Narcis Avarvari

Strategies towards chiral molecular conductors

Introduction of chirality into conducting systems is a topic of much current interest as it allows the preparation of multifunctional materials in which the chirality may modulate the structural disorder or expresses its influence through the electrical magneto-chiral anisotropy effect. The access to various chiral electroactive precursors for molecular conductors is therefore of paramount importance. Different families of chiral tetrathiafulvalene (TTF) derivatives are reviewed together with the corresponding synthetic strategies. Systems based on stereogenic carbon or sulfur atoms, axial and supramolecular chirality have been developed. In other systems the use of achiral TTFs with chiral anions has provided molecular conductors, although in most of them to date the anion is present in racemic form. Starting from some of these precursors several chiral conductors have been prepared and in two cases, involving either chiral TTF-oxazoline salts or BEDT-TTF salts with metal-oxalate anions and chiral solvent molecules, differences between the conductivity of the racemic and enantiopure forms have been found, as a consequence of the structural disorder in the solid state. Further developments in this field are expected to be directed especially towards helical architectures, possibly based on supramolecular chirality, and systems combining conductivity, magnetism and chirality in both organic and inorganic lattices.

Electrical magnetochiral anisotropy in a bulk chiral molecular conductor

So far, no effect of chirality on the electrical properties of bulk chiral conductors has been observed. Introduction of chiral information in tetrathiafulvalene precursors represents a powerful strategy towards the preparation of crystalline materials in which the combination of chirality and conducting properties might allow the observation of the electrical magnetochiral anisotropy effect. Here we report the synthesis by electrocrystallization of both enantiomers of a bulk chiral organic conductor based on an enantiopure tetrathiafulvalene derivative. The enantiomeric salts crystallize in enantiomorphic hexagonal space groups. Single crystal resistivity measurements show metallic behaviour for the enantiopure salts down to 40 K, in agreement with band structure calculations. We describe here the first experimental evidence of electrical magnetochiral anisotropy in these crystals, confirming the chiral character of charge transport in our molecular materials.

Tetrathiafulvalene based phosphino-oxazolines: a new family of redox active chiral ligands

Reaction of the lithium salt of EDT-TTF-2-(4-methyl)oxazoline with chloro-diphenylphosphine afforded the novel redox active chiral chelating ligands, EDT-TTF-phosphino-oxazolines, for which a palladium (II) dichloride complex was synthesized and structurally characterized.

Chirality driven metallic versus semiconducting behavior in a complete series of radical cation salts based on dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF).

Enantiopure (S,S) and (R,R) dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF) 1 donors are synthesized by cross coupling followed by decarboxylation reactions. In the solid state the methyl groups are arranged in axial positions within sofa-type conformation for the six-membered rings. Crystalline radical cation salts formulated as [(S,S)-1]2PF6, [(R,R)-1]2PF6, and [(rac)-1]2PF6 are obtained by electrocrystallization. When the experiment is conducted with enantioenriched mixtures both enantiopure and racemic phases are obtained. The monoclinic enantiopure salts, containing four independent donors in the unit cell, show semiconducting behavior supported by band structure calculations of extended Hückel type. The racemic salt contains only one independent donor in the mixed valence oxidation state +0.5. Under ambient pressure the racemic material is metallic down to 120 K, while an applied pressure of 11.5 kbar completely suppresses the metal-insulator transition. Band structure calculations yield an open Fermi surface, typical for a pseudo-one-dimensional metal, with unperfected nesting, thus ruling out the possibility of charge or spin density modulations to be at the origin of the transition. Raman spectroscopy measurements, in agreement with structural analysis at 100 K, show no indication of low-temperature charge ordering in the racemic material at ambient pressure, thus suggesting Mott-type charge localization for the observed metal-insulator transition.

