Luminescent Pt(II) Complexes Using Unsymmetrical Bis(2-pyridylimino)isoindolate Analogues.

A series of ligands based upon a 1,3-diimino-isoindoline framework have been synthesized and investigated as pincer-type (N∧N∧N) chelates for Pt(II). The synthetic route allows different combinations of heterocyclic moieties (including pyridyl, thiazole, and isoquinoline) to yield new unsymmetrical ligands. Pt(L1-6)Cl complexes were obtained and characterized using a range of spectroscopic and analytical techniques: 1H and 13C NMR, IR, UV-vis and luminescence spectroscopies, elemental analyses, high-resolution mass spectrometry, electrochemistry, and one example via X-ray crystallography which showed a distorted square planar environment at Pt(II). Cyclic voltammetry on the complexes showed one irreversible oxidation between +0.75 and +1 V (attributed to Pt2+/3+ couple) and a number of ligand-based reductions; in four complexes, two fully reversible reductions were noted between -1.4 and -1.9 V. Photophysical studies showed that Pt(L1-6)Cl absorbs efficiently in the visible region through a combination of ligand-based bands and metal-to-ligand charge-transfer features at 400-550 nm, with assignments supported by DFT calculations. Excitation at 500 nm led to luminescence (studied in both solutions and solid state) in all cases with different combinations of the heterocyclic donors providing tuning of the emission wavelength around 550-678 nm.

Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC.

A series of mixed ligand, photoluminescent organometallic Ir(III) complexes have been synthesized to incorporate substituted 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands. The structures of three example complexes were categorically confirmed using X-ray crystallography each sharing very similar structural traits including evidence of interligand hydrogen bond contacts that account for the shielding effects observed in the 1H NMR spectra. The structural iterations of the cyclometalated ligand provide tuning of the principal electronic transitions that determine the visible absorption and emission properties of the complexes: emission can be tuned in the visible region between 550 and 610 nm and with triplet lifetimes up to 10 µs. The nature of the emitting state varies across the series of complexes, with different admixtures of ligand-centered and metal-to-ligand charge transfer triplet levels evident. Finally, the use of the complexes as photosensitizers in triplet-triplet annihilation energy upconversion (TTA-UC) was investigated in the solution state. The study showed that the complexes possessing the longest triplet lifetimes showed good viability as photosensitizers in TTA-UC. Therefore, the use of an electron-withdrawing group on the 2-phenyl-1H-naphtho[2,3-d]imidazole ligand framework can be used to rationally promote TTA-UC using this class of complex.

Increasing the versatility of the biphenyl-fused-dioxacyclodecyne class of strained alkynes.

Biphenyl-fused-dioxacyclodecynes are a promising class of strained alkyne for use in Cu-free 'click' reactions. In this paper, a series of functionalised derivatives of this class of reagent, containing fluorescent groups, are described. Studies aimed at understanding and increasing the reactivity of the alkynes are also presented, together with an investigation of the bioconjugation of the reagents with an azide-labelled protein.

Organometallic Platinum(II) Photosensitisers that Demonstrate Ligand-Modulated Triplet-Triplet Annihilation Energy Upconversion Efficiencies.

A series of 2-phenylquinoxaline ligands have been synthesised that introduce either CF3 or OCF3 electron-withdrawing groups at different positions of the phenyl ring. These ligands were investigated as cyclometalating reagents for platinum(II) to give neutral complexes of the form [Pt(C^N)(acac)] (in which C^N=cyclometalating ligand; acac=acetyl acetonate). X-ray crystallographic studies on three examples showed that the complexes adopt an approximate square planar geometry. All examples revealed strong Pt-Pt linear contacts of 3.2041(6), 3.2199(3) and 3.2586(2) Å. The highly coloured complexes display efficient visible absorption at 400-500 nm (ϵ ≈5000 M-1  cm-1 ) and orange red photoluminescent characteristics (λem =603-620 nm; Φem ≤37 %), which were subtly tuned by the ligand. Triplet emitting character was confirmed by microsecond luminescence lifetimes and the photogeneration of singlet oxygen with quantum efficiencies up to 57 %. Each complex was investigated as a photosensitiser for triplet-triplet annihilation energy upconversion using 9,10-diphenylanthracene as the annihilator species: a range of good upconversion efficiencies (ΦUC 5.9-14.1 %) were observed and shown to be strongly influenced by the ligand structure in each case.

Polycationic Ru(II) Luminophores: Syntheses, Photophysics, and Application in Electrostatically Driven Sensitization of Lanthanide Luminescence.

