Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation.

6,13-Bis(tri(isopropyl)silylethynyl)pentacene, a particularly stable acene derivative important for (opto)electronic materials, turns reactive upon electrochemical one-electron oxidation. One of the typically stabilizing tri(isopropyl)silylethynyl substituents becomes involved in a (4 + 2) cycloaddition after redox umpolung. The electrosynthetic dimerization of the title compound provides easy access under mild conditions to a complex scaffold, which includes an intact pentacene, an anthracene, and a phenylene unit, all electronically separated. The product's electrochemical redox properties are explained by superimposed cyclic voltammetric features of the pentacene and the anthracene moieties. The reaction path is analyzed on the basis of electroanalytical and ESR data, and an oxidation-cycloaddition-reduction sequence is elaborated. The contribution of homogeneous electron transfers (electron transfer chain reaction) is negligible, in accordance with the relative formal redox potentials of the starting compound and the product. Quantum chemical calculations indicate that the central cycloaddition should be described as a two-step process with a distonic radical cation intermediate. We suggest an extended notation to define the contribution of the components with respect to electron count in the two-step cycloaddition, [3 + 1, 1 + 1].

Bright Luminescence by Combining Chiral [2.2]Paracyclophane with a Boron-Nitrogen-Doped Polyaromatic Hydrocarbon Building Block.

Novel BN-doped compounds based on chiral, tetrasubstituted [2.2]paracyclophane and NBN-benzo[f,g]tetracene were synthesized by Sonogashira-Hagihara coupling. Conjugated ethynyl linkers allow electronic communication between the π-electron systems through-bond, whereas through-space interactions are provided by strong π-π overlap between the pairs of NBN-building blocks. Excellent optical and chiroptical properties in racemic and enantiopure conditions were measured, with molar absorption coefficients up to ϵ=2.04×105  M-1 cm-1 , fluorescence quantum yields up to ΦPL =0.70, and intense, mirror-image electronic circular dichroism and circularly polarized luminescence signals of the magnitude of 10-3 for the absorption and luminescence dissymmetry factors. Computed glum,calcd. values match the experimental ones. Electroanalytical data show both oxidation and reduction of the ethynyl-linked tetra-NBN-substituted paracyclophane, with an overlap of two redox processes for oxidation leading to a diradical dication.

Electron-Deficient Imidazolium Substituted Cp Ligands and their Ru Complexes.

The synthesis of electron-poor mono-, di- and tri(imidazolium)-substituted Cp-ylides is presented and their electronic properties are discussed based on NMR spectroscopy, X-ray structure analyses, electrochemical investigations and DFT calculations as well as by their reactivity toward [Ru(CH3 CN)3 Cp*](PF6 ). With mono- and di(imidazolium)-substituted cyclopentadienides the respective monocationic and dicationic ruthenocences are formed (X-ray), whereas tri(imidazolium) cyclopentadienides are too electron-poor to form the ruthenocenes. Cyclic voltammetric analysis of the ruthenocenes shows reversible oxidation at a potential that increases with every additional electron-withdrawing imidazolium substituent at the Cp ligand by 0.53-0.55 V in an electrolyte based on a weakly coordinating anion. A reversible oxidation can be observed for the free 1,3-disubstituted ligand as well.

Modulating the Electronic and Solid-State Structure of Organic Semiconductors by Site-Specific Substitution: The Case of Tetrafluoropentacenes.

