Lewis-Pairing-Induced Electrochemiluminescence Enhancement from Electron Donor-Acceptor Diads Decorated with Tris(pentafluorophenyl)borane as an Electrochemical Protector.

This study reports an effective peripheral decoration of organic donor-acceptor diads with B(C6 F5 )3 for stabilizing electrogenerated radical ions. By employing a common p-type organic semiconductor benzothienobenzothiophene (BTBT) as the donor, tetracoordinate boron complexes showed improved solution electrochemiluminescence (ECL) intensity, reaching a 156-fold increase compared to that of the parent diad. The unprecedented Lewis-pairing-induced ECL enhancement is attributed to the multiple roles of B(C6 F5 )3 : 1) redistributing frontier orbitals, 2) facilitating electrochemical excitation, and 3) restricting molecular motions. Furthermore, B(C6 F5 )3 converted the molecular arrangement of BTBT from conventional 2D herringbones into 1D π-stacks. This robust, highly ordered columnar nanostructure allowed red-shifting of the crystalline film ECL with electrochemical doping through the electronic coupling pathways of BTBT. Our approach will facilitate the development of elaborate metal-free ECL systems.

Immobilizing a π-Conjugated Catecholato Framework on Surfaces of SiO2 Insulator Films via a One-Atom Anchor of a Platinum Metal Center to Modulate Organic Transistor Performance.

Chemical modification of insulating material surfaces is an important methodology to improve the performance of organic field-effect transistors (OFETs). However, few redox-active self-assembled monolayers (SAMs) have been constructed on gate insulator film surfaces, in contrast to the numerous SAMs formed on many types of conducting electrodes. In this study, we report a new approach to introduce a π-conjugated organic fragment in close proximity to an insulating material surface via a transition metal center acting as a one-atom anchor. On the basis of the reported coordination chemistry of a catecholato complex of Pt(II) in solution, we demonstrate that ligand exchange can occur on an insulating material surface, affording SAMs on the SiO2 surface derived from a newly synthesized Pt(II) complex containing a benzothienobenzothiophene (BTBT) framework in the catecholato ligand. The resultant SAMs were characterized in detail by water contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and cyclic voltammetry. The SAMs served as good scaffolds of π-conjugated pillars for forming thin films of a well-known organic semiconductor C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), accompanied by the engagements of the C8-BTBT molecules with the SAMs containing the common BTBT framework at the first layer on SiO2. OFETs containing the SAMs displayed improved performance in terms of hole mobility and onset voltage, presumably because of the unique interfacial structure between the organic semiconducting and inorganic insulating layers. These findings provide important insight into creating new elaborate interfaces through installing coordination chemistry in solution to solid surfaces, as well as OFET design by considering the compatibility between SAMs and organic semiconductors.

Ferrocene on Insulator: Silane Coupling to a SiO2 Surface and Influence on Electrical Transport at a Buried Interface with an Organic Semiconductor Layer.

A silane coupling-based procedure for decoration of an insulator surface containing abundant hydroxy groups by constructing redox-active self-assembled monolayers (SAMs) is described. A newly synthesized ferrocene (Fc) derivative containing a triethoxysilyl group designated FcSi was immobilized on SiO2/Si by a simple operation that involved immersing the substrate in a toluene solution of the Fc silane coupling reagent and then rinsing the resulting substrate. X-ray photoelectron spectroscopy (XPS) measurements confirmed that the Fc group was immobilized on SiO2/Si in the Fe(II) state. Cyclic voltammetry measurements showed that the Fc groups were electrically insulated from the Si electrode by the SiO2 layer. The FcSi on SiO2/Si structures were found to serve as a good scaffold for formation of organic semiconductor thin films by vacuum thermal evaporation of C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), which is well-known as an organic field-effect transistor (OFET) material. The X-ray diffraction profile indicated that the conventional standing-up conformation of the C8-BTBT molecules perpendicular to the substrates was maintained in the thin films formed on FcSi@SiO2/Si. Further vacuum thermal evaporation of Au provided an FcSi-based OFET structure with good transfer characteristics. The FcSi-based OFET showed pronounced source-drain current hysteresis between the forward and backward scans. The degree of this hysteresis was varied reversibly via gate bias manipulation, which was presumably accompanied by trapping and detrapping of hole carriers at the Fc-decorated SiO2 surface. These findings provide new insights into application of redox-active SAMs to nonvolatile OFET memories while also creating new interfaces through junctions with functional thin films, in which the underlying redox-active SAMs play supporting roles.

