Amphoteric Redox Research Articles

A “Cyanine−Cyanine” Salt Exhibiting Photovoltaic Properties

Association between cationic and anionic heptamethine dyes led to the formation of a new organic salt (3) displaying exceptional light-harvesting properties in the NIR spectral range and having amphoteric redox character. Preliminary results of molecular bulk heterojunction solar cells based on the title compound 3 and [60]PCBM as the only active layer reveal this new dye as a promising light-harvesting material for photovoltaics.

Synthesis and Redox Behavior of 1,3‐Bismethylthio‐) and 1,3‐Bis(phenylthio)azulenes Bearing 2‐ and 3‐Thienyl Substituents by Palladium‐Catalyzed Cross‐Coupling Reaction of 2‐ and 6‐Haloazulenes with Thienylmagnesium Ate Complexes

AbstractPreparation of thienylazulenes 3–6 was established by the palladium‐catalyzed cross‐coupling reaction of the corresponding haloazulenes with thienylmagnesium ate complexes, which were readily prepared from the corresponding bromothiophenes. The reaction of 3–6 with several sulfoxides in the presence of Tf2O, followed by treatment with triethylamine afforded the corresponding 1,3‐bis(methylthio)‐ and 1,3‐bis(phenylthio)‐2‐ and 6‐thienylazulenes 7–12 in good yields. Redox behavior of the novel azulene derivatives with 2‐ and 3‐thienyl‐substituted 3–12 was examined by cyclic voltammetry and differential pulse voltammetry, which revealed amphoteric redox behavior. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Synthesis and redox behavior of ene–diyne scaffolds that bear ferrocenes at the periphery

The ene–diyne systems 1 and 2 possessing ferrocenyl groups at the periphery were prepared by the simple one-pot Sonogashira–Hagihara coupling reaction of ethynylferrocene with 9-dibromomethylene-9 H-fluorene ( 4) and 9,10-bis(dibromomethylene)-9,10-dihydroanthracene ( 5). The novel ferrocene-substituted ene–diyne compounds exhibited amphoteric redox behavior with a reversible one-stage electrochemical oxidation upon cyclic voltammetry.

Synthesis, Crystal Structure, and Physical Properties of 7,14-Disubstituted Pentacene-5,12-diones Containing a Methylenequinoid Structure

Title 7,14-disubsituted pentacene-5,12-diones were prepared for the first time by a nucleophilic substitution reaction of a pentacene-5,7,12,14-tetraone with aryl- or ethynyllithiums followed by a dehydroxylation reaction. The methylenequinoid structures with two conjugated carbonyl groups were clearly observed in their crystal structures. They showed intense absorptions in the visible region and amphoteric redox properties with high reduction potentials.

An Extremely Simple Dibenzopentalene Synthesis from 2‐Bromo‐1‐ethynylbenzenes Using Nickel(0) Complexes: Construction of Its Derivatives with Various Functionalities

Nice and easy: A very simple synthesis for dibenzopentalenes, which starts from 1-bromo-2-ethynylbenzenes, has been developed. It uses Ni(0) complexes (see scheme), from which a relatively stable Ni(II) complex as an important intermediate has been isolated. Dibenzopentalenes with various functional groups can be prepared by the procedure, and their electronic properties are consistent with theoretical calculations.An extremely simple dibenzopentalene synthesis from readily available 2-bromo-1-ethynylbenzenes using a nickel(0) complex is described. Although the yields are moderate, the formation of three C-C bonds in a single process and the high availability of the starting materials are important advantages of this reaction. The corresponding aryl-nickel(II) complex as an important intermediate was isolated as relatively stable crystals, and the structure was confirmed by X-ray crystallographic analysis. The high stability of this complex should play a key role in this reaction. The reaction is applicable to the preparation of dibenzopentalenes bearing various functional groups. Their electronic properties are consistent with theoretical calculations. The cyclic voltammograms of these compounds reveal highly amphoteric redox properties. In particular, the electron-donating property of a tetramethoxy derivative is greater than that of oligothiophenes and dibenzodithiophenes and almost comparable to that of pentacene.

Efficient electron transfer in β-substituted porphyrin-C 60 dyads connected through a p-phenylenevinylene dimer

Two new porphyrin-C 60 dyads have been synthesized in which the electroactive moieties have been connected through a p-phenylenevinylene dimer. The electrochemical study confirms the amphoteric redox behavior of these dyads. Irradiation of these compounds gives rise to the corresponding radical pair confirming that substitution on the β-position of the porphyrin facilitates the electronic communication between the porphyrin and C 60.

