An n-Type Open-Shell Conjugated Polymer with High-Spin Ground-State and High Intrinsic Electrical Conductivity.

Open-shell conjugated polymers with a high intrinsic conductivity and high-spin ground state hold considerable promise for applications in organic electronics and spintronics. Herein, two novel acceptor-acceptor (A-A) conjugated polymers based on a highly electron-deficient quinoidal benzodifurandione unit have been developed, namely DPP-BFDO-Th and DPP-BFDO. The incorporation of the quinoidal moiety into the polymers backbones enables deeply aligned lower-lying lowest unoccupied molecular orbital (LUMO) levels of below -4.0 eV. Notably, DPP-BFDO exhibits an exceptionally low LUMO (-4.63 eV) and a high-spin ground state characterized by strong diradical characters. Moreover, a self-doping through intermolecular charge-transfer is observed for DPP-BFDO, as evidenced by X-ray photoelectron spectroscopy (XPS) studies. The high carrier concentration in combination with a planar and linear conjugated backbone yields a remarkable electrical conductivity (σ) of 1.04 S cm-1 in the "undoped" native form, ranking among the highest values reported for n-type radical-based conjugated polymers. When employed as an n-type thermoelectric material, DPP-BFDO achieves a power factor of 12.59 µW m-1 K-2. Furthermore, upon n-doping, the σ could be improved to 65.68 S cm-1. This study underscores the great potential of electron-deficient quinoidal units in constructing dopant-free n-type conductive polymers with a high-spin ground state and exceptional intrinsic conductivity.

On-surface synthesis of graphene nanostructures with π-magnetism.

Graphene nanostructures (GNs) including graphene nanoribbons and nanoflakes have attracted tremendous interest in the field of chemistry and materials science due to their fascinating electronic, optical and magnetic properties. Among them, zigzag-edged GNs (ZGNs) with precisely-tunable π-magnetism hold great potential for applications in spintronics and quantum devices. To improve the stability and processability of ZGNs, substitutional groups are often introduced to protect the reactive edges in organic synthesis, which renders the study of their intrinsic properties difficult. In contrast to the conventional wet-chemistry method, on-surface bottom-up synthesis presents a promising approach for the fabrication of both unsubstituted ZGNs and functionalized ZGNs with atomic precision via surface-catalyzed transformation of rationally-designed precursors. The structural and spin-polarized electronic properties of these ZGNs can then be characterized with sub-molecular resolution by means of scanning probe microscopy techniques. This review aims to highlight recent advances in the on-surface synthesis and characterization of a diversity of ZGNs with π-magnetism. We also discuss the important role of precursor design and reaction stimuli in the on-surface synthesis of ZGNs and their π-magnetism origin. Finally, we will highlight the existing challenges and future perspective surrounding the synthesis of novel open-shell ZGNs towards next-generation quantum technology.

Solution-Phase Synthesis and Isolation of An Aza-Triangulene and Its Cation in Crystalline Form.

Synthesis of triangulene and its derivatives is challenging due to their intrinsic high spin nature. Herein, we report solution-phase synthesis and isolation of a nitrogen-doped triangulene (i.e., aza-triangulene) (NT) and its cation (NT+ ) in single-crystal form. Notably, the cation NT+ can be regarded as an isoelectronic structure of the corresponding all-carbon triangulene. Both NT and NT+ show reasonable stability due to kinetic blocking by bulky and electron-withdrawing aryl substituents, and intramolecular donor-acceptor interaction. Bond length analysis, magnetic measurements and theoretical calculations reveal that the neutral NT has a doublet ground state with a zwitterionic character, while the cation NT+ exhibits a triplet ground state with a singlet-triplet energy gap of +0.84 kcal mol-1 . This study provides a rational strategy to access high-spin systems by heteroatom doping of pure π-conjugated polycyclic hydrocarbons.

Facile Synthesis of Nitrogen-Doped [(6.)m8]nCyclacene Carbon Nanobelts by a One-Pot Self-Condensation Reaction.

