Progress in mechanochromic luminescence of gold(I) complexes

Photophysical properties of organic and organometallic luminophors are closely related with their molecular packings, enabling the exploitation of stimuli-responsive functional luminescent molecules. Mechanochromic molecules, which can change their luminescence characteristics after mechanical stimulus, have received an increasing interest due to their promising applications in multifunctional sensors and molecular switches. During the past two decades, the development of gold(I) chemistry has been attracting the attention of plenty of researchers. Indeed, a variety of gold(I) complexes with fascinating photophysical behaviors have been discovered. This review focuses on the research progress in the different types of mechanoluminochromic gold(I) complexes, including mono-, bi- and multi-nuclear gold(I) systems. Their interesting luminescence behaviors of these gold(I)-containing luminogens upon mechanical stimulus and the proposed mechanisms of their observed mechanochromic luminescence are summarized systematacially. Moreover, this review will put forward an outlook about the possible opportunities and challenges in this significative scientific field.     

Synthesis and X-ray structures of dinuclear and trinuclear gold(I) and dinuclear gold(II) amidinate complexes

The structures of the trinuclear gold(I), [Au(3)(2,6-Me(2)-form)(2)-(THT)Cl], the dinuclear [Au(2)(2,6-Me(2)-form)(2)], and the oxidative-addition product [Au(2)(2,6-Me(2)-form)(2)Cl(2)] formamidinate complexes are reported. The trinuclear complex is stable with gold-gold distances 3.01 and 3.55 A. The gold-gold distance in the dinuclear complex decreases upon oxidative-addition with halogens from 2.7 to 2.5 A, similar to observations made with the dithiolates and ylides.     

Therapeutic applications of gold complexes: lipophilic gold(III) cations and gold(I) complexes for anti-cancer treatment

Gold and its complexes have long been known to display unique biological and medicinal properties. Extensive cell-based (in vitro) and animal (in vivo) studies have revealed the potent anti-cancer activities of diverse classes of gold(I) and gold(III) complexes. Most of the reported anti-cancer active gold complexes are highly cytotoxic and unstable under physiological conditions, which hamper their development to be launched clinically. Several clinical reports showed that lipophilic organic cations are promising anti-cancer drug candidates targeting to mitochondria. Through metal-ligand coordination, gold(I) and gold(III) ions can form stable lipophilic cations containing organic ligands having tunable lipophilicity and diverse functionalities. The present highlight summarizes the recent development of lipophilic gold(III) cations and gold(I) complexes with promising anti-cancer activities.     

Proton-induced luminescence of mono- and dinuclear rhenium(I) tricarbonyl complexes containing 4-pyridinealdazine

New mono- and dinuclear rhenium(I) tricarbonyls, of formulas [Re(bpy)(CO)3(PCA)]+ (1), [(bpy)(CO)3Re(I)(PCA)Re(I)(CO)3(bpy)]2+ (2), and [(bpy)(CO)3Re(I)(PCA)Ru(II)(NH3)5]3+ (3) (bpy = 2,2'-bipyridine, PCA = 4-pyridinecarboxaldehydeazine), have been synthesized as PF6- salts and characterized by spectroscopic, electrochemical, and photophysical techniques. These species do not emit at room temperature in CH(3)CN; however, in aqueous solutions, a decrease in pH induces luminescence in all of them, due to protonation of one of the N atoms of the -C=N-N=C- chain of PCA, as indicated by the pKa values of the ground states, obtained by absorption measurements, which are ca. 3 orders of magnitude lower than the pKa value of the pyridine N of PCA in complex 1. On the other hand, the values of pKa* of the excited states, obtained by emission measurements, of complexes 1 and 2 are similar (pKa* = 2.7 +/- 0.1 at I = 0.1 M) and higher than those of the corresponding ground states. At low values of pH, chemical decomposition takes place rapidly in complex 3, but not in 1 and 2, supporting the possible use of these latter species as luminescent sensors of pH. The heterodinuclear complex, of formula [(bpy)(CO)3Re(I)(PCA)Ru(III)(NH3)5]4+, was obtained by bromine oxidation of the [Re(I), Ru(II)] precursor in CH3CN solution; from spectral and electrochemical measurements, the recombination charge-transfer reaction [Re(II), Ru(II) ] --> [Re(I), Ru(III)], which occurs after photoexcitation, is predicted to lie in the Marcus inverted region.     

