Lowest Energy Electronic Absorption Research Articles

Evidence of Different Charging Behavior of Conductive and Dielectric Materials in Low-Temperature Plasmas and a New Diagnostic for Low-Energy Electron Absorption.

In any physical system where a surface is hit by electrons, the sticking probability s of the electrons is a central parameter governing, for example, the charging of the surface. For dielectrics, it could previously only be measured for high energies (>100 eV), while it is well-known for metals even at energies of only a few eV. Recent theoretical investigations concerning dielectrics such as silica predict values for s significantly below 1. With precision charge measurements on microparticles in the sheath of a low-pressure plasma, a difference in charge between silica and gold-coated particles is found, challenging the long-standing assumption in dusty plasma physics that dielectric and metallic particles charge in the same way for the first time. Based on the measured charging difference, the low-energy sticking coefficient of silica is obtained, validating the theoretical predictions and offering a new diagnostic for the otherwise quasi-inaccessible low-energy electron sticking of dielectric materials.

Effect of Solvent Polarity on the Spectral Characteristics of 5,10,15,20-Tetrakis(p-hydroxyphenyl)porphyrin.

The electronic absorption and vibrational spectra of deprotonated 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin (THPP) are studied as a function of solvent polarity in H2O-DMF, H2O-acetone, H2O-methanol, and DMF-acetone mixtures. The maximum absorption wavelength (λmax) of the lowest energy electronic absorption band of deprotonated THPP shows an unusual solvatochromism-a bathochromic followed by a hypsochromic shift with reduced polarity. According to the correlation analysis, both specific interactions (H-bonds) and nonspecific interactions affect the spectral changes of this porphyrin. Furthermore, the solvent polarity scale ET(30) can explain both shifts very well. At higher polarity (ET(30) > 48), THPP exists as a hyperporphyrin. The ET(30) is linear with λmax and a decrease in solvent polarity is accompanied by a bathochromic shift of λmax. These results can be rationalized in terms of the cooperative effects of H-bonds and nonspecific interactions on the spectra of hyperporphyrin. At relatively low polarity (45.5 < ET(30) < 48), hyperporphyrin gradually becomes Na2P as ET(30) reaches the critical value of 45.5. The spectrum of the hyperporphyrin turns into the three-band spectrum of the metalloporphyrin, which is accompanied by a hypsochromic shift of λmax.

Torsional Disorder, Symmetry Breaking, and the Crystal Violet Shoulder Controversy.

The nature of the lowest-energy electronic absorption band of crystal violet (CV) and particularly the origin of its high-energy shoulder have been debated since the middle of the past century. The most recent studies invoke a splitting of the S1 state upon symmetry breaking induced by interactions with the solvent and/or the counterion. Using a combination of stationary and time-resolved polarized spectroscopy together with quantum-chemical calculations, we show that torsional disorder in the ground-state results in an inhomogeneous broadening of the absorption band of CV. The center of the band is mostly due to symmetric molecules with a degenerate S1 state, whereas the edges originate from transitions to the S1 and S2 states of distorted symmetry-broken molecules. Transient-absorption measurements with different excitation wavelengths reveal that these two groups of molecules interconvert rapidly in liquid but not in a rigid environment.

Hysteresis control using a DC magnetic field in an argon inductively coupled plasma

Control of the hysteresis phenomenon in a high-pressure (250 mTorr) argon inductively coupled plasma was experimentally investigated by applying a DC magnetic field. Electron energy probability functions (EEPFs) were measured with and without DC magnetic fields to obtain electron densities. Without the magnetic field, a hysteresis loop is clearly observed during the E and H mode transitions, but surprisingly, when 20 G of DC magnetic field is applied, the hysteresis loop gets smaller, and it vanishes completely when the applied DC magnetic field is increased to over 40 G. Measured EEPFs show that there is a significant evolution of the EEPFs by DC magnetic field. The EEPF without magnetic field is a Druyvesteyn distribution, but evolves to a Maxwellian-like distribution under a strong DC magnetic field condition. This evolution of the EEPF causes significant reduction in the collisional energy loss εc in E-mode. The evolution of EEPFs is explained by a decrease in Ohmic power absorption of low-energy electrons and an increase in electron–electron collisions, and the vanishing of the hysteresis is explained by the suppression of nonlinear changes in EEPF and nonlinear changes in collisional energy loss.

