High-Spin Diradical Dication of Chiral π-Conjugated Double Helical Molecule

Polycationic Open-Shell Cyclophanes: Synthesis of Electron-Rich Chiral Macrocycles, and Redox-Dependent Electronic States

π-Conjugated chiral nanorings with intriguing electronic structures and chiroptical properties have attracted considerable interests in synthetic chemistry and materials science. We present the design principles to access new chiral macrocycles (1 and 2) that are essentially built on the key components of main-group electron-donating carbazolyl moieties or the π-expanded aza[7]helicenes. Both macrocycles show the unique molecular conformations with a (quasi) figure-of-eight topology as a result of the conjugation patterns of 2,2',7,7'-spirobifluorenyl in 1 and triarylamine-coupled aza[7]helicene-based building blocks in 2. This electronic nature of redox-active, carbazole-rich backbones enabled these macrocycles to be readily oxidized chemically and electrochemically, leading to the sequential production of a series of positively charged polycationic open-shell cyclophanes. Their redox-dependent electronic states of the resulting multispin polyradicals have been characterized by VT-ESR, UV/Vis-NIR absorption and spectroelectrochemical measurements. The singlet (ΔES-T=-1.29 kcal mol-1) and a nearly degenerate singlet-triplet ground state (ΔES-T(calcd)=-0.15 kcal mol-1 and ΔES-T(exp)=0.01 kcal mol-1) were proved for diradical dications 12+2⋅ and 22+2⋅, respectively. Our work provides an experimental proof for the construction of electron-donating new chiral nanorings, and more importantly for highly charged polyradicals with potential applications in chirospintronics and organic conductors.

Observation of SOMO-HOMO Inversion in a Neutral Polycyclic Conjugated Hydrocarbon

We report the generation of a nonbenzenoid polycyclic conjugated hydrocarbon, which consists of a biphenyl moiety substituted by indenyl units at the 4,4' positions, on ultrathin sodium chloride films by tip-induced chemistry. Single-molecule characterization by scanning tunneling and atomic force microscopy reveals an open-shell biradical ground state with a peculiar electronic configuration wherein the singly occupied molecular orbitals (SOMOs) are lower in energy than the highest occupied molecular orbital (HOMO).

N-Heterocyclic Carbene-Derived 1,3,5-Trimethylenebenzene: On-Surface Synthesis and Electronic Structure

1,3,5-Trimethylenebenzene (1,3,5-TMB), a 3-fold-symmetric triradical with a high-spin ground state, is an attractive platform for investigating the unique spin properties of π-conjugated triangular triradicals. Here, we report the on-surface synthesis of N-heterocyclic carbene (NHC)-derived 1,3,5-TMB (N-TMB) via surface-assisted C-C and C-N coupling reactions on Au(111). The chemical and electronic structures of N-TMB on the Au(111) surface are revealed with atomic precision using scanning tunneling microscopy and noncontact atomic force microscopy, combined with density functional theory (DFT) calculations. It is demonstrated that there is substantial charge transfer between N-TMB and the substrate, resulting in a positively charged N-TMB on Au(111). DFT calculations at the UB3LYP/def2-TZVP level of theory and multireference method, e.g., CASSCF/NEVPT2, indicate that N-TMB possesses a doublet ground state with reduced Cs symmetry in the gas phase, contrasting the quartet ground state of 1,3,5-TMB with D3h symmetry, and exhibits a doublet-quartet energy gap of -0.80 eV. The incorporation of NHC structures and the extended π-conjugation promote the spin-orbital overlaps in N-TMB, leading to Jahn-Teller distortion and the formation of a robust doublet state. Our results not only demonstrate the fabrication of polyradicals based on NHC but also shed light on the effect of NHC and π-conjugation on the electronic structure and spin coupling, which opens up new possibilities for precisely regulating the spin-spin exchange coupling of organic polyradicals.

