Modern History of Organic Conductors: An Overview

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

Quantum spin-liquid states in an organic magnetic layer and molecular rotor hybrid

The exotic properties of quantum spin liquids (QSLs) have continually been of interest since Anderson's 1973 ground-breaking idea. Geometrical frustration, quantum fluctuations, and low dimensionality are the most often evoked material's characteristics that favor the long-range fluctuating spin state without freezing into an ordered magnet or a spin glass at low temperatures. Among the few known QSL candidates, organic crystals have the advantage of having rich chemistry capable of finely tuning their microscopic parameters. Here, we demonstrate the emergence of a QSL state in [EDT-TTF-CONH₂]₂ ⁺[[Formula: see text]] (EDT-BCO), where the EDT molecules with spin-1/2 on a triangular lattice form layers which are separated by a sublattice of BCO molecular rotors. By several magnetic measurements, we show that the subtle random potential of frozen BCO Brownian rotors suppresses magnetic order down to the lowest temperatures. Our study identifies the relevance of disorder in the stabilization of QSLs.

Charge localization in 1D tetramerized organic conductors: the special case of (tTTF)2ClO4

We report a detailed structural and spectroscopic study of the 1D 2:1 cation radical salt (tTTF)₂ClO₄, where tTTF  =  trimethylenetetrathiafulvalene, which exhibits a semiconductor-semiconductor phase transition at ca. T  =  137 K. Crystal structures are determined above and below the transition; the tTTF molecules in stacks are grouped into weakly interacting tetramers. The reorganization of tTTF stacks is accompanied with an order-disorder transition in anion sublattice. Polarized infrared and Raman spectra of (tTTF)₂ClO₄ are measured in the broad frequency range as a function of the temperature (10-293 K). The structural and vibrational features are investigated to elucidate the origin of the semiconductor-semiconductor phase transition. We discuss the electron-intramolecular vibration coupling effects in the vibrational spectra of (tTTF)₂ClO₄ and identify signatures of high- and low-temperature states of charge localization in the tetramerized system. Both the C=C and C-S stretching modes of tTTF give evidence of strong charge distribution fluctuations in conducting stacks for T  >  137 K, which are responsible for the appearance of molecules with charge  +1e, and charge localization in tTTF tetramers for T  <  137 K. The uniqueness of the salt (tTTF)₂ClO₄ in comparison with other tetramerized 1D systems is discussed.

Static Modulation Wave of Arrays of Halogen Interactions Transduced to a Hierarchy of Nanoscale Change Stimuli of Crystalline Rotors Dynamics

Here we present a study where what can be seen as a static modulation wave encompassing four successive arrays of interacting iodine atoms in crystalline 1,4-Bis((4'-(iodoethynyl)phenyl) ethynyl)bicyclo[2,2,2]octane rotors changes the structure from one-half molecule to three-and-a-half molecules in the asymmetric unit below a phase transition at 105 K. The remarkable finding is that the total ¹H spin-lattice relaxation rate, T₁^-1, of unprecedented complexity to date in molecular rotors, is the weighted sum of the relaxation rates of the four contributing rotors relaxation rates, each with distinguishable exchange frequencies reflecting Arrhenius parameters with different activation barriers ( E_a) and attempt frequencies (τ_o^-1). This allows us to show in tandem with rotor-environment interaction energy calculations how the dynamics of molecular rotors are able to decode structural information from their surroundings with remarkable nanoscale precision.

Reversible phase transition and switchable dielectric behaviors triggered by rotation and order-disorder motions of crowns

Compound 1 shows apparent step-like dielectric changes near the phase transition, which are triggered by the synergetic rotation motion of 18-C-6 crown ether cycles and hexafluorophosphate anions.

Correlating conduction properties with the molecular symmetry: segregation of Z and E isomers in the charge-assisted, halogen-bonded cocrystal [(Z,E)-Me2I2TTF]2Br

Co-crystallization of theZandEisomers of Me₂I₂TTF in a mixed-valence bromide salt leads to segregated stacks with two different charge order patterns and associated charge-assisted halogen bonding.