Synthesis of fully conjugated carbon nanobelts (CNBs) remains one of the biggest challenges in organic chemistry. Herein, we report a facile synthesis of four nitrogen-doped [(6.)m8]ncyclacene CNBs (m = 1-3; n = 3,4) with different sizes by a one-pot self-condensation reaction of three bis(o-aminobenzophenone) precursors. The belt-shaped structure was confirmed by X-ray crystallographic analysis. The existence of eight-membered [1,5]diazocine rings releases the strain while maintaining weak π-conjugation throughout the belt backbone, which is supported by electronic absorption spectra and frontier molecular orbital analysis. NMR measurements and magnetic shielding calculations suggest an alternating aromatic-nonaromatic ring structure, with a slightly more shielded chemical environment in the cavity. Our method opens the opportunities to access more sophisticated π-conjugated 2D/3D belt-/cage-like molecules in a simple way.

Machine Vision Automated Chiral Molecule Detection and Classification in Molecular Imaging.

Scanning probe microscopy (SPM) is recognized as an essential characterization tool in a broad range of applications, allowing for real-space atomic imaging of solid surfaces, nanomaterials, and molecular systems. Recently, the imaging of chiral molecular nanostructures via SPM has become a matter of increased scientific and technological interest due to their imminent use as functional platforms in a wide scope of applications, including nonlinear chiroptics, enantioselective catalysis, and enantiospecific sensing. Due to the time-consuming and error-prone image analysis process, a highly efficient analytic framework capable of identifying complex chiral patterns in SPM images is needed. Here, we adopted a state-of-the-art machine vision algorithm to develop a one-image-one-system deep learning framework for the analysis of SPM images. To demonstrate its accuracy and versatility, we employed it to determine the chirality of the molecules comprising two supramolecular self-assemblies with two distinct chiral organization patterns. Our framework accurately detected the position and labeled the chirality of each molecule. This framework underpins the tremendous potential of machine learning algorithms for the automated recognition of complex SPM image patterns in a wide range of research disciplines.

Tailoring long-range superlattice chirality in molecular self-assemblies via weak fluorine-mediated interactions.

Controllable fabrication of enantiospecific molecular superlattices is a matter of imminent scientific and technological interest. Herein, we demonstrate that long-range superlattice chirality in molecular self-assemblies can be tailored by tuning the interplay of weak intermolecular non-covalent interactions between hexaphenylbenzene-based enantiomers. By means of high-resolution scanning tunneling microscopy measurements, we demonstrate that the functionalization of a hexaphenylbenzene-based molecule with fluorine (F) atoms leads to the formation of molecular self-assemblies with distinct long-range chiral recognition patterns. We employed density functional theory calculations to quantify F-mediated lone pair F⋯π, C-H⋯F, and F⋯F interactions attributed to the distinct enantiospecific molecular self-organizations. Our findings underpin a viable route to fabricate long-range chiral recognition patterns in supramolecular assemblies by engineering the weak non-covalent intermolecular interactions.

Fused Quinoidal Dithiophene-Based Helicenes: Synthesis by Intramolecular Radical-Radical Coupling Reactions and Dynamics of Interconversion of Enantiomers.

A series of fused quinoidal dithiophene-based double and triple helicenes (1-M, 2-M, 2-M-Cl, 3-M, 3-M-Cl) were synthesized by intramolecular radical-radical coupling followed by oxidative dehydrogenation reaction. These helical molecules show dynamic interconversion of enantiomers in solution as revealed by variable-temperature NMR measurements, and the energy barriers are correlated to the substituents and topological structures. Notably, dynamic high performance liquid chromatography was used to quantitatively investigate the room-temperature racemization process between the (P,P,M)- and (P,M,M)- enantiomers of the triple helical 3-M-Cl, which gave an interconversion energy barrier in consistent with density functional theory calculations. Their optical and electrochemical properties are dependent on the fusion mode. Our studies provide both new synthetic strategy and new dynamic analytical method for helicenes with unique electronic structure.

Synthesis and Structural Elucidation of Bisdibenzocorannulene in Multiple Redox States.

We report an anti-folded bowl-shaped bisdibenzocorannulene (BDBC) featuring a new chair-cyclohexane-like hexagon as a bridge of two dibenzocorannulene moieties. The neutral compound showed multiple redox-active properties and could be converted to the corresponding redox states through chemical reduction or oxidation. Chemical reduction of BDBC by stoichiometric addition of metallic potassium in the presence of [18]crown-6 ether, provided a radical anion BDBC.- and a dianion BDBC2- , respectively; while chemical oxidation by silver hexafluoroantimonate(V), converted the neutral compound to an open-shell singlet diradical dication (BDBC.. )2+ . The structural consequences of both electron-reduction and oxidation were closely related to the release of ring-strain of the bowl-shaped π-scaffold and imposed steric hindrance of the hexagonal bridge. In addition, the unusual open-shell nature of the dication could mainly be attributed to the changing of localized antiaromaticity in the closed-shell structure to delocalized character in the biradical, and thus the emergence of weakly bonded π-electrons.

