Photo- and Thermal Interconversion of Multiconfigurational Strained Hydrocarbons Exhibiting Completely Switchable Oxidation to Stable Dicationic Dyes

Carbon-based Biradicals: Structural and Magnetic Switching

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

One-Pot Synthesis of Helical Azaheptalene and Chiroptical Switching of an Isolable Radical Cation

A nitrogen-centered heptalene, azaheptalene, was designed as a representative of a new class of redox-responsive molecules with a large steric strain that originates from the adjacent seven-membered rings. The pentabenzo derivative of azaheptalene was efficiently synthesized by a palladium-catalyzed one-pot reaction of commercially available reagents. Bromination led to mono- and dibrominated derivatives, the latter of which is interconvertible with isolable radical cation species exhibiting near-infrared absorption. Since the azaheptalene skeleton shows configurationally stable helicity with a large torsion angle, enantiomers could be successfully separated. Thus, optically pure azaheptalenes with P- or M-helicity showed strong chiroptical properties (|gabs |≥0.01), which could be changed by an electric potential.

Air and Moisture Stable para- and ortho-Quinodimethane Derivatives Derived from bis-N-Heterocyclic Olefins

Herein we report the development of a new methodology for the synthesis of various quinodimethane derivatives under two-electron oxidation of bis-N-heterocyclic olefins linked by different π-conjugated aromatic spacers. In case of para- and ortho-phenylene bridge, we obtained air and moisture stable diimidazolium para- and ortho-quinodimethane derivatives. Analogues of the para-phenylene spacer such as tetrafluoro-p-phenylene and p-anthracene also led to the corresponding air and moisture stable quinodimethane derivatives. This emphasizes the influence of imidazolium substituents which facilitate the air and moisture stability of the quinodimethane derivatives. Differences were observed for the electron transfer processes: two one-electron vs one two-electron redox transitions between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes depending on the employed π-conjugated aromatic spacer. The formation of the π-conjugated radical-cations, transient redox intermediates between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes, was addressed by an EPR investigation.

A Nitrogen Out-of-Plane (NOOP) Mechanism for Imine-Based Light-Driven Molecular Motors

Light-driven molecular motors have generated considerable interest due to their potential applications in material and biological systems. Recently, Greb and Lehn reported a new class of molecular motors, chiral N-alkyl imines, which undergo unidirectional rotation induced by light and heat. The mechanism of unidirectional motion in molecular motors containing a C═N group has been assumed to consist of photoinduced torsion about the double bond. In this work, we present a computational study of the photoisomerization dynamics of a chiral N-alkyl imine motor. We find that the location and energetics of minimal energy conical intersections (MECIs) alone are insufficient to understand the mechanism of the motor. Furthermore, a key part of the mechanism consists of out-of-plane distortions of the N atom (followed by isomerization about the double bond). Dynamic effects and out-of-plane distortions are critical to understand the observed (rather low) quantum yield for photoisomerization. Our results provide hints as to how the photoisomerization quantum yield might be increased, improving the efficiency of this class of molecular motors.

Synthesis and structure elucidation of triarylmethyl radicals with anthryl substitution

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

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

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

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

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

Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs

Overcrowded ethylenes composed of 10-methyleneanthrone and two bulky aromatic rings contain a twisted carbon-carbon double (C=C) bond as well as a folded anthrone unit. As such, they are unique frustrated aromatic enes (FAEs). Various colored crystals of these FAEs, obtained in different solvents, correspond to multiple metastable conformations of the FAEs with various twist and fold angles of the C=C bond, as well as various dihedral angles of attached aryl units with respect to the C=C bond. The relationships between color and these parameters associated with conformational features around the C=C bond were elucidated in experimental and computational studies. Owing to the fact that they are separated by small energy barriers, the variously colored conformations in the FAE crystal change in response to various external stimuli, such as mechanical grinding, hydrostatic pressure and thermal heating.