Analysis of weak interactions in the crystal packing of inorganic metalorganic hybrids based on Keggin polyoxometalates and dinuclear copper(II)-acetate complexes

Reaction of in situ generated copper-monosubstituted Keggin polyoxometalate (POM) and copper-phenanthroline complexes in potassium or sodium acetate buffers led to the formation of the potassium salt of [[SiW(11)O(39)Cu(H(2)O)][Cu(2)(phen)(2)(H(2)O)(ac)(2)]](4-) (1) and [[Si(2)W(22)Cu(2)O(78)(H(2)O)][u(2)(phen)(2)(H(2)O)(ac)(2)](2)](8-) (2, where phen = phenanthroline and ac = acetate) hybrid polyanions, respectively. Both compounds are the first discrete mono- and bimolecular transition-metal-substituted Keggin POMs that support a binuclear copper-acetate complex. Despite the different nature of the POMs, the crystal packing of the two compounds is closely related, being formed of hybrid parallel layers that give rise to an alternate sequence of inorganic and metalorganic regions. This packing type seems to be determined by the extensive network of weak intermolecular interactions established by the dicopper complexes, as a Hirshfeld surface analysis shows. Electron paramagnetic resonance studies indicate that both the supported [Cu(2)(ac)(2)(phen)(2)(H(2)O)](2+) complexes and the copper(II)-monosubstituted POMs are magnetically isolated.     

Synthesis, structural characterization, and theoretical studies of gold(I) and gold(I)-gold(III) thiolate complexes: quenching of gold(I) thiolate luminescence

The gold(I) thiolate complexes [Au(2-SC6H4NH2)(PPh3)] (1), [PPN][Au(2-SC6H4NH2)2] (2) (PPN = PPh3=N=PPh3), and [{Au(2-SC6H4NH2)}2(mu-dppm)] (3) (dppm = PPh2CH2PPh2) have been prepared by reaction of acetylacetonato gold(I) precursors with 2-aminobenzenethiol in the appropriate molar ratio. All products are intensely photoluminescent at 77 K. The molecular structure of the dinuclear derivative 3 displays a gold-gold intramolecular contact of 3.1346(4) A. Further reaction with the organometallic gold(III) complex [Au(C6F5)3(tht)] affords dinuclear or tetranuclear mixed gold(I)-gold(III) derivatives with a thiolate bridge, namely, [(AuPPh3){Au(C6F5)3}(mu2-2-SC6H4NH2)] (4) and [(C6F5)3Au(mu2-2-SC6H4NH2)(AudppmAu)(mu2-2-SC(6)H4NH2)Au(C6F5)3] (5). X-ray diffraction studies of the latter show a shortening of the intramolecular gold(I)-gold(I) contact [2.9353(7) or 2.9332(7) A for a second independent molecule], and short gold(I)-gold(III) distances of 3.2812(7) and 3.3822(7) A [or 3.2923(7) and 3.4052(7) A] are also displayed. Despite the gold-gold interactions, the mixed derivatives are nonemissive compounds. Therefore, the complexes were studied by DFT methods. The HOMOs and LUMOs for gold(I) derivatives 1 and 3 are mainly centered on the thiolate and phosphine (or the second thiolate for complex 2), respectively, with some gold contributions, whereas the LUMO for derivative 4 is more centered on the gold(III) fragment. TD-DFT results show a good agreement with the experimental UV-vis absorption and excitation spectra. The excitations can be assigned as a S --> Au-P charge transfer with some mixture of LLCT for derivative 1, an LLCT mixed with ILCT for derivative 2, and a S --> Au...Au-P charge transfer with LLCT and MC for derivative 3. An LMCT (thiolate --> Au(III) mixed with thiolate --> Au-P) excitation was found for derivative 4. The differing nature of the excited states [participation of the gold(III) fragment and the small contribution of sulfur] is proposed to be responsible for quenching the luminescence.     

