Visible and Near-Infrared Luminescence of Lanthanide-Containing Dimetallic Triple-Stranded Helicates: Energy Transfer Mechanisms in the SmIII and YbIII Molecular Edifices

Optical Amplification at 1.5 µm in ErIII Coordination Polymer-Doped Waveguides Based on Intramolecular Energy Transfer

9,9-bis (diphenylphosphorylphenyl) fluorene (FDPO) and dibenzotetrathienoacene (DBTTA), are synthesized as the neutral and anionic ligands, respectively, to prepare the ErIII coordination polymer [Er(DBTTA)3(FDPO)]n. Based on the intramolecular energy transfer, optical gains at 1.5 µm are demonstrated in [Er(DBTTA)3(FDPO)]n-doped polymer waveguides under excitations of low-power light-emitting diodes (LEDs) instead of laser pumping. A ligand-sensitization scheme between organic ligands and Er3+ ions under an excitation of an ultraviolet (UV) LED is established. Relative gains of 10.5 and 8.5 dB cm-1 are achieved at 1.53 and 1.55 µm, respectively, on a 1-cm-long SU-8 channel waveguide with a cross-section of 2 × 3 µm2 and a 1.5-µm-thick [Er(DBTTA)3(FDPO)]n-doped polymethylmethacrylate (PMMA) as upper cladding. The ErIII coordination polymer [Er(DBTTA)3(FDPO)]n can be conveniently integrated with various low-loss inorganic waveguides to compensate for optical losses in the C-band window. Moreover, by relying on the intramolecular energy transfer and UV LED top-pumping technology, it is easy to achieve coupling packaging of erbium-doped waveguide amplifiers (EDWAs) with pump sources in planar photonic integrated chips, effectively reducing the commercial costs.

A trivalent 4f complex with two bis-silylamide ligands displaying slow magnetic relaxation

The best-performing single-molecule magnets (SMMs) have historically relied on pseudoaxial ligands delocalized across several coordinated atoms. This coordination environment has been found to elicit strong magnetic anisotropy, but lanthanide-based SMMs with low coordination numbers have remained synthetically elusive species. Here we report a cationic 4f complex bearing only two bis-silylamide ligands, Yb(III)[{N(SiMePh₂)₂}₂][Al{OC(CF₃)₃}₄], which exhibits slow relaxation of its magnetization. The combination of the bulky silylamide ligands and weakly coordinating [Al{OC(CF₃)₃}₄]^- anion provides a sterically hindered environment that suitably stabilizes the pseudotrigonal geometry necessary to elicit strong ground-state magnetic anisotropy. The resolution of the m_J states by luminescence spectroscopy is supported by ab initio calculations, which show a large ground-state splitting of approximately 1,850 cm^-1. These results provide a facile route to access a bis-silylamido Yb(III) complex, and further underline the desirability of axially coordinated ligands with well-localized charges for high-performing SMMs.

Magnetic and Luminescence Properties of 8-Coordinated Pyridyl Adducts of Samarium(III) Complexes Containing 4,4,4-Trifluoro-1-(naphthalen-2-yl)-1,3-butanedionate

A novel series of polypyridyl adducts, [Sm(ntfa)3(NN)] (2–4), with ntfa = 4,4,4-trifluoro-1-(naphthalen-2-yl)-1,3-butanedionate, NN = 2,2′-bipyridine (bipy), 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me2bipy), and 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me2bipy) were synthesized from the precursor complex [Sm(ntfa)3(MeOH)2] (1) and the corresponding pyridyl ligands. Single X-ray crystallography showed that the complexes displayed 8-coordinated geometry. The solid pyridyl adducts 2–4 exhibited emission of luminescence in the NIR and visible regions with close quantum yields (QY = 0.20–0.25%). The magnetic data of 1–4 showed larger values than those expected for magnetically noncoupled Sm(III) complexes in the 6H5/2 ground state, with no saturation on the applied high magnetic field static at a temperature of 2 K.

