Stable Metallic State of a Neutral-Radical Single-Component Conductor at Ambient Pressure

Mixed-ligand, radical, gold bis(dithiolene) complexes: from single-component conductors to controllable NIR-II absorbers

Neutral radical bis(dithiolene) gold complexes [Au(dt)2]˙ are known to exhibit a strong absorption in the 1400-2000 nm NIR absorption range. Here, we demonstrate that the NIR signature of mixed-ligand bis(dithiolene) gold complexes [Au(dtA)(dtD)]˙ associating two different dithiolene, dtA and dtD, is found at higher energy, out of the range of the homoleptic analogs [Au(dtA)2]˙ and [Au(dtD)2]˙, in the looked-after NIR-II 1000-1400 nm absorption range. An efficient synthetic approach towards precursor mixed-ligand monoanionic gold bis(dithiolene) complexes [Au(dtA)(dtD)]-1 is reported. Using this strategy, no symmetrical complexes are formed and, upon electrocrystallization, no scrambling was observed in solution, allowing for the isolation of radical gold bis(dithiolene) complex such as [Au(bdt)(Et-thiazdt)]˙ (bdt: benzene-1,2-dithiolate; Et-thiazdt: N-ethyl-thiazoline-2-thione-3,4-dithiolate), which behaves as a single-component conductor. It is shown from theoretical calculations that the spin polarization induced by electron repulsions leads to a strong localization of the spin-orbitals, and provides a sound basis to understand, (i) the different ligand-based oxidation potentials, (ii) the NIR optical absorption at notably higher energies and (iii) the larger potential difference of the two redox processes than in the parent symmetric complexes. The solid-state properties of the radical complex [Au(bdt)(Et-thiazdt)]˙ are the consequence of a strongly 1D electronic structure with weakly dimerized chains and electronic localization favoring a semiconducting behavior, stable under pressures up to 18.2 GPa. Altogether, the versatility of the preparation method of [Au(dtA)(dtD)]-1 salts opens the route for a wide library of different mixed-ligand radical complexes [Au(dtA)(dtD)]˙ with simultaneously an adaptable absorption in the NIR-II range and the rich structural chemistry of single-component conductors.

Structure-activity relationship of anticancer and antiplasmodial gold bis(dithiolene) complexes

Monoanionic gold bis(dithiolene) complexes were recently shown to display activity against ovarian cancer cells, Gram-positive bacteria, Candida strains and the rodent malaria parasite, P. berghei. To date, only monoanionic gold(III) bis(dithiolene) complexes with a thiazoline backbone substituted with small alkyl chains have been evaluated for biomedical applications. We now analyzed the influence of the length and the hydrophobicity vs. hydrophilicity of these complexes' alkyl chain on their anticancer and antiplasmodial properties. Isomer analogues of these monoanionic gold(III) bis(dithiolene) complexes, this time with a thiazole backbone, were also investigated in order to assess the influence of the nature of the heterocyclic ligand on their overall chemical and biological properties. In this report we present the total synthesis of four novel monoanionic gold(III) bis(dithiolene) complexes with a long alkyl chain and a polyoxygenated (PEG) chain aiming to improve their solubility and biological properties. Our results showed that the complexes with a PEG chain showed promising anticancer and antiplasmodial activities beside improved solubility, a key parameter in drug discovery and development.

Mixed-Valence Conductors from Ni Bis(diselenolene) Complexes with a Thiazoline Backbone

