Orbital hybridization of donor and acceptor to enhance the conductivity of mixed-stack complexes

Mixed-stack complexes which comprise columns of alternating donors and acceptors are organic conductors with typically poor electrical conductivity because they are either in a neutral or highly ionic state. This indicates that conductive carriers are insufficient or are mainly localized. In this study, mixed-stack complexes that uniquely exist at the neutral-ionic boundary were synthesized by combining donors (bis(3,4-ethylenedichalcogenothiophene)) and acceptors (fluorinated tetracyanoquinodimethanes) with similar energy levels and orbital symmetry between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Surprisingly, the orbitals were highly hybridized in the single-crystal complexes, enhancing the room-temperature conductivity (10-4-0.1 S cm-1) of mixed-stack complexes. Specifically, the maximum conductivity was the highest reported for single-crystal mixed-stack complexes under ambient pressures. The unique electronic structures at the neutral-ionic boundary exhibited structural perturbations between their electron-itinerant and localized states, causing abrupt temperature-dependent changes in their electrical, optical, dielectric, and magnetic properties.

Electron Localization Induced by Disordered Anions in an Organic Conductor

We report on a new organic conductor κ″-(ET)2Cu[N(CN)2]Br (κ″-Br), which is the first polymorph of an organic superconductor κ-(ET)2Cu[N(CN)2]Br (κ-Br), where ET denotes bis(ethylenedithio)tetrathiafulvalene. κ″-Br has a similar κ-type arrangement of ET molecules to κ-Br, but, in contrast to the orthorhombic κ-Br, which has ordered polyanion chains, presents a monoclinic crystal structure with disordered polymeric anion chains. To elucidate the electronic state of κ″-Br, we performed band calculations as well as transport, magnetic, and optical measurements. The calculated band dispersion, magnitude of electron correlation, and room-temperature optical conductivity spectra of κ″-Br were comparable to those of κ-Br. Despite these similarities, the κ″-Br salt exhibited a semiconducting behavior. The electron spin resonance and Raman spectroscopies indicated that there is neither magnetic nor charge order in κ″-Br, suggesting the occurrence of Anderson localization due to disordered anion layers.

Efficient Charge Separation and Transport in Fullerene-CuPcOC8 Donor-Acceptor Nanorod Enhancing Photocatalytic Hydrogen Generation

Photocatalytic hydrogen generation via water decomposition is a promising avenue in the pursuit of large-scale, cost-effective renewable hydrogen energy generation. However, the design of an efficient photocatalyst plays a crucial role in achieving high yields in hydrogen generation. Herein, we have engineered a fullerene-2,3,9,10,16,17,23,24-octa(octyloxy)copper phthalocyanine (C60-CuPcOC8) photocatalyst, achieving both efficient hydrogen generation and high stability. The significant donor-acceptor (D-A) interactions facilitate the efficient electron transfer from CuPcOC8 to C60. The rate of photocatalytic hydrogen generation for C60-CuPcOC8 is 8.32 mmol·g-1·h-1, which is two orders of magnitude higher than the individual C60 and CuPcOC8. The remarkable increase in hydrogen generation activity can be attributed to the development of a robust internal electric field within the C60-CuPcOC8 assembly. It is 16.68 times higher than that of the pure CuPcOC8. The strong internal electric field facilitates the rapid separation within 0.6 ps, enabling photogenerated charge transfer efficiently. Notably, the hydrogen generation efficiency of C60-CuPcOC8 remains above 95%, even after 10 h, showing its exceptional photocatalytic stability. This study provides critical insight into advancing the field of photocatalysis.

Magnetic behavior and ground spin states for coordination {L·[MII(Hal)2]3}3- assemblies (Hal = Cl or I) of radical trianion hexacyanohexaazatriphenylenes (L) with three coordinated high-spin FeII (S = 2) or CoII (S = 3/2) centers