A series of photoluminescent Ru(II) polypyridine complexes have been synthesized in a manner that varies the extent of the cationic charge. Two ligand systems (L1 and L2), based upon 2,2'-bipyridine (bipy) mono- or difunctionalized at the 5- or 5,5'-positions using N-methylimidazolium groups, were utilized. The resulting Ru(II) species therefore carried +3, +4, +6, and +8 complex moieties based on a [Ru(bipy)3]2+ core. Tetra-cationic [Ru(bipy)2(L2)][PF6]4 was characterized using XRD, revealing H-bonding interactions between two of the counteranions and the cationic unit. The ground-state features of the complexes were found to closely resemble those of the parent unfunctionalized [Ru(bipy)3]2+ complex. In contrast, the excited state properties produce a variation in emission maxima, including a bathochromic 44 nm shift of the 3MLCT band for the tetra-cationic complex; interestingly, further increases in overall charge to +6 and +8 produced a hypsochromic shift in the 3MLCT band. Supporting DFT calculations suggest that the trend in emission behavior may, in part, be due to the precise nature of the LUMO and its localization. The utility of a photoactive polycationic Ru(II) complex was then demonstrated through the sensitization of a polyanionic Yb(III) complex in free solution. The study shows that electrostatically driven ion pairing is sufficient to facilitate energy transfer between the 3MLCT donor state of the Ru(II) complex and the accepting 2F5/2 excited state of Yb(III).

Upconversion in a d-f [RuYb3] Supramolecular Assembly.

We have prepared a hetero-tetrametallic assembly consisting of three ytterbium ions coordinated to a central [Ru(bpm)3]2+ (bpm = 2,2'-bipyrimidine) motif. Irradiation into the absorption band of the peripheral ytterbium ions at 980 nm engenders emission of the 3MLCT state of the central [Ru(bpm)3]2+ core at 636 nm, which represents the first example of f → d molecular upconversion (UC). Time-resolved measurements reveal a slow rise of the UC emission, which was modeled with a mathematical treatment of the observed kinetics according to a cooperative photosensitization mechanism using a virtual Yb centered doubly excited state followed by energy transfer to the Ru centered 1MLCT state.

Cooperative Luminescence and Cooperative Sensitisation Upconversion of Lanthanide Complexes in Solution.

Upconversion materials have led to various breakthrough applications in solar energy conversion, imaging, and biomedicine. One key impediment is the facilitation of such processes at the molecular scale in solution where quenching effects are much more pronounced. In this work, molecular solution-state cooperative luminescence (CL) upconversion arising from a Yb excited state is explored and the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/Tb systems is investigated. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with a quantum yield of 2.8×10-6 at a low laser power density of 2.86 W cm-2 .

Cyclam-Based Chelators Bearing Phosphonated Pyridine Pendants for 64Cu-PET Imaging: Synthesis, Physicochemical Studies, Radiolabeling, and Bioimaging.

Herein we present the preparation of two novel cyclam-based macrocycles (te1pyp and cb-te1pyp), bearing phosphonate-appended pyridine side arms for the coordination of copper(II) ions in the context of 64Cu PET imaging. The two ligands have been prepared through conventional protection-alkylation sequences on cyclam, and their coordination properties have been thoroughly investigated. The corresponding copper complexes have been fully characterized in the solid state (X-ray diffraction analysis) and in solution (EPR and UV-vis spectroscopies). Potentiometric studies combined with spectrometry have also allowed us to determine their thermodynamic stability constants, confirming their high affinity for copper(II) cations. The kinetic inertness of the complexes has been verified by acid-assisted dissociation experiments, enabling their use in 64Cu-PET imaging in mice for the first time. Indeed, the two ligands could be quantitatively radiolabeled under mild conditions, and the resulting 64Cu complexes have demonstrated excellent stability in serum. PET imaging demonstrated a set of features emerging from the combination of picolinates and phosphonate units: high stability in vivo, fast clearance from the body via renal elimination, and most interestingly, very low fixation in the liver. This is in contrast with what was observed for monopicolinate cyclam (te1pa), which had a non-negligible accumulation in the liver, owing probably to its different charge and lipophilicity. These results thus pave the way for the use of such phosphonated pyridine chelators for in vivo 64Cu-PET imaging.

Luminescent Anion Sensing by Transition-Metal Dipyridylbenzene Complexes Incorporated into Acyclic, Macrocyclic and Interlocked Hosts.

A series of novel acyclic, macrocyclic and mechanically interlocked luminescent anion sensors have been prepared by incorporation of the isophthalamide motif into dipyridylbenzene to obtain cyclometallated complexes of platinum(II) and ruthenium(II). Both the acyclic and macrocyclic derivatives 7⋅Pt, 7⋅Ru⋅PF6 , 10⋅Pt and 10⋅Ru⋅PF6 are effective sensors for a range of halides and oxoanions. The near-infra red emitting ruthenium congeners exhibited an increased binding strength compared to platinum due to the cationic charge and thus additional electrostatic interactions. Intramolecular hydrogen-bonding between the dipyridylbenzene ligand and the amide carbonyls increases the preorganisation of both acyclic and macrocyclic metal derivatives resulting in no discernible macrocyclic effect. Interlocked analogues were also prepared, and preliminary luminescent chloride anion spectrometric titrations with 12⋅Ru⋅(PF6 )2 demonstrate a marked increase in halide binding affinity due to the complementary chloride binding pocket of the [2]rotaxane. 1 H NMR binding titrations indicate the interlocked dicationic receptor is capable of chloride recognition even in competitive 30 % aqueous mixtures.