Small anion-assisted electrochemical potential splitting in a new series of bistriarylamine derivatives: organic mixed valency across a urea bridge and zwitterionization.

We report the synthesis of a new bistriarylamine series having a urea bridge and investigate its mixed-valence (MV) states by electrochemical and spectroelectrochemical methods. We found that the supporting electrolytes had unusual effects on potential splitting during electrochemical behavior, in which a smaller counteranion thermodynamically stabilized a MV cation more substantially than did a bulky one. The effects contrary to those reported in conventional MV systems were explained by zwitterionization through hydrogen bonding between the urea bridge and the counteranions, increasing the electronic interactions between two triarylamino units. Furthermore, we clarified the intervalence charge transfer characteristics of the zwitterionic MV state.

Zwitterionic Mixed Valence: Internalizing Counteranions into a Biferrocenium Framework toward Molecular Expression of Half-Cells in Quantum Cellular Automata.

Realization of molecular quantum cellular automata (QCA), a promising architecture for molecular computing through current-free processes, requires improved understanding and application of mixed-valence (MV) molecules. In this report, we present an electrostatic approach to creating MV subspecies through internalizing opposite charges in close proximity to MV ionic moieties. This approach is demonstrated by unsymmetrically attaching a charge-responsive boron substituent to a well-known organometallic MV complex, biferrocenium. Guest anions (CN- and F- ) bind to the Lewis acidic boron center, leading to unusual blue-shifts of the intervalence charge-transfer (IVCT) bands. To the best of our knowledge, this is the first reported example of a zwitterionic MV series in which the degree of positive charge delocalization can be varied by changing the bound anions, and serves to clarify the interplay between IVCT parameters. The key underlying factor is the variable zero-level energy difference in the MV states. This work provides new insight into imbuing MV molecules with external charge-responsiveness, a prerequisite of molecular QCA techniques.

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units.

In this study, we report the generation of new mixed-valence (MV) subspecies with charge-separated (CS) characters from an unsymmetrical acceptor-donor-donor (A-D-D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D-D conjugate that consisted of triarylamine (NAr3) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IVCT) absorption derived from the bis(NAr3)•+ moiety in the near-IR region. The charge-separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr3 to NAr2 sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into counterion-free MV systems and their applications in molecular devices.

New π-extended catecholato complexes of Pt(ii) and Pd(ii) containing a benzothienobenzothiophene (BTBT) moiety: synthesis, electrochemical behavior and charge transfer properties.

Benzothienobenzothiophene (BTBT) and derivatives have received increasing attention as organic field-effect transistor materials and molecular conductors. This report presents the first synthesis of metal complexes involving a BTBT moiety, which was achieved by complexation of 2,2'-bipyridyl complexes of Pt(ii) and Pd(ii) with dihydroxy-substituted BTBT (1) as a new π-extended catecholato ligand (tBu2Bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). The resulting complexes M(tBu2Bpy)(O2BTBT) (M = Pt (3Pt) and Pd (3Pd)) were characterized by UV-vis spectroscopy, density functional theory (DFT) calculations, and cyclic voltammetry. The electron donating ability of BTBT was substantially enhanced upon including two oxygen substituents followed by metal coordination. This enabled chemical oxidation of 3Pt and 3Pd with a mild chemical oxidant (ferrocenium hexafluorophosphate) and formation of the one-electron-oxidized state. While 3Pt and 3Pd exhibited an absorption band originating from a catecholate → Bpy ligand-to-ligand charge transfer transition typical of this class of catecholato complexes, the radical cations exhibited a unique π-π* intramolecular charge transfer (ICT) transition absorption in which the π and π* orbitals were the newly incorporated benzothienothiophene-based donor and semiquinonato-based acceptor, respectively. The BTBT+ skeleton was electronically divided into two sites by the present chemical modification. The ICT properties of the complexes were found to be modulated by varying the metal ions. These findings offer a new approach to molecular design for (semi)conducting materials using optical properties.

Learning from B12 enzymes: biomimetic and bioinspired catalysts for eco-friendly organic synthesis.

