We report a study of redox reactions of uranium in model conditions using luminescence spectroscopy, which with its ease and wide availability has the potential to offer new insights into a bioremediation strategy of particular interest – the enzymatic reduction of UVIO22+ by bacteria such as Geobacter sulfurreducens. The inherent luminescent properties of UVIO22+ have been combined with confocal fluorescence microscopy techniques and lifetime image mapping to report directly on uranium concentration, localisation and oxidation state in cellular systems during uranium bioreduction, suggesting that localisation of uranyl species on the cell membrane surface plays an important role and that extracellular biogenic features form alongside uranyl sorbed cellular species during early stages of the bioreduction. The use of confocal microscopy in tandem with lifetime image mapping offers both improved temporal and spatial resolution (nanoseconds to microseconds and sub-micron respectively) than more conventional X-ray based techniques and offers the potential to image redox reactions occurring in situ. Together, these techniques provide an excellent and sensitive probe to assess the coordination environment of uranium during bioreduction processes that are currently being considered for remediation strategies of redox active radionuclides present in contaminated land.

Room temperature detection of neptunyl(VI) LMCT emission in a coordination compound and in the presence of uranyl(VI) is reported for the first time. Differences in the excitation profiles of the complexes enable spectral editing so either exclusively neptunyl(VI) or uranyl(VI) emission is observed or a sum of the two.

A new Ir(III) cyclometallated complex bearing a fluorenyl 5-substituted-1,10-phenanthroline ligand ([Ir(ppy)2(L1)][PF6], ppy = 2-phenylpyridine) is presented which exhibits enhanced triplet oxygen sensing properties. The efficacy of this complex to act as a photosensitiser for altering the morphology of C6 Glioma cells that represent malignant nervous tumours has been evaluated. The increased heavy metal effect and related spin–orbit coupling parameters on the photophysical properties of this complex are evidenced by comparison with Ru(II) analogues. The complex [Ir(ppy)2(L1)][PF6] is shown to exhibit relatively high two-photon absorption efficiencies for the lowest energy MLCT electronic transitions with two-photon absorption cross sections that range from 50 to 80 Goeppert–Mayer units between 750 to 800 nm. Quantum yields for the complex were measured up to 23% and the Stern–Volmer quenching constant, KSV was determined to be 40 bar−1 in acetonitrile solution, confirming the high efficiency of the complex as a triplet oxygen sensitiser. Preliminary in vitro experiments with C6 Glioma cells treated with [Ir(ppy)2(L1)][PF6], show that the complex is an efficient sensitizer for triplet oxygen, producing cytotoxic singlet oxygen (1O2) by two-photon excitation at 740 nm resulting in photodynamic effects that lead to localised cell damage and death.

The redox chemistry of the actinide elements plays a central role in many aspects of nuclear fission technology including the reprocessing of spent fuel, safe disposal strategies and in the ability to reliably predict the mobility of actinides in natural and engineered environmental conditions. In both aqueous and non-aqueous conditions, the redox chemistry of the actinides can be complicated and diverse and speciation is governed by both the actinide in question and many environmental factors. Although, historically, actinyl(VI) and (V) ions have been the subject of the most in depth research, the study of actinide ions in the +IV oxidation state (principally for U, Np and Pu) is inherently important in governing speciation in all aspects of the nuclear fuel cycle. Importantly, reactions involving reduction, disproportionation and re-oxidation tend to involve actinide ions in the +IV oxidation state leading to complex systems particularly in aqueous solution that control the solubility and migratory behaviour of actinide containing species. In this review, we focus on recent developments in the coordination and redox chemistry of the actinides involving actinide(IV) species in terms of fundamental coordination chemical studies, mineral chemistry, biogeochemistry and the implications of hydrolysis chemistry on the chemical and physical behaviour of actinide(IV) ions in the natural and engineered environment.

