Synthesis of π-Extended Thiele’s and Chichibabin’s Hydrocarbons and Effect of the π-Congestion on Conformations and Electronic States

1,1'-Biolympicenyl: A Stable Non-Kekulé Diradical with a Small Singlet and Triplet Energy Gap

Dimerization of delocalized polycyclic hydrocarbon radicals is a simple and versatile method to create diradicals with tailored electronic structures and accessible high-spin states. However, the synthesis is challenging, and the stability issue of the diradicals remains a concern. In this study, we present the synthesis of a stable non-Kekulé 1,1'-biolympicenyl diradical 1 using a protection-oxidation-protection strategy. Diradical 1 demonstrated exceptional stability, with a solution half-life time exceeding 3.5 years and a solid state thermal decomposition temperature above 300 °C. X-ray crystallographic analysis revealed its intersected molecular structure and tightly bound dimer configuration. A singlet ground state with a small singlet-triplet energy gap is consistently identified using electron paramagnetic resonance (EPR) and a superconducting quantum interference device (SQUID) in a rigid matrix, and the triplet state is thermally accessible at room temperature. The solution phase properties were systematically examined through EPR, absorption spectroscopy, and cyclic voltammetry, revealing a rotational motion in the slow-motion regime and multistage redox characteristics. This study presents an efficient synthetic and stabilization strategy for organic diradicals, enabling the development of various high-spin functional materials.

Double Spin with a Twist: Synthesis and Characterization of a Neutral Mixed-Valence Organic Stable Diradical

A convenient design strategy opens access to neutral open-shell mixed-valence species via the redox transformation of charged stable precursors, i.e., the spiro-fused borate anions. We have implemented this strategy for the synthesis of the first neutral mixed-valence diradical: two neutral mixed-valence radical fragments were assembled via a twisted biphenyl bridge. The diradical is a crystalline solid obtained in almost quantitative yield by using a facile synthetic procedure. It is stable at room temperature in the triplet ground state with a very small singlet/triplet gap. This metal-free diradical can reversibly form five redox states. The diradical exhibits an intense IVCT band in the NIR region and can be assigned as a Class 2 Robin-Day MV (mixed valence) system with weakly interacting redox centers. Computations suggest that this diradical finds itself in a unique tug-of-war between two electron delocalization patterns, Kekulé and non-Kekulé, which gives rise to two geometric isomers that are close in energy but drastically different in spin distribution and polarity. Such bistable spin-systems should be intrinsically switchable and promising for the design of functional spin devices. The scope and limitations of the new redox-strategy for the neutral MV radicals were also tested on other types of spiro-fused borates, revealing structural factors responsible for the evolution from transient to persistent and then to stable radicals.

Twisted diphenoquinones fused with thiophene rings: thiophene analogs of bianthrone

The twisted conformer of bistricyclic aromatic enes (BAEs) has a small HOMO-LUMO gap owing to the twisted double bond. In this study, we synthesized diphenoquinones fused with thiophene rings as a new twisted conformer-predominant BAE. They exhibited deep LUMO energy levels and apparent n-type semiconductor properties.

Synthesis and reactivity of the di(9-anthryl)methyl radical

The di(9-anthryl)methyl (DAntM) radical was synthesized and investigated to elucidate its optical, electrical properties, and reactivity. The generation of the DAntM radical was confirmed by its ESR spectrum, which showed two broad signals. The unpaired electron is primarily localized on the central sp2 carbon and slightly delocalized over the two anthryl moieties. Although the DAntM radical undergoes dimerization in solution, the radical still remains even at 190 K due to the bulky nature of the two anthryl groups. Interestingly, upon exposure to air, the purple color of the radical solution quickly fades to orange, resulting in decomposition to give 9-anthryl aldehyde and anthroxyl radical derivatives.

