Open-shell Singlet Ground State Research Articles

Radical Cations of Bilayer Nanographenes.

Chemical redox reactions of bilayer nanographene complexes, (C96H24Ar6)2 (Ar = 2,6-dimethylphenyl) (12) and (C42H12R6)2 (R = tBu) (22), were investigated. Upon two-electron oxidation reactions, 12 and 22 were transformed to radical cations 122•+ and 222•+, respectively. SQUID and EPR measurements on 122•+ and 222•+ indicate that they possess an open-shell singlet ground state with antiferromagnetic interactions between two layers. The shortest separation distance between bilayers in 222•+ (3.30 Å) is shorter than that in 22 (3.44 Å) and 22•+ (3.40 Å), illustrating stronger interaction upon loss of electrons.

Multiconfigurational actinide nitrides assisted by double Möbius aromaticity.

Understanding the bonding nature between actinides and main-group elements remains a key challenge in actinide chemistry due to the involvement of f orbitals. Herein, we propose a unique "aromaticity-assisted multiconfiguration" (AAM) model to elucidate the bonding nature in actinide nitrides (An2N2, An = Ac, Th, Pa, U). Each planar four-membered An2N2 with equivalent An-N bonds possesses four delocalized π electrons and four delocalized σ electrons, forming a new family of double Möbius aromaticity that contributes to the molecular stability. The unprecedented aromaticity further supports actinide nitrides to exhibit multiconfigurational characters, where the unpaired electrons (2, 4 or 6 in naked Th2N2, Pa2N2 or U2N2, respectively) either are spin-free and localized on metal centres or form metal-ligand bonds. High-level multiconfigurational computations confirm an open-shell singlet ground state for actinide nitrides, with small energy gaps to high spin states. This is consistent with the antiferromagnetic nature observed experimentally in uranium nitrides. The novel AAM bonding model can be authenticated in both experimentally identified compounds containing a U2N2 motif and other theoretically modelled An2N2 clusters and is thus expected to be a general chemical bonding pattern between actinides and main-group elements.

Fused Indacene Dimers.

Polycyclic hydrocarbons consisting of two or more directly fused antiaromatic subunits are rare due to their high reactivity. However, it is important to understand how the interactions between the antiaromatic subunits influence the electronic properties of the fused structure. Herein, we present the synthesis of two fused indacene dimer isomers: s-indaceno[2,1-a]-s-indacene (s-ID) and as-indaceno[3,2-b]-as-indacene (as-ID), containing two fused antiaromatic s-indacene or as-indacene units, respectively. Their structures were confirmed by X-ray crystallographic analysis. 1 H NMR/ESR measurements and DFT calculations revealed that both s-ID and as-ID have an open-shell singlet ground state. However, while localized antiaromaticity was observed in s-ID, as-ID showed weak global aromaticity. Moreover, as-ID exhibited a larger diradical character and a smaller singlet-triplet gap than s-ID. All the differences can be attributed to their distinct quinoidal substructures.

Bisazapentalene Dication: Global Aromaticity and Open-Shell Singlet Ground State

Antiaromatic moieties fused in polycyclic π-conjugated molecules usually exhibit strong localized antiaromaticiy. Herein, we reported the synthesis and properties of a bisazapentalene dication (BAP2+) obtained from in situ two-electron oxidation of neutral species 8. Noteworthily, it possesses global aromaticity and an open-shell singlet ground state. This study underlines the importance of heteroatoms in determining the delocalization of π-electrons and the aromaticity of molecules in their oxidized states.

The Fully Oxidized State of the Glutamate Coordinated O2-Tolerant [NiFe]-Hydrogenase Shows a Ni(III)/Fe(III) Open-Shell Singlet Ground State.

