Thermally controlling the singlet-triplet energy gap of a diradical in the solid state

Effect of Oriented External Electric Fields on the Electronic Properties of Linear Acenes: A Thermally Assisted Occupation DFT Study

Recently, oriented external electric fields (OEEFs) have earned much attention due to the possibility of tuning the properties of electronic systems. From a theoretical perspective, one can resort to electronic structure calculations to understand how the direction and strength of OEEFs affect the properties of electronic systems. However, for multi-reference (MR) systems, calculations employing the popular Kohn-Sham density functional theory with the traditional semilocal and hybrid exchange-correlation energy functionals can yield erroneous results. Owing to its decent compromise between accuracy and efficiency for MR systems at the nanoscale (i.e., MR nanosystems), in this study, thermally assisted occupation density functional theory (TAO-DFT) is adopted to explore the electronic properties of n-acenes (n = 2-10), containing n linearly fused benzene rings, in OEEFs, where the OEEFs of various electric field strengths are applied along the long axes of n-acenes. According to our TAO-DFT calculations, the ground states of n-acenes in OEEFs are singlets for all the cases examined. The effect of OEEFs is shown to be significant on the vertical ionization potentials and vertical electron affinities of ground-state n-acenes with odd-number fused benzene rings. Moreover, the MR character of ground-state n-acenes in OEEFs increases with the increase in the acene length and/or the electric field strength.

Electronic Properties of Graphene Nano-Parallelograms: A Thermally Assisted Occupation DFT Computational Study

In this computational study, we investigate the electronic properties of zigzag graphene nano-parallelograms (GNPs), which are parallelogram-shaped graphene nanoribbons of various widths and lengths, using thermally assisted occupation density functional theory (TAO-DFT). Our calculations revealed a monotonic decrease in the singlet-triplet energy gap as the GNP length increased. The GNPs possessed singlet ground states for all the cases examined. With the increase of GNP length, the vertical ionization potential and fundamental gap decreased monotonically, while the vertical electron affinity increased monotonically. Some of the GNPs studied were found to possess fundamental gaps in the range of 1-3 eV, lying in the ideal region relevant to solar energy applications. Besides, as the GNP length increased, the symmetrized von Neumann entropy increased monotonically, denoting an increase in the degree of the multi-reference character associated with the ground state GNPs. The occupation numbers and real-space representation of active orbitals indicated that there was a transition from the nonradical nature of the shorter GNPs to the increasing polyradical nature of the longer GNPs. In addition, the edge/corner localization of the active orbitals was found for the wider and longer GNPs.

Extremely Stable Perylene Bisimide-Bridged Regioisomeric Diradicals and Their Redox Properties

Excellent stability is an essential premise for organic diradicals to be used in organic electronic and spintronic devices. We have attached two tris(2,4,6-trichlorophenyl)methyl (TTM) radical building blocks to the two sides of perylene bisimide (PBI) bridges and obtained two regioisomeric diradicals (1,6-TTM-PBI and 1,7-TTM-PBI). Both of the isomers show super stability rather than the monomeric TTM under ambient conditions, due to the increased conjugation and the electron-withdrawing effects of the PBI bridges. The diradicals show distinct and reversible multistep redox processes, and a spectro-electrochemistry investigation revealed the generation of organic mixed-valence (MV) species during reduction processes. The two diradicals have singlet ground states, very small singlet-triplet energy gaps (ΔES-T ) and a pure open-shell character (with diradical character y0 =0.966 for 1,6-TTM-PBI and 0.967 for 1,7-TTM-PBI). This work opens a window to developing very stable diradicals and offers the opportunity of their further application in optical, electronic and magnetic devices.

Structure-property-function relationships of stabilized and persistent C- and N-based triaryl radicals

Structurally similar C- and N-based triaryl radicals are among the most commonly used structural motifs in stable, open-shell, organic molecules. The application of such species is associated with their stability, properties and structural design. This study summarizes the basic stabilization and persistence principles of C- and N-based triaryl radicals and highlights recent advances in design strategies of radicals tailored for specific applications.