Metal-insulator phase transition in the δ−(BEDT−TTF)4 [2,6-anthracene- bis(sulfonate)] ·4H2O studied by infrared spectroscopy

Temperature dependence of polarized infrared (IR) reflectance spectra of two-dimensional weakly dimerized organic conductor $\ensuremath{\delta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}$[2,6-anthracene-bis(sulfonate)]$\ifmmode\cdot\else\textperiodcentered\fi{}4{\mathrm{H}}_{2}\mathrm{O}$ with $\frac{1}{4}$-filled band has been measured to investigate the metal-insulator phase transition at ${T}_{\mathrm{MI}}=85\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ linked to charge ordering (CO). Vibrational bands related to $\mathrm{C}=\mathrm{C}$ stretching vibrations of the BEDT-TTF donor molecule give evidence of the CO and charge fluctuations both in the metallic and insulating phases above and below 85 K, respectively. Considerable modifications of the vibrational features due to electron-molecular vibration coupling are also observed. The phase transition is associated with the reorganization of hydrogen $\mathrm{OH}\ensuremath{\cdots}\mathrm{O}$ bonds in anion layers between the sulfonate moieties of the anion and the trapped water molecules, which is transmitted to conducting BEDT-TTF layers through weaker C-$\mathrm{H}\ensuremath{\cdots}\mathrm{O}$ bonds. The analysis of electronic bands confirms the presence of strong electronic correlations in the material. Two electronic absorptions are observed in the mid-IR region, attributed to transitions between Hubbard bands and transitions inside BEDT-TTF dimers. A low-energy electronic absorption in the far-IR region is attributed to collective excitations of the short-range charge order. Below 85 K, the opening of a charge gap of \ensuremath{\sim}24 meV is observed both parallel and perpendicular to the BEDT-TTF stacking direction. The spectroscopic data indicate that the CO phase transition is a consequence of both Coulomb interactions between charge carriers and donor-acceptor interactions.

Coupling of Vibrational Modes to the Electron-Hole Continuum in Doped Single-Wall Carbon Nanotubes

Electron-phonon coupling is at the heart of many physical phenomena like superconductivity, charge transport and the Raman effect (Ferrari2007). Previously, it has been shown that a coupling of vibrational modes to the low-energy electronic continuum absorption in graphene and carbon nanotubes results in transparency windows in the mid-infrared spectral region (Lapointe2012, Lapointe2017). The reduced absorption at the vibrational energies can be attributed to so-called Fano (anti-)resonances (Fano1961).Here, we present a systematic study of the infrared continuum absorption and such antiresonances in purely semiconducting, monochiral carbon nanotube (CNT) thin-films. We observe strong antiresonances for the Raman active D- and G-phonons, which acquire oscillator strength via coupling to the bright Drude-like intraband continuum in doped CNTs. Both the spectrum of the continuum absorption and the amplitude of the Fano resonances critically depend on the doping level. With increasing carrier concentration, we find a redshift of the Drude peak accompanied with a non-monotonic doping level dependence of the antiresonance intensity. Our observations show that carrier localization and delocalization as well as disorder induced by defects play a critical role for both charge transport and electron-phonon coupling as evidenced by the Fano resonances. We suggest that our findings also have implications for other low-dimensional systems with a low-energy electronic continuum absorption.C. Ferrari, Solid State Commun. 2007, 143, 47-57.Lapointe et al., Phys. Rev. Lett. 2012, 109, 097402.Lapointe et al., J. Phys. Chem. C 2017, 121, 9053-9062.Fano, Phys. Rev. 1961, 124, 1866-1878.