Stable Olympicenyl Radicals and Their π-Dimers.

An olympicenyl radical, a spin 1/2 hydrocarbon radical with C2v symmetry and uneven spin distribution, remains elusive despite the considerable theoretical research interest. Herein, we report syntheses of two air-stable olympicenyl radical derivatives, OR1 and OR2, with half-life times (τ1/2) in air-saturated solution of 7 days and 34 days. The high stability was ascribed to kinetic blocking of reactive sites with high spin densities. X-ray crystallographic analysis revealed unique 20-center-2-electron head-to-tail π-dimer structures with intermolecular distances shorter than the sum of van der Waals radius of carbon. The ground state of the π-dimers was found to be singlet, with singlet-triplet energy gaps estimated to be -2.34 kcal/mol and -3.28 kcal/mol for OR1 and OR2, respectively, by variable-temperature electron spin resonance (ESR) spectroscopy. The monomeric radical species were in equilibrium with the π-dimer in solution, and the optical and electrochemical properties of the monomers and π-dimers in solution were investigated by UV-vis-NIR spectroscopy and cyclic voltammetry, revealing a concentration-dependent nature. Theoretical calculations illustrated that upon formation of a π-dimer the local aromaticity of each monomer was enhanced, and spatial ring current between the monomers was present, which resulted in an increment of aromaticity of the interior of the π-dimer.

Real-Space Imaging of a Single-Molecule Monoradical Reaction.

Organic radicals consisting of light elements exhibit a low spin-orbit coupling and weak hyperfine interactions with a long spin coherence length, which are crucial for future applications in molecular spintronics. However, the synthesis and characterization of these organic radicals have been a formidable challenge due to their chemical instability arising from unpaired electrons. Here, we report a direct imaging of the surface chemical transformation of an organic monoradical synthesized via the monodehydrogenation of a chemically designed precursor. Bond-resolved scanning tunneling microscopy unambiguously resolves various products formed through a complex structural dissociation and rearrangement of organic monoradicals. Density functional theory calculations reveal detailed reaction pathways from the monoradical to different cyclized products. Our study provides unprecedented insights into complex surface reaction mechanisms of organic radical reactions at the single molecule level, which may guide the design of stable organic radicals for future quantum technology applications.

Global Aromaticity in Macrocyclic Polyradicaloids: Hückel's Rule or Baird's Rule?

Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic π-conjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Hückel's rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Möbius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Hückel's rule. When the molecules are excited into the first triplet excited state, the Hückel (anti)aromaticity observed in the ground state will become reversed according to Baird's rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar's aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Hückel's rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Hückel's rule and Baird's rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will mainly discuss their synthesis, global aromaticity, and the fundamental structure-radical character-aromaticity-properties relationships. Various experimental methods (e.g., NMR, X-ray crystallographic analysis, and electronic absorption spectroscopy) and theoretical calculations (e.g., anisotropy of the induced current density, nucleus independent chemical shift, and isochemical shielding surface) have been used to elaborate their (anti)aromatic character. At the end, a perspective on the possible three-dimensional global aromaticity in fully conjugated cagelike diradicaloids or polyradicaloids will be also discussed.

Formation of Azulene-Embedded Nanographene: Naphthalene to Azulene Rearrangement During the Scholl Reaction.

Incorporation of a non-hexagonal ring into a nanographene framework can lead to new electronic properties. During the attempted synthesis of naphthalene-bridged double [6]helicene and heptagon-containing nanographene by the Scholl reaction, an unexpected azulene-embedded nanographene and its triflyloxylated product were obtained, as confirmed by X-ray crystallographic analysis and 2D NMR spectroscopy. A 5/7/7/5 ring-fused substructure containing two formal azulene units is formed, but only one of them shows an azulene-like electronic structure. The formation of this unique structure is explained by arenium ion mediated 1,2-phenyl migration and a naphthalene to azulene rearrangement reaction according to an in-silico study. This report represents the first experimental example of the thermodynamically unfavorable naphthalene to azulene rearrangement and may lead to new azulene-based molecular materials.

