The excited-state absorption of Ru(bpy)2+3 reexamined

The excited-state absorption of Ru (bpy)₃²⁺ ( RBY ) in the range 240 to 2200 nm is reported, extending the range and improving the resolution of previous measurements. The excitation power dependence of the excited-state populations are quantified and absorption measurements made in H₂O and D₂O solutions at 300 K and in poly (vinyl-alcohol) (PVA) films at 20 K. No evidence is found for an intervalence charge-transfer type transition in the near IR, but the Ψ→ χ ligand excitation transition is clearly seen near 11000 and 9000 cm⁻¹ in solution and in PVA respectively. The visible and UV excited-state spectra are similar in the two environments and differences are attributed to the effects of photoselection in PVA. The Ψ→ χ transition shows a marked shift in passing from a rigid to a fluid medium, indicating the possibility of a change in ligand geometry.     

Absolute Asymmetric Synthesis: Viedma Ripening of [Co(bpy)3]2+ and Solvent‐Free Oxidation to [Co(bpy)3]3+

Syntheses of [Co(bpy)₃]²⁺ yield racemic solutions because the Δ‐ and Λ‐enantiomers are stereochemically labile. However, crystallization and attrition‐enhanced deracemization can give homochiral crystal batches of either handedness in quantitative yield. Subsequently, solvent‐free oxidation with bromine vapour fixes the chirality because [Co(bipy)₃]³⁺ does not enantiomerize in solution at ambient temperature. This combination of Viedma ripening and the labile/inert Co^II/Co^III couple constitutes a convenient method of absolute asymmetric synthesis.     

A model electronic Hamiltonian to study low-lying electronic states of [Fe(bpy)3]2+ in aqueous solution

A simple model electronic Hamiltonian to describe the potential energy surfaces of several low-lying d-d states of the [Fe(bpy)(3)](2+) complex is developed for use in molecular dynamics (MD) simulation studies. On the basis of a method proposed previously for first-row transition metal ions in aqueous solution, the model Hamiltonian is constructed using density functional theory calculations for the lowest singlet and quintet states. MD simulations are then carried out for the two spin states in aqueous solution in order to examine the performance of the model Hamiltonian. The simulation results indicate that the present model electronic Hamiltonian reasonably describes the potential energy surfaces of the two spin states of the aqueous [Fe(bpy)(3)](2+) system, while retaining sufficient simplicity for application in simulation studies on excited state dynamics.     

High-spin to low-spin relaxation kinetics in the [Fe(TRIM)2]Cl2 complex

A quasi-quantitative photo-induced low-spin (LS)-->high-spin (HS) conversion of FeII ions has been observed in the [Fe(TRIM)2]Cl2 complex by irradiating the sample with blue light (488 nm) at 10 K. The time dependence of the HS-->LS relaxation has been studied between 10 K and 44 K by means of magnetic susceptibility measurements. These relaxation curves could be satisfactorily fitted by mono-exponential decays including tunnelling effect except for temperatures below 30 K. The introduction of a distribution of vibrational frequencies into this model improved significantly the fits in the low-temperature range and gave a good agreement with the experimental data in the whole temperature range suggesting a multi-rate relaxation process in this complex.     

UV-Induced Formation of [Fe(bpy)3]2+ Complex and its Confinement into Silica Fine Particles

Silica fine particles in a single or sub-micrometer regime, containing FeCl₃ and 2,2-bipyridine (bpy), were prepared by utilizing reverse micellar method. Formation of tris-bpy iron(II) ([Fe(bpy)₃]²⁺) complex was induced in the silica fine particles by UV irradiation at 365 nm. The starting micellar solution was obtained by an injection of 0.04 cm³ HCl aqueous solution into 5 cm³ tetraethyl orthosilicate (TEOS)/cyclohexane (2 cm³/3 cm³) mixed solution containing 4–14 vol% of nonionic surfactant, polyoxyethylene(5) nonylphenyl ether (NP-5). Size of the silica particles was regulated by the concentration of NP-5.     

Photoinitiated DNA Binding by cis-[Ru(bpy)2(NH3)2]2+

The ligand-loss photochemistry of cis-[Ru(bpy)(2)(NH(3))(2)](2+) (bpy = 2,2'-bipyridine) was investigated in water and in the presence of added ligands such as bipyridine and chloride. Irradiation of the complex results in the covalent binding to 9-methyl- and 9-ethylguanine, as well as to single-stranded and double-stranded DNA. This photoinduced DNA binding is not observed for the control complex [Ru(bpy)(2)(en)](2+) (en = ethylenediamine) under similar irradiation conditions. The results presented here show that octahedral Ru(II) complexes with photolabile ligands may prove useful as photoactivated cisplatin analogs.     