Spectroscopic aspects for the Yb3+ coordination compound with a large energy gap between the ligand and Yb3+ excited states

We present the experimental and theoretical results that made it possible to propose the energy transfer mechanism for a Yb complex with a large energy gap between the ligand and Yb excited states using a theoretical model and experimental data. Absorption and emission spectroscopy in the 300-4 K range is used for the study of the Yb³⁺ compound with N-phosphorylated sulfonamide (Na[YbL₄]), which, despite the large energy gap, is characterized by high emission sensitization efficiency (η_sens = 40%) and relatively long Yb³⁺ emission lifetime (27 μs). The crystal structure of Na[YbL₄], radiative lifetime (930 μs), refractive index (1.46), intrinsic (3.0%), and overall (1.3%) emission quantum yield were determined. To obtain the electronic properties of the Na[YbL₄], a time-dependent density functional theory (TD-DFT) was performed. The intramolecular energy transfer (IET) rates from the excited states S₁ and T₁ to the Yb³⁺ ion as well as between the ligand and the ligand-to-metal charge transfer (LMCT) states were calculated. Once the intersystem crossing S₁ → T₁ is not so effective due to a large energy gap between S₁ and T₁ (≈10000 cm^-1), it has been shown that the LMCT state acts as an additional channel to feed the T₁ state. Then, the T₁ can transfer energy to the Yb^{3+ 2}F_5/2 energy level (W_T), where W_T is dominated by the exchange mechanism. Based on IET and a rate equation model, the overall emission quantum yield Q^L_Ln was simulated with and without the LMCT, this also confirmed that the pathway S₁ → LMCT → T₁ → Yb³⁺ is more likely than the S₁ → T₁ → Yb³⁺ one.

Near-infrared circular dichroism of the ytterbium DOTMA complex: an ab initio investigation

The electronic structure and circular dichroism spectra of the ytterbium(III) complex [Yb(DOTMA)]^- are calculated using complete and restricted active space self-consistent field wavefunction methods with the spin-orbit coupling treated by the state interaction approach. The influence of the dynamical correlation effect is then included via the 2nd order perturbation method. The experimental circular dichroism spectrum is well reproduced by calculations, both in terms of relative energy excitations and in terms of rotatory strength intensities. The results allow highlighting the mechanism that drives the chiroptical properties in Yb(III) complexes and reveal the importance of taking into account the 4f¹²5d¹ electronic configurations in the calculated wavefunctions to properly describe the chiroptical properties of the 4f-4f transitions.

Mesoporous Silica Nanoparticles-Embedded Lanthanide Organic Polyhedra for Enhanced Stability, Luminescence and Cell Imaging

We report here a simple but efficient “ship-in-a-bottle” synthetic strategy for increasing the stability and luminescence performance of LOPs by embedding them into mesoporous silica nanoparticles (MSNs). Three types of...

Tuning Excited-State Properties of [2.2]Paracyclophane-Based Antennas to Ensure Efficient Sensitization of Lanthanide Ions or Singlet Oxygen Generation

The multistep synthesis of original antennas incorporating substituted [2.2]paracyclophane (pCp) moieties in the π-conjugated skeleton is described. These antennas, functionalized with an electron donor alkoxy fragment (A¹) or with a fused coumarin derivative (A²), are incorporated in a triazacyclonane macrocyclic ligand L¹ or L², respectively, for the design of Eu(III), Yb(III), and Gd(III) complexes. A combined photophysical/theoretical study reveals that A¹ presents a charge transfer character via through-space paracyclophane conjugation, whereas A² presents only local excited states centered on the coumarin-paracyclophane moiety, strongly favoring triplet state population via intersystem crossing. The resulting complexes EuL¹ and YbL² are fully emissive in red and near-infrared, respectively, whereas the GdL² complex acts as a photosensitizer for the generation of singlet oxygen.

Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand

EuIII , TbIII , GdIII and YbIII complexes of the nonadentate bispidine derivative L2 (bispidine=3,7-diazabicyclo[3.3.1]nonane) were successfully synthesized and their emission properties studied. The X-ray crystallography reveals full encapsulation by the nonadentate ligand L2 that enforces to all LnIII cations a common highly symmetrical capped square antiprismatic (CSAPR) coordination geometry (pseudo C4v symmetry). The well-resolved identical emission spectra in solid state and in solution confirm equal structures in both media. As therefore expected, this results in long-lived excited states and high emission quantum yields ([EuIII L2 ]+ , H2 O, 298 K, τ=1.51 ms, ϕ=0.35; [TbIII L2 ]+ , H2 O, 298 K, τ=1.95 ms, ϕ=0.68). Together with the very high kinetic and thermodynamic stabilities, these complexes are a possible basis for interesting biological probes.

From Mono- to Polynuclear Coordination Complexes with a 2,2'-Bipyrimidine-4,4'-dicarboxylate Ligand

The coordination properties of the ligand 2,2'-bipyrimidine-4,4'-dicarboxylic acid (H₂bpd) with lanthanide(III) ions (Ln = Eu, Tb, or Lu) were investigated. The syntheses of the H₂bpd ligand and its salts, [K₂(bpd)(H₂O)₂] (1) and [(AlkNH)Lu(bpd)₂] (Alk = Et, Hex, or en), are described. In the presence of LnCl₃ salts (Ln = Lu, Eu, or Tb), the formation of [Ln(bpd)₂]^- and [Ln(bpd)(H₂O)_x]⁺ species was assessed by ¹H nuclear magnetic resonance (NMR), spectrophotometry, and spectrofluorometric titrations in aqueous solution. The solid state structure of 1, [K(H₂O)₂][Lu(bpd)₂] (2), and [(Et₃NH)Lu(bpd)₂] (3) could be determined by X-ray diffraction, showing the ligand to act as a tetradentate unit with formation of three five-membered chelate rings around the central Ln(III). With the aim of building polynuclear assemblies, the coordination between [Lu(bdp)₂]^- and [Lu(tta)₃(H₂O)] units (tta = thenoyltrifluoroacetylacetonate) was also investigated. In methanol, ¹H NMR titration experiments revealed the formation of complex mixtures from which two new species could be identified, [Lu₂(bpd)(tta)₄] (4) and H[Lu(bpd)(tta)₂] (5), as confirmed by their solid state structure analysis. Using highly lipophilic cations in chloroform, the octametallic complex [enH]₄[Lu₈(bpd)₄(tta)₁₈] (6) could be isolated and its X-ray structure determined.

Base- and Metal-Dependent Self-Assembly of Lathanide-Organic Coordination Polymers or Macrocycles with Tetradentate Acylhydrazone-based Ditopic Ligands

Herein, we report a comprehensive study on the lanthanide-directed coordination self-assembly with two bis-tetradentate acylhydrazone ligands H₄ L¹ and H₄ L² . Multifarious outcomes, which are base- and metal-dependent, were revealed by NMR, ESI-TOF-MS and X-ray crystallography. In the absence of base, bent H₄ L¹ was assembled into dinuclear double-strand helicate Ln₂ (H₂ L¹ )₂ by partially-deprotonated assembly with La, Sm or Eu, while trinuclear Ln₃ (H₂ L¹ )₃ with Yb or Lu. For linear H₄ L² , infinite 1D zig-zag metal-organic polymeric chain (Ln₂ H₂ L² )_n was obtained. However, complete deprotonated L¹ and L² assembled into discrete trinuclear Ln₃ (L^{1 /2} )₃ and tetranuclear Ln₄ (L^{1 /2} )₄ macrocyclic structures under the basic condition. For these, there are multiple possible isomers coexisting in the solution which were enumerated and simulated with molecular mechanic modeling. Visible-light sensitized NIR emissions on the Yb complexes have been observed, endowing them potential application in photofunctional materials.

