Photophysics of Cage/Guest Assemblies: Photoinduced Electron Transfer between a Coordination Cage Containing Osmium(II) Luminophores, and Electron-Deficient Bound Guests in the Central Cavity.

A heterometallic octanuclear coordination cage [Os4Zn4(Lnap)12]X16 (denoted Os•Zn; X = perchlorate or chloride) has been prepared (Lnap is a bis-bidentate bridging ligand containing two pyrazolyl-pyridine chelating units separated by a 1,5-naphthalenediyl spacer group). The {Os(NN)3}2+ units located at four of the eight vertices of the cube have a long-lived, phosphorescent 3MLCT excited state which is a stronger electron donor than [Ru(bipy)3]2+. The chloride form of Os•Zn is water-soluble and binds in its central cavity the hydrophobic electron-accepting organic guests 1,2,4,5-tetracyanobenzene, 1,4-naphthoquinone and 1-nitronaphthalene, with binding constants in the range 103-104 M-1, resulting in quenching of the phosphorescence arising from the Os(II) units. A crystal structure of an isostructural Co8 cage containing one molecule of 1,2,4,5-tetracyanobenzene as a guest inside the cavity has been determined. Ultrafast transient absorption measurements show formation of a charge-separated Os(III)/guest•- state arising from cage-to-guest photoinduced electron transfer; this state is formed within 13-21 ps, and decays on a time scale of ca. 200 ps. In the presence of a competing guest with a large binding constant (cycloundecanone) which displaces each electron-accepting quencher from the cage cavity, the charge-separated state is no longer observed. Further, a combination of mononuclear {Os(NN)3}2+ model complexes with the same electron-accepting species showed no evidence for formation of charge-separated Os(III)/guest•- states. These two control experiments indicate that the {Os(NN)3}2+ chromophores need to be assembled into the cage structure to bind the electron-accepting guests, and for PET to occur. These results help to pave the way for use of photoactive coordination cages as hosts for photoredox catalysis reactions on bound guests.

Catalysis in a Cationic Coordination Cage Using a Cavity-Bound Guest and Surface-Bound Anions: Inhibition, Activation, and Autocatalysis.

The Kemp elimination (reaction of benzisoxazole with base to give 2-cyanophenolate) is catalyzed in the cavity of a cubic M8L12 coordination cage because of a combination of (i) benzisoxazole binding in the cage cavity driven by the hydrophobic effect, and (ii) accumulation of hydroxide ions around the 16+ cage surface driven by ion-pairing. Here we show how reaction of the cavity-bound guest is modified by the presence of other anions which can also accumulate around the cage surface and displace hydroxide, inhibiting catalysis of the cage-based reaction. Addition of chloride or fluoride inhibits the reaction with hydroxide to the extent that a new autocatalytic pathway becomes apparent, resulting in a sigmoidal reaction profile. In this pathway the product 2-cyanophenolate itself accumulates around the cationic cage surface, acting as the base for the next reaction cycle. The affinity of different anions for the cage surface is therefore 2-cyanophenolate (generating autocatalysis) > chloride > fluoride (which both inhibit the reaction with hydroxide but cannot deprotonate the benzisoxazole guest) > hydroxide (default reaction pathway). The presence of this autocatalytic pathway demonstrates that a reaction of a cavity-bound guest can be induced with different anions around the cage surface in a controllable way; this was confirmed by adding different phenolates to the reaction, which accelerate the Kemp elimination to different extents depending on their basicity. This represents a significant step toward the goal of using the cage as a catalyst for bimolecular reactions between a cavity-bound guest and anions accumulated around the surface.

Porphyrin/Platinum(II) C

A new class of substituted porphyrins has been developed in which a different number of cyclometalated Pt(II) C^N^N acetylides and polyethylene glycol (PEG) chains are attached to the meso positions of the porphyrin core, which are meant for photophysical, electrochemical, and in vitro light-induced singlet oxygen ((1)O2) generation studies. All of these Zn(II) porphyrin-Pt(II) C^N^N acetylide conjugates show moderate to high (ΦΔ =0.55 to 0.63) singlet oxygen generation efficiency. The complexes are soluble in organic solvents but, despite the PEG substituents, slowly aggregate in aqueous solvent systems. These conjugates also exhibit interesting photophysical properties, including near-complete photoinduced energy transfer (PEnT) through the rigid acetylenic bond(s) from the Pt(II) C^N^N antenna units to the Zn(II) porphyrin core, which shows sensitized luminescence, as shown by quenching of Pt(II) C^N^N-based luminescence. Electrochemical measurements show a set of redox processes that are approximately the sum of what is observed for the Pt(II) C^N^N acetylide and Zn(II) porphyrin units. UV/Vis spectroscopic properties are supported by DFT calculations.

Stepwise assembly of mixed-metal coordination cages containing both kinetically inert and kinetically labile metal ions: introduction of metal-centred redox and photophysical activity at specific sites.