Nafion-stabilized magnetic nanoparticles (Fe3O4) for [Ru(bpy)3]2+(bpy = bipyridine) electrogenerated chemiluminescence sensor

A highly sensitive and stable [Ru(bpy)3]2+ ECL sensor has been fabricated based on the multilayer films of Nafion-stabilized magnetic nanoparticles (Nafion/Fe3O4) formed on a platinum electrode surface by means of an external magnet.     

NRIS study on the [FeN6] core in photo-induced high-spin state of [Fe(2-pic)3]Cl2.EtOH

A comparison is reported for thermally and photo-induced high-spin phases of [Fe(2-pic)3]Cl2.EtOH (2-pic: 2-picolylamine) using the nuclear resonant inelastic scattering (NRIS) technique.     

Relative energy of the high-(5T2g) and low-(1A1g) spin states of [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+: CASPT2 versus density functional theory

High-level ab initio calculations using the CASPT2 method and extensive basis sets were performed on the energy differences of the high-[(5)T(2g):t(2g) (4)e(g) (2)] and low-[(1)A(1g):t(2g) (6)] spin states of the pseudo-octahedral Fe(II) complexes [Fe(H(2)O)(6)](2+), [Fe(NH(3))(6)](2+), and [Fe(bpy)(3)](2+). The results are compared to the results obtained from density functional theory calculations with the generalized gradient approximation functional BP86 and two hybrid functionals B3LYP and PBE0, and serve as a calibration for the latter methods. We find that large basis set CASPT2 calculations may provide results for the high-spin/low-spin splitting DeltaE(HL) that are accurate to within 1000 cm(-1), provided they are based on an adequately large CAS[10,12] reference wave function. The latter condition was found to be much more stringent for [Fe(bpy)(3)](2+) than for the other two complexes. Our "best" results for DeltaE(HL) (including a zero-point energy correction) are -17 690 cm(-1) for [Fe(H(2)O)(6)](2+), -8389 cm(-1) for [Fe(NH(3))(6)](2+), and 3820 cm(-1) for [Fe(bpy)(3)](2+).     

Ultrafast nonadiabatic dynamics of [Fe(II)(bpy)(3)](2+) in solution

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at approximately 600 nm, which decays in < or =20 fs, and a weak emission at approximately 660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the 3MLCT, the second is identified as the population time of the 5T2 state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate approximately 2000 cm-1/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.     

Intermolecular electron transfer from photogenerated Ru(bpy)3+ to [2Fe2S] model complexes of the iron-only hydrogenase active site

Visible light-driven intermolecular electron transfer was observed from a reduced species Ru(bpy)3+, photogenerated via a reductive quenching of the ruthenium photosensitizer by a diethyldithiocarbamate anion, to bioinspired [2Fe2S] model complexes of the iron-only hydrogenase active site. The results indicate that Ru(bpy)32+ can act as a photoactive functional model of the [4Fe4S] cluster, playing the role of an electron-transfer relay. The photogenerated FeIFe0 species, which is proposed to be a crucial intermediate for proton reduction catalyzed electrochemically by the [2Fe2S] complexes, gives promise in the light-driven dihydrogen evolution using diiron complexes as surrogates of noble platinum catalysts.     

Salt and solvent effects on the kinetics of the oxidation of the excited state of the [Ru(bpy)3]2+ complex by S2O8(2-)

The title reaction was studied in different reaction media: aqueous salt solutions (NaNO3) and water-cosolvent (methanol) mixtures. The observed rate constants, k(obs), show normal behavior in the solutions containing the electrolyte, that is, a negative salt effect. However, the solvent effect is abnormal, because a decrease of the rate constant is observed when the dielectric constant of the reaction medium decreases. These effects (the normal and the abnormal) can be explained using the Marcus-Hush treatment for electron transfer reactions. To apply this treatment, the true, unimolecular, electron-transfer rate constants, k(et), have been obtained from k(obs) after calculation of the rate constants corresponding to the formation of the encounter complex from the separate reactants, k(D), and the dissociation of this complex, k(-D). This calculation has been carried out using an exponential mean spherical approach (EMSA).     

