Bimetallic Au2Cu6 Nanoclusters: Strong Luminescence Induced by the Aggregation of Copper(I) Complexes with Gold(0) Species

The concept of aggregation‐induced emission (AIE) has been exploited to render non‐luminescent Cu^ISR complexes strongly luminescent. The Cu^ISR complexes underwent controlled aggregation with Au⁰. Unlike previous AIE methods, our strategy does not require insoluble solutions or cations. X‐ray crystallography validated the structure of this highly fluorescent nanocluster: Six thiolated Cu atoms are aggregated by two Au atoms (Au₂Cu₆ nanoclusters). The quantum yield of this nanocluster is 11.7 %. DFT calculations imply that the fluorescence originates from ligand (aryl groups on the phosphine) to metal (Cu^I) charge transfer (LMCT). Furthermore, the aggregation is affected by the restriction of intramolecular rotation (RIR), and the high rigidity of the outer ligands enhances the fluorescence of the Au₂Cu₆ nanoclusters. This study thus presents a novel strategy for enhancing the luminescence of metal nanoclusters (by the aggregation of active metal complexes with inert metal atoms), and also provides fundamental insights into the controllable synthesis of highly luminescent metal nanoclusters.     

Metal Complexes with Azolate‐Functionalized Multidentate Ligands: Tactical Designs and Optoelectronic Applications

This review is aimed at updating the recent development on the metal complexes bearing azolate‐containing chelates that have received a growing attention from both the industrial and academic sectors. Particular emphasis is given to the luminescent metal complexes, for which tridentate and multidentate bonding interactions give rise to both higher ligand field strength and better rigidity versus their bidentate counterparts—consequently, this is beneficial to the chemical stability and emission efficiency needed for applications such as organic light‐emitting diodes and bio‐imaging. Their basic designs involve chelates, such as monoanionic 6‐azolyl 2,2′‐bipyridine, dianionic 2,6‐diazolylpyridine, and 2‐azolyl‐6‐phenylpyridine, and the core metal ion spanning from main group elements, such as Ga^III and In^III, to the late transition metal ions such as Ru^II, Os^II, Ir^III, and Pt^II and even the lanthanides. Furthermore, the great versatility of these azolate chelates for assembling the robust and emissive metal complexes, provides bright prospect in future optoelectronic investigations.     

Applications of luminescent inorganic and organometallic transition metal complexes as biomolecular and cellular probes

The rich photophysical properties of luminescent inorganic and organometallic transition metal complexes, such as their intense, long-lived, and environment-sensitive emission, render them excellent candidates for biological and cellular studies. In this Perspective, we review examples of biological probes derived from luminescent transition metal complexes with a d(6), d(8), or d(10) metal center. The design of luminescent covalent labels and noncovalent probes for protein molecules is discussed. Additionally, the recent applications of these complexes as cellular probes and bioimaging reagents are described. Emphasis is put on the structural features, photophysical behavior, biomolecular interactions, cellular uptake, and intracellular localization properties of luminescent transition metal complexes.     

Tuning the Cellular Uptake Properties of Luminescent Heterobimetallic Iridium(III)–Ruthenium(II) DNA Imaging Probes

The synthesis of two new luminescent dinuclear Ir^III–Ru^II complexes containing tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine (tpphz) as the bridging ligand is reported. Unlike many other complexes incorporating cyclometalated Ir^III moieties, these complexes display good water solubility, allowing the first cell‐based study on Ir^III–Ru^II bioprobes to be carried out. Photophysical studies indicate that emission from each complex is from a Ru^II excited state and both complexes display significant in vitro DNA‐binding affinities. Cellular studies show that each complex is rapidly internalised by HeLa cells, in which they function as luminescent nuclear DNA‐imaging agents for confocal microscopy. Furthermore, the uptake and nuclear targeting properties of the complex incorporating cyclometalating 2‐(4‐fluorophenyl)pyridine ligands around its Ir^III centre is enhanced in comparison to the non‐fluorinated analogue, indicating that fluorination may provide a route to promote cell uptake of transition‐metal bioprobes.     

Metal–Carbonyl Units for Vibrational and Luminescence Imaging: Towards Multimodality

Metal–carbonyl complexes are attractive structures for bio‐imaging. In addition to unique vibrational properties due to the CO moieties enabling IR and Raman cell imaging, the appropriate choice of ancillary ligands opens up the opportunity for luminescence detection. Through a classification by techniques, past and recent developments in the application of metal–carbonyl complexes for vibrational and luminescence bio‐imaging are reviewed. Finally, their potential as bimodal IR and luminescent probes is addressed.     

