Highly conjugated p-quinonoid pi-extended tetrathiafulvalene derivatives: a class of highly distorted electron donors

A new class of pi-extended TTF-type electron donors (11 a-c) has been synthesized by Wittig-Horner olefination of bianthrone (9) with 1,3-dithiole phosphonate esters (10 a-c). In cyclic voltammetry experiments, donors 11 a-c reveal a single, electrochemically irreversible oxidation-yielding the corresponding dicationic products-at relatively low oxidation potentials (approximately 0.7-0.8 V). Theoretical calculations, performed at the DFT level (B3 P86/6-31 G*), predict a highly-folded C(2h) structure for 11 a. In the ground state, the molecule adopts a double saddle-like conformation to compensate the steric hindrance. The calculations suggest that the intramolecular charge transfer associated with the HOMO-->LUMO transition is responsible for an absorption band observed above 400 nm. While the radical cation 11 a*+ retains the folded C(2h) structure predicted for the neutral molecule as the most stable conformation, the dication 11 a(2+) has a fully aromatic D(2) structure, formed by an orthogonal 9,9'-bianthryl central unit to which two singly-charged dithiole rings are attached. The drastic conformational changes that compounds 11 undergo upon oxidation account for their electrochemical properties. By means of pulse radiolysis measurements, radical-induced one-electron oxidation of 11 a-c was shown to lead to the radical cation species (11 a-c*+), which were found to disproportionate with generation of the respective dication species (11 a-c(2+)) and the neutral molecules (11 a-c).     

Tetrathiafulvalene-Inserted Diphenoquinone: Synthesis,Structure, and Dynamic Redox Property

A new tetrathiafulvalene (TTF) derivative is synthesized, which is substituted with two phenoxy radicals on one 1,3-dithiole ring, and may have either open-shell diradical or closed-shell extended-quinoidal ground states. X-ray single crystal analysis and NMR measurements prove that this molecule has a closed-shell extended quinoidal structure both in the solid state and in solution. DFT calculations show the donor–acceptor electronic properties of this molecule with a well-separated HOMO–LUMO distribution and a small HOMO–LUMO energy gap. Because of this donor–acceptor character, this molecule gives both the dication and the dianion species by electrochemical oxidation and reduction. Furthermore, during the redox process between the neutral and dication states, this molecule exhibits unique changes in the cyclic voltammogram upon repeating the cycles or varying the scan rate. The observed electrochemical behavior is explained by the conformational changes in the electrochemically generated species, thus indicating that this molecule is classified as a dynamic redox system.     

Synthesis and properties of Bingel-type methanofullerene-pi-extended-TTF diads and triads

Novel C(60)/pi-extended tetrathiafulvalene (exTTF) diads (12a-c) and triads [D(2)A (14a-c) and DA(2) (25, 27a-c)] have been synthesized by the Bingel cyclopropanation reaction of the respective exTTF-containing malonates and [60]fullerene. The reaction of exTTF-bismalonates with C(60) affords the respective C(60)-exTTF diads (26a-c) together with the triad C(60)-exTTF-C(60) (25, 27a-c) and a regioisomeric mixture of bisadducts (28b-c). Theoretical calculations (PM3) predict the favored geometry for triads 14a-c depending upon the orientation (up and down) of the 1,3-dithiole rings in the exTTFs, as well as the more stable regioisomers for the bisadducts 28. Cyclic voltammetry measurements reveal that C(60) and exTTF units do not intereact in the ground state. Compounds 26a-c and 27a-c are not electrochemically stable. A photoinduced electron transfer leading to the formation of the radical pair (C(60)(-)-exTTF(*+)) has been observed for compounds 14a-c.     