Formation of Heteropolynuclear Lanthanide Complexes Using Macrocyclic Phosphonated Cyclam-Based Ligands.

Ligands L1 and L2, respectively based on a cyclam and a cross-bridged cyclam scaffold functionalized at N1 and N8 by 6-phosphonic-2-methylene pyridyl groups, are described. While complexation of lanthanide (Ln) cations with L2 was not possible, a family of complexes has been prepared with L1, of the general formulae [LnL1H2]Cl (Ln3+ = Lu, Tb, Yb) or [LnL1H] (Ln3+ = Eu). The solution, structural, potentiometric, and photophysical data for these novel ligands and their complexes have been investigated, including a solid-state study by X-ray diffraction (L1, L2, and [EuL1H]), 1H NMR complexation investigations (Lu3+), as well as UV-vis absorption and luminescence spectroscopy in water and D2O (pH ≈ 7). L1 forms 1:1 metal-ligand stoichiometric octadentate complexes in solution. Importantly, the pyridyl phosphonate functions are capable of simultaneous chelation to the metal center and of interaction with a second metal center. 1H NMR (Lu3+) and spectrophotometric titrations of the isolated [TbL1]- complex by EuCl3 salts demonstrated the formation of high-order (hetero)polymetallic species in aqueous solution (H2O, pH = 7). Global analysis of the luminescence titration experiment points to the formation of 4:1, 3:1, and 3:2 [TbL1]/Eu heteropolynuclear assemblies, exhibiting a strong preference to forming [TbL1]3Eu2 at increased europium concentrations, with energy transfer occurring between the kinetically inert terbium complex and added europium cations.

Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes.

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2'-biphenyl)(PR3 )2 ][BArF4 ] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1 H and 31 P NMR spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3 >PCy3 >PiPr3 >PPh3 . In addition to these structure-property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH2 Cl2 ) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M-H-C distances.

Synthesis and cycloaddition reactions of strained alkynes derived from 2,2'-dihydroxy-1,1'-biaryls.

A series of strained alkynes, based on the 2,2'-dihydroxy-1,1'-biaryl structure, were prepared in a short sequence from readily-available starting materials. These compounds can be readily converted into further derivatives including examples containing fluorescent groups with potential for use as labelling reagents. The alkynes are able to react in cycloadditions with a range of azides without the requirement for a copper catalyst, in clean reactions with no observable side reactions.

Potassium Binding Adjacent to Cationic Transition-Metal Fragments: Unusual Heterobimetallic Adducts of a Calix[4]arene-Based Thione Ligand.

The synthesis of cationic rhodium and iridium complexes of a bis(imidazole-2-thione)-functionalized calix[4]arene ligand and their surprising capacity for potassium binding are described. In both cases, uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition-metal fragment [Rh+···K+ = 3.715(1) Å; Ir+···K+ = 3.690(1) Å]. The formation and constituent bonding of these unusual heterobimetallic adducts have been interrogated through extensive solution and solid-state characterization, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalized calix[4]arene analogue, and use of density functional theory based energy decomposition analysis.

Halotriazolium axle functionalised [2]rotaxanes for anion recognition: investigating the effects of halogen-bond donor and preorganisation.

The anion-templated synthesis of three novel halogen-bonding 5-halo-1,2,3-triazolium axle containing [2]rotaxanes is described, and the effects of altering the nature of the halogen-bond donor atom together with the degree of inter-component preorganisation on the anion-recognition properties of the interlocked host investigated. The ability of the bromotriazolium motif to direct the halide-anion-templated assembly of interpenetrated [2]pseudorotaxanes was studied initially; bromide was found to be the most effective template. As a consequence, bromide anion templation was used to synthesise the first bromotriazolium axle containing [2]rotaxane, the anion-binding properties of which, determined by (1) H NMR spectroscopic titration experiments, revealed enhanced bromide and iodide recognition relative to a hydrogen-bonding protic triazolium rotaxane analogue. Two halogen-bonding [2]rotaxanes with bromo- and iodotriazolium motifs integrated into shortened axles designed to increase inter-component preorganisation were also synthesised. Anion (1) H NMR spectroscopic titration experiments demonstrated that these rotaxanes were able to bind halide anions even more strongly, with the iodotriazolium axle integrated rotaxane capable of recognising halides in aqueous solvent media. Importantly, these observations suggest that a halogen-bonding interlocked host binding domain, in combination with increased inter-component preorganisation, are requisite design features for a potent anion receptor.