Stepwise preparation of the heterometallic octanuclear coordination cages [(M(a))4(M(b))4L12](16+) is reported, in which M(a) = Ru or Os and M(b) = Cd or Co (all in their +2 oxidation state). This requires initial preparation of the kinetically inert mononuclear complexes [(M(a))L3](2+) in which L is a ditopic ligand with two bidentate chelating pyrazolyl-pyridine units: in the complexes [(M(a))L3](2+) one terminus of each ligand is bound to the metal ion, such that the complex has three pendant bidentate sites at which cage assembly can propagate by coordination to additional labile ions M(b) in a separate step. Thus, combination of four [(M(a))L3](2+) units and four [M(b)](2+) ions results in assembly of the complete cages [(M(a))4(M(b))4L12](16+) in which a metal ion lies at each of the eight vertices, and a bridging ligand spans each of the twelve edges, of a cube. The different types of metal ion necessarily alternate around the periphery with each bridging ligand bound to one metal ion of each type. All four cages have been structurally characterised: in the Ru(ii)/Cd(ii) cage (reported in a recent communication) the Ru(ii) and Cd(ii) ions are crystallographically distinct; in the other three cages [Ru(ii)/Co(ii), Os(ii)/Cd(ii) and Os(ii)/Co(ii), reported here] the ions are disordered around the periphery such that every metal site refines as a 50 : 50 mixture of the two metal atom types. The incorporation of Os(ii) units into the cages results in both redox activity [a reversible Os(ii)/Os(iii) couple for all four metal ions simultaneously, at a modest potential] and luminescence [the Os(ii) units have luminescent (3)MLCT excited states which will be good photo-electron donors] being incorporated into the cage superstructure.

Serum Albumin Binding Inhibits Nuclear Uptake of Luminescent Metal-Complex-Based DNA Imaging Probes.

The DNA binding and cellular localization properties of a new luminescent heterobimetallic Ir(III) Ru(II) tetrapyridophenazine complex are reported. Surprisingly, in standard cell media, in which its tetracationic, isostructural Ru(II) Ru(II) analogue is localized in the nucleus, the new tricationic complex is poorly taken up by live cells and demonstrates no nuclear staining. Consequent cell-free studies reveal that the Ir(III) Ru(II) complex binds bovine serum albumin, BSA, in Sudlow's Site I with a similar increase in emission and binding affinity to that observed with DNA. Contrastingly, in serum-free conditions the complex is rapidly internalized by live cells, where it localizes in cell nuclei and functions as a DNA imaging agent. The absence of serum proteins also greatly alters the cytotoxicity of the complex, where high levels of oncosis/necrosis are observed due to this enhanced uptake. This suggests that simply increasing the lipophilicity of a DNA imaging probe to enhance cellular uptake can be counterproductive as, due to increased binding to serum albumin protein, this strategy can actually disrupt nuclear targeting.

Tuning the excited state of water-soluble Ir(III)-based DNA intercalators that are isostructural with [Ru(II)(NN)2(dppz)] light-switch complexes.

The synthesis of two new Ir(III)  complexes which are effectively isostructural with well-established [Ru(NN)2(dppz)](2+) systems is reported (dppz=dipyridophenazine; NN=2,2'-bipyridyl, or 1,10-phenanthroline). One of these Ir(III) complexes is tricationic and has a conventional N6 coordination sphere. The second dicationic complex has a N5C coordination sphere, incorporating a cyclometalated analogue of the dppz ligand. Both complexes show good water solubility. Experimental and computational studies show that the photoexcited states of the two complexes are very different from each other and also differ from their Ru(II) analogues. Both of the complexes bind to duplex DNA with affinities that are two orders of magnitude higher than previously reported Ir(dppz)-based systems and are comparable with Ru(II)(dppz) analogues.

Modulating the electron-transfer properties of a mixed-valence system through host-guest chemistry.

Metal directed self-assembly has become a much-studied route towards complex molecular architectures. Although studies on mixed valence, MV, systems accessible through this approach are almost non-existent, the potential applications of such systems are very exciting as MV states provide the basis of a number of molecular-scale devices, including single electron wires and switches. Furthermore, while many novel hosts for guest ions and molecules have been developed through metal directed self-assembly, as these products tend to be kinetically labile, very few electrochemical studies have been reported. Herein, we report that the interplay between the binding properties and redox activity of a self-assembled trinuclear RuII macrocycle leads to an hitherto unreported phenomenon, in which access to specific MV states can be gated by host-guest chemistry. Thus, this system is the first in which MV states and the extent of electron delocalisation are switched by an ion without any change in electrochemical potential.

Correction: Modulating the electron-transfer properties of a mixed-valence system through host-guest chemistry.

[This corrects the article DOI: 10.1039/C4SC02799J.].