Luminescent gold(I) complexes for chemosensing

With the rich spectroscopic and luminescence properties associated with aurophilic Au?Au interactions, gold(I) complexes have provided an excellent platform for the design of luminescent chemosensors. This review concentrates on our recent exploration of luminescent gold(I) complexes in host–guest chemistry. Through the judicious design and choice of the functional receptor groups, specific chemosensors for cations and/or anions have been obtained. Utilization of sensing mechanisms based on the on–off switching of Au?Au interactions and photoinduced electron transfer (PET) has been successfully demonstrated. The two-coordinate nature of gold(I) complexes has also been utilized for the design of ditopic receptors through connecting both cation- and anion-binding sites within a single molecule.     

Design and synthesis of luminescence chemosensors based on alkynyl phosphine gold(I)-copper(I) aggregates

Receptor-containing polynuclear mixed-metal complexes of gold(I)-copper(I) 1-3 based on a [{Au(3)Cu(2)(C≡CPh)(6)}Au(3){PPh(2)-C(6)H(4)-PPh(2)}(3)](2+) (Au(6)Cu(2)) core with benzo-15-crown-5, oligoether and urea binding sites were designed and synthesized, respectively. These complexes exhibited remarkably strong red emission at ca. 619-630 nm in dichloromethane solution at room temperature upon photoexcitation at λ > 400 nm, with the emission quantum yield in the range 0.59-0.85. The cation-binding properties of 1 and 2 and the anion-binding properties of 3 were studied using UV-vis, emission and (1)H NMR techniques. Complex 1, with six benzo-15-crown-5 pendants, was found to show a higher binding preference for K(+), with a selectivity trend of K(+)? Cs(+) > Na(+) > Li(+). The addition of metal ions (Li(+), Na(+), K(+) and Cs(+)) to complex 1 led to a modest emission enhancement with a concomitant slight blue shift in energy and well-defined isoemissive points, which is attributed to the rigidity of the structure and the inhibited PET (photo-induced electron transfer) process from the oxygen to the aggregate as a result of the binding of the metal ion. The six urea receptor groups on complex 3 were found to form multiple hydrogen bonding interactions with anions, with the positive charge providing additional electrostatic interaction for anion-binding. The anion selectivity of 3 follows the trend F(-) > Cl(-)≈ H(2)PO(4)(-) > Br(-) and the highest affinity towards F(-) is attributed to the stronger basicity of F(-), as well as its good size match with the cavity of the urea pocket.     

Synthesis, characterization, electrochemistry and luminescence studies of heterometallic gold(I)-rhenium(I) alkynyl complexes

The present work provides a brief summary review of the chemistry of luminescent gold(I) alkynyls and their ability to form heterometallic complexes. A series of luminescent heterometallic gold(I)-rhenium(I) alkynyl complexes has been synthesized and characterized. Their electrochemical and photophysical properties have been studied and their emission origins elucidated.     

Fine-tuning the luminescence and HOMO-LUMO energy levels in tetranuclear gold(I) fluorinated amidinate complexes

Tetranuclear gold(I) fluorinated amidinate complexes have been synthesized and their photophysical properties and structures described. DFT calculations were carried out to illustrate how a minor change in the ligand resulted in a loss of emission in the perfluorophenyl amidinate complex compared with nonfluorinated phenyl amidinate complexes reported previously. The fluorinated complexes reported here [Au(ArN)(2)C(H)](4) (1, Ar = 4-FC(6)H(4); 2, 3,5-F(2)C(6)H(3); 3, 2,4,6-F(3)C(6)H(2); 4, 2,3,5,6-F(4)C(6)H) emit in the blue-green region at 470, 1, 478, 2, 508, 3, and 450 nm, 4, by excitation at ca. 375 nm at room temperature with nanosecond lifetimes. The emissions observed at 77 K in the solid state show structured emission for complexes 1 and 2, with a vibrational spacing of ca. 1200 and 1500 cm(-1), corresponding to the vibrational modes of the amidinate ligand. The pentafluorophenyl derivative 5, Ar = C(6)F(5,) shows no photoluminescence in the solid state nor in the solution. This result is different from results in which the pentafluorophenyl group is attached to a phenylpyridine ligand in an Ir(III) complex and other organics. This quenching appears to be related to a nonradiative de-excitation process caused by the ππ*-πσ* crossover in the excited state of the pentafluorophenyl amidinate ligand. With increasing numbers of fluorine atoms, there is a progressive decrease in the contribution of the amidinate ligands to the corresponding HOMO orbital. There also is a slight decrease in the ligand contribution to the LUMO with increased numbers of fluorine atoms and an exchange of the character of the orbitals of the gold centers.     

