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Several conjugates between folic acid and a series of kinetically stable lanthanide complexes have been synthesized, using amide coupling and azide–alkyne cycloaddition methodologies to link the metal-binding domain to folate through a variety of spacer groups. While all these complexes exhibit affinity for the folate receptor, it is clear that the point of attachment to folate is essential, with linkage through the γ-carboxylic acid giving rise to significantly enhanced receptor affinity. All the conjugates studied show affinities consistent with displacing biological circulating folate derivatives, 5-methyltetrahydrofolate, from folate receptors. All the complexes exhibit luminescence with a short-lived component arising from ligand fluorescence overlaid on a much longer lived terbium-centered component. These can be separated using time-gating methods. From the results obtained, the most promising approach to achieve sensitized luminescence in these systems requires incorporating a sensitizing chromophore close to the lanthanide.

Hypoxia (low oxygen levels) occurs in a range of biological contexts, including plants, bacterial biofilms, and solid tumors; it elicits responses from these biological systems that impact their survival. For example, conditions of low oxygen make treating tumors more difficult and have a negative impact on patient prognosis. Therefore, chemical probes that enable the study of biological hypoxia are valuable tools to increase the understanding of disease-related conditions that involve low oxygen levels, ultimately leading to improved diagnosis and treatment. While small-molecule hypoxia-sensing probes exist, the majority of these image only very severe hypoxia (<1% O2) and therefore do not give a full picture of heterogeneous biological hypoxia. Commonly used antibody-based imaging tools for hypoxia are less convenient than small molecules, as secondary detection steps involving immunostaining are required. Here, we report the synthesis, electrochemical properties, photophysical analysis, and biological validation of a range of indolequinone-based bioreductive fluorescent probes. We show that these compounds image different levels of hypoxia in 2D and 3D cell cultures. The resorufin-based probe 2was activated in conditions of 4% O2and lower, while the Me-Tokyo Green-based probe 4was only activated in severe hypoxia─0.5% O2and less. Simultaneous application of these compounds in spheroids revealed that compound 2images similar levels of hypoxia to pimonidazole, while compound 4images more extreme hypoxia in a manner analogous to EF5. Compounds 2and 4are therefore useful tools to study hypoxia in a cellular setting and represent convenient alternatives to antibody-based imaging approaches.

A complex containing the unstable isotope 254Es has been synthesized on a nanogram scale. Analysis of the fundamental bonding and spectroscopic characteristics of this einsteinium compound shows a blue shift of Es(iii) luminescence upon complexation, and the ligand serves as an antenna to sensitize the excited state.

Kinetically inert lanthanide complexes are proving to be highly effective building blocks for the preparation of complex heterometallic architectures, allowing complete control of metal ion domains, which cannot be achieved under thermodynamic control. Kinetic stability may render perceivable labile coordination bonds more durable than several types of covalent interactions. For complexes in clinical use, the significance of kinetic stability cannot be overstated, and this Account treats the topic accordingly.

A novel pretargeted SPECT imaging strategy based on the HaloTag enzyme has been evaluated for the first time in a living system. To determine the efficacy of this approach, two clinically relevant cancer biomarkers, HER2 and TAG-72, were selected to represent models of internalizing and noninternalizing antigens, respectively. In MDA-MB-231/H2N (HER2-expressing) and LS174T (TAG-72-expressing) xenograft tumors in mice, pretargeting experiments were performed in which HaloTag-conjugated derivatives of the antibodies trastuzumab (anti-HER2) or CC49 (anti-TAG-72) were utilized as primary agents, and the small molecule HaloTag ligands 111In-HTL-1, -2, and -3 were evaluated as secondary agents. While this approach was not sufficiently sensitive to detect the internalizing HER2 antigen, pretargeting experiments involving the most optimal secondary agent, 111In-HTL-3, were successful in detecting the noninternalizing antigen TAG-72 and provided high-contrast SPECT images at 4 and 24 h postinjection.

