Organic superconductors

Charge and Spin Transfer Dynamics in a Weakly Coupled Porphyrin Dimer

The dynamics of electron and spin transfer in the radical cation and photogenerated triplet states of a tetramethylbiphenyl-linked zinc-porphyrin dimer were investigated, so as to test the relevant parameters for the design of a single-molecule spin valve and the creation of a novel platform for the photogeneration of high-multiplicity spin states. We used a combination of multiple techniques, including variable-temperature continuous wave EPR, pulsed proton electron-nuclear double resonance (ENDOR), transient EPR, and optical spectroscopy. The conclusions are further supported by density functional theory (DFT) calculations and comparison to reference compounds. The low-temperature cw-EPR and room-temperature near-IR spectra of the dimer monocation demonstrate that the radical cation is spatially localized on one side of the dimer at any point in time, not coherently delocalized over both porphyrin units. The EPR spectra at 298 K reveal rapid hopping of the radical spin density between both sites of the dimer via reversible intramolecular electron transfer. The hyperfine interactions are modulated by electron transfer and can be quantified using ENDOR spectroscopy. This allowed simulation of the variable-temperature cw-EPR spectra with a two-site exchange model and provided information on the temperature-dependence of the electron transfer rate. The electron transfer rates range from about 10.0 MHz at 200 K to about 53.9 MHz at 298 K. The activation enthalpies Δ‡H of the electron transfer were determined as Δ‡H = 9.55 kJ mol-1 and Δ‡H = 5.67 kJ mol-1 in a 1:1:1 solvent mixture of CD2Cl2/toluene-d8/THF-d8 and in 2-methyltetrahydrofuran, respectively, consistent with a Robin-Day class II mixed valence compound. These results indicate that the interporphyrin electronic coupling in a tetramethylbiphenyl-linked porphyrin dimer is suitable for the backbone of a single-molecule spin valve. Investigation of the spin density distribution of the photogenerated triplet state of the Zn-porphyrin dimer reveals localization of the triplet spin density on a nanosecond time scale on one-half of the dimer at 20 K in 2-methyltetrahydrofuran and at 250 K in a polyvinylcarbazole film. This establishes the porphyrin dimer as a molecular platform for the formation of a localized, photogenerated triplet state on one porphyrin unit that is coupled to a second redox-active, ground-state porphyrin unit, which can be explored for the formation of high-multiplicity spin states.

DNA as a perfect quantum computer based on the quantum physics principles

DNA is a complex multi-resolution molecule whose theoretical study is a challenge. Its intrinsic multiscale nature requires chemistry and quantum physics to understand the structure and quantum informatics to explain its operation as a perfect quantum computer. Here, we present theoretical results of DNA that allow a better description of its structure and the operation process in the transmission, coding, and decoding of genetic information. Aromaticity is explained by the oscillatory resonant quantum state of correlated electron and hole pairs due to the quantized molecular vibrational energy acting as an attractive force. The correlated pairs form a supercurrent in the nitrogenous bases in a single band π -molecular orbital ( π -MO). The MO wave function ( Φ ) is assumed to be the linear combination of the n constituent atomic orbitals. The central Hydrogen bond between Adenine (A) and Thymine (T) or Guanine (G) and Cytosine (C) functions like an ideal Josephson Junction. The approach of a Josephson Effect between two superconductors is correctly described, as well as the condensation of the nitrogenous bases to obtain the two entangled quantum states that form the qubit. Combining the quantum state of the composite system with the classical information, RNA polymerase teleports one of the four Bell states. DNA is a perfect quantum computer.

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.

Intermolecular Resonance Correlates Electron Pairs Down a Supermolecular Chain: Antiferromagnetism in K-Doped p-Terphenyl

Recent interest in potassium-doped p-terphenyl has been fueled by reports of superconductivity at T_c values surprisingly high for organic compounds. Despite these interesting properties, studies of the structure-function relationships within these materials have been scarce. Here, we isolate a phase-pure crystal of potassium-doped p-terphenyl: [K(222)]₂[p-terphenyl₃]. Emerging antiferromagnetism in the anisotropic structure is studied in depth by magnetometry and electron spin resonance. Combining these experimental results with density functional theory calculations, we describe the antiferromagnetic coupling in this system that occurs in all 3 crystallographic directions. The strongest coupling was found along the ends of the terphenyls, where the additional electron on neighboring p-terphenyls antiferromagnetically couple. This delocalized bonding interaction is reminiscent of the doubly degenerate resonance structure depiction of polyacetylene. These findings hint toward magnetic fluctuation-induced superconductivity in potassium-doped p-terphenyl, which has a close analogy with high T_c cuprate superconductors. The new approach described here is very versatile as shown by the preparation of two additional salts through systematic changing of the building blocks.