A series of trianion assemblies of hexaazatriphenylenehexacarbonitrile {HAT(CN)6} and hexaazatrinaphthylenehexacarbonitrile {HATNA(CN)6} with three Fe(II) or Co(II) ions: {cryptand(K+)}3·{HATNA(CN)6·(FeIII2)3}3-·2C6H4Cl2 (1), {cryptand(K+)}3·{HATNA(CN)6·(CoIII2)3}3-·2C6H4Cl2 (2), and (CV+)3·{HAT(CN)6·(CoIICl2)3}3-·0.5(CVCl)·2.5C6H4Cl2 (3) are synthesized (CVCl = crystal violet). Salt 1 has a χMT value of 9.80 emu K mol-1 at 300 K, indicating a contribution of three high-spin FeII (S = 2) and one S = 1/2 of HATNA(CN)6˙3-. The χMT value increases with cooling up to 12.92 emu K mol-1 at 28 K, providing a positive Weiss temperature of +20 K. Such behavior is described using a strong antiferromagnetic coupling between S = 2 and S = 1/2 with J1 = -82.1 cm-1 and a weaker FeII-FeII antiferromagnetic coupling with J2 = -7.0 cm-1. As a result, the spins of three Fe(II) ions (S = 2) align parallel to each other forming a high-spin S = 11/2 system. Density functional theory (DFT) calculations support a high-spin state of CoII (S = 3/2) for 2 and 3. However, the χMT value of 2 and 3 is 2.25 emu K mol-1 at 300 K, which is smaller than 6 emu K mol-1 calculated for the system with three independent S = 3/2 and one S = 1/2 spins. In contrast to 1, the χMT values decrease with cooling to 0.13-0.36 emu K mol-1 at 1.9 K, indicating that spins of cobalt atoms align antiparallel to each other. Data fitting using PHI software for the model consisting of three high-spin Co(II) ions and an S = 1/2 radical ligand shows very large CoII-L˙3- coupling for 2 and 3 with J1 values of -442 and -349 cm-1. The CoII-CoII coupling via the ligand (J2) is also large, being -100 and -84 cm-1, respectively, which is more than 10 times larger than that of 1. One of the reasons for the J2 increase may be the shortening of the Co-N(L) bonds in 3 and 2 to 2.02(2) and 1.993(12) Å. DFT calculations support the population of the quartet state for the Co3 system, whereas the high-spin decet (S = 9/2) state is positioned higher by 680 cm-1 and is not populated at 300 K. This is explained by the large CoII-CoII coupling. Thus, a balance between J1 and J2 couplings provides parallel or antiparallel alignment of the FeII and CoII spins, leading to high- or low-spin ground states of {L·[MII(Hal)2]3}3-.

The role of molecular oxygen (O2) and UV light in the anion radical formation and stability of TCNQ and its fluorinated derivatives

We report the electronic absorption spectroscopy of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives (F2TCNQ and F4TCNQ), well-known electron-accepting molecules in common organic solvents (toluene, chlorobenzene, acetonitrile, and ethanol) under controlled exposure to air (O2) and UV light. All compounds (FxTCNQ (x = 0, 2, 4)) were stable in a neutral state (FxTCNQ0) in toluene and chlorobenzene, even under both O2 and UV light. On the other hand, in EtOH, the formation of FxTCNQ·− was monitored upon controlled exposure to O2 or UV light. Especially in air-equilibrated ethanol upon the UV-illumination, efficient α,α-dicyano-p-toluoylcyanide anion (DCTC−) and its fluorinated derivatives were generated evinced by the absorption peak near 480 nm, whereas the reaction was shut off by removing O2 or blocking UV light, thereby keeping FxTCNQ·− stable. However, even in deaerated ethanol, upon the UV-illumination, the anion formation of TCNQ and its fluorinated derivatives (FxTCNQ·−, x = 0, 2, 4) was inevitable, showing the stability of FxTCNQ0 depends on the choice of solvent.

Diphenoquinhydrones and Related Hydrogen-Bonded Charge-Transfer Complexes

Benzoquinone and hydroquinone cocrystallize to form quinhydrone, a 1:1 complex with a characteristic structure in which the components are positioned by hydrogen bonds and charge-transfer interactions. We have found that analogous diphenoquinhydrones can be made by combining 4,4'-diphenoquinones with the corresponding 4,4'-dihydroxybiphenyls. In addition, mixed diphenoquinhydrones can be assembled from components with different substituents, and mismatched quinhydrones can be made from benzoquinones and dihydroxybiphenyls. In all cases, the components of the resulting structures are linked in alternation by O-H···O hydrogen bonds to form essentially planar chains, which stack to produce layers in which π-donors and π-acceptors are aligned by charge-transfer interactions. Geometric parameters, computational studies, and spectroscopic properties of diphenoquinhydrones show that the key intermolecular interactions are stronger than those in simple quinhydrone analogues. These findings demonstrate that the principles of modular construction underlying the formation of classical quinhydrones can be generalized to produce a broad range of hydrogen-bonded charge-transfer materials in which the components are positioned by design.

