Robert D. Peacock

Ligand-polarization contributions to the intensity of hypersensitive trivalent lanthanide transitions

The oscillator strengths of the hypersensitive f-electron transitions in lanthanide(III) complexes, the quasi-quadrupolar transitions with intensities sensitive to the ligand environment, are found to arise principally from the coulombic correlation between transient induced electric dipoles in the ligands and the f-electron quadrupole moments of the metal ion. The correlation gives a non-zero resultant electric dipole transition moment in complexes belonging to the point groups, Dp and Cpv , with p unrestricted, C 3h , D 3h , Td , and their subgroups, the mechanism being forbidden where there is inversion symmetry or an Sp axis with p ⩾ 5. The theory accounts quantitatively for the hypersensitive oscillator strengths of the lanthanide trihalides, which range over two orders of magnitude, and the substituent effects observed in the lanthanide tris-β-dicarbonyl chelate complexes. The restriction of hypersensitivity to the qudrupolar f-electron transitions is discussed.

Synthesis and characterisation of highly emissive and kinetically stable lanthanide complexes suitable for usage ‘in cellulo’

The synthesis and photophysical characterisation are reported of a series of cationic, neutral and anionic europium and terbium complexes based on structurally related, nonadentate ligands based on the cyclen macrocycle. Each complex incorporates a tetraazatriphenylene moiety and overall absolute emission quantum yields are in the range 15-40% in aerated aqueous media. Dynamic quenching of the lanthanide excited state occurs with electron-rich donors, e.g. iodide, ascorbate and urate, and a mechanistic interpretation is put forward involving an electron transfer process. The cationic lanthanide complexes are taken up by NlH/3T3 cells and tend to localise inside the cell nucleus.

Reversible Redox Reactions in an Extended Polyoxometalate Framework Solid

Back and forth: A concerted reversible redox reaction occurs in a pure metal oxide extended polyoxometalate framework when the accessible pockets are filled with a suitable redox agent. Direct control over the framework properties is demonstrated by repeated reversible switching between an expanded and a contracted structure. Successive recrystallizations from hot water repeatedly destroys and regenerates the framework.

Dynamic coupling contributions to the intensity of hypersensitive lanthanide transitions

Abstract The intensities of the pseudoquadrupolar hypersensitive f-electron transitions in Ln(III) complexes are found to arise principally from the coulombic correlation of induced electric dipoles in the ligands by the transition quadrupole moment of the metal ion. The correlation gives a non-zero resultant electric dipole transition moment in complexes devoid of inversion symmetry or S p axes for p > 5. The theory quantitatively reproduces the observed intensities of the 4 I 9/2 → 4 G 5/2 transition near 17 kK of gaseous NdBr 3 and NdI 3 .

Metal-Directed Synthesis of Enantiomerially Pure Dimetallic Lanthanide Luminescent Triple-Stranded Helicates

The synthesis and photophysical evaluation of two enatiomerially pure dimetallic lanthanide luminescent triple-stranded helicates is described. The two systems, formed from the chiral (R,R) ligand 1 and (S,S) ligand 2, were produced as single species in solution, where the excitation of either the naphthalene antennae or the pyridiyl units gave rise to Eu(III) emission in a variety of solvents. Excitation of the antennae also gave rise to circularly polarized Eu(III) luminescence emissions for Eu(2):1(3) and Eu(2):2(3) that were of equal intensity and opposite sign, confirming their enantiomeric nature in solution providing a basis upon which we were able to assign the absolute configurations of Eu(2):1(3) and Eu(2):2(3).

Chiral Schiff base complexes of copper (II), vanadium (IV) and nickel (II) as oxidation catalysts. X-ray crystal structures of [Cu (R-salpn) (OH2)] and [Cu (±-busalcx)]

Abstract Several new complexes of chiral and achiral tetradentate Schiff bases with copper (II), nickel (II) and oxovanadyl (IV) ions have been synthesised and characterised and the structures of [Cu ( R -salpn) (OH 2 )] and [Cu (±-busalcx)] have been elucidated by X-ray analysis. [Cu ( R -salpn) (OH 2 )] is essentially square pyramidal with an unusually long bond [2.494 (4) A] from Cu to the oxygen of the apical water molecule. Molecules are linked by hydrogen bonding between the coordinated water and the salpn oxygens of neighbouring molecules. A long contact between a salpn CH 2 hydrogen and a neighbouring copper atom [H(3B). . .Cu = 2.966 (1) A] might be regarded as a CH···Cu hydrogen bond and completes a pseudo-octahedral coordination about copper. [Cu (±-busalcx)] displays tetrahedrally-distorted planar coordination about the copper atom and has no significant intermolecular contacts. The copper, nickel and vanadyl complexes were screened as homogeneous catalysts for the oxidation by Bu t OOH or H 2 O 2 of PhMeS to PhMeSO. All of them are active catalysts but of the chiral complexes only [Cu ( R , R -busalcx)] produced a reasonable enantiomeric excess (14%). Supporting the complexes on silica improved the enantioselectivity of the less sterically hindered [Cu ( R -salpn) (OH 2 )] ( from 2 to 10%), but diminished that of the more hindered [Cu ( R , R -busalcx)], probably by slowing the reaction to such an extent that the uncatalysed oxidation could compete effectively. Finally [Cu (salen)], [Cu ( R -salpn)] and [Ni ( R -salpn)] were synthesised in the pores of zeolites X and Y. These were shown to be active heterogeneous catalysts for the sulfide to sulfoxide oxidations, but no significant e.e. resulted from the use of the chiral catalysts.

