Structural Studies on the Radical Cations of Benzene, Naphthalene, Biphenylene, and Anthracene Fully Annelated with Bicyclo[2.2.2]octene Frameworks

A Triply Negatively Charged Nanographene Bilayer with Spin Frustration

We report on the largest open-shell graphenic bilayer and also the first example of triply negatively charged radical π-dimer. Upon three-electron reduction, bilayer nanographene fragment molecule (C₉₆ H₂₄ Ar₆ )₂ (Ar=2,6-dimethylphenyl) (1₂ ) was transformed to a triply negatively charged species 1₂ ^3.- , which has been characterized by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy and magnetic properties on a superconducting quantum interference device (SQUID). 1₂ ^3.- features a 96-center-3-electron (96c/3e) pancake bond with a doublet ground state, which can be thermally excited to a quartet state. It consists of 34 π-fused rings with 96 conjugated sp² carbon atoms. Spin frustration is observed with the frustration parameter f>31.8 at low temperatures in 1₂ ^3.- , which indicates graphene upon reduction doping may behave as a quantum spin liquid.

Robust Radical Cations of Hexabenzoperylene Exhibiting High Conductivity and Enabling an Organic Nonvolatile Optoelectronic Memory

Herein, we report robust π-conjugated radical cations resulting from the oxidation of hexabenzoperylene (HBP) derivatives, HBP-B and HBP-H, which have butyl and hexyl groups, respectively, attached to the same twisted double helicene π-backbone. The radical cation of HBP-B was successfully crystallized in the form of hexafluorophosphate, which exhibited conductivity as high as 1.32 ± 0.04 S cm^-1. Photochemical oxidation of HBP-H by molecular oxygen led to the formation of its radical cation in the solid state, as found with different techniques. This allowed the organic field effect transistor of HBP-H to function as a nonvolatile optoelectronic memory, with the memory switching contrast above 10³ and long-term stability without using a floating gate, an electret layer, or photochromic molecules.

S = 1/2 tetracene monoradical cation/anion: ion-based one-dimensional antiferromagnetic chains

The monoradical cation 1˙+ and anion 1˙- based on tetracene were generated by one-electron oxidation and one-electron reduction of the bulky tetracene (1), respectively, which contain the largest π-fused skeletons reported to date. For monoradical species, 1˙+ and 1˙-, EPR spectra and DFT calculation results indicate the spin density delocalized over the whole molecules. Notably, in the solid state, 1˙+ and 1˙-, respectively, pack into (1˙+)_n and (1˙--K-crown)_n, characterized by single-crystal X-ray diffraction studies. The intermolecular interactions of 1˙+ and 1˙- are, respectively, through van der Waals forces and exchange couplings supported by metal ions. The monoradical cation polymer (1˙+)_n was the first example based on PAHs. The EPR spectra at 90 K of 1˙+ and 1˙- all show forbidden transitions (Δm_s = ±2), indicating the existence of electronic coupling between the neighboring radicals, with respective 2J = -6.54 K and 2J = -0.22 K characterized by SQUID measurements.

Red-to-NIR emissive radical cations derived from simple pyrroles

Red-to-near-infrared (NIR) fluorophores are highly desirable in bio-imaging studies with advantages of high tissue penetration ability and less interference from auto-fluorescence. However, their preparation usually requires tedious synthetic procedures, which seriously restrict their applications. Thus, the direct preparation of red-to-NIR fluorophores from easily available substrates is highly desirable. Compared with the conventional closed-shell fluorophores, radical cations feature a large red-shift absorption, but only very few of them are fluorescent and they suffer from high instability. Herein, we proposed a convenient strategy for the preparation of red-to-NIR fluorophores through air oxidation of electron-rich 2,5-dimethylpyrroles to in situ generate red-to-NIR emissive radical cations, which can be stabilized by adsorption on silica gel-coated thin layer chromatography (TLC) plates or encapsulated in cucurbit[7]uril (CB[7]). The radical cations derived from pyrroles were verified using electron paramagnetic resonance (EPR) spectroscopy, theoretical calculations and one-electron oxidation experiments. Moreover, the pyrrole-derived radical cations encapsulated in CB[7] can be used for mitochondrial imaging in living cells with high specificity and in vivo imaging with long-term stability. The easily available pyrrole-derived radical cations with red-to-NIR emission are thus promising for biomedical applications.

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 BDBC^2- , 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.