Needlestick-Stimulation-Induced Conversion of Short-Wave Infrared-Light Transparency Using a Liquescent Radical Anion

A liquescent salt consisting of a 7,7,8,8-tetracyanquinodimethane (TCNQ) radical anion and a tetra-n-decylammonium ion, 1+•TCNQ•-, exhibits rapid changes in the short-wave infrared (SWIR) light transparency at 1000-1400 nm upon the application of a one-shot needlestick-stimulus. Radical anion salt 1+•TCNQ•- transforms from a blue solid to a green liquid at 90 °C without decomposition under aerated conditions, and remains in the liquid state upon cooling to 70 °C. After applying pressure with a needlestick on a cover glass at 70 °C, the liquid transforms rapidly into the solid state over a timescale of seconds across a centimeter scale of area. Along with the liquid-solid transition, the SWIR-light transparency at 1200 nm completely switches from the "on" to the "off" states. Experimental results, such as electronic spectra and crystal structure analysis, indicates that the SWIR-light absorption in the solid state is due to the existence of a slipped-stacking π-dimer structure for TCNQ•-. The rapid rearrangement is induced by the formation of the π-dimer structures from the monomers of TCNQ•- and the subsequent generations of the solid-state seed.

On-Surface Covalent Synthesis of Carbon Nanomaterials by Harnessing Carbon gem-Polyhalides

The design of innovative carbon-based nanostructures stands at the forefront of both chemistry and materials science. In this context, π-conjugated compounds are of great interest due to their impact in a variety of fields, including optoelectronics, spintronics, energy storage, sensing and catalysis. Despite extensive research efforts, substantial knowledge gaps persist in the synthesis and characterization of new π-conjugated compounds with potential implications for science and technology. On-surface synthesis has emerged as a powerful discipline to overcome limitations associated with conventional solution chemistry methods, offering advanced tools to characterize the resulting nanomaterials. This review specifically highlights recent achievements in the utilization of molecular precursors incorporating carbon geminal (gem)-polyhalides as functional groups to guide the formation of π-conjugated 0D species, as well as 1D, quasi-1D π-conjugated polymers, and 2D nanoarchitectures. By delving into reaction pathways, novel structural designs, and the electronic, magnetic, and topological features of the resulting products, the review provides fundamental insights for a new generation of π-conjugated materials.

A Stable Chichibabin Diradicaloid with Near-Infrared Emission

Conjugated molecules with multiple radical centers such as the iconic Chichibabin diradicaloid hold promise as building blocks in materials for quantum sensing and quantum information processing. However, it is a considerable challenge to design simple analogues of the Chichibabin hydrocarbon that are chemically inert, exhibit high diradical character and emit light at a distinct wavelength that may offer an optical readout of the spin state in functional ensembles. Here we describe the serendipitous discovery of the stable TTM-TTM diradicaloid, which exhibits high diradical character, a striking sky-blue color and near-infrared (NIR) emission (in solution). This combination of properties is unique among related diradicaloids and is due to the presence of hydrogen and chlorine atoms in "just the right positions", allowing a perfectly planar, yet predominantly benzenoid bridge to connect the two sterically stabilized radical centers. In-depth studies of the optical and magnetic properties suggest that this structural motif could become a mainstay building block of organic spin materials.

Polycationic Open-Shell Cyclophanes: Synthesis of Electron-Rich Chiral Macrocycles, and Redox-Dependent Electronic States

π-Conjugated chiral nanorings with intriguing electronic structures and chiroptical properties have attracted considerable interests in synthetic chemistry and materials science. We present the design principles to access new chiral macrocycles (1 and 2) that are essentially built on the key components of main-group electron-donating carbazolyl moieties or the π-expanded aza[7]helicenes. Both macrocycles show the unique molecular conformations with a (quasi) figure-of-eight topology as a result of the conjugation patterns of 2,2',7,7'-spirobifluorenyl in 1 and triarylamine-coupled aza[7]helicene-based building blocks in 2. This electronic nature of redox-active, carbazole-rich backbones enabled these macrocycles to be readily oxidized chemically and electrochemically, leading to the sequential production of a series of positively charged polycationic open-shell cyclophanes. Their redox-dependent electronic states of the resulting multispin polyradicals have been characterized by VT-ESR, UV/Vis-NIR absorption and spectroelectrochemical measurements. The singlet (ΔES-T=-1.29 kcal mol-1) and a nearly degenerate singlet-triplet ground state (ΔES-T(calcd)=-0.15 kcal mol-1 and ΔES-T(exp)=0.01 kcal mol-1) were proved for diradical dications 12+2⋅ and 22+2⋅, respectively. Our work provides an experimental proof for the construction of electron-donating new chiral nanorings, and more importantly for highly charged polyradicals with potential applications in chirospintronics and organic conductors.