Luminescence, chiroptical, magnetic and ab initio crystal-field characterizations of an enantiopure helicoidal Yb(iii) complex

The electronic structure of a chiral Yb(iii)-based complex is fully determined by taking advantage of experimental magnetic, luminescence, and chiroptical (NIR-ECD and CPL) characterizations in combination with ab initio wavefunction calculations.

Ship-in-a-Bottle Preparation of Long Wavelength Molecular Antennae in Lanthanide Metal-Organic Frameworks for Biological Imaging

While metal-organic frameworks (MOFs) have been identified as promising materials for sensitizing near-infrared emitting lanthanide ions (Ln³⁺) for biological imaging, long-wavelength excitation of such materials requires large, highly delocalized organic linkers or guest-chromophores. Incorporation of such species generally coincides with fewer Ln³⁺ emitters per unit volume. Herein, the excitation bands of ytterbium-based MOFs are extended to 800 nm via the postsynthetic coupling of acetylene units to form a high density of conjugated π-systems throughout MOF pores. The resulting long wavelength excitation/absorption bands are a synergistic property of the composite material as they are not observed in the individual organic components after disassociation of the MOFs, thus circumventing the need for large organic chromophores. We demonstrate that the long wavelength excitation and emission properties of these modified MOFs are maintained in the biological conditions of cell culture (aqueous environment, salts, heating), pointing toward their promising use for biological imaging applications.

Proton Conductive Luminescent Thermometer Based on Near-Infrared Emissive {YbCo2} Molecular Nanomagnets

Lanthanide(III)-based coordination complexes have been explored as a source of bifunctional molecular materials combining Single-Molecule Magnet (SMM) behavior with visible-to-near-infrared photoluminescence. In pursuit of more advanced multifunctionality, the next target is to functionalize crystalline solids based on emissive molecular nanomagnets toward high proton conductivity and an efficient luminescent thermometric effect. Here, a unique multifunctional molecule-based material, (H₅O₂)₂(H)[Yb^III(hmpa)₄][Co^III(CN)₆]₂·0.2H₂O (1, hmpa = hexamethylphosphoramide), composed of molecular {YbCo₂}^3- anions noncovalently bonded to acidic H₅O₂⁺ and H⁺ ions, is reported. The resulting Yb^III complexes present a slow magnetic relaxation below 6 K and room temperature NIR 4f-centered photoluminescence sensitized by [Co(CN)₆]^3- ions. The microporous framework, built on these emissive magnetic molecules, exhibits a high proton conductivity of the H-hopping mechanism reaching σ of 1.7 × 10^-4 S·cm^-1 at 97% relative humidity, which classifies 1 as a superionic conductor. Moreover, the emission pattern is strongly temperature-dependent which was utilized in achieving a highly sensitive single-center luminescent thermometer with a relative thermal sensitivity, S_r > 1% K^-1 in the 50-175 K range. This work shows an unprecedented combination of magnetic, optical, and electrical functionalities in a single phase working as a proton conductive NIR-emissive thermometer based on Single-Molecule Magnets.

Luminescent europium(iii) and terbium(iii) complexes of β-diketonate and substituted terpyridine ligands: synthesis, crystal structures and elucidation of energy transfer pathways

A series of coordinatively saturated Ln^III complexes: [Ln(R-TPY)(TTA)₃] (1–6) were designed and structurally characterized and plausible energy transfer (ET) pathways determined using a theoretical method.

Misconceptions in electronic energy transfer: bridging the gap between chemistry and physics

Many treatments of energy transfer (ET) phenomena in current literature employ incorrect arguments and formulae and are not quantitative enough. This is unfortunate because we witness important breakthroughs from ET experiments in nanoscience. This review aims to clarify basic principles by focusing upon Förster-Dexter electric dipole-electric dipole (ED-ED) ET. The roles of ET in upconversion, downconversion and the antenna effect are described and the clichés and simple formulae to be avoided in ET studies are highlighted with alternative treatments provided.