The barcode-like spectrum of lanthanide-centered emission has been used in imaging and to make responsive luminescent reporters. The intensities and the shapes of each line in the luminescence spectrum can also report on the coordination environment of the lanthanide ion. Here, we used lanthanide-centered emission to report on the binding of potassium in an 18-crown-6 binding pocket. The responsive systems were made by linking a crown ether to a kinetically inert lanthanide binding pocket using a molecular building block approach. Specifically, an alkyne-appended Ln.DO3A was used as a building block in a copper(I)-catalyzed alkyne–azide cycloaddition (CuAAC) “click” reaction with azide-functionalized crown ethers. The resulting complexes were investigated using NMR and optical methods. Titrations with potassium chloride in methanol observing the sensititzed europium- and terbium-centered emissions were used to investigate the response of the systems. The molecular reporters based on aliphatic crown ethers were found to have strongly inhibited binding of potassium, while the benzo-18-crown-6 derived systems had essentially the same association constants as the native crown ethers. The shape of the lanthanide emission spectra was shown to be unperturbed by the binding of potassium, while the binding was reported by an overall increased intensity of the lanthanide-centered emission. This observation was contrasted to the change in spectral shape between propargyl-Ln.DO3A and the triazolyl-Ln.DO3A complexes. The solution structure of the lanthanide complexes was found to be determining for the observed physical chemical properties of these systems.

Mesenchymal stem cells (MSCs) hold great promise as therapeutic agents in regenerative medicine. Numerous animal studies have documented the multipotency of MSCs, showing their capabilities for differentiating into orthopedic tissues such as muscle, bone, cartilage, and tendon. However, the safety of culture expanded MSCs for human use has only just begun to be reported. Methods: Between 2006 and 2010, two groups of patients were treated for various orthopedic conditions with cultureexpanded, autologous, bone marrow-derived MSCs (group 1: n50; group 2: n290-one patient in both groups). Cells were cultured in monolayer culture flasks using an autologous platelet lysate technique and re-injected into peripheral joints or into intervertebral discs with use of c-arm fluoroscopy. While both groups had prospective surveillance for complications, Group 1 additionally underwent 3.0T MRI tracking of the re-implant sites. Results: The mean age of patients treated was 53 /- 13.85 years; 214 were males and 125 females with mean follow-up time from any procedure being 435 days /- 261 days. Number of contacts initiated based on time from first procedure was 482 at 3 months, 433 at 6 months, 316 contacts at 12 months, 110 contacts at 24 months, and 22 contacts at 36 months. For Group 1, 50 patients underwent 210 MRI surveillance procedures at 3 months, 6 months, 1, 2, and 3 years which failed to demonstrate any tumor formation at the re-implant sites. Formal disease surveillance for adverse events based on HHS criteria documented significantly less morbidity than is commonly reported for more invasive surgical procedures, all of which were either self-limited or were remedied with therapeutic measures. Two patients were diagnosed with cancer out of 339 patients treated since study inception; however, this was almost certainly unrelated to the MSC therapy and the neoplasm rate in similar to that seen in the U.S. Caucasian population. Knee outcome data was collected on a subset of patients. Here, >75 improvement was reported in 41.4 while decreasing the improvement threshold to >50 improvement, 63.2 reported an improvement. At an average reporting time of 11.3 months from first procedure average reported relief in the knee sample equaled 53.1 (n133 reporting). Conclusions: Using both intensive high field MRI tracking and complications surveillance in 339 patients, no neoplastic complications were detected at any stem cell re-implantation site. These findings are consistent with our prior publication and other published reports that also show no evidence of malignant transformation in vivo, following implantation of MSCs for orthopedic use.

We have used time‐resolved luminescence methods to study rates of photoinduced energy transfer (PEnT) from [M(bipy)3]2+(M=Ru, Os) chromophores to LnIIIions with low‐energy f–f states (Ln=Yb, Nd, Er) in d–f dyads in which the metal fragments are separated by a saturated CH2CH2 spacer, a p‐C6H4spacer, or a p‐(C6H4)2spacer. The finding that d→f PEnT is much faster across a conjugated p‐C6H4spacer than it is across a shorter CH2CH2spacer points unequivocally to a Dexter‐type energy transfer, involving electronic coupling mediated by the bridging ligand orbitals (superexchange) as the dominant mechanism. Comparison of the distance dependence of the Ru→Nd energy‐transfer rate across different conjugated spacers [p‐C6H4or p‐(C6H4)2groups] is also consistent with this mechanism. Observation of Ru→Nd PEnT (as demonstrated by partial quenching of the RuII‐based 3MLCT emission (MLCT=metal‐to‐ligand charge transfer), and the growth of sensitised NdIII‐based emission at 1050 nm) over approximately 20 Å by an exchange mechanism is a departure from the normal situation with lanthanides, in which long‐range energy transfer often involves through‐space Coulombic mechanisms.