Rational Construction of Molecular Electron-Conducting Nanowires Encapsulated in a Proton-Conducting Matrix in a Charge Transfer Salt

Insulated molecular wires have gained significant attention owing to their potential contribution in the fields of nanoelectronics and low-dimensional chemistry/physics. Based on molecular charge transfer salts, we demonstrate, for the first time, the rational construction of molecular electron-conducting wires encapsulated in a proton-conducting matrix, which possibly paves the way to ionoelectronics. As expected from the molecular structure of the newly designed complex anion (i.e., propeller-shaped structure with hydrogen-bonding sites at four edges), a three-dimensional hydrogen-bonded framework was constructed within the crystal, which contains a one-dimensional array of an electron donor, tetrathiafulvalene (TTF). From the single-crystal crystallographic and spectroscopic studies, it was clarified that the nonstoichiometric deprotonation of anions and partial oxidation of TTFs occur, whereas the anion is electronically inert. Moderate conductivities of electron and proton were confirmed by dc and ac conductivity measurements. In addition, the electronic isolation of TTF wires was confirmed by the magnetic susceptibility data.

Charge Transfer Complexes of New Sulfur- and Selenium-Rich Aromatic Donors

Two new sulfur- and selenium-rich pentacyclic aromatic compounds were prepared by short chemical syntheses. The two donors readily formed charge transfer (CT) complexes upon reaction with antimony pentachloride or tris(4-bromophenyl)ammoniumyl hexachloroantimonate. The X-ray structures of the heterocyclic donors and their CT complexes were determined. The donors flattened considerably upon CT complex formation.

Subsurface oxygen defects electronically interacting with active sites on In2O3 for enhanced photothermocatalytic CO2 reduction

Oxygen defects play an important role in many catalytic reactions. Increasing surface oxygen defects can be done through reduction treatment. However, excessive reduction blocks electron channels and deactivates the catalyst surface due to electron-trapped effects by subsurface oxygen defects. How to effectively extract electrons from subsurface oxygen defects which cannot directly interact with reactants is challenging and remains elusive. Here, we report a metallic In-embedded In₂O₃ nanoflake catalyst over which the turnover frequency of CO₂ reduction into CO increases by a factor of 866 (7615 h⁻¹) and 376 (2990 h⁻¹) at the same light intensity and reaction temperature, respectively, compared to In₂O₃. Under electron-delocalization effect of O-In-(O)V_o-In-In structural units at the interface, the electrons in the subsurface oxygen defects are extracted and gather at surface active sites. This improves the electronic coupling with CO₂ and stabilizes intermediate. The study opens up new insights for exquisite electronic manipulation of oxygen defects.

How to effectively extract electrons from subsurface oxygen defects is challenging in heterogeneous catalysis. Here the authors demonstrate that Metallic In-embedded In2O3 nanoflake catalyst promotes the delocalization of electrons among subsurface oxygen defects, obviously increasing electron density of surface active sites.

Weak Antiferromagnetic Exchange and Ferromagnetic Alignment of FeII (S=2) Spins in Differently Charged {HAT ⋅ (FeII Cl2 )3 }n (n=2- and 3-) Assemblies of Hexaazatriphenylenes (HAT)