Chiral π-Conjugated Double Helical Aminyl Diradical with the Triplet Ground State

We report a neutral high-spin diradical of chiral C2-symmetric bis[5]diazahelicene with ΔEST ≈ 0.4 kcal mol-1, as determined by EPR spectroscopy/SQUID magnetometry. The diradical is the most persistent among all high-spin aminyl radicals reported to date by a factor of 20, with a half-life of up to 6 days in 2-MeTHF at room temperature. Its triplet ground state and excellent persistence may be associated with the unique spin density distribution within the dihydrophenazine moiety, which characterizes two effective 3-electron C-N bonds analogous to the N-O bond of a nitroxide radical. The enantiomerically enriched (ee ≥ 94%) (MM)- and (PP)-enantiomers of the precursors to the diradicals are obtained by either preparative chiral supercritical fluid chromatography or resolution via functionalization with the chiral auxiliary of the C2-symmetric racemic tetraamine. The barrier for the racemization of the solid tetraamine is ΔG‡ = 43 ± 0.01 kcal mol-1 in the 483-523 K range. The experimentally estimated lower limit of the barrier for the racemization of a diradical, ΔG‡ ≥ 26 kcal mol-1 in 2-MeTHF at 293 K, is comparable to the DFT-determined barrier of ΔG‡ = 31 kcal mol-1 in the gas phase at 298 K. While the enantiomerically pure tetraamine displays strong chiroptical properties, with anisotropy factor |g| = |Δε|/ε = 0.036 at 376 nm, |g| ≈ 0.005 at 548 nm of the high-spin diradical is comparable to that recently reported triplet ground-state diradical dication. Notably, the radical anion intermediate in the generation of diradical exhibits a large SOMO-HOMO inversion, SHI = 35 kcal mol-1.

Triple para-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes

Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E1/2red -1.50 V → -0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λabs ∼ 700-900 nm) with gabs values above 10-3. The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.

Chiral Benzene Triimide (BTI) Radical Anions for Probing the Interplay of Unpaired Electron Spin and Chirality

Herein a series of chiral BTI radical anions bearing different chiral substituents were efficiently prepared by chemical reduction. X-ray crystallography revealed finely-tuned packing and helix assemblies of the radicals by the size of chiral substituents in crystalline state. In accordance with the crystalline-state packing, the powder ESR spectra indicate that 4 a- ⋅CoCp2 + and 4 c- ⋅CoCp2 + π-dimers exhibit thermally excited triplet states arising from strong spin-spin interactions, while discrete 4 b- ⋅CoCp2 + shows a broad doublet-state signal reflecting weak spin-spin interactions. The interplay between the unpaired electron spin and chiral substituents was studied by UV-Vis-NIR spectra, electronic circular dichroism (ECD) and TD DFT calculations. Different NIR absorptions of the radicals attributing to isolated SOMO→LUMO+1 (~889 nm) transitions were recorded. The emergence of Cotton effects (CEs) at the NIR region for 4 c- ⋅CoCp2 + radical enantiomers suggest the interplay between chirality and unpaired electron spin. The origin of the different circularly polarized light absorptions regarding SOMO derived transitions (around 880 nm) was attributed to chiral substitutes regulated electric and magnetic transition dipole moments of the unpaired electron participated transition.

From Stable Radicals to Thermally Robust High-Spin Diradicals and Triradicals

Stable radicals and thermally robust high-spin di- and triradicals have emerged as important organic materials due to their promising applications in diverse fields. New fundamental properties, such as SOMO/HOMO inversion of orbital energies, are explored for the design of new stable radicals, including highly luminescent ones with good photostability. A relation with the singlet-triplet energy gap in the corresponding diradicals is proposed. Thermally robust high-spin di- and triradicals, with energy gaps that are comparable to or greater than a thermal energy at room temperature, are more challenging to synthesize but more rewarding. We summarize a number of high-spin di- and triradicals, based on nitronyl nitroxides that provide a relation between the experimental pairwise exchange coupling constant J/k in the high-spin species vs experimental hyperfine coupling constants in the corresponding monoradicals. This relation allows us to identify outliers, which may correspond to radicals where J/k is not measured with sufficient accuracy. Double helical high-spin diradicals, in which spin density is delocalized over the chiral π-system, have been barely explored, with the sole example of such high-spin diradical possessing alternant π-system with Kekulé resonance form. Finally, we discuss a high-spin diradical with electrical conductivity and derivatives of triangulene diradicals.

