Structural and Physical Properties of a Series of 7,7,8,8-Tetracyanoquinodimethane Salts with Dications Bearing Two Terminal Pyridinium Rings

A series of radical anion salts composed of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and dications with -C_<i>n</i>H_2<i>n</i>- (<i>n</i> = 1–6) chains between two pyridinium rings are synthesized. The dication and TCNQ formed either 1:4 or 1:3 stoichiometries with no correlation between the composition and <i>n</i>. The 1:4 stoichiometry salts were obtained when <i>n</i> = 1, 3, 4, 5, 6, where TCNQ formed a one-dimensional column with either dimerization or tetramerization. In these salts, the individual dication length varied considerably, but the effective dication length (the distance between two adjacent dications in the lattice) was independent of <i>n</i>. For <i>n</i> = 1, solvent molecules were involved. However, the 1:4 stoichiometry could not be formed for <i>n</i> = 2 since an appropriate effective dication length could not be attained. The 1:3 stoichiometry salts were obtained when <i>n</i> = 2 and 6. In both crystals, TCNQ assembled into a trimer. For both stoichiometries, the crystals were moderately conducting (resistivity at room temperature: 10¹–10⁴ Ω cm) and had a semiconducting temperature dependence, demonstrating nonuniform TCNQ stacking. The charge-localized feature was revealed by the magnetic properties. All of the salts showed antiferromagnetic interactions between the localized spins with varying exchange interaction parameters, reflecting the structural features

Carrier Dynamics in a Series of Organic Magnetic Superconductors

Understanding how electrons behave, i.e., carrier dynamics, in superconductors is indispensable for understanding the mechanism of superconductivity. Recently we have reported the carrier dynamics of κ- and λ-(BETS)₂MCl₄ (BETS = bis(ethylenedithio)tetraselenafulvalene, M = Ga, Fe) based on ultrafast spectroscopy, but the interpretation of the results remains an open question. In this paper we interpreted the results with the aid of newly measured magnetic susceptibility, X-ray single crystal structural analysis, and band calculation. Observation of coherent phonons only in the λ-type salts indicated that <i>e</i>-ph interaction should be characteristically strong in the λ-type salts. By comparison of the observed carrier dynamics with a two-temperature model, the temperature-dependence of carrier dynamics is consistently explained by the different strengths of the <i>e</i>-ph interaction between the λ- and the κ-type salts. The difference in strength of the <i>e</i>-ph interactions is related to their crystal structures. In conclusion, their carrier dynamics is consistently interpreted and classified by their crystal structures

Molecular Motion, Dielectric Response, and Phase Transition of Charge-Transfer Crystals: Acquired Dynamic and Dielectric Properties of Polar Molecules in Crystals

Molecules in crystals often suffer from severe limitations on their dynamic processes, especially on those involving large structural changes. Crystalline compounds, therefore, usually fail to realize their potential as dielectric materials even when they have large dipole moments. To enable polar molecules to undergo dynamic processes and to provide their crystals with dielectric properties, weakly bound charge-transfer (CT) complex crystals have been exploited as a molecular architecture where the constituent polar molecules have some freedom of dynamic processes, which contribute to the dielectric properties of the crystals. Several CT crystals of polar tetrabromophthalic anhydride (TBPA) molecules were prepared using TBPA as an electron acceptor and aromatic hydrocarbons, such as coronene and perylene, as electron donors. The crystal structures and dielectric properties of the CT crystals as well as the single-component crystal of TBPA were investigated at various temperatures. Molecular reorientation of TBPA molecules did not occur in the single-component crystal, and the crystal did not show a dielectric response due to orientational polarization. We have found that the CT crystal formation provides a simple and versatile method to develop molecular dielectrics, revealing that the molecular dynamics of the TBPA molecules and the dielectric property of their crystals were greatly changed in CT crystals. The TBPA molecules underwent rapid in-plane reorientations in their CT crystals, which exhibited marked dielectric responses arising from the molecular motion. An order–disorder phase transition was observed for one of the CT crystals, which resulted in an abrupt change in the dielectric constant at the transition temperature

What Happens at the Interface between TTF and TCNQ Crystals (TTF = Tetrathiafulvalene and TCNQ = 7,7,8,8-Tetracyanoquinodimethane)?

The interface between tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) crystals was prepared by treating a TCNQ single crystal surface with TTF powder. Optical measurements and atomic force microscopy (AFM) observation of the interface indicated that not only are TTF–TCNQ nanocrystals formed at the interface, but also direct charge injection from TTF powder to the TCNQ single crystal surface may be responsible for the high conductivity of the interface

Electrochemical Crystallization of Organic Molecular Conductors: Electrode Surface Conditions for Crystal Growth

An electrochemical cell designed to allow in situ observation of crystal growth of organic conductors was fabricated, and the initial growth process of a partially oxidized salt of bis­(ethylenedithio)­tetrathiafulvalene (ET) was investigated for different current values. The shape, number, and size of the single crystals grown on the electrode depended on the current. However, our results suggested that the presence of trap sites on the electrode surfaces, which promote crystal nucleation, is more important for crystal growth, since flawless surfaces prepared by melting the electrode gave no crystals. The flawless electrode modified with an alkanethiol self-assembled monolayer (SAM) did not give crystals, while with a terminal carboxylate SAM, it yielded quality crystals over the entire surface, indicating that, when modified by anionic species, the electrode surfaces promote clustering of the oxidized ET radical cations and succeed in nucleating crystal growth of the partially oxidized salt