Hexaazatrianthracene (HATA) and hexaazatriphenylenehexacarbonitrile {HAT(CN)₆ } are reduced by metallic iron in the presence of crystal violet (CV⁺ )(Cl^- ). Anionic ligands are produced, which simultaneously coordinate three Fe^II Cl₂ to form (CV⁺ )₂ {HATA ⋅ (Fe^II Cl₂ )₃ }^2-  ⋅ 3 C₆ H₄ Cl₂ (1) and (CV⁺ )₃ {HAT(CN)_6. (Fe^II Cl₂ )₃ }^3-  ⋅ 0.5CVCl ⋅ 2.5 C₆ H₄ Cl₂ (2). High-spin (S=2) Fe^II atoms in both structures are arranged in equilateral triangles at a distance of 7 Å. An antiferromagnetic exchange is observed between Fe^II in {HATA ⋅ (Fe^II Cl₂ )₃ }^2- (1) with a Weiss temperature (Θ) of -80 K, the PHI estimated exchange interaction (J) is -4.7 cm^-1 . The {HAT(CN)₆  ⋅ (Fe^II Cl₂ )₃ }^3- assembly is obtained in 2. The formation of HAT(CN)₆ ^.3- is supported by the appearance of an intense EPR signal with g=2.0037. The magnetic behavior of 2 is described by a strong antiferromagnetic coupling between the Fe^II and HAT(CN)₆ ^.3- spins with J₁ =-164 cm^-1 (-2 J formalism) and by a weaker antiferromagnetic coupling between the Fe^II spins with J₂ =-15.4 cm^-1 . The stronger coupling results in the spins of the three Fe^II Cl₂ units to be aligned parallel to each other in the assembly. As a result, an increase of the χ_M T values is observed with the decrease of temperature from 9.82 at 300 K up to 15.06 emu ⋅ K/mol at 6 K, and the Weiss temperature is also positive being at +23 K. Thus, a change in the charge and spin state of the HAT-type ligand to ⋅3^- results in ferromagnetic alignment of the Fe^II spins, yielding a high-spin (S=11/2) system. DFT calculations showed that, due to the high symmetry and nearly degenerated LUMO of both HATA and HAT(CN)₆ , their complexes with Fe^II Cl₂ have a variety of closely lying excited high-spin states with multiplicity up to S=15/2.

Molecular solid solutions for advanced materials – homeomorphic or heteromorphic

Crystalline molecular solid solutions are discussed on the basis of homeomorphism and heteromorphism of blended molecules.

Conjugation Length Effect on the Conducting Behavior of Single-crystalline Oligo(3,4-ethylenedioxythiophene) (nEDOT) Radical Cation Salts

The conjugation length is a unique structural factor for oligomer-based π-conjugated conductors as it modulates their electronic structures. Herein, we demonstrated the conjugation length effects on conductivity by comparing a...

A Mountaineering Strategy to Excited States: Highly Accurate Energies and Benchmarks for Bicyclic Systems

Pursuing our efforts to define highly accurate estimates of the relative energies of excited states in organic molecules, we investigate, with coupled-cluster methods including iterative triples (CC3 and CCSDT), the vertical excitation energies of 10 bicyclic molecules (azulene, benzoxadiazole, benzothiadiazole, diketopyrrolopyrrole, furofuran, phthalazine, pyrrolopyrrole, quinoxaline, tetrathiafulvalene, and thienothiophene). In total, we provide aug-cc-pVTZ reference vertical excitation energies for 91 excited states of these relatively large systems. We use these reference values to benchmark various wave function methods, i.e., CIS(D), EOM-MP2, CC2, CCSD, STEOM-CCSD, CCSD(T)(a)*, CCSDR(3), CCSDT-3, ADC(2), ADC(2.5), and ADC(3), as well as some spin-scaled variants of both CC2 and ADC(2). These results are compared to those obtained previously on smaller molecules. It turns out that while the accuracy of some methods is almost unaffected by system size, e.g., CIS(D) and CC3, the performance of others can significantly deteriorate as the systems grow, e.g., EOM-MP2 and CCSD, whereas others, e.g., ADC(2) and CC2, become more accurate for larger derivatives.

Alkylated Benzodithienoquinolizinium Salts as Possible Non-Fullerene Organic N-Type Semiconductors: An Experimental and Theoretical Study

Three photobicyclized benzodithienoquinolizinium tetrafluoroborates (BPDTQBF4) were prepared and evaluated by UV-Vis and fluorescence spectral, electrochemical analysis, and by theoretical calculations as possible organic n-type semiconductors. Evaluation and comparison of their LUMO levels, HOMO-LUMO energy gaps as monomeric and π-stacked dimers with those of other materials, suggest their potential as organic n-type semiconductors. Calculations of their relative charge carrier mobilities confirmed this potential for one derivative with a long (C-14) alkyl chain appended to the polycyclic planar π-system.