Synthesis and Reduction of Nitrogen-Substituted Diaryl Dihydrophenazine Diradical Dications

A one-pot synthetic approach to form core-extended N,N'-disubstituted diaryl dihydrophenazine (DADHP) diradical dications (DRDCs) via chemical oxidation from aryl-substituted ortho-phenyldiamines is reported. The isolated N,N'-disubstituted DADHP DRDCs were reduced to their neutral counterparts with hydrazine. The model system featuring an unsubstituted fluorene aryl group, 2a, was tested as a photocatalyst for the polymerization of methyl methacrylate using organocatalyzed atom transfer polymerization (O-ATRP), which yielded a polymer with a controlled molecular weight and narrow polydispersity.

Thermally Ultrarobust S = 1/2 Tetrazolinyl Radicals: Synthesis, Electronic Structure, Magnetism, and Nanoneedle Assemblies on Silicon Surface

Open-shell organic molecules, including S = 1/2 radicals, may provide enhanced properties for several emerging technologies; however, relatively few synthesized to date possess robust thermal stability and processability. We report the synthesis of S = 1/2 biphenylene-fused tetrazolinyl radicals 1 and 2. Both radicals possess near-perfect planar structures based on their X-ray structures and density-functional theory (DFT) computations. Radical 1 possesses outstanding thermal stability as indicated by the onset of decomposition at 269 °C, based on thermogravimetric analysis (TGA) data. Both radicals possess very low oxidation potentials <0 V (vs. SCE) and their electrochemical energy gaps, Ecell ≈ 0.9 eV, are rather low. Magnetic properties of polycrystalline 1 are characterized by superconducting quantum interference device (SQUID) magnetometry revealing a one-dimensional S = 1/2 antiferromagnetic Heisenberg chain with exchange coupling constant J'/k ≈ -22.0 K. Radical 1 in toluene glass possesses a long electron spin coherence time, Tm ≈ 7 μs in the 40-80 K temperature range, a property advantageous for potential applications as a molecular spin qubit. Radical 1 is evaporated under ultrahigh vacuum (UHV) forming assemblies of intact radicals on a silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy (XPS). Scanning electron microscope (SEM) images indicate that the radical molecules form nanoneedles on the substrate. The nanoneedles are stable for at least 64 hours under air as monitored by using X-ray photoelectron spectroscopy. Electron paramagnetic resonance (EPR) studies of the thicker assemblies, prepared by UHV evaporation, indicate radical decay according to first-order kinetics with a long half-life of 50 ± 4 days at ambient conditions.

Two-ring chirality generated by the alignment of two achiral phenylacetylene macrocycles

When two achiral rings are bound mechanically, a chiral source is generated in the assembly. The chiroptical properties could be modulated according to the relative occupation of each ring in the assembly. In fact, we have found that two isomeric assemblies (1 and 2) show unique properties in each assembly with two achiral rings of phenylacetylene macrocycle (PAM). When considering the difference in the chiroptical properties of these two isomeric assemblies (6PAM × 2), no comparison was available based on no activity of the achiral component element itself (6PAM). In this work, we synthesized a two-ring chiral analog (4) by the ring-fusion of two 6PAMs to an 11PAM, and examined the chiroptical properties of 4, since the single helix was imparted as a chiral source. By comparison of the chiroptical properties (molar circular dichroism and molar optical rotation) of 1 and 2 to those of 4, we demonstrated that the disparity was related to the alignment of the two achiral rings.

A Dualistic Arrangement of a Chiral [1]Rotaxane Based on the Assembly of Two Rings and Two Rods

We demonstrate the synthesis and chiroptical properties of doubled molecules of a chiral [1]rotaxane, based on the assembly of an achiral ring of a phenylacetylene macrocycle (6PAM) and a p-phenylene ethynylene rod. Two molecules of [1]rotaxane constituted the doubled molecule through the ring fusion of 6PAMs to a 10PAM, which assured stationary occupation relative to each optically active unit. The absorption properties of the 10PAM-based doubled molecule and 6PAM-based original unit were consistently characterized by the independent existence of m-phenylene ethynylene ring(s) and p-phenylene ethynylene rod(s). Thus, molar circular dichroism (CD) was directly compared between the doubled molecule (n = 2) and the original unit (n = 1) to show that molar CD was increased more than expected by an increase in the number of units, or by an increase in absorbance. Due to the invariance of the configuration and the relative occupation of two units arranged adjacent to each other in 10PAM, one more comparison was available with an isomeric molecule of two rings and two rods in a threaded-and-unthreaded form. The additional arrangement of an optically inactive unit in an unthreaded form also led to an increase in molar CD, compared to that of the original chiral unit in a threaded form.