Fragmentation of Polyfunctional Compounds Recorded Using Automated High-Throughput Desorption Electrospray Ionization

Using desorption electrospray ionization (DESI) as part of an automated high-throughput system, tandem mass spectra of the compounds in a pharmaceutical library were recorded in the positive mode under standardized conditions. Quality control filtering yielded an MS/MS library of 16 662 spectra. Fragmentation of subsets of the compounds in the library chosen to contain a single instance of a particular functional group (amide, piperazine, sulfonamide) was predicted by experts, and the results were compared with the experimental data. Expert performance was good to excellent for all the cases evaluated. Substituents on the functional groups were found to exert important secondary control over the fragmentation, with the main effect observed being product ion stabilization by aromatic substitution, which was consistent across the different groups evaluated. These substituent effects are generally explicable in terms of standard physical organic chemistry considerations of product ion stability as controlling fragmentation. A somewhat unexpected feature was the incidence of homolytic cleavages, driven by the stability of substituted amine radical cations. The findings of this study are intended to lay the groundwork for machine learning approaches to performing MS/MS spectrum → structure and structure → MS/MS spectrum operations on the same experimental data set. The effort involved and the success achieved in computer-aided interpretation, now underway, will be compared with the expert performance as described here.

Oxidation of a Dithieno[3,4-b:3',4'-d]thiophene Cyclic Dimer Containing a Planar Cyclooctatetraene Ring: Retention of High Antiaromaticity During Reactions

One-electron and peracid oxidations of dithieno[3,4-b : 3',4'-d]thiophene cyclic dimer, which contains an antiaromatic planar cyclooctatetraene (COT) core, were conducted. The reaction of the cyclic dimer with SbCl₅ produced isolable radical cation salts. Density functional theory (DFT) calculations showed that the spin density of the radical cation resides not on the COT ring but on the peripheral sulfur and carbon atoms in the thiophene unit with retention of high antiaromaticity based on the nucleus-independent chemical shift (NICS). The peracid oxidation of the cyclic dimer was found to proceed not on the COT ring but on the bridging sulfur atom in the dithienothiophene moiety. The retention of the high antiaromaticity of the COT ring after the sulfoxide formation was experimentally confirmed based on the relative hardness, and also was theoretically supported by NICS calculations. Interestingly, the DFT calculations suggested that the high antiaromaticity does not enhance the reactivity towards the epoxidation on the COT ring.

Radical cations of twisted acenes: chiroptical properties and spin delocalization

We introduce the first series of enantiopure twistacene radical cations, which form reversibly upon chemical or electrochemical oxidation. Their vis-NIR chiroptical properties (Cotton effect and anisotropy factor) increase systematically with the backbone twist. The hyperfine constants observed by EPR demonstrate significant spin delocalization even for large backbone twist angles.

The solution plasma process for heteroatom-carbon nanosheets: the role of precursors

The solution plasma process (SPP), known as non-equilibrium cold plasma at atmospheric pressure and room temperature, was used to investigate the synthesis of nitrogen-carbon nasnosheets (NCNs). To verify the effect of elementary composition and structure of N-methyl-2-pyrrolidone (NMP), various precursors were used in the SPP to synthesize NCNs via the bottom-up synthesis method for the first time. The NCNs were analyzed by transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Among the various precursors, SPP of 2-pyrrolidone was demonstrated to facilitate the formation of highly ordered NCNs. On the other hand, the SPP with cyclopentanone, cyclohexanone and pyrrole did not lead to the formation of carbon nanosheets. The results of this study would uncover new parameter fields for the growth of heteroatom-carbon nanosheets using this synthesis system. In addition, the study is expected to contribute toward research in improving the large-area growth and quality of two-dimensional nanostructures, such as heteroatom-carbon nanosheets or graphene, for various applications in other synthesis methods.

Fastest Formation Routes of Nanocarbons in Solution Plasma Processes

Although solution-plasma processing enables room-temperature synthesis of nanocarbons, the underlying mechanisms are not well understood. We investigated the routes of solution-plasma-induced nanocarbon formation from hexane, hexadecane, cyclohexane, and benzene. The synthesis rate from benzene was the highest. However, the nanocarbons from linear molecules were more crystalline than those from ring molecules. Linear molecules decomposed into shorter olefins, whereas ring molecules were reconstructed in the plasma. In the saturated ring molecules, C-H dissociation proceeded, followed by conversion into unsaturated ring molecules. However, unsaturated ring molecules were directly polymerized through cation radicals, such as benzene radical cation, and were converted into two- and three-ring molecules at the plasma-solution interface. The nanocarbons from linear molecules were synthesized in plasma from small molecules such as C2 under heat; the obtained products were the same as those obtained via pyrolysis synthesis. Conversely, the nanocarbons obtained from ring molecules were directly synthesized through an intermediate, such as benzene radical cation, at the interface between plasma and solution, resulting in the same products as those obtained via polymerization. These two different reaction fields provide a reasonable explanation for the fastest synthesis rate observed in the case of benzene.