Thermally Induced Persistent Covalent-Organic Frameworks Radicals

Persistent covalent-organic framework (COF) radicals hold important applications in magnetics and spintronics; however, their facile synthesis remains a daunting challenge. Here, three p-phenylenediacetonitrile-based COFs (named CityU-4, CityU-5, and CityU-6) were synthesized. Upon heat treatment (250 °C for CityU-4 and CityU-5 or 220 °C for CityU-6), these frameworks were brought into their persistent radical forms (no obvious changes after at least one year), together with several observable factors, including color changes, red-shifted absorption, the appearance of electron spin resonance (ESR) signals, and detectable magnetic susceptibility. The theoretical simulation suggests that after heat treatment, lower total energy and nonzero spin density are two main factors to guarantee persistent COFs radicals and polarized spin distributions. This work provides an efficient method for the preparation of persistent COF radicals with promising potentials.

Chemical Tailoring Assisted non-TADF to TADF Switching in Carbazole-Benzophenone Emitter - An In-silico Investigation

Organic light-emitting diodes (OLEDs) have become one of the most popular lighting technologies since they offer several advantages over conventional devices. In carbazole-benzophenone (CzBP) OLED devices, the polymeric form of the compound is previously reported to be Thermally Activated Delayed Fluorescence (TADF)-active (ΔEST ≈0.12 eV), while the monomer (CzBP) (ΔEST ≈0.39 eV) does not. The present study examines the effect of chemical tailoring on the optical and photophysical properties of CzBP using DFT and TDDFT methods. The introduction of a single -NO2 group or di-substitution (-NO2 , -COOH or -CN) in the selected LUMO region of the reference CzBP monomer significantly reduces ΔEST ≈0.01 eV, projecting these systems as potential TADF-active emitters. Furthermore, the chemical modification of CzBP-LUMO alters the two-step TADF mechanism (T1 →T2 →S1 ) in CzBP (ES₁ >ET2 >ET₁ ) to the Direct Singlet Harvesting (T1 →S1 ) mechanism (ET2 >ES₁ >ET₁ ), which has recently been identified in the fourth-generation OLED materials.

A highly efficient open-shell singlet luminescent diradical with strong magnetoluminescence properties

Developing open-shell singlet (OS) diradicals with high luminescent properties and exceptional single-molecule magnetoluminescence (ML) performance is extremely challenging. Herein, we propose a concept to enhance luminescent efficiency by adjusting the donor conjugation of OS diradicals, thereby achieving a highly luminescent diradical, DR1, with outstanding stability and making it a viable option for use in the emitting layer of organic light-emitting diodes (OLEDs). More importantly, the 0.5 wt%-DR1 doped film demonstrates significant single-molecule magnetoluminescence (ML) properties. A giant ML value of 210% is achieved at a magnetic field of 7 T, showing the great potential of DR1 in magneto-optoelectronic devices.

Minimal Active Space for Diradicals Using Multistate Density Functional Theory

This work explores the electronic structure as well as the reactivity of singlet diradicals, making use of multistate density functional theory (MSDFT). In particular, we show that a minimal active space of two electrons in two orbitals is adequate to treat the relative energies of the singlet and triplet adiabatic ground state as well as the first singlet excited state in many cases. This is plausible because dynamic correlation is included in the first place in the optimization of orbitals in each determinant state via block-localized Kohn-Sham density functional theory. In addition, molecular fragment, i.e., block-localized Kohn-Sham orbitals, are optimized separately for each determinant, providing a variational diabatic representation of valence bond-like states, which are subsequently used in nonorthogonal state interactions (NOSIs). The computational procedure and its performance are illustrated on some prototypical diradical species. It is shown that NOSI calculations in MSDFT can be used to model bond dissociation and hydrogen-atom transfer reactions, employing a minimal number of configuration state functions as the basis states. For p- and s-types of diradicals, the closed-shell diradicals are found to be more reactive than the open-shell ones due to a larger diabatic coupling with the final product state. Such a diabatic representation may be useful to define reaction coordinates for electron transfer, proton transfer and coupled electron and proton transfer reactions in condensed-phase simulations.

