Wai-Pong To

Luminescent Organogold(III) Complexes with Long‐Lived Triplet Excited States for Light‐Induced Oxidative CH Bond Functionalization and Hydrogen Production

All that glitters is gold: highly phosphorescent gold(III) complexes with extended π-conjugated cyclometalating ligands exhibit rich photophysical and photochemical properties. They act as efficient photocatalysts/photosensitizers for oxidative functionalizations of secondary and tertiary benzylic amines and homogeneous hydrogen production from a water/acetonitrile mixture.

Strongly Luminescent Gold(III) Complexes with Long‐Lived Excited States: High Emission Quantum Yields, Energy Up‐Conversion, and Nonlinear Optical Properties

Strongly Luminescent Gold(III) Complexes with Long-Lived Excited States: High Emission Quantum Yields, Energy Up-Conversion, and Nonlinear Optical Properties Photochemistry : A series of emissive gold(III) complexes with fluorene-containing cyclometalating ligands exhibits strong phosphorescence and long-lived excited states with emission quantum yields and lifetimes up to 58 % and 305 ms, respectively. These complexes can sensitize energy up-conversion of 9,10-diphenylanthracene (DPA; see picture) and display rich two-photon absorption properties (TPA; TTA = triplet–triplet annihilation). Angewandte Chemie

A Robust Palladium(II)–Porphyrin Complex as Catalyst for Visible Light Induced Oxidative CH Functionalization

A series of palladium(II)-porphyrin complexes that display dual emissions with lifetimes up to 437 μs have been synthesized. Among the four complexes, PdF20TPP is an efficient and robust catalyst for photoinduced oxidative C-H functionalization by using oxygen as terminal oxidant. α-Functionalized tertiary amines were obtained in good to excellent yields by light irradiation (λ>400 nm) of a mixture of PdF20TPP, tertiary amine, and nucleophile (cyanide, nitromethane, dimethyl malonate, diethyl phosphite, and acetone) under aerobic conditions. Four examples of intramolecular cyclized amine compounds could be similarly prepared. Comparison of the UV-visible absorption spectra before and after the photochemical reaction revealed that PdF20TPP was highly robust (>95 % recovery). The practical application of PdF20TPP has been revealed by the photochemical reactions performed by using a low catalyst loading (0.01 mol %) and on a 10 mmol scale. The PdF20TPP catalyst could sensitize photoinduced oxidation of sulfides to sulfoxides in excellent yields. Mechanistic studies revealed that the photocatalysis proceeded by singlet-oxygen oxidation.

A Binuclear Gold(I) Complex with Mixed Bridging Diphosphine and Bis(N-Heterocyclic Carbene) Ligands Shows Favorable Thiol Reactivity and Inhibits Tumor Growth and Angiogenesis In Vivo†

In the design of anticancer gold(I) complexes with high in vivo efficacy, tuning the thiol reactivity to achieve stability towards blood thiols yet maintaining the thiol reactivity to target cellular thioredoxin reductase (TrxR) is of pivotal importance. Herein we describe a dinuclear gold(I) complex (1-PF6) utilizing a bridging bis(N-heterocyclic carbene) ligand to attain thiol stability and a diphosphine ligand to keep appropriate thiol reactivity. Complex 1-PF6 displays a favorable stability that allows it to inhibit TrxR activity without being attacked by blood thiols. In vivo studies reveal that 1-PF6 significantly inhibits tumor growth in mice bearing HeLa xenograft and mice bearing highly aggressive mouse B16-F10 melanoma. It inhibits angiogenesis in tumor models and inhibits sphere formation of cancer stem cells in vitro. Toxicology studies indicate that 1-PF6 does not show systemic anaphylaxis on guinea pigs and localized irritation on rabbits.

Water‐Soluble Luminescent Cyclometalated Gold(III) Complexes with cis‐Chelating Bis(N‐Heterocyclic Carbene) Ligands: Synthesis and Photophysical Properties

A new class of cyclometalated Au(III) complexes containing various bidentate C-deprotonated C^N and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized and characterized. These are the first examples of Au(III) complexes supported by cis-chelating bis-NHC ligands. [Au(C^N)(bis-NHC)] complexes display emission in solutions under degassed condition at room temperature with emission maxima (λmax ) at 498-633 nm and emission quantum yields of up to 10.1 %. The emissions are assigned to triplet intraligand (IL) π→π* transitions of C^N ligands. The Au(III) complex containing a C^N (C-deprotonated naphthalene-substituted quinoline) ligand with extended π-conjugation exhibits prompt fluorescence and phosphorescence of comparable intensity with λmax at 454 and 611 nm respectively. With sulfonate-functionalized bis-NHC ligand, four water-soluble luminescent Au(III) complexes, including those displaying both fluorescence and phosphorescence, were prepared. They show similar photophysical properties in water when compared with their counterparts in acetonitrile. The long phosphorescence lifetime of the water-soluble AuIII complex with C-deprotonated naphthalene-substituted quinoline ligand renders it to function as ratiometric sensor for oxygen. Inhibitory activity of one of these water-soluble Au(III) complexes towards deubiquitinase (DUB) UCHL3 has been investigated; this complex also displayed a significant inhibitory activity with IC50 value of 0.15 μM.