Novel Bis-Fused pi-Electron Donors for Organic Metals: 2-(1,3-Dithiol-2-ylidene)-5-(thiopyran-4-ylidene)-1,3,4,6-tetrathiapentalene

Bis-fused pi-electron donors composed of tetrathiafulvalene (TTF) and 2-(thiopyran-4-ylidene)-1,3-dithiole (TPDT), 2-(1,3-dithiol-2-ylidene)-5-(thiopyran-4-ylidene)-1,3,4,6-tetrathiapentalene (1a, TPDT-TTP), and its derivatives (1b-d, 2a-d) have been synthesized as donor components for organic conductors. An X-ray structure analysis of bis(methylthio)-1 (1c) revealed that the TPDT-TTP skeleton is almost planar except for the outer 1,3-dithiole ring, and that the crystal has a two-dimensional "theta-type" arrangement of molecules. The cyclic voltammograms of TPDT-TTPs exhibit four pairs of single-electron redox waves. The first oxidation potential (E(1)) of 1a (+0.37 V vs SCE, in PhCN) is comparable to that of TTF (+0.35 V) and is higher by 0.1 V than that of TPDT (+0.27 V). The observed substituent effect on E(1) values suggests that the first one-electron oxidation mainly occurs in the 2-(thiopyran-4-ylidene)-1,3-dithiole (TPDT) moiety. On the other hand, on-site coulombic repulsion estimated from the E(2) - E(1) value is lower than in TTF and TPDT. MNDO MO calculations reveal that all the sulfur atoms in the 1,3-dithiole rings have the same phase in the HOMO, a condition necessary for realization of effective transverse intermolecular interaction. The present donors have produced many charge-transfer complexes and cation radical salts showing relatively high conductivity (sigma(rt) = 10(-)(1)-10(1) S cm(-)(1)), several of which display metallic temperature dependence.     

Intramolecular charge-transfer interactions in pi-extended tetrathiafulvalene derivatives

New electron-donor (D)-electron-acceptor (A) TTF architectures are presented in which two electron-donating 1,3-dithiole moieties are connected by a pi bridge to the weak electron-accepting quinoxaline moiety (D-pi-A compounds 9a and 9b and also two 1,3-dithiole-2-ylidene moieties are connected by a pi bridge to the electron-accepting thieno[3,4-b]quinoxaline bridge (D-pi-A-pi-D compounds 12a-c). There are through-bond intramolecular charge-transfer (ICT) interactions, predicted in theoretical calculations, and confirmed by UV-vis spectroscopy and cyclic voltammetry measurements. This work constitutes the first use of quinoxalines as electron-accepting moieties in D-pi-A compounds.     

Synthesis and properties of conjugated hybrid tetrathiafulvalene dimers

The synthesis of new hybrid tetrathiafulvalene (TTF) dimers (11a-c) has been carried out by a Wittig-Horner reaction of the respective phosphonate esters (10a-c) with 2-(tetrathiafulvalenylvinyl)-9, 10-anthraquinone (9) prepared by olefination of formyltetrathiafulvalene (7) and the phosphonium salt of anthraquinone 8. Electrochemical studies show that the dimers 11a-c mainly retain the electrochemical properties of both TTF and the pi-extended TTF components, and most importantly, intramolecular electronic interactions between the two moieties are observed by cyclic voltammetry and Osteryoung square wave voltammetry. Semiempirical PM3 calculations reveal an almost planar geometry for the TTF and the benzene ring connected through the vinyl spacer. These compounds can form stable charge-transfer complexes with 2, 3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) showing a stoichiometry of 1:3 (D:A). Attempts to electrocrystallize the dimeric donors with different counteranions are discussed.     

Electronic and structural effects on the nonlinear optical behavior in push-pull TTF/tricarbonyl chromiun arene complexes

A novel D-pi-A system in which tetrathiafulvalene (TTF) and pi-extended TTFs as strong electron donors are covalently connected to a tricarbonyl (eta(6)-arene)chromium complex as the acceptor moiety through a systematically increased conjugated bridge of vinylene units (12a-c, 16a-c) have been synthesized by Wittig-Horner olefination reaction. The electronic spectra as well as the electrochemical data reveal a different behavior of TTF derivatives (12a-c) and of exTTF derivatives (16a-c). Cyclic voltammetry shows the influence of the tricarbonylchromium arene on the oxidation potentials in compounds 12a-c, and no remarkable effect is observed for exTTFs (16a-c). The nonlinear optical properties of 12a-c and 16a-c have been calculated by using the ab initio CPHF/6-31G//B3P86/6-31G model, and the time-dependent density functional theory (TD-DFT) method has been used for the calculation of the electronic transitions. The calculations reveal that an intraligand charge-transfer transition (ILCT) and the metal to ligand charge-transfer transition (MLCT) are responsible for the nonlinear response. In addition, the large angles formed by the ground-state dipole moment and the vectorial hyperpolarizability are responsible for the mubeta values determined experimentally by the EFISH technique.     