The complexes [Pt(bipy){CC‐(4‐pyridyl)}2] (1) and [Pt(tBu2bipy){CC‐(4‐pyridyl)}2] (2) and [Pt(tBu2‐bipy)(CC‐phen)2] (3) all contain a Pt(bipy)(diacetylide) core with pendant 4‐pyridyl (1and 2) or phenanthroline (3) units which can be coordinated to {Ln(diketonate)3} fragments (Ln = a lanthanide) to make covalently‐linked PtII/LnIIIpolynuclear assemblies in which the PtIIchromophore, absorbing in the visible region, can be used to sensitise near‐infrared luminescence from the LnIIIcentres. For 1and 2one‐dimensional coordination polymers [1⋅Ln(tta)3]∞and [2⋅Ln(hfac)3]∞are formed, whereas 3forms trinuclear adducts [3⋅{Ln(hfac)3}2] (tta=anion of thenoyl‐trifluoroacetone; hfac=anion of hexafluoroacetylacetone). Complexes 1–3show typical PtII‐based 3MLCT luminescence in solution at ≈510 nm, but in the coordination polymers [1⋅Ln(tta)3]∞and [2⋅Ln(hfac)3]∞the presence of stacked pairs of PtIIunits with short Pt⋅⋅⋅Pt distances means that the chromophores have 3MMLCT character and emit at lower energy (≈630 nm). Photophysical studies in solution and in the solid state show that the 3MMLCT luminescence in [1⋅Ln(tta)3]∞and [2⋅Ln(hfac)3]∞in the solid state, and the 3MLCT emission of [3⋅{Ln(hfac)3}2] in solution and the solid state, is quenched by Pt→Ln energy transfer when the lanthanide has low‐energy f–f excited states which can act as energy acceptors (Ln=Yb, Nd, Er, Pr). This results in sensitised near‐infrared luminescence from the LnIIIunits. The extent of quenching of the PtII‐based emission, and the Pt→Ln energy‐transfer rates, can vary over a wide range according to how effective each LnIIIion is at acting as an energy acceptor, with YbIIIusually providing the least quenching (slowest Pt→Ln energy transfer) and either NdIIIor ErIIIproviding the most (fastest Pt→Ln energy transfer) according to which one has the best overlap of its f–f absorption manifold with the PtII‐based luminescence.

The first example of assembling novel heteronuclear lanthanide–transition metal pseudorotaxanes where interpenetration of the lanthanide stoppered thread into the transition metal sensitiser containing macrocycle is signalled viasensitised near-infrared (NIR) lanthanide luminescence is described.

Luminescent lanthanide complexes have been prepared by exploiting the interaction between lanthanide DO3A complexes and chromophore bearing carboxylates or phosphonates. This interaction can be utilised to probe the choice of sensitising chromophore suited to a given lanthanide. Furthermore, ternary complexes obtained from chromophore appended carboxylates dissociate in the presence of phosphate, while those obtained from phosphonates do not.

The complexes [M(L1)2(NO3)] and [M(L2)(NO3)2] (M = Pr, Er; L1= the tetradentate ligand dihydrobis-[3-(2-pyridyl)pyrazolyl]borate; L2= the hexadentate ligand hydrotris-[3-(2-pyridyl)pyrazolyl]borate) were prepared and their structural and photophysical properties studied. All complexes are 10-coordinate. Crystallographic analysis of [M(L1)2(NO3)] (M = Pr, Er) showed that for the smaller Er(iii) ions steric congestion at the metal centre results in two of the Er–N(pyridyl) distances being particularly long, which does not occur with the larger Pr(iii) ion that is better able to accommodate 10-fold coordination. On UV irradiation, both Pr(iii) complexes show, in the visible region of their luminescence spectra, transitions originating from both the 3P0level (at ca.21 000 cm-1) and the 1D2level (at ca.17 000 cm-1), a consequence of the fact that the lowest triplet state of the coordinated pyrazolylborate ligands lies at ca.24 000 cm-1in each case so is high enough in energy to populate both levels. This contrasts with Pr(iii) complexes based on diketonate ligands in which the lower triplet energies of the ligands result in emission from the 1D2level only. At longer wavelengths, near-infrared luminescence arising from the 1D2emissive level is observed with lifetimes (in both the solid state and solution) being in the range 50–110 ns. For both Er(iii) complexes, luminescence at 1530 nm occurs following UV excitation of ligand-centred transitions. In CH2Cl2both complexes gave dual-exponential luminescence, with the major component having a lifetime characteristic of an intact Er(iii) complex (≈ 1.5 μs) and the minor component being much shorter lived (0.2–0.5 μs), suggestive of a species in which a ligand is partially detached and the metal is solvated, with the two forms interconverting slowly. This behaviour is consistent with the steric congestion and long M–N(pyridyl) bonds that were observed in [Er(L1)2(NO3)]. In the solid state both Er(iii) complexes gave very weak luminescence, which could be fitted to a single exponential decay with a lifetime similar to the longer-lived of the solution components.