The catenation of a singlet diradical dication and modulation of diradical character by metal coordination

A singlet bis(triarylamine) diradical dication and its zigzag 1D magnetic chain catenated by silver cations were isolated and characterized by single-crystal X-ray crystallography, EPR spectroscopy, SQUID measurements, cyclic voltammetry and...

Diamidocarbene-Based Thiele and Tschitschibabin Hydrocarbons: Carbonyl Functionalized Kekulé Diradicaloids

Herein, we report diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, diradicaloids that contain four carbonyl/amido functional groups. The impact of two different π-conjugated spacers, p-phenylene vs p,p'-biphenylene, has been realized. The quantum chemical calculations suggest diamidocarbene (DAC)-based Thiele hydrocarbon (p-phenylene bridged) closed-shell singlet is the ground state, whereas for the diamidocarbene (DAC)-based Tschitschibabin hydrocarbon (p,p'-biphenylene bridged), open-shell singlet is the ground state. The influence of two different π-conjugated spacers also has been reflected in their UV-vis spectra. To gain more information on the diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, we have also carried out cyclic voltammetry investigations along with UV-vis-NIR-spectroelectrochemical studies of their corresponding 2-e oxidized product.

Normal & reversed spin mobility in a diradical by electron-vibration coupling

π−conjugated radicals have great promise for use in organic spintronics, however, the mechanisms of spin relaxation and mobility related to radical structural flexibility remain unexplored. Here, we describe a dumbbell shape azobenzene diradical and correlate its solid-state flexibility with spin relaxation and mobility. We employ a combination of X-ray diffraction and Raman spectroscopy to determine the molecular changes with temperature. Heating leads to: i) a modulation of the spin distribution; and ii) a “normal” quinoidal → aromatic transformation at low temperatures driven by the intramolecular rotational vibrations of the azobenzene core and a “reversed” aromatic → quinoidal change at high temperatures activated by an azobenzene bicycle pedal motion amplified by anisotropic intermolecular interactions. Thermal excitation of these vibrational states modulates the diradical electronic and spin structures featuring vibronic coupling mechanisms that might be relevant for future design of high spin organic molecules with tunable magnetic properties for solid state spintronics.

In this manuscript, Negri, Zheng, Casado et al develop a stable and flexible diradical. Using a combination of experimental and theoretical techniques, they show how heating leads to change in the electronic and spin delocalizations ocurring between quinoidal and aromatic forms, and elucidate a unique spin-vibrational coupling.

Electronic Properties of Carbon Nanobelts Predicted by Thermally-Assisted-Occupation DFT

Accurate prediction of properties of large-scale multi-reference (MR) electronic systems remains difficult for traditional computational methods (e.g., the Hartree-Fock theory and Kohn-Sham density functional theory (DFT)). Recently, thermally-assisted-occupation (TAO)-DFT has been demonstrated to offer reliable description of electronic properties of various large-scale MR electronic systems. Consequently, in this work, TAO-DFT is used to unlock the electronic properties associated with C-Belt[n] (i.e., the carbon nanobelts containing n fused 12-membered carbon rings). Our calculations show that for all the system sizes reported (n = 4-24), C-Belt[n] have singlet ground states. In general, the larger the size of C-Belt[n], the more pronounced the MR character of ground-state C-Belt[n], as evident from the symmetrized von Neumann entropy and the occupation numbers of active TAO-orbitals. Furthermore, the active TAO-orbitals are delocalized along the circumference of C-Belt[n], as evident from the visualization of active TAO-orbitals.