Synthesis of tetrahydroxy-π-extended tetrathiafulvalenes as new supramolecular redox building blocks

A novel p-quinonoid π-extended tetrathiafulvalene (exTTF) endowed with four hydroxy groups with different reactivity (phenol and alcohol) has been synthesized as a supramolecular redox building block. The redox properties, studied by cyclic voltammetry, reveal a strong donor ability and, despite the different substitution pattern on the 1,3-dithiole rings, only one oxidation wave involving two electrons to form the dication species.     

Molecular saddles. 4. Redox-active cyclophanes by bridging the 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene system: synthesis, electrochemistry, and X-ray crystal structures of neutral species and a dication salt.

We report the synthesis of a new series of cyclophanes 11a-d by ester-forming macrocyclization reactions of diol 9 with the dicarbonyl chloride derivatives of benzene, thiophene, ferrocene, and diphenyl ether, 10a-d, respectively. Compounds 11a-d display a two-electron, quasireversible oxidation wave in the cyclic voltammogram to yield the dication species at Eoxpa = 0.70-0.72 V (for 11a-c) and 0.47 V (for 11d) (vs Ag/AgCl in acetonitrile). The raised oxidation potentials for 11a-c reflect the reduced stability of the twisted dication structure within the steric constraints of the smaller cyclophanes. Consistent with this, the value of delta E (defined as Eoxpa - Eoxpc) decreases (i.e., reversibility of the oxidation process increases) in the sequence 11d > 11c > 11a > 11b as the bridging chain becomes shorter. X-ray crystal structures are reported for compounds 11a-d and the dication salt 11d2+(I3-)2.(CH2Cl2)2.25. For 11a-d the typical saddle-shaped conformation of the 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene moiety is observed, with the strain imposed by the cyclophane ring being accommodated in the structure of the bridging unit. In the dication 11d2+ both dithiolium rings are planar and the anthracene unit is essentially aromatic, with the conformation of the bridge basically the same as in neutral 11d.     

Synthesis, electrochemistry and self-assembled monolayers of novel tetrathiafulvalene (TTF) and pi-extended TTF (exTTF) disulfides

The synthesis of a new series of tetrathiafulvalene (TTF) and pi-extended TTF (exTTF) disulfides and the electrochemical properties of self-assembled monolayers derived from these compounds are described. When the intermediate bromides 3 and 7 were reacted with thiourea, followed by basic hydrolysis, the expected thiol formation was not observed and only disulfides were obtained. A mechanism is proposed to explain the self-oxidation process of these compounds. For the first time SAMs of exTTF derivatives were prepared. Electrochemical data for SAMs of 6 and 8 reveal a single two-electron chemically reversible oxidation process to form a dicationic state, typical of the exTTF system. The SAMs are stable over extended periods of time and show electrochemical stability upon repeated potential scans.     

Bis(1,3-dithiole-2-chalcogenones) and tetrathiafulvalenes in the synthesis of bridged tetrathiafulvalene-containing structures

Bis(1,3-dithiole-2-chalcogenones) in which the 1,3-dithiole fragments are linked through various bridging groups were synthesized by different methods. Some of these compounds were converted into substituted tetrathiafulvalenes with bridged 1,3-dithiole rings. The same structures were synthesized from preliminarily prepared symmetric tetrathiafulvalenes containing 2-cyanoethylsulfanyl groups in both 1,3-dithiole rings. Similar spacers were used to bridge two tetrathiafulvalene fragments. Syntheses of the involved initial compounds were described.     

