Structural and spectroscopic properties of Ru(II) complexes of 4-(aryl)thiosemicarbazones of thiophen-2-carbaldehyde

Six Ru(II) complexes of formula [Ru(L)₂(PPh₃)₂] have been prepared where LH = 4-(aryl)thiosemicarbazones of thiophen-2-carbaldehyde. X-ray crystal structures of five of the complexes are reported. In all the complexes ruthenium is six coordinate with a distorted octahedral cis-P₂, cis-N₂, trans-S₂ donor environment, and each of the two thiosemicarbazone ligands are coordinated in a bidentate fashion forming a four membered chelate ring. The complexes undergo a one-electron oxidation at ～0.5 V vs. Ag/AgCl. The EPR spectrum of the electrochemically oxidized solution at 100 K shows a rhombic signal, with transitions at g₁ = 2.27, g₂ = 2.00 and g₃ = 1.80. DFT calculations on one of the complexes suggest that there is 35% ruthenium and 17% sulfur orbital contribution to the HOMO. These results suggest that the assignment of metal atom oxidation states in these compounds is not unambiguous.     

Activity coefficients in concentrated electrolytes: a comparison of the Brunauer-Emmett-Teller (BET) model with experimental values

Electrolyte mean ionic activity coefficients, γ_±, are calculated from the Stokes-Robinson application of the Brunauer-Emmett-Teller (BET) adsorption model for seven electrolytes (NaOH, HCl, KOH, CaCl₂, LiCl, LiBr and Ca(NO₂)₃). Only two model parameters are needed, which are derived from water vapor pressure measurements. Results were compared with experimental mean ionic activity coefficients (γ_±). Because the standard state for the BET model is the anhydrous electrolyte rather than the infinitely dilute solution, it is necessary to adjust for the differing standard states by comparing the BET and experimental γ_± values at one ‘anchoring’ concentration. The higher this ‘anchoring’ concentration, the better is the agreement between the BET and experimental mean ionic activity coefficients over the entire concentration (molality, mol kg⁻¹) range except at the two extremes of nearly pure water and nearly pure electrolyte. This is because the BET model lacks a finite limit at infinite dilution, and the experimental data are referenced to the infinite dilute solution. The BET model is well-behaved and in good agreement with experimental data on γ_±.     

High-resolution X-ray photoelectron spectroscopic studies of alkylated silicon(111) surfaces

Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si-H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl5 in chlorobenzene exhibited a Si 2p(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si-Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with CnH(2n+1)- (n = 1 or 2) or C6H5CH2- groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 1 monolayer. This positively shifted Si 2p(3/2) peak is consistent with the presence of Si-C and Si-H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group.     

Distal metal effects in cobalt porphyrins related to CcO

Cobalt(II) porphyrins were studied to determine the influence of distal site metalation and superstructure upon dioxygen reactivity in active site models of cytochrome c oxidase (CcO). Monometallic, Co(II)(P) complexes when ligated by an axial imidazole react with dioxygen to form reversible Co-superoxide adducts, which were characterized by EPR and resonance Raman (RR). Unexpectedly, certain Co porphyrins with Cu(I) metalated imidazole pickets do not form mu-peroxo Co(III)/Cu(II) products even though the calculated intermetallic distance suggests this is possible. Instead, cobalt-porphyrin-superoxide complexes are obtained with the distal copper remaining as Cu(I). Moreover, distal metals (Cu(I) or Zn(II)) greatly enhance the stability of the dioxygen adduct, such that Co superoxides of bimetallic complexes demonstrate minimal reversibility. The "trapping" of dioxygen by a second metal is attributed to structural and electrostatic changes within the distal pocket upon metalation. EPR evidence suggests that the terminal oxygen in these bimetallic Co-superoxide systems is H-bonded to the NH of an imidazole picket amide linker, which may contribute to enthalpic stabilization of the dioxygen adduct. Stabilization of the dioxygen adduct in these bimetallic systems suggests one possible role for the distal copper in the Fe/Cu bimetallic active site of terminal oxidases, which form a heme-superoxide/copper(I) adduct upon oxygenation.     

