Molecular Design of Luminescence Ion Probes for Various Cations Based on Weak Gold(I)···Gold(I) Interactions in Dinuclear Gold(I) Complexes

Digold(I) Thianthrenyl Complexes. Effect of Diphosphine Ligands on Molecular Structures in the Solid State and in Solution

A series of digold complexes possessing two thianthrenyl ligands, Au₂(Thi)₂(Ph₂P(CH₂) _n PPh₂) (Thi: 1-thianthrenyl; 1: n = 1, 2: n = 2, 3: n = 3, 4: n = 4), were prepared and characterized by crystallographic and spectroscopic measurements. X-ray crystallography of complexes 1 and 3 revealed U-shaped structures with short Au-Au distances [3.2171(3) Å and 3.0735(2) Å]. Complex 2 and three of the four structure-determined molecules of complex 4 showed structures without Au-Au contacts. UV-vis spectroscopic measurements of 1-4 and TD-DFT calculations of the two conformers of 1 revealed that complexes 1 and 3 in the solution phase contained conformers with Au(I)-Au(I) interactions in a much higher proportion than complexes 2 and 4. As a result, complexes with diphosphine ligands containing an odd number of methylene groups preferred structures with Au-Au interactions in the solid state and in solution. Oxidation of 1 with 2 equiv of PhICl₂ yielded a mixture of monomeric and dimeric thianthrenes and its dimer via ligand elimination and C-C coupling, respectively.

Systematic investigation of the influence of electronic substituents on dinuclear gold(I) amidinates: synthesis, characterisation and photoluminescence studies

Dinuclear gold(I) compounds are of great interest due to their aurophilic interactions that influence their photophysical properties. Herein, we showcase that gold-gold interactions can be influenced by tuning the electronic properties of the ligands. Therefore, various para substituted (R) N,N'-bis(2,6-dimethylphenyl)formamidinate ligands (pRXylForm; Xyl = 2,6-dimethylphenyl and Form = formamidinate) were treated with Au(tht)Cl (tht = tetrahydrothiophene) to give via salt metathesis the corresponding gold(I) compounds [pRXylForm₂Au₂] (R = -OMe, -Me, -Ph, -H, -SMe, and -CO₂Me). All complexes showed intense luminescence properties at low temperatures. Alignment with the Hammett parameter σ_p revealed the trends in the ¹H and ¹³C NMR spectra. These results showed the influence of the donor-acceptor abilities of different substituents on the ligand system which were confirmed with calculated orbital energies. Photophysical investigations showed their lifetimes in the millisecond range indicating phosphorescence processes and revealed a redshift with the decreasing donor ability of the substituents in the solid state.

Hetero-bimetallic Lanthanide-Coinage Metal Compounds Featuring Possible Metal-Metal Interactions in the Excited State

Heterometallic complexes, combining metals of the outer rims of the d-block, for example lanthanides(III) (Ln) and coinage metals(I) (M) are scarcely reported, synthetically challenging and highly interesting in terms of their interactions. In this context, we synthesized hetero-bimetallic Ln-M compounds ligated by the phosphine functionalized amidinate system (N,N'-bis[(2-diphenylphosphino)phenyl]formamidinate, "dpfam"). The resulting compounds [dpfam3 LnM][OTf] (Ln = La, Nd and M = Ag, Au) feature a close proximity of the two metal centres and were investigated experimentally by photoluminescence spectroscopy and quantum chemical calculations. The latter showed rare La-Au interactions for the first excited state.

Stepwise coordination isomerism of 2D networks: adsorption of diiodomethane into crystals and recognition in SCSC mode

3CH₂Cl₂·2C₂H₅OH@[CdI₂L] with a new 2D topology of {4³·6²·8} are isomerized into new single crystals of 4C₄H₈O@[CdI₂L]. Interestingly, both crystals are integral to an efficient and tolerant matrix for recognition of diiodomethane in the SCSC mode.

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.