The effects of chelating N4 ligand coordination on Co(II)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

Here we describe the synthesis and X-ray crystal structures of a panel of cis-[CoII(N4)Cl2] complexes (N4 = tetradentate N atom donor ligand). We also examine the catalytic activities of these complexes in the photochemical and electrochemical reduction of CO2 to CO using [Ir(ppy)3] as the photosensitizer. Among the complexes studied, cis-[CoII(PDP)Cl2] (C1) (PDP = 1,1′-bis(2-pyridinylmethyl)-2,2′-bipyrrolidine) displayed the highest catalytic activity. This Co(II) complex was able to effectively mediate the reduction of CO2 to CO under either electrochemical or visible light photocatalytic conditions. For the electrocatalysis, C1 catalysed CO2 to CO with up to 96% Faraday efficiency at −1.70 V (vs. SCE, SCE = saturated calomel electrode). A selectivity of up to 95% for CO production was achieved in a photocatalytic CO2 reduction system by using C1 as the catalyst, Ir(ppy)3 as the photosensitizer and triethylamine as the electron donor. The Co(I) species in situ generated by the one electron reduction of cis-[CoII(PDP)Cl]+ is suggested to be directly responsible for CO2 activation. Ultrafast time (ns) resolved absorption spectroscopy revealed that the photoinduced electron transfer from the triplet excited state of Ir(ppy)3 to C1 is a key step in the generation of active Co(I) species. The electronic structure and redox properties of the Co(I) species, [CoI(N4)Cl], as well as its role in the catalytic reaction were investigated by DFT calculations. The presence of one chloride ligand cis to the CO2 coordination site neutralizes the positive charge on the Co(I) centre, therefore assisting the bound CO2 molecule in attracting protons. The reaction mechanism for CO2 reduction to CO catalysed by the recently reported [CoII(TPA)Cl]+ (TPA = tris(2-pyridylmethyl)amine) catalyst was also computed. Subtle modifications of the chelating N4 ligand from cis-[CoII(N4)Cl2] were found to have a profound effect on the efficiency of CO2 reduction by DFT calculations.

Deciphering Photoluminescence Dynamics and Reactivity of the Luminescent Metal–Metal‐Bonded Excited State of a Binuclear Gold(I) Phosphine Complex Containing Open Coordination Sites

Luminescent metal complexes having open coordination sites hold promise in the design of sensory materials and photocatalysts. As a prototype example, [Au2 (dcpm)2)](2+) (dcpm = bis(dicyclohexylphosphanyl) is known for its intriguing environmental sensitive photoluminescence. By integrating a range of complementary ultrafast time-resolved spectroscopy to interrogate the excited state dynamics, this study uncovers that the events occurring in extremely rapid timescales and which are modulated strongly by environmental conditions play a pivotal role in the luminescence behavior and photochemical outcomes. Formed independent of the phase and solvent property within ∼0.15 ps, the metal-metal bonded (3)5dσ*6pσ state is highly reactive possessing strong propensity toward increasing coordination number at Au(I) center, and with ∼510 ps lifetime in dichloromethane is able to mediate light induced C-X bond cleavage.

Light-induced catalytic and cytotoxic properties of phosphorescent transition metal compounds with a d8 electronic configuration

Transition metal compounds are well documented to have diverse applications such as in catalysis, light-emitting materials and therapeutics. In the areas of photocatalysis and photodynamic therapy, metal compounds of heavy transition metals are highly sought after because they can give rise to triplet excited states upon photoexcitation. The long lifetimes (more than 1 μs) of the triplet states of transition metal compounds allow for bimolecular reactions/processes such as energy transfer and/or electron transfer to occur. Reactions of triplet excited states of luminescent metal compounds with oxygen in cells may generate reactive oxygen species and/or induce damage to DNA, leading to cell death. This article recaps the recent findings on photochemical and phototoxic properties of luminescent platinum(II) and gold(III) compounds both from the literature and experimental results from our group.

Highly Luminescent Pincer Gold(III) Aryl Emitters: Thermally Activated Delayed Fluorescence and Solution-Processed OLEDs

Herein are described the synthesis, photophysical properties and applications of a series of luminescent cyclometalated AuIII complexes having an auxiliary aryl ligand. These complexes show photoluminescence with emission quantum yields of up to 0.79 in solution and 0.84 in thin films (4 wt % in PMMA) at room temperature, both of which are the highest reported values among AuIII complexes. Thermally activated delayed fluorescence (TADF) is the emission origin for some of these complexes. Solution-processed OLEDs made with these complexes showed sky-blue to green electroluminescence with external quantum efficiencies (EQEs) of up to 23.8 %, current efficiencies of up to 70.4 cd A-1 , and roll-off of down to 1 %, highlighting the bright prospect of AuIII -TADF emitters in OLEDs.