Mesomorphism and Photophysics of Some Metallomesogens Based on Hexasubstituted 2,2′:6′, 2′′‐Terpyridines

The luminescent and mesomorphic properties of a series of metal complexes based on hexacatenar 2,2′:6′,2′′‐terpyridines are investigated using experimental methods and density functional theory (DFT). Two types of ligand are examined, namely 5,5′′‐di(3,4,5‐trialkoxyphenyl)terpyridine with or without a fused cyclopentene ring on each pyridine and their complexes were prepared with the following transition metals: Zn^II, Co^III, Rh^III, Ir^III, Eu^III and Dy^III. The exact geometry of some of these complexes was determined by single X‐ray diffraction. All complexes with long alkyl chains were found to be liquid crystalline, which property was induced on complexation. The liquid‐crystalline behaviour of the complexes was studied by polarising optical microscopy and small‐angle X‐ray diffraction. Some of the transition metal complexes (for example, those with Zn^II and Ir^III) are luminescent in solution, the solid state and the mesophase; their photophysical properties were studied both experimentally and using DFT methods (M06‐2X and B3LYP).     

Recent progress of luminescent metal complexes with photochromic units

Photo-responsive molecules have been studied extensively because of their light irradiation abilities that enable modulation of certain physical and chemical properties in emerging molecular electronic and photonic devices. For advanced photonic applications, photochromic metal complexes that have photochromic units as the photo-responsive ligand are highly desirable, as they allow improvement of the photochromic properties and their photo-switching functionality. This article focuses on recent progress in luminescent metal complexes with photochromic units. Luminescence-switching properties of photochromic metal complexes depend on characteristic electronic transitions. The electronic transitions of photochromic metal complexes can be divided into three categories: (1) π–π* transition of the ligand, (2) metal to ligand charge transfer (MLCT) in transition-metal complex, and (3) f–f transition in lanthanide complex. Luminescence modulation using various metal complexes with photochromic units has been studied extensively in recent years, and various applications for future molecular switching devices are expected in the field of advanced photonics. Based on the literature and our studies on luminescent metal complexes with photochromic units, we report on the recent progress of luminescent metal complexes with photochromic units.     

Unexpected Luminescence Behavior of Coinage Metal π‐Diborene Complexes

A number of unprecedented photophysical phenomena were observed in the study of luminescent π‐diborene complexes of Cu and Ag. These observations included unusually high fluorescence quantum yields (up to 100%) in solution for complexes of these metals. This result indicates that very little or no intersystem crossing between S₁ and T_n occurs in the complexes, despite the strong spin–orbit coupling of the metal atoms. The replacement of carbon with boron thus yields luminescent isolobal analogues of otherwise non‐emissive olefin complexes of Cu and Ag.     

NEW TRENDS IN THE DESIGN OF LUMINESCENT METAL COMPLEXES*

The requirements needed for an ideal luminophore are discussed with reference to the luminescent properties of organic molecules and metal complexes. Recent strategies for the design of luminescent metal complexes are discussed, with particular emphasis on cage-type complexes and oligonuclear homo- and hetero-metallic complexes.     

Luminescent Ionic Transition‐Metal Complexes for Light‐Emitting Electrochemical Cells

Higher efficiency in the end‐use of energy requires substantial progress in lighting concepts. All the technologies under development are based on solid‐state electroluminescent materials and belong to the general area of solid‐state lighting (SSL). The two main technologies being developed in SSL are light‐emitting diodes (LEDs) and organic light‐emitting diodes (OLEDs), but in recent years, light‐emitting electrochemical cells (LECs) have emerged as an alternative option. The luminescent materials in LECs are either luminescent polymers together with ionic salts or ionic species, such as ionic transition‐metal complexes (iTMCs). Cyclometalated complexes of Ir^III are by far the most utilized class of iTMCs in LECs. Herein, we show how these complexes can be prepared and discuss their unique electronic, photophysical, and photochemical properties. Finally, the progress in the performance of iTMCs based LECs, in terms of turn‐on time, stability, efficiency, and color is presented.     