Highly conducting, mixed-valence, multi-component nickel bis(diselenolene) salts were obtained by electrocrystallization of the monoanionic species [Ni(Me-thiazds)₂]^-1 (Me-thiazds: N-methyl-1,3-thiazoline-2-thione-4,5-diselenolate), with 1:2 and 1:3 stoichiometries depending of the counter ion used (Et₄N⁺ and nBu₄N⁺ vs Ph₄P⁺, respectively). This behavior strongly differs from that of the corresponding monoanionic dithiolene complexes whose oxidation afforded the single component neutral species. This provides additional rare examples of mixed-valence conducting salts of nickel diselenolene complexes, only known in two examples with the dsit (1,3-dithiole-2-thione-4,5-diselenolate) and dsise (1,3-dithiole-2-selone-4,5-diselenolate) ligands. The mixed-valence salts form highly dimerized or trimerized bi- and trimetallic units, rarely seen with such nickel complexes. Transport measurements under a high pressure (up to 10 GPa) and band structure calculations confirm the semiconducting character of [Ph₄P][Ni(Me-thiazds)₂]₃ and the quasi metallic character of [Et₄N][Ni(Me-thiazds)₂]₂ and [NBu₄]_x[Ni(Me-thiazds)₂]₂ salts (0 < x < 1).

Magnetic coupling and spin ordering in bisdithiazolyl, thiaselenazolyl, and bisdiselenazolyl molecular materials

The use of purely organic materials is a promising approach for the miniaturization of devices due to their interesting optical, electronic and magnetic properties. Bisdithiazolyl-based bisDTA compounds have emerged as promising candidates for radical-based single component conductors exhibiting simultaneously magnetic properties. Our computational work focuses on the intriguing magnetism of 4 isostructural pyridine-bridged bisDTA-multifunctional materials triggered by their magnetic and conducting properties being strongly dependent on the different S/Se ratios in the neutral radical skeleton: specifically, bisdithiazolyl (S,S) displays no magnetic order at low temperatures, thiaselenazolyl (Se,S) exhibits spin-canted antiferromagnetism (AFM), and both (S,Se) and bisdiselenazolyl (Se,Se) behave as bulk ferromagnets (FM). Our results reveal that (1) the magnetic response depends on the existence of an intricate network of both AFM and FM spin exchange J_AB couplings between neighbouring radicals; and (2) the structural arrangement of π-stacked pairs of radicals sits on a point in the configurational space that is very close to a crossover region where J_AB switches from AFM to FM. Indeed, for bulk FM, the experimental response is only accounted for when considering an ab initio optimised crystal structure able to portray adequately the electronic structure of bisDTAs in the region close to the temperature at which magnetic ordering emerges. Magneto-structural correlation maps show the large sensitivity of J_AB to very small structural changes with temperature along the π-stacks that lead to drastic changes in the magnetic properties. Clearly, the understanding of magnetism in the title bisDTA compounds is decisive to rationally tailor the properties of multifunctional materials by subtle structural modifications of their crystal packing.

Broad Spectrum Functional Activity of Structurally Related Monoanionic Au(III) Bis(Dithiolene) Complexes

The biological properties of sixteen structurally related monoanionic gold (III) bis(dithiolene/diselenolene) complexes were evaluated. The complexes differ in the nature of the heteroatom connected to the gold atom (AuS for dithiolene, AuSe for diselenolene), the substituent on the nitrogen atom of the thiazoline ring (Me, Et, Pr, iPr and Bu), the nature of the exocyclic atom or group of atoms (O, S, Se, C(CN)₂) and the counter-ion (Ph₄P⁺ or Et₄N⁺). The anticancer and antimicrobial activities of all the complexes were investigated, while the anti-HIV activity was evaluated only for selected complexes. Most complexes showed relevant anticancer activities against Cisplatin-sensitive and Cisplatin-resistant ovarian cancer cells A2780 and OVCAR8, respectively. After 48 h of incubation, the IC₅₀ values ranged from 0.1–8 µM (A2780) and 0.8–29 µM (OVCAR8). The complexes with the Ph₄P⁺ ([P]) counter-ion are in general more active than their Et₄N⁺ ([N]) analogues, presenting IC₅₀ values in the same order of magnitude or even lower than Auranofin. Studies in the zebrafish embryo model further showed that, despite their marked anticancer effect, the complexes with [P] counter-ion exhibited low in vivo toxicity. In general, the exocyclic exchange of sulfur by oxygen or ylidenemalononitrile (C(CN)₂) enhanced the compounds toxicity. Most complexes containing the [P] counter ion exhibited exceptional antiplasmodial activity against the Plasmodium berghei parasite liver stages, with submicromolar IC₅₀ values ranging from 400–700 nM. In contrast, antibacterial/fungi activities were highest for most complexes with the [N] counter-ion. Auranofin and two selected complexes [P][AuSBu(=S)] and [P][AuSEt(=S)] did not present anti-HIV activity in TZM-bl cells. Mechanistic studies for selected complexes support the idea that thioredoxin reductase, but not DNA, is a possible target for some of these complexes. The complexes [P] [AuSBu(=S)], [P] [AuSEt(=S)], [P] [AuSEt(=Se)] and [P] [AuSeiPr(=S)] displayed a strong quenching of the fluorescence intensity of human serum albumin (HSA), which indicates a strong interaction with this protein. Overall, the results highlight the promising biological activities of these complexes, warranting their further evaluation as future drug candidates with clinical applicability.