Stimuli-Induced Controls of Magnetic and Photophysical Properties Using Liquescent Open-Shell Ionic Molecular Systems

This study explores the remarkable properties of liquescent open-shell ionic molecular systems, emphasizing the magnetic and photophysical characteristics arising from their associated structures in the condensed state under various conditions. Well-investigated open-shell molecules, namely, phenothiazine, dihydrophenazine, and tetrathiafulvalene radical cations, and bis(malononitriledithiolato)nickel(III) anionic complexes were examined, and the concept of liquescent open-shell ionic molecular systems was devised. Transformations in their associated structures are induced by external stimuli, resulting in significant variations in their physical properties. These experimental findings open new avenues for exploring and applying stimuli-responsive molecule-based materials.

Thermal Equilibrium Between Quinoid/Biradical Forms Enhancing Electrochemical Amphotericity

Upon dibenzo annulation on Thiele's hydrocarbon (tetraphenyl-p-quinodimethane), the quinoid form and the biradical form adopt quite different geometries, and thus are no longer resonance structures. When these two forms can interconvert rapidly due to the small energy barrier (ΔG≠), the equilibrated mixture contains both forms in a ratio that is determined by the energy difference (ΔGo) between the two forms. For a series of tetrakis[5-(4-methoxyphenyl)-2-thienyl]-substituted derivatives, the more stable quinoid form and the metastable biradical form coexist in solution as an equilibrated mixture due to small ΔG≠ (<15 kcal mol-1) and ΔGo (1-4 kcal mol-1), in which the proportion of the two forms can be regulated by temperature. Since the biradical form can undergo easy two-electron (2e) oxidation to the corresponding dications as well as easy 2e-reduction to the dianions, it exhibits very high electrochemical amphotericity. This character with a record-small span for not only the first oxidation and reduction potentials but also the second those, [E1 sum≈E2 sum=E2 ox-E2 red=ca. 1.4 V], is attained through thermally enhanced conversion to the biradical form from the corresponding quinoid form, the latter of which is less amphoteric due to higher Eox and lower Ered values.

Breaking Global Diatropic Current to Tame Diradicaloid Character: Thiele's Hydrocarbon Under Macrocyclic Constraints

A diradical/biradical character of organic derivatives is one of the key aspects of contemporary research focusing on the fundamental studies followed by potential applicability relying on the unique optical, electronic, or magnetic properties assigned to unpaired electrons. A precise involvement of two p-phenylenes into a cyclophane-like conjugated, diatropic system creates a flexible molecule with the two different characters of both subunits (benzene and quinone) imprinting into the structure a Kekulé delocalized system. A dynamic of both carbocyclic subunits, and their mutual interaction generates a singlet open-shell state (J=-1.25 kcal/mol) as documented spectroscopically (NMR and EPR). The extended theoretical analysis has proved a correlation between dihedral angle and the diradicaloid character that shifts from a closed-shell singlet to an open-shell state, eventually showing the y0=0.86 for 78 degrees and ΔEST=-0.34 kcal/mol.