Gd4O2S:Yb:Tm rare-earth upconversion phosphors have been utilised to monitor the redox behaviour of flavin mononucleotide and report on the turnover of a flavo-protein, (pentaerythritol tetranitrate reductase). The presence of two bands separated by over 300 nm in the UCP emission spectra allows ratiometric signalling of these processes with high sensitivity.

A 5f2 uranium(IV) complex of the macrocycle DO3A (DO3A = [4,7,10-tris-carboxymethyl-,1,4,7,10-tetraaza-cyclododec-1-yl]-acetic acid) has been prepared and characterised in the solid state and in solution. The DO3A scaffold containing no strongly absorbing chromophores enables the facile detection of relatively long-lived (8–13 ns) UV-visible emission that possesses significant charge transfer character tentatively assigned to deactivation of the excited 3F2 5f1 6d1 electronic configuration. This study demonstrates, for the first time, that luminescence of simple U(IV) chelates is detectable in the absence of an antenna, potentially serving as a diagnostic tract for environmental U(IV) species.

We describe the synthesis, solid state and solution properties of two families of uranyl(VI) complexes that are ligated by neutral monodentate and anionic bidentate PO, PNH and AsO ligands bearing pendent phenyl chromophores. The uranyl(VI) ions in these complexes possess long-lived photoluminescent LMCT 3Πu excited states, which can be exploited as a sensitive probe of electronic structure, bonding and aggregation behaviour in non-aqueous media. For a family of well defined complexes of given symmetry in trans-[UO2Cl2(L2)] (L = Ph3PO (1), Ph3AsO (2) and Ph3PNH (3)), the emission spectral profiles in CH2Cl2 are indicative of the strength of the donor atoms bound in the equatorial plane and the uranyl bond strength; the uranyl LMCT emission maxima are shifted to lower energy as the donor strength of L increases. The luminescence lifetimes in fluid solution mirror these observations (0.87–3.46 μs) and are particularly sensitive to vibrational and bimolecular deactivation. In a family of structurally well defined complexes of the related anion, tetraphenylimidodiphosphinate (TPIP), monometallic complexes, [UO2(TPIP)(thf)] (4), [UO2(TPIP)(Cy3PO)] 5), a bimetallic complex [UO2(TPIP)2]2 (6) and a previously known trimetallic complex, [UO2(TPIP)2]3 (7) can be isolated by variation of the synthetic procedure. Complex 7 differs from 6 as the central uranyl ion in 7 is orthogonally connected to the two peripheral ones viauranyl → uranium dative bonds. Each of these oligomers exhibits a characteristic optical fingerprint, where the emission maxima, the spectral shape and temporal decay profiles are unique for each structural form. Notably, excited state intermetallic quenching in the trimetallic complex 7 considerably reduces the luminescence lifetime with respect to the monometallic counterpart 5 (from 2.00 μs to 1.04 μs). This study demonstrates that time resolved and multi-parametric luminescence can be of value in ascertaining solution and structural forms of discrete uranyl(VI) complexes in non-aqueous solution.

Four structurally related iridium(III) and ruthenium(II) complexes bearing two polar terpyridyl–stilbene derived chromophores 4-(4-{2-[4-(methoxy)phenyl]ethenyl}phenyl)-2,2′-6′,2′′-terpyridine (ttpyeneanisole) and 4-(4-{2-[phenyl]ethenyl}phenyl)-2,2′-6′,2′′-terpyridine (tpystilbene) have been synthesised and characterised in the solid state and in solution. In the solid state, the dihedral angle subtending the pyridyl and tolyl groups of 27.1° in the Ir(III) complex [Ir(ttpyeneanisole)2]·3PF6 is more acute than in the Ru(II) derivative [Ru(tpystilbene)2]·2PF6 (35.5°), indicating the presence of a greater degree of π-delocalisation across the terpyridine unit in the former compound. Their luminescence properties in fluid solution have been investigated following both resonant and non-resonant excitation. We have shown that each of the complexes undergoes two-photon excitation when excited in the near infrared (740 to 820 nm), with two-photon absorption cross sections in the range 11–67 × 10−50 cm4 s photon−1. The larger cross sections for the Ir(III) complexes reflect the differences observed in the solid state. This work therefore demonstrates that such complexes are promising as luminescent markers for 3D imaging and illustrates that simple functionalisation of the chromophores and the choice of metal can lead to marked enhancements in the two-photon cross sections (σ2) compared to those of simpler heteroleptic polypyridyl based derivatives.