Energy Transfer in Oligofluorene-C60 and C60-Oligofluorene-C60 Donor−Acceptor Conjugates

Two complementary series of C(60)-(Fl)(n) and C(60)-(Fl)(n)-C60 (Fl = 9,9-dihexylfluorene-2,7-diyl; n = 1-5) derivatives with terminal N-methylfulleropyrrolidine units have been synthesized from CHO-(Fl)(n) and CHO-(Fl)(n)-CHO precursors via 1,3-dipolar cycloaddition reaction of in situ generated azomethine ylides with an excess of C60. In solution electrochemical experiments, these conjugates give rise to amphoteric redox behavior. Three consecutive quasireversible reduction waves have been observed at the expected potentials for the N-methylfulleropyrrolidine cores. For the C(60)-(Fl)(n)-C(60) series, each reduction wave is a two-electron process with no observable interaction between the C(60) units. Two or, in some cases, three oxidation waves--most of them irreversible--are ascribed to the oligofluorene system. These waves are cathodically shifted with an increasing number of fluorene units and anodically shifted by the conjugated terminal aldehyde units, compared to the N-methylfulleropyrrolidine termini. Steady-state and time-resolved photolytic techniques show that an efficient transduction of singlet excited-state energy transfer prevails from the photoexcited oligofluorene to the energy accepting fullerene.

1,1,4,4-Tetra(2-furyl, 2-thienyl, and 2-selenienyl)butatrienes: Synthesis, Properties, and Molecular Structures

Abstract Tetra(2-furyl, 2-thienyl, and 2-selenienyl)butatrienes and their derivatives have been prepared by dimerization of ate-type complexes derived from the corresponding 1,1-dichloro-2,2-diarylethenes using appropriate copper complexes in moderate to high yields. These new butatrienes are relatively stable crystalline substances with intense absorption bands at long wavelengths. The electronic properties of the substituents at the 5-position of aryl groups remarkably influence the wavelength of the longest absorption and the π-electron distribution of butatriene moiety. Their redox potentials, measured by cyclic voltammetry, have higher amphoteric redox properties than those of tetraphenylbutatriene. The crystal structures of tetrakis(5-trimethylsilyl)-substituted derivatives show the following two features: (1) the thiophene and selenole derivatives have pseudo-D2 structures, while the furan derivative has a C2 symmetric structure probably due to the counterbalance between the energy of conjugation and the nonbonded chalcogen–chalcogen interaction; (2) the central double bonds of these butatrienes are considerably short.

Synthesis and Characterization of a Novel Terthiophene-Based Quinodimethane Bearing a 3,4-Ethylenedioxythiophene Central Unit

The synthesis and a combined spectroscopic and density functional theoretical characterization of a 3',4'-ethylenedioxy-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene analogue of 7,7,8,8-tetracyanoquinodimethane (TCNQ) are presented. Electrochemical data show that this novel trimer can be both reversibly reduced and oxidized at relatively low potentials. Quantum-chemical calculations show that the compound exhibits a quinoidal structure in its ground electronic state and that a certain degree of intramolecular charge transfer takes place from the central terthienyl moiety toward both =C(CN)2 end-caps. Therefore, the amphoteric redox behavior of this novel material can be related to the coexistence of an electron-impoverished terthienyl core endowed by two electron-enriched =C(CN)2 substituents. The UV-vis spectrum is dominated by the appearance of a strong absorption near 660 nm, attributable to the highest occupied molecular orbital (HOMO) --> lowest unoccupied molecular orbital (LUMO) pi-pi electronic transition of the terthienyl spine on the basis of time-dependent density functional theory (DFT) computations. The DFT calculations performed on the minimum-energy molecular geometry about the equilibrium atomic charge distribution, topologies, and energies of the frontier orbitals around the gap and about the Raman-active vibrations associated with the strongest Raman features are also consistent with a rather effective pi-electron conjugation and the partial degree of intramolecular charge transfer mentioned above. Our study reveals this novel heteroquinoid trimer could act as a promising candidate in organic field-effect transistor (OFET) applications.

Extensive Quinoidal Oligothiophenes with Dicyanomethylene Groups at Terminal Positions as Highly Amphoteric Redox Molecules

A series of highly extensive quinoidal oligothiophenes carrying a dicyanomethylene group at both terminal positions is synthesized. As the quinoidal structures extend, they have highly amphoteric abilities and show strong electronic absorptions in the visible to near-infrared region. The higher homologues, quinquethiophene and sexithiophene, exist as equilibrium mixtures with the biradical species.