Benzidine/Quinoidal-Benzidine-Linked, Superbenzene-Based π-Conjugated Chiral Macrocycles and Cyclophanes.

Synthesis of fully conjugated cyclophanes containing large-size polycyclic aromatics is challenging. Now, three benzidine-linked, hexa-peri-hexabenzocoronene (superbenzene)-based ortho-, para-, and meta-cyclophanes are synthesized through intermolecular Yamamoto coupling reaction of structurally pre-organized precursors. Subsequent oxidative dehydrogenation gave the corresponding quinoidal benzidine-linked cyclophanes. Their geometries were confirmed by X-ray crystallographic analysis and their electronic properties were investigated by electronic absorption, cyclic voltammetry, and DFT calculations. The quinoidal benzidine-linked cyclophanes show thermally populated paramagnetic activity with a relatively large singlet-triplet energy gap. Two enantiomers for the ortho-cyclophanes (1-NH and 1-N) were isolated and their chiral figure-of-eight macrocyclic structures were identified. The cage-like cyclophanes 2-NH and 3-NH with concave surface can selectively encapsulate fullerene C70 .

Perylene-Fused, Aggregation-Free Polycyclic Aromatic Hydrocarbons for Solution-Processed Distributed Feedback Lasers.

Perylene-fused, aggregation-free polycyclic aromatic hydrocarbons with partial zigzag periphery (ZY-01, ZY-02, and ZY-03) were synthesized. X-ray crystallographic analysis reveals that there is no intermolecular π-π stacking in any of the three molecules, and as a result, they show moderate-to-high photoluminescence quantum yield in both solution and in the solid state. They also display the characteristic absorption and emission spectra of perylene dyes. ZY-01 and ZY-02 with a nearly planar π-conjugated skeleton exhibit amplified spontaneous emission (ASE) when dispersed in polystyrene thin films. Solution-processed distributed feedback lasers have been fabricated using ZY-01 and ZY-02 as active gain materials, both showing narrow emission linewidth (<0.4 nm) at wavelengths around 515 and 570 nm, respectively. In contrast, ZY-03 did not show ASE and lasing, presumably due to its highly twisted backbone, which facilitates nonradiative internal conversion and intersystem crossing.

Dearomatization Approach Toward a Superbenzoquinone-Based Diradicaloid, Tetraradicaloid, and Hexaradicaloid.

Reported here is the step-by-step dearomatization of a highly aromatic polycyclic aromatic hydrocarbon (PAH), the hexa-peri-hexabenzocoronene (also called as "superbenzene"), to give a series of superbenzoquinones containing two, four, and six ketone groups. Different from traditional PAH-based quinones, these superbenzoquinones show open-shell multiradical character by rearomatization in the open-shell forms as experimentally validated by X-ray crystallographic analysis, NMR and ESR spectroscopy, and FT-IR measurements, as well as theoretically supported by restricted active space spin-flip calculations. These compounds exhibit structure- and molecular-symmetry-dependent optical, electrochemical, and magnetic properties.

Effects of bone marrow mesenchymal stem cells transfected with Ang-1 gene on hyperoxia-induced optic nerve injury in neonatal mice.

Optic nerve injury triggered retinal ganglion cell (RGC) death and optic nerve atrophy lead to visual loss. Bone marrow mesenchymal stem cells (BMSCs) are stromal cells, capable of proliferating and differentiating into different types of tissues. This aims of this study is to investigate the role of BMSCs transfected with angiopoietin-1 (Ang-1) in optic nerve injury induced by hyperoxia in a neonatal mice model. Ang-1 overexpression vector was constructed and used to transfect BMSCs. Reverse transcription-quantitative polymerase chain reaction was performed to detect Ang-1 expression in BMSCs. The hyperoxia-induced optic nerve injury model was established. The optic nerves at 6-7 mm posterior to the eyeball were extracted, and were treated with luxol fast blue staining, immunohistochemistry, immunofluorescence, and transmission electron microscopy to examine the effects of Ang-1-modified BMSCs on optic nerve injury induced by hyperoxia. The mice in the Ang-1 + BMSCs and BMSCs groups showed remarkably improved myelin sheaths of nerve fibers compared to the hyperoxia saline group. The positive expression and integrated optic density of Ang-1 in the Ang-1 + BMSCs group were significantly higher compared to the air control, hyperoxia saline and BMSCs groups. The number and diameter of myelinated nerve fibers, the diameter of axons and the thickness of myelin sheath in the air control and Ang-1 + BMSCs groups were higher compared to the hyperoxia saline group. Our study provides evidence supporting that Ang-1-modified BMSCs may have preventive and therapeutic effects on hyperoxia-induced optic nerve injury in neonatal mice.