The complexes ML12NO3 and ML2NO32 M Pr, Er; L1 the tetradentate ligand dihydrobis-3-2-pyridylpyrazolylborate; L2 the hexadentate ligand hydrotris-3-2-pyridylpyrazolylborate were prepared and their structural and photophysical properties studied. All complexes are 10-coordinate. Crystallographic analysis of ML12NO3 M Pr, Er showed that for the smaller Eriii ions steric congestion at the metal centre results in two of the Er–Npyridyl distances being particularly long, which does not occur with the larger Priii ion that is better able to accommodate 10-fold coordination. On UV irradiation, both Priii complexes show, in the visible region of their luminescence spectra, transitions originating from both the 3P0level at ca.21 000 cm−1 and the 1D2level at ca.17 000 cm−1, a consequence of the fact that the lowest triplet state of the coordinated pyrazolylborate ligands lies at ca.24 000 cm−1in each case so is high enough in energy to populate both levels. This contrasts with Priii complexes based on diketonate ligands in which the lower triplet energies of the ligands result in emission from the 1D2level only. At longer wavelengths, near-infrared luminescence arising from the 1D2emissive level is observed with lifetimes in both the solid state and solution being in the range 50–110 ns. For both Eriii complexes, luminescence at 1530 nm occurs following UV excitation of ligand-centred transitions. In CH2Cl2both complexes gave dual-exponential luminescence, with the major component having a lifetime characteristic of an intact Eriii complex 1.5 µs and the minor component being much shorter lived 0.2–0.5 µs, suggestive of a species in which a ligand is partially detached and the metal is solvated, with the two forms interconverting slowly. This behaviour is consistent with the steric congestion and long M–Npyridyl bonds that were observed in ErL12NO3. In the solid state both Eriii complexes gave very weak luminescence, which could be fitted to a single exponential decay with a lifetime similar to the longer-lived of the solution components.

The new pro-ligand 4-methyl-4′-carbonylamino2-tert-butoxycarbonylaminoethyl-2,2′-bipyridyl L1 has been prepared and used to synthesise the complex fac-ReICl(CO)3L11and the complex salts [MII(bipy)2(L1)](PF6)2M RuII8or OsII15. Deprotection with trifluoroacetic acid affords the amine-functionalised derivatives fac-ReICl(CO)3L22, [MII(bipy)2(L2)](PF6)2M RuII9or OsII16 which react with the dianhydride of diethylenetriamine pentaacetic acid DTPA to give the binuclear complex fac-ReICl(CO)32L33and the complex salts [MII(bipy)22(L3)](PF6)4M RuII10or OsII17. The latter react with salts LnOTf3to afford a series of 12 heterotrimetallic compounds that contain a lanthanide Ln ion in the DTPA binding site; fac-ReICl(CO)32L3LnIIILn Nd 4, Er 5, Yb 6or Y 7) and [MII(bipy)22(L3)LnIII](PF6)(OTf)3(M RuII, Ln Nd 11, Er 12, Yb 13or Y 14; M OsII, Ln Nd 18, Er 19, Yb 20or Y 21. All of these trimetallic species display absorption bands ascribed to metal-to-ligand charge-transfer MLCT excitations, and luminescence measurements show that these excited states can be used to sensitise near-infrared emission from LnIIILn Nd, Er or Yb ions. Single crystal X-ray structures of L1and [RuII(bipy)2(L2H)](H2PO4)3·CH32CO·0.8H2O were obtained, the latter revealing the presence of H2PO4−counter anions, the source of which is presumed to be hydrolysis of PF6−ions.