Band-Like Carrier Transport at the Single-Crystal Contact Interfaces between 2,5-Difluoro-7,7,8,8-tetracyanoquinodimethane and Electron Donors

Some heterojunction interfaces formed with molecular solids show metal-like transport behavior. In order to clarify the requirement, interfaces are fabricated by lamination of single-crystal electron-accepting 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F₂TCNQ) and electron-donating molecules with a wide range of ionization potentials. Carrier injection between the acceptor and donor crystals leads to highly conducting interfaces, some of which exhibited band-like charge transport behaviors. Combinations with weak donors also resulted in interfaces with band-like transport properties. Accordingly, band-like conduction was achieved for interfaces where the donor and acceptor crystals do not have well-matched band energies. The results indicate that the wide range of candidates have great potential for modification of the electronic structure of organic crystals. The present method is expected to enable control of the electronic properties of the interface

Charge Carrier Doping into the Peierls Insulator of the TCNQ Anion Radical Salt (TCNQ = 7,7,8,8-Tetracyanoquinodimethane)

Hole-doping into K–TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) crystals with segregated TCNQ anion radical columns with dimeric deformation (Peierls state) has been performed by a contact doping method using F₄TCNQ (F₄TCNQ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) crystals or powder. The sheet resistance of the K–TCNQ surface has been found to decrease by the F₄TCNQ contact. Formation of K–F₄TCNQ nanocrystals at the contact interface has been observed, but conductive AFM images indicate that current paths form along the hole-doped K–TCNQ surface. Interestingly, hole-doping into K–TCNQ suppresses the phase transition to the high-temperature phase (Mott insulator). This is considered to result from the energy gain by the delocalization of the doped carriers

Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8-Tetracyanoquinodimethane Single-Component and Charge-Transfer Complex Single Crystals: Nucleation, Crystal Growth, Morphology, and Charge Transport

Electrically conducting TTF–TCNQ thin films are fabricated on various molecular crystals containing 7,7,8,8-tetracyanoquinodimethane (TCNQ) by exposing a tetrathiafulvalene (TTF) vapor under ambient conditions. To systematically investigate the properties of the films, mixed-stack TCNQ charge-transfer (CT) complex crystals with nine kinds of donors have been prepared as the substrates, and the morphology change of the films on the surfaces at the initial stage of the TTF vapor contact has been observed. When the substrate is a TCNQ single-component crystal, randomly oriented TTF–TCNQ nanometer-size needle crystals are grown by the reaction with a TTF vapor. However, when the substrate is a TCNQ CT complex crystal, TTF–TCNQ crystals are grown with alignment of their needle axis along the mixed-stack direction of the substrate. The surface roughness, the size of the needle crystals, and the degree of the dense packing of the needles have been found to systematically depend on the strength of the CT interactions in the substrate, and the sheet resistance also exhibits a systematic change. The resistance drop is rapid and remarkable when the donor of the substrate CT complex is weak. The difference in the morphology and the properties is considered to arise from the difference in the ease of nucleus formation and the rate of crystal growth of the TTF–TCNQ nanocrystals

Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8-Tetracyanoquinodimethane Single-Component and Charge-Transfer Complex Single Crystals: Nucleation, Crystal Growth, Morphology, and Charge Transport

Electrically conducting TTF–TCNQ thin films are fabricated on various molecular crystals containing 7,7,8,8-tetracyanoquinodimethane (TCNQ) by exposing a tetrathiafulvalene (TTF) vapor under ambient conditions. To systematically investigate the properties of the films, mixed-stack TCNQ charge-transfer (CT) complex crystals with nine kinds of donors have been prepared as the substrates, and the morphology change of the films on the surfaces at the initial stage of the TTF vapor contact has been observed. When the substrate is a TCNQ single-component crystal, randomly oriented TTF–TCNQ nanometer-size needle crystals are grown by the reaction with a TTF vapor. However, when the substrate is a TCNQ CT complex crystal, TTF–TCNQ crystals are grown with alignment of their needle axis along the mixed-stack direction of the substrate. The surface roughness, the size of the needle crystals, and the degree of the dense packing of the needles have been found to systematically depend on the strength of the CT interactions in the substrate, and the sheet resistance also exhibits a systematic change. The resistance drop is rapid and remarkable when the donor of the substrate CT complex is weak. The difference in the morphology and the properties is considered to arise from the difference in the ease of nucleus formation and the rate of crystal growth of the TTF–TCNQ nanocrystals

Charge Conduction Properties at the Contact Interface between (Phthalocyaninato)nickel(II) and Electron Acceptor Single Crystals

Single-component crystals of both (phthalocyaninato)­nickel­(II) (Ni­(<i>Pc</i>)) and 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F₂TCNQ) are typical band insulators. However, the contact interface between them demonstrates metal-like transport properties. Although Ni­(<i>Pc</i>) and F₂TCNQ are an electron donor and an acceptor, respectively, the combination of these two components does not yield any charge transfer (CT) complex crystals. Infrared spectra show that the highly conductive feature originates from charge injection at the contact interface. The thermoelectric power of the mixed powder reveals that the transport at the contact interface is dominated by the holes in the Ni­(<i>Pc</i>) crystal