Impact of Diradical Character on Two-Photon Absorption: Bis(acridine) Dimers Synthesized from an Allenic Precursor

Anion⋯anion self-assembly under the control of σ- and π-hole bonds

The electrostatic attraction between charges of opposite signs and the repulsion between charges of the same sign are ubiquitous and influential phenomena in recognition and self-assembly processes. However, it has been recently revealed that specific attractive forces between ions with the same sign are relatively common. These forces can be strong enough to overcome the Coulomb repulsion between ions with the same sign, leading to the formation of stable anion⋯anion and cation⋯cation adducts. Hydroden bonds (HBs) are probably the best-known interaction that can effectively direct these counterintuitive assembly processes. In this review we discuss how σ-hole and π-hole bonds can break the paradigm of electrostatic repulsion between like-charges and effectively drive the self-assembly of anions into discrete as well as one-, two-, or three-dimensional adducts. σ-Hole and π-hole bonds are the attractive forces between regions of excess electron density in molecular entities (e.g., lone pairs or π bond orbitals) and regions of depleted electron density that are localized at the outer surface of bonded atoms opposite to the σ covalent bonds formed by atoms (σ-holes) and above and below the planar portions of molecular entities (π-holes). σ- and π-holes can be present on many different elements of the p and d block of the periodic table and the self-assembly processes driven by their presence can thus involve a wide diversity of mono- and di-anions. The formed homomeric and heteromeric adducts are typically stable in the solid phase and in polar solvents but metastable or unstable in the gas phase. The pivotal role of σ- and π-hole bonds in controlling anion⋯anion self-assembly is described in key biopharmacological systems and in molecular materials endowed with useful functional properties.

Dianionic and Neutral Diboron-Centered Classical Diradicaloids

Herein, we report the syntheses and electronic structures of crystalline dianionic as well as neutral diboron-centered classical diradicaloids as boron analogues of classical Thiele, Chichibabin, and Müller (this only for dianionic diradicaloids!) hydrocarbons. These are based on borane radical anion and NHC-stabilized boryl radical spin carriers, respectively. All these dianionic diboron-centered diradicaloids exhibit triplet population at room temperature regardless of the π-conjugated spacer: p-phenylene, p,p'-biphenylene, or p,p″-terphenylene. In the case of neutral diboron-centered diradicaloids, the employed π-conjugated spacer plays a crucial role for the triplet population at room temperature: EPR inactive for p-phenylene vs EPR active for p,p'-biphenylene. The findings emphasize the importance of the spin carriers for the resulting ground-state: borane radical anion vs NHC-stabilized boryl radical along with the pivotal role of the π-conjugated spacer as spin-coupler between two spins. Notably, 100 years (a century) after the first report by Krause of the triphenyl borane radical-anion, being isoelectronic to the triphenylmethyl radical, we convey borane radical anion-based diradicaloids. Furthermore, while donor-stabilized boryl radicals were introduced in the 1980s by Giles and Roberts, said concept is herewith being extended to NHC-stabilized boryl radical-based diradicaloids.

Isomerism tunes the diradical character of difluorenopyrroles at constant Hückel-level anti-aromaticity

A new diradical based on diindenocarbazole or difluorenopyrrole was synthesized and experimentally characterized by optical, electrochemical, and magnetic techniques, as well as quantum chemical calculations. The isomerism of these structures tunes the diradical character and the associated properties, representing a unique case of such important modulation. A full study of the electronic structure was carried out considering the perturbative interactions between different canonical forms as well as the anti-aromatic character of the molecular cores. Such a study reveals how we can tune diradical character simply by reorganizing the bonding patterns at constant chemical costs (composition).

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.

Unveiling a new aspect of oxocarbons: open-shell character of 4- and 5-membered oxocarbon derivatives showing near-infrared absorption

Oxocarbon derivatives consisting of 4- and 5-membered rings, referred to as croconaine and squaraine dyes and regarded as closed-shell molecules, are found to have an intermediate open-shell character from the experimental results of ¹H-NMR, ESR spectroscopy, SQUID magnetometric analysis, and X-ray crystallography. We employed two chalcogenopyrylium moieties with O and S chalcogen atoms as substitutions on oxocarbons. The singlet-triplet energy gaps (ΔE _S-T) associated with the degree of diradical nature are smaller for croconaines than for squaraines and smaller for thiopyrylium than for pyrylium groups. The diradical nature impacts the electronic transition energy that decreased with a decreasing degree of diradical contribution. They exhibit substantial two-photon absorption in the region over 1000 nm. The diradical character y of the dye was determined experimentally from the observed one- and two-photon absorption peaks and the triplet energy level. The present finding provides new insight into diradicaloids with the contribution of non-Kekulé oxocarbon and also showcases the correlation between the electronic transition energy and their diradical character.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF6 ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC8 results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

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.

