarylpropynone, namely 1-(naphthalenyl)-3-(pyren-1-yl)prop-2-yn-1-one (PYNAP). Unlike the monomer, which exhibits poor photoluminescence in solution (φ_f =2 % in ACN), the twisted manifold of PYNAP allows the orientation of the molecules in a slip-stack fashion during the course of aggregation, which not only avoids a direct co-facial arrangement, but also induces augmented rigidity, leading to restricted intramolecular rotation (RIR) and enhanced emission quantum yield (φ_f =5 % in ACN/H₂ O). The aggregation behavior of PYNAP's congener, 1-phenyl-3-(pyren-1-yl)prop-2-yn-1-one (PYPH) reinforces the hypothesis that slip-stack assembly is a useful strategy for AIEE in polycyclic hydrocarbon luminogens.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

Adsorption Promoted Aggregation-Induced Emission Showing Strong Dye Lateral Interactions

Aggregation-induced emission (AIE) is a powerful method to produce fluorescence for a diverse range of applications. While most previous work induced aggregation by change of solvent, ionic strength, pH, or self-assembly, we herein explored adsorption-induced aggregation using 4,4'-(hydrazine-1,2-diylidene bis(methanylylidene)) bis(3-hydroxybenzoic acid) (HDBB) as an AIE luminogen. HDBB is known to aggregate with AIE at low pH but not at neutral pH, and its aggregation facilitates excited state intramolecular proton transfer for enhanced emission. Using a nonquenching nanomaterial, Y₂O₃ nanoparticles, HDBB showed sevenfold fluorescence increase at pH 7.0. Fluorescence lifetime showed that HDBB was in the aggregated state in the presence of Y₂O₃. For comparison, a fluorescent porphyrin compound showed that adsorption caused quenching after mixing with Y₂O₃, whereas other dyes such as fluorescein, calcein, and rhodamine B failed to be adsorbed by Y₂O₃. Adsorption did not follow a Langmuir isotherm, but it showed strong lateral HDBB interactions because adsorption was only achieved with a high concentration of HDBB. Adsorption was inhibited by salt and by phosphate, indicating the importance of electrostatic and metal-binding interactions. Comparisons were made with other nanomaterials, where graphene oxide and CeO₂ quenched HDBB and a cationic liposome also enhanced its emission, although with a less red-shifted peak wavelength. This study provides a simple method to induce aggregation of an AIE dye and its aggregation in turn-enhanced adsorption.

Excitation Wavelength-Dependent Nearly Pure White Light-Emitting Crystals from a Single Gold(I)-Containing Complex

A gold(I) complex is reported. Interestingly, crystals of the gold(I) complex exhibit an excellent excitation wavelength-dependent emission effect at room temperature. Notably, a nearly pure white emission with Commission Internationale de L'Eclairage (CIE) 1931 chromaticity coordinates of (0.32, 0.33) is obtained upon excitation with 406 nm light.