Electronic structure and magnetic coupling in selenium substituted pyridine-bridged bisdithiazolyl multifunctional molecular materials

Bisdithiazolyl radicals have furnished in recent years multiple examples of molecular materials with promising conductive and magnetic properties. The electronic band structure and magnetic ordering in four different isostructural pyridine-bridged bisdithiazolyl and Selenium substituted compounds have been studied by means of hybrid DFT based methods as implemented in the CRYSTAL code. The full rationalization of the properties of these multifunctional magnetic molecular materials requires a careful description of their complex open-shell electronic structure. The results describe the systems as narrow band (0.2-0.3 eV dispersion) open-shell semiconductors with a gap of 1.15-1.40 eV between the valence and conducting bands. The bands defining the insulating gap are dominated by orbital contributions arising from the heteroatoms sitting in the outer rings. A low energy closed-shell metallic solution is found at 0.25-0.35 eV above the magnetic solutions thus suggesting a complex mechanism for electric conduction with band and hopping contributions. The observed trend of the conductivity is in line with the variation of the insulating gap but more rigorous modelling is required to take into account the details of the band structure of the systems. For all the systems the spin density is well localised on the molecular units and is independent of the magnetic solution. Thus the system can be described as an ensemble of well-defined S = 1/2 magnetic centres using a two-body Heisenberg-Dirac-van Vleck spin Hamiltonian. The lowest energy electronic solutions are in line with the observed magnetic behaviour at low temperature. The set of competing magnetic exchange interactions that emerges from using a suitable mapping to consistently describe the low energy magnetic solutions explains the variety of magnetic responses (absence of long-range magnetic order, antiferromagnetism or ferromagnetism) of the four studied compounds at low temperatures.

Proton-electron-coupled functionalities of conductivity, magnetism, and optical properties in molecular crystals

Proton-electron-coupled reactions, specifically proton-coupled electron transfer (PCET), in biological and chemical processes have been extensively investigated for use in a wide variety of applications, including energy conversion and storage. However, the exploration of the functionalities of the conductivity, magnetism, and dielectrics by proton-electron coupling in molecular materials is challenging. Dynamic and static proton-electron-coupled functionalities are to be expected. This feature article highlights the recent progress in the development of functionalities of dynamic proton-electron coupling in molecular materials. Herein, single-unit conductivity by self-doping, quantum spin liquid state coupled with quantum fluctuation of protons, switching of conductivity and magnetism triggered by the disorder-order transition of deuterons, and their external responses under pressure and in the presence of an electric field are introduced. In addition, as for the functionalities of proton-d/π-electron coupling in metal dithiolene complexes, magnetic switching with multiple PCET and vapochromism induced by electron transfer through hydrogen-bond (H-bond) formation is introduced experimentally and theoretically. We also outlined the basic and applied issues and potential challenges for development of proton-electron-coupled molecular materials, functionalities, and devices.

