Strategies towards chiral molecular conductors

Abstract

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 have 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-oxazolines 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.2

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…Typically, the structure of these materials is formed by segregated stacks of the organic donors and the inorganic counterions which add the second functionality to the conducting material. [143], which shows a high room-temperature conductivity (ca. 10 S cm −1 ) and a metallic behavior down to ca.…”

Electrical magnetochiral anisotropy in a bulk chiral molecular conductor

“…Nevertheless, as mentioned earlier, no such evidence has been provided so far in bulk conductors, including the TTF-based materials, although the influence of chirality on the conducting properties of TTF conductors has been indirectly expressed through the modulation of the structural disorder in a few series of conducting radical cation salts based on chiral TTFoxazoline 17,18 or TM-BEDT-TTF donors 19 , in which the enantiopure materials were more conducting than the racemic ones because of the structural disorder present in the latter. Compared with the relatively large number of chiral TTF precursors 16,20 , only a few examples of derived conducting materials have been described, such as those based on TM-BEDT-TTF 13,19,21 dimethyl-bis(ethylenedithio)-TTF (DM-BEDT-TTF) 22,23 , or TTF-oxazoline 17,18 . Moreover, complete series of conducting salts containing both enantiomers and the racemic forms, in order to compare their conducting properties, are even more scarce [17][18][19] , and in all of them the enantiomeric pairs crystallize within the same space group, although there is in principle the possibility that enantiomers crystallize in one of the 11 enantiomorphic space groups, which remains, however, a relatively rare event.…”

“…Although the first chiral TTF, namely the (S,S,S,S) enantiomer of tetramethyl-bis(ethylenedithio)-TTF (TM-BEDT-TTF), has been synthesized in the middle 80s (ref. 15), only much more recently there has been an increasing interest in the introduction of chiral information in TTF derivatives 16 , mainly motivated by the possible detection of the eMChA effect. Nevertheless, as mentioned earlier, no such evidence has been provided so far in bulk conductors, including the TTF-based materials, although the influence of chirality on the conducting properties of TTF conductors has been indirectly expressed through the modulation of the structural disorder in a few series of conducting radical cation salts based on chiral TTFoxazoline 17,18 or TM-BEDT-TTF donors 19 , in which the enantiopure materials were more conducting than the racemic ones because of the structural disorder present in the latter.…”

Enantiopure Radical Cation Salt Based on Tetramethyl-Bis(ethylenedithio)-Tetrathiafulvalene and Hexanuclear Rhenium Cluster

“…The latter was also described as (R) enantiomer and racemate. The interest in chiral TTF precursors and derived materials [8] is mainly related to the combination of chirality with conducting properties through the electrical magneto-chiral anisotropy effect [9], recently described for enantiopure crystalline metallic salts of the dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF) donor [10]. Nevertheless, differences in conducting properties between the enantiopure and racemic (S)-1 represents the first example of a chiral TTF derivative [4], and its synthesis allowed the preparation of several TTF based chiral conducting radical cation salts by electrocrystallization [1,5] [7].…”