Molecular gold strings: aurophilicity, luminescence and structure-property correlations

This review covers the compound class of one-dimensional gold strings. These compounds feature a formally infinite repetition of gold complexes as monomers/repeating units that are held together by aurophilic interactions, i.e. direct gold-gold contacts. Their molecular structures are primarily determined in the solid state using single crystal X-ray diffraction. The chemical composition of the employed gold complexes is diverse and furthermore plays a key role in terms of structure characteristics and the resulting properties. One of the most common features of gold strings is their photoluminescence upon UV excitation. The emission energy is often dependent on the distance of adjacent gold ions and the electronic structure of the whole string. In terms of gold strings, these parameters can be fine-tuned by external stimuli such as solvent, pH value, pressure or mechanical stress. This leads to direct structure-property correlations, not only with regard to the photophysical properties, but also electric conductivity for potential application in nanoelectronics. Concerning these correlations, gold strings, consisting of self-assembled individual complexes as building blocks, are the ideal compound class to look at, as perturbations by an inhomogeneity in the ligand sphere (such as the end of a molecule) can be neglected. Therefore, the aim of this review is to shed light on the past achievements and current developments in this area.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Electronic and optical properties of [Au(CH3CSS)]4 cluster. A quantum chemistry study

The [Au(dta)]₄ (dta = dithioacetate, S₂CCH₃) cluster is modeled and the electronic and optical properties described.

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.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Luminescence mechanisms of ultrasmall gold nanoparticles

The past decade has witnessed a burst of study on ultrasmall gold nanoparticles. Unlike semiconductor quantum dots, ultrasmall gold nanoparticles have very diverse emission mechanisms, which are often involved in many structural factors such as size, valence state, surface ligands and crystallinity. In this frontier, we summarize our latest advancement in the fundamental understanding of emission mechanisms of ultrasmall gold nanoparticles, which are expected to help us more precisely control their emissions and broaden their applications from energy technologies to disease detection.

Incorporation of redox-inactive cations promotes iron catalyzed aerobic C-H oxidation at mild potentials

The synthesis and characterization of the Schiff base complexes Fe(ii) (2M) and Fe(iii)Cl (3M), where M is a K+ or Ba2+ ion incorporated into the ligand, are reported. The Fe(iii/ii) redox potentials are positively shifted by 440 mV (2K) and 640 mV (2Ba) compared to Fe(salen) (salen = N,N'-bis(salicylidene)ethylenediamine), and by 70 mV (3K) and 230 mV (3Ba) compared to Fe(Cl)(salen), which is likely due to an electrostatic effect (electric field) from the cation. The catalytic activity of 3M towards the aerobic oxidation of allylic C-H bonds was explored. Prior studies on iron salen complexes modified through conventional electron-donating or withdrawing substituents found that only the most oxidizing derivatives were competent catalysts. In contrast, the 3M complexes, which are significantly less oxidizing, are both active. Mechanistic studies comparing 3M to Fe(salen) derivatives indicate that the proximal cation contributes to the overall reactivity in the rate determining step. The cationic charge also inhibits oxidative deactivation through formation of the corresponding Fe2-μ-oxo complexes, which were isolated and characterized. This study demonstrates how non-redox active Lewis acidic cations in the secondary coordination sphere can be used to modify redox catalysts in order to operate at milder potentials with a minimal impact on the reactivity, an effect that was unattainable by tuning the catalyst through traditional substituent effects on the ligand.

[Au12(PPh2)2S4(L2)4]2+ (L2 = 3,4-bis(diphenylphosphino)-2,5-bis(trimethylsilyloxy)furan): an Au12 unit protected by modified maleic anhydride phosphine ligands

The reaction of the bis(dichlorogold) complex of 2,3-bis(diphenylphosphino)maleic anhydride (L1) with S(SiMe₃)₂ in the presence of NaBPh₄ yields the ionic dodecanuclear cluster compound [Au₁₂(PPh₂)₂S₄(L2)₄](BPh₄)₂ (1). During the formation of 1, the initial ligand L1 is transformed into the doubly sylilated derivative, 3,4-bis(diphenylphosphino)-2,5-bis(trimethylsilyloxy)-furan (L2). The presence of bridging PPh₂ groups is a result of the transformation of L1 in the coordination sphere of Au(i). Compound 1 was characterized by single crystal X-ray diffraction and electrospray-ionization (ESI) mass spectrometry.

Solvent-induced stable pseudopolymorphism of Au(i)–thiolate lamellar assemblies: a model system for understanding the environment acclimation of biomacromolecules

Two pseudopolymorphs are achieved in two solvents and exhibit high structure preservation but have distinct optical properties, morphology and thermal stability.