Structural and electronic control of the metal to insulator transition and local orderings in the θ-(BEDT-TTF)2X organic conductors

A first-principles density functional theory (DFT) study of [Formula: see text]-(BEDT-TTF)2X molecular conductors with X  =  I3, CsCo(SCN)4 (ambient pressure, 7.5 kbar and 10 kbar), CsZn(SCN)4, TlCo(SCN)4, RbCo(SCN)4 and RbZn(SCN)4 (220 K and 90 K) is reported. It is shown that these salts exhibit three different types of band structure each of them associated with a different physical behavior. In contrast with previous proposals it is found that the key electronic parameter behind the differences in the band structures is the intrastack transfer integral, t c . A new mechanism for the metal to insulator transition in the [Formula: see text]-(BEDT-TTF)2MM'(SCN)4 ([Formula: see text], Tl; [Formula: see text], Co) salts is proposed, where an order-disorder structural transition of the ethylenedithio groups doubling the periodicity along the stack direction drives the system into an electronically pseudo-1D system along the interstack direction that is subject to a 4k F charge localization of holes. The structural rearrangement is such that the holes are not distributed equally between the two donors; the larger hole density is associated with the B donors which establish the strongest hydrogen bonds with the anion layers. A detailed microscopic description of how disorder of the ethylenedithio groups, the θ dihedral angle and the electronic structure intermingle and lead to the unusual phase diagram of these salts is presented. In this framework the role of pressure and uniaxial strain in controlling the physical behavior of these salts is discussed.

Dielectric spectroscopy on organic charge-transfer salts

This topical review provides an overview of the dielectric properties of a variety of organic charge-transfer salts, based on both, data reported in literature and our own experimental results. Moreover, we discuss in detail the different processes that can contribute to the dielectric response of these materials. We concentrate on the family of the 1D (TMTTF)2 X systems and the 2D BEDT-TTF-based charge-transfer salts, which in recent years have attracted considerable interest due to their often intriguing dielectric properties. We will mainly focus on the occurrence of electronic ferroelectricity in these systems, which also includes examples of multiferroicity.

Exploring the electronic structure of an organic semiconductor based on a compactly fused electron donor-acceptor molecule

A Mott-type semiconductor based on a compactly fused and partially oxidized electron donor-acceptor (D-A) molecule was recently prepared and identified to exhibit a large room-temperature conductivity of 2 S cm(-1) . In a marked contrast to the organic conductors characterized by relatively well decoupled and segregated uniform stacks of D and A moieties, the formally half-oxidized tetrathiafulvalene donors of the actual compound are organized in columnar π stacks only, whereby the coplanar electron-acceptor units, namely benzothiadiazole, are closely annulated along their ridges. Herein, we present a theoretical study that explores the electronic structure of this novel type of organic semiconductor. The highly symmetric-solid state material behaves as a one-dimensional electronic system with strong antiferromagnetic interactions (coupling constant>200 cm(-1) ). The unique shape and local dipole of this redox-active fused electron D-A molecule lays the basis for further investigations of the collective electronic structure, mainly in the function of different counterions embedded in the crystalline lattice.

Characterization of the quasi-one-dimensional compounds δ-(EDT-TTF-CONMe2)2X, X=AsF6 and Br by vibrational spectroscopy and density functional theory calculations

We have investigated the infrared spectra of the quarter-filled charge-ordered insulators δ-(EDT-TTF-CONMe2)2X (X= AsF6, Br) along all three crystallographic directions in the temperature range from 300 to 10 K. DFT-assisted normal mode analysis of the neutral and ionic EDT-TTF-CONMe2 molecule allows us to assign the experimentally observed intramolecular modes and to obtain relevant information on the charge ordering and intramolecular interactions. From frequencies of charge-sensitive vibrations we deduce that the charge-ordered state is already present at room temperature and does not change on cooling, in agreement with previous NMR measurements. The spectra taken along the stacking direction clearly show features of vibrational overtones excited due to the anharmonic electronic molecule potential caused by the large charge disproportionation between the molecular sites. The shift of certain vibrational modes indicates the onset of the structural transition below 200 K.

Charge-ordering transition in (TMTTF)2X explored via dilatometry

Charge-ordering phenomena have been highly topical over the past few years. A phase transition towards a charge-ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one-dimensional organic charge-transfer salts (TMTTF)2X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF6, AsF6 and SbF6). However, the relationship between the electron localization phenomena and the role of the lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the literature. Here we present a brief overview of selected literature results, with emphasis placed on recent thermal expansion experiments probing the charge-ordering transition of these salts.