Circularly Polarized Lanthanide Luminescence from Langmuir–Blodgett Films Formed from Optically Active and Amphiphilic EuIII‐Based Self‐Assembly Complexes

The development of functional nanomaterials and supramolecular systems is an active area of research, particularly for molecular recognition/sensing, catalysis, optical devices, and magnetically active compounds for switching and data storage. While much attention has been focused on transition-metal-based supramolecular systems, there has been a recent insurgence of lanthanide-based systems. These ions possess rich coordination environments and unique physical properties, such as long-lived and long wavelength emission in the visible or the NIR regions, as well as magnetic properties, which have been exploited for use in the developments of MRI contrast agents. Hence, these properties make them ideal and highly desirable candidates for the formation of functional supramolecular systems. The development of supramolecular assemblies that can be further organized into functional devices is also of great current interest. These assemblies can be achieved by covalently attaching appropriate ligands and complexes to nanoparticles or flat surfaces, through the formation of polymers, or by forming thin films using Langmuir–Blodgett (LB) techniques. Herein we describe our efforts in bridging these two areas of research by employing lanthanide-directed synthesis (using ligands 1 and 2) in the formation of chiral luminescent lanthanide amphiphilic complexes, and their use in the formation of LB films, the properties of which can be probed by using circularly polarized luminescence (CPL). The ligands were designed to include a terdentate coordination pocket with a closely associated sensitizing antenna (i.e. the R- or S-naphthylamine moieties) for the lanthanide ions such as EuIII and TbIII—an approach that has been extremely successful for the development of luminescent supramolecular self-assembly structures, such as chiral “bundles” and dilanthanide triple-stranded helicates. Additionally, a long hydrophobic hexadecyl chain was included to allow the formation of Langmuir–Blodgett films. Ligands 1 and 2 were prepared in yields of 74% and 82%, respectively, by employing EDCI·HCl peptide coupling reactions (EDCI·HCl=1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride) between the R and the S isomers of precursors 3 and 4, respectively, and N-hexadecylamine.

Chiral phosphinoylalcohol complexes of monooxobis(peroxo)molybdenum(VI) and their use as asymmetric oxidants

Abstract Complexes of the type [MoO(O 2 ) 2 (L*)(ROH)], where L*=chiral β-phosphinoylalcohol, have been synthesised and used as stoichiometric oxidants for a number of unfunctionalised alkenes. In all of the complexes the chiral auxiliary is bound through the phosphinoyl oxygen as a monodentate ligand. The coordination about the metal atom in these pseudo-pentagonal bipyramidal molecules is completed by a solvent molecule (ethanol/water) lying opposite the axial MoO bond. Oxidation of small-chain non-functionalised alkenes occurs in variable yield to give epoxides with an enantiomeric excess of up to 40%. These compounds also behave as catalysts for the Bu t OOH oxidations of alkenes, but with similar modest enantioselectivities. The modest enantioselectivities are explained on the basis of the mode of coordination of the chiral ligand, and it is argued that there may be inherent limits in the use of these systems in asymmetric oxidations.

Circularly Polarized Luminescence from Helically Chiral N,N,O,O‐Boron‐Chelated Dipyrromethenes

Abstract Helically chiral N,N,O,O‐boron chelated dipyrromethenes showed solution‐phase circularly polarized luminescence (CPL) in the red region of the visible spectrum (λ em(max) from 621 to 663 nm). The parent dipyrromethene is desymmetrised through O chelation of boron by the 3,5‐ortho‐phenolic substituents, inducing a helical chirality in the fluorophore. The combination of high luminescence dissymmetry factors (|g lum| up to 4.7 ×10−3) and fluorescence quantum yields (Φ F up to 0.73) gave exceptionally efficient circularly polarized red emission from these simple small organic fluorophores, enabling future application in CPL‐based bioimaging.

The charge-transfer contribution to the intensity of hypersensitive trivalent lanthanide transitions

By considering the variation of charge-transfer energy and of the intensity parameters across isostructural series it is concluded that charge-transfer mixing is of negligible importance in accounting for the intensity of the hypersensitive transitions of the trivalent lanthanides. The experimental relationship between hypersensitive transition intensity and charge-transfer energy found for a particular member of the lanthanide series in various environments can be as satisfactorily explained by the ligand polarization theory as by charge-transfer mixing.