Functionalised oligoenes with unusual topologies: synthesis, electrochemistry and structural studies on redox-active

New [3]- and [4]-dendralenes bearing electron-donor 1,3-dithiole and ferrocene substituents have been synthesised. Compounds 8, 15 and 17 have been characterised by single-crystal X-ray diffraction. Two of the dithiole rings of 8 are conjugated (dihedral angle 9 degrees), while the third dithiole ring is almost orthogonal to this plane, and hence its pi-electron system is isolated. For the dendralene precursor molecule 15, the substituted cyclopentadienyl ring, two C=C bonds and fused dithiole and dithiine rings comprise an extended pi-conjugated system. In molecule 17 the potential conjugation path C(6)C(3) C(4)C(5)-C5Hs is distorted by an 8 degrees twist around the C(3)-C(4) bond and a 7 degrees twist around the C(5)-C(21) bond, and the delocalisation along the chain is insignificant. Solution electrochemical data demonstrate that the dendralenes are strong pi-electron donors, which give rise to dication, radical trication or tetracation species. Spectroelectrochemical studies on compounds 7 and 10 suggest that the radical species are situated within the linear 1,2-ethylenediylidene moieties and that a conformational change may occur at the dication redox stage. UV/Vis spectroscopic data are consistent with poor cross-conjugation in these systems.     

Intramolecular electronic interactions in conjugated ferrocene-pi-extended-tetrathiafulvalene donor-pi-donor molecular hybrids

The synthesis of new hybrid ferrocene and pi-extended tetrathiafulvalene (TTF) donor(1)-pi-donor(2) molecular assemblies 16a-c has been carried out by a Wittig-Horner reaction of the respective phosphonate esters 15a-c with 2-(2-ferrocenylvinyl)-9, 10-anthraquinone (18) prepared by olefination of ferrocenecarboxaldehyde (14) and the anthraquinone phosphonium salt 17. Electrochemical studies show that the D(1)-pi-D(2) (D = donor) molecular assemblies 16a-c essentially retain the redox characteristics of both ferrocene and the pi-extended TTF components and the effects of solvent, temperature, scan rate, and working electrode are significant. Most importantly, pronounced intramolecular electronic interactions between the two donor moieties were observed by cyclic voltammetry and Osteryoung square wave voltammetry in both the ground and charged states. Semiempirical calculations support the electrochemical observations.     

Synthesis,structure, and oxidation of 3,4-dihydro-1,2,5-benzotrithiepins

Stable benzene-fused polysulfide compounds, 3,4-dihydro-1,2,5-benzotrithiepins ( 1a-c ), have been prepared, and the structure of 1a has been determined by X-ray crystallographic analysis. While the electrophilic oxidation of compounds 1 with m-chloroperbenzoic acid gave the corresponding 3,4-dihydro-1,2,5-benzotrithiepin 5-oxides ( 2 ) in moderate yields, the oxidation of 1 with N-bromosuccinimide afforded a mixture of 5-oxides 2 , unexpected, inseparable 3,4-dihydro-1,2,5-benzotrithiepin 2,2-dioxides ( 3 ), and 3,4-dihydro-1,2,5-benzotrithiepin 1,1-dioxides ( 4 ). Semiempirical PM3 calculations were carried out, and the computed HOMO of 1a suggested a significant favoring of electrophilic reactions at the sulfur atom at the 5-position. The treatment of 5-oxides 2 with acetyl bromide or oxalyl dibromide as halogenating reagents gave 2,2-dioxides 3 and 1,1-dioxides 4 , suggesting that an intramolecular halogen transfer from the 5-position (sulfide moiety) to the 1- and 2-positions (disulfide moiety) took place in the reactions.     