An Approach to an Ideal Molecule-Based Mixed Conductor with Comparable Proton and Electron Conductivity

We synthesized a molecule-based proton-electron mixed conductor (PEMC), a Pt(III) dithiolate complex with 1,4-naphthoquinone skeletons. The π-planar Pt complex involves a π-stacking column, which is connected by one-dimensional hydrogen bonding chains composed of water molecules. The room-temperature (RT) proton conductivity is 8.0 × 10^-5 S cm^-1 under ambient conditions, which is >2 orders of magnitude higher than that of the isomorphous Ni complex (7.2 × 10^-7 S cm^-1). The smaller activation energy (0.23 eV) compared to that of the Ni complex (0.42 eV) possibly originates from the less dense water, which promotes the reorientational dynamics, in the Pt complex with an expanded lattice, namely, negative chemical pressure upon substitution of Ni with the larger Pt. In addition, the Pt complex shows a relatively high RT electronic conductivity of 1.0 × 10^-3 S cm^-1 caused by the π-columns, approaching an ideal PEMC with comparable proton and electron conduction.

Indeno[2,1-a]fluorene-11,12-dione radical anions:synthesis,characterization and property

The one-electron reduction reactions of indeno[2,1-a]fluorene-11,12-dione ( IF ) with various alkali metals bring about the radical anion salts. The different structures and properties are characterized by single crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) measurements and physical property measurement system (PPMS). IF •- K + (18-c-6) is regarded as a one-dimensional magnetic chain through C-H-C interaction. Theoretical calculations and magnetic results prove that [ IF •- K + (15-c-5)] 2 is a dimer with an open-shell ground state. IF •- Na + (15-c-5) and IF •- K + (cryptand) are monoradical anion salts. IF 2 •- Li + possesses unique π-stack structure with an interplanar separation less than 3.46 Å, making it a semiconductor ( δ RT = 1.9 Χ 10 -4 S•cm -1 ). This work gives a wealth of insights into multifunctional radical anions, and makes the design and development of different functional radicals attractive.

Gold(iii) bis(dithiolene) complexes: from molecular conductors to prospective anticancer, antimicrobial and antiplasmodial agents

The anticancer, antimicrobial and antiplasmodial activities of six gold(iii) bis(dithiolene) complexes were studied. Complexes 1-6 showed relevant anticancer properties against A2780/A2780cisR ovarian cancer cells (IC50 values of 0.08-2 μM), also being able to overcome cisplatin resistance in A2780cisR cells. Complex 1 also exhibited significant antimicrobial activity against Staphylococcus aureus (minimum inhibitory concentration (MIC) values of 12.1 ± 3.9 μg mL-1) and both Candida glabrata and Candida albicans (MICs of 9.7 ± 2.7 and 19.9 ± 2.4 μg mL-1, respectively). In addition, all complexes displayed antiplasmodial activity against the Plasmodium berghei parasite liver stages, even exhibiting better results than the ones obtained using primaquine, an anti-malarial drug. Mechanistic studies support the idea that thioredoxin reductase, but not DNA, is a possible target of these complexes. Complex 1 is stable under biological conditions, which would be important if this compound is ever to be considered as a drug. Overall, the results obtained evidenced the promising biological activity of complex 1, which might have potential as a novel anticancer, antimicrobial and antiplasmodial agent to be used as an alternative to current therapeutics.

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.