Sulfone-Functionalized Chichibabin's Hydrocarbons: Stable Diradicaloids with Symmetry Breaking Charge Transfer Contributing to NIR Emission beyond 900 nm

While monoradical emitters have emerged as a new route toward efficient organic light-emitting diodes, the luminescence property of organic diradicaloids is still scarcely explored. Herein, by devising a novel radical-radical coupling-based synthetic approach, we report a new class of sulfone-functionalized Chichibabin's hydrocarbon derivatives, SD-1-3, featuring varied substituent patterns and moderate to high diradical characters of 0.44-0.70, as highly stable diradicaloids with rarely seen NIR emission beyond 900 nm. Via comprehensive experimental and theoretical investigations, we reveal that the optoelectronic and magnetic properties of these materials are significantly tuned by the variations of substitutions (H/CF3/OMe) on the molecular skeletons. More importantly, quantum chemical computations indicate that the embedding of sulfone groups has contributed to a breaking of their quasi-C2 symmetry of these diradicaloid molecules and results in an excited-state charge transfer character. Therefore, a remarkably deep NIR emissive wavelength of up to 998 nm, together with a large Stokes shift (∼386 nm), is achieved for the CF3-based SD-2 molecule in tetrahydrofuran. To the best of our knowledge, such a luminescent wavelength of SD-2 has represented the longest wavelengths among the currently reported organic fluorescent radicals. Overall, our work not only establishes a new synthetic approach toward stable Chichibabin's hydrocarbons but also paves the way for designing NIR emissive open-shell materials with both fundamental understanding and feasible control of their luminescent properties.

Carbon-based Biradicals: Structural and Magnetic Switching

Sterically hindered C═C double bonds often deform into a bent or twisted geometry. Thus, many overcrowded ethylenes or anthraquinodimethanes can adopt multiple conformations, such as a folded form or a twisted form, which are interconvertible under the application of external stimuli. A perpendicular form with biradical character can also be adopted when designed to incorporate a stable carbon-based radical unit, which is involved in stimuli-responsive magnetic switching accompanied by a structural change. This review focuses on recent advances in the development of such strained π-electron systems and reveals the factors that affect the mutual interconversion and switching behavior. The energy barrier for the interconversion of conformational isomers is affected by the tricyclic skeleton or bulky substituents on the C═C double bonds, whereas the relative stability of the perpendicular biradical form increases with the additional insertion of 9,10-anthrylene units into the C═C double bonds.

Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure

Herein, we report the synthesis of two "partially embedded fused-dihydropyridazine N-aryl aza[5]helicene derivatives" (PDHs) and the demonstration of their intrinsic photo-triggered multi-functional properties based on a Kekulé biradical structure. Introducing bulky electron-withdrawing trifluoromethyl or pentafluoroethyl groups into the aza[5]helicene framework (PDH-CF3 and -C2 F5 ) gives PDH axial chirality based on the helicity of the P and M forms, even at room temperature. Upon photo-irradiation of PDH-CF3 in a frozen solution, an ESR signal from the triplet biradical with zero-field splitting values, generated by N-N bond dissociation, was observed. However, when the irradiation was turned off, the ESR signal became silent, thus indicating the existence of two equilibria: between the biradical and quinoidal forms based on the Kekulé structure, and between N-N bond cleavage and recombination. The observed photo- and thermally induced behaviors indicate that T-type photochromic molecules are involved in the photoisomerization mechanism involving the two equilibria. Inspired by the photoisomerization, chirality control of PDH by photoracemization was achieved. Multiple functionalities, such as T-type photochromism, photo-excitation-mediated triplet biradical formation, and photoracemization, which are attributed to the "partially embedded dihydropyridazine" structure, are demonstrated.

Tetrafluorenofulvalene as a sterically frustrated open-shell alkene

Electronic and steric effects are known to greatly influence the structure, characteristics and reactivity of organic compounds. A typical π bond is weakened by oxidation (corresponding to the removal of electrons from bonding orbitals), by reduction (through addition of electrons to antibonding orbitals) and by unpairing of the bonding electrons, such as in the triplet state. Here we describe tetrafluorenofulvalene (TFF), a twisted, open-shell alkene for which these general rules do not hold. Through the synthesis, experimental characterization and computational analysis of its charged species spanning seven redox states, the central alkene bond in TFF is shown to become substantially stronger in the tri- and tetraanion, generated by chemical reduction. Furthermore, although its triplet state contains a weaker alkene bond than the singlet, in the quintet state its bond order increases substantially, yielding a flatter structure. This behaviour originates from the doubly bifurcated topology of the underlying spin system and can be rationalized by the balancing effects of benzenoid aromaticity and spin pairing.

