Introduction of Electron Donor Groups into the Azulene Structure: The Appearance of Intense Absorption and Emission in the Visible Region

In this work, through the Suzuki-Miyaura cross-coupling reaction with high yields, new π-conjugated azulene compounds containing diphenylaniline groups at positions 2 and 6 of azulene were synthesized. The obtained diphenylaniline-azulenes have intensely visible-light absorbing and emitting (in the wavelength range from 400 to 600 nm) properties. It has been shown that such unique optical properties, in particular fluorescent emission in the region of blue and green photoluminescence (λem at 495 and 525 nm), which were absent in the original azulene, are the result of the electron donor effect of diphenylaniline groups, which significantly changes the electronic structure of azulene and leads to the allowed HOMO → LUMO electron transition.

Synthesis and Properties of Cyclic π-Conjugated Molecules and Their Dication and Monoradical Cation

We synthesized cyclic π-conjugated molecules by double Friedel-Crafts reaction of amino group-substituted 1,2-bis(2-phenylethynyl)benzene with Meldrum's acid derivative. The structures of the cyclic π-conjugated molecules were determined by single-crystal X-ray structure analysis. The oxidation of the dimethylamino group-substituted π-conjugated molecule with NOBF4 gave a closed-shell dication that is stable at >210 °C. The monoradical cation of the di(4-methoxyphenyl)amino group-substituted π-conjugated molecule is stable in dichloromethane solution (half-life of nearly 15 days) and shows near-infrared absorption.

Intense absorption of azulene realized by molecular orbital inversion

The introduction of diarylamino groups at the 2- and 6-positions of azulene was found to invert the order of the orbital energy levels and allowed the HOMO-LUMO transition, resulting in a substantial increase in absorbance in the visible region. In addition, the stability of their one-electron oxidised species was improved by introducing bromine or methoxy groups at the 1- and 3-positions.

Oxidation State Tuning of Room Temperature Phosphorescence and Delayed Fluorescence in Phenothiazine and Phenothiazine-5,5-dioxide Dimers

Heterocyclic dimers consisting of combinations of butterfly-shaped phenothiazine (PTZ) and its chemically oxidized form phenothiazine-5,5-dioxide (PTZ(SO2 )) have been synthesized. A twist is imposed across the dimers by ortho-substituents including methyl ethers, sulfides and sulfones. X-ray crystallography, cyclic voltammetry and optical spectroscopy, underpinned by computational studies, have been employed to study the interplay between the oxidation state, conformational restriction, and emission mechanisms including thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). While the PTZ(SO2 ) dimers are simple fluorophores, the presence of PTZ induces triplet-mediated emission with a mixed PTZ-PTZ(SO2 ) dimer displaying concentration dependent hallmarks of both TADF and RTP.

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.

Crystalline radical cations of bis-BN-based analogues of Thiele's hydrocarbon

Two radical cations of bis-BN-based analogues of Thiele's hydrocarbons were facilely synthesized, fully characterized, and theoretically investigated. One-electron oxidation leads to the reduced bond length alternation and NICS values of the central C₄N₂ rings, suggesting the decreasing antiaromatic character. The spin density of the radical cations is significantly delocalized over the central linkers with a small contribution from two terminal N-heterocyclic boryl units.

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...

S,C,C- and O,C,C-Bridged Triarylamines and Their Persistent Radical Cations

This work reports the synthesis, crystal structures, and electronic properties of structurally constrained S,C,C- and O,C,C-bridged triarylamine derivatives and their persistent radical cations. O,C,C-Bridged triphenylamines and a dinaphthylphenylamine were obtained through a straightforward synthetic protocol. Similar to a previously reported S,C,C-bridged triphenylamine, the O,C,C-bridged triarylamines were easily oxidized to afford the corresponding radical cations, which were obtained as hexachloroantimonate salts. X-ray crystallographic analyses showed almost planar structures for these O,C,C-bridged triarylamine radical cations, which represent new members of the family of planar triarylamine radical cations without substituents on the aryl rings. Detailed investigations of the electronic properties of the S,C,C- and O,C,C-bridged triarylamine radical cations demonstrated that the spin and positive charge are sufficiently delocalized over the planar triarylamine scaffolds. The results provide the following insights into the effects of the bridging unit (sulfur vs oxygen) and the dibenzo-annulation on the spin delocalization in the bridged triarylamine radical cations: (1) An effective decrease of the spin density on the nitrogen atom is observed for the sulfur bridge relative to the oxygen bridge; and (2) a moderate decrease of the spin density on the oxygen atom rather than the nitrogen atom is induced by the dibenzo-annulation.