Role of low-energy electrons in the solubility switch of Zn-based oxocluster photoresist for extreme ultraviolet lithography.

The electron-induced chemistry of a resist material for extreme ultraviolet lithography (EUVL) consisting of Zn oxoclusters with methacrylate (MA) and trifluoroacetate (TFA) ligands (Zn(MA)(TFA)) has been studied. Electron energies of 80 eV and 20 eV mimic the effect of photoelectrons released by the absorption of EUV photons and low-energy secondary electrons (LESEs) produced by those photoelectrons. The chemical conversion of the resist is studied by mass spectrometry to monitor the volatile species that desorb during electron irradiation, combined with reflection absorption infrared spectra (RAIRS) measured before and after irradiation. The observed reactions are closely related to those initiated upon EUV absorption. Also, the conversion of the Zn(MA)(TFA) resist layer that is required in EUVL is achieved by a similar energy input upon electron irradiation. The dominant component of the desorbing gas is CO2, but CO detection also suggests Zn oxide formation during electron irradiation. In contrast, species deriving from the ligand side chains predominantly remain within the resist layer. RAIRS gives direct evidence that, during electron irradiation, C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds of the MA ligands are more rapidly consumed than the carboxylate groups. This supports that chain reactions occur and contribute to the solubility switch in the resist in EUVL. Remarkably, 20 eV electrons still evolve roughly 50% of the amount of the gas that is observed at 80 eV for the same electron dose. The present results thus provide complementary and new insight to the EUV-induced chemistry in the Zn(MA)(TFA) resist and point towards the important contribution of low-energy electrons therein.

Characterization of Conjugation Effects in the Series of Quinoxaline-2-ones by Means of Vibrational Raman Spectroscopy

A broad series of quinoxalinone-based π-conjugated donor-acceptor fluoro- and NLO-phores is characterized by means of Raman spectroscopy and single-crystal X-ray analysis supported by quantum chemical computations. Intense Raman spectroscopic markers that allow the differentiation of even closely related structures are identified. The intensities of these bands are shown to be related to the conjugation of the different molecular moieties, and they can provide an estimation of its extent. The intensity redistribution between these markers serves as a source of auxiliary structural information capable of pointing to a distortion of the conjugation or to the influence of aggregation effects in the condensed state. A simple relation between the intensity of the marker and the position and oscillator strength of the lowest-energy electronic absorption band of quinoxalinones allows a linking of the Raman effect with the optical properties of these compounds, which can be used for the rational design of novel species with improved optical characteristics.

Tailoring low energy electron absorption via surface nano-engineering of cesiated chromium films

In this letter, we demonstrate that improved low energy electron absorption is achieved by suppressing the crystallinity of chromium thin-films grown on W[110], which points to a promising route for achieving highly efficient thermionic energy converters. Using low energy electron microscopy (LEEM) and in situ film growth, we show that substrate temperature control permits well-controlled fabrication of either epitaxial Cr[110] films or nanocrystalline Cr layers. We show that the work function of cesium saturated nanocrystalline Cr thin-films is ∼0.20 eV lower than that of epitaxial Cr[110] films. Our LEEM measurements of absorbed and reflected currents as a function of electron energy demonstrate that nanocrystallinity of cesiated chromium films results in 96% electron absorption in the range up to 1 eV above the work function, compared to just 79% absorption in cesiated crystalline Cr[110] films. These results point to metal films with suppressed crystallinity as an economical and scalable means to synthesize nanoengineered surfaces with optimized properties for next generation anode materials in high performance thermionic energy converters.