Reaction of 1-hydrazinophthalazine with chloroacetyl chloride yields 3-chloromethyl-1,2,4-triazolo-phthalazine. Reaction of this product with the tris tert-butyl ester of DO3A yields a triazolophthalazine appended macrocycle. Hydrolysis and complexation with lanthanide ions gives access to a series of lanthanide complexes Ln Nd, Eu, Yb, Er; these are all luminescent and exhibit sensitisation of the lanthanide centre by the chromophore.

The ligands tris3-2-pyridylpyrazol-1-ylhydroborate L1, potentially hexadentate and bis3-2-pyridylpyrazol-1-yldihydroborate L2, potentially tetradentate have been used to prepare ternary lanthanide complexes in which the remaining ligands are dibenzoylmethane anions dbm. EuL1dbm2 is eight-coordinate, with L1acting only as a tetradentate chelate with one potentially bidentate arm pendant and two bidentate dbm ligands. NdL1dbm2 was also prepared but on recrystallisation some of it rearranged to NdL12Nddbm4, which contains a twelve-coordinate NdL12cation two interleaved hexadentate podand ligands and the eight-coordinate anion Nddbm4−which, uniquely amongst eight-coordinate complexes having four diketonate ligands, has a square prismatic structure with near-perfect O8cubic coordination. Formation of this sterically unfavourable geometry is assumed to arise from favourable packing with the pseudo-spherical cation. The isostructural series of complexes LnL2dbm2 Ln Pr, Nd, Eu, Gd, Tb, Er, Yb was also prepared and all members structurally characterised; again the metal ions are eight-coordinate, from one tetradentate ligand L2and two bidentate dbm ligands. Photophysical studies on the complexes with Ln Pr, Nd, Er, and Yb were carried out; all show the near-IR luminescence characteristic of these metal ions, with longer lifetimes in CD3OD than in CH3OH. For YbL2dbm2, two species with different luminescence lifetimes were observed in CH3OH solution, corresponding to species with zero or one coordinated solvent molecules, in slow exchange on the luminescence timescale. For NdL2dbm2 a single average solvation number of 0.7 was observed in MeOH. For PrL2dbm2 a range of emission lines in the visible and NIR regions was detected; time-resolved measurements show a particularly high susceptibility to quenching by solvent CH and OH oscillators.

Ytterbium and neodymium have been shown to exhibit sensitised emission following excitation of pyrene chromophores. Sensitised emission is demonstrated in self-assembled complexes and in azamacrocycle derivatives bearing pendent pyrene groups. Energy transfer in these systems is dependent on the nature of the link between the ligand and the complex.

Reaction of 1-hydrazinophthalazine with chloroacetyl chloride yields 3-chloromethyl-1,2,4-triazolo-phthalazine. Reaction of this product with the tris tert-butyl ester of DO3A yields a triazolophthalazine appended macrocycle. Hydrolysis and complexation with lanthanide ions gives access to a series of lanthanide complexes (Ln = Nd, Eu, Yb, Er); these are all luminescent and exhibit sensitisation of the lanthanide centre by the chromophore.

The ligands tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate (L1, potentially hexadentate) and bis[3-(2-pyridyl)pyrazol-1-yl]dihydroborate (L2, potentially tetradentate) have been used to prepare ternary lanthanide complexes in which the remaining ligands are dibenzoylmethane anions (dbm). [Eu(L1)(dbm)₂] is eight-coordinate, with L1 acting only as a tetradentate chelate (with one potentially bidentate arm pendant) and two bidentate dbm ligands. [Nd(L1)(dbm)₂] was also prepared but on recrystallisation some of it rearranged to [Nd(L1)₂][Nd(dbm)₄], which contains a twelve-coordinate [Nd(L1)₂]+ cation (two interleaved hexadentate podand ligands) and the eight-coordinate anion [Nd(dbm)₄]− which, uniquely amongst eight-coordinate complexes having four diketonate ligands, has a square prismatic structure with near-perfect O₈ cubic coordination. Formation of this sterically unfavourable geometry is assumed to arise from favourable packing with the pseudo-spherical cation. The isostructural series of complexes [Ln(L2)(dbm)₂] (Ln = Pr, Nd, Eu, Gd, Tb, Er, Yb) was also prepared and all members structurally characterised; again the metal ions are eight-coordinate, from one tetradentate ligand L2 and two bidentate dbm ligands. Photophysical studies on the complexes with Ln = Pr, Nd, Er, and Yb were carried out; all show the near-IR luminescence characteristic of these metal ions, with longer lifetimes in CD₃OD than in CH₃OH. For [Yb(L2)(dbm)₂], two species with different luminescence lifetimes were observed in CH₃OH solution, corresponding to species with zero or one coordinated solvent molecules, in slow exchange on the luminescence timescale. For [Nd(L2)(dbm)₂] a single average solvation number of 0.7 was observed in MeOH. For [Pr(L2)(dbm)₂] a range of emission lines in the visible and NIR regions was detected; time-resolved measurements show a particularly high susceptibility to quenching by solvent CH and OH oscillators.