Sensing of 2,4,6‐Trinitrotoluene (TNT) and 2,4‐Dinitrotoluene (2,4‐DNT) in the Solid State with Photoluminescent RuII and IrIII Complexes

A series of metal–organic chromophores containing Ru^II or Ir^III were studied for the luminometric detection of nitroaromatic compounds, including trinitrotoluene (TNT). These complexes display long‐lived, intense photoluminescence in the visible region and are demonstrated to serve as luminescent sensors for nitroaromatics. The solution‐based behavior of these photoluminescent molecules has been studied in detail in order to identify the mechanism responsible for metal‐to‐ligand charge‐transfer (MLCT) excited state quenching upon addition of TNT and 2,4‐dinitrotoluene (2,4‐DNT). A combination of static and dynamic spectroscopic measurements unequivocally confirmed that the quenching was due to a photoinduced electron transfer (PET) process. Ultrafast transient absorption experiments confirmed the formation of the TNT radical anion product following excited state electron transfer from these metal complexes. Reported for the first time, photoluminescence quenching realized through ink‐jet printing and solid‐state titrations was used for the solid‐state detection of TNT; achieving a limit‐of‐quantitation (LOQ) as low as 5.6 ng cm^?2. The combined effect of a long‐lived excited state and an energetically favorable driving force for the PET process makes the Ru^II and Ir^III MLCT complexes discussed here particularly appealing for the detection of nitroaromatic volatiles and related high‐energy compounds.     

Efficient Formation of Luminescent Lanthanide(III) Complexes by Solid‐Phase Synthesis and On‐Resin Screening

Time‐resolved luminescence measurements of luminescent lanthanide complexes have advantages in biological assays and high‐throughput screening, owing to their high sensitivity. In spite of the recent advances in their energy‐transfer mechanism and molecular‐orbital‐based computational molecular design, it is still difficult to estimate the quantum yields of new luminescent lanthanide complexes. Herein, solid‐phase libraries of luminescent lanthanide complexes were prepared through amide‐condensation and Pd‐catalyzed coupling reactions and their luminescent properties were screened with a microplate reader. Good correlation was observed between the time‐resolved luminescence intensities of the solid‐phase libraries and those of the corresponding complexes that were synthesized by using liquid‐phase chemistry. This method enabled the rapid and efficient development of new sensitizers for Sm^III, Eu^III, and Tb^III luminescence. Thus, solid‐phase combinatorial synthesis combined with on‐resin screening led to the discovery of a wide variety of luminescent sensitizers.     

A Ratiometric Luminescent Thermometer Co‐doped with Lanthanide and Transition Metals

Herein, we report the fabrication of a sensitive ratiometric and colorimetric luminescent thermometer with a wide operating‐temperature range, from cryogenic temperatures up to high temperatures, through the combination of lanthanide and transition metal complexes. Benefiting from the transition metal complex as a self‐reference, the lanthanide content in the mixed‐coordination complex, Eu_0.05(Mebip‐mim bromine)_0.15Zn_0.95(Mebip‐mim bromine)_1.9, was lowered to 5 %.     

Developing Luminescent Ratiometric Thermometers Based on a Covalent Organic Framework (COF)

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are proposed as a new type of support for grafting lanthanide ions (Ln³⁺) and employing these hybrid materials as ratiometric luminescent thermometers. A TpBpy‐COF—prepared from 1,3,5‐triformylphloroglucinol (Tp) and 2,2′‐bipyridine‐5,5′‐diamine (Bpy) grafted with Eu/Tb and Dy acetylacetone (acac) complexes can be successfully used as a luminescent thermometer in the 10–360 K (Eu) and 280–440 K (Tb) ranges with good sensing properties (thermal sensitivity up to 1.403 % K^?1, temperature uncertainty δT<1 K above 110 K). For the Eu/Tb systems, we observe an unusual and rarely reported behavior, that is, no thermal quenching of the Tb³⁺ emission, a result of the absence of ion‐to‐ligand/host energy back‐transfer. The LnCOF materials proposed here could be a new class of materials employed for temperature‐sensing applications following up on the well‐known luminescent metal–organic framework thermometers.     

Bimetallic Lanthanide Complexes Derived from Macrocycle‐Appended m‐Xylyl Derivatives: Synthesis and Spectroscopic Properties

A series of bimetallic lanthanide complexes was prepared from a bimacrocyclic system in which two DO3A units are linked by a m‐xylyl unit appended with either a NO₂ or an NH₂ group (DO3A=1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid). The Nd‐, Eu‐, Tb‐ and Yb‐complexes were all luminescent: time‐resolved studies indicated that the lipophilic xylyl group restricts close approach of H₂O to the metal centre.     