Optical response of laser-driven charge-transfer complex described by Holstein-Hubbard model coupled to heat baths: Hierarchical equations of motion approach

We investigate the optical response of a charge-transfer complex in a condensed phase driven by an external laser field. Our model includes an instantaneous short-range Coulomb interaction and a local optical vibrational mode described by the Holstein-Hubbard (HH) model. Although characterization of the HH model for a bulk system has typically been conducted using a complex phase diagram, this approach is not sufficient for investigations of dynamical behavior at finite temperature, in particular for studies of nonlinear optical properties, where the time irreversibility of the dynamics that arises from the environment becomes significant. We therefore include heat baths with infinite heat capacity in the model to introduce thermal effects characterized by fluctuation and dissipation to the system dynamics. By reducing the number of degrees of freedom of the heat baths, we derive numerically "exact" hierarchical equations of motion for the reduced density matrix of the HH system. As demonstrations, we calculate the optical response of the system in two- and four-site cases under external electric fields. The results indicate that the effective strength of the system-bath coupling becomes large as the number of sites increases. Excitation of electrons promotes the conductivity when the Coulomb repulsion is equivalent to or dominates the electron-phonon coupling, whereas excitation of optical vibrations always suppresses the conductivity.

Conductive Metal-Organic Frameworks: Electronic Structure and Electrochemical Applications

Smarter and minimization of devices are consistently substantial to shape the energy landscape. Significant amounts of endeavours have come forward as promising steps to surmount this formidable challenge. It is undeniable that material scientists were contemplating smarter material beyond purely inorganic or organic materials. To our delight, metal-organic frameworks (MOFs), an inorganic-organic hybrid scaffold with unprecedented tunability and smart functionalities, have recently started their journey as an alternative. In this review, we focus on such propitious potential of MOFs that was untapped over a long time. We cover the synthetic strategies and (or) post-synthetic modifications towards the formation of conductive MOFs and their underlying concepts of charge transfer with structural aspects. We addressed theoretical calculations with the experimental outcomes and spectroelectrochemistry, which will trigger vigorous impetus about intrinsic electronic behaviour of the conductive frameworks. Finally, we discussed electrocatalysts and energy storage devices stemming from conductive MOFs to meet energy demand in the near future.

Dynamics of proton, ion, molecule, and crystal lattice in functional molecular assemblies

Dynamic molecular processes, such as short- or long-range proton (H⁺) and ion (M⁺) motions, and molecular rotations in electrical conducting and magnetic molecular assemblies enable the fabrication of electron-H⁺ (or M⁺) coupling systems, while crystal lattice dynamics and molecular conformation changes in hydrogen-bonded molecular crystals have been utilised in external stimuli responsive reversible gas-induced gate opening and molecular adsorption/desorption behavior. These dynamics of the polar structural units are responsible for the dielectric measurements. The H⁺ dynamics are formed from ferroelectrics and H⁺ conductors, while the dynamic M⁺ motions of Li⁺ and Na⁺ involve ionic conductors and coupling to the conduction electrons. In n-type organic semiconductors, the crystal lattices are modulated by replacing M⁺ cations, with cations such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The use of polar rotator or inversion structures such as alkyl amides, m-fluoroanilinium cations, and bowl-shaped trithiasumanene π-cores enables the formation of ferroelectric molecular assemblies. The host-guest molecular systems of ESIPT fluorescent chromic molecules showed interesting molecular sensing properties using various bases, where the dynamic transformation of the crystal lattice and the molecular conformational change were coupled to each other.

A Full-Spectrum Porphyrin-Fullerene D-A Supramolecular Photocatalyst with Giant Built-In Electric Field for Efficient Hydrogen Production

A full-spectrum (300-850 nm) responsive donor-acceptor (D-A) supramolecular photocatalyst tetraphenylporphinesulfonate/fullerene (TPPS/C₆₀ ) is successfully constructed. The theoretical spectral efficiency of TPPS/C₆₀ is as high as 70%, offering the possibility of full-solar-spectrum light harvesting. The TPPS/C₆₀ performs a highly efficient photocatalytic H₂ evolution rate of 276.55 µmol h^-1 (34.57 mmol g^-1 h^-1 ), surpassing many reported organic photocatalysts. The D-A structure effectively promotes electron transfer from TPPS to C₆₀ , which is beneficial to the photocatalytic reaction. Specifically, a giant internal electric field in the D-A structure is built via the enhanced molecular dipole, which dramatically promotes the charge separation (CS) efficiency by 2.35 times. Transient absorption spectra results show a long-lived CS state TPPS^•+ -C₆₀ ^•- in the D-A structure, which effectively promotes participation of photogenerated electrons in the reduction reaction. Briefly, this work provides a novel approach for designing high-performance photocatalytic materials via enhancing the interfacial electric field.

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.