Ytterbium and neodymium have been shown to exhibit sensitised emission following excitation of pyrene chromophores. Sensitised emission is demonstrated in self-assembled complexes and in azamacrocycle derivatives bearing pendent pyrene groups. Energy transfer in these systems is dependent on the nature of the link between the ligand and the complex.

A series of dinuclear platinum(II)–lanthanide(iii)complexes has been prepared in which a square‐planar PtIIunit, either [(PPh3)2Pt(pdo)] (H2pdo=5,6‐dihydroxyphenanthroline) or [Cl2Pt(dppz)] [dppz=2,3‐bis(2‐pyridyl)pyrazine], is connected to a {Ln(dik)3} unit (“dik”=a 1,3‐diketonate ligand). The mononuclear complexes [(PPh3)2Pt(pdo)] and [Cl2Pt(dppz)] both have external, vacant N,N‐donor diimine‐type binding sites that react with various [Ln(dik)3(H2O)2] units to give complexes [(PPh3)2Pt(μ‐pdo)Ln(tta)3] (series A; Htta=thenoyltrifluoroacetone), [Cl2Pt(μ‐dppz)Ln(tta)3] (series B); and [Cl2Pt(μ‐dppz)Ln(btfa)3] (series C; Hbtfa=benzoyltrifluoroacetone); in all of these the lanthanide centres are eight‐coordinate. The lanthanides used exhibit near‐infrared luminescence (Nd, Yb, Er). Crystal structures of members of each series are described. In all complexes, excitation into the Pt‐centred absorption band (at 520 nm for series Acomplexes; 440 nm for series Band Ccomplexes) results in characteristic near‐IR luminescence from the Nd, Yb or Er centres in both the solid state and in CH2Cl2, following energy‐transfer from the Pt antenna chromophore. This work demonstrates how d‐block‐derived chromophores, with their intense and tunable electronic transitions, can be used as sensitisers to achieve near‐infrared luminescence from lanthanides in suitably designed heterodinuclear complexes based on simple bridging ligands.

A series of dinuclear platinum(II)–lanthanide(iii) complexes has been prepared in which a square-planar Pt^II unit, either [(PPh3)2Pt(pdo)] (H2pdo=5,6-dihydroxyphenanthroline) or [Cl2Pt(dppz)] [dppz=2,3-bis(2-pyridyl)pyrazine], is connected to a {Ln(dik)3} unit (“dik”=a 1,3-diketonate ligand). The mononuclear complexes [(PPh3)2Pt(pdo)] and [Cl2Pt(dppz)] both have external, vacant N,N-donor diimine-type binding sites that react with various [Ln(dik)3(H2O)2] units to give complexes [(PPh3)2Pt(μ-pdo)Ln(tta)3] (series A; Htta=thenoyltrifluoroacetone), [Cl2Pt(μ-dppz)Ln(tta)3] (series B); and [Cl2Pt(μ-dppz)Ln(btfa)3] (series C; Hbtfa=benzoyltrifluoroacetone); in all of these the lanthanide centres are eight-coordinate. The lanthanides used exhibit near-infrared luminescence (Nd, Yb, Er). Crystal structures of members of each series are described. In all complexes, excitation into the Pt-centred absorption band (at 520 nm for series A complexes; 440 nm for series B and C complexes) results in characteristic near-IR luminescence from the Nd, Yb or Er centres in both the solid state and in CH2Cl2, following energy-transfer from the Pt antenna chromophore. This work demonstrates how d-block-derived chromophores, with their intense and tunable electronic transitions, can be used as sensitisers to achieve near-infrared luminescence from lanthanides in suitably designed heterodinuclear complexes based on simple bridging ligands.