Sensitization, energy transfer and infra-red emission decay modulation in Yb3+-doped NaYF4 nanoparticles with visible light through a perfluoroanthraquinone chromophore

Infra-red emission (980 nm) of sub 10 nm Yb3+-doped NaYF4 nanoparticles has been sensitized through the excitation of 2-hydroxyperfluoroanthraquinone chromophore (1,2,3,4,5,6,7-heptafluro-8-hydroxyanthracene-9,10-dione) functionalizing the nanoparticle surface. The sensitization is achieved with a broad range of visible light excitation (400-600 nm). The overall near infra-red (NIR) emission intensity of Yb3+ ions is increased by a factor 300 as a result of the broad and strong absorption of the chromophore compared with ytterbium's intrinsic absorption. Besides the Yb3+ NIR emission, the hybrid composite shows organic chromophore-based visible emission in the orange-red region of the spectrum. We observe the energy migration process from the sensitized Yb3+ ions at the surface to those in the core of the particle using time-resolved optical spectroscopy. This highlights that the local environments for emitting Yb3+ ions at the surface and center of the nanoparticle are not identical, which causes important differences in the NIR emission dynamics. Based on the understanding of these processes, we suggest a simple strategy to control and modulate the decay time of the functionalized Yb3+-doped nanoparticles over a relatively large range by changing physical or chemical parameters in this model system.

Photophysics of Coumarin and Carbostyril-Sensitized Luminescent Lanthanide Complexes: Implications for Complex Design in Multiplex Detection

Luminescent lanthanide (Ln(III)) complexes with coumarin or carbostyril antennae were synthesized and their photophysical properties evaluated using steady-state and time-resolved UV-vis spectroscopy. Ligands bearing distant hydroxycoumarin-derived antennae attached through triazole linkers were modest sensitizers for Eu(III) and Tb(III), whereas ligands with 7-amidocarbostyrils directly linked to the coordination site could reach good quantum yields for multiple Ln(III), including the visible emitters Sm(III) and Dy(III), and the near-infrared emitters Nd(III) and Yb(III). The highest lanthanide-centered luminescence quantum yields were 35% (Tb), 7.9% (Eu), 0.67% (Dy), and 0.18% (Sm). Antennae providing similar luminescence intensities with 2-4 Ln-emitters were identified. Photoredox quenching of the carbostyril antenna excited states was observed for all Eu(III)-complexes and should be sensitizing in the case of Yb(III); the scope of the process extends to Ln(III) for which it has not been seen previously, specifically Dy(III) and Sm(III). The proposed process is supported by photophysical and electrochemical data. A FRET-type mechanism was identified in architectures with both distant and close antennae for all of the Lns. This mechanism seems to be the only sensitizing one at long distance and probably contributes to the sensitization at shorter distances along with the triplet pathway. The complexes were nontoxic to either bacterial or mammalian cells. Complexes of an ester-functionalized ligand were taken up by bacteria in a concentration-dependent manner. Our results suggest that the effects of FRET and photoredox quenching should be taken into consideration when designing luminescent Ln complexes. These results also establish these Ln(III)-complexes for multiplex detection beyond the available two-color systems.

Double-decker luminescent ytterbium and erbium SMMs with symmetric and asymmetric Schiff base ligands

Multifunctional molecules that respond both to magnetic fields and light are subject of study due to possible applications in fields as diverse as imaging or information processing and storage.

Friend or foe? The role of solvents in non-triplet, intraligand charge transfer sensitization of lanthanide(iii) luminescence

A comprehensive photophysical and solvatochromic study is conducted to evaluate the luminescence properties of our Eu(iii), Sm(iii) and Yb(iii) complexes.