The interplay of non-covalent interactions determining the antiparallel conformation of (isocyanide)gold(i) dimers

Ligand⋯ligand contacts, including a novel n → π* interaction involving an isocyanide group as the acceptor, determine the persistent antiparallel conformation of dimers of (isocyanide)Au(i) complexes in their crystal structures.

π-Backbonding and non-covalent interactions in the JohnPhos and polyfluorothiolate complexes of gold(i)

We studied the influence of changing the degree of fluorination in eight new gold(i) derivatives containing both JohnPhos phosphine and polyfluorinated thiolates: [Au(SR_F)(JPhos)], JPhos = P(C₆H₄-C₆H₅)(t-But)₂ and R_F = C₆F₅ (1), C₆HF₄ (2), C₆H₃F₂-3,5 (3), C₆H₃F₂-2,4 (4), C₆H₄F-2 (5), C₆H₄F-3 (6), C₆H₄F-4 (7) and CF₃ (8). We determined the molecular and crystal structures of all new compounds by single crystal X-ray diffraction. Later, we characterised the chemical bonding scenario with quantum chemical topology tools, specifically the Quantum Theory of Atoms in Molecules (QTAIM) and the analysis of the NCI-index. Our QTAIM results indicate that while the linear S-Au-P moiety is unaffected by the variation of the fluorine content on the thiolates and that Au-S and Au-P bond strengths are mostly constant for all compounds in the series, the π character of gold bonds seems to be modified by the fluorination of the substituents at the thiolate ligand. Besides, the examination of the NCI-index reveals the presence of weak Au-π_Phenyl non-covalent interactions in all compounds. Overall, this study shows the relevance of (i) the π-backbonding properties of the metal centre and (ii) different non-covalent interactions in the stability of JohnPhos gold(i) compounds.

Weaving an infinite 3-D supramolecular network via AuI⋯AuIII aurophilicity and C–H⋯Cl hydrogen bonding

The use of ligand-unsupported Au^I⋯Au^III interactions to increase the structural dimensionality to a 3-D array was demonstrated.

“Host–guest” binding of a luminescent dinuclear Au(i) complex based on cyclic diphosphine with organic substrates as a reason for luminescence tuneability

A dinuclear Au(i) complex with cyclic diphosphine is a versatile platform for different luminescence responses to nucleophilic and electrophilic molecules in solutions.

Re(I) derivatives functionalised with thioether crowns containing the 1,10-phenanthroline subunit as a new class of chemosensors

A series of luminescent fac-[Re(CO)3(L)(NN)](+) complexes, where L is a pyridine or an imidazole and NN is the 1,10-phenanthroline subunit of mixed donor pentadentate thioether crowns have been synthesised and their luminescence properties have been analysed. Then, heterometallic Re(i)/Au(i) complexes, with the Au(i) fragment bonded directly to the imidazole ligand, and heterometallic Re(i)/Ag(i) complexes, with the silver fragment coordinating the S-donor thioether linker of the rings have also been prepared. Analysis of their luminescence properties showed a considerable blue shift of the emission maxima for the Re(i)/Ag(i) derivatives, upon coordination of the silver centre to the S-donor atoms of the aliphatic chain of the macrocyclic units.

Luminescent cation sensors: from host–guest chemistry, supramolecular chemistry to reaction-based mechanisms

This highlight provides a brief overview on luminescent cation detection strategies derived from a wide variety of organic and organometallic architectures, including those based on the ion-receptor complementarity, integrated with the extension of the concept of supramolecular chemistry and those using the irreversible analyte-specific reactions.

A colorimetric and luminescent dual-modal assay for Cu(II) ion detection using an iridium(III) complex

A novel iridium(III) complex-based chemosensor bearing the 5,6-bis(salicylideneimino)-1,10-phenanthroline ligand receptor was developed, which exhibited a highly sensitive and selective color change from colorless to yellow and a visible turn-off luminescence response upon the addition of Cu(II) ions. The interactions of this iridium(III) complex with Cu2+ ions and thirteen other cations have been investigated by UV-Vis absorption titration, emission titration, and 1H NMR titration.