[M(bpy)3] (M = Fe,Ru, Os): New Crystalline Materials from the reductive electrocrystallization of [M(bpy)3](PF6)2

Reductive electrocrystallization at a constant current density (11.0–11.5 μA/cm²) of millimolar solutions of [M(bpy)₃](PF₆)₂, where M = Fe, Ru, or Os, and bpy = 2,2′-bipyridine in acetonitrile containing 0.1M Bu₄NPF₆ results in the formation of dark crystals on the Pt cathode. The crystals grow as long, thin, and shiny needles having a hexagonal cross section of 0.1–0.5 mm in diameter. Combustion microanalyses results are consistent with the composition for [Fe(bpy)₃], [Ru(bpy)₃], and [Os(bpy)₃]. In addition, the chromophores are conserved, as confirmed by recording both the electronic and the ¹H-NMR spectra after reoxidation of the electrocrystals in humid air. The spectra are identical to those for authentic samples of [Fe(bpy)₃]²⁺, [Ru(bpy)₃]²⁺, and [Os(bpy)₃]²⁺. A ratio of 2.0 ± 0.1 e^?/molecule is observed upon completion of the controlled potential electrolysis of a solution of [M(bpy)₃]²⁺, which results in the precipitation of a dark solid and the almost complete fading of the color of the original solution. Unexpectedly, the crystals do not exhibit an ESR signal. These data indicate the formation of novel materials, crystalline [Fe(bpy)₃], [Ru(bpy)₃], and [Os(bpy)₃].     

Base-Modified DNA Labeled by [Ru(bpy)3]2+ and [Os(bpy)3]2+ Complexes: Construction by Polymerase Incorporation of Modified Nucleoside Triphosphates,Electrochemical and Luminescent Properties,and Applications

Modified 2′-deoxynucleoside triphosphates (dNTPs) bearing [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ complexes attached via an acetylene linker to the 5-position of pyrimidines (C and U) or to the 7-position of 7-deazapurines (7-deaza-A and 7-deaza-G) have been prepared in one step by aqueous cross-couplings of halogenated dNTPs with the corresponding terminal acetylenes. Polymerase incorporation by primer extension using Vent (exo-) or Pwo polymerases gave DNA labeled in specific positions with Ru²⁺ or Os²⁺ complexes. Square-wave voltammetry could be efficiently used to detect these labeled nucleic acids by reversible oxidations of Ru^2+/3+ or Os^2+/3+. The redox potentials of the Ru²⁺ complexes (1.1–1.25 V) are very close to that of G oxidation (1.1 V), while the potentials of Os²⁺ complexes (0.75 V) are sufficiently different to enable their independent detection. On the other hand, Ru²⁺-labeled DNA can be independently analyzed by luminescence. In combination with previously reported dNTPs bearing ferrocene, aminophenyl, and nitrophenyl tags, the Os-labeled dATP has been successfully used for “multicolor” redox labeling of DNA and for DNA minisequencing.     

Femtosecond transient absorption anisotropy study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast interligand randomization of the MLCT state

It is known that the relaxed excited state of [Ru(bpy)3]2+ is best described as a metal to ligand charge transfer (MLCT) state having one formally reduced bipyridine and two neutral. Previous reports have suggested [Malone, R. et al. J. Chem. Phys. 1991, 95, 8970] that the electron "hops" from ligand to ligand in the MLCT state with a time constant of about 50 ps in acetonitrile. However, we have done transient absorption anisotropy measurements indicating that already after one picosecond the molecule has no memory of which bipyridine was initially photoselected, which suggests an ultrafast interligand randomization of the MLCT state.     

Room-temperature photochromism in cis- and trans-[Ru(bpy)2(dmso)2]2+

We report on phototriggered Ru-S --> Ru-O and thermal Ru-O --> Ru-S intramolecular linkage isomerizations in cis- and trans-[Ru(bpy)2(dmso)2]2+. The cis complex features only S-bonded sulfoxides (cis-[S,S]), whereas the trans isomer is characterized by S- and O-bonded dmso ligands. Both cis-[S,S] and trans-[S,O] exhibit photochromism at room temperature in dmso solution and ionic liquid (IL). Rates of reaction in IL were monitored by UV-visible spectroscopy and are similar to those reported in dmso solution (k(O-->S) ranges from approximately 10(-3) to 10(-4) s(-1)). Cyclic voltammetric measurements of cis-[S,S] and trans-[S,O] are consistent with an electrochemically triggered linkage isomerism mechanism. While both cis-[S,S] and trans-[S,O] are photochromic at room temperature, neither complex is emissive. However, upon cooling to 77 K, cis-[S,S] exhibits LMCT (ligand-to-metal charge transfer) emission typical of many ruthenium polypyridine complexes. In contrast to cis-[S,S], trans-[S,O] does not show any detectable emission even at 77 K.