Protonation reactions of dinuclear pyrazolato iridium(I) complexes

The complex [[Ir(mu-Pz)(CNBu(t))(2)](2)] (1) undergoes double protonation reactions with HCl and with HO(2)CCF(3) to give the neutral dihydride complexes [[Ir(mu-Pz)(H)(X)(CNBu(t))(2)](2)] (X = Cl, eta(1)-O(2)CCF(3)), in which the hydride ligands were located trans to the X groups and in the boat of the complexes, both in the solid state and in solution. The complex [[Ir(mu-Pz)(H)(Cl)(CNBu(t))(2)](2)] evolves in solution to the cationic complex [[Ir(mu-Pz)(H)(CNBu(t))(2)](2)(mu-Cl)]Cl. Removal of the anionic chloride by reaction with methyltriflate allows the isolation of the triflate salt [[Ir(mu-Pz)(H)(CNBu(t))(2)](2)(mu-Cl)]OTf. This complex undergoes a metathesis reaction of hydride by chloride in CDCl(3) under exposure to the direct sunlight to give the complex [[Ir(mu-Pz)(Cl)(CNBu(t))(2)](2)(mu-Cl)]OTf. Protonation of both metal centers in [[Ir(mu-Pz)(CO)(2)](2)] with HCl occurs at low temperature, but eventually the mononuclear compound [IrCl(HPz)(CO)(2)] is isolated. The related complex [[Ir(mu-Pz)(CO)(P[OPh](3))](2)] reacts with HCl and with HO(2)CCF(3) to give the neutral Ir(III)/Ir(III) complexes [[Ir(mu-Pz)(H)(X)(CO)(P[OPh](3))](2)], respectively. Both reactions were found to take place stepwise, allowing the isolation of the intermediate monohydrides. They are of different natures, i.e., the metal-metal-bonded Ir(II)/Ir(II) compound [(P[OPh](3))(CO)(Cl)Ir(mu-Pz)(2)Ir(H)(CO)(P[OPh](3))] and the mixed-valence Ir(I)/Ir(III) complex [(P[OPh](3))(CO)Ir(mu-Pz)(2)Ir(H)(eta(1)-O(2)CCF(3))(CO)(P[OPh](3))].     

Selenolate gold complexes with aurophilic Au(I)-Au(I) and Au(I)-Au(III) interactions

The gold(I) selenolate compound [Au(2)(SePh)(2)(mu-dppf)] (dppf = 1,1'-bis(diphenylphosphino)ferrocene) has been prepared by reaction of [Au(2)Cl(2)(mu-dppf)] with PhSeSiMe(3) in a molar ratio 1:2. This complex reacts with gold(I) or gold(III) derivatives to give polynuclear gold(I)-gold(I) or gold(I)-gold(III) complexes of the type [Au(4)(mu-SePh)(2)(PPh(3))(2)(mu-dppf)](OTf)(2), [Au(3)(C(6)F(5))(3)(mu-SePh)(2)(mu-dppf)], or [Au(4)(C(6)F(5))(6)(mu-SePh)(2)(mu-dppf)], with bridging selenolate ligands. The reaction of [Au(2)(SePh)(2)(mu-dppf)] with 1 equiv of AgOTf leads to the formation of the insoluble Ag(SePh) and the compound [Au(2)(mu-SePh)(mu-dppf)]OTf. The complexes [Au(4)(C(6)F(5))(6)(mu-SePh)(2)(mu-dppf)] and [Au(2)(mu-SePh)(mu-dppf)]OTf (two different solvates) have been characterized by X-ray diffraction studies and show the presence of weak gold(I)-gold(III) interactions in the former and intra- and intermolecular gold(I)-gold(I) inter-actions in the later.     