Furan-Extended Helical Rylenes with Fjord Edge Topology and Tunable Optoelectronic Properties

Lateral furan-expansion of polycyclic aromatics, which enables multiple O-doping and peripheral edge evolution of rylenes, is developed for the first time. Tetrafuranylperylene TPF-4CN and octafuranylquaterrylene OFQ-8CN were prepared as model compounds bearing unique fjord edge topology and helical conformations. Compared to TPF-4CN, the higher congener OFQ-8CN displays a largely red-shifted (≈333 nm) and intensified absorption band (λ_max =829 nm) as well as a narrowed electrochemical band gap (≈1.08 eV) due to its pronounced π-delocalization and emerging of open-shell diradicaloid upon the increase of fjord edge length. Moreover, strong circular dichroism signals in a broad range until 900 nm are observed for open-shell chiral OFQ-8CN, owing to the excellent conformational stability of its central bis(tetraoxa[5]helicene) fragments. Our studies provide insights into the relationships between edge topologies and (chir)optoelectronic properties for this novel type of O-doped PAHs.

A bis-NHC-CAAC dimer derived dicationic diradical

The isolation of carbon-centered diradicals is always challenging due to synthetic difficulties and their limited stability. Herein we report the synthesis of a trans-1,4-cyclohexylene bridged bis-NHC-CAAC dimer derived thermally stable dicationic diradical. The diradical character of this compound was confirmed by EPR spectroscopy. The variable temperature EPR study suggests the singlet state to be marginally more stable than the triplet state (2J = -5.5 cm-1 (ΔE ST = 0.065 kJ mol-1)). The presence of the trans-1,4-cyclohexylene bridge is instrumental for the successful isolation of this dicationic diradical. Notably, in the case of ethylene or propylene bridged bis-NHC-CAAC dimers, the corresponding dicationic diradicals are transient and rearrange to hydrogen abstracted products.

High-Spin S = 3/2 Ground-State Aminyl Triradicals: Toward High-Spin Oligo-Aza Nanographenes

We report high-spin aminyl triradicals with near-planar triphenylene backbones. Near-planarity of the fused aminyl radicals and the 2,6,10-triphenylene ferromagnetic coupling unit (FCU), magnetically equivalent to three fused 3,4'-biphenyl FCUs, assures an effective 2pπ-2pπ overlap within the cross-conjugated π-system, leading to an S = 3/2 (quartet) ground state that is well separated from low-spin excited doublet states. Thermal populations of the low-spin (S = 1/2) excited states are detectable both by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy, providing doublet-quartet energy gaps, ΔEDQ, corresponding to >85% population of the quartet ground states at room temperature. Notably, EPR-based determination of ΔEDQ relies on direct detection of the quartet ground state and doublet excited states. The ΔEDQ values are 1.0-1.1 kcal mol-1, with the more sterically shielded triradical having the larger value. The half-life of the more sterically shielded triradical in 2-methyltetrahydrofuran (2-MeTHF) is about 6 h at room temperature. The less sterically shielded triradical in 2-MeTHF decomposes at 158 K with a half-life of about 4 h, while at 195 K, the half-life is still about 2 h. The dominant products of the decay of triradicals are the corresponding triamines, suggesting hydrogen atom abstraction from the solvent as the primary mechanism. This study expands the frontier of the open-shell PAHs/nanographenes, of which the unique electronic, nonlinear optical, and magnetic properties could be useful in the development of novel organic electronics, photonics, and spintronics.

Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics

Organic radicals possessing an electronic configuration in which the energy of the singly occupied molecular orbital (SOMO) is below the highest doubly occupied molecular orbital (HOMO) level have recently attracted significant interest, both theoretically and experimentally. The peculiar orbital energetics of these SOMO-HOMO inversion (SHI) organic radicals set their electronic properties apart from the more common situation where the SOMO is the highest occupied orbital of the system. This review gives a general perspective on SHI, with key fundamental aspects regarding the electronic and structural factors that govern this particular electronic configuration in organic radicals. Selected examples of reported compounds with SHI are highlighted to establish molecular guidelines for designing this type of radical, and to showcase the potential of SHI radicals in organic spintronics as well as for the development of more stable luminescent radicals for OLED applications.

Benzo-Extended Cyclohepta[def]fluorene Derivatives with Very Low-Lying Triplet States

Open-shell non-alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene-based diradicaloids (1-3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (E_g ^opt =0.52-0.69 eV) and persistent stability under ambient conditions (t_1/2 =11.7-33.3 h). More importantly, this new type of diradicaloids possess a low-lying triplet state with an extremely small singlet-triplet energy gap, as low as 0.002 kcal mol^-1 , with a clear dependence on the molecular size. This family of compounds thus offers a new route to create non-alternant open-shell PHs with high-spin ground states, and opens up novel possibilities and insights into understanding the structure-property relationships.

Highly Oxidized Cobalt Porphyrin Dimer: Control of Spin Coupling via a Bridge

A cobalt porphyrin dimer is constructed in which two Co(II)porphyrins are connected covalently through a redox-active diethylpyrrole moiety via a flexible but "nonconjugated" methylene bridge. Upon oxidation with even a mild oxidant such as iodine, each cobalt(II) center and porphyrin ring undergo 1e^- oxidation, leading to the formation of a 4e^--oxidized cobalt(III)porphyrin dication diradical complex. Other oxidants such as Cl₂ and Br₂ also produce similar results. To stabilize such highly oxidized dication diradicals, the "nonconjugated" methylene spacer undergoes a facile and spontaneous oxidation to form a methine group with a drastic structural change, thereby making the bridge fully π-conjugated and enabling through-bond communication. This results in a strong spin coupling between two π-cation radicals which stabilizes the singlet state. The experimental observations are also strongly supported by extensive density functional theory calculations. The present study highlights the crucial role played by the nature of the bridge in the long-range electronic communication.

Carbazole Isomerism in Helical Radical Cations: Spin Delocalization and SOMO-HOMO Level Inversion in the Diradical State

We report a new molecular design to afford persistent chiral organic open-shell systems with configurational stability and an inversion in energy of the singly occupied molecular orbital (SOMO) and the highest doubly occupied molecular orbital (HOMO) for both mono- and diradical states. The unpaired electron delocalization within the designed extended helical π-conjugated systems is a crucial factor to reach chemical stabilities, which is not obtained using the classical steric protection approach. The unique features of the obtained helical monoradicals allow an exploration of the chiral intramolecular electron transfer (IET) process in solvents of different polarity by means of optical and chiroptical spectroscopies, resulting in an unprecedented electronic circular dichroism (ECD) sign inversion for the radical transitions. We also characterized the corresponding helical diradicals, which show near-infrared electronic circular dichroism at wavelengths up to 1100 nm and an antiferromagnetic coupling between the spins, with an estimated singlet-triplet gap (ΔE_ST) of about -1.2 kcal mol^-1. The study also revealed an intriguing double SOMO-HOMO inversion (SHI) electronic configuration for these diradicals, providing new insight regarding the peculiar energetic ordering of radical orbitals and the impact on the corresponding (chiral) optoelectronic properties.

High-Spin (S = 1) Blatter-Based Diradical with Robust Stability and Electrical Conductivity

