Tetranuclear Au2Cu2 Clusters with Butterfly- and Planar-Shaped Metal Cores: Strong Rigidochromism Induced by Jahn–Teller Distortion in Two-Coordinated Gold(I) Centers

A Phosphorescent P/N/S Hybrid Ligand Stabilized Au2Cu Complex Selectively Senses Ammonia and Amines

Reaction of a P/N/S hybrid ligand dpppyatc (N,N-bis((diphenylphosphaneyl)methyl)-N-(pyridin-2-yl)-amino-thiocarbamide) with Au(tht)Cl (tht=tetrahydrothiophene) and [Cu(MeCN)4]BF4 afforded cluster complex [Au2Cu(dpppyatc)2](BF4)2Cl (1). Upon excitation at 480 nm, 1 emitted orange phosphorescence at 646 nm, which was red-shifted to ~698 nm selectively in the presence of ammonia or amine vapor. This chromic photoluminescent response toward ammonia was sensitive and reversible. Complex1 could detect ammonia in aqueous solution down to concentrations of 2 ppm (w/w).

Dynamic Motions of Ligands around the Metal Centers Afford a Fidget Spinner-Type AIE Luminogen

A new type of aggregation-induced emission (AIE) luminogen containing a dimeric metal fragment and two or three phthalazine ligands is described, which shows dynamic motions of ligands around the metal centers in solution. Based on the variable-temperature and EXSY NMR spectroscopy data, X-ray crystallography structures, and computational results, three different pathways (i.e., reversible exchange with haptotropic shifts, circulation of ligands around the dimeric metal fragment, and walking on the spot of ligands on the metal centers) were considered for this dynamic behavior. Restriction of these dynamic processes in the aggregate forms of the compounds (in H2O/CH3CN solvent mixtures) contributes to their AIE. DFT calculations and NMR analysis showed that bright excited states for these molecules are not localized on isolated molecules, and the emission of them stemmed from π-dimers or π-oligomers. The morphologies and the mode of associations in the solvent mixtures were determined by using transmission electron microscopy (TEM) and concentration-dependent NMR spectroscopy. The computational results showed the presence of a conical intersection (CI) between the S0 and S1 excited state, which provides an accessible pathway for nonradiative decay in these systems.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Vapochromic behaviour of a gold(I)-lead(II) complex as a VOC sensor

The reaction among [Au2Ag2(C6F5)4(OEt2)2]n, PbCl2 and terpyridine leads to the polymeric complex [{Au(C6F5)2}2{Pb(terpy)}]n (1). Its crystal structure reveals potential voids close to the lead centres large enough to hold different molecules. The availability of these free sites allows complex 1 to act as a VOC sensor. Thus, when 1 is exposed to different solvent vapours such as acetonitrile, toluene or THF, variations in its solid appearance and its photophysical properties are observed as a consequence of the formation of the new polymorphs [{Au(C6F5)2}2{Pb(terpy)(CH3CN)2}]n (2), [{Au(C6F5)2}2{Pb(terpy)}]n·Tol (3) and [{Au(C6F5)2}2{Pb(terpy)(THF)}]n·THF (4). Each polymorph displays a different emission energy depending on its structure and the presence of metallophilic interactions. In addition, the reversible solvent molecule exchange allows the tuning of the luminescence emissions in the greenish yellow-red range. DFT and TD-DFT calculations were performed to explain the origin of the luminescence of all these complexes.

Heterometallic Au(I)-Cu(I) Clusters: Luminescence Studies and 1O2 Production

Two different organometallic gold(I) compounds containing naphthalene and phenanthrene as fluorophores and 2-pyridyldiphenylphosphane as the ancillary ligand were synthesized (compounds 1 with naphthalene and 2 with phenanthrene). They were reacted with three different copper(I) salts with different counterions (PF₆^-, OTf^-, and BF₄^-; OTf = triflate) to obtain six Au(I)/Cu(I) heterometallic clusters (compounds 1a-c for naphthalene derivatives and 2a-c for phenanthrene derivatives). The heterometallic compounds present red pure room-temperature phosphorescence in both solution, the solid state, and air-equilibrated samples, as a difference with the dual emission recorded for the gold(I) precursors 1 and 2. The presence of Au(I)-Cu(I) metallophilic contacts has been identified using single-crystal X-ray diffraction structure resolution of two of the compounds, which play a direct role in the resulting red-shifted emission with respect to the gold(I) homometallic precursors. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) polymeric matrices were doped with our luminescent compounds, and the resulting changes in their emissive properties were analyzed and compared with those previously recorded in the solution and the solid state. All complexes were tested to analyze their ability to produce ¹O₂ and present very good values of Φ_Δ up to 50%.