On-off-on Control of Molecular Inversion Symmetry via Multi-stage Protonation: Elucidating Vibronic Laporte Rule

The Laporte rule dictates that one- and two-photon absorption spectra of inversion-symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric- and non-inversion symmetric two-photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single- and double-protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two-photon absorption. By performing detailed one- and two-photon titration experiments, with supporting quantum-chemical model calculations, we explain how certain low-frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on-off-on inversion symmetry switch, opening new avenues for two-photon sensing applications.

Controlling Diradical Character of Photogenerated Colored Isomers of Phenoxyl-Imidazolyl Radical Complexes

We propose a rational method for evaluating the diradical character of the photochromic phenoxyl-imidazolyl radical complex (PIC) derivatives based on their radical-radical coupling reaction rates. PIC consists of an imidazole ring, a phenoxyl ring, and a bridging unit that structurally connects them. The C-N bond formed between the imidazole and phenoxyl rings can be dissociated photochemically in a homolytic manner. The photochromism of PIC differs significantly from other photochromic molecules in that the transient colored open-ring isomer has a diradical character. The colored open-ring isomer returns promptly to the initial colorless closed-ring isomer by the intramolecular radical recombination reaction. By changing the aromaticity and substitution position of the bridging unit, it is possible to control the degree of contribution of the open-shell diradical and closed-shell quinoidal structures to the open-ring isomer. Systematic investigation of the photochromic reactions of several PIC derivatives revealed that the half-life of the open-ring isomers reflects the diradical character. Thus, the radical recombination reaction rate of the open-ring isomer of the PIC derivatives is an excellent parameter of the diradical character.

Tuning the Diradical Character of Indolocarbazoles: Impact of Structural Isomerism and Substitution Position

In this study, a set of 10 positional indolocarbazole (ICz) isomers substituted with dicyanomethylene groups connected via para or meta positions are computationally investigated with the aim of exploring the efficiency of structural isomerism and substitution position in controlling their optical and electronic properties. Unrestricted density functional theory (DFT), a spin-flip time-dependent DFT approach, and the multireference CASSCF/NEVPT2 method have been applied to correlate the diradical character with the energetic trends (i.e., singlet-triplet energy gaps). In addition, the nucleus-independent chemical shift together with ACID plots and Raman intensity calculations were used to strengthen the relationship between the diradical character and (anti)aromaticity. Our study reveals that the substitution pattern and structural isomerism represent a very effective way to tune the diradical properties in ICz-based systems with meta-substituted systems with a V-shaped structure displaying the largest diradical character. Thus, this work contributes to the elucidation of the challenging chemical reactivity and physical properties of diradicaloid systems, guiding experimental chemists to produce new molecules with desirable properties.

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...

An Air-Stable Organic Radical from a Controllable Photoinduced Domino Reaction of a Hexa-aryl Substituted Anthracene

Air-stable organic radicals and radical ions have attracted great attention for their far-reaching application ranging from bioimaging to organic electronics. However, because of the highly reactive nature of organic radicals, the design and synthesis of air-stable organic radicals still remains a challenge. Herein, an air-stable organic radical from a controllable photoinduced domino reaction of a hexa-aryl substituted anthracene is described. The domino reaction involves a photoinduced [4 + 2] cycloaddition reaction, rearrangement, photolysis, and an elimination reaction; ¹H/¹³C NMR spectroscopy, high resolution mass spectrometry, single-crystal X-ray diffraction, and EPR spectroscopy were exploited for characterization. Furthermore, a photoinduced domino reaction mechanism is proposed according to the experimental and theoretical studies. In addition, the effects of employing push and pull electronic groups on the controllable photoinduced domino reaction were investigated. This article not only offers a new blue emitter and novel air-stable organic radical compound for potential application in organic semiconductor applications, but also provides a perspective for understanding the fundamentals of the reaction mechanism on going from anthracene to semiquinone in such anthracene systems.