Polyradical character assessment using multireference calculations and comparison with density-functional derived fractional occupation number weighted density analysis

The biradicaloid character of different types of polycyclic aromatic hydrocarbons (PAHs) based on small band gaps is an important descriptor to assess their opto-electronic properties. In this work, the unpaired electron densities and numbers of unpaired electrons (NU values) calculated at the high-level multireference averaged quadratic coupled-cluster (MR-AQCC) method are used to develop a test set to assess the capabilities of different biradical descriptors based on density functional theory. A benchmark collection of 29 different compounds has been selected. The DFT descriptors contain primarily the fractional occupation number weighted electron density (FOD) based on simplified thermally-assisted-occupation density functional theory (TAO-DFT) calculations, but the singlet-triplet energy difference and other descriptors denoted as y0 and nLUNO have been considered as well. After adjustment of the literature-recommended finite temperatures, a very good, detailed agreement between unpaired density and FOD analysis is observed which is also manifested in excellent statistical correlations. The other two descriptors also show good correlations even though the absolute scaling is not satisfactory. A new linear fit of FOD data to the MR-AQCC reference values leads to an improved regression relation for determining the recommended finite temperature value in dependence of the Hartree-Fock exchange. This provides the basis for fast and reliable assessment of the biradical character of many classes of PAHs without the need for performing computationally extended MR calculations.

Dimeric tetrabromo-p-quinodimethanes: synthesis and structural/electronic properties

Despite their great potential as molecular building blocks for organic synthesis, tetrabromo-p-quinodimethanes (TBQs) are a relatively unknown family of compounds. Herein, we showcase a series of five derivatives incorporating two tetrabromo-anthraquinodimethane (TBAQ) units linked by π-conjugated spacers of different nature and length. The resulting dimers TBQ1-5 are fully characterised by means of thorough spectroscopic measurements and theoretical calculations. Interestingly, owing to the steric hindrance imposed by the four bulky bromine atoms, the TBAQ fragments adopt a characteristically warped geometry, somehow resemblant of a butterfly, and the novel dimers show a complex NMR pattern with signal splittings. To ascertain whether dynamic processes regarding fluxional inversion of the butterfly configurations are involved, first-principles calculations assessing the interconversion energy barriers are performed. Three possible stereoisomers are predicted involving two diastereomers, thus accounting for the observed NMR spectra. The rotational freedom of the TBAQ units around the π-conjugated linker influences the structural and electronic properties of TBQ1-5 and modulates the electronic communication between the terminal TBAQ moieties. The role of the linker on the electronic properties is investigated by Raman and UV-vis spectroscopies, theoretical calculations and UV-vis measurements at low temperature. TBQ1-5 are of interest as less-explored structural building precursors for a variety of scientific areas. Finally, the sublimation, self-assembly and reactivity on Au(111) of TBQ3 is assessed.

Dark State of the Thiele Hydrocarbon: Efficient Solvatochromic Emission from a Nonpolar Centrosymmetric Singlet Diradicaloid