Crystal Engineering Using Polyiodide Halogen and Chalcogen Bonding to Isolate the Phenothiazinium Radical Cation and Its Rare Dimer, 10-(3-Phenothiazinylidene)phenothiazinium

Utilizing facile one-electron oxidation of 10H-phenothiazine by molecular diiodine, the solid-state structure of the 10H-phenothiazinium radical cation was obtained in three cation:iodide ratios, as well as its THF and acetone solvates. Oxidation of 10H-phenothiazine with molecular diiodine in DMSO or DMF provided the structure of the radical coupling product 10-(3-phenothiazinyldene)phenothiazinium, which has not been crystallographically characterized to date. The radical cations were balanced by a mixture (I₇ )^- , (I₅ )^- , (I₃ )^- , and I^- anions, where a variety of chalcogen, halogen, and hydrogen bonding interactions stabilize the structures to reveal these interesting cationic species.

Rational design and syntheses of aniline-based diradical dications: isolable congeners of quinodimethane diradicals

Two-electron oxidation of five aniline-based compounds 4,4′′-p/m-terphenyldiamines afforded the first isolable aniline-based diradical dications 1²⁺–5²⁺.

An Air-Stable, Neutral Phenothiazinyl Radical with Substantial Radical Stabilization Energy

The vital effect of radical states on the pharmacological activity of phenothiazine-based drugs has long been speculated. Whereas cationic radicals of N-substituted phenothiazines show high stability, the respective neutral radicals of N-unsubstituted phenothiazines have never been isolated. Herein, the 1,9-diamino-3,7-di-tert-butyl-N1 ,N9 -bis(2,6-diisopropylphenyl)-10H-phenothiazin-10-yl radical (SQH2 . ) is described as the first air-stable, neutral phenothiazinyl free radical. The crystalline dark-blue species is characterized by means of EPR and UV/Vis/near-IR spectroscopy, as well as cyclic voltammetry, spectro-electrochemical analysis, single-crystal XRD, and computational studies. The SQH2 . radical stands out from other aminyl radicals by an impressive radical stabilization energy and its parent amine has one of the weakest N-H bond dissociation energies ever determined. In addition to serving as open-shell reference in medicinal chemistry, its tridentate binding pocket or hydrogen-bond-donor ability might enable manifold uses as a redox-active ligand or proton-coupled electron-transfer reagent.

Bis[1]benzothieno[1,4]thiazines: Planarity, Enhanced Redox Activity and Luminescence by Thieno-Expansion of Phenothiazine

Twofold Buchwald-Hartwig aminations selectively furnish three regioisomers of bis[1]benzothieno[1,4]thiazines; X-ray structure analyses and DFT calculations were corroborated for correlation of their electronic properties. All regioisomers outscore the parent compound phenothiazine with respect to a low-lying oxidation potential and reversible redox activity. The anti-anti bis[1]benzothieno[3,2-b:2',3'-e][1,4]thiazines possess the lowest oxidation potentials in this series and displayed pronounced green luminescence in solution (Φ_F ≈20 %) and in the solid state. Syn-anti regioisomers were only weakly luminescent in solution, but showed aggregation-induced emission enhancement and solid-state luminescence. Most interestingly, X-ray structure analyses revealed that anti-anti derivatives have an amazingly coplanar structure of the pentacyclic anellated 1,4-thiazine system, emphasizing a structural similarity to heteroacenes. The calculated theoretical nucleus-independent chemical shifts additionally suggested that these 8π-electron core systems can be considered as the first electronically unbiased anellated 1,4-thiazines with antiaromatic character.

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.

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.

Oligomers of cyclopentadithiophene-vinylene in aromatic and quinoidal versions and redox species with intermediate forms

Quinoidal-vs.-aromatic synergy: Cyclopentadithiophene-vinylene oligomers with different sizes, in different oxidation states and as pro-aromatic quinoidals are studied considering balanced contributions of aromatic and quinoidal forms.

A new series of π-conjugated oligomers based on the 4,4 dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene vinylene repeating unit has been prepared and characterized by X-ray, electrochemical, spectroscopic (UV-Vis absorption, emission and Raman) and density functional theory methods. The oligomers in their neutral, oxidized and reduced forms have been investigated. The neutral compounds show a longer mean conjugation length than oligothiophenes and oligothiophene-vinylenes and display very rich redox chemistry with the stabilization of polycationic states of which the radical cations and dications are strong NIR absorbers, the latter displaying singlet diradicaloid character. An interesting complementarity between the sequence of aromatic-quinoidal structural segments in the radical cations and dications has been described and interpreted. Two derivatives with the 4,4 dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene vinylene unit, disubstituted either with electron donor, bis(triaryl amino) groups, or acceptors bis(dicyano-methylene) caps enforcing a quinoidal structure in the dithiophene-vinylene bridge, have been also synthesized and characterized. The radical cation of the triarylamine compound and the radical anion of the tetracyano compound similarly display hole and electron charge localization, or confinement, in the nitrogen and dicyano surrounding parts, or class II mixed valence systems, while their dication and dianion species, conversely, are open-shell diradical (i.e., polaron pair) and closed-shell (i.e., bipolaron), respectively. The preparation of these new π-conjugated oligomers gives way to the realization of compounds with new electronic properties and unique structures potentially exploitable in organic electronics.