Reaction of the potentially bis-bidentate bridging ligand 3,6-bis(2-pyridyl)tetrazine (BPTZ) with various lanthanide complexes [Ln(tta)₃(H₂O)₂] [Htta = thenoyl(trifluoro)acetone; Ln = La, Nd, Gd, Er, Yb] in aqueous ethanol afforded the mononuclear complexes [Ln(tta)₃(BPTZ)] (Ln = La, Nd) or the dinuclear complexes [{Ln(tta)₃}₂(μ-BPTZ)] (Ln = Gd, Er, Yb) in which one or two, respectively, lanthanide tris-diketonate {Ln(tta)₃} units are bound to the N,N-bidentate compartments of BPTZ. Crystal structures of the dinuclear complexes [{Yb(tta)₃}₂(μ-BPTZ)]·CH₂Cl₂ and [{Gd(tta)₃}₂(μ-BPTZ)]·2CH₂Cl₂ show that the metal centres have an approximately square-antiprismatic eight-coordinate geometry; there are close contacts above and below the plane of the BPTZ bridging ligand between peripheral trifluoromethyl groups from a tta ligand associated with each metal centre. It is not apparent why the larger lanthanides La and Nd only give a mononuclear complex whereas the smaller lanthanides Gd, Er and Yb give the dinuclear complex in each case. UV/Vis spectroscopic titrations of a solution of BPTZ in CH₂Cl₂ with increasing amounts of [Ln(tta)₃(H₂O)₂] (Ln = La and Yb) show very similar behaviour, with stepwise binding constants K₁ and K₂ for association of the two {Ln(tta)₃} units of ca. 106 M−1 and 105 M−1; allowing for the expected statistical factor of 4 there is an additional reduction in the value of K₂ compared to K₁ which may be associated with a steric interaction between the two {Ln(tta)₃} units when the dinuclear complex forms. Steady-state and time-resolved luminescence studies on the complexes with Yb, Nd and Er, both in the solid state and CH₂Cl₂ solution, show that near-IR luminescence on the microsecond timescale can be sensitised by irradiation either at 337 nm into the tta-based transition, or at 520 nm into the low-energy BPTZ-centred transition.

A series of homo-binuclear lanthanide complexes have been prepared from DO3A derived systems containing seven-coordinate binding domains linked viaan aromatic. The luminescence properties of xylyl bridged complexes show that the lanthanide ions behave as isolated centres, while the combination of the lanthanide contraction and the lipophilicity of the linker group limits inner-sphere hydration around the metal centres for later lanthanides such as ytterbium.

A series of homo-binuclear lanthanide complexes have been prepared from DO3A derived systems containing seven-coordinate binding domains linked via an aromatic. The luminescence properties of xylyl bridged complexes show that the lanthanide ions behave as isolated centres, while the combination of the lanthanide contraction and the lipophilicity of the linker group limits inner-sphere hydration around the metal centres for later lanthanides such as ytterbium.

Intramolecular energy transfer from a non-protonated phenanthridine chromophore to ytterbium occurs rapidly despite negligible overlap of the triplet state of phenanthridine with the absorption band of the metal centre. This can be explained by a sequential electron transfer and back electron transfer, or redox, mechanism. When the phenanthridine is protonated, this energy transfer pathway ceases to be thermodynamically feasible. Under these circumstances, energy transfer to the metal becomes rate determining and is mediated by the phenanthridinium triplet state, despite its near-zero spectral overlap with the metal-centred absorption band.

We have prepared neodymium(iii) and ytterbium(iii) complexes from a range of poly(pyrazolyl)borate ligands and studied their near-IR luminescence properties, in particular using solution lifetimes obtained in protonated and deuterated methanol to determine the extent of solvation of the complexes. For the neodymium complexes, the luminescence lifetime in methanolic solutions increases as solvent is excluded from the inner coordination sphere. However, for a given inner sphere coordination number, these complexes have longer luminescence lifetimes than complexes with aminocarboxylate ligands: there are fewer proximate C–H oscillators in the pyrazolylborate complexes compared to the aminocarboxylate complexes, and non-radiative quenching is therefore reduced. This bears out earlier suggestions that long luminescence lifetimes can be obtained by minimising the number of close diffusing X–H oscillators.