Cobalamins (B12) play various important roles in vivo. Most B12-dependent enzymes are divided into three main subfamilies: adenosylcobalamin-dependent isomerases, methylcobalamin-dependent methyltransferases, and dehalogenases. Mimicking these B12 enzyme functions under non-enzymatic conditions offers good understanding of their elaborate reaction mechanisms. Furthermore, bio-inspiration offers a new approach to catalytic design for green and eco-friendly molecular transformations. As part of a study based on vitamin B12 derivatives including heptamethyl cobyrinate perchlorate, we describe biomimetic and bioinspired catalytic reactions with B12 enzyme functions. The reactions are classified according to the corresponding three B12 enzyme subfamilies, with a focus on our recent development on electrochemical and photochemical catalytic systems. Other important reactions are also described, with a focus on radical-involved reactions in terms of organic synthesis.

Reprint of: Impact of the corrin framework of vitamin B12 on the electrochemical carbon-skeleton rearrangement in comparison to an imine/oxime planar ligand; tuning selectivity in 1,2-migration of a functional group by controlling electrolysis potential.

Among the coenzyme B12-dependent enzymes, methylmalonyl-CoA mutase (MMCM) catalyzes the carbon-skeleton rearrangement reaction between R-methylmalonyl-CoA and succinyl-CoA. Diethyl 2-bromomethyl-2-phenylmalonate, an alkyl bromide substrate having two different migrating groups (phenyl and carboxylic ester groups) on the ß-carbon, was applied to the electrolysis mediated by a hydrophobic vitamin B12 model complex, heptamethyl cobyrinate perchlorate in this study. The electrolysis of the substrate at -1.0V vs. Ag-AgCl by light irradiation afforded the simple reduced product (diethyl 2-methyl-2-phenylmalonate) and the phenyl migrated product (diethyl 2-benzyl-2-phenylmalonate), as well as the electrolysis of the substrate at -1.5V vs. Ag-AgCl in the dark. The electrolysis of the substrate at -2.0V vs. Ag-AgCl afforded the carboxylic ester migrated product (diethyl phenylsuccinate) as the major product. The selectivity for the migrating group was successfully tuned by controlling the electrolysis potential. We clarified that the cathodic chemistry of the Co(III) alkylated heptamethyl cobyrinate is critical for the selectivity of the migrating group through mechanistic investigations and comparisons to the simple vitamin B12 model complex, an imine/oxime-type cobalt complex.

Impact of the corrin framework of vitamin B12 on the electrochemical carbon-skeleton rearrangement in comparison to an imine/oxime planar ligand; tuning selectivity in 1,2-migration of a functional group by controlling electrolysis potential.

Among the coenzyme B12-dependent enzymes, methylmalonyl-CoA mutase (MMCM) catalyzes the carbon-skeleton rearrangement reaction between R-methylmalonyl-CoA and succinyl-CoA. Diethyl 2-bromomethyl-2-phenylmalonate, an alkyl bromide substrate having two different migrating groups (phenyl and carboxylic ester groups) on the ß-carbon, was applied to the electrolysis mediated by a hydrophobic vitamin B12 model complex, heptamethyl cobyrinate perchlorate in this study. The electrolysis of the substrate at -1.0V vs. Ag-AgCl by light irradiation afforded the simple reduced product (diethyl 2-methyl-2-phenylmalonate) and the phenyl migrated product (diethyl 2-benzyl-2-phenylmalonate), as well as the electrolysis of the substrate at -1.5V vs. Ag-AgCl in the dark. The electrolysis of the substrate at -2.0V vs. Ag-AgCl afforded the carboxylic ester migrated product (diethyl phenylsuccinate) as the major product. The selectivity for the migrating group was successfully tuned by controlling the electrolysis potential. We clarified that the cathodic chemistry of the Co(III) alkylated heptamethyl cobyrinate is critical for the selectivity of the migrating group through mechanistic investigations and comparisons to the simple vitamin B12 model complex, an imine/oxime-type cobalt complex.

Cerasomes: Soft Interface for Redox Enzyme Electrochemical Signal Transmission.