Absence of HOMO/LUMO Transition in Charge-Transfer Complexes of Thienoacenes

In charge-transfer complexes, transition from the donor highest occupied molecular orbital (HOMO) to the acceptor lowest unoccupied molecular orbital (LUMO) gives the charge-transfer absorption. However, in tetracyanoquinodimethane (TCNQ) complexes of thienoacenes, comparison of the observed and calculated charge-transfer absorption demonstrates that the HOMO/LUMO transition is absent in the solid state owing to the orbital symmetry, and the first near-infrared band comes from the transition from the donor next HOMO to the TCNQ LUMO. Maps of the oscillator strength in rotated and translated molecular geometries are calculated on the basis of the time-dependent density functional theory, in which the absence of the HOMO/LUMO transition is approximately maintained even in the general molecular geometry.

Solid-State Properties of Hexaazatriphenylenehexacarbonitrile HAT(CN)6 .- Radical Anions in Crystalline Salts Containing Cryptand(M+ ) and Crystal Violet Cations

Crystalline {Cryptand[2.2.2](Na⁺ )}{HAT(CN)₆ ^.- }⋅0.5C₆ H₄ Cl₂ (1), {Cryptand[2.2.2](K⁺ )}{HAT(CN)₆ ^.- } (2), (CV⁺ ){HAT(CN)₆ ^.- } (3), and (CV⁺ ){HAT(CN)₆ ^.- }⋅2C₆ H₄ Cl₂ (4) salts (where CV⁺ is the crystal violet cation) containing hexaazatriphenylenehexacarbonitrile radical anions have been obtained. The solid-state molecular structure as well as the optical and magnetic properties of HAT(CN)₆ ^.- are studied. The formation of HAT(CN)₆ ^.- in 1-4 leads to the appearance of new bands in the visible range, at 694 and 740 nm. The HAT(CN)₆ ^.- radical anions have spin state S=1/2 and are packed in one-dimensional stacks containing the {HAT(CN)₆ ^.- }₂ dimers alternated with weaker interacting pairs of HAT(CN)₆ ^.- in 1 and nearly isolated {HAT(CN)₆ ^.- }₂ dimers in 2. The {HAT(CN)₆ ^.- }₂ dimers are diamagnetic in 1 but they effectively mediate one-dimensional antiferromagnetic coupling of spins within the stacks with moderate exchange interaction of J/k_B = -80 K. The behaviour of salt 2 is described by a singlet-triplet model for the {HAT(CN)₆ ^.- }₂ dimers with an energy gap of 434(±7) K. Magnetic behaviour of both salts agree well with the data of extended Hückel calculations. Salts 3 and 4 contain isolated stacks of alternated HAT(CN)₆ ^.- and CV⁺ ions, and in this case, nearly paramagnetic behaviour is observed with Weiss temperatures of -1 and -7 K, respectively. Narrow Lorentzian EPR signals with g = 2.0033-2.0039 were found for the HAT(CN)₆ ^.- radical anions in 1 and 4 but in solution g-factor shifts to 1.9964. The electronic structure of HAT(CN)₆ ^.- is analysed based on X-ray diffraction data for 2, showing a Jahn-Teller distortion of the radical anion that reduces the symmetry from D_3h to C_s and splits the initially degenerated LUMOs.

Magnetism, Conductivity and Spin-Spin Interactions in Layered Hybrid Structure of Anionic Radicals [Ni(dmit)2] Alternated by Iron(III) Spin-Crossover Complex [Fe(III)(3-OMe-Sal2trien)] and Ferric Moiety Precursors

In this study, crystals of the hybrid layered structure, combined with Fe(III) Spin-Crossover (SCO) complexes with metal-dithiolate anionic radicals, and the precursors with nitrate and iodine counterions, are obtained and characterized. [Fe(III)(3-OMe-Sal₂trien)][Ni(dmit)₂] (1), [Fe(III)(3-OMe-Sal₂trien)]NO₃·H₂O (2), [Fe(III)(3-OMe-Sal₂trien)]I (3) (3-OMe-Sal₂trien = hexadentate N₄O₂ Schiff base is the product of the condensation of triethylenetetramine with 3-methoxysalicylaldehyde; H₂dmit = 2-thioxo-1,3-dithiole-4,5-dithiol). Bulk SCO transition was not achieved in the range 2.0–350 K for all three compounds. Alternatively, the hybrid system (1) exhibited irreversible segregation into the spatial fractions of Low-Spin (LS) and High-Spin (HS) phases of the ferric moiety, induced by thermal cycling. Fractioning was studied using both SQUID and EPR methods. Magnetic properties of the LS and HS phases were analyzed in the framework of cooperative interactions with anionic sublattice: Anion radical layers Ni(dmit)₂ (1), and H-bonded chains with NO₃ and I (2,3). LS phase of (1) exhibited unusual quasi-two-dimensional conductivity related to the Arrhenius mechanism in the anion radical layers, ρ_||c = 2 × 10⁵ Ohm·cm and ρ_⊥c = 7 × 10² Ohm·cm at 293 K. Ground spin state of the insulating HS phase was distinctive by ferromagnetically coupled spin pairs of HS Fe³⁺, S = 5/2, and metal-dithiolate radicals, S = 1/2.