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.

Anion-Coordination-Assisted Assembly of Supramolecular Charge-Transfer Complexes Based on Tris(urea) Ligands

Charge-transfer (CT) complexes, formed by noncovalent bonding between electron-rich (donor, D) and electron-deficient (acceptor, A) molecules (or moieties) have attracted considerable attention due to their fascinating structures and potential applications. Herein, we demonstrate that anion coordination is a promising strategy to promote CT complex formation between anion-binding, electron-rich tris(urea) donor ligands (D) and electron-deficient viologen cation acceptors (A), which form co-crystals featuring infinite ⋅⋅⋅DADA⋅⋅⋅ or discrete (circular DADA or three-decker DAD) π-stacking interactions. These CT complexes were studied by X-ray diffraction, UV/Vis spectroscopy, electric conductivity measurements, charge displacement curve (CDC) calculations, and DFT computations.

Structural diversity in conducting bilayer salts (CNB-EDT-TTF)4A

The family of recently described salts based on the electron donor CNB-EDT-TTF and different anions A, with general formula (CNB-EDT-TTF)₄A, constitutes an unprecedented type of molecular conductor based on a bilayer structure of the donors.

Crystal structures of two charge-transfer com-plexes of benzo[1,2-c:3,4-c':5,6-c'']tri-thio-phene (D 3h -BTT)

Benzo[1,2-c:3,4-c':5,6-c'']tri-thio-phene (D 3h -BTT) is an easily prepared electron donor that readily forms charge-transfer complexes with organic acceptors. We report here two crystal structures of its charge-transfer complexes with 7,7,8,8-tetra-cyano-quinodi-methane (TCNQ) and buckminsterfullerene (C60). The D 3h -BTT·TCNQ complex, C12H6S3·C12H4N4, crystallizes with mixed layers of donors and acceptors, with an estimated degree of charge transfer at 0.09 e. In the D 3h -BTT·C60·toluene complex, C12H6S3·C60·C7H8, the central ring of BTT is 'squeezed' by the C60 mol-ecules from both faces. However, the degree of charge transfer is low. The C60 unit is disordered over two sites in a 0.766 (3):0.234 (3) ratio and was refined as a two-component inversion twin.

[Cu3(C6Se6)] n : The First Highly Conductive 2D π-d Conjugated Coordination Polymer Based on Benzenehexaselenolate

Nanocrystals of a 2D π-d conjugated copper bis(diselenolene) coordination polymer (Cu-BHS, BHS = benzenehexaselenolate) are synthesized via a simple homogeneous reaction between cupric ions and benzenehexaselenol (H6BHS). Its 2D extended hexagonal lattice is confirmed by powder X-ray diffraction, and further characterized by scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The electrical conductivity measured on compressed powder sample reaches 110 S cm-1 at 300 K, which is among the highest value ever reported for coordination polymers. Furthermore, the intrinsic metallic characteristics of Cu-BHS are confirmed by ultraviolet photoelectron spectroscopy and band structure calculation.

Donor-Donor'-Acceptor Triads Based on [3.3]Paracyclophane with a 1,4-Dithiafulvene Donor and a Cyanomethylene Acceptor: Synthesis, Structure, and Electrochemical and Photophysical Properties

Donor-donor'-acceptor triads (1, 2), based on [3.3]paracyclophane ([3.3]PCP) as a bridge, with electron-donating properties (D') using 1,4-dithiafulvene (DTF; TTF half unit) as a donor and dicyanomethylene (DCM; TCNE half unit) or an ethoxycarbonyl-cyanomethylene (ECM) as an acceptor were designed and synthesized. The pulse radiolysis study of 1 a in 1,2-dichloroethane allowed the clear assignment of the absorption bands of the DTF radical cation (1 a^.+ ), whereas the absorption bands due to the DCM radical anion could not be observed by γ-ray radiolysis in 2-methyltetrahydrofuran rigid glass at 77 K. Electrochemical oxidation of 1 a first generates the DTF radical cation (1 a^.+ ), the absorption bands of which are in agreement with those observed by a pulse radiolysis study, followed by dication (1 a²⁺ ). The ESR spectrum of 1 a^.+ showed a symmetrical signal with fine structure and an ESR simulation predicted that the spin of 1 a^.+ is delocalized over S and C atoms of the DTF moiety and the central C atom of the trimethylene bridge bearing the DTF moiety. Pulse radiolysis, ESR, and electrochemical studies indicate that the DTF radical cation of 1 a^.+ is more stable than that of 6^.+ , and the latter shows a strong tendency to dimerize. This result indicates that the [3.3]PCP moiety as a bridge can stabilize the DTF radical cation more than the 1,3-diphenylpropane moiety because of kinetic stability due to its rigid structure and the weak electronic interaction of DTF and DCM moieties through [3.3]PCP.