Anionic cerasomes, which consist of a liposomal lipid bilayer and a ceramic surface, were used as a soft interface for the construction of an integrated modified electrode to achieve the transmission of chemical information from a redox enzyme through electrical signals. The morphological properties of the cerasomes were systematically compared with those of two structural analogues, namely, liposomes and silica nanoparticles. The results indicated that the cerasomes combined the advantages of liposomes and silica nanoparticles. The lipid bilayer gave excellent biocompatibility, as in the case of liposomes, and high structural stability, similar to that of silica nanoparticles, was derived from the silicate framework on the cerasome surface. The performance at the electrochemical interface created by means of a combination of cerasomes and horseradish peroxidase on a glassy carbon electrode was much better than those achieved with liposomes or silica nanoparticles instead of cerasomes. The potential use of cerasomes in the construction of supramolecular devices for mediator-free biosensing was evaluated.

Tuning of intramolecular charge transfer properties and charge distributions in ferrocene-appended catechol derivatives by chemical substitution.

In this study, we report intramolecular charge transfer (ICT) properties and charge distributions in a series of FcC derivatives (FcC = 4-ferrocenylcatecholate where Fc = ferrocene and C = catecholate). This series consists of a previously reported complex FcV (4-ferrocenylveratrole) and newly synthesized complexes FcA (4-ferrocenylcatechol bis(acetate) and Pt((t)Bu2bpy)(FcC) ((t)Bu2bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). An electrochemical analysis of Pt((t)Bu2bpy)(FcC) using cyclic voltammetry revealed two well-defined, reversible waves which were assigned to the sequential oxidation of the Pt((t)Bu2bpy)(C) and Fc moieties. The potential splitting between the waves (524 mV) indicated that there was an electronic interaction between both moieties. ICT property and charge distribution of [Pt((t)Bu2bpy)(FcC)]˙(+) were rationalized by comparison with the [FcV]˙(+) and [FcA]˙(+) (4-ferrocenylcatechol bis(acetate)). DFT calculations and UV-vis-NIR spectroscopy revealed that [Pt((t)Bu2bpy)(FcC)]˙(+), [FcV]˙(+), and [FcA]˙(+) were ferrocenium (Fc(+))-centered rather than semiquinone ligand-centered and that these complexes exhibited ICT transition bands from the catechol-derivatized framework to the Fc(+) moiety in the near infrared (NIR) region. Both the electronic coupling parameter (HAB) and delocalization parameter (α) increased in value as the electron-donating strength of the substituent groups in the catechol-derivatized framework increased (OCOCH3 ([FcA]˙(+)) < OCH3 ([FcV]˙(+)) < O(-) ([Pt((t)Bu2bpy)(FcC)]˙(+))). The electronic interactions between the organometallic center and the non-innocent framework were tuned by changing the substituents. The potential energy surfaces of the Fc(+) derivatives, obtained using two-state Marcus-Hush theory, can be modulated by changing the energy level of the molecular orbitals of the appended catechol-derivatized moieties.

Organic mixed valency in quadruple hydrogen-bonded triarylamine dimers bearing ureido pyrimidinedione moieties.

Quadruple hydrogen-bonding interactions stabilized the mixed-valence states of dimers of triarylamine derivatives bearing an ureido pyrimidinedione moiety without any electronic coupling. This study represents the first example of proton-coupled "organic" mixed valency in solution with different substituents being used to control the stability.

Construction of covalent- and hydrogen-bonded assemblies from 1',1'''-biferrocenediboronic acid as a new organobimetallic building block.

1',1'''-Biferrocenediboronic acid () was synthesized from 1',1'''-dibromobiferrocene by a typical procedure of converting Br to B(OH)2 groups in 76% yield and identified by (1)H-, (13)C- and (11)B-NMR and ESI-MS. X-ray diffraction (XRD) studies showed that, in non-solvated crystals (Form I), the new organobimetallic building block formed 1D hydrogen-bonded networks (i.e., chain) with octaatomic rings composed of the neighbouring two molecules. In solvated crystals with a composition of ()3(THF)2 (Form II), exists in two conformers (Conformers A and B) with respect to the rotation of the CpB(OH)2 moieties relative to the Cp rings of the fulvalenide moieties; Conformer A formed 1D hydrogen-bonded networks laterally hydrogen-bonding with THF molecules while Conformer B formed a new planar hydrogen-bonded motif involving four B(OH)2 groups and stepwise laminated networks of the planar motif. A macrocyclic tetraferrocenyl boronate ester was synthesized by cyclocondensation between and pentaerythritol in 33% yield and identified by (1)H-, (13)C- and (11)B-NMR, ESI-MS and XRD. In electrochemical measurements, the cyclocondensed compound exhibited four defined reversible waves with a total spread of 756 mV in CH2Cl2 containing n-Bu4NBArF4 (ArF = 3,5-bis(trifluoromethyl)phenyl), displaying both intra- and inter-biferrocenyl interactions.