The properties as well as solid-state structures, singlet fission, and organic field-effect transistor (OFET) performance of three tetrafluoropentacenes (1,4,8,11: 10, 1,4,9,10: 11, 2,3,9,10: 12) are compared herein. The novel compounds 10 and 11 were synthesized in high purity from the corresponding 6,13-etheno-bridged precursors by reaction with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate at elevated temperatures. Although most of the molecular properties of the compounds are similar, their chemical reactivity and crystal structures differ considerably. Isomer 10 undergoes the orbital symmetry forbidden thermal [4+4] dimerization, whereas 11 and 12 are much less reactive. The isomers 11 and 12 crystallize in a herringbone motif, but 10 prefers π-π stacking. Although the energy of the first electric dipole-allowed optical transition varies only within 370 cm-1 (0.05 eV) for the neutral compounds, this amounts to roughly 1600 cm-1 (0.20 eV) for radical cations and 1300 cm-1 (0.16 eV) for dications. Transient spectroscopy of films of 11 and 12 reveals singlet-fission time constants (91±11, 73±3 fs, respectively) that are shorter than for pentacene (112±9 fs). OFET devices constructed from 11 and 12 show close to ideal thin-film transistor (TFT) characteristics with electron mobilities of 2×10-3 and 6×10-2  cm2 V-1 s-1 , respectively.

Bridging the Gap between Pentacene and Perfluoropentacene: Synthesis and Characterization of 2,3,9,10-Tetrafluoropentacene in the Neutral, Cationic, and Dicationic States.

The thermal and photochemical syntheses of 2,3,9,10-tetrafluoropentacene (F4PEN) from 6,13-etheno bridged precursors were investigated computationally and experimentally. A computational study of the retro-Diels-Alder reaction to give 2,3,9,10-tetrasubstituted pentacenes and pyridazine revealed a linear correlation between barrier height and substituent constant (σp) indicative of an electronic effect that could diminish the yield of electron-poor 2,3,9,10-tetrasubstituted pentacenes in this reaction. The photochemical route from the corresponding bridged α-diketone yields F4PEN, which was characterized photophysically, electrochemically, and structurally. The compound crystallizes in a herringbone motif with quite short intermolecular F-F contacts that are, however, only very weakly bonding according to computations. The electrochemical and photophysical data show that the HOMO-LUMO gap of F4PEN is increased compared to that of PEN. This is due to an increase of the oxidation potential of F4PEN by 0.18 V in combination with an essentially unchanged reduction potential. The radical cation and dication of F4PEN could be generated in oxidizing solvents and characterized by optical spectroscopy and ESR or NMR, respectively. Both charged F4PEN species persist for days in solution.

Mechanistic Aspects of Redox-Induced Assembly and Disassembly of S-Bridged [2M-2S] Structures.

Sulfur-bridged binuclear structures [2M-2S] play a pivotal role in a variety of chemical processes such as bond breaking and formation and electron transfer. In general, structural persistence is deemed essential to the respective function but owing to the lack of a suitable molecular model system, the current understanding of the factors that control the thermodynamic and kinetic stability of [2M-2S] cores clearly is limited. This work reports a series of binuclear complexes of nickel derived from a 1,4-terphenyldithiophenol ligand platform that is ideally suited for mechanistic work to overcome this limitation. Redox-induced assembly and disassembly of S-bridged [2M-2S] fragments have been investigated at the molecular level. As part of an extended square scheme, metastable binuclear structures that are significant mechanistically have been identified, characterized, and their reactivity studied quantitatively. Electronic properties that are inherent to [2M-2S] structures and determine thermodynamic and kinetic stability are differentiated from steric effects imposed by co-ligands.

(Electro)chemical Oxidation of 6,13-Bis[tri(isopropyl)silylethynyl]pentacene to its Radical Cation and Dication.

6,13-Bis[tri(isopropyl)silylethynyl]pentacene is a prototypical molecule for organic semiconductor and photovoltaic materials, which makes its electrochemical (redox) properties highly interesting. However, previous cyclic voltammetric studies have provided only limited information. Kinetic and persistence information and identification of the oxidation product(s) and their further reaction or oxidation have not been reported. Thus, an extended electrochemical and spectroscopic investigation of this compound was conducted in CH2 Cl2 and THF electrolytes at Pt electrodes. The electrochemically and chemically generated radical cation of the title compound was characterized by using ESR and UV/Vis/NIR spectroscopy and quantum-chemical modeling. In CH2 Cl2 , further oxidation to a dication with chemical reversibility at fast timescales but follow-up reactivity at slow timescales was observed. Pertinent parameters of the electron transfers (formal potentials E0 , electron transfer rate constants ks , electron stoichiometry n) were determined. The diffusion coefficients, D, in the two electrolytes were estimated from electrochemical and pulse gradient spin echo (PGSE) NMR spectroscopy data. Simulations of cyclic voltammograms supported the proposed oxidation mechanism and allowed the estimation of further reaction parameters.