We describe the synthesis, solid state and solution properties of two families of uranyl(VI) complexes that are ligated by neutral monodentate and anionic bidentate PO, PNH and AsO ligands bearing pendent phenyl chromophores. The uranyl(VI) ions in these complexes possess long-lived photoluminescent LMCT 3Πu excited states, which can be exploited as a sensitive probe of electronic structure, bonding and aggregation behaviour in non-aqueous media. For a family of well defined complexes of given symmetry in trans-[UO2Cl2(L2)] (L = Ph3PO (1), Ph3AsO (2) and Ph3PNH (3)), the emission spectral profiles in CH2Cl2 are indicative of the strength of the donor atoms bound in the equatorial plane and the uranyl bond strength; the uranyl LMCT emission maxima are shifted to lower energy as the donor strength of L increases. The luminescence lifetimes in fluid solution mirror these observations (0.87–3.46 μs) and are particularly sensitive to vibrational and bimolecular deactivation. In a family of structurally well defined complexes of the related anion, tetraphenylimidodiphosphinate (TPIP), monometallic complexes, [UO2(TPIP)(thf)] (4), [UO2(TPIP)(Cy3PO)] 5), a bimetallic complex [UO2(TPIP)2]2 (6) and a previously known trimetallic complex, [UO2(TPIP)2]3 (7) can be isolated by variation of the synthetic procedure. Complex 7 differs from 6 as the central uranyl ion in 7 is orthogonally connected to the two peripheral ones viauranyl → uranium dative bonds. Each of these oligomers exhibits a characteristic optical fingerprint, where the emission maxima, the spectral shape and temporal decay profiles are unique for each structural form. Notably, excited state intermetallic quenching in the trimetallic complex 7 considerably reduces the luminescence lifetime with respect to the monometallic counterpart 5 (from 2.00 μs to 1.04 μs). This study demonstrates that time resolved and multi-parametric luminescence can be of value in ascertaining solution and structural forms of discrete uranyl(VI) complexes in non-aqueous solution.

Four structurally related iridium(III) and ruthenium(II) complexes bearing two polar terpyridyl–stilbene derived chromophores 4-(4-{2-[4-(methoxy)phenyl]ethenyl}phenyl)-2,2′-6′,2′′-terpyridine (ttpyeneanisole) and 4-(4-{2-[phenyl]ethenyl}phenyl)-2,2′-6′,2′′-terpyridine (tpystilbene) have been synthesised and characterised in the solid state and in solution. In the solid state, the dihedral angle subtending the pyridyl and tolyl groups of 27.1° in the Ir(III) complex [Ir(ttpyeneanisole)2]·3PF6 is more acute than in the Ru(II) derivative [Ru(tpystilbene)2]·2PF6 (35.5°), indicating the presence of a greater degree of π-delocalisation across the terpyridine unit in the former compound. Their luminescence properties in fluid solution have been investigated following both resonant and non-resonant excitation. We have shown that each of the complexes undergoes two-photon excitation when excited in the near infrared (740 to 820 nm), with two-photon absorption cross sections in the range 11–67 × 10−50 cm4 s photon−1. The larger cross sections for the Ir(III) complexes reflect the differences observed in the solid state. This work therefore demonstrates that such complexes are promising as luminescent markers for 3D imaging and illustrates that simple functionalisation of the chromophores and the choice of metal can lead to marked enhancements in the two-photon cross sections (σ2) compared to those of simpler heteroleptic polypyridyl based derivatives.