Synthesis, Structures, and Electronic Properties of 2,7-Anthrylene-Based Azacyclophanes Bearing o-, m-, and p-Phenylenediamine Linkers

A series of novel azacyclophanes consisting of 2,7-anthrylene and phenylene units were designed and synthesized by the Buchwald-Hartwig coupling reaction to investigate their unique electronic properties in multiple oxidized states. Cyclic voltammetry showed that the p-phenylene derivative exhibited three reversible oxidation waves, whereas the o- and m-phenylene derivatives showed two quasi-reversible oxidation waves due to the complicated intramolecular interaction between the oxidized units and neutral units. Moreover, the absorption spectra of the p-phenylene derivative in different oxidation states showed absorption bands at 865 and 1025 nm, which were attributed to intramolecular charge-transfer interactions. The photophysical and electrochemical properties of the p-phenylene analog were also compared with those of the o- and m-phenylene derivatives based on theoretical calculations for further evaluation of the intramolecular electronic interactions.

Controlling pairing of π-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials.

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies but the external dynamical control of these states still awaits experimental realization. Here, via quantum chemical calculations, the authors demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units induces controlled pairing of π-conjugated electrons in a reversible way.

Twistable dipolar aryl rings as electric field actuated conformational molecular switches

The ability to control the chemical conformation of a system via external stimuli is a promising route for developing molecular switches. For eventual deployment as viable sub-nanoscale components that are compatible with current electronic device technology, conformational switching should be controllable by a local electric field (i.e. E-field gateable) and accompanied by a rapid and significant change in conductivity. In organic chemical systems the degree of π-conjugation is linked to the degree of electronic delocalisation, and thus largely determines the conductivity. Here, by means of accurate first principles calculations, we study the prototypical biphenyl based molecular system in which the dihedral angle between the two rings determines the degree of conjugation. In order to make this an E-field gateable system we create a net dipole by asymmetrically functionalising one ring with: (i) electron withdrawing (F, Br and CN), (ii) electron donating (NH2), and (iii) mixed (NH2/NO2) substituents. In this way, the application of an E-field interacts with the dipolar system to influence the dihedral angle, thus controlling the conjugation. For all considered substituents we consider a range of E-fields, and in each case extract conformational energy profiles. Using this data we obtain the minimum E-field required to induce a barrierless switching event for each system. We further extract the estimated switching speeds, the conformational probabilities at finite temperatures, and the effect of applied E-field on electronic structure. Consideration of these data allow us to assess which factors are most important in the design of efficient gateable electrical molecular switches.

Hole-mediated photoredox catalysis: tris(p-substituted)biarylaminium radical cations as tunable, precomplexing and potent photooxidants

Triarylamines are demonstrated as novel, tunable electroactivated photocatalysts that use dispersion precomplexation to harness the full potential of the visible photon (>4.0 V vs. SCE) in anti-Kasha photo(electro)chemical super-oxidations of arenes.

Noble metal nanoclusters-decorated NiFe layered double hydroxide superstructure as nanoreactors for selective hydrogenation catalysis

The ability to conduct the self-assembly of nanometer-scale building blocks is the core issue in achieving "bottom-up" fabrications of desired superstructures. However, the general fabrication strategy of a superstructure at an ultrathin nanoscale is not yet fully developed. Herein, we report a facile synthesis route for 3d transition metal layered double hydroxide (LDH) based laminar superstructures via a concise hydrothermal process. The LDH laminar superstructures are further decorated by noble metal clusters to form a new kind of 2-dimensional nanoreactor, which presents outstanding catalytic performance under mild conditions. It is suggested that dodecyl sulfate anions play a key role in the self-assembly of well-defined laminar superstructures, which provide a confinement effect and enhance the activity and selectivity of the catalytic process. The fabrication of an as-prepared nanoreactor reveals a new strategy to tailor the performance of catalysts with high selectivity.