Wetland restoration projects are frequently evaluated by their hydrologic roles and vegetation characteristics, but their success in restoring biogeochemical processes, such as denitrification, is less well known. To determine how restoration of structure affects specific ecosystem processes, denitrification potential, soluble organic C (SOC) concentrations, and soil moisture were measured seasonally over a 1‐yr period in replicated natural bottomland hardwood (BLH) wetlands (NAT), restored BLH wetlands with hydrology reestablished (RWH), and restored BLH wetlands without hydrology reestablished (RWOH). Denitrification potential was significantly higher in NAT wetlands (657 ng N2O‐N g−1soil h−1) than in RWOH (167 ng N2O‐N g−1soil h−1) (P= 0.07). Soil moisture was highest in NAT wetlands and lowest in RWOH (P= 0.01), but no differences were measured in SOC concentrations in three out of the four seasons sampled. Because RWOH wetlands exhibited denitrification potentials which were significantly lower than NAT wetland sites, these results demonstrate that a BLH wetland restored without the natural hydrologic regime reestablished will not be a replacement for a natural BLH wetland in terms of biogeochemical processes such as denitrification.

The synthesis of C-aryl monosubstituted derivatives of 1,4,7,10-tetraazacyclododecane (cyclen) by metal templated reductive amination of triethylenetetramine with aryl glyoxals has been developed and shown to be widely applicable. Octadentate ligands derived from these C-substituted macrocycles form relatively labile eight- coordinate complexes, which dissociate when challenged with EDTA in aqueous solution at pH 5.5, suggesting their use in metal-transport processes rather than for applications invivo.

The radiative rate constants for depopulation of the excited states of closely-related series of anionic, neutral and cationic europium, terbium and ytterbium complexes have been measured in H2O and D2O. With the aid of selective ligand deuteriation, the relative contributions of OH, NH (both amide and amine) and CH oscillators have been measured and critically assessed. Quenching of the Eu 5D0 excited state by amine NH oscillators is more than twice as efficient as OH quenching. The importance of the distance between the excited Ln ion and the XH oscillator is described with recourse to published crystallographic information. The general equation, q 4;= 4;A′(ΔkH2O–kD2O)corr is presented and revised values of A′ for Eu (1.2 ms), Tb (5 ms) and Yb (1 µs) given, which allow for the quenching contribution of closely diffusing OH oscillators. The relevance of such studies to the hydration state of certain gadolinium complexes is described and clear evidence provided for a break in hydration at gadolinium.

This article reviews and expands upon our observations of neodymium and ytterbium-centered luminescence in the near-IR. A variety of neodymium (III) and ytterbium (III) complexes with aminocarboxylate ligands was synthesized and their photophysical properties were investigated in aqueous solutions. Metal-centered emission was observed in the near-IR for complexes of both ions and time-resolved studies were used to show how quenching of the excited states is dependent on both inner and outer sphere coordinated water molecules.

Depth of rusting on steel rods has been proposed as an inexpensive means of determining depth to the water table and the reducing zone in wetland soils. The suitability of steel‐rod oxidation as a hydrologic indicator in wetland soils was tested in a series of laboratory and field experiments in both Histosols and a mineral soil (Typic Fluvaquent). In laboratory microcosms, the steel‐rod rusting depth exactly matched water‐table levels under both permanently flooded and drained conditions. But under a rapidly fluctuating hydrology, once heavy rust formed on the rods it did not dissolve upon partial reflooding. Field experiments showed a good correspondence between steel‐rod rusting depth and water‐table depth under both relatively constant and seasonally changing hydrology in Histosols (r2= 0.80, slope = 0.81). In the Typic Fluvaquent, a close correspondence between steel‐rod rusting depth and water‐table depth occurred under relatively constant hydrological conditions but, with a rapidly dropping water table, rod oxidation showed a lag period in response. We conclude that the steel‐rod technique is valuable as a hydrologic indicator of the reducing zone in basic science studies. We recommend frequent sampling (monthly) and the use of other hydrological measures to ensure unambiguous results. The technique is unsuitable for jurisdictional delineation in areas with a fluctuating hydrology due to prolonged lag periods in rod oxidation under some conditions and the inability of previously formed rust to dissolve upon reflooding.

The relaxivity of [GdL1]3+ in aqueous solution has been studied as a function of temperature and magnetic field strength. The complex possesses a high relaxivity at pH 6 (12.8 mmol-1 s-1) that diminishes as successive water molecules are deprotonated, leading to a complex species whose relaxivity is typical of a purely outer-sphere complex. Measurements of the luminescence lifetime of a very similar complex, [YbL2]3+, in H2O and D2O were used to confirm that the complex possesses three bound waters at pH 5.5, but above pH 11 there are no bound OH groups in the putative di-oxo-bridged dimer.