Highly Ag+ selective tripodal gold(I) acetylide-based "off-on" luminescence chemosensors based on 3(ππ*) emission switching

Newly prepared gold(I) acetylide-based luminescent cation sensors 1c and 1d, which bear a novel three-armed flexible conformation, together with control molecule 2c, were synthesized in four steps. 1c and 1d exhibit the π → π* absorption of the acetylide ligands in fluid solutions and produce the (3)(ππ*) emission of the acetylide ligands in the solid state and degassed THF solution. The (3)(ππ*) emission of 1c in DMSO can be turned on upon addition of Ag(+). The binding ratio between 1c and Ag(+) as well as the binding constant log K were determined as 1:1 and 4.35 ± 0.12 respectively by UV-vis titration experiments. The formation of [1c·Ag](+) adduct, which leads to the appearance of new up-field peaks, was confirmed by (1)H NMR spectroscopic titrations. The control (1)H NMR titration experiments for the acetylide-free analogue 1a and single-armed analogue 2c indicate the acetylide groups and tripodal structures are responsible for the binding of Ag(+). The control experiment for tripodal gold(I) acetylide analogue 1d suggests the change of PPh3 with P(2-Py)Ph2 induces similar binding affinity toward Ag(+) but less selectivity toward Ag(+). Free 1c also exhibits the anion binding affinity toward F(-), but the Ag(+) adduct [1c·Ag](+) shows less affinity toward F(-).

Photofunctional organometallics—from fundamentals to design, assembly and functions

A number of soluble platinum(II) and gold(I) complexes have been designed and synthesized. Their luminescence and photophysical behavior have been studied. Their assembly properties, which were governed by the noncovalent metal–metal interactions, have also been investigated with the introduction of various external stimuli. The UV–vis and emission spectral changes of these complexes, which arose from the modulation of their assembly and disassembly behavior, have demonstrated the establishment of a convenient and sensitive detection strategy for various types of analytes. These studies suggested the potential applications of these metal complexes as detection probes for a diverse range of target substrates.

Metal coordination in photoluminescent sensing

Coordination chemistry plays an essential role in the design of photoluminescent probes for metal ions. Metal coordination to organic dyes induces distinct optical responses which signal the presence of metal species of interest. Luminescent lanthanide (Ln(3+)) and transition metal complexes of d(6), d(8) and d(10) configurations often exhibit unique luminescence properties different from organic dyes, such as high quantum yield, large Stokes shift, long emission wavelength and emission lifetimes, low sensitivity to microenvironments, and can be explored as lumophores to construct probes for metal ions, anions and neutral species. In this review, the design principles and coordination chemistry of metal probes based on mechanisms of PeT, PCT, ESIPT, FRET, and excimer formation will be discussed in detail. Particular attention will be given to rationales for the design of turn-on and ratiometric probes. Moreover, phosphorescent probe design based on Ln(3+) and d(6), d(8) and d(10)-metal complexes are also presented via discussing certain factors affecting the phosphorescence of these metal complexes. A survey of the latest progress in photoluminescent probes for identification of essential metal cations in the human body or toxic metal cations in the environment will be presented focusing on their design rationales and sensing behaviors. Metal complex-based photoluminescent probes for biorelated anions such as PPi, and neutral biomolecules ATP, NO, and H(2)S will be discussed also in the context of their metal coordination-related sensing behaviors and design approaches.

Protein-directed synthesis of NIR-emitting, tunable HgS quantum dots and their applications in metal-ion sensing

The development of luminescent mercury sulfide quantum dots (HgS QDs) through the bio-mineralization process has remained unexplored. Herein, a simple, two-step route for the synthesis of HgS quantum dots in bovine serum albumin (BSA) is reported. The QDs are characterized by UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, luminescence, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), circular dichroism (CD), energy dispersive X-ray analysis (EDX), and picosecond-resolved optical spectroscopy. Formation of various sizes of QDs is observed by modifying the conditions suitably. The QDs also show tunable luminescence over the 680-800 nm spectral regions, with a quantum yield of 4-5%. The as-prepared QDs can serve as selective sensor materials for Hg(II) and Cu(II), based on selective luminescence quenching. The quenching mechanism is found to be based on Dexter energy transfer and photoinduced electron transfer for Hg(II) and Cu(II), respectively. The simple synthesis route of protein-capped HgS QDs would provide additional impetus to explore applications for these materials.