Introducing Selenium in Single-Component Molecular Conductors Based on Nickel Bis(dithiolene) Complexes

Two selenated analogues of the all-sulfur single-component molecular conductor [Ni(Et-thiazdt)₂] (Et-thiazdt = N-ethylthiazoline-2-thione-4,5-dithiolate) have been prepared from their precursor radical-anion complexes. Replacement of the thione by a selenone moiety gives the neutral [Ni(Et-thiazSedt)₂] complex. It adopts an unprecedented solid-state organization (for neutral nickel complexes), with the formation of perfectly eclipsed dimers and very short intermolecular Se···Se contacts (81% of the van der Waals contact distance). Limited interactions between dimers leads to a large semiconducting gap and low conductivity (σ_RT = 1.7 × 10^-5 S cm^-1). On the other hand, going from the neutral [Ni(Et-thiazdt)₂] dithiolene complex to the corresponding [Ni(Et-thiazds)₂] diselenolene complex gives rise to a more conventional layered structure built out of uniform stacks of the diselenolene complexes, different, however, from the all-sulfur analogue [Ni(Et-thiazdt)₂]. Band structure calculations show an essentially 1D electronic structure with large band dispersion and a small HOMO-LUMO gap. Under high pressures (up to 19 GPa), the conductivity increases by 4 orders of magnitude and the activation energy is decreased from 120 meV to only 13 meV, with an abrupt change observed around 10 GPa, suggesting a structural phase transition under pressure.

A radical mixed-ligand gold bis(dithiolene) complex

Asymmetry in the electronic structure of a mixed-ligand gold bis(dithiolene) complex explains its peculiar optical, electrochemical and structural features.

Electrochemical deposition of a semiconducting gold dithiolene complex with NIR absorption

Here, the synthesis of a new anionic gold dithiolene complex, NBu₄·[1-i], and that of its corresponding neutral gold complex 2 is reported. Complex 2 shows strong absorption into the IR, semiconductivity (σ_RT = 3.06 × 10^-7 S cm^-1) with an activation energy of 0.25 eV, and weakly temperature dependent paramagnetic susceptibility, indicating strong intermolecular interactions in the solid state. As a consequence of their strong intermolecular interactions, neutral gold dithiolene complexes are often highly insoluble, limiting their utility and processability. Electrochemical deposition is used to deposit conductive films of complex 2, which retain the NIR properties present in the bulk material, indicting that the strong intermolecular interactions are retained in the film.

Conducting neutral gold bisdithiolene complex [Au(dspdt)2]˙

[Au(dspdt)2] (dspdt = 2,3-dihydro-5,6-selenophenedithiolate) is an unprecedented example of a neutral gold bisdithiolene complex with a unique structure composed of interacting dimer and trimer chains displaying relatively high electrical conductivity (0.1 S cm-1 at room temperature).

Prototype Material for New Strategy of Photon Energy Storage

The smart utilization of photons is paid global attention from the viewpoint of renewable energy and information technology. However, it is still impossible to store photons as batteries and condensers do for electrons. All the present technologies utilize (the energy of) photons in situ, such as solar panels, or in spontaneous relaxation processes, such as photoluminescence. If we can store the energy of photons over an arbitrary period and utilize them on demand, not only we will make an innovative progress in energy management, but we will also be able to replace a part of electrons by photons in the information technology for more efficient performance. In this article, we review a prototype of such a material including the current status of related research as well as where we are heading for.

Probing the structural and electronic response of Magnus green salt compounds [Pt(NH2R)4][PtCl4] (R = H, CH3) to pressure

Despite possessing the desirable crystal packing and short Pt⋯Pt stacking distances required for a large piezoresistive response, we explain why the conductivity-pressure response of the Magnus green salt [Pt(NH₃)₄][PtCl₄] is extremely sluggish.

Heterometallic coordination polymers based on homo- and heteroleptic Au(iii) dithiolene complexes

Two novel hetero- and homoleptic Au(iii) dithiolene complexes have been designed, synthesized, characterized and employed for the elaboration of heterometallic coordination networks.

Halogen bonded metal bis(dithiolene) 2D frameworks

Short and directional I⋯S halogen bonding interactions in iodinated bis(dithiolene) complexes lead to the crystallization of 2D or 3D anionic frameworks.