Synthesis and luminescent properties of hetero-bimetallic and hetero-trimetallic Ru(II)/Au(I) or Ir(III)/Au(I) complexes.

A series of Ru(II) and Ir(III) based photoluminescent complexes were synthesised that incorporate an ancillary 2,2'-bipyridine ligand adorned with either one or two pendant N-methyl imidazolium groups. These complexes have been fully characterised by an array of spectroscopic and analytical techniques. One Ir(III) example was unequivocally structurally characterised in the solid state using single crystal X-ray diffraction confirming the proposed formulation and coordination sphere. These complexes were then transformed into their heterometallic, Au(I)-containing, analogues in two steps to yield either bi- or trimetallic complexes that integrate {Au(PPh3)}+ units. X-ray diffraction was used to corroborate the solid state structure of the hetero bimetallic complex, based upon a Ru(II)-Au(I) species. The heterometallic complexes all displayed red photoluminescent features (λem = 616-629 nm) that were consistent with the parent Ru(II) or Ir(III) lumophores in each case. The modulation of the emission from the Ru(II)-Au(I) complexes was much more strongly evident than for the Ir(III)-Au(I) analogues, which is ascribed to the inherent differences in the specific triplet excited state character of the emitting states within each heterometallic species.

Upconverting photons at the molecular scale with lanthanide complexes.

In this perspective, we summarise the major milestones to date in the field of molecular upconversion (UC) with lanthanide based coordination complexes. This begins from the leap firstly from solid-state to nanoparticular regimes, and further down the scale to the molecular domain. We explain the mechanistic intricacies of each differing way of generating upconverted photons, critiquing them and outlining our views on the benefits and limitations of each process, also offering our perspective and opinion on where these new molecular UC edifices will take us. This nascent area is already rapidly expanding and improving, having increased in luminance efficiency by more than four orders of magnitude in the last decade: we conclude that the future is bright for molecular UC.

Upconversion in molecular hetero-nonanuclear lanthanide complexes in solution.

Here we show that nonanuclear lanthanide complexes respresent a new class of solution state upconversion (UC) molecules. For a composition of one Tb per eight Yb the nonanuclear complexes display a very efficient UC phenomenon with Tb luminescence in the visible region upon 980 nm NIR excitation of Yb. An unprecedented value of 1.0 × 10-7 was obtained for the UC efficiency at only 2.86 W cm-2, demonstrating these new molecular complexes to be up to 26 times more efficient than the best current molecular systems, the UC being observed down to a concentration of 10 nM.

Synthesis and Reactivity of a Bis-Strained Alkyne Derived from 1,1'-Biphenyl-2,2',6,6'-tetrol.

The novel "double strained alkyne" 3 has been prepared and evaluated in strain-promoted azide-alkyne cycloaddition reactions with azides. The X-ray crystallographic structure of 3, which was prepared in one step from 1,1'-biphenyl-2,2',6,6'-tetrol 4, reveals the strained nature of the alkynes. Dialkyne 3 undergoes cycloaddition reactions with a number of azides, giving mixtures of regiosiomeric products in excellent yields. The monoaddition products were not observed or isolated from the reactions, suggesting that the second cycloaddition proceeds at a faster rate than the first, and this is supported by molecular modeling studies. Dialkyne 3 was successfully employed for "peptide stapling" of a p53-based diazido peptide, whereby two azides are bridged to give a product with a stabilized conformation.

A strained alkyne-containing bipyridine reagent; synthesis, reactivity and fluorescence properties.

We report the synthesis of a bipyridyl reagent containing a strained alkyne, which significantly restricts its flexibility. Upon strain-promoted alkyne-azide cycloaddition (SPAAC) with an azide, which does not require a Cu catalyst, the structure becomes significantly more flexible and an increase in fluorescence is observed. Upon addition of Zn(ii), the fluorescence is enhanced further. The reagent has the potential to act as a fluorescent labelling agent with azide-containing substrates, including biological molecules.

Combining Electronic and Steric Effects To Generate Hindered Propargylic Alcohols in High Enantiomeric Excess.

Tethered ruthenium-TsDPEN complexes have been applied to the catalysis of the asymmetric transfer hydrogenation of a range of aryl/acetylenic ketones. The introduction of an ortho- substituent to the aryl ring of the substrate results in a reversal of the enantioselectivity, while the introduction of two o-fluoro substituents results in an improvement to the reduction enantioselectivity, as does the replacement of a phenyl ring on the alkyne with a trimethylsilyl group. These effects are rationalized as resulting from a change in the steric properties of the aryl ring and the electronic properties of the alkyne which, when matched in the reduction transition state, combine within a "window" of substrate/catalyst matching to generate products of high ee.