Synthesis and photophysical properties of luminescent mononuclear and dinuclear gold(I) complexes with ethynyl-substituted phenanthrolines

A novel asymmetric dinuclear gold(I) complex with 3,6-diethynylphenanthroline, 3,6-bis{(PPh₃)–Au–C≡C}₂-phen, has been synthesized from Au(PPh₃)Cl (PPh₃ = triphenylphosphine) and 3,6-diethynyl-1,10-phenanthroline. The asymmetrical dinuclear gold(I) complex, 3,6-bis{(PPh₃)–Au–C≡C}₂-phen, demonstrated a weak phosphorescence assignable to the metal-perturbed ³ π–π* transition in the long wavelength region compared to an intense emission of the symmetrical dinuclear complex with 3,8-diethynylphenanthroline, 3,8-bis{(PPh₃)–Au–C≡C}₂-phen. A similar tendency of phosphorescent bands for the mononuclear gold(I) complexes with 5-ethynylphenanthroline, 5-{(PPh₃)–Au–C≡C}-phen, and 3-ethynylphenanthroline, 3-{(PPh₃)–Au–C≡C}-phen was observed. The absorption bands assignable to the π–π*(C≡Cphen) transition and phosphorescent emission assignable to the metal-perturbed ³ π–π* transition for these four gold(I) complexes were reasonably consistent with the results calculated by DFT and TD-DFT.     

Syntheses, structures, and photophysical properties of mono- and dinuclear sulfur-rich gold(I) complexes

The dinuclear gold complexes [{Au(PPh 3)} 2(mu- dmid)] ( 1) ( dmid = 1,3-dithiole-2-one-4,5-dithiolate) and [{Au(PPh 3)} 2(mu- dddt)] ( 2) ( dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate) were synthesized and characterized by X-ray crystallography. Both complexes exhibit intramolecular aurophilic interactions with Au...Au distances of 3.1984(10) A for 1 and 3.1295(11) A for 2. A self-assembly reaction between 4,5-bis(2-hydroxyethylthio)-1,3-dithiole-2-thione ( (HOCH 2 CH 2 ) 2 dmit) and [AuCl(tht)] affords the complex [AuCl{ (HOCH 2 CH 2 ) 2 dmit}] 2 ( 4), which possesses an antiparallel dimeric arrangement resulting from a short aurophilic contact of 3.078(6) A. This motif is extended into two dimensions due to intra- and intermolecular hydrogen bonds via the hydroxyethyl groups, giving rise to a supramolecular network. Three compounds were investigated for their rich photophysical properties at 298 and 77 K in 2-MeTHF and in the solid state; [Au 2(mu- dmid)(PPh 3) 2] ( 1), [Au 2(mu- dddt)(PPh 3) 2] ( 2), and [AuCl{( HOCH 2 CH 2 ) 2 dmit}] ( 4). 1 exhibits relatively long-lived LMCT (ligand-to-metal charge transfer) emissions at 298 K in solution (370 nm; tau e approximately 17 ns, where M is a single gold not interacting with the other gold atom; i.e., the fluxional C-SAuPPh 3 units are away from each other) and in the solid state (410 nm; tau e approximately 70 mus). At 77 K, a new emission band is observed at 685 nm (tau e = 132 mus) and assigned to a LMCT emission where M is representative for two gold atoms interacting together consistent with the presence of Au...Au contacts as found in the crystal structure. In solution at 77 K, the LMCT emission is also red-shifted to 550 nm (tau e approximately 139 mus). It is believed to be associated to a given rotamer. 2 also exhibits LMCT emissions at 380 nm at 298 K in solution and at 470 nm in the solid state. 4 exhibits X/MLCT emission (halide/metal to ligand charge transfer) where M is a dimer in the solid state with obvious Au...Au interactions, resulting in red-shifted emission band, and is a monomer in solution in the 10 (-5) M concentration (i.e., no Au...Au interactions) resulting in blue-shifted luminescence. Both fluorescence and phosphorescence are observed for 4.     