Organoplatinum(II) Complexes with Chromophore–Acceptor Dyad Studied by Ultrafast Time‐Resolved Absorption Spectroscopy

Photoinduced electron-transfer (ET) reactions are fundamental steps in photosynthesis, and which have led to extensive investigations into charge-separated species in artificial mimics. In the literature, many molecular systems based on d metal-to-ligand charge transfer (MLCT) and porphyrin p–p* chromophores have been developed for light-induced photocatalysis. These chromophores have several advantages, including high absorptivity in the visible spectral region, favorable redox properties, and relatively long-lived excited states, the properties of which can be tuned by varying the auxiliary ligands. Early developments in photoinduced ET catalysis focused on biomolecular reactions of triplet excited states of metal complexes. Studies on photoinduced multi-electron-transfer reactions of transition-metal complexes have led to many important observations, such as photoreduction of water using multicomponent systems, [Ru ACHTUNGTRENNUNG(bpy)3]2+/EDTA/MV2+ (EDTA=ethylenediaminetetraacetic acid disodium salt) or [Pt ACHTUNGTRENNUNG(terpy)(arylacetylide)]+/ TEOA/MV (terpy=2,2’:6’,2’’-terpyridine, TEOA= triethanolamine), along with single-component system, EDTA/ Ru-Pt . In the past decades, there have been extensive studies on the design of “chromophore-quencher” assemblies that comprise metal complex chromophores covalently linked to an organic electron donor or acceptor. A key obstacle for light-to-chemical energy conversion is the efficient back electron-transfer reaction between the highly oxidized and reduced species generated by ET reactions of excited-state species. A way to minimize the back electrontransfer reaction is that the latter is in the Marcus inverted region, allowing the photochemically generated oxidized and reduced species to have sufficient lifetimes to undergo multi-electron-transfer reactions other than the back ET reaction. Alternatively, the back ET reaction could be slowed down by the large reorganization energies required. Direct observation of inverted region behavior has been achieved by Gray and Winkler in a study of the back electron-transfer reactions between the products generated from the quenching of an electronically excited iridium(I) complex by pyridinium acceptors. Fukuzumi et al. also reported a long-lived charge-separated state with a lifetime of 10 ms involving the zinc(II) porphyrin-goldACHTUNGTRENNUNG(III) porphyrin dyad in cyclohexane. In recent years, Eisenberg and coworkers have reported a class of dyads containing a cationic [Pt ACHTUNGTRENNUNG(terpy) ACHTUNGTRENNUNG(C CAr)] chromophore. Herein we report the photoinduced ET reaction from Pt chromophore to viologen acceptor in a new [ClPt ACHTUNGTRENNUNG{C6N6NACHTUNGTRENNUNG(PhMV2+)}] (1; HCNN ACHTUNGTRENNUNG(PhMV2+)= 4’-[p-(1-methyl-4,4’-bipyridinium-1’-yl)methylphenyl]-6-phenyl-2,2’-bipyridine) dyad (Scheme 1). Despite the large driving force, the backward electron-transfer reaction is around 10 times slower than the forward electron-transfer reaction. For comparison, the Pt ACHTUNGTRENNUNG(diimine)-methylviologen bis(arylacetylide) dyad, [Pt ACHTUNGTRENNUNG(MV-bpy)ACHTUNGTRENNUNG(C CPh)2] ACHTUNGTRENNUNG(PF6)2 (3 ; MV-bpy= 1-(4-(4’-methyl-2,2’-bipyridin-4-yl)butyl)-1’-methyl-4,4’-bipyridinediium), was synthesized, characterized, and examined for light-induced ET reaction (Scheme 2, and see the Supporting Information). Complexes [ClPt ACHTUNGTRENNUNG{C6N6N ACHTUNGTRENNUNG(PhMe)}] (2) and [Pt ACHTUNGTRENNUNG(Me2bpy) ACHTUNGTRENNUNG(C CPh)2] (4 ; Me2bpy=4,4’-dimethyl-2,2’[a] Dr. S.-W. Lai, Dr. Y. Chen, Dr. X.-J. Zhao, Prof. Dr. W.-F. Fu Technical Institute of Physics and Chemistry Chinese Academy of Sciences Peking, 100080 (China) Fax: (+86) 10-6255-4670 E-mail : wf_fu@yahoo.com.cn [b] Dr. S.-W. Lai, Dr. W.-M. Kwok, W.-P. To, Prof. Dr. C.-M. Che Department of Chemistry and HKU-CAS Joint Laboratory on New Materials The University of Hong Kong Pokfulam Road (Hong Kong) Fax: (+852) 2857-1586 E-mail : cmche@hku.hk [c] Dr. W.-M. Kwok Department of Applied Biology and Chemical Technology The Hong Kong Polytechnic University Hung Hom, Kowloon (Hong Kong) [] Drs. S.-W. Lai and Y. Chen contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.200900304.