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

The biradicaloid of Chichibabin’s hydrocarbon exits in a unique thermal equilibrium between closed-shell singlet and open-shell triplet forms. Conceptually, the incorporation of nonplanar aromatic groups, such as anthraquinodimethane (AQD), in these species could bring about stabilization of the individual singlet and triplet spin biradicaloids by creating a high energy barrier for conformational interconversion between folded (singlet) and twisted (triplet) forms. Moreover, this alteration could introduce the possibility of controlling spin states through conformational changes induced by chemical and physical processes. Herein, we report the preparation of AQD-containing, π-extended Thiele’s (A-TH) and Chichibabin’s (A-CH) hydrocarbons, which have highly π-congested structures resulting from the presence of bulky 9-anthryl units. The π-congestion in these substances leads to steric frustration about carbon–carbon double bonds and creates flexible dynamic motion with a moderate activation barrier between folded singlet and twisted triplet states. These constraints make it possible to isolate the twisted triplet state of A-CH. In addition, simple mechanical grinding of the folded singlet of A-CH produces the twisted triplet.

Overcrowded Ethylene-Bridged Nanohoop Dimers: Regioselective Synthesis, Multiconfigurational Electronic States, and Global Hückel/Möbius Aromaticity

The design and preparation of molecular systems with multiple geometric and electronic configurations are the cornerstones for multifunctional materials with stimuli-responsive behaviors. We describe here the regioselective and facile synthesis of two types of overcrowded ethylene-bridged nanohoop dimers, with folded and twisted geometric structures as well as closed-shell, diradical and dication electronic structures. The strained nanohoop structures have a profound effect on the overall molecular and electronic configurations, which resulted in the destabilized diradical state. X-ray crystallographic analysis revealed the folded molecular geometry for the neutral species and twisted geometry for the dication species. The unique molecular dynamics, optical properties, and dynamic redox properties were disclosed in the solution phase by spectroscopic and electrochemical methods. Furthermore, the global Hückel and Möbius aromaticity were revealed by a combination of experimental and theoretical approaches. Our studies shed light on the design of nanohoop-incorporated multiconfigurational materials with unique topologies and functions.

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.

Redox-active tetraaryldibenzoquinodimethanes

Quinodimethanes (QDs) are a class of non-aromatic π-conjugated compounds that are well-known to be interconvertible scaffolds in many response systems. While parent ortho- and para-QDs (o-QD and p-QD) can be easily converted to benzocyclobutenes or oligomers/polymers by the formation of C-C bonds at α-positions, the attachment of four phenyl groups to these reactive sites makes o-Ph₄QD and p-Ph₄QD long-lived. We have demonstrated that further dibenzo-annulation of such tetraaryl QD units also drastically increases their stability, and many tetraarylated dibenzoquinodimethane derivatives have been developed. This Feature Article shows our milestones in creating functional redox systems, where drastic changes in structure occur upon electron transfer (dynamic redox "dyrex" behaviour).

Multi-Redox Active Carbons and Hydrocarbons: Control of their Redox Properties and Potential Applications

Precise control over redox properties is essential for high-performance organic electronic devices such as organic batteries, electrochromic devices, and information storage devices. In this context, multi-redox active carbons and hydrocarbons, represented as C_x H_y molecules (x≥1, y≥0), are highly sought after, because they can switch between multiple redox states. Herein, we outline the redox properties of C_x H_y molecules as solutes and adsorbed species. Furthermore, the limitations of evaluating their redox properties and the possible solutions are summarized. Additionally, the theoretical capacity (mAh/g) and gravimetric energy density (Wh/kg) of secondary batteries were estimated based on the redox properties of 185 C_x H_y molecules, which have primarily been reported in the last decade. Among them, seven C_x H_y molecules were found to have the potential to surpass the energy density of LiNi_0.6 Mn_0.2 Co_0.2 O₂ /graphite batteries. The use of C_x H_y molecules in multielectrochromic devices and multi-bit memory is also explained. We believe that this review will encourage further utilization of C_x H_y molecules thereby promoting its applications in organic electronic devices.