The oxygen tolerance of the [NiFe]-hydrogenase from H. thermoluteolus was recently assigned to originate from an unusual coordination sphere of the active site nickel atom (Shomura et al. Science 2017, 357, 928-932, 10.1126/science.aan4497). In the oxidized state, a terminal cysteine residue is displaced by a bidentate coordinating nearby Glu32 and thus moves to occupy a third μ-cysteine bridging position. Spectral features of the oxidized state were assigned to originate from a closed-shell Ni(IV)/Fe(II) state (Kulka-Peschke et al. J. Am. Chem. Soc. 2022, 144, 17022-17032, 10.1021/jacs.2c06400). Such a high-valent nickel oxidation state is unprecedented in biological systems. The spectral properties and the coordination sphere of that [NiFe]-hydrogenase can, however, also be rationalized by an energetically lower broken-symmetry Ni(III)/Fe(III) state of the active site which was not considered. In this open-shell singlet, the ligand-mediated antiferromagnetic spin-coupling leads to an overall S = 0 spin state with evenly distributed spin densities over the metal atoms. Experiments are suggested that may clarify the final assignment of redox states.

Open-Shell Donors and Closed-Shell Acceptors in Organic Solar Cells

The active materials of organic solar cells are recognized to show a singlet ground state, and their photo-excited states have been widely investigated in previous work. Herein, we disclose the inherent open-shell singlet ground state of all the donors with obvious electron spin resonance signals. In contrast, the well-known highly efficient non-fullerene acceptors are almost electron spin resonance-silent. These results are carefully studied and confirmed via the combination of variable-temperature nuclear magnetic resonance, electron spin resonance, superconducting quantum interference device, and theoretical calculation. More interestingly, the ground state can be readily tunable between open-shell and closed-shell by changing the quinoidal character of their conjugated backbone. This work provides a new perspective to understand the intrinsic molecular structure of active materials in organic solar cells.

M-Quinodimethane-based fused-ring triplet hydrocarbons

Abstract Hydrocarbons with open-shell singlet and triplet ground states have long been studied. In contrast to studies of Kekulé hydrocarbons with an open-shell singlet ground state, studies of non-Kekulé and Kekulé hydrocarbons with a triplet ground state are quite limited, and no hydrocarbon with a triplet ground state has been isolated as single crystals. In this review, our work on the synthesis, isolation, and properties of m-quinodimethane-based non-Kekulé polycyclic hydrocarbon, a kinetically stabilized triangulene derivative, and Kekulé polycyclic hydrocarbon, a kinetically stabilized bisdibenzo[3,4:5,6]cyclohepta[1,2-a:2,1-d]benzene derivative, are described. Triplet ground states of these hydrocarbons were experimentally confirmed by ESR and magnetic measurements, and these are the first example of polycyclic hydrocarbons with a triplet ground state whose structures were characterized by X-ray crystal structural analysis. These studies will enable the development of various polycyclic hydrocarbon multi-radicals with high spin multiplicity.

A Highly Stable Organic Luminescent Diradical.

It is very challenging to obtain stable room-temperature luminescent open-shell singlet diradicals. Herein we report the first stable Müller's hydrocarbon TTM-PhTTM with luminescent properties. Variable-temperature electron paramagnetic resonance spectroscopy measurements and theoretical calculations show that TTM-PhTTM has an open-shell singlet ground state with a diradical character of 90 %. Because of a small singlet-triplet energy gap, the open-shell singlet ground state can be thermally excited to a triplet state. TTM-PhTTM shows room-temperature deep-red emission in various solutions. Unusually high stability of TTM-PhTTM was also observed owing to effective steric hindrance and spin delocalization. Our results are beneficial to the rational design and discovery of more stable luminescent diradical materials.

Aromaticity in Fully π-Conjugated Open-Cage Molecules.