TAO-DFT Study on the Electronic Properties of Diamond-Shaped Graphene Nanoflakes

At the nanoscale, it has been rather troublesome to properly explore the properties associated with electronic systems exhibiting a radical nature using traditional electronic structure methods. Graphene nanoflakes, which are graphene nanostructures of different shapes and sizes, are typical examples. Recently, TAO-DFT (i.e., thermally-assisted-occupation density functional theory) has been formulated to tackle such challenging problems. As a result, we adopt TAO-DFT to explore the electronic properties associated with diamond-shaped graphene nanoflakes with n = 2-15 benzenoid rings fused together at each side, designated as n-pyrenes (as they could be expanded from pyrene). For all the n values considered, n-pyrenes are ground-state singlets. With increasing the size of n-pyrene, the singlet-triplet energy gap, vertical ionization potential, and fundamental gap monotonically decrease, while the vertical electron affinity and symmetrized von Neumann entropy (which is a quantitative measure of radical nature) monotonically increase. When n increases, there is a smooth transition from the nonradical character of the smaller n-pyrenes to the increasing polyradical nature of the larger n-pyrenes. Furthermore, the latter is shown to be related to the increasing concentration of active orbitals on the zigzag edges of the larger n-pyrenes.

CAAC-Based Thiele and Schlenk Hydrocarbons

Diradicals have been of tremendous interest for over a century ever since the first reports of p- and m-phenylene-bridged diphenylmethylradicals in 1904 by Thiele and 1915 by Schlenk. Reported here are the first examples of cyclic(alkyl)(amino)carbene (CAAC) analogues of Thiele's hydrocarbon, a Kekulé diradical, and Schlenk's hydrocarbon, a non-Kekulé diradical, without using CAAC as a precursor. The CAAC analogue of Thiele's hydrocarbon has a singlet ground state, whereas the CAAC analogue of Schlenk's hydrocarbon contains two unpaired electrons. The latter forms a dimer, by an intermolecular double head-to-tail dimerization. This straightforward synthetic methodology is modular and can be extended for the generation of redox-active organic compounds.

Electronic Properties of Linear and Cyclic Boron Nanoribbons from Thermally-Assisted-Occupation Density Functional Theory

It remains rather difficult for traditional computational methods to reliably predict the properties of nanosystems, especially for those possessing pronounced radical character. Accordingly, in this work, we adopt the recently formulated thermally-assisted-occupation density functional theory (TAO-DFT) to study two-atom-wide linear boron nanoribbons l-BNR[2,n] and two-atom-wide cyclic boron nanoribbons c-BNR[2,n], which exhibit polyradical character when the n value (i.e., the number of boron atoms along the length of l-BNR[2,n] or the circumference of c-BNR[2,n]) is considerably large. We calculate various electronic properties associated with l-BNR[2,n] and c-BNR[2,n], with n ranging from 6 to 100. Our results show that l-BNR[2,n] and c-BNR[2,n] have singlet ground states for all the n values examined. The electronic properties of c-BNR[2,n] exhibit more pronounced oscillatory patterns than those of l-BNR[2,n] when n is small, and converge to the respective properties of l-BNR[2,n] when n is sufficiently large. The larger the n values, the stronger the static correlation effects that originate from the polyradical nature of these ribbons. Besides, the active orbitals are found to be delocalized along the length of l-BNR[2,n] or the circumference of c-BNR[2,n]. The analysis of the size-dependent electronic properties indicates that l-BNR[2,n] and c-BNR[2,n] can be promising for nanoelectronic devices.

The Diradical-Dication Strategy for BODIPY- and Porphyrin-Based Dyes with Near-Infrared Absorption Maxima from 1070 to 2040 nm

Four stable boron dipyrromethene (BODIPY)- and porphyrin-based bis-arylamine diradical dications were synthesized by two-electron oxidation of their neutral molecules. The two BODIPY-based dications have open-shell singlet ground states. UV/Vis absorption spectra of all four dications showed large redshifts in the NIR region compared to their neutral precursors with absorption maxima at 1274 and 1068 nm for the two BODIPY-based dications and 1746 and 2037 nm for the two porphyrin-based dications. Thus, two new types of NIR dyes with longer wavelengths are provided by the diradical-dication strategy, which can be applied for the generation of other NIR dyes with a range of different chromophores and auxochromes.

Carbene derived diradicaloids - building blocks for singlet fission?