Sensitised near-infrared luminescence from lanthanide(III) centres using Re(I) and Pt(II) diimine complexes as energy donors in d-f dinuclear complexes based on 2,3-bis(2-pyridyl)pyrazine

The luminescent transition metal complexes [Re(CO)(3)Cl(bppz)] and [Pt(CC-C(6)H(4)CF(3))(2)(bppz)] [bppz = 2,3-bis(2-pyridyl)pyrazine], in which one of the diimine binding sites of the potentially bridging ligand bppz is vacant, have been used as 'complex ligands' to make heterodinuclear d-f complexes by attachment of a {Ln(dik)(3)} fragment (dik = a 1,3-diketonate) at the vacant site. When Ln = Pr, Nd, Er or Yb the lanthanide centre has low-energy f-f excited states capable of accepting energy from the (3)MLCT excited state of the Pt(II) or Re(I) centre, quenching the (3)MLCT luminescence and affording sensitised lanthanide(III)-based luminescence in the near-IR region. UV/Vis and luminescence spectroscopic titrations allowed measurement of (i) the association constants for binding of the {Ln(dik)(3)} fragment at the vacant diimine site of [Re(CO)(3)Cl(bppz)] or [Pt(CC-C(6)H(4)CF(3))(2)(bppz)], and (ii) the degree of quenching of the (3)MLCT luminescence according to the nature of the Ln(III) centre. In all cases Nd(III) was found to be the most effective of the series at quenching the (3)MLCT luminescence of the d-block component because the high density of f-f excited states of the appropriate energy make it a particularly effective energy-acceptor.     

Metallophilic bonding and agostic interactions in gold(I) and silver(I) complexes bearing a thiotetrazole unit

Gold(I) and silver(I) complexes of 1-methyl-5-thio-tetrazole (1) have been prepared and the coordination chemistry of this ligand toward metal-phosphine frameworks has been explored. As indicated by IR and Raman data, ligand 1 is deprotonated and the resulted anion acts as a bidentate (S,N)-tetrazole-5-thiolato unit in the new gold(I) complexes, [Au(SCN(4)Me)(PPh(3))] (2), [{Au(SCN(4)Me)}(2)(μ-dppm)] (3), and [{Au(SCN(4)Me)}(2)(μ-dppe)] (4), while it is coordinated only through the sulfur atom as its neutral tetrazole-5-thione form in the silver(I) derivative, [Ag(HSCN(4)Me)(PPh(3))](2)(OTf)(2) (5). Further characterization of the new compounds was performed using multinuclear ((1)H, (13)C, (31)P, (19)F) NMR spectroscopy, mass spectrometry, and DSC measurements. Single-crystal X-ray diffraction studies revealed basically linear P-M-S arrangements in complexes 3-5. The bidentate (S,N) coordination pattern results in a T-shaped (S,N)PAu core in 3 and 4, whereas, in 5, a similar coordination geometry is achieved in the dimer association based on S-bridging ligand 1. Herein, weak (C)H···Au and (C)H···Ag agostic interactions were observed. An intramolecular Au···Au contact occurs in 3, while in 4 intermolecular aurophilic bonds lead to formation of a chain polymer. An intermolecular Ag···Ag contact is also present in the dimer unit of 5. Low-temperature (31)P NMR data for 5 evidenced the presence of monomer and dimer units in solution. Theoretical calculations on model of the complexes 2 and 4 are consistent with the geometries found by X-ray diffraction studies.     

Silver(I) and gold(I) complexes of rhodanine

Summary Some rhodanine (HL) complexes of silver(I) and gold(1) have been prepared and studied by conductivity measurements and by i.r. spectroscopy. Structures for the complexes are proposed.     

Coordination of pyridinethiols in gold(I) complexes

High-yield synthesis of gold(I) thionato complexes, bis(pyridine-2-thionato)gold(I) chloride (1) and bis(pyridine-4-thionato)gold(I) chloride (2), are described. According to their solid-state structures, a linear coordination of Au(I), equiplanar coordination of the ligands and two weak gamma-agostic interactions are found in both of these complexes despite of different relative positions of N and S atoms in the pyridinethionato ligands. Density functional theory calculations on 1 and 2 reproduce the observed X-ray structures. Even though the C-H...Au interactions of Au(I) and two pyridine moieties (2.83 and 2.88 A in 1 and 2.86 A in 2) are relatively weak, according to calculations they seem to provide further stabilization for the coordination and orientation of the ligands. In 1 the shortest Au...Au distances of 3.50 A indicate that aurophilic interactions, even though weak, are present in the solid state, whereas in 2 these interactions are absent.