Although aromaticity in 2D π-conjugated monocyclic and polycyclic molecules has been intensively studied, aromaticity in 3D fully π-conjugated molecular cages remains largely unexplored mainly due to the synthetic challenges. Herein, we report the facile synthesis of a π-conjugated molecular cage (1) containing four dimethylmethylene-bridged triphenylamine (DTPA) units via platinum-mediated assembly of four molecules of a pinacol borate trisubstituted DTPA derivative, followed by reductive elimination. 1 has an open-cage structure, consisting of two isomeric trimers in trans- and cis-configurations, and an additional macrocycle across four DTPA units. Accordingly, the trans- (2) and cis- (3) macrocyclic trimers were also synthesized for comparison. 1-3 can be facilely oxidized into their respective cations in which electrons are effectively delocalized at two or three dimensions. The detailed experimental measurements and theoretical calculations reveal that (1) the neutral cage 1 shows localized aromaticity in individual benzene rings; (2) the dication 12+·2SbF6- displays bicyclic (anti)aromaticity with one macrocycle being aromatic (38π) and another macrocycle being antiaromatic (28π); on the other hand, the dications of the model compounds 2 and 3 are globally antiaromatic and nonaromatic, respectively; (3) the tetracation 14+·4SbF6- exhibits dominant 2D Hückel antiaromaticity in one of the macrocycles (36π). In addition, 12+·2SbF6-, 14+·4SbF6-, and 22+·2SbF6- possess open-shell singlet ground state with significant diradical character, while 32+·2SbF6- adopts a triplet ground state to release strain.

Design of an open-shell nitrogen-centered diradicaloid with tunable stimuli-responsive electronic properties

Organic diradicaloids usually display an open-shell singlet ground state with significant singlet diradical character (y0) which endow them with intriguing physiochemical properties and wide applications. In this study, we present the design of an open-shell nitrogen-centered diradicaloid which can reversibly respond to multiple stimuli and display the tunable diradical character and chemo-physical properties. 1a was successfully synthesized through a simple and high-yielding two-step synthetic strategy. Both experimental and calculated results indicated that 1a displayed an open-shell singlet ground state with small singlet-triplet energy gap (ΔES−T = −2.311 kcal mol−1) and a modest diradical character (y0 = 0.60). Interestingly, 1a was demonstrated to undergo reversible Lewis acid-base reaction to form acid-base adducts, which was proven to effectively tune the ground-state electronic structures of 1a as well as its diradical character and spin density distributions. Based on this, we succeeded in devising a photoresponsive system based on 1a and a commercially available photoacid merocyanine (MEH). We believe that our studies including the molecular design methodology and the stimuli-responsive organic diradicaloid system will open up a new way to develop organic diradicaloids with tunable properties and even intelligent-responsive diradicaloid-based materials.

The Lewis Acid Induced Formation of a Stable Diradical with an Intramolecular Ion Pairing State.

A stable cross-conjugated diradical was prepared by the reaction of a donor-acceptor-donor (D-A-D) molecule with B(C6F5)3. Its geometry and electronic structure were characterized by single crystal X-ray diffraction, EPR spectroscopy, SQUID measurement, UV/vis spectroscopy, and DFT calculation. It has an open-shell singlet ground state with a thermally excited triplet state. It can be viewed as an intramolecular radical ion pair, and the formation mechanism is proposed as an intramolecular single electron transfer that occurs from the bis(triarylamine) donor fragment to the central dioxophenyl acceptor moiety, induced by the acidic boron atom. This work provides a Lewis acid induced approach to the formation of neutral and cross-conjugated diradicals.

Generation and Characterization of the Charge-Transferred Diradical Complex CaCO2 with an Open-Shell Singlet Ground State

The CaCO2 complex is generated via the reaction of excited-state calcium atom with carbon dioxide in a solid neon matrix. Infrared absorption spectroscopy and quantum chemical calculations reveal that the complex has a planar four-membered ring structure with a strongly bent CO2 ligand side-on coordinated to the calcium center in an η2-O, O manner. The complex has an open-shell singlet ground state, which can be described as the bonding interactions between a Ca+ (4s1) cation in the doublet ground state and a doublet ground state CO2- anion. The analysis of the bonding situation suggests that the Ca-O2C bonds have a large (75%) electrostatic character. The covalent (orbital) interactions come from the coupling of the unpaired electrons of Ca+ and CO2- giving rise to electron-sharing bonding and a stronger contribution from dative bonding (Ca+)←(CO2-). The atomic orbitals (AOs) of Ca+ that are engaged in the covalent bonds are the 4s AO for the electron-sharing bonds and the 3d AOs for the dative bonds. This is further evidence for the assignment of the heavier alkaline-earth atoms as transition metals rather than main-group elements.