A Novel Photoluminescent Ag/Cu Cluster Exhibits a Chromic Photoluminescence Response towards Volatile Organic Vapors

A new Ag/Cu bimetallic cluster [Ag₁₀Cu₆(bdppthi)₂(C≡CPh)₁₂(EtOH)₂](ClO₄)₄ (1, bdppthi = N,N'-bis(diphenylphosphanylmethyl)-tetrahydroimidazole) exhibited strong phosphorescent (PL) emission at 644 nm upon excitation at 400 nm. Removal of the coordinated EtOH molecules in 1 resulted in derivative 1a, which exhibited significant red-shifted emission at 678 nm. The structure and PL of 1 was restored on exposure to EtOH vapor. Cluster 1a also exhibited a vapor-chromic PL response towards other common organic solvent vapors including acetone, MeOH and MeCN. A PMMA film of 1a was developed as a reusable visible sensor for MeCN.

Real-Time and In Situ Viscosity Monitoring in Industrial Adhesives Using Luminescent Cu(I) Phenanthroline Molecular Sensors

Monitoring the viscosity of polymers in real-time remains a challenge, especially in confined environments where traditional rheological measurements are hard to apply. In this study, we have utilized the luminescent complex [Cu(diptmp)2]+ (diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline) as an optical probe for real-time sensing of viscosity in various adhesives during the curing process (viscosity increases). The emission lifetime of the triplet metal-to-ligand charge transfer (3MLCT) state of [Cu(diptmp)2]+ in epoxy adhesive increased exponentially during curing, similar to viscosity values obtained from oscillatory rheology. The longer lifetime in higher viscosity materials was attributed to changes in the excited-state deactivation processes from a known Jahn-Teller distortion in the Cu(I) geometry from tetrahedral in the ground state to square planar in the excited state. The real-time viscosity was also monitored reversibly by emission lifetime during polymer swelling (viscosity and lifetime decrease) and unswelling (viscosity and lifetime increase). Monitoring emission lifetime, unlike measuring the excited-state lifetime via transient absorption measurements in our previous study, allowed us to measure viscosity in opaque samples which scatter light. The optical probe [Cu(diptmp)2]+ in Gorilla Glue adhesive showed a clear correlation of the emission intensity or lifetime to viscosity during the curing process. We have also compared these lifetime changes using [Ru(bpy)3]2+ (bpy = bipyridine) as a control. [Cu(diptmp)2]+ showed not only a higher emission lifetime but also more ubiquity as a real-time viscosity sensor.

Controllable Synthesis and Luminescence Behavior of Tetrahedral Au@Cu4 and Au@Ag4 Clusters Supported by tris(2-Pyridyl)phosphine

We report herein a family of polynuclear complexes, [Au@Ag₄(Py₃P)₄]X₅ and [Au@Cu₄(Py₃P)₄]X₅ [X = NO₃, ClO₄, OTf, BF₄, SbF₆], containing unprecedented Au-centered Ag₄ and Cu₄ tetrahedral cores supported by tris(2-pyridyl)phosphine (Py₃P) ligands. The [Au@Ag₄]⁵⁺ clusters are synthesized via controlled substitution of the central Ag(I) ion in all-silver [Ag@Ag₄]⁵⁺ precursors by the reaction with Au(tht)Cl, while the [Au@Cu₄]⁵⁺ cluster is assembled through the treatment of a pre-organized [Au(Py₃P)₄]⁺ metallo-ligand with 4 equiv of a Cu(I) source. The structure of the Au@M₄ clusters has been experimentally and theoretically investigated to reveal very weak intermolecular Au-M metallophilic interactions. At ambient temperature, the designed compounds emit a modest turquoise-to-yellow luminescence with microsecond lifetimes. Based on the temperature-dependent photophysical experiments and DFT/TD-DFT computations, the emission observed has been assigned to an MLCT or LLCT type depending on composition of the cluster core.

Selective coordination of coinage metals using orthogonal ligand scaffolds

Group 11 metal complexes with their ability to form metallophilic interations are widely pursued to develop multifunctional luminescent materials. Heteronuclear coinage metal complexes are promising candidates to tune electronic and optical properties which are not readily accessed by their homometallic congeners. In this review, we present the concept of orthogonal ligands which are rationally designed to access heteronuclear coinage metal complexes and studied in terms of their photophysical properties. Bifunctional ligands containing soft and hard donor atoms have the potential of providing different coordination modes to selectively synthesise heterobimetallic complexes in a predictable manner. This review deals with ligand sets composed of pyridine, bipyridine- or iminopyridine-substituted NHCs featuring C-N coordination modes, phosphine-based N-heterocycles and amidinate ligand scaffolds comprising of P-N functionalities and mixed phosphine-phosphine oxide with P-O donor sites. Therefore, the scope of this perspective is the discussion of heteronuclear coinage metal complexes supported by recently developed bifunctional ligands in terms of their synthesis, coordination geometries and tunability of optical properties when compared to their homometallic analogues.