Novel extended tetrathiafulvalenes based on acetylenic spacers: synthesis and electronic properties

A selection of mono- and diacetylenic dithiafulvalenes was synthesized and employed for the construction of extended tetrathiafulvalenes (TTFs) with hexa-2,4-diyne-1,6-diylidene or deca-2,4,6,8-tetrayne-1,10-diylidene spacers between the two 1,3-dithiole rings. By stepwise acetylenic scaffolding using (E)-1,2-diethynylethene (DEE) building blocks, an extended TTF containing a total of 18 C(sp) and C(sp(2)) atoms in the spacer was prepared. The versatility of the acetylenic dithiafulvene modules was also established by the efficient synthesis of a thiophene-spaced TTF, employing a palladium-catalyzed cross-coupling reaction. The developed synthetic protocols allow functionalization of the extended TTFs in three general ways: with 1) peripheral substituents on the fulvalene cores, 2) alkynyl moieties laterally appended to the spacer, and 3) cobalt clusters involving acetylenic moieties. Strong chromophoric properties of the extended TTFs were revealed by linear and nonlinear optical spectroscopies. Extensive electrochemical studies and calculations on these compounds are also reported, as well as X-ray crystallographic analyses.     

Electronic interactions in a new pi-extended tetrathiafulvalene dimer

The first pi-extended tetrathiafulvalene (exTTF) dimer in which the two exTTF units are covalently connected by 1,3-dithiole rings has been obtained in a multistep synthetic procedure involving the Ullmann cross-coupling reaction by using copper(I) thiophene-2-carboxylate (CuTC). The electronic spectrum reveals a significant electronic interaction between the exTTF units. The electrochemical study carried out by cyclic voltammetry in solution and in thin-layer conditions, and the electrochemical simulation and spectroelectrochemical (SEC) measurements confirm the electronic communication and show that the oxidation of dimer 14 occurs as two consecutive 2 e(-) processes D(0)-D(0)-->D(2+)-D(0)-->D(2+)-D(2+). Theoretical calculations, performed at the B3P86/6-31G* level, confirm the experimental findings and predict that 14(2+) exists as a delocalized D(.+)-D(.+) species in the gas phase and as a localized D(2+)-D(0) species in solution (CH(3)CN or CH(2)Cl(2)). Oxidation of 14(2+) forms the tetracation 14(4+) which is constituted by two aromatic anthracene units bearing four aromatic, almost orthogonal 1,3-dithiolium cations.     

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).     

Selenium heterocycles. XXXI. Reaction of ethyl propiolate with 5-phenyl-2-thioxo-1,3-thiaselenole and 4-phenyl-2-thioxo-1,3-dithiole

Reaction of ethyl propiolate with 4-phenyl-2-thioxo-1,3-dithiole afforded 4-carbethoxy-2-thioxo-1,3-dithiole in high yield. Reaction of ethyl propiolate with 5-phenyl-2-thioxo-1,3-thiaselenole gave 4-carbethoxy-2-thioxo-1,3-thiaselenone (IX), 4-carbethoxy-2-selenoxo-1,3-dithiole (X) and 5-carbethoxy-2-thioxo-1,3-thiaselenole (XI). A possible mechanism for the formation of these compounds is given.     

Mono‐ and Bis(tetrathiafulvalene)‐1,3,5‐Triazines as Covalently Linked Donor–Acceptor Systems: Structural,Spectroscopic, and Theoretical Investigations

Reaction of 2,4,6‐trichloro‐1,3,5‐triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono‐ and bis(TTF)–triazines as new covalently linked (multi)donor–acceptor systems. Single‐crystal X‐ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)–triazine compound, while mixed TTF–triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time‐dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)–triazine is paramagnetic, but no spin–spin exchange interaction could be detected.     

Solution Structure of One-Electron Reduced and Oxidized Molecules with Twisted Donor and Acceptor Moieties

The solution structures of the radical anion and the radical cation of the donor-acceptor molecules 3,4-di(1,3-dithiol-2-ylidene)-4-phenylbut-1-ene-1,1-dicarbonitrile ( 1 ) and 3,4-di(1,3-dithiol-2-ylidene)-4-phenylbut-1-ene-1,1,2-tricarbonitrile ( 2 ) are discussed based on cyclovoltammetric and ESR/ENDOR measurements. It is shown that the spin population of the radical anions is limited to the di- and tricyanoethene moiety and the coplanar 1,3-dithiole at C(3), whereas that of the radical cations resides mainly inside the two 1,3-dithiole rings. The energies of the long-wave bands in the electronic-absorption spectra of 1 and 2 correspond to the differences between the oxidation and reduction potentials and thus point to a charge-transfer character of these transitions.