Azulene-Substituted Aromatic Amines. Synthesis and Amphoteric Redox Behavior of N,N-Di(6-azulenyl)-p-toluidine and N,N,N‘,N‘-Tetra(6-azulenyl)-p-phenylenediamine and Their Derivatives

[reaction: see text] N,N-Di(6-azulenyl)-p-toluidine (1a) and N,N,N',N'-tetra(6-azulenyl)-p-phenylenediamine (2a) and their derivatives with 1,3-bis(ethoxycarbonyl) substituents on each 6-azulenyl group (1b and 2b) were prepared by Pd-catalyzed amine azulenylation and characterized as a study into new aromatic amines for multistage amphoteric redox materials. The redox behavior of each compound was characterized by cyclic voltammetry. These compounds undergo facile reduction to stable anion radicals and dianion diradicals owing to the resonance stabilization between the 6-azulenyl groups and exhibit electrochemical oxidation depending on the amine subunits. The ESR measurement of anion radicals and a dianion diradical generated by the electrochemical reduction of amine 1b and diamine 2b revealed that the unpaired electron of these radicals delocalizes over the entire azulene ring including the central nitrogen atoms. UV-vis spectral analysis of amines 1a,b and diamines 2a,b, taken during the electrochemical reduction, exhibited a gradual decrease of the absorption bands of the neutral species along with an increase of the new absorption maxima at 625, 605, 640, and 610 nm, respectively, with the development of well-defined isosbestic points at 502, 562, 478, and 545 nm, respectively. As indicated by a combined ESR and UV-vis spectral study, the species giving rise to the new absorption maxima are concluded to be the generation of anion radicals and dianion diradicals of aromatic amines and diamines with high thermodynamic stability.

Connecting two C60stoppers to molecular wires: ultrafast intramolecular deactivation reactions

A new series of soluble dumbbell-type architectures constituted by π-conjugated oligophenyleneethynylenes (OPEs) and two C60 stoppers have been prepared by using a convergent synthetic strategy. In particular, we base our approach on the systematic cross-coupling, palladium catalyzed Hagihara–Sonogashira reaction between aryl iodides and alkynyl derivatives. The cyclic voltammetry of the correspondingly synthesized system reveals amphoteric redox behavior and the lack of significant electronic communication between the oligo-arylalkynyl bridge and the two C60 moieties, which behave as two independent units. Excited state studies carried out by fluorescence and transition absorption spectroscopy show that, although a weak competitive electron transfer could operate in the deactivation process of the excited oligo-arylalkynyl bridges, their deactivation is mainly governed by energy transfer.

FT-Raman spectroscopic study, aided by quantum chemical DFT calculations, of a series of oligothiophenes end-capped by nitriles

We report on the FT-Raman spectroscopic characterization, aided by DFT model chemistry calculations, of a bunch of α,ω-dicyano end-capped oligothiophenes, ranging in length from the dimer to the hexamer, which have been previously shown to display quite promissing amphoteric redox properties for their potential use in new technological applications. The attention is mainly focused to the role played by the CN⋯H hydrogen bonding interactions between adjacent molecules on various molecular properties, such as the minimum-energy geometry, atomic charges equilibrium distribution,topologies/energies of the frontier molecular orbitals and the Raman scattering spectral profile. To this end we have performed to two sets of quantum chemical calculations, at the DFT//B3LYP/3-21G* level, on: (a) a single terthiophene in the vacuum and (b) a suited dimeric model with a fully coplanar array of the two terthienyl segments interacting through the cyano end-caps.

Multi-Stage Amphoteric Redox Hydrocarbons Based on a Phenalenyl Radical

Abstract Molecular design and properties of amphoteric redox hydrocarbons are reviewed. Our molecular design is based on a phenalenyl radical, which has a non-bonding molecular orbital with an occupation number of one and is known to behave as a two-stage amphoteric redox system. We have prepared eight amphoteric redox compounds containing two or three phenalenyl units. Cyclic voltammetry showing small E1sum values confirms the high amphotericity of these compounds. The properties of ionic redox species generated are also investigated. At the last part of this account, we show some aspects of a singlet biradicaloid character that originates from a small HOMO–LUMO gap.

2,6‐Di‐tert‐butyl‐4‐(3,3‐diarylpropadienylidene)‐2,5‐cyclohexadien‐1‐ones, the First Stable p‐Quinopropadienes

AbstractTreatment of 2,6‐di‐tert‐butyl‐4‐ethynylphenol with 2 equivalents of nBuLi, followed by addition of diaryl ketones, afforded the corresponding p‐quinopropadienes 2a−f upon appropriate dehydrative treatments. These compounds are fairly stable, orange to purple, crystalline substances, exhibiting intense absorptions at longer wavelengths. In particular, the bis(dimethylamino) derivative 2a absorbs across the whole of the visible spectrum and into the near infrared. Unlike the quinone methides 1, compounds 2 exhibited irreversible oxidation waves together with irreversible reduction waves on cyclic voltammetry. The very small E1sum value of 2a also indicates the highly amphoteric redox properties of 2a; however, both the anion and cation radicals of 2 are highly reactive. The molecular structures of 2a and 2e exhibit relatively small dihedral angles between two aromatic ring planes (2a for 47° and 2e for 52°), and each has a short central double bond and long side double bonds. The short central double bond (2a for 1.228 Å and 2e for 1.235 Å) indicates the importance of the resonance contribution of the dipolar structure with an acetylenic bond. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Probing charge separation in structurally different C60/exTTF ensembles.