Synthesis and Characterization of peri-Heptacene on a Metallic Surface.

The synthesis of long n-peri-acenes (n-PAs) is challenging as a result of their inherent open-shell radical character, which arises from the presence of parallel zigzag edges beyond a certain n value. They are considered as π-electron model systems to study magnetism in graphene nanostructures; being potential candidates in the fabrication of optoelectronic and spintronic devices. Here, we report the on-surface formation of the largest pristine member of the n-PA family, i.e. peri-heptacene (n=7, 7-PA), obtained on an Au(111) substrate under ultra-high vacuum conditions. Our high-resolution scanning tunneling microscopy investigations, complemented by theoretical simulations, provide insight into the chemical structure of this previously elusive compound. In addition, scanning tunneling spectroscopy reveals the antiferromagnetic open-shell singlet ground state of 7-PA, exhibiting singlet-triplet spin-flip inelastic excitations with an effective exchange coupling (Jeff ) of 49 meV.

Hückel- and Baird-Type Global Aromaticity in a 3D Fully Conjugated Molecular Cage.

Global aromaticity in 3D π-conjugated molecular cages remains largely unexplored. Herein, we report the facile synthesis of a fully conjugated molecular cage (1) containing two bridged triphenylamine units and three quinoidal bithiophene arms. X-ray crystallographic analysis, NMR/ESR measurements and theoretical calculations reveal that: 1) its dication (12+ ) has an open-shell singlet ground state and is 3D globally aromatic, with individual macrocycles being 2D Hückel aromatic; 2) its tetracation (14+ ) has a triplet ground state and is also 3D globally aromatic, with individual macrocycles being 2D Baird aromatic; and 3) its hexacation (16+ ) has a closed-shell nature and shows local aromaticity. The study demonstrated a close relationship between 2D Hückel/Baird aromaticity and 3D global π-aromaticity.

Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments.

Low-valent titanium species were prepared by reaction of [TiCp*X3 ] (Cp*=η5 -C5 Me5 ; X=Cl, Br, Me) with LiEH4 (E=Al, B) or BH3 (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X3 ] with LiAlH4 in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(μ-H)}2 {(μ-H)2 AlX(L)}2 ] (L=thf, X=Cl, Br; L=OEt2 , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH2 XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X3 ] with LiBH4 (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH4 )(μ-X)}2 ] (X=Cl, Br) and [{TiCp*(BH4 )(μ-BH4 )}2 ], respectively. The treatment of [TiCp*Me3 ] with 4 and 5 equiv. of BH3 (thf) produces the diamagnetic [{TiCp*(BH3 Me)}2 (μ-B2 H6 )] and paramagnetic [{TiCp*(μ-B2 H6 )}2 ] complexes, respectively.

Antiaromatic Dicyclopenta[b,g]/[a,f]naphthalene Isomers Showing an Open-Shell Singlet Ground State with Tunable Diradical Character.

Since the first isolation of 1,3,5,7-tetra-tert-butyl-s-indacene in 1986, core-expanded s- and as-indacenes have attracted intensive interest. However, there is no reported synthesis of such type of molecules due to their high reactivity for over 30 years. Herein, we report the successful synthesis of two relatively stable, core-expanded indacene isomers, dicyclopenta[b,g]-naphthalene (5) and dicyclopenta[a,f]naphthalene (6). X-ray crystallographic analyses reveal that the backbone of 5 adopts a bond-delocalized structure, while that of 6 exhibits a bond-localized character. Variable-temperature 1H NMR/ESR measurements, electronic absorption spectra, and theoretical calculations confirm that both molecules are globally antiaromatic and have an open-shell singlet ground state. However, 6 shows stronger antiaromaticity, a larger diradical character (y0 = 48%), and a smaller singlet-triplet energy gap (ΔES-T = -0.99 kcal mol-1) compared to 5 (y0 = 30%, ΔES-T = -6.88 kcal mol-1), which can be explained by their different quinoidal structures.