In this work, a comprehensive investigation of the photoinduced processes and mechanisms linked to the luminescence of a novel nonperchlorinated Thiele hydrocarbon (TTH) is presented. Despite the comparable diradical character of TTH (y0 = 0.32-0.44) and the unsubstituted Thiele hydrocarbon (TH) (y0 = 0.30), the polyhalogenated species is inert and photostable, showing an intense deep-red/near-infrared (NIR) fluorescence (photoluminescence quantum yield (PLQY) = 0.84 in toluene) even at room temperature and in the solid state (PLQY = 0.19). TTH displays a large Stokes shift (307 nm in benzonitrile) and solvatochromic behavior, which is unusual for a centrosymmetric, nonpolar, and low-conjugated species. These outstanding emission features are interpreted through quantum-chemical calculations, indicating that its fluorescence arises from the low-lying dark doubly excited zwitterionic state, typically found at low excitation energies in diradicaloids, acquiring dipole moment and intensity by state mixing via twisting around the strongly elongated exocyclic CC bonds of the excited p-quinodimethane (pQDM) core, with a mechanism similar to sudden polarization occurring in olefins. Such a mechanism is derived from ns and fs transient absorption measurements.

Synthesis, structural analysis, and properties of highly twisted alkenes 13,13'-bis(dibenzo[a,i]fluorenylidene) and its derivatives

The rotation of a C = C bond in an alkene can be efficiently accelerated by creating the high-strain ground state and stabilizing the transition state of the process. Herein, the synthesis, structures, and properties of several highly twisted alkenes are comprehensively explored. A facile and practical synthetic approach to target molecules is developed. The twist angles and lengths of the central C = C bonds in these molecules are 36-58° and 1.40-1.43 Å, respectively, and confirmed by X-ray crystallography and DFT calculations. A quasi-planar molecular half with the π-extended substituents delivers a shallow rotational barrier (down to 2.35 kcal/mol), indicating that the rotation of the C = C bond is as facile as that of the aryl-aryl bond in 2-flourobiphenyl. Other versatile and unique properties of the studied compounds include a broad photoabsorption range (from 250 up to 1100 nm), a reduced HOMO-LUMO gap (1.26-1.68 eV), and a small singlet-triplet energy gap (3.65-5.68 kcal/mol).

Controlling the Diradical Character of Thiele Like Compounds

Organic diradicals play an important role in many fields of chemistry, biochemistry, and materials science. In this work, by means of high-level theoretical calculations, we have investigated the effect of representative chemical substituents in p-quinodimethane (pQDM) and Thiele's hydrocarbons with respect to the singlet-triplet energy gap, a feature characterizing their diradical character. We show how the nature of the substituents has a very important effect in controlling the singlet-triplet energy gap so that several compounds show diradical features in their ground electronic state. Importantly, steric effects appear to play the most determinant role for pQDM analogues, with minor effects of the substituents in the central ring. For Thiele like compounds, we found that electron-withdrawing groups in the central ring favor the quinoidal form with a low or almost null diradical character, whereas electron-donating group substituents favor the aromatic-diradical form if the electron donation does not exceed 6-π electrons. In this case, if there is an excess of electron donation, the diradical character is reduced. The electronic spectrum of these compounds is also calculated, and we predict that the most intense bands occur in the visible region, although in some cases characteristic electronic transition in the near-IR region may appear.

Synthesis and structure elucidation of triarylmethyl radicals with anthryl substitution

Two stable triarylmethyl radicals with one or two anthryl substitutions are synthesized in gram scale, and are isolated in the crystalline state. Detailed structural elucidation with X-ray crystallographic analysis and DFT calculations revealed that the twisted structure is more energetically favorable than the folded structure, and consequently, the spin density is mainly localized at the methyl carbon. The spin distribution leads to unique physical properties, making them promising open-shell organic materials.

6,6'-Biindeno[1,2-b]anthracene: An Open-Shell Biaryl with High Diradical Character

We report in situ generation of a 6,6'-biindeno[1,2-b]anthracene (BIA) derivative as an open-shell biaryl with high diradical character, which could be identified by mass spectrometry, NMR spectroscopy, single-crystal X-ray analysis, UV-vis-NIR absorption spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. Theoretical calculations by various methods and variable-temperature EPR analyses were performed to tackle the elusive ground state of BIA diradical, suggesting a singlet ground state with a nearly degenerate triplet state. These results provide insight into the design of unique open-shell biaryls.

