The synthesis of mono- and novel bis-methylated pyrrolo[1,2-a]quinoxalines through the addition of unstable methyl radicals to aryl isocyanides is described contingent upon the reaction conditions employed. The strategy has been effectively employed in the total synthesis of the natural product marinoquinoline A.

A series of substituted aryl phosphate esters have been synthesized from their alpha-hydroxyphosphonates substrates, using DBN (1,5 diazabicyclo(4.3.0)non-5-ene) at room temperature, via a phospha-Brook rearrangement. The aryl-substrate dependence of the rearrangement was explored, and excellent yields of the phosphate esters were achieved irrespective of whether the aryl moiety was activated or unactivated. A plausible mechanism for the rearrangement has been proposed. Based on the low temperature P-31-NMR, the mechanism of the phospha-Brook rearrangement is proposed to take place via an oxaphosphirane intermediate.

Figure 3. Two-photon (lex = 780 nm) confocal (a, b) and PLIM (c–e) imaging of live HDF cells labelled with 1·Eu. a), b) Steady state confocal microscopy images (lem = 500–550 nm) using cells incubated with 1·Eu at 100 and 50 mm, respectively. c)–e) PLIM images as follows: c) black and white intensity image (all emitted photons binned into a single channel) ; d) lifetime map showing Ir-based emission lifetime across cells ; and e) overlaid emission decay curves and lifetimes of the major emission component from cellular locations 1–4. Figure 4. Two-photon (lex = 780 nm) confocal a) and PLIM b)–d) imaging of live HDF cells labelled with 1·Eu (50 mm, 0.25 % DMSO in PBS). a) Steadystate confocal microscopy images (lem = 500–550 nm). b)–d) PLIM images as follows: b) black and white intensity image (all emitted photons binned into a single channel) ; c) lifetime map showing variations of Ir-based emission lifetime across cells ; and d) overlaid emission decay curves and lifetimes of major emission component from cellular locations 1–3.

The first example of cell imaging using two independent emission components from a dinuclear d/f complex is reported. A water-stable, cell-permeable Ir(III) /Eu(III) dyad undergoes partial Ir→Eu energy transfer following two-photon excitation of the Ir unit at 780 nm. Excitation in the near-IR region generated simultaneously green Ir-based emission and red Eu-based emission from the same probe. The orders-of-magnitude difference in their timescales (Ir ca. μs; Eu ca. 0.5 ms) allowed them to be identified by time-gated detection. Phosphorescence lifetime imaging microscopy (PLIM) allowed the lifetime of the Ir-based emission to be measured in different parts of the cell. At the same time, the cells are simultaneously imaged by using the Eu-based emission component at longer timescales. This new approach to cellular imaging by using dual d/f emitters should therefore enable autofluorescence-free sensing of two different analytes, independently, simultaneously and in the same regions of a cell.

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.

A study of the anion-binding properties of three structurally related lanthanide complexes which all contain chemically identical anion-binding motifs has revealed dramatic differences in their anion affinity. These arise as a consequence of changes in the substitution pattern on the periphery of the molecule, at a substantial distance from the binding pocket. In this paper, we explore these remote substituent effects, and explain the observed behaviour through discussion of the way in which remote substituents can influence and control the global structure of a molecule through their demands upon conformational space. Peripheral modifications to a binuclear lanthanide motif derived from ,'-bis(DO3A)-mxylene are shown to result in dramatic changes to the binding constant for isophthalate. In this system, the parent compound displays considerable conformational flexibility, yet can be assumed to bind to isophthalate through a well-defined conformer. Addition of steric bulk remote from the binding site restricts conformational mobility, giving rise to an increase in binding constant on entropic grounds as long as the ideal binding conformation is not excluded from the available range of conformers.