Numerical study on ionization characteristics of CH4/Air mixture employing plate dielectric barrier discharge

To investigate the effect of the equivalence ratio on ionization characteristics, as well as the variations in electrons and discharge parameters, a quantitative study of the dielectric barrier discharge of CH4/air mixtures was carried out by employing two electrode plates with an ionization space distance of 10 mm under a 6000 V DC discharge voltage. The influence of the equivalence ratio on electron density and active particle concentration were analyzed, and the variation of active particles with time has been discussed. The result shows that, regardless of the active particle species with the increase in the equivalence ratio, there is a postponement in the time required to reach the maximum value of the molar fraction. H and CH3 appear to have the same change tendency, and the molar fraction can reach the maximum value and then maintain a rough balance. The NO molar fraction reaches the maximum value first, then it sharply decreases and finally has a slight increase. Because of the secondary electron emission from the cathode surface caused by the bombardment of heavy particles to the cathode, as well as the acceleration of free electrons, the number of electrons grows exponentially around the cathode. Since new electrons are generated, low-energy electron absorption and velocity reduction are caused by collision of electrons nearby the cathode, and the maximum value of the electron density appears to have different variation tendencies. Although the equivalence ratio changed, it has no obvious influence on the electron density, which has a very beneficial effect on the methane-air combustion reaction under conditions with a low equivalence ratio. Discharge greatly improved the molar concentrations of H, O3, CH3 and NO, which play major roles in the reaction for methane combustion.

Synthesis and properties of new mononuclear Ru(II)-based photocatalysts containing 4,4'-diphenyl-2,2'-bipyridyl ligands.

The synthesis and characterisation of a series of eleven new 4,4'-diphenyl-2,2'-bipyridyl derivatives (N^N) with varying 4- or 3,5-substituents on the phenyl rings are reported. The molecular structures of two of these compounds, 4,4'-bis(3,5-diheptyloxyphenyl)-2,2'-bipyridyl and 4,4'-bis(3,5-di-4,4,5,5,5-pentafluoropentyloxyphenyl)-2,2'-bipyridyl are confirmed by X-ray crystallography. Fourteen neutral complexes trans-Ru(II)Cl2(N^N)(CO)2 are prepared by reacting the new proligands and three known ones with the polymeric precursor [Ru(II)Cl2(CO)2]n, and their optical and electrochemical properties are studied. Density functional theory (DFT) and time-dependent DFT calculations have been carried out on selected complexes in order to rationalise their electronic structures and absorption properties. The low energy electronic absorption bands have metal-to-ligand charge-transfer and ligand-to-ligand charge-transfer (LLCT) character, but these dominate only in the 4-substituted species. In the 3,5-disubstituted ones, intraligand charge-transfer (ILCT) within the bpy ligands becomes more important. The complexes show only irreversible reductive electrochemistry and no signs of polymerisation. The photocatalytic oxidation behaviour of selected trans-Ru(II)Cl2(N^N)(CO)2 complexes and their mono-aquo derivatives [Ru(II)(H2O)(CO)2Cl(N^N)](+) with 4-methoxybenzyl alcohol is studied. Turnover numbers of up to ca. 130 are achieved when using [Ru(II)(bpy)3](2+) (bpy = 2,2'-bipyridyl) as photosensitizer and [Co(III)(NH3)5Cl](2+) as a sacrificial oxidant.

Measurements of low-energy electron reflection at a plasma boundary

It is demonstrated that low-energy (&amp;lt;3 eV) electron reflection from a solid surface in contact with a low-temperature plasma can have significant variation with time. An uncontaminated, i.e., “clean,” metallic surface (just after heating up to glow) in a plasma environment may have practically no reflection of low-energy incident electrons. However, a contaminated, i.e., “dirty,” surface (in some time after cleaning by heating) that has a few monolayers of absorbent can reflect low-energy incident electrons and therefore significantly affect the net electron current collected by the surface. This effect may significantly change plasma properties and should be taken into account in plasma experiments and models. A diagnostic method is demonstrated for measurements of low-energy electron absorption coefficient in plasmas with a mono-energetic electron group.

Theoretical modeling of low-energy electronic absorption bands in reduced cobaloximes.

The reduced Co(I) states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we analyze the low-energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.