Redox Active cAAC-Fluorene/Indene Systems Displaying Solvatochromism, Green Luminescence and pH Sensing: Functionalization of Fluorenyl/Indenyl Rings with Radical Carbene

Two new series of air stable compounds of cAAC^X = fluorene/indene (X = Me₂ , Et₂ , Cy) [cAAC = cyclic (alkyl) amino carbene] have been isolated and well characterized by X-ray single crystal diffraction, photoluminescence, cyclic voltammogram (CV) and electron paramagnetic resonance (EPR) studies. Fluorescence studies reveals green light emission of cAAC bonded fluorene, whereas free fluorene generally displays a violet emission. Interestingly, the sterically crowded cAAC-fluorene analogue display solvatochromism and CF₃ CO₂ H sensing in solution. CV of the these compounds show a quasi-reversible electron transfer process, indicating the functionalization of fluorene/indene with radical anionic form of carbene, confirmed by CV/EPR measurements. DFT/TDDFT calculations and energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) have been carried out to study different aspects of bonding and electronic transitions. Such a class of redox active and thermally stable organic molecules may be suitable for molecule based spin memory devices in future.

Dibenzotropylium-Capped Orthogonal Geometry Enabling Isolation and Examination of a Series of Hydrocarbons with Multiple 14π-Aromatic Units

A series of six dications composed of pure hydrocarbons with one to six non-substituted 9,10-anthrylene units end-capped with two dibenzotropyliums were designed and synthesized to elucidate the electronic properties of huge oligo(9,10-anthrylene) backbones. Their structures were successfully determined by X-ray analyses even in the case of eight planar 14π-electron units, revealing that all dications adopt almost orthogonally twisted structures between neighboring units. Spectroscopic and voltammetric analyses show that neither the significant overlap of orbitals nor the delocalization of electrons between 14π-electron units occurs due to the orthogonally twisted geometry even in solution. As a result, sequential oxidation processes were observed with the reversible formation of multivalent cations with the release of the same number of electrons as the number of anthrylene units. Upon two-electron reduction, a closed-shell butterfly-shaped form was obtained from the dication containing one anthrylene unit, whereas open-shell twisted biradicals were isolated as stable entities in the cases of derivatives containing three to six anthrylene units. Notably, from the derivative with two anthrylene units, a metastable open-shell isomer was obtained quantitatively and underwent slow thermal conversion to the most stable closed-shell isomer (E_a = 23.1 kcal mol^-1). There is a drastic change in oxidation potentials between two neutral species (ΔE = 1.32 V in CH₂Cl₂). Since the present dications were regenerated upon oxidation of the isolated reduction products, these systems may contribute to the development of advanced response systems capable of switching color, magnetic properties, and oxidative properties by using a "cation-capped orthogonal geometry".

Multiple stable redox states and tunable ground states via the marriage of viologens and Chichibabin's hydrocarbon†

Chichibabin's hydrocarbon and viologens are among the most famous diradicaloids and organic redox systems, respectively. However, each has its own disadvantages: the instability of the former and its charged species, and the closed-shell nature of the neutral species derived from the latter, respectively. Herein, we report that terminal borylation and central distortion of 4,4′-bipyridine allow us to readily isolate the first bis-BN-based analogues (1 and 2) of Chichibabin's hydrocarbon with three stable redox states and tunable ground states. Electrochemically, both compounds exhibit two reversible oxidation processes with wide redox ranges. One- and two-electron chemical oxidations of 1 afford the crystalline radical cation 1˙⁺ and dication 1²⁺, respectively. Moreover, the ground states of 1 and 2 are tunable with 1 as a closed-shell singlet and the tetramethyl-substituted 2 as an open-shell singlet, the latter of which could be thermally excited to its triplet state because of the small singlet-triplet gap.

Herein, we report the isolation of bis-BN-based species 1 and 2 with multiple stable redox states. Their ground states are tunable with 1 as a closed-shell singlet and 2 as an open-shell singlet with a small singlet-triplet gap.