Diindeno[2,1-b:2',1'-h]biphenylenes: Syntheses, Structural Analyses, and Properties.

A series of diindeno[2,1-b:2',1'-h]biphenylenes with open-shell singlet ground states and interesting properties were prepared. The studied compounds consist of p-quinodimethane moieties, which suffer from geometric perturbation with bond angles of around 90°. The substituent effects on structural parameters, local aromaticity, and properties were systematically explored.

On-Surface Synthesis and Characterization of Super-nonazethrene.

Beginning with the early work of Clar et al. in 1955, zethrenes and their laterally extended homologues, super-zethrenes, have been intensively studied in the solution phase and widely investigated as optical and charge transport materials. Superzethrenes are also considered to exhibit an open-shell ground state and may thus serve as model compounds to investigate nanoscale π-magnetism. However, their synthesis is extremely challenging due to their high reactivity. We report here the on-surface synthesis of the hitherto largest zethrene homologue-super-nonazethrene-on Au(111). Using single-molecule scanning tunneling microscopy and spectroscopy, we show that super-nonazethrene exhibits an open-shell singlet ground state featuring a large spin polarization-driven electronic gap of 1 eV. Consistent with the emergence of an open-shell ground state, high-resolution tunneling spectroscopy reveals singlet-triplet spin excitations in super-nonazethrene, characterized by a strong intramolecular magnetic exchange coupling of 51 meV. Given the paucity of zethrene chemistry on surfaces, our results therefore provide unprecedented access to large, open-shell zethrene compounds amenable to scanning probe measurements, with potential application in molecular spintronics.

DFT Calculations of InP Quantum Dots: Model Chemistries, Surface Passivation, and Open-Shell Singlet Ground States

DFT Calculations of InP Quantum Dots: Model Chemistries, Surface Passivation, and Open-Shell Singlet Ground States

Kinetic stability and NBO analysis of the C20-nAln nanocages (n = 1–5) using DFT investigation

Following our quest for novel heterofullerenes, we have compared and contrasted substitution effects on C20-nAln structures with n = 1–5, from kinetically view point at density functional theory (DFT). Structures with the odd numbered heteroatom (C19Al1, C17Al3 and C15Al5) appear open shell singlet ground state while the even ones (C18Al2-a, C18Al2-b, C16Al4-a and C16Al4-b) show closed shell singlet ground state. The high NBO charge distribution on the surface of C15Al5 nanocage provokes the more investigation for hydrogen storage. The overall trend for kinetic stability based on their band gap (ΔEHOMO-LUMO) in eV is C15Al5 (3.36) > C20 (2.31) > C18Al2-a (2.25) > C17Al3 (1.96) ≥ C19Al1 (1.95) > C16Al4-b (1.86) > C16Al4-a (1.56) > C18Al2-b (1.03). Evidently, the C15Al5 heterofullerene is as the most kinetic stable species against electronic excitation from the occupied HOMO orbital to the corresponding unoccupied LUMO orbital and contains five aluminum atom alternatively in equatorial. Furthermore, C15Al5 displays the highest energy of HOMO (EHOMO = -0.23321 a.u.), the lowest energy of LUMO (ELUMO = -0.10981 a.u.), the lowest nucleophilicity (N = 3.11 eV), the lowest electrophilicity (ω = 3.24 eV), the highest hardness (η = 3.36 eV) and the lowest softness (S = 0.15 eV) among the scrutinized species. Isolation of the two Al substitutions from each other by means of strong CC double bonds, increases N (4.48 eV), ω (9.69 eV) making C18Al2-b as the most nucleophilic and electrophilic species.