Synthesis of new hybrid 1,4-thiazinyl-1,2,3-dithiazolyl radicals via Smiles rearrangement

The condensation reaction of 2-aminobenzenethiols and 3-aminopyrazinethiols with 2-amino-6-fluoro-N-methylpyridinium triflate afforded thioether derivatives that were found to undergo Smiles rearrangement and cyclocondensation with sulphur monochloride to yield new hybrid 1,4-thiazine-1,2,3-dithiazolylium cations. The synthesized cations were readily reduced to the corresponding stable neutral radicals with spin densities delocalized over both 1,4-thiazinyl and 1,2,3-dithiazolyl moieties.

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.

Bis(aminoaryl) Carbon-Bridged Oligo(phenylenevinylene)s Expand the Limits of Electronic Couplings

Carbon-bridged bis(aminoaryl) oligo(para-phenylenevinylene)s have been prepared and their optical, electrochemical, and structural properties analyzed. Their radical cations are class III and class II mixed-valence systems, depending on the molecular size, and they show electronic couplings which are among the largest for the self-exchange reaction of purely organic molecules. In their dication states, the antiferromagnetic coupling is progressively tuned with size from quinoidal closed-shell to open-shell biradicals. The data prove that the electronic coupling in the radical cations and the singlet-triplet gap in the dications show similar small attenuation factors, thus allowing charge/spin transfer over rather large distances.

Neutral Aminyl Radicals Derived from Azoimidazolium Dyes

The synthesis and characterization of a new class of neutral aminyl radicals is reported. Monoradicals were obtained by reduction of azoimidazolium dyes with potassium. Structural, spectroscopic, and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation of a biradical with largely independent unpaired electrons. Both the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

Role of a singlet diradical character in carbon nanomaterials: a novel hot spot for efficient nonlinear optical materials

Carbon atoms have the potential to produce a variety of fascinating all-carbon structures with amazing electronic and mechanical properties. Over the last few decades, several efforts have been made using experimental and computational techniques to functionalize graphene, carbon nanotubes and fullerenes for potential use in modern hi-tech electronic, medicinal, optical and nonlinear optical (NLO) applications. Since photons replaced electrons as a carrier of information, the field of NLO material design has drawn immense interest in contemporary materials science. There have been several reports of bridging the gap between the exciting fields of carbon nanomaterials and NLO materials by functionalizing carbon nanomaterials for potential NLO applications. In contrast to previous reports of the design of third-order NLO materials using conventional closed-shell materials, a novel strategy using open-shell diradical molecular systems has recently been proposed. Quantum chemically, diradical character is explained in terms of the instability of the chemical bonds in open-shell molecular systems. Interestingly, several carbon nanomaterials, which naturally possess open-shell singlet configurations, have recently gained momentum in the design of these classes of open-shell NLO materials with excellent NLO properties, stability and diversity. The present review establishes a systematic sequence of different studies (spanning over two decades of intense research efforts), starting from the simplest theoretical two-site diradical model, continuing to its experimental and theoretical realization in actual chemical systems, and finally applying the abovementioned model/rule to novel carbon nanomaterials to tune their NLO properties, particularly their second hyperpolarizability (γ). In the present report, we highlight several recent efforts to functionalize carbon nanomaterials by exploiting their open-shell diradical character to achieve efficient third-order NLO properties. Several issues and opportunities are discussed, including the inherited disadvantages of both experimental (the high reactivity and short life of diradical compounds) and quantum (need for multi-reference methodology) techniques when dealing with carbon nanomaterials. A comparative analysis of several quantum chemical investigations, along with contemporary experimental results, will be performed to emphasize the core issues and opportunities related to carbon nanomaterials with singlet open-shell diradical character. Thus, the present review will highlight carbon nanomaterials with diradical/biradical character for their prospective applications in the NLO field.

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

Diradicals, molecules with two unpaired electrons, are reactive intermediates that play an important role in many fields. Their defining feature is the energy difference between their singlet and triplet states, which provides direct information on the extent of their electron exchange interactions. Such knowledge is essential for understanding their diradical character, which is controllable internally by modification of the electronic and steric properties of the substituents. We now report that the energy gap of a diradical in the solid state can also be controlled by an external stimulus. The dication diradical of 4,4''-di(bisphenylamino)-p-terphenyl exhibits two singlet states with different exchange coupling constants at different temperatures as determined by SQUID and EPR measurements. The behavior is induced by the conformation change of the terphenyl bridge, the key structural unit of the species. The work presents an unprecedented instance of a thermally controllable singlet-triplet gap for a crystalline diradical and provides a novel diradical material relevant to the design of functional materials.

The synthesis and structure of a carbene-stabilized iminocarboranyl-boron(I) compound

A carbene-stabilized iminocarboranylboron(I) compound has been synthesized and structurally characterized. Single-crystal X-ray analyses and DFT calculations show that the π back donation of the lone pair of electrons on the boron(I) center onto the π* orbital of the imine unit is crucial for removing the electron density of the boron center thereby stabilizing such species, in which the carbene serves solely as a σ donor. This work also demonstrates that imines play a similar role to that of carbenes in the stabilization of low valent boron compounds.