[6,6]-Open and [6,6]-closed isomers of C70(CF2): synthesis, electrochemical and quantum chemical investigation.

Novel difluoromethylenated [70]fullerene derivatives, C70(CF2 )n (n=1-3), were obtained by the reaction of C70 with sodium difluorochloroacetate. Two major products, isomeric C70(CF2 ) mono-adducts with [6,6]-open and [6,6]-closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X-ray analysis and high-level spectroscopic techniques. The [6,6]-open isomer of C70(CF2 ) constitutes the first homofullerene example of a non-hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C70(CF2 ) isomers showed that it is substantially higher for the [6,6]-open isomer (the 70-electron π-conjugated system is retained) than the [6,6]-closed form, the latter being similar to the electron affinity of pristine C70. In situ ESR spectroelectrochemical investigation of the C70(CF2 ) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter-conversion between the [6,6]-closed and [6,6]-open forms of a cage-modified fullerene driven by an electrochemical one-electron transfer. Thus, [6,6]-closed C70(CF2 ) constitutes an interesting example of a redox-switchable fullerene derivative.

Anti-[2.2](1,4)pentacenophane: a covalently coupled pentacene dimer.

Two in a row: A pentacene dimer in which both units are covalently linked through a [2.2]paracyclophane bridge, has been synthesized. The electronic properties of the molecule were elucidated by a combination of experimental and computational methods. Such molecules could lead to materials with improved charge-transport properties.

High-temperature chlorination-reduction sequence for the preparation of silicon hydride modified silica surfaces.

A general method for the functionalization of silica surfaces with silicon hydride (Si-H) groups is described for four different preparations of silica. The silica surface is reduced in a two-step chlorination-reduction procedure within a simple gas-flow system at high temperatures. After initial dehydroxylation of the silica surface, silicon chloride groups are formed by the reaction with thionyl chloride. The chlorination activates otherwise inaccessible surface siloxane moieties. A high silicon-hydride surface concentration results from the subsequent reduction of the chlorinated surface with hydrogen. The physical properties of the resulting silica are analyzed using scanning electron microscopy, as well as dynamic light scattering and Brunauer-Emmet-Teller measurements. The chlorination-reduction sequence has no significant impact on the structure, surface area and mesopore size of the silica materials used. The surface of the materials is characterized by diffuse reflectance infrared Fourier transform (DRIFT) and (29)Si CP/MAS NMR spectroscopy. The silicon-hydride groups are mostly of the ${{\rm T}{{3\hfill \atop {\rm H}\hfill}}}$-type. The use of high temperatures (>800 degrees C) results in the condensation of internal and surface silanol groups. Therefore, materials with both a fully condensed silica matrix as well as a surface free of silanol groups are obtained. The materials are ideal precursors for further molecular silica surface modification, as demonstrated with a ferrocene derivative.

Thermally induced radical hydrosilylation for synthesis of C18 HPLC phases from highly condensed SiH terminated silica surfaces.