Organic singlet diradicaloids promise application in non-linear optics, electronic devices and singlet fission. The stabilization of carbon allotropes/cumulenes (C1, C2, C4) by carbenes has been equally an area of high activity. Combining these fields, we showed recently that carbene scaffolds allow as well for the design of diradicaloids. Herein, we report a comprehensive computational investigation (CASSCF/NEVPT2; fractional occupation DFT) on the electronic properties of carbene-bridge-carbene type diradicaloids. We delineate how to adjust the properties of these ensembles through the choice of carbene and bridge and show that already a short C2 bridge results in remarkable diradicaloid character. The choice of the carbene separately tunes the energies of the S1 and T1 excited states, whereas the bridge adjusts the overall energy level of the excited states. Accordingly, we develop guidelines on how to tailor the electronic properties of these molecules. Of particular note, fractional occupation DFT is an excellent tool to predict singlet-triplet gaps.

Isomerism, Diradical Signature, and Raman Spectroscopy: Underlying Connections in Diamino Oligophenyl Dications

A diradical dication of a 4,4'-di(bis(1,4-methylphenyl)amino)-p-terphenyl oligomer has been characterized in solid-state by Raman spectroscopy and thermo-spectroscopy together with quantum chemical calculations. The diradical character has been evaluated on the basis of the Raman spectra and as a function of temperature. A complete understanding of the nature of the changes in solid state has been provided based on a pseudo-Jahn-Teller effect, which is feasible owing to the fine balance between quinoidal/aromatic extension among consecutive rings and steric crowding. This study contributes to the further comprehension of the molecular and electronic structures of these particular diradical molecules with strong implications on the understanding of the nature of chemical bonds in the limits of high electronic correlation or π-conjugation.

S = 1 Tetraazacyclophane Diradical Dication with Robust Stability: A Case of Low-Temperature One-Dimensional Antiferromagnetic Chain

One-dimensional (1D) spin-1 ( S = 1) chain of organic radicals with low local magnetic anisotropy may provide a better understanding of the low-dimensional magnetism. We report solid-state studies, including single crystal X-ray crystallography, of air-stable tetraazacyclophane diradical dication salt 1^2·2+·2[Al(OC(CF₃)₂CH₃)₄]^- with a triplet ground state (Δ E_ST ≈ 0.5 kcal mol^-1). The magnetic behavior for 1^2·2+ at low temperature is best modeled by 1D spin S = 1 Heisenberg chain with intrachain antiferromagnetic coupling of J'/ k = -5.4 K, which is associated with the interaryl C···C contacts, including π-π interactions. Zero-field splitting value, | D/ hc| ≈ 5.6 × 10^-3 cm^-1, for 1^2·2+ is rather small; thus, the 1D chains are characterized by the high degree of isotropicity | D/2 J'| ≈ 7.5 × 10^-4. The diradical dication salt possesses extraordinary stability with onset of decomposition at temperature of about 180 °C (∼450 K), based on thermogravimetric analysis and EPR spectroscopy.

Porphyrinoids as a platform of stable radicals

The non-innocent ligand nature of porphyrins was observed for compound I in enzymatic cycles of cytochrome P450. Such porphyrin radicals were first regarded as reactive intermediates in catabolism, but recent studies have revealed that porphyrinoids, including porphyrins, ring-contracted porphyrins, and ring-expanded porphyrins, display excellent radical-stabilizing abilities to the extent that radicals can be handled like usual closed-shell organic molecules. This review surveys four types of stable porphyrinoid radical and covers their synthetic methods and properties such as excellent redox properties, NIR absorption, and magnetic properties. The radical-stabilizing abilities of porphyrinoids stem from their unique macrocyclic conjugated systems with high electronic and structural flexibilities.

Tetrabenzo-Chichibabin's hydrocarbons: substituent effects and unusual thermochromic and thermomagnetic behaviours

A series of new Chichibabin's hydrocarbons was studied, and a cationic ammonium substituted product was obtained, showing unusual thermochromic and thermomagnetic behaviours.