Highlighting the role of the medium in DFT analysis of the photophysical properties of ruthenium(II) polypyridine-type complexes

In order to test the pertinence of the density functional theory to interpret the photophysical properties of ruthenium(II) polypyridine-type complexes, DFT and TDDFT calculations are performed both on the isolated molecule and in solution media described by the dielectric-like polarized continuum model (PCM). This study is focused on three isoelectronic complexes: [Ru(bpy)(2)(PhenImHPh)](2+) (II), where PhenImHPh represents the 2-(3,5-ditertbutylphenyl)imidazo[4,5-f][1,10]phenanthroline ligand, as well as [Ru(bpy)2(PhenImPh)]+ (I), and [Ru(bpy)(2)(PhenImH2Ph)](3+) (III), obtained by changing the protonic state of the imidazole ring. The structural and electronic properties of the ground and lowest triplet states are fully characterized in vacuo and in water solution, and the absorption spectra in the visible region are also investigated by TDDFT. The theoretical data are compared to the electrochemistry, UV-visible, and photophysical experiments to assess the validity and limits of each type of calculation. The choice of the functional is also discussed.     

Influence of the protonic state of an imidazole-containing ligand on the electrochemical and photophysical properties of a ruthenium(II)-polypyridine-type complex

The synthesis and characterisation of [Ru(bpy)2(PhenImHPh)]2+ where PhenImHPh represents the 2-(3,5-di-tert-butylphenyl)imidazo[4,5-f][1,10]phenanthroline ligand are described. The compounds issued from the three different protonic states of the imidazole ring [Ru(bpy)2(PhenImPh)]+ (I), [Ru(bpy)2(PhenImHPh)]2+ (II) and [Ru(bpy)2(PhenImH2Ph)]3+ (III) were isolated and spectroscopically characterised. The X-ray structures of [Ru(bpy)2(PhenImPh)](PF6)H2O.6 MeOH, [Ru(bpy)2(PhenImHPh)](NO3)2H2O.3 MeOH and [Ru(bpy)2(PhenImH2Ph)](PF6)3 5 H2O are reported. Electrochemical data obtained on these complexes indicate almost no potential shift for the Ru(III/II) redox couple. Therefore a Coulombic effect between the imidazole ring and the metal centre can be ruled out. The monooxidised forms of I and II have been characterised by EPR spectroscopy and are reminiscent of the presence of a radical species. The emission properties of the parent compound [Ru(bpy)2(PhenImHPh)]2+ were studied as a function of pH and both the lifetimes and intensities decreased upon deprotonation. Photophysical properties, investigated in the absence and presence of an electron acceptor (methylviologen), were distinctly different for the three compounds. Transient absorption features indicate that unique excited states are involved. Theoretical data obtained from DFT calculations in water on the three protonic forms are presented and discussed in the light of the experimental results.     

A theoretical investigation into the photophysical properties of ruthenium polypyridine-type complexes

Excited states of ruthenium polypyridine-type complexes have always attracted the interest of chemists. We have recently found evidence of a remarkable long-lived excited state (30 micros) for a Ru(II) complex containing a heteroditopic ligand that can be viewed as a fused phenanthroline and salophen ligand.1 To unravel this intriguing electronic property, we have used density functional theory (DFT) calculations to understand the ground-state properties of [(bpy)(2)Ru(LH(2))](2+), where LH(2) represents N,N'-bis(salicylidene)-(1,10-phenanthroline)diamine. Excited singlet and triplet states have been examined by the time-dependent DFT (TDDFT) formalism and the theoretical findings have been compared with those for the parent complex [Ru(bpy)(3)](2+). The outstanding result is the presence of excited states lower in energy than the metal-to-ligand charge-transfer states, originating from intraligand charge transfer (ILCT) from the phenolic rings to the phenanthroline part of the coordinated LH(2). The spin density distribution for the lowest triplet state provides evidence that it is in fact the lowest triplet state of the free ligand. Correlation between the energy level diagram of orbitals for the ground state and that for the (3)ILCT state clearly establishes that the ruthenium retains its formal Ru(II) oxidation state. The quenching of the luminescence and the evidence of the long-lived excited state observed for [(bpy)(2)Ru(LH(2))](2+) are discussed in the light of the computational results.     

Detachment force of particles from air-liquid interfaces of films and bubbles

The detachment force required to pull a microparticle from an air-liquid interface is measured using atomic force microscopy (AFM) and the colloidal probe technique. Water, solutions of sodium dodecyl sulfate (SDS), and silicone oils are tested in order to study the effects of surface tension and viscosity. Two different liquid geometries are considered: the air-liquid interface of a bubble and a liquid film on a solid substrate. It was shown that detaching particles from liquid films is fundamentally different than from bubbles or drops due to the restricted flow of the liquid phase. Additional force is required to detach a particle from a film, and the maximum force during detachment is not necessarily at the position where the particle breaks away from the interface (as seen in bubble or drop systems). This is due to the dynamics of meniscus formation and viscous effects, which must be considered if the liquid is constrained in a film. The magnitude of these effects is related to the liquid viscosity, film thickness, and detachment speed.