Anionic Boron- and Carbon-Based Hetero-Diradicaloids Spanned by a p-Phenylene Bridge

Herein we report the synthesis and characterization of anionic boron- and carbon-based Kekulé diradicaloids spanned by a p-phenylene bridge. In contrast to Thiele's hydrocarbon, a closed-shell singlet system, they show an appreciable population of the triplet state at room temperature, as evidenced by both NMR and EPR spectroscopy. Moreover, en route to these anionic boron- and carbon-based hetero-diradicaloids, the formation of an isolable diamino(4-diarylboryl-phenyl)methyl radical was observed.

Benzothiadiazole-Substituted Aza-BODIPY Dyes: Two-Photon Absorption Enhancement for Improved Optical Limiting Performances in the Short-Wave IR Range

Aza-boron dipyrromethenes (aza-BODIPYs) presenting a benzothiadiazole substitution on upper positions are described. The strong electron-withdrawing effect of the benzothiadiazole moiety permits enhancement of the accepting strength and improves the delocalization of the aza-BODIPY core to attain a significant degree of electronic communication between the lower donating groups and the upper accepting groups. The nature of the intramolecular charge transfer is studied both experimentally and theoretically. Linear spectroscopy highlighted the strongly redshifted absorption and emission of the synthesized molecules with recorded fluorescence spectra over 1000 nm. Nonlinear optical properties were also investigated. Strong enhancement of the two-photon absorption of the substituted dyes compared with the unsubstituted one (up to 4520 GM at 1300 nm) results in an approximately 15-20 % improvement of the optical power limiting performances. These dyes are therefore a good starting point for further improvement of optical power limiting in the short-wave IR range.

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⁺.

Near-infrared dyes for two-photon absorption in the short-wavelength infrared: strategies towards optical power limiting

The recent advances in the field of two-photon absorbing chromophores in the short-wavelength infrared spectral range (SWIR 1100–2500 nm) are summarized, highlighting the development of optical power limiting devices in this spectral range.

Acyclic diaminocarbene-based Thiele, Chichibabin, and Müller hydrocarbons

Thiele, Chichibabin and Müller hydrocarbons are considered as classical Kekulé diradicaloids. Herein we report the synthesis and characterization of acyclic diaminocarbene (ADC)-based Thiele, Chichibabin, and Müller hydrocarbons. The calculated singlet–triplet energy gaps are ΔE_S–T = −27.96, −3.70, −0.37 kcal mol⁻¹, respectively, and gradually decrease with the increasing length of the π-conjugated spacer (p-phenylene vs. p,p′-biphenylene vs. p,p′′-terphenylene) between the two ADC-scaffolds. In agreement with the calculations, we also experimentally observed the enhancement of paramagnetic diradical character as a function of the length of the π-conjugated spacer. ADC-based Thiele's hydrocarbon is EPR silent and exhibits very well resolved NMR spectra, whereas ADC-based Müller's hydrocarbon displays EPR signals and featureless NMR spectra at room temperature. The spacer also has a strong influence on the UV-Vis-NIR spectra of these compounds. Considering that our methodology is modular, these results provide a convenient platform for the synthesis of an electronically modified new class of carbon-centered Kekulé diradicaloids.

We report the synthesis of acyclic diaminocarbene (ADC)-scaffold based Thiele, Chichibabin, and Müller hydrocarbons. Studies support that the singlet-triplet energy gap depends on the π-conjugated spacer between the ADC scaffolds.

Crystalline Diradical Dianions of Pyrene-Fused Azaacenes

Although diradicals and azaacenes have been greatly attractive in fundamental chemistry and functional materials, the isolable diradical dianions of azaacenes are still unknown. Herein, we describe the first isolation of pyrene-fused azaacene diradical dianion salts [(18-c-6)K(THF)₂ ]⁺ [(18-c-6)K]⁺ ⋅1^2-.. and [(18-c-6)K(THF)]²⁺ ⋅2^2-.. by reduction of the neutral pyrene-fused azaacene derivatives 1 and 2 with excess potassium graphite in THF in the presence of 18-crown-6. Their electronic structures were investigated by various experiments, in conjunction with theoretical calculations. It was found that both dianions are open-shell singlets in the ground state and their triplet states are thermally readily accessible owing to the small singlet-triplet energy gap. This work provides the first examples of crystalline diradical dianions of azaacenes with considerable diradical character.

Fine-tuning the diradical character of molecular systems via the heteroatom effect

Heteroatoms were introduced as a novel modification strategy for fine-tuning the diradical character of molecular systems. Both the diradical character and the singlet-triplet energy gaps of diketopyrrolopyrrole based phenoxyl diradicaloids decreased as the size of the substituted heteroatoms (from O, S to Se atom) increased.