Triplet ground-state organic molecules are of interest with respect to several emerging technologies but usually show limited stability, especially as thin films. We report an organic diradical, consisting of two Blatter radicals, that possesses a triplet ground state with a singlet-triplet energy gap, ΔEST ≈ 0.4-0.5 kcal mol-1 (2J/k ≈ 220-275 K). The diradical possesses robust thermal stability, with an onset of decomposition above 264 °C (TGA). In toluene/chloroform, glassy matrix, and fluid solution, an equilibrium between two conformations with ΔEST ≈ 0.4 kcal mol-1 and ΔEST ≈ -0.7 kcal mol-1 is observed, favoring the triplet ground state over the singlet ground-state conformation in the 110-330 K temperature range. The diradical with the triplet ground-state conformation is found exclusively in crystals and in a polystyrene matrix. The crystalline neutral diradical is a good electrical conductor with conductivity comparable to the thoroughly optimized bis(thiazolyl)-related monoradicals. This is surprising because the triplet ground state implies that the underlying π-system is cross-conjugated and thus is not compatible with either good conductance or electron delocalization. The diradical is evaporated under ultra-high vacuum to form thin films, which are stable in air for at least 18 h, as demonstrated by X-ray photoelectron and electron paramagnetic resonance (EPR) spectroscopies.

Long-Range Intramolecular Spin Coupling through a Redox-Active Bridge upon Stepwise Oxidations: Control and Effect of Metal Ions

Dinickel(II) and dicopper(II) porphyrin dimers have been constructed in which two metalloporphyrin units are widely separated by a long unconjugated dipyrrole bridge. Two macrocycles are aligned somewhat orthogonally to each other, while oxidation of the bridge generates a fully π-conjugated butterfly-like structure, which, in turn, upon stepwise oxidations by stronger oxidants result in the formation of the corresponding one- and two-electron-oxidized species exhibiting unusual long-range charge/radical delocalization to produce intense absorptions in the near-infrared (NIR) region and electron paramagnetic resonance (EPR) signals of a triplet state due to interaction between the unpaired spins on the Cu(II) ions. Although the two metal centers have a large physical separation through the bridge (more than 16 Å), they share electrons efficiently between them, behaving as a single unit rather than two independent centers. Detailed UV-vis-NIR, electrospray ionization mass spectrometry, IR, variable-temperature magnetic study, and EPR spectroscopic investigations along with X-ray structure determination of unconjugated, conjugated, and one electron-oxidized complexes have been exploited to demonstrate the long-range electronic communication through the bridge. The experimental observations are also supported by density functional theory (DFT) and time-dependent DFT calculations. The present study highlights the crucial roles played by a redox-active bridge and metal in controlling the long-range electronic communication.

Porphyrinoids, a unique platform for exploring excited-state aromaticity

Recently, Baird (anti)aromaticity has been referred to as a description of excited-state (anti)aromaticity. With the term of Baird's rule, recent studies have intensively verified that the Hückel aromatic [4n + 2]π (or antiaromatic [4n]π) molecules in the ground state are reversed to give Baird aromatic [4n]π (or Baird antiaromatic [4n + 2]π) molecules in the excited states. Since the Hückel (anti)aromaticity has great influence on the molecular properties and reaction mechanisms, the Baird (anti)aromaticity has been expected to act as a dominant factor in governing excited-state properties and processes, which has attracted intensive scientific investigations for the verification of the concept of reversed aromaticity in the excited states. In this scientific endeavor, porphyrinoids have recently played leading roles in the demonstration of the aromaticity reversal in the excited states and its conceptual development. The distinct structural and electronic nature of porphyhrinoids depending on their (anti)aromaticity allow the direct observation of excited-state aromaticity reversal, Baird's rule. The explicit experimental demonstration with porphyrinoids has contributed greatly to its conceptual development and application in novel functional organic materials. Based on the significant role of porphyrinoids in the field of excited-state aromaticity, this review provides an overview of the experimental verification of the reversal concept of excited-state aromaticity by porphyrinoids and the recent progress on its conceptual application in novel functional molecules.

A stable triplet diradical emitter

Molecules with luminescence have been extensively investigated, but the luminescence of a stable molecule with a triplet ground state has not been observed. Synthesis of boron-containing radicals has attracted lots of interest because of their unique electronic structures and potential applications in organic semiconductors. Though some boron-based diradicals have been reported, neutral boron-containing diradicals with triplet ground states are rare. Herein two borocyclic diradicals with different substituents (3 and 4) have been isolated. Their electronic structures were investigated by EPR and UV spectroscopy, and SQUID magnetometry, in conjunction with DFT calculations. Both experiment and calculation suggest that 3 is an open shell singlet diradical while 4 is a triplet ground state diradical with a large singlet–triplet gap (0.25 kcal mol⁻¹). Both diradicals show multi fluorescence peaks (3: 414, 431, and 470 nm; 4: 420, 433, and 495 nm). 3 displays multiple redox steps and is a potential material towards the design of high-density memory devices. 4 represents the first example of a neutral triplet boron-containing diradical with a strong ferromagnetic interaction, and also is the first stable triplet diradical emitter.