Lanthanide ion and tetrathiafulvalene-based ligand as a "magic" couple toward luminescence, single molecule magnets, and magnetostructural correlations

The synthesis of molecules featuring different properties is a perpetual challenge for the chemists' community. The coexistence and even more the synergy of those properties open new perspectives in the field of molecular devices and molecular electronics. In that sense, coordination chemistry contributed to the development of new functional molecules through, for instance, single-molecule magnets (SMMs) and light emitting molecules with potential applications in high capacity data storage and OLEDs, respectively. The appealing combination of both electronic properties into one single object may offer the possibility to have magnetized luminescent entities at nanometric scale. To that end, lanthanides seem to be one of the key ingredients since their peculiar electronic structures endow them with specific magnetic and luminescence properties. Indeed, lanthanides cover a wide range of emission wavelengths, from infrared to UV, which add up to a large variety of magnetic behaviors, from the fully isotropic spin (e.g., Gd(III)) to highly anisotropic magnetic moments (e.g., Dy(III)). In lanthanide complexes, ligands play a fundamental role because on one hand they govern the orientation of the magnetic moment of anisotropic lanthanides and on the other hand they can sensitize efficiently the luminescence. The design of appropriate organic ligands to elaborate such chemical objects with the desired property appears to be essential but remains a perpetual challenge. In this Account, we describe the design of lanthanide-based complexes that emit light, behave as SMMs, or combine both properties. We have paid peculiar attention to the design of ligands based on the tetrathiafulvalene (TTF) moiety. TTF and its derivatives are well-known chemical entities, stable at different oxidation states, and employed mainly in the synthesis of molecular conductors and superconductors. In addition to their redox properties, TTF-based derivatives act as organic chromophores for the sensitization of visible and near-infrared (NIR) luminescence of lanthanides. The mechanism of sensitization involves either antenna effect (energy transfer from the excited state) or photoinduced electron transfer. TTF-based ligands act also as structural agents in the conception of SMM in crystals. Such objects are obtained with the highly anisotropic Dy(III) ion in crystalline phase as well as in frozen solution with magnetic memory at helium-4 temperature (4 K). We highlight the influence of the magnetic dilution (both in amorphous solution and in diamagnetic crystalline matrix) and, particular case of dysprosium based SMMs, the effect of metal-centered isotope enrichment on the SMM properties. Our aim is not only to realize functional molecules but to rationalize both luminescence and magnetic properties on the basis of the structure of the molecules. These two properties are intimately intricate and governed by the electronic structure, which can be calculated and interpreted using modern quantum chemistry tools.

Luminescence and single-molecule magnet behavior in lanthanide complexes involving a tetrathiafulvalene-fused dipyridophenazine ligand

The reaction between the TTF-fused dipyrido[3,2-a:2',3'-c]phenazine (dppz) ligand (L) and 1 equiv of Ln(hfac)3·2H2O (hfac(-) = 1,1,1,5,5,5-hexafluoroacetyacetonate) or 1 equiv of Ln(tta)3·2H2O (tta(-) = 2-thenoyltrifluoroacetonate) (Ln(III) = Dy(III) or Yb(III)) metallic precursors leads to four mononuclear complexes of formula [Ln(hfac)3(L)]·C6H14 (Ln(III) = Dy(III) (1), Yb(III) (2)) and [Ln(tta)3(L)]·C6H14 (Ln(III) = Dy(III) (3), Yb(III) (4)), respectively. Their X-ray structures reveal that the Ln(III) ion is coordinated to the bischelating nitrogenated coordination site and adopts a D4d coordination environment. The dynamic magnetic measurements show a slow relaxation of the Dy(III) magnetization for 1 and 3 with parameters highlighting a slower relaxation for 3 than for 1 (τ0 = 4.14(±1.36) × 10(-6) and 1.32(±0.07) × 10(-6) s with Δ = 39(±3) and 63.7(±0.7) K). This behavior as well as the orientation of the associated magnetic anisotropy axes have been rationalized on the basis of both crystal field splitting parameters and ab initio SA-CASSCF/RASSI-SO calculations. Irradiation of the lowest-energy HOMO → LUMO ILCT absorption band induces a (2)F5/2 → (2)F7/2 Yb-centered emission for 2 and 4. For these Yb(III) compounds, Stevens operators method has been used to fit the thermal variation of the magnetic susceptibilities, and the resulting MJ splittings have been correlated with the emission lines.