Construction of di- and tetra-ferrocenyl spiroborate complexes from catechol building blocks and their redox behaviors.

In this paper, we report a method for the construction of multi-ferrocenyl spiroborate assemblies from discrete mono- and di-ferrocenyl complexes. To prepare the building blocks for the spiroborate assemblies, 4-ferrocenylveratrole (1) and 4,5-diferrocenylveratrole (3) were first synthesized by Negishi cross-coupling between ferrocene and the corresponding aryl halides. Compounds 1 and 3 were characterized by NMR and UV-vis spectroscopies and electrochemical methods, namely cyclic voltammetry and differential pulse voltammetry. The potential splittings of 167 and 263 mV for the oxidations of the two ferrocenyl groups of 3 in CH2Cl2-n-Bu4NPF6 and n-Bu4NB(C6F5)4, together with the presence of an intervalence charge-transfer transition of 3(+) in the NIR region, indicate a strong inter-ferrocenyl interaction through the ortho connection in the veratrole bridge. The treatment of 1 and 3 with BBr3, followed by complexation with B(OH)3 in the presence of KOH, afforded potassium salts of di- and tetra-ferrocenyl spiroborate assemblies, K2 and K4, respectively. The spiroborate assemblies K2 and K4 were characterized by NMR and UV-vis spectroscopies, ESI-MS, and electrochemical methods. K2 exhibited a potential splitting value of 302 mV in DMF-n-Bu4NPF6, indicating electronic interactions over the boron center. K4 exhibited four one-electron-oxidation waves in DMF-n-Bu4NPF6 and n-Bu4NB(C6F5)4; the one-electron-oxidized species 4 is thermodynamically more stable than 4(+) and 4(2+). The overall E(1/2)(1)-E(1/2)(4) spread for K4 is 201 mV larger than the E(1/2)(1)-E(1/2)(2) splitting for 3 in DMF-n-Bu4NPF6, indicating that the spiroborate link between diferrocenyl units led to electronic interactions over the boron center.

Synthesis, characterization, Co-S bond reactivity of a vitamin B12 model complex having pentafluorophenylthiolate as an axial ligand.

Heptamethyl (aquo)(pentafluorophenylthiolate)cobyrinate perchlorate, [(H2O)(C6F5S)Cob(III)7C1ester]ClO4, was synthesized as a B12 model complex having a thiolate ligand in the axial position. The axial ligand change in heptamethyl (diaquo)cobyrinate diperchlorate, [(H2O)2Cob(III)7C1ester](ClO4)2, from H2O to C6F5S(-) afforded the B12-thiolate complex. The B12-thiolate model complex was characterized by UV-vis, NMR and ESI-mass spectroscopies. The coordination of C6F5S(-) to the cobalt center affected the spectroscopic properties of the corrin ring through the electronic interaction between the axial ligand (C6F5S(-)) and the equatorial ligand (corrin). The photolysis of the B12-thiolate model complex led to the homolytic cleavage of the Co(iii)-S bond to form the Co(II) complex and the phenyl thiyl radical. The thermolysis of the B12-thiolate model complex also led to the homolytic cleavage of the Co(III)-S bond. Furthermore, the reactivity of the Co(III)-S bond of the B12-thiolate model complex was applied to the catalytic oxidation of C6F5SH to C6F5S-SC6F5.

Synthesis, characterization and catalytic function of a B12-hyperbranched polymer.

A new hybrid catalyst composed of a vitamin B(12) derivative and a hyperbranched polymer (HBP) was synthesized and characterized by UV-vis and ESR spectroscopy as well as AFM. The B(12)-HBP showed good properties as a homogenous catalyst. The covalently-immobilized B(12) moieties were efficiently solvated and the cobalt centers were accessible for pyridine guests. The B(12)-HBP showed high reactivity to 2-phenethylbromide and increased selectivity for reductive dimerization in the presence of TiO(2) with UV light irradiation.