Gold(i) complexes bearing a PNP-type pincer ligand: photophysical properties and catalytic investigations

The synthesis and characterization of two dinuclear and five tetranuclear gold(i) complexes bearing the 2,6-bis(diphenylphosphinomethyl)pyridine diphosphane ligand (DPPMPY) are herein reported.

A photoluminescent thermometer made from a thermoresponsive tetranuclear gold complex and phosphor N630

Reaction of [(3-bdppmapy)(AuCl)₂] with NaHmba (3-bdppmapy = N,N'-bis-(diphenylphosphanylmethyl-3-aminopytidine, H₂mba = 2-mercaptobenzoic acid) resulted in a new tetranuclear Au/P/S complex [(3-bdppmapy)₂(AuHmba)₃(AuCl)] (1). Upon excitation at 370 nm, 1 exhibited solid state, room temperature, green fluorescent emission (QY = 4.7%, τ = 2.58 ns) which was significantly enanced at lower temperatures due to strengthening of the Au-Au interaction. Different ratios of 1 with phosphor N630 in PMMA were used to make thermochromic photoluminescent films and fibres that could be fabricated into an optical thermometer sensitive over temperature ranges 80-300 K and 300-370 K.

Study of a phosphorescent cationic iridium(III) complex displaying a blue-shift in crystals

We report the synthesis and the characterization of a new cationic iridium(III) complex featuring two 1-(p-methoxyphenyl)-5-methoxybenzimidazole cyclometallating ligands and a dimethylbipyridine ancillary ligand. The complex has been fully characterized by 1D and 2D NMR (¹H, ¹³C, ¹⁹F and ³¹P), elemental analysis and high-resolution mass spectrometry (HRMS). The photoluminescence studies performed in a solution, on amorphous powder and on crystals revealed an unexpected behavior. Indeed, the emission spectra observed in both solution (CH₂Cl₂) and amorphous powder samples are centered at around 580 nm, whereas in crystals the emission displays a large hypsochromic shift of ∼800 cm^-1 (λ_em = 558 nm). X-ray diffraction experiments, photophysical studies and DFT calculations allow for rationalizing the hypsochromic shift.

Keep it tight: a crucial role of bridging phosphine ligands in the design and optical properties of multinuclear coinage metal complexes

Copper subgroup metal ions in the +1 oxidation state are classical candidates for aggregation via non-covalent metal-metal interactions, which are supported by a number of bridging ligands. The bridging phosphines, soft donors with a relatively labile coordination to coinage metals, serve as convenient and essential components of the ligand environment that allow for efficient self-assembly of discrete polynuclear aggregates. Simultaneously, accessible and rich modification of the organic spacer of such P-donors has been used to generate many fascinating structures with attractive photoluminescent behavior. In this work we consider the development of di- and polynuclear complexes of M(i) (M = Cu, Ag, Au) and their photophysical properties, focusing on the effect of phosphine bridging ligands, their flexibility and denticity.

Reversible Mechanochromic Luminescence of Tetranuclear Cuprous Complexes

Mechanochromic luminescence materials have attracted rapidly growing interest. Nevertheless, the designed synthesis of such materials remains a challenge, and there have been few examples based on weak intramolecular interactions. Herein, we report a new approach for preparing mechanochromic luminescence materials of Cu(I) complexes, i.e., constructing a photoluminescence system that bears a large coplanar multinuclear Cu(I) unit showing weak intramolecular π···π interactions with the planar rings of the coordinated ligands in the molecule. Using it, a series of novel mechanochromic luminescent tetranuclear Cu(I) complexes have been successfully designed and synthesized. As revealed by single-crystal X-ray crystallography, these Cu(I) complexes share an identical {Cu₄[μ₃-η²(N,N),η¹(N),η¹(N)-pyridyltetrazole]₂}²⁺ planar fragment whose coplanar pyridyl rings exhibit weak intramolecular π···π interactions with the phenyl rings of the coordinated phosphine ligands in the molecule. All of these Cu(I) complexes exhibit reversible mechanochromic luminescence, which can be attributed to the change in the rigidity of the molecular structure resulting from the disruption and restoration of intramolecular π···π interactions between the pyridyl and phenyl rings triggered by grinding and CH₂Cl₂ vapor, as supported by powder X-ray diffraction and Fourier transform infrared spectrometry. In addition, the results might provide a new route for developing mechanochromic luminescence materials of Cu(I) complexes for intelligent responsive luminescent devices.