Singlet-Fission-Induced Enhancement of Third-Order Nonlinear Optical Properties of Pentacene Dimers

Using quantum chemical calculations and exciton dynamics simulations, we investigate the static second hyperpolarizability γ [the third-order nonlinear optical (NLO) property at the molecular scale] of slip-stacked pentacene dimer models in the correlated-triplet-pair [1(TT)] state created from the singlet excited state in the singlet fission (SF) process. It is found that the SF induces significant (∼20 times at maximum) enhancement of γ/monomer in the 1(TT) state as compared to that in the singlet ground state. The origin of the remarkable enhancement of γ/monomer is revealed by analyzing the γ density distribution and the intermolecular orbital interaction. Furthermore, we clarify molecular packings suitable for highly efficient SF and largely enhanced γ values of a novel class of SF-induced NLO systems, which have promising potential to surpass the conventional NLO systems.

Perylenetetracarboxy-3,4:9,10-diimide derivatives with large two-photon absorption activity

Perylenetetracarboxy-3,4:9,10-diimides with large TPA cross-sections.

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.

Theoretical Study on Open-Shell Singlet Character and Second Hyperpolarizabilities in Cofacial π-Stacked Dimers Composed of Weak Open-Shell Antiaromatic Porphyrins

From the analysis based on the broken-symmetry density functional theory (DFT) calculations, we in this study propose a strategy to enhance the open-shell characters and third-order nonlinear optical (NLO) properties of π-stacked dimers composed of antiaromatic molecules with weak open-shell characters. For this purpose, we here constructed cofacial π-stacked dimer models composed of aromatic and antiaromatic Ni^II porphyrins in order to examine the π-π stacking distance (R) dependence of the diradical characters (y) and static second hyperpolarizabilities (γ). The antiaromatic porphyrin dimers are found to have intermediate y around R∼3.3 Å, the result of which originates in the unique intermolecular interactions between the antiaromatic monomers. Static γ along the stacking direction of such antiaromatic porphyrin dimers with intermediate diradical characters are shown to be enhanced significantly as compared to those of the isolated monomers and the aromatic porphyrin dimers.

Exploring the novel donor-nanotube archetype as an efficient third-order nonlinear optical material: asymmetric open-shell carbon nanotubes

Contrary to the enormous number of previous studies on carbon nanotubes (CNTs), herein, we realized the origin of the intrinsic open-shell diradical character and second hyperpolarizability γ using a broken symmetry approach. This study was inspired by our recent findings (S. Muhammad, et al., Nanoscale, 2016, 8, 17998 and Nakano, et al., J. Phys. Chem. C, 2016, 120, 1193). We performed structural modifications through a unique asymmetric donor-nanotube framework, which led to a novel paradigm of modified CNTs with tunable open-shell diradical character and remarkably superior NLO response properties. Interestingly, asymmetry and diradical character were found to be the crucial factors to modulate the second hyperpolarizability γ. We initially performed a comparative analysis of the diradical characters and γ amplitudes of boron nitride nanotubes (BNNTs) and CNTs possessing significant ionic characters and covalent characters, respectively. The basic findings for these simple configurations were further extended to the donor-acceptor CNT paradigm, which finally led to excellent asymmetric donor-CNT configurations with remarkably larger γ amplitudes. Furthermore, among the CNTs, finite length zigzag CNT(6,0)3 were modified with different donor-acceptor configurations. Interestingly, for the first time, unique donor-nanotube configurations [1,4-(NH2)2CNT-(6,0)3 and 1,4-(NH2)2CNT-(6,0)5] were found; they showed significantly robust γ amplitudes as large as 2519 × 103 and 4090 × 103 a.u. at the LC-UBLYP(μ = 0.33)/6-31G* level of theory. Additionally, several molecular level insights have been obtained for these novel donor-nanotube configurations using their odd electron densities, molecular electrostatic maps, densities of states and γ density analyses to highlight the realization of these novel materials for highly efficient optical and NLO applications.