From Open-Shell Singlet Diradicaloid to Closed-Shell Global Antiaromatic Macrocycles.

A dithieno[a,h]-s-indacene- (DTI-) based diradicaloid DTI-2Br was synthesized and its open-shell singlet diradical character was validated by magnetic measurements. On the other hand, its macrocyclic trimer DTI-MC3 and tetramer DTI-MC4 turned out to be closed-shell compounds with global antiaromaticity, which was supported by X-ray crystallographic analysis and NMR spectroscopy, assisted by ACID and 2D-ICSS calculations. Such change can be explained by a subtle balance between two types of antiferromagnetic spin-spin coupling along the π-conjugated macrocycles. The dications of DTI-MC3 and DTI-MC4 turned out to be open-shell singlet diradical dications, with a singlet-triplet energy gap of -2.90 and -2.60 kcal mol-1 , respectively. At the same time, they are both global aromatic. Our studies show that intramolecular spin-spin interactions play important roles on electronic properties of π-conjugated macrocycles.

Global Aromaticity in Macrocyclic Cyclopenta-Fused Tetraphenanthrenylene Tetraradicaloid and Its Charged Species.

A stable cyclopenta-fused tetraphenanthrenylene macrocycle, CPTP-M, was synthesized, and the structure was confirmed by X-ray crystallographic analysis. It exhibits a large radical character (number of unpaired electron, NU =3.52) and a small singlet-triplet energy gap (ΔES-T =-2.8 kcal mol-1 by SQUID). Its backbone contains 60 ([4n]) conjugated π electrons and is globally antiaromatic. NMR measurements and theoretical calculations revealed that its dication/dianion is globally aromatic owing to the existence of [4n-2]/[4n+2] π-conjugated electrons. Remarkably, the ring-current map of the tetraanion shows a unique ring-in-ring structure, with a diamagnetic outer ring and a paramagnetic inner ring. Accordingly, both the inner-rim and outer-rim protons are deshielded in its 1 H NMR spectrum. The tetraanion can be regarded as an isoelectronic structure of the known octulene, which shows similar electronic properties.

Diazuleno-s-indacene Diradicaloids: Syntheses, Properties, and Local (anti)Aromaticity Shift from Neutral to Dicationic State.

Non-alternant, non-benzenoid π-conjugated polycyclic hydrocarbons (PHs) are expected to exhibit very different electronic properties from all-benzenoid PHs. Reported herein are the syntheses and physical properties of four derivatives of two azulene-fused s-indacene isomers, the diazuleno[2,1-a:2',1'-g]-s-indacene (DAI-1) and diazuleno[2,1-a:1',2'-h]-s-indacene (DAI-2). The backbone of both isomers contains 28π electrons and is a 7-5-5-6-5-5-7 fused ring system. X-ray crystallographic analysis, NMR spectra, and theoretical calculations (ACID, NICS) reveal a structure with two aromatic azulene units fused with a central anti-aromatic s-indacene moiety. All compounds exhibit open-shell diradical character and are magnetically active, but the derivatives of DAI-2 show larger radical character than the respective ones of DAI-1. Their dications were isolated in crystalline form and all experimental and theoretical analyses disclose a shift of local (anti)aromaticity along the backbone, with two aromatic tropylium rings at the termini.

Stable Oxindolyl-Based Analogues of Chichibabin's and Müller's Hydrocarbons.

Chichibabin's and Müller's hydrocarbons are classical open-shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR-2 and OxR-3, based on the newly developed oxindolyl radical. X-ray crystallographic analysis on OxR-2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y0 =11.1 %) and large singlet-triplet gap (ΔES-T =-10.8 kcal mol-1 ). Variable-temperature NMR studies on OxR-2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG≠298K =15-16 kcal mol-1 ). OxR-3 exhibits a much larger diradical character (y0 =80.6 %) and a smaller singlet-triplet gap (ΔES-T =-3.5 kcal mol-1 ), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon-centered monoradical and diradicaloid.