Structural and electrochemical study of metal carbonyl complexes with chelating bis- and tetrakis(diphenylphosphino)tetrathiafulvalenes

Abstract Several mono- and bimetallic complexes involving organometallic fragments such as M(CO)4 (M=Mo, W), Re(CO)3Cl and Fe(CO)3 coordinated to the chelating diphosphine 3,4-dimethyl-3′,4′-bis(diphenylphosphino)tetrathiafulvalene (P2) or tetraphosphine tetrakis(diphenylphosphino)tetrathiafulvalene (P4) have been synthesized and characterized. X-ray diffraction structures have been determined for P4[W(CO)4]2, P2Re(CO)3Cl, P2Fe(CO)3 and P4[Fe(CO)3]2. Octahedral geometries around the metallic center are observed for the tungsten and rhenium complexes, with a fac arrangement of the ligands for the latter, whereas slightly distorted trigonal bipyramid are found for both iron counter parts. Although the complexes are more difficult to oxidize than the corresponding free phosphines, as evidenced by cyclic voltammetry measurements, the first oxidation potentials remain in the usual range, thus offering the opportunity to generate stable radical cation salts. In the case of Fe complexes, the first oxidation wave corresponds to the formation of paramagnetic d7 Fe(I) species, whereas in the Mo, W and Re complexes the metallic center is much more difficult to oxidize than the TTF core.

First cation radical salt of a tetrathiafulvalene-based phosphine metal complex

Electrocrystallization of (o-P2)Mo(CO)4, where o-P2 is the tetrathiafulvalene-based chelating diphosphine o-Me2TTF-(PPh2)2, in the presence of the Linquist anion Mo6O192− affords a 2:1 salt, formulated as [(o-P2)Mo(CO)4]2[Mo6O19], with a Heisenberg spin chain magnetic behaviour.

Supramolecular electroactive organogel and conducting nanofibers with C3-symmetrical architectures

Two C3 symmetric tris(TTF) derivatives, based on a central unit containing the rigid core 1,3,5-tricarbonyl-benzene substituted with three 3,3′-diamino-2,2′-bipyridines, have been synthesized by a convergent strategy. Single crystal X-ray analysis of the precursor 3′-[(ethylenedithio-tetrathiafulvalenyl)formylamino]-2,2′-bipyridine-3-amine shows a planar transoid conformation for the bipyridine unit, favored by intramolecular hydrogen bonding. The compound N,N′,N″-tris{3[3′-[bis(ethylthio)-tetrathiafulvalenyl]formylamino]-2,2′-bipyridyl}-benzene-1,3,5-tricarboxamide, having C3 symmetry, presents gelator properties in chlorinated solvents. The gel formed in ortho-dichlorobenzene provided—after evaporation of the solvent—a xerogel constituted by a complex network of thick and thinner fibers as demonstrated by TEM and AFM microscopies. The thick fibers were about 100 nm wide and between 1 and 5 µm long, and the thinner ones between 12 and 18 nm wide and 50 to 500 nm long. Iodine doping of the material induced the formation of a mixed valence system with charge transfer, as indicated by IR-NIR spectroscopic measurements. The doped material has a TTF:I3 ratio of 2.4:1 ten minutes after doping, but slowly loses iodine over days. The morphology of the gel did not change after the doping process, as revealed by SEM and AFM experiments. Current sensing AFM measurements showed that the thicker fibers are more conducting than the thinner ones, a likely consequence of the better ordering and/or more effective interfiber contacts in the former.

Ethylenedithio-tetrathiafulvalene-helicenes: electroactive helical precursors with switchable chiroptical properties.

Electroactive fused ethylenedithio-tetrathiafulvalene-[4]helicene and -[6]helicenes have been synthesized through a strategy that involved the preparation of 2,3-dibromo-helicene derivatives as intermediates. The dihedral angles between the terminal helicenes, as determined by single-crystal X-ray analysis, are 22.7° and 50.7° for the [4]helicene and [6]helicene, respectively. Their solid-state architectures show interplay between S⋅⋅⋅S and π⋅⋅⋅π intermolecular interactions. The chiroptical properties of the enantiopure EDT-TTF-[6]helicene derivatives have been investigated and supported by TDDFT calculations. Remarkable redox switching of the circular dichroism (CD) signal between the neutral and radical-cation species has been achieved.

Order versus disorder in chiral tetrathiafulvalene-oxazoline radical-cation salts: structural and theoretical investigations and physical properties.

Electrocrystallization experiments with the chiral ethylenedithio-tetrathiafulvalene-methyl-oxazoline (EDT-TTF-OX) donors (R)-, (S)-, and (rac)-1 have provided two series of mixed-valence salts with the PF(6) (-) and [Au(CN)(2)](-) anions. Within each series the cell parameters are the same for the three R, S, and rac compounds, except for the space group, which is centrosymmetric triclinic P

TetrathiafulvaleneBenzothiadiazoles as Redox‐Tunable Donor–Acceptor Systems: Synthesis and Photophysical Study

\bar 1