Room temperature detection of neptunyl(VI) LMCT emission in a coordination compound and in the presence of uranyl(VI) is reported for the first time. Differences in the excitation profiles of the complexes enable spectral editing so either exclusively neptunyl(VI) or uranyl(VI) emission is observed or a sum of the two.

A new Ir(III) cyclometallated complex bearing a fluorenyl 5-substituted-1,10-phenanthroline ligand ([Ir(ppy)2(L1)][PF6], ppy = 2-phenylpyridine) is presented which exhibits enhanced triplet oxygen sensing properties. The efficacy of this complex to act as a photosensitiser for altering the morphology of C6 Glioma cells that represent malignant nervous tumours has been evaluated. The increased heavy metal effect and related spin–orbit coupling parameters on the photophysical properties of this complex are evidenced by comparison with Ru(II) analogues. The complex [Ir(ppy)2(L1)][PF6] is shown to exhibit relatively high two-photon absorption efficiencies for the lowest energy MLCT electronic transitions with two-photon absorption cross sections that range from 50 to 80 Goeppert–Mayer units between 750 to 800 nm. Quantum yields for the complex were measured up to 23% and the Stern–Volmer quenching constant, KSV was determined to be 40 bar−1 in acetonitrile solution, confirming the high efficiency of the complex as a triplet oxygen sensitiser. Preliminary in vitro experiments with C6 Glioma cells treated with [Ir(ppy)2(L1)][PF6], show that the complex is an efficient sensitizer for triplet oxygen, producing cytotoxic singlet oxygen (1O2) by two-photon excitation at 740 nm resulting in photodynamic effects that lead to localised cell damage and death.

A 5f2 uranium(IV) complex of the macrocycle DO3A (DO3A = [4,7,10-tris-carboxymethyl-,1,4,7,10-tetraaza-cyclododec-1-yl]-acetic acid) has been prepared and characterised in the solid state and in solution. The DO3A scaffold containing no strongly absorbing chromophores enables the facile detection of relatively long-lived (8–13 ns) UV-visible emission that possesses significant charge transfer character tentatively assigned to deactivation of the excited 3F2 5f1 6d1 electronic configuration. This study demonstrates, for the first time, that luminescence of simple U(IV) chelates is detectable in the absence of an antenna, potentially serving as a diagnostic tract for environmental U(IV) species.

Gd4O2S:Yb:Tm rare-earth upconversion phosphors have been utilised to monitor the redox behaviour of flavin mononucleotide and report on the turnover of a flavo-protein, (pentaerythritol tetranitrate reductase). The presence of two bands separated by over 300 nm in the UCP emission spectra allows ratiometric signalling of these processes with high sensitivity.

We report a study of redox reactions of uranium in model conditions using luminescence spectroscopy, which with its ease and wide availability has the potential to offer new insights into a bioremediation strategy of particular interest – the enzymatic reduction of UVIO22+ by bacteria such as Geobacter sulfurreducens. The inherent luminescent properties of UVIO22+ have been combined with confocal fluorescence microscopy techniques and lifetime image mapping to report directly on uranium concentration, localisation and oxidation state in cellular systems during uranium bioreduction, suggesting that localisation of uranyl species on the cell membrane surface plays an important role and that extracellular biogenic features form alongside uranyl sorbed cellular species during early stages of the bioreduction. The use of confocal microscopy in tandem with lifetime image mapping offers both improved temporal and spatial resolution (nanoseconds to microseconds and sub-micron respectively) than more conventional X-ray based techniques and offers the potential to image redox reactions occurring in situ. Together, these techniques provide an excellent and sensitive probe to assess the coordination environment of uranium during bioreduction processes that are currently being considered for remediation strategies of redox active radionuclides present in contaminated land.