Spatially Resolved Investigation of Mixed Valence and Insulator-to-Metal Transition in an Organic Salt

Using scanning tunneling microscopy/spectroscopy (STM/STS), we investigate the evolution of electronic structures across the boundaries of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and K-TCNQ assemblies on a weakly interacting substrate. Despite the semiconducting/insulating nature of TCNQ (TCNQ⁰) and K-TCNQ (TCNQ^-1), a continuum metallic-like density of states extending deep (∼1.5 nm) into the TCNQ assembly is observed near the domain boundary. We attribute the formation of these states to the abrupt change of molecular valence, which perturbs the electrostatics of the junction and creates local electric fields as evidenced by the band bending near the domain boundary. To the best of our knowledge, this study provides the first microscopic understanding of the crucial physics occurring near domain boundaries of mixed valence in K-TCNQ, or broadly speaking charge-transfer complexes, which highlights these boundaries as potential "weak" points to initiate the electric field-induced insulator-to-metal transition.

Fine tuning of intra-lattice electron transfers through site doping in tetraoxolene-bridged iron honeycomb layers

The precise control of intra-lattice multiple electron transfers was demonstrated in the solvated and desolvated species of the tetraoxolene-bridged Fe honeycomb layer system, (NPr4)2[Fe2(Cl2An)3]·(solv) (Cl2Ann- = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate; NPr4+ = tetrapropylammonium cation), by the site-doping of the Cl2Ann- bridging unit using X2Ann- units with X = Br or F.

Crystal structure and metallization mechanism of the π-radical metal

Radical electrons tend to localize on individual molecules, resulting in an insulating (Mott-Hubbard) bandgap in the solid state. Herein, we report the crystal structure and intrinsic electronic properties of the first single crystal of a π-radical metal, tetrathiafulvalene-extended dicarboxylate (TED). The electrical conductivity is up to 30 000 S cm-1 at 2 K and 2300 S cm-1 at room temperature. Temperature dependence of resistivity obeys a T 3 power-law above T > 100 K, indicating a new type of metal. X-ray crystallographic analysis clarifies the planar TED molecule, with a symmetric intramolecular hydrogen bond, is stacked along longitudinal (the a-axis) and transverse (the b-axis) directions. The π-orbitals are distributed to avoid strong local interactions. First-principles electronic calculations reveal the origin of the metallization giving rise to a wide bandwidth exceeding 1 eV near the Fermi level. TED demonstrates the effect of two-dimensional stacking of π-orbitals on electron delocalization, where a high carrier mobility of 31.6 cm2 V-1 s-1 (113 K) is achieved.

One-dimensional electronic systems: metal-chain complexes and organic conductors

One-dimensional (1D) metal-chain complexes and organic conductors show many similarities as well as striking differences in structural and electronic properties, although constituent elements and orbitals that contribute to charge transfer in these systems are quite different. In this review, we highlighted the structural and electronic properties of neutral MMX-chain complexes (M = Pt2+/3+, X = I-) and tetramethyltetrathiafulvalene-based cation radical salts as typical examples of each group while comparing them with each other. This review primarily aims to construct a coherent body of knowledge of 1D electronic materials that might have been separately investigated. We have proposed future directions for the exploration of new and more advanced electronic materials not only having 1D character, but also residing in the dimensional crossover regime.