Isolable Bis(triarylamine) Dications: Analogues of Thiele's, Chichibabin's, and Müller's Hydrocarbons

Since the pioneering work by Thiele and Chichibabin, who synthesized the first diradicals bridged by phenylene and biphenylene groups in 1904 and 1907, respectively, numerous efforts have been devoted to synthesizing stable diradicals during the last few decades, and several strategies have been developed to stabilize these highly reactive diradicals. In this Account, we describe the synthesis and characterization of isolable bis(triarylamine) dications, nitrogen analogues of Thiele's, Chichibabin's, and Müller's hydrocarbons, which represent facilely accessible, stable diradicals by replacing carbinyl centers with isoelectronic aminium centers. Along with discussing the molecular structures and electronic structures of the isolated bis(triarylamine) dications, their spectroscopic and magnetic properties are also introduced. Since 2011, we have reported the stabilization of a variety of radical cations bearing the weakly coordinating anion Al(OR_F)₄^- (R_F = polyfluorinated alkyl group), which we have recently successfully applied for the stabilization and crystallization of bis(triarylamine) dications, analogues of Thiele's, Chichibabin's, and Müller's hydrocarbons. Prior to our and Kamada's work, there have been only three stable bis(triarylamine) dications isolated in the solid state. The facile access of bis(triarylamine) dications in their crystalline forms allowed us to pursue a deep investigation of their solid-state structures, electronic structures, and physical properties. Similar to their hydrocarbon analogues, bis(triarylamine) dications possess characteristic resonance structures between open-shell singlet (OS) diradicals and closed-shell (CS) quinoidal forms. The combination of single-crystal X-ray diffraction (XRD) analysis and density functional theory (DFT) calculations has proved to be a robust strategy to gain a better understanding of the electronic structures of the obtained diradicals. The structural parameters obtained from XRD analysis reflect the overall contribution of each resonance structure to the crystal structure. The comparison of the parameters from the crystal structures with those from DFT calculations for the pure electronic configurations (CS, OS, and triplet states) affords an overview of the ground-state structures of the diradicals. To justify the "degree" of singlet diradical character, the diradical parameter y is applied, which is estimated by the occupancy of the lowest unoccupied natural orbital (LUNO) having antibonding nature (y = 0 for the closed-shell and y = 1 for the pure singlet diradical). In addition, magnetic susceptibility measurements serve as a practical experimental method to determine the singlet-triplet energy gaps of the isolable diradical dications. Through detailed studies on isolable bis(triarylamine) dications, magnetic bistability caused by intramolecular electron-exchange interactions was observed. Moreover, we also found that the singlet-triplet energy gaps of the diradicals could be thermally controlled. These investigations highlight the potential of bis(triarylamine) dications as building blocks for functional materials.

Para-Quinodimethanes: A Unified Review of the Quinoidal-Versus-Aromatic Competition and its Implications

In this article, some quinoidal p-quinodimethanes compounds that convert partially or completely to diradicals or biradicaloids are analyzed. The aromatic/quinoidal balance is revisited with the objective of providing a common interpretation for most of them. For that purpose, important structural and energetic parameters such as the bond length alternation pattern and the singlet-triplet gaps are analyzed and interpreted in the framework of double spin polarization and π-conjugation. p-Quinodimethanes based in oligothiophenes, polycyclic aromatic hydrocarbons, oligophenylenes, thienothiophenes, charged dications and cyclic conjugated molecules are discussed. There are excellent reviews in the field of singlet diradicals; however, a revision similar to that proposed here can help the reader to have another perspective on these promising new functional materials. The focus has been put on molecules which are well known by the author and another of relevance in the field. In this regard, the article finishes with a discussion of some important applications of these diradicals in organic electronics. New chemical systems based on the p-quinodimethane building blocks are waiting us around the corner, bringing us new and challenging structures and fascinating novel properties, which describe a very rich field of research in chemistry and in physics with an excellent present and a bright future.