Theoretical Modeling of Low-Energy Electronic Absorption Bands in Reduced Cobaloximes

Theoretical Modeling of Low-Energy Electronic Absorption Bands in Reduced Cobaloximes

Theoretical Modelling of Photoswitching of Hyperpolarisabilities in Ruthenium Complexes

Static excited-state polarisabilities and hyperpolarisabilities of three RuII ammine complexes are computed at the density functional theory (DFT) and several correlated ab initio levels. Most accurate modelling of the low energy electronic absorption spectrum is obtained with the hybrid functionals B3LYP, B3P86 or M06 for the complex [RuII(NH3)5(MeQ+)]3+ (MeQ+=N-methyl-4,4′-bipyridinium, 3) in acetonitrile. The match with experimental data is less good for [RuII(NH3)5L]3+ (L=N-methylpyrazinium, 2; N-methyl-4-{E,E-4-(4-pyridyl)buta-1,3-dienyl}pyridinium, 4). These calculations confirm that the first dipole- allowed excited state (FDAES) has metal-to-ligand charge-transfer (MLCT) character. Both the solution and gas-phase results obtained for 3 by using B3LYP, B3P86 or M06 are very similar to those from restricted active-space SCF second-order perturbation theory (RASPT2) with a very large basis set and large active space. However, the time-dependent DFT λmax predictions from the long-range corrected functionals CAM-B3LYP, LC-ωPBE and wB97XB and also the fully ab initio resolution of identity approximate coupled-cluster method (gas-phase only) are less accurate for all three complexes. The ground state (GS) two-state approximation first hyperpolarisability β2SA for 3 from RASPT2 is very close to that derived experimentally via hyper-Rayleigh scattering, whereas the corresponding DFT-based values are considerably larger. The β responses calculated by using B3LYP, B3P86 or M06 increase markedly as the π-conjugation extends on moving along the series 2→4, for both the GS and FDAES species. All three functionals predict substantial FDAES β enhancements for each complex, increasing with the π-conjugation, up to about sevenfold for 4. Also, the computed second hyperpolarisabilities γ generally increase in the FDAES, but the results vary between the different functionals.

Twisting-Based Spectroscopic Measure of Solvent Polarity: ThePTScale

The P(T) scale, which is correlated well with the E(T)(30), Y, and Z scales, is the first twisting-based spectroscopic measure of solvent polarity. It is based on two combined mechanisms: (1) the solvent-dependent intramolecular charge-transfer absorption that displays a regular hypsochromic band shift in polar solvents and (2) overcoming the intramolecular hydrogen-bond of o-1 by its differentiated solvation in polar solvents, causing a C-C bond to twist that leads to a regular hypsochromic shift of its lowest energy electronic absorption band.

Coordination of N,N-Chelated Re(CO)3Cl Units Across a Mo2 Quadruple Bond: Synthesis, Characterization, and Photophysical Properties of a Re–Mo2–Re Triad and Its Component Pieces

2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound fac-Re(PMT-H)(CO)3Cl, I, which is an analog of the well-known Re(bpy)(CO)3Cl molecule, where bpy is 2,2'-bipyridine. The acids PMT-H (2 equiv) and Re(PMT-H)(CO)3Cl (2 equiv) also react with Mo2(T(i)PB)4 (T(i)PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound trans-Mo2(T(i)PB)2(PMT)2, II, and the royal blue compound trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2, III, respectively. The X-ray and spectroscopic characterization of I confirms its close relationship with Re(bpy)(CO)3Cl, as does the spectroscopic characterization of compounds II and III as analogs of other compounds of the form trans-M2(TiPB)2L2, where L is a π-acceptor ligand. Electronic structure calculations on model compounds II' and III', where formate ligands substitute for T(i)PB, show that the highest occupied molecular orbital (HOMO) in II is Mo2δ. When the Re(CO)3Cl unit is attached to the PMT ligand to form III, this orbital is stabilized significantly and now becomes associated with a close in energy band of Re d(6), t2g type orbitals. Oxidation of III is shown to be Mo2-based, as evident by EPR spectroscopy, and the lowest-energy electronic absorption corresponds to a Mo2δ-to-PMT π* transition. The S1 states in both II and III are metal-to-ligand charge-transfer (MLCT), and the lowest-energy triplet sate, T1 is (3)MoMoδδ*, as evidenced by its steady state emission spectral features. The excited states of compounds I (T1) and III (S1 and T1) have been investigated by time-resolved infrared spectroscopy (TRIR). The spectral features of I parallel those for Re(bpy)(CO)3Cl, with the lowest-energy T1 state corresponding to Re dπ to PMT-H π* charge transfer, producing higher-energy CO stretching vibrations relative to the ground state. For III, the CO vibrations are shifted to lower energy, consistent with charge being located on the PMT ligand, which enhances Re-to-CO backbonding. In the MoMoδδ* T1 state, however, the backbonding is reduced to the PMT ligand, and the CO stretches are at slightly higher energy relative to the ground state.