Electroluminochromic Materials and Devices Based on Metal Complexes

Electroluminochromism (ELC) refers to an interesting phenomenon exhibited by a material whose luminescent properties can be reversibly modulated under an electrical stimulus. Such a luminescence‐switching property has been widely used in various organic optoelectronic devices because it can simultaneously detect electrical and optical signals. Metal complexes are the promising candidates for ELC materials due to their sensitivity to an electrical stimulus. Herein, recent progress on electroluminochromic materials and devices based on various metal complexes has been summarized. Meanwhile, the applications of these complexes in data recording and security protection have also been discussed. Finally, a brief conclusion and outlook are presented, pointing out that the development of electroluminochromic metal complexes with excellent performance is important because they play a vital role in future intelligent optoelectronic devices.     

Reversible Phase Transition of Robust Luminescent Hybrid Hydrogels

We report herein on remote control over a reversible phase transition of robust luminescent hybrid hydrogels as enabled by the rational selection and incorporation of photoswitches. Azobenzene units functionalized with a guanidinium group were utilized as the photoswitches and incorporated through a host–guest inclusion method involving α‐cyclodextrins functionalized with 2,6‐pyridinedicarboxylic acid (PDA) groups. While the guanidinium functional groups bind to the negatively charged Laponite matrix surface to connect organic and inorganic components, the PDA groups enable simultaneous coordination with different lanthanide metal ions, thus rendering the hydrogel broadly luminescent. Owing to its conformation‐dependent binding behavior with α‐cyclodextrin, the isomerization of azobenzene induced association or dissociation of the inclusion complexes and thus lead to a reversible photocontrolled sol?gel phase transition of the luminescent hybrid hydrogels.     

Luminescent metal complexes of d6, d8 and d10 transition metal centres

This highlight focuses on various luminescent complexes with different transition metal centres of d(6), d(8) and d(10) electronic configurations. Through the systematic study on the variation of ligands, structural and bonding modes of different metal centres, the structure-property relationships of the various classes of luminescent transition metal complexes can be obtained. With the knowledge and fundamental understanding of their photophysical behaviours, their electronic absorption and luminescence properties can be fine-tuned. Introduction of supramolecular assembly with hierarchical complexity involving non-covalent interactions could lead to research dimensions of unlimited possibilities and opportunities. The approach of "function by design" could be employed to explore and exploit the potential applications of such luminescent transition metal complexes for future development of luminescent materials.     

Protein Labelling with Versatile Phosphorescent Metal Complexes for Live Cell Luminescence Imaging

To take advantage of the luminescent properties of d⁶ transition metal complexes to label proteins, versatile bifunctional ligands were prepared. Ligands that contain a 1,2,3‐triazole heterocycle were synthesised using Cu^I catalysed azide–alkyne cycloaddition “click” chemistry and were used to form phosphorescent Ir^III and Ru^II complexes. Their emission properties were readily tuned, by changing either the metal ion or the co‐ligands. The complexes were tethered to the metalloprotein transferrin using several conjugation strategies. The Ir^III/Ru^II–protein conjugates could be visualised in cancer cells using live cell imaging for extended periods without significant photobleaching. These versatile phosphorescent protein‐labelling agents could be widely applied to other proteins and biomolecules and are useful alternatives to conventional organic fluorophores for several applications.     

Bioconjugated Luminescent Nanoparticles for Biological Applications

Abstract

Inorganic nanostructures that interface with biological systems have recently attracted widespread interest in biology and medicine. Nanoparticles are thought to have potential as novel luminescent probes for both diagnostic (e.g., imaging) and therapeutic (e.g., drug delivery) purposes because of their size comparable to biomolecules and their novel optical, electronic, and magnetic properties. Critical issues for successful nanoparticle delivery include the ability to target specific tissues and cell types and escape from the biological particulate filter known as reticuloendothelial system. Three distinct types of luminescent nanoparticles have been identified which show promise in bioanalysis, namely dye‐doped nanoparticles, semiconductor and metal nanoparticles. In this article we examine the recent advances in the development of dye‐doped nanoparticles, metal and semiconductor nanoparticles, bioconjugation schemes to attach these nanoparticles to biomolecules and a few biological applications.