Open-Shell Character Dependences of the Second Hyperpolarizability in Two-Dimensional Tetraradicaloids

The open-shell character dependences of the second hyperpolarizability, γ, are investigated for rectangular-shaped tetraradicaloid models, such as diradical dimers, using numerically exact solutions of the extended Hubbard model. The newly defined local diradical characters for intra- and intermolecular interactions (referred to as y_intra and y_inter, respectively) are compared to conventional global ones ( y₀ and y₁) and provide a comprehensive understanding of the electronic structure of the system. The system shows two kinds of enhancements of the γ components, γ_intra and γ_inter (caused, respectively, by intra- and intermolecular diradical interactions): (i) a system with a large y_intra ( y_inter) (>∼0.4) exhibits the enhancement of a single component of γ, γ_inter (γ_intra), at the intermediate y_inter ( y_intra) region (∼0.3-0.4), and (ii) in contrast to conventional diradical systems, the system exhibits a further enhancement of both components of γ (γ_intra and γ_inter) at the region where y_intra ∼ y_inter with small values (≤∼0.3). The obtained relationships are verified by using ab initio quantum chemical calculations of the realistic tertraradical models of a 4,4'-bis(1,2,3,5-dithiazdiazolyl) (BDTDA) dimer and a disilene dimer. The present results are expected to pioneer an alternative class of two-dimensional multiradical NLO systems, which potentially cause further enhancement of γ as compared with conventional intermediate diradical NLO systems.

Stable Nitrogen-Centered Bis(imino)rylene Diradicaloids

The synthesis of stable open-shell singlet diradicaloids is critical for their practical material application. So far, most reported examples are based on carbon-centered radicals, which are intrinsically reactive, and there are very few examples of stable nitrogen-centered diradicaloids. In this full paper, a series of soluble and stable bis(imino)rylenes up to octarylene were synthesized on the basis of newly developed dibromorylene intermediates. It was found that from hexarylene onward, these quinoidal rylenes showed open-shell singlet ground states and could be thermally populated to paramagnetic triplet aminyl diradicals. They are stable due to efficient spin delocalization onto the rylene backbone as well as kinetic blocking of the aminyl sites by the bulky and electron-deficient 2,4,6-trichlorophenyl groups. They exhibited very different electronic structures, diradical character, excited-state dynamics, one-photon absorption, two-photon absorption, and electrochemical properties from their respective aromatic rylene counterparts. These bis(imino)rylenes represent a rare class of stable, neutral, nitrogen-centered aminyl diradicaloids.

Tunability of Open-Shell Character, Charge Asymmetry, and Third-Order Nonlinear Optical Properties of Covalently Linked (Hetero)Phenalenyl Dimers

Tunability of the open-shell character, charge asymmetry, and third-order nonlinear optical (NLO) properties of covalently linked (hetero)phenalenyl dimers are investigated by using the density functional theory method. By changing the molecular species X and substitution position (i, j) for the linker part, a variety of intermonomer distances R and relative alignments between the phenalenyl dimers can be realized from the geometry optimizations, resulting in a wide-range tuning of diradical character y and charge asymmetry. It is found that the static second hyperpolarizabilities along the stacking direction, γ_yyyy , are one-order enhanced for phenalenyl dimer systems exhibiting intermediate y, a feature that is in good agreement with the "y-γ correlation". By replacing the central carbon atoms of the phenalenyl rings with a boron or a nitrogen, we have also designed covalently linked heterophenalenyl dimers. The introduction of such a charge asymmetry to the open-shell systems, which leads to closed-shell ionic ground states, is found to further enhance the γ_yyyy values of the systems having longer intermonomer distance R with intermediate ionic character, that is, charge asymmetry. The present results demonstrate a promising potential of covalently linked NLO dimers with intermediate open-shell/ionic characters as a new building block of highly efficient NLO systems.

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.

Third-Order Nonlinear Optical Properties of One-Dimensional Quinoidal Oligothiophene Derivatives Involving Phenoxyl Groups

The diradical characters (y) and third‐order nonlinear optical (NLO) properties of open‐shell quinoidal oligothiophene derivatives with phenoxyl groups, and the corresponding reduced (hydrogenated)‐state oligomers, are investigated by using the broken‐symmetry density functional theory method. The oxidized (dehydrogenated) states are predicted to have an open‐shell singlet ground state and their y values increase with the number of units. Static second hyperpolarizabilities (γ) of the open‐shell oligomers with intermediate y are shown to be enhanced significantly compared with those of the closed‐shell analogues. Furthermore, owing to the effective diradical distances, the γ values of open‐shell oligomers are found to exceed that of s‐indaceno[1,2,3‐cd;5,6,7‐c′d′]diphenalene, which is known as an organic molecule with the largest two‐photon absorption cross‐section in this size of the pure hydrocarbons. This feature extends the range of efficient open‐shell third‐order NLO materials to a novel class of one‐dimensional conjugated oligomers with redox‐based high tunability of third‐order NLO properties.