Stabilization of oxidovanadium(iv) by organic radicals

o-Imino-p-R'-benzosemiquinone anion radical (L(R')(IS)(˙-)) complexes of oxidovanadium(IV) of type [(L(1)(R-))(VO(2+))(L(R')(IS)(˙-))] (R = H, R' = H, 1; R = H, R' = -CMe(3), 2; R = -CMe(3), R' = H, 3 and R = -CMe(3), R' = -CMe(3), 4) incorporating the redox-innocent tridentate NNO-donor L(1)(R-) ligands (L(1)(R)H = 2,4-di-R-6-{(2-(pyridin-2-yl)hydrazono)methyl}phenol) were isolated and substantiated by elemental analyses, IR, mass, NMR and UV-vis spectra including the single crystal X-ray structure determinations. The V-O(phenolato) (cis to the V=O) lengths spanning 1.905(3)-1.9355(15) Å in 1-4 are consistent with the coordination to the [VO](2+) state. The V-O(IS) (trans to the V=O) lengths, 2.1505(17)-2.1869(15) Å, in 1-4 are longer due to the trans influence of the V=O bond. The V-N(IS) lengths, 1.906(3)-1.924(2) Å, in 1-4 are comparatively shorter due to the higher affinity of the paramagnetic [VO](2+) ion towards the L(R')(IS)(˙-) anion radicals. Density functional theory (DFT) calculations using B3LYP, B3PW91 and PBE1PBE functionals on 1 and 2 authenticated that the closed shell singlet (CSS) solutions (dianionic o-amido-p-R'-phenolates (L(R')(AP)(2-)) coordinated to VO(3+), Type I) of 1-4 are unstable with respect to the open shell singlet (OSS) perturbations. Broken symmetry, BS (1,1) M(s) = 0 (L(R')(IS)(˙-) coordinated to the VO(2+) ion, Type III) solutions of 1-4 are stable and reproduce the experimental bond parameters. Frozen glasses EPR spectra of [1-4](+) ions (e.g. g(||) = 1.948, g(⊥) = 1.978, A(||) = 184 (22 G), A(⊥) = 62(15 G) for [2](+)) and unrestricted DFT calculations on [1](+), [2](+), [1](-) and [2](-) ions using doublet spin state elucidated that the reversible anodic waves at [0.15-0.31] V of 1-4 complexes are due to the oxidation of L(R')(IS)(˙-) generating [(L(1)(R-))(VO(2+))(L(R')(IQ))]+ complexes (L(R')(IQ) = o-imino-p-R'-benzoquinone) coordinated to the [VO](2+) ion (Type V) while the irreversible cathodic waves at -[1.08-1.49] V are due to the formation of unstable [(L(1)(R-))(VO(2+))(L(R')(AP)(2-))](-) complexes (Type II). The second anodic waves at [0.76-0.89] V are assigned to a [VO](3+)-[VO](2+) couple affording diamagnetic [(L(1)(R-))(VO(3+))(L(R')(IQ))](2+), [1-4](2+) complexes (Type VI) which are identified by UV-vis spectra, DFT and time dependent (TD) DFT calculations. Spectro-electrochemical measurements and TD DFT calculations on 1 and 2 disclosed that lower energy electronic absorption bands of 1-4 are due to the LMCT and CSS-OSS perturbation which disappear in [1-4](+) ions. [1-4](+) absorb at 600-650 nm due to d-d transitions and MLCT which are absent in VO(3+) complexes, [1-4](2+).