Simultaneous manifestation of metallic conductivity and single-molecule magnetism in a layered molecule-based compound

Single-molecule magnets (SMMs) show superparamagnetic behaviour below blocking temperature at the molecular scale, so they exhibit large magnetic density compared to the conventional magnets. Combining SMMs and molecular conductors in one compound will bring about new physical phenomena, however, the synergetic effects between them still remain unexplored. Here we present a layered molecule-based compound, β′′-(BEDO-TTF)₄ [Co(pdms)₂]·3H₂O (BO4), (BEDO-TTF (BO) and H₂pdms are bis(ethylenedioxy)tetrathiafulvalene and 1,2-bis(methanesulfonamido)benzene, respectively), which was synthesized by using an electrochemical approach and studied by using crystal X-ray diffraction. This compound simultaneously exhibited metallic conductivity and SMM behaviour up to 11 K for the first time. The highest electrical conductivity was 400–650 S cm⁻¹ at 6.5 K, which is the highest among those reported so far for conducting SMM materials. Furthermore, antiferromagnetic ordering occurred below 6.5 K, along with a decrease in conductivity, and the angle-independent negative magnetoresistance suggested an effective electron correlation between the conducting BO and Co(pdms)₂ SMM layers (d–π interactions). The strong magnetic anisotropy and two-dimensional conducting plane play key roles in the low-temperature antiferromagnetic semiconducting state. BO4 is the first compound exhibiting antiferromagnetic ordering among SMMs mediated by π-electrons, demonstrating the synergetic effects between SMMs and molecular conductors.

A metallic single-molecule magnet was synthesised demonstrating simultaneous metallic conduction and excellent SMM properties at the same temperature range for the first time, with potential applications in molecule-based quantum spintronics.

A Highly Crystalline Perylene Imide Polymer with the Robust Built-In Electric Field for Efficient Photocatalytic Water Oxidation

A highly crystalline perylene imide polymer (Urea-PDI) photocatalyst is successfully constructed. The Urea-PDI presents a wide spectrum response owing to its large conjugated system. The Urea-PDI performs so far highest oxygen evolution rate (3223.9 µmol g^-1 h^-1 ) without cocatalysts under visible light. The performance is over 107.5 times higher than that of the conventional PDI supramolecular photocatalysts. The strong oxidizing ability comes from the deep valence band (+1.52 eV) which is contributed by the covalent-bonded conjugated molecules. Besides, the high crystallinity and the large molecular dipoles of the Urea-PDI contribute to a robust built-in electric field promoting the separation and transportation of photogenerated carriers. Moreover, the Urea-PDI is very stable and has no performance attenuation after 100 h continuous irradiation. The Urea-PDI polymer photocatalyst provides with a new platform for the use of photocatalytic water oxidation, which is expected to contribute to clean energy production.

Highly Conducting and Flexible Radical Crystals

Together with high conductivity, high flexibility is an important property required for next generation organic electronic components. Both properties are difficult to achieve together especially when the components are crystalline because of the intrinsic high brittleness of organic molecular crystals. We report an organic radical crystal system that has both high flexibility and high conductivity. The crystal consists of 9,10-bis(phenylethynyl)anthracene radical cation (BPEA^.+ ) units, and shows flexibility under pressure with high conductivity in ambient condition exhibiting average conductivity of 2.68 S cm^-1 when normal linear shape, as well as 2.43 S cm^-1 when bent. The structural analysis reveals that both a short π-π distance (3.290 Å) between BPEA^.+ units that are aligned along the crystal length direction, and the presence of PF₆ ^- counter ions induce flexibility and high electrical conductivity.

Incorporating Spacer Molecules into the Tetrathiafulvalene-p-Chloranil Charge-Transfer Framework: Modulating the Neutral-Ionic Phase Transition

The charge-transfer (CT) tetrathiafulvalene-p-chloranil (TTF-CA) crystal, a representative functional organic electronic material, has been the subject of both basic and applied research. This material shows a neutral-ionic phase transition (NIPT) that induces drastic changes in its physical properties. Here, we use this crystal as a framework and demonstrate a method for modulating physical properties of TTF-CA. A number of multicomponent (ternary) CT crystals were obtained by crystallizing TTF-CA with a third molecular species. These complexes all contain molecular sheets formed with TTF-CA; however, the third molecules were differently inserted between these sheets as spacers to induce a variety of physical properties in the CT crystals. Some showed spacer-modified NIPT, while the transition to the ionic state was suppressed in one complex despite the presence of TTF-CA sheets, which indicates that spacer molecules can modulate the physical properties or functions of CT crystals.