Controlling dimensionality via a dual ligand strategy in Ln-thiophene-2,5-dicarboxylic acid-terpyridine coordination polymers

Eleven new lanthanide-2,5-TDC-terpyridine coordination polymers which employ a dual ligand strategy have been synthesized and characterized by single crystal and powder X-ray diffraction as well as luminescence spectroscopy.

Magnetic and photo-physical investigations into DyIII and YbIII complexes involving tetrathiafulvalene ligand

Three TTF-based complexes have been elaborated using β-diketonate Dy^III and Yb^III precursors. [Yb(tta)₃(L)]·2CH₂Cl₂ enlarges the scarce series of redox-active luminescent single-molecule magnets with correlation between emissive and magnetic properties.

Sensitisation of Eu(III)- and Tb(III)-based luminescence by Ir(III) units in Ir/lanthanide dyads: evidence for parallel energy-transfer and electron-transfer based mechanisms

A series of blue-luminescent Ir(III) complexes with a pendant binding site for lanthanide(III) ions has been synthesized and used to prepare Ir(III)/Ln(III) dyads (Ln = Eu, Tb, Gd). Photophysical studies were used to establish mechanisms of Ir→Ln (Ln = Tb, Eu) energy-transfer. In the Ir/Gd dyads, where direct Ir→Gd energy-transfer is not possible, significant quenching of Ir-based luminescence nonetheless occurred; this can be ascribed to photoinduced electron-transfer from the photo-excited Ir unit (*Ir, (3)MLCT/(3)LC excited state) to the pendant pyrazolyl-pyridine site which becomes a good electron-acceptor when coordinated to an electropositive Gd(III) centre. This electron transfer quenches the Ir-based luminescence, leading to formation of a charge-separated {Ir(4+)}˙-(pyrazolyl-pyridine)˙(-) state, which is short-lived possibly due to fast back electron-transfer (<20 ns). In the Ir/Tb and Ir/Eu dyads this electron-transfer pathway is again operative and leads to sensitisation of Eu-based and Tb-based emission using the energy liberated from the back electron-transfer process. In addition direct Dexter-type Ir→Ln (Ln = Tb, Eu) energy-transfer occurs on a similar timescale, meaning that there are two parallel mechanisms by which excitation energy can be transferred from *Ir to the Eu/Tb centre. Time-resolved luminescence measurements on the sensitised Eu-based emission showed both fast and slow rise-time components, associated with the PET-based and Dexter-based energy-transfer mechanisms respectively. In the Ir/Tb dyads, the Ir→Tb energy-transfer is only just thermodynamically favourable, leading to rapid Tb→Ir thermally-activated back energy-transfer and non-radiative deactivation to an extent that depends on the precise energy gap between the *Ir and Tb-based (5)D4 states. Thus, the sensitised Tb(iii)-based emission is weak and unusually short-lived due to back energy transfer, but nonetheless represents rare examples of Tb(III) sensitisation by a energy donor that could be excited using visible light as opposed to the usually required UV excitation.

Lighting up cells with lanthanide self-assembled helicates

Lanthanide bioprobes and bioconjugates are ideal luminescent stains in view of their low propensity to photobleaching, sharp emission lines and long excited state lifetimes permitting time-resolved detection for enhanced sensitivity. We show here how the interplay between physical, chemical and biochemical properties allied to microfluidics engineering leads to self-assembled dinuclear lanthanide luminescent probes illuminating live cells and selectively detecting biomarkers expressed by cancerous human breast cells.