Dinuclear osmium(II) probes for high-resolution visualisation of cellular DNA structure using electron microscopy.

Two dinuclear osmium polypyridyl complexes function as convenient, easy to handle TEM contrast agents and facilitate the high-resolution visualisation of intracellular structure, particularly sub-nuclear detail.

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.

From intercalation to groove binding: switching the DNA-binding mode of isostructural transition-metal complexes.

The interaction with duplex DNA of a small library of structurally related complexes that all contain a d6-metal ion coordinated to either the 2,2':4,4'':4',4'''-quaterpyridyl ligand or its methylated derivative are reported. This library is made up of a mixture of newly synthesised and previously reported systems. Despite their structural similarities the complexes display an almost 20-fold variation in binding affinities. Although effects due to the overall charge of the complexes are apparent, the differences in binding characteristics are deeper than this; indeed, in a number of cases, changes in overall charge have little effect on binding affinity. Intriguingly, despite interacting with DNA through unfused ring systems, although two of the complexes studied are groove binders, the majority are non-classical intercalators. A rationale for these effects has been obtained through a combination of experimental and computational studies.

Solvent-dependent modulation of metal-metal electronic interactions in a dinuclear cyanoruthenate complex: a detailed electrochemical, spectroscopic and computational study.

The dinuclear complex [{Ru(CN)(4)}(2)(µ-bppz)](4-) shows a strongly solvent-dependent metal-metal electronic interaction which allows the mixed-valence state to be switched from class 2 to class 3 by changing solvent from water to CH(2)Cl(2). In CH(2)Cl(2) the separation between the successive Ru(II)/Ru(III) redox couples is 350 mV and the IVCT band (from the UV/Vis/NIR spectroelectrochemistry) is characteristic of a borderline class II/III or class III mixed valence state. In water, the redox separation is only 110 mV and the much broader IVCT transition is characteristic of a class II mixed-valence state. This is consistent with the observation that raising and lowering the energy of the d(π) orbitals in CH(2)Cl(2) or water, respectively, will decrease or increase the energy gap to the LUMO of the bppz bridging ligand, which provides the delocalisation pathway via electron-transfer. IR spectroelectrochemistry could only be carried out successfully in CH(2)Cl(2) and revealed class III mixed-valence behaviour on the fast IR timescale. In contrast to this, time-resolved IR spectroscopy showed that the MLCT excited state, which is formulated as Ru(III)(bppz(˙-))Ru(II) and can therefore be considered as a mixed-valence Ru(II)/Ru(III) complex with an intermediate bridging radical anion ligand, is localised on the IR timescale with spectroscopically distinct Ru(II) and Ru(III) termini. This is because the necessary electron-transfer via the bppz ligand is more difficult because of the additional electron on bppz(˙-) which raises the orbital through which electron exchange occurs in energy. DFT calculations reproduce the electronic spectra of the complex in all three Ru(II)/Ru(II), Ru(II)/Ru(III) and Ru(III)/Ru(III) calculations in both water and CH(2)Cl(2) well as long as an explicit allowance is made for the presence of water molecules hydrogen-bonded to the cyanides in the model used. They also reproduce the excited-state IR spectra of both [Ru(CN)(4)(µ-bppz)](2-) and [{Ru(CN)(4)}(2)(µ-bppz)](4-) very well in both solvents. The reorganization of the water solvent shell indicates a possible dynamical reason for the longer life time of the triplet state in water compared to CH(2)Cl(2).

Synthesis, characterization, and DNA binding properties of ruthenium(II) complexes containing the redox active ligand benzo[i]dipyrido[3,2-a:2',3'-c]phenazine-11,16-quinone.

Synthetic methods toward ruthenium(II) complexes incorporating the benzo[i]dipyrido[3,2-a:2',3'-c]phenazine-11,16-quinone ligand, qdppn, are reported. In several cases, it was found that complexes containing coordinated benzo[i]dipyrido[3,2-a:2',3'-c]phenazine, dppn, could be chemically or photochemically oxidized to their qdppn analogues. Since this method was not possible in all the cases, a new, higher yielding, convenient synthesis of qdppn was developed. The crystal structure of the complex [Ru(phen)(2)(qppn)](PF(6))(2) (phen = 1,10-phenanthroline) which was synthesized from free qdppn reveals that a combination of π-π stacking between coordinated phen and qdppn units, as well as anion-ligand hydrogen bonding, define large hexagonal channels which are occupied by anions and solvent molecules. Electrochemical and photophysical studies reveal that the new qdppn-based complexes are not luminescent and, in contrast to their dppn analogues, they are also poor singlet oxygen sensitizers. Time-resolved studies and density functional theory (DFT) calculations indicate that optical properties of the new complexes are due to a short-lived charge separated state involving the quinone moiety of qdppn. The DNA binding properties of the new complexes have also been investigated. It was found that they are intercalators, displaying binding affinities which are comparable to their dppn analogues.