Stable borocyclic diradical emitters with a tunable ground state.

Axially Chiral Stable Radicals: Resolution and Characterization of Blatter Radical Atropisomers

Atropisomers of three Blatter radicals were obtained by the addition of 8-substituted 1-naphthyllithiums to 3-phenyl and 3-t-butylbenzo[e][1,2,4]triazine and separated by chiral high-performance liquid chromatography. Their absolute configurations were assigned by a comparison of experimental and time-dependent density functional theory calculated electronic circular dichroism spectra. The free energy of activation, ΔG^‡₂₉₈, and the half life of racemization, t_1/2, at 298 K were determined at ∼25 kcal mol^-1 and <130 h, respectively. Intramolecular π-π interactions in radicals were evident from single-crystal X-ray diffraction, density functional theory, and electrochemical analyses.

Thiophene-Based Double Helices: Radical Cations with SOMO-HOMO Energy Level Inversion†

We report relatively persistent, open-shell thiophene-based double helices, radical cations 1^•+ -TMS₁₂ and 2^•+ -TMS₈ . Closed-shell neutral double helices, 1-TMS₁₂ and 2-TMS₈ , have nearly identical first oxidation potentials, E^+/0  ≈ +1.33 V, corresponding to reversible oxidation to their radical cations. The radical cations are generated, using tungsten hexachloride in dichloromethane (DCM) as an oxidant, E^+/0  ≈ +1.56 V. EPR spectra consist of a relatively sharp singlet peak with an unusually low g-value of 2.001-2.002, thus suggesting exclusive delocalization of spin density over π-conjugated system consisting of carbon atoms only. DFT computations confirm these findings, as only negligible fraction of spin density is found on sulfur and silicon atoms and the spin density is delocalized over a single tetrathiophene moiety. For radical cation, 1^•+ -TMS₁₂ , energy level of the singly occupied molecular orbital (SOMO) lies below the four highest occupied molecular orbitals (HOMOs), thus indicating the SOMO-HOMO inversion (SHI) and therefore, violating the Aufbau principle. 1^•+ -TMS₁₂ has a half-life of the order of only 5 min at room temperature. EPR peak intensity of 2^•+ -TMS₈ , which does not show SHI, is practically unchanged over at least 2 h.

1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 0-5), was investigated to understand the effects of the curvature and system size on the spin-spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1,3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller-sized diradicals DR-(n+3)CPPs (n = 0-3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.

A giant spin molecule with ninety-six parallel unpaired electrons

Unpaired electrons which are essential for organic radicals and magnetic materials are hardly to align parallel, especially upon the increasing of spin numbers. Here, we show that the antiferromagnetic interaction in the largest Cr(III)-RE (rare earth) cluster {Cr₁₀RE₁₈} leads to 96 parallel electrons, forming a ground spin state S_T of 48 for RE = Gd. This is so far the third largest ground spin state achieved in one molecule. Moreover, by using the classical Monte Carlo simulation, the exchange coupling constants Jij can be determined. Spin dynamics simulation reveals that the strong Zeeman effects of 18 Gd(III) ions stabilize the ground ferrimagnetic state and hinder the magnetization reversals of these spins. In addition, the dysprosium(III) analog is an exchange-biasing single-molecule magnet. We believe that the ferrimagnetic approach and analytical protocol established in this work can be applied generally in constructing and analyzing giant spin molecules.

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The largest {Cr₁₀RE₁₈} molecular clusters were assembled for RE = Gd, Dy, and Y

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The {Cr₁₀Gd₁₈} cluster shows a large ground spin state of S_T = 48

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The exchange coupling constants were determined by Classical Monte Carlo simulation

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Spin dynamics simulation reveals a ferrimagnetic ground state of {Cr₁₀Gd₁₈}.