Silicon hydride terminated silica surfaces were prepared at high temperatures by a chlorination-reduction sequence. SiH groups are desired for further surface modification as an alternative to the native silanol groups which are unfavorable for RPLC applications. Only few silanol groups remain in these materials and mostly SiH moieties with the highest degree of cross-linking are obtained. The retention properties of basic analytes on the SiH terminated material confirm that the surfaces is mostly free of silanols and that therefore the remaining SiOH groups are bulk species. A reagentless, radical initiated hydrosilylation reaction is introduced for the functionalization of the hydride terminated surface with 1-octadecene. (13)C CP/MAS NMR and DRIFT spectroscopy demonstrate the reaction of the carbon-carbon double bond and the SiH group as well as the linkage of C18 groups to the silica surface. These novel C18 materials show promising performance in RPLC separation, especially for the separation of organic bases.

Synthesis and Characterization of a Boron-Nitrogen-Boron Zigzag-Edged Benzo[fg]tetracene Motif.

The boron-nitrogen-boron (BNB) zigzag edged benzo[fg]tetracene is accessible from 4-butyl-2,6-diphenylaniline in four steps in good yields. The two mesityl groups stabilize the boron centers toward nucleophilic attack and result in two enantiomeric forms in the solid state. The title compound has a large optical gap, shows blue fluorescence, and is quite resistant toward oxidation and reduction.

Ligand Structural Effects on the Electrochemistry of Chromium(III) Amino Carboxylate Complexes.

The aqueous electrochemical behavior of 10 Cr(III) complexes with potentially tri- and hexadentate amino carboxylate ligands is reported and is shown to depend on the composition and spatial arrangement of the donor atom set. Complexes with two amine and four carboxylate donors (N(2)O(4)) and two amine, one aquo, and three carboxylate donors (N(2)O(3)O') in which the N atoms are coordinated cis to one another undergo chemically and electrochemically reversible reduction at ca. -1.4 and ca. -1.2 V vs SCE, respectively. However, complexes with a trans-N(2)O(4) donor atom set, as exemplified by Cr(MIDA)(2)(-) (MIDA(2)(-) = N-methyliminodiacetate), undergo quasi-reversible Cr(III/II) reduction at ca. -1.4 V that is followed by a sequence of reactions which establishes an electrochemical square scheme. The chemical reactions in the scheme involve displacement of a bound carboxylate group following reduction to Cr(II) and its reattachment after reoxidation to Cr(III). This mechanistic sequence is analyzed by digital simulation, and values of formal potentials, transfer coefficients, and chemical and electrochemical rate constants are reported for Cr(MIDA)(2)(-) and its N-ethyl homolog. The difference in electrochemical behavior between cis- and trans-N(2)O(4) complexes is attributed to differences in the Jahn-Teller distortions experienced by these structures upon reduction to Cr(II). It is proposed that simultaneous N-Cr-N bond elongation, which is possible only for trans species, leads to greater strain in the facially coordinated N-alkyliminodiacetate ligand and thus increases the barrier to electron transfer and facilitates Cr-carboxylate bond cleavage after reduction.

Electrochemistry of Oxygenation Catalysts. 3.(1) Thermodynamic Characterization of Electron Transfer and Solvent Exchange Reactions of Co(salen)/[Co(salen)](+) in DMF, Pyridine, and Their Mixtures.

Redox and ligand exchange reactions of the oxygenation catalyst (N,N '-bis(salicylidene)ethylenediaminato)cobalt(II), Co(salen), and its one-electron oxidation product, Co(salen)(+), are investigated in DMF, pyridine, and mixtures of these solvents. Electron transfers and solvent exchange reactions involving three neutral Co(II) and three cationic Co(III) complexes with different axially bound solvent molecules (two DMF, one DMF and one pyridine, or two pyridine molecules) form a three-rung ladder scheme. All formal potentials E(0) and equilibrium constants K in this scheme are determined from electrochemical or spectrophotometric experiments or the construction of thermodynamic cycles. The latter are also used to prove consistency of the results. Values for the E(0) and K are discussed in terms of the Co coordination geometry, solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition. Some peculiarities found in the oxygenation of flavonols and indoles are explained.

Estimation of diffusion coefficients from voltammetric signals by support vector and gaussian process regression.