Dibenzosuberenone-Based Photo- and Thermochromic Switches, a Transformation: Rearrangement of Dibenzosuberenones to Spiro Anthrones

In this study, a new class of photo- and thermochromic compounds was discovered. Fluorescent dihydropyridazine-appended dibenzosuberenone derivatives with central seven-membered rings were converted to colorless spiro anthrones with central six-membered rings by thermal isomerization or visible-light (420 nm) irradiation. Under UV light (350 nm), the spiro anthrones converted back to benzosuberenones. The chemical structures of the spiro anthrones were determined by NMR, UV-vis spectroscopy, and HRMS analyses. It was also observed that the dibenzosuberenone derivatives were oxidized on exposure to UV irradiation for a long time.

Hysteretic Three-State Redox Interconversion among Zigzag Bisquinodimethanes with Non-fused Benzene Rings and Twisted Tetra-/Dications with [5]/[3]Acenes Exhibiting Near-Infrared Absorptions

Octaaryl-substituted bisquinodimethanes (BQDs) with a zigzag structure were designed as redox-switchable molecules that undergo four-electron oxidation to produce tetracationic pentacenes with a doubly twisted structure. In contrast to one-stage four-electron oxidation of BQDs, stepwise two-electron reduction of tetracationic pentacenes occurs to give dicationic anthracenes and then the original BQDs, step-by-step. Since both tetracations and dications exhibit near-infrared (NIR) absorptions (-1400 nm) based on an intramolecular charge-transfer interaction, changes in not only their structures but also their UV-vis-NIR spectra can be controlled by redox stimuli. In this Communication, we present an unprecedented one-step π-extension to pentacene from non-fused benzene rings by oxidation, and subsequent two-stage deannulation to benzene rings via anthracene upon reduction. All structures were determined by single-crystal X-ray analyses, and their properties were characterized by spectroscopic and theoretical studies.

Dibenzocycloheptatriene as end-group of Thiele and tetrabenzo-Chichibabin hydrocarbons

Thiele (Th) and tetrabenzo-Chichibabin (TBC) derivatives with terminal dibenzocycloheptatriene (DBHept) units were prepared. A clear correlation between their electronic and molecular structures was stablished. Insights into their closed- or open-shell ground states were gained, where particular contribution of the heptagonal carbocycles as end-groups was proved. Remarkably, a thermally accessible triplet diradical configuration was confirmed for the DBHept-TBC compound.

Flexible C-C Bonds: Reversible Expansion, Contraction, Formation, and Scission of Extremely Elongated Single Bonds

Since carbon-carbon (C-C) covalent bonds are rigid and robust, the bond length is, in general, nearly constant and depends only on the bond order and hybrid orbitals. We report herein direct visualization of the reversible expansion and contraction of a C(sp³ )-C(sp³ ) single bond by light and heat. This flexibility of a C-C bond was demonstrated by X-ray analysis and Raman spectroscopy of hexaphenylethane (HPE)-type hydrocarbons with two spiro-dibenzocycloheptatriene units, the intramolecular [2+2] photocyclization of which and thermal cleavage of the resulting cyclobutane ring both occur in a single-crystalline phase. The force constant of the contracted C-C bond is 1.6 times greater than that of the expanded bond. Since formation of the cyclobutane ring and contraction of the C-C bond lower the HOMO level by approximately 1 eV, the oxidative properties of these HPEs with a flexible C-C bond can be deactivated/activated by light/heat.

Switching of Redox Properties Triggered by a Thermal Equilibrium between Closed-Shell Folded and Open-Shell Twisted Species

Thermally switchable redox properties have been reported to be due to a change in the spin state of newly designed overcrowded ethylenes, which can adopt closed-shell folded and open-shell twisted forms. In this study, tetrathienylanthraquinodimethane derivatives were designed to be in thermal equilibrium between a more stable folded form and less stable but more donating twisted diradical in solution, so that the oxidation potential can be controlled by heating/cooling. This is the first example of a switching of redox properties based on a thermally equilibrated twisted diradical, which can be more readily oxidized to the twisted dication.

Ring-expansion approach towards extended asymmetric benzopentafulvalenes: overcrowded olefinic structure and chain length-dependent properties

A strain energy-induced ring-expansion is described to construct novel asymmetric benzopentafulvalenes precisely, which show chain length-dependent physical properties and air-stable ambipolar carrier transport.