Synthesis and Thin Films of Thermally Robust Quartet (S = 3/2) Ground State Triradical

High-spin (S = 3/2) organic triradicals may offer enhanced properties with respect to several emerging technologies, but those synthesized to date typically exhibit small doublet quartet energy gaps and/or possess limited thermal stability and processability. We report a quartet ground state triradical 3, synthesized by a Pd(0)-catalyzed radical-radical cross-coupling reaction, which possesses two doublet-quartet energy gaps, ΔE_DQ ≈ 0.2-0.3 kcal mol^-1 and ΔE_DQ2 ≈ 1.2-1.8 kcal mol^-1. The triradical has a 70+% population of the quartet ground state at room temperature and good thermal stability with onset of decomposition at >160 °C under an inert atmosphere. Magnetic properties of 3 are characterized by SQUID magnetometry in polystyrene glass and by quantitative EPR spectroscopy. Triradical 3 is evaporated under ultrahigh vacuum to form thin films of intact triradicals on silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy. AFM and SEM images of the ∼1 nm thick films indicate that the triradical molecules form islands on the substrate. The films are stable under ultrahigh vacuum for at least 17 h but show onset of decomposition after 4 h at ambient conditions. The drop-cast films are less prone to degradation in air and have a longer lifetime.

Axially and Helically Chiral Cationic Radical Bicarbazoles: SOMO-HOMO Level Inversion and Chirality Impact on the Stability of Mono- and Diradical Cations

We report persistent chiral organic mono- and diradical cations based on bicarbazole molecular design with an unprecedented stability dependence on the type of chirality, namely, axial versus helical. An unusual chemical stability was observed for sterically unprotected axial bicarbazole radical in comparison with monocarbazole and helical bicarbazole ones. Such results were experimentally and theoretically investigated, revealing an inversion in energy of the singly occupied molecular orbital (SOMO) and the highest (doubly) occupied molecular orbital (HOMO) in both axial and helical bicarbazole monoradicals along with a subtle difference of electronic coupling between the two carbazole units, which is modulated by their relative dihedral angle and related to the type of chirality. Such findings allowed us to explore in depth the SOMO-HOMO inversion (SHI) in chiral radical molecular systems and provide new insights regarding its impact on the stability of organic radicals. Finally, these specific electronic properties allowed us to prepare a persistent, intrinsically chiral, diradical which notably displayed near-infrared electronic circular dichroism responses up to 1100 nm and almost degenerate singlet-triplet ground states with weak antiferromagnetic interactions evaluated by magnetometry experiments.

Rhodium-Catalyzed Highly Diastereo- and Enantioselective Synthesis of a Configurationally Stable S-Shaped Double Helicene-Like Molecule

An S-shaped double helicene-like molecule (>99 % ee), possessing stable helical chirality, has been synthesized by the rhodium(I)/difluorphos complex-catalyzed highly diastereo- and enantioselective intramolecular double [2+2+2] cycloaddition of a 2-naphthol- and benzene-linked hexayne. The collision between two terminal naphthalene rings destabilizes the helical chirality of the S-shaped double helicene-like molecule, but the introduction of two additional fused benzene rings significantly increases the configurational stability. Thus, no epimerization and racemization were observed even at 100 °C. The enantiopure S-shaped double helicene-like molecule forms a trimer through the multiple C-H⋅⋅⋅π and C-H⋅⋅⋅O interactions in the solid-state. The trimers stack to form columnar packing structures, in which neighboring stacks have opposite dipole directions. The accumulation of helical structures in the same direction in the S-shaped double helicene-like molecule enhanced the chiroptical properties.

Furan-containing double tetraoxa[7]helicene and its radical cation

An unprecedented furan-based double oxa[7]helicene 1 was achieved, featuring a stable twisted conformation with π-overlap at both helical ends. The excellent conformational stability allowed for optical resolution of 1, which provided a pair of enantiomers exhibiting pronounced mirror-imaged circular dichroism and circularly polarized luminescence activity. The radical cation of 1 was obtained by chemical oxidation as evidenced by UV-Vis-NIR absorption, electron paramagnetic resonance spectroscopy and in situ spectroelectrochemistry. The present work is the starting point for the investigation of open-shell oxahelicenes.