The bridging ligand L(14Nap), which contains two chelating pyrazolyl-pyridine units separated by a naphthalene-1,4-diyl spacer, has been used in self-assembly of polyhedral coordination cages. The largest such cage is [Cd16(L(14Nap))24](BF4)32 which has a tetra-capped truncated tetrahedral Cd16 core with a bridging ligand spanning every edge. The complex is indefinitely stable in dilute solution, which makes it quite different from the previously-reported isostructural cage [Cd16(L(14Ph))24](BF4)32 (based on a 1,4-phenyl bridge) that forms on crystallisation but slowly rearranges to smaller cages in solution. The additional inter-ligand π-stacking between ligand fragments associated with replacement of a phenyl group by a naphthyl group allows the complex to be stable in solution, providing conclusive proof of the importance of inter-ligand π-stacking in the assembly of these cages. With Cu(II) in place of Cd(II) a smaller cage [Cu12(L(14Nap))15](ClO4)24 was formed which contains a mixture of tris-chelated (six-coordinate) and bis-chelated (four-coordinate, or five-coordinate if an additional monodentate ligand is present) Cu(II) ions; the difference between the two structures arises in part from the different stereoelectronic preferences of the two metal ions. Despite this difference both the Cd16 and Cu12 cages contain {M3(L(14Nap))3}(6+) triangular helical units as subcomponents which form the triangular faces of the polyhedra. By using a 1 : 1 ligand : metal ratio in the synthesis examples of these can be isolated and characterised; the structures of the trinuclear cyclic helicates [Cd3(L(14Nap))3(BF4)4(EtOAc)2](BF4)2 and [Cu3(L(14Nap))3(BF4)(MeCN)2](BF4)5 have also been determined.

A series of luminescent complexes based on {Ir-(phpy)(2)} (phpy = cyclometallating anion of 2-phenylpyricline) or {Ir(F(2)phpy)(2)} [F(2)phpy = cyclometallating anion of 2-(2',4'-difluorophenyl)pyridine] units, with an additional 3-(2-pyridyl)-pyrazole (pypz) ligand, have been prepared; fluorination of the phenylpyridine ligands results in a blue-shift of the usual (MLCT)-M-3/(LC)-L-3 luminescence of the Ir unit from 477 to 455 nm. These complexes have pendant from the coordinated pyrazolyl ring an additional chelating 3-(2-pyridyl)-pyrazole unit, separated via a flexible chain containing a naphthalene-1,4-diyl or naphthalene-1,5-diyl spacer. Crystal structures show that the flexibility of the pendant chain allows the naphthyl group to lie close to the Ir core and participate in a it-stacking interaction with a coordinated phpy or F(2)phpy ligand. Luminescence spectra show that, whereas the {Ir(phpy)(2)(pypz)} complexes show typical Ir-based emission-albeit with lengthened lifetimes because of interaction with the stacked naphthyl group the {Ir(F(2)phpy)(2)(pypz)} complexes are nearly quenched. This is because the higher energy of the Ir-based (MLCT)-M-3/(LC)-L-3 excited state can now be quenched by the adjacent naphthyl group to form a long-lived naphthyl-centered triplet ((3)nap) state which is detectable by transient absorption. Coordination of an {Eu(hfac)(3)} unit (hfac = 1,1,1,5,5,5-hexafluoro-pentane-2,4-dionate) to the pendant pypz binding site affords Ir-naphthyl-Eu triads. For the triads containing a {Ir(phpy)(2)} core, the unavailability of the 3nap state (not populated by the Ir-based excited state which is too low in energy) means that direct Ir -> Eu energy-transfer occurs in the same way as in other flexible Ir/Eu complexes. However for the triads based on the{Ir(F(2)phpy)(2)} core, the initial Ir-)3nap energy-transfer step is followed by a second, slower, (3)nap -> Eu energy-transfer step: transient absorption measurements clearly show the (3)nap state being sensitized by the Ir center (synchronous Jr-based decay and (3)nap rise-time) and then transferring its energy to the Eu center (synchronous (3)nap decay and Eu-based emission rise time). Thus the (3)nap state, which is energetically intermediate in the {Ir(F(2)phpy)(2)}-naphthyl-Eu systems, can act as a "stepping stone" for two-step d -> f energy-transfer.

Invited for this month’s cover are the groups of Prof. Daniele Passarella, Prof. Graziella Cappelletti, Dr. Stephen Faulkner and Prof. John S. Snaith based at the Universities of Milano, Oxford and Birmingham. The image shows that a fluorescent tubulin binder could evidence the tubulin/microtubule network.

Luminescence spectroscopy provides a window into conformational space and its influence on the anion binding properties of a series of binuclear lanthanide complexes. Bulky remote substituents are shown to have a profound influence on the affinity of such complexes for a given guest. For more information see the Full Paper by T. J. Sørensen, S. Faulkner et al. on page 16566 ff.