The scope of the present work is to highlight the effects stemming from different C60/exTTF linkages (exTTF = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene)-either via an anthracene unit or a dithiole ring. Particular emphasis is placed on photoinduced electron-transfer features. Therefore, we devised a new series of C60-exTTF ensembles, synthesized via 1,3-dipolar cycloaddition and Diels-Alder cycloaddition reactions, in which exTTF units are separated from C60 by two single bonds (3a-c, 4), one vinylene unit (5a), or two vinylene units (5b). The cyclic voltammetry reveals an amphoteric redox behavior with remarkably strong electron-donor ability of the trimethyl-substituted exTTF moiety in 4 and 5a,b. Steady-state and time-resolved photolytic techniques show that the fullerene singlet excited state in (3a-c, 4, and 5a,b) is subject to a rapid electron-transfer quenching. The resulting charge-separated states, that is C60*(-)-exTTF*+, were identified by transient absorption spectroscopy. We determined radical pair lifetimes of the order of 200 ns in benzonitrile. This suggests (i) that the positive charge of the exTTF*+ is delocalized over the entire donor rather than localized on one of the 1,3-dithiole rings and (ii) that linking exTTF via the anthracene or 1,3-dithiole ring has no appreciable influence. Increasing the donor-acceptor separation via implementing one or two vinylene units as spacers led to improved radical pair lifetimes (5a: tau = 725 ns; 5b: tau = 1465 ns).

Addition reaction of azido-exTTFs to C60: synthesis of fullerotriazoline and azafulleroid electroactive dyads.

[structure: see text] The addition reaction of azido-exTTFs to C(60) affords electroactive fullerotriazoline and azafulleroid dyads, which behave as amphoteric redox systems. Fluorescence experiments and transient absorption spectroscopy reveal that excitation of the fullerene moiety leads to radical pair lifetimes that are 2 orders of magnitude higher than those previously reported for related fullerotriazolines.

Engineering a remarkably low HOMO-LUMO gap by covalent linkage of a strong pi-donor and a pi-acceptor--tetrathiafulvalene-sigma-polynitrofluorene diads: their amphoteric redox behavior, electron transfer and spectroscopic properties.

Novel R3TTF-sigma-A compounds 14, 16 and 19 (R3TTF = trial-kyletrathiafulvalene, sigma = saturated spacer, A = polynitrofluoren-9-dicyano-methylene acceptor) incorporating very strong donor and acceptor moieties have been synthesized by condensation of the corresponding R3TTF-sigma-fluoren-9-one diads with malononitrile. Reversible five-step amphoteric redox behavior has been observed with an extremely low HOMO-LUMO gap (approximately 0.3 eV). For compound 14 a strong EPR signal is observed in the solid state, ascribed to intermolecular complexation: a less intense signal is seen in solution, corresponding to ca. 2% of the molecules existing in a radical form at room temperature. Intramolecular charge transfer in diads 14 and 16 is manifested in strong absorption bands in the near-IR region of their electronic spectra. Spectroelectrochemical data reveal marked electrochromic behavior in the visible and near-IR region of both compounds. The first X-ray crystal structure of a fluorene radical-anion salt is reported, namely the copper salt of 2,4,5,7-tetranitro-9- dicyanomethylenefluorene (1:1 stoichiometry).

4,7,11,14,18,21-Hexa-t-butyltribenzodecacyclenyl Radical: A Six-Stage Amphoteric Redox System

Abstract A six-stage amphoteric redox hydrocarbon radical 3• containing three phenalenyl units was prepared. Cyclic voltammogram of 3• exhibits six reversible redox waves with a small numerical sum (E1sum) of first oxidation (E1ox) and reduction (E1red) potentials. Some redox states of 3• were successfully generated from the neutral 3• and characterized by NMR, ESR, UV-vis-NIR spectroscopies, and theoretical calculations. The spin and the charges are delocalized over the entire C3 molecule of 33+, 3•2+, 3•, and 33−. On the other hand, 3•2− shows the localization of the spin on one phenalenyl unit; these results are consistent with the distorted C2 structure caused by the Jahn-Teller effect.

Tetra- tert-butyl- as-indaceno[1,2,3- cd:6,7,8- c′d′]diphenalene: a four-stage amphoteric redox system

Tetra- tert-butyl- as-indaceno[1,2,3- cd:6,7,8- c′ d′]diphenalene (TTB- as-IDPL) was prepared and found to behave as a four-stage amphoteric redox compound. The properties of its five redox states were investigated.