BACKGROUND: Support vector regression (SVR) and Gaussian process regression (GPR) were used for the analysis of electroanalytical experimental data to estimate diffusion coefficients. RESULTS: For simulated cyclic voltammograms based on the EC, Eqr, and EqrC mechanisms these regression algorithms in combination with nonlinear kernel/covariance functions yielded diffusion coefficients with higher accuracy as compared to the standard approach of calculating diffusion coefficients relying on the Nicholson-Shain equation. The level of accuracy achieved by SVR and GPR is virtually independent of the rate constants governing the respective reaction steps. Further, the reduction of high-dimensional voltammetric signals by manual selection of typical voltammetric peak features decreased the performance of both regression algorithms compared to a reduction by downsampling or principal component analysis. After training on simulated data sets, diffusion coefficients were estimated by the regression algorithms for experimental data comprising voltammetric signals for three organometallic complexes. CONCLUSIONS: Estimated diffusion coefficients closely matched the values determined by the parameter fitting method, but reduced the required computational time considerably for one of the reaction mechanisms. The automated processing of voltammograms according to the regression algorithms yields better results than the conventional analysis of peak-related data.

Stöber silica particles as basis for redox modifications: particle shape, size, polydispersity, and porosity.

The synthesis of Stöber silica particles as basis for redox modifications is optimized for desired properties, in particular diameter in a wide sub-micrometer range, spherical shape, monodispersity, the absence of porosity, and aggregation free isolability for characterization and later covalent modification. The materials are characterized by SEM, DLS, nitrogen sorption isotherms, helium as well as Gay-Lussac (water) pycnometry, and DRIFT spectroscopy. Particles with diameters between approximately 50 and 800 nm are obtained by varying the concentrations of the reagents and reactants, the type of solvent as well as the temperature. The use of high water concentrations and post-synthetic calcination at 600 °C results in silica particles that can be considered as nonporous with respect to the size of the active molecules to be immobilized. The effect of reaction temperature on size distribution is identified. Low polydispersity is achieved by performing the reaction in a temperature range in which a change in temperature has only a weak or no effect on the final particle diameter. Upon optimization of the sol-gel process, the shape of the particles is still spherical. The agreement between experimental and geometric data is within the expected precision of the characterization techniques.

Electrochemistry and DFT simulation of optical spectra of N-ferrocenoyl-N'-ω-decenoyl-ethylenediamine.

A new hybrid organic-inorganic chromophore based on ferrocene was synthesized. In addition to a detailed synthesis description the article describes complex quantum chemical and experimental (UV-VIS and electrochemical) studies of the redox and chromophore characteristics. The properties studied include the diffusion coefficient as well as solvatochromic effects in solvents differing in polarity, namely dichloromethane, propylene carbonate and acetonitrile. The experimental results are discussed within a framework of theoretical description at the DFT and TDDFT (6-31G(df,p)) levels of theory, with the PCM solvation model. Accuracy of B3LYP and BLYP functionals in reproducing orbital energy levels and optical transitions of the title molecule is briefly discussed.

Redox-active silica nanoparticles. Part 1. Electrochemistry and catalytic activity of spherical, nonporous silica particles with nanometric diameters and covalently bound redox-active modifications.

Nonporous spherical silica nanoparticles resulting from a controlled Stöber process are covalently surface modified with redox-active molecules. Ferrocene, a ruthenium(II) complex with an N2P2Cl2 ligand set, and a sterically hindered biphenylamine are used as modifiers. The resulting materials are characterized by physical, spectroscopic, electrochemical, and chemical methods. The cyclic voltammetric behavior is studied in detail and reveals effects of charge transport by electron hopping along the surface of particles adsorbed on a Pt electrode. The ruthenium(II) complex remains catalytically active with respect to hydrogenation upon immobilization on the particles. Thus, the respective material provides a heterogenized homogeneous hydrogenation catalyst on a solid support.