Structure and characterization of charge transfer complexes of benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene [C3h-BTT]

Four charge-transfer complexes of C_3h-BTT (2) with the organic acceptors TCNQ, F₄TCNQ, chloranil, and fluoranil were prepared and crystallographically characterized.

Structural Conformers of (1,3-Dithiol-2-ylidene)ethanethioamides: The Balance between Thioamide Rotation and Preservation of Classical Sulfur-Sulfur Hypervalent Bonds

The reaction of N-(2-phthalimidoethyl)-N-alkylisopropylamines and S2Cl2 gave 4-N-(2-phthalimidoethyl)-N-alkylamino-5-chloro-1,2-dithiol-3-thiones that quantitatively cycloadded to dimethyl or diethyl acetylenedicarboxylate to give stable thioacid chlorides, which in turn reacted with one equivalent of aniline or a thiole to give thioanilides or a dithioester. Several compounds of this series showed atropisomers that were studied by a combination of dynamic NMR, simulation of the signals, conformational analysis by DFT methods, and single crystal X-ray diffraction, showing a good correlation between the theoretical calculations, the experimental values of energies, and the preferred conformations in the solid state. The steric hindering of the crowded substitution at the central amine group was found to be the reason for the presence of permanent atropisomers in this series of compounds and the cause of a unique disposition of the thioxo group at close-to-right angles with respect to the plane defined by the 1,3-dithiole ring in the dithiafulvene derivatives, thus breaking the sulfur-sulfur hypervalent bond that is always found in this kind of compounds.

Metallic and Mott insulating spin-frustrated antiferromagnetic states in ionic fullerene complexes with a two-dimensional hexagonal C60·- packing motif

(MDABCO(+))(C60(·-))(TPC) (1), in which MDABCO(+) is N-methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene-based quasi-2D metal and contains closely packed hexagonal fullerene layers with interfullerene center-to-center distances of 10.07 Å at 300 K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m0 =6.7). The latter indicates a narrow-band nature, which is consistent with the calculated bandwidth of 0.10-0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200-230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ(+))(C60(·-))(TPC) (2) with a hexagonal arrangement of C60(·-) was obtained by increasing the interfullerene distance with the bulkier N-methylquinuclidinium cations (MQ(+)) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center-to-center distances in the layers (10.124, 10.155, and 10.177 Å at 250 K). Unit-cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1, 2 exhibits a nonmetallic spin-frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is -27 K) and no magnetic ordering down to 1.9 K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2, and thus a Mott-Hubbard insulating state with antiferromagnetically frustrated spins results.

Synthesis of pyrrolidine-fused 1,3-dithiolane oligomers by the cycloaddition of polycyclic dithiolethiones to maleimides and evaluation as mercury(II) indicators

The scandium triflate-catalyzed cycloaddition reaction of polycyclic 1,2-dithiolethiones to maleimides is described. The reaction constitutes an easy approach to linear as well as branched oligomeric cis-fused dihydro[1,3]dithiolo[4,5-c]pyrrole-4,6-dione rings interconnected by 3,5-diylidenethiomorpholine-2,6-dithione or ylidene-6-thioxo[1,2]dithiolo[3,4-b][1,4]thiazin-3-one groups. The presence of highly colored, highly polarized push-pull α,β-unsaturated thione groups in their structures make these compounds sensitive to the presence of mercury(II) cation in organic or mixed organic/aqueous solvents.

Transition metal complexes and radical anion salts of 1,10-phenanthroline derivatives annulated with a 1,2,5-tiadiazole and 1,2,5-tiadiazole 1,1-dioxide moiety: multidimensional crystal structures and various magnetic properties

Advances in the molecular variety and the elucidation of the physical properties of 1,10-phenanthroline annulated with 1,2,5-thiadiazole and 1,2,5-thiadiazole 1,1-dioxide moieties have been achieved, and are described herein. A 1,2,5-thiadiazole compound, [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline (tdap), was used as a ligand to create multidimensional network structures based on S•••S and S•••N intermolecular interactions. A 1,2,5-thiadiazole 1,1-dioxide compound, [1,2,5] thiadiazolo[3,4-f][1,10]phenanthroline, 1,1-dioxide (tdapO₂), was designed to create a stable radical anion, as well as good network structures. Single crystal X-ray structure analyses revealed that transition metal complexes of tdap, and radical anion salts of tdapO₂ formed multidimensional network structures, as expected. Two kinds of tdap iron complexes, namely [Fe(tdap)₂(NCS)₂] and [Fe(tdap)₂(NCS)₂]•MeCN exhibited spin crossover transitions, and their transition temperatures showed a difference of 150 K, despite their similar molecular structures. Magnetic measurements for the tdapO₂ radical anion salts revealed that the magnetic coupling constants between neighboring radical species vary from strongly antiferromagnetic (J = −320 K) to ferromagnetic (J = 24 K), reflecting the differences in their π overlap motifs.