Hydrogen bonding interactions in single component molecular conductors based on metal (Ni, Au) bis(dithiolene) complexes

Nickel (closed-shell) or gold (radical) bis(dithiolene) neutral complexes, functionalized with hydroxyethyl and thiazole moieties, afford hydrogen-bonded single component conductors.

A molecular crystal with an unprecedentedly long-lived photoexcited state

The Au(iii)-complex anions in a newly synthesised compound BPY[Au(dmit)₂]₂ (BPY = N,N'-ethylene-2,2'-bipyridinium, dmit = 1,3-dithiole-2-thione-4,5-dithiolate) reversibly exhibit a molecular distortion in the solid state under UV-radiation. The photoexcited state is maintained for a week at 298 K, during which time molecules relax to their original structures and energy is gradually released as heat without decomposition or light emission. Most Au atoms adopt square planar (SP) coordination geometries, but some anions have unusual non-planar (NP) coordination geometries that produce disorder at the Au sites. The total (Gibbs) energy of the system depends on the proportion of Au atoms of NP geometry, which is directly determined from the occupancy (Occ (%)) by X-ray diffractometry. Due to phase transition, Occ substantially changes at a critical temperature (T_C) of ∼280 K without other structural changes; however it remains almost constant in each phase. In addition, due to UV-promoted charge-transfer transitions between BPY and Au(dmit)₂, Occ can be controlled by UV irradiation (∼250-450 nm). The UV-excited states have unprecedentedly long relaxation times (t_1/2 > 36 h at 298 K), which is attributed to the close connection between the degrees of freedom on charge, spin, and molecular structures.

High Pressure Crystal Structure and Electrical Properties of a Single Component Molecular Crystal [Ni(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate)

Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)₂] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)₂], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)₂] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)₂], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)₂] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)₂] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures.

The Importance of Electronic Dimensionality in Multiorbital Radical Conductors

The exceptional performance of oxobenzene-bridged bis-1,2,3-dithiazolyls 6 as single-component neutral radical conductors arises from the presence of a low-lying π-lowest unoccupied molecular orbital, which reduces the potential barrier to charge transport and increases the kinetic stabilization energy of the metallic state. As part of ongoing efforts to modify the solid-state structures and transport properties of these so-called multiorbital materials, we report the preparation and characterization of the acetoxy, methoxy, and thiomethyl derivatives 6 (R = OAc, OMe, SMe). The crystal structures are based on ribbonlike arrays of radicals laced together by S···N' and S···O' secondary bonding interactions. The steric and electronic effects of the exocyclic ligands varies, affording one-dimensional (1D) π-stacked radicals for R = OAc, 1D cofacial dimer π-stacks for R = SMe, and a pseudo two-dimensional (2D) brick-wall arrangement for R = OMe. Variable-temperature magnetic and conductivity measurements reveal strong antiferromagnetic interactions and Mott insulating behavior for the two radical-based structures (R = OAc, OMe), with lower room-temperature conductivities (σ_RT ≈ 1 × 10^-4 and ∼1 × 10^-3 S cm^-1, respectively) and higher thermal activation energies ( E_act = 0.24 and 0.21 eV, respectively) than found for the ideal 2D brick-wall structure of 6 (R = F), where σ_RT ≈ 1 × 10^-2 S cm^-1 and E_act = 0.10 eV. The performance of R = OMe, OAc relative to that of R = F, is consistent with the results of density functional theory band electronic structure calculations, which indicate a lower kinetic stabilization energy of the putative metallic state arising from their reduced electronic dimensionality.

Construction of three-dimensional anionic molecular frameworks based on hydrogen-bonded metal dithiolene complexes and the crystal solvent effect

Three-dimensional hydrogen-bonded anionic molecular frameworks based on a metal dithiolene complex were constructed with a significant solvent effect.

Charge-transfer complexes of sulfur-rich acceptors derived from birhodanines

The title acceptors form charge-transfer complexes with mixed stacks, whose transistors are affected by the S–S interaction between the acceptors.