A study of the anion-binding properties of three structurally related lanthanide complexes, which all contain chemically identical anion-binding motifs, has revealed dramatic differences in their anion affinity. These arise as a consequence of changes in the substitution pattern on the periphery of the molecule, at a substantial distance from the binding pocket. Herein, we explore these remote substituent effects and explain the observed behaviour through discussion of the way in which remote substituents can influence and control the global structure of a molecule through their demands upon conformational space. Peripheral modifications to a binuclear lanthanide motif derived from α,α'-bis(DO3 Ayl)-m-xylene are shown to result in dramatic changes to the binding constant for isophthalate. In this system, the parent compound displays considerable conformational flexibility, yet can be assumed to bind to isophthalate through a well-defined conformer. Addition of steric bulk remote from the binding site restricts conformational mobility, giving rise to an increase in binding constant on entropic grounds as long as the ideal binding conformation is not excluded from the available range of conformers.

In a pair of Ir/Eu and Ir/Tb dyads, two-photon excitation of the Ir-phenylpyridine chromophore at 780 nm is followed by partial d → f energy-transfer to give a combination of short-lived Ir-based (blue) and long-lived lanthanide-based (red or green) emission; these components can be selected separately by time-gated detection.

An extensive series of blue-luminescent iridium(III) complexes has been prepared containing two phenylpyridine-type ligands and one ligand containing two pyrazolylpyridine units, of which one is bound to Ir(III) and the second is pendant. Attachment of {Ln(hfac)(3)} (Ln = Eu, Gd; hfac = anion of 1,1,1,5,5,5,-hexafluoropentanedione) to the second coordination site affords Ir(III)/Ln(III) dyads. Crystallographic analysis of several mononuclear iridium(III) complexes and one Ir(III)/Eu(III) dyad reveals that in most cases the complexes can adopt a folded conformation involving aromatic π stacking between a phenylpyridine ligand and the bis(pyrazolylpyridine) ligand, but in one series, based on CF(3)-substituted phenylpyridine ligands coordinated to Ir(III), the steric bulk of the CF(3) group prevents this and a quite different and more open conformation arises. Quantum mechanical calculations well reproduce these two types of "folded" and "open" conformations. In the Ir(III)/Eu(III) dyads, Ir → Eu energy transfer occurs with varying degrees of efficiency, resulting in partial quenching of the Ir(III)-based blue emission and the appearance of a sensitized red emission from Eu(III). Calculations based on consideration of spectroscopic overlap integrals rule out any significant contribution from Förster (dipole-dipole) energy transfer over the distances involved but indicate that Dexter-type (exchange) energy transfer is possible if there is a small electronic coupling that would arise, in part, through π stacking between components. In some cases, an initial photoinduced electron-transfer step could also contribute to Ir → Eu energy transfer, as shown by studies on isostructural iridium/gadolinium model complexes. A balance between the blue (Ir-based) and red (Eu-based) emission components can generate white light.

An isophthalate-bearing DOTA monoamide derivative has been synthesised and used to prepare a family of lanthanide complexes. Luminescence and NMR studies in solution show that the predominant form of the complexes in solution is a mono-capped square antiprism about the lanthanide centre, in which a solvent molecule occupies the ninth coordination site. The crystal structure of the terbium complex is presented and is in close agreement with the solution state data.

Luminescent iridium(iii) complex units bearing pendant 2,2'-bipyridyl-type binding sites can be used to generate Ir/Ln dyads in which the Ir(iii) luminophore acts as an energy donor to the lanthanide by the Dexter mechanism, generating sensitised emission in the visible (from Eu) or near-infrared (Nd, Yb) regions.

The photophysical properties of the neptunyl (V) ion in aqueous solution have been studied using time-resolved luminescence spectroscopy. While any f-f transitions in emission are too weak to detect using available technology, the ligand to metal charge transfer state is emissive in the visible part of the spectrum. Both the aquo ion and its complexes with bidentate ligands exhibit biexponential decay kinetics, which can be rationalised by slow exchange on the timescale of the experiment.

In Ir(III)/Tb(III) dyads in which the excited state energy of the Ir(III) unit lies above 22,000 cm(-1), visible-light excitation of the Ir(III) chromophore results in sensitised emission from Tb(III) following Ir → Tb energy-transfer.