Charge Delocalization in a Heterobimetallic Ferrocene−(Vinyl)Ru(CO)Cl(PiPr3)2 System†Dedicated to Prof. Dr. Helmut Werner on the occasion of his 75th birthday

Ru(CH═CHFc)Cl(CO)(PiPr3)2 (Fc = ferrocenyl, (η5-C5H4)Fe(η5-C5H5)), 1, has been prepared by hydroruthenation of ethynylferrocene and characterized by NMR, IR, ESI-MS, and Moessbauer spectroscopy and by X-ray crystallography. Complex 1 features conjoined ferrocene and (vinyl)ruthenium redox sites and undergoes two consecutive reversible oxidations. Pure samples of crystalline, monooxidized 1•+ have been prepared by chemical oxidation of 1 with the ferrocenium ion. Structural comparison with 1 reveals an increase of Fe−C and Fe−Cpcentr. bond lengths and ring tilting of the Cp decks, as is typical of ferrocenium ions, but also a discernible lengthening of the Ru−C(CO) and Ru−P bonds and a shortening of the Ru−C(vinyl) bond upon oxidation. This supports the general idea of charge delocalization over both redox sites in 1•+. Band shifts of the charge-sensitive IR labels (ν(CO) for Ru, ν(C−H, Cp) for Fc), the rather small g-anisotropy in the ESR spectrum of 1•+, and the results of quantum chemical calculations indicate that in solution the positive charge partly resides on the vinyl ruthenium moiety. Comparison of IR shifts in the solid state and in solution and the quadrupole splitting in the Moessbauer spectrum of powdered 1•+ point to a larger extent of charge localization on the ferrocenyl site in solid samples. This is probably due to CH···F hydrogen bonding interactions between the cyclopentadienyl hydrogen atoms of the radical cations and the PF6− counterions. Monooxidized 1•+ displays low-energy electronic absorption bands at 1370 and 2150 nm. According to quantum chemical calculations, the underlying transitions are largely localized on the ferrocene part of the molecule with only little charge transfer into the vinyl ruthenium subunit. The second oxidation is more biased toward the (vinyl)ruthenium site.

Platinum Dithiolene Complexes of π-Coordinating and π-Interacting η4-Cyclobutadiene Ligands

Five new organometallic platinum dithiolene complexes of the η4-cyclobutadiene ligand [(η4-C4Me4)Pt(dithiolene)] (dithiolene = mnt (2), dcmedt (3), tdt (4), dddt (5), dmit (6)) and one platinum diselenolene complex, [(η4-C4Me4)Pt(dsit)] (7), were prepared from [(η4-C4Me4)Pt(Cl)(μ-Cl)]2 (1) and Na2(mnt), O═C(dcmedt), H2tdt, O═C(dddt), (NBu4)2[Zn(dmit)2], or (NEt4)2[Zn(dsit)2], respectively. The (η4-C4Me4)Pt complexes 2−7 were characterized by NMR, UV−vis spectra, and CV. Those 1H and 13C NMR spectra showed 195Pt satellite coupling at the C4Me4 (JPt−H = 13−15 Hz), at the C4Me4 (JPt−C = ca. 100 Hz), and at the dithiolene carbons. The complexes having an electron-rich dichalcogenolene ligand (5−7) resulted in lower energy electronic absorption compared with the electron-poor series 2−4. The η4-C4Me4 ligand was replaced by the nucleophilic substitution of bis(diphenylphosphino)ethane (DPPE) to form the square-planar [Pt(dithiolene)(dppe)] complex. 2, 3, and 5−7 were structurally determined by X-ray diffraction...