Concepts relating magnetic interactions, intertwined electronic orders, and strongly correlated superconductivity

Unconventional superconductivity (SC) is said to occur when Cooper pair formation is dominated by repulsive electron-electron interactions, so that the symmetry of the pair wave function is other than an isotropic s-wave. The strong, on-site, repulsive electron-electron interactions that are the proximate cause of such SC are more typically drivers of commensurate magnetism. Indeed, it is the suppression of commensurate antiferromagnetism (AF) that usually allows this type of unconventional superconductivity to emerge. Importantly, however, intervening between these AF and SC phases, intertwined electronic ordered phases (IP) of an unexpected nature are frequently discovered. For this reason, it has been extremely difficult to distinguish the microscopic essence of the correlated superconductivity from the often spectacular phenomenology of the IPs. Here we introduce a model conceptual framework within which to understand the relationship between AF electron-electron interactions, IPs, and correlated SC. We demonstrate its effectiveness in simultaneously explaining the consequences of AF interactions for the copper-based, iron-based, and heavy-fermion superconductors, as well as for their quite distinct IPs.

Ferrocene-dithiolene hybrids: control of strong donor-acceptor electronic communication to reverse the charge transfer direction

We prepared a novel class of ferrocene-dithiolene hybrid molecules, FcS4dt(Me)2 and FcS4dt[Pt((t)Bu2bpy)] (where FcS4dt indicates 2-(1,3-dithia[3]ferrocenophane-2-ylidene)-1,3-dithiole-4,5-dithiolate and (t)Bu2bpy indicates 4,4'-di-tert-butyl-2,2'-bipyridine), in which the ferrocene moiety was bound to the planar conjugated dithiolene skeleton via two sulfur atoms such that the cyclopentadienyl rings were perpendicular to the dithiolene backbone. The physical properties and electronic structures of the complexes and their oxidized species [FcS4dt(Me)2](•+) and [FcS4dt[Pt((t)Bu2bpy)]](•+) were investigated by means of single-crystal X-ray diffraction (XRD) analysis, cyclic voltammetry, electron paramagnetic resonance (EPR), and UV-vis near infrared (UV-vis-NIR) spectroscopy. The electron density distributions of the highest occupied molecular orbitals (HOMOs) of FcS4dt(Me)2 and FcS4dt[Pt((t)Bu2bpy)] differed remarkably in that the HOMO of the former was ferrocene-based whereas that of the latter was dithiolene-based. The differences in the HOMO distributions originated from the energy level of the dithiolene-based π-orbital in each of the complexes, which was controlled by changing R in FcS4dt(R)2 (R = Me for FcS4dt(Me)2; 2R = Pt((t)Bu2bpy) for FcS4dt[Pt((t)Bu2bpy)]). We succeeded in analyzing the crystal structure of [FcS4dt[Pt((t)Bu2bpy)]](F4TCNQ)·C6H14·CH2Cl2 (where F4TCNQ indicates 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), which provided a rare example of the crystal structure of a [Pt(diimine)(dithiolate)](•+) ion-based complex. A comparison of the bond lengths in FcS4dt[Pt((t)Bu2bpy)] and [FcS4dt[Pt((t)Bu2bpy)]](•+) suggested that the latter complex displayed a conjugated dithiolene-based π-radical character. These considerations agreed well with the electronic structures calculated using density functional theory (DFT) and time-dependent(TD)-DFT methods. Significant electronic communication between the ferrocene and dithiolene moieties was detected for both [FcS4dt(Me)2](•+) and [FcS4dt[Pt((t)Bu2bpy)]](•+) in the appearance of an intramolecular charge transfer band, which was hardly observed for previously reported ferrocene-dithiolene hybrid molecules. The charge transfer direction was reversed between the two cations. The electron coupling parameter HAB and the potential energy curves of the oxidized complexes were estimated based on the classical two-state Marcus-Hush theory. These results suggest that FcS4dt-based metalladithiolenes can exhibit controllable electronic structures expressed as double-minimum potential energy curves.