Calculating X-ray Absorption Spectra of Open-Shell Molecules with the Unrestricted Algebraic-Diagrammatic Construction Scheme for the Polarization Propagator

X-ray absorption spectroscopy (XAS) is a powerful tool that provides information about the electronic structure of molecules via excitation of electrons from the K-shell core region to the unoccupied molecular levels. These high-lying electronic core-excited states can be accurately calculated using the algebraic-diagrammatic construction scheme of second order ADC(2) by applying the core-valence separation (CVS) approximation to the ADC(2) working equations. For the first time, an efficient implementation of an unrestricted CVS-ADC(2) variant CVS-UADC(2) is presented for the calculation of open-shell molecules by treating α and β spins separately from each other. The potential of the CVS-UADC(2) method is demonstrated with a set of small organic radicals by comparison with standard TD-DFT/B3LYP values and experimental data. It turns out that the extended variant CVS-UADC(2)-x, in particular, provides the most accurate results with errors of only 0.1% compared to experimental values. This remarkable agreement justifies the prediction of yet nonrecorded experimental XAS spectra like the one of the anthracene cation. The cation exhibits additional peaks due to the half-filled single-occupied molecular orbital, which may help to distinguish cation from the neutral species.

9,9′-Anthryl-anthroxyl radicals: strategic stabilization of highly reactive phenoxyl radicals

Stable anthroxyl radical with a half-life over 10 days in solution.

Professor Tetsuo Nozoe and my tropylium ion chemistry

Needless to say, the discovery of hinokitiol with its "unconventional" aromaticity by Professor Tetsuo Nozoe is one of the most important achievements in organic chemistry in the last century. The essence of this "non-benzenoid" aromaticity in hinokitiol is of course that of tropolone, and it is further related to the aromaticity of tropone and the tropylium ion, i.e., the cycloheptatrienyl cation. In this account, details of the study conducted by the author's group, particularly on the synthesis and properties of tropylium ion derivatives with various unique structures pursuing the ultimately high carbocation stability, are described.

The question of aromaticity in open-shell cations and anions as ion-radical offsprings of polycyclic aromatic and antiaromatic hydrocarbons

Arene cation-radicals and anion-radicals result directly from the one-electron oxidation and reduction of many aromatic hydrocarbons, yet virtually nothing is known of their intrinsic (thermodynamic) stability and hence "aromatic character". Since such paramagnetic ion radicals lie intermediate between aromatic (Hückel) hydrocarbons with 4n + 2-electrons and antiaromatic analogues with 4n-electrons, we can now address the question of pi-delocalization in these odd-electron counterparts. Application of the structure-based "harmonic oscillator model of aromaticity" or the HOMA method leads to the surprising conclusion that the aromaticity of these rather reactive, kinetically unstable arene cation and anion radicals (as measured by the HOMA index) is actually higher than that of their (diamagnetic) parent-contrary to conventional expectations.

Persistent ? radical cations: self-association and its steric control in the condensed phase

Pi Radical cations, which are highly reactive in general, can be made persistently stable by appropriate structural modification with heteroatoms, pi-conjugated systems, and alkyl substituents. Many of these pi radical cations undergo self-association in the condensed phase. The steric control of such self-association of stabilized pi radical cations is the subject of the present article. Such an association can result in the formation of pi- and/or sigma-dimers. The pi-dimerization in particular is now considered as an important intermolecular interaction for model studies of a charge-transport phenomenon in positively doped conducting polymers. On the other hand, the intermolecular interactions can be suppressed when the pi-system is modified with sterically demanding structural units, for example, by annelation with bicycloalkene frameworks. This structural modification not only brings about unusual stabilization of the radical cations but provides valuable information on the electronic structure/properties of the positively charged pi-systems in a segregated state.

Radical Cation of Dibenzothiophene Fully Annelated with Bicyclo[2.2.2]octene Units: X-ray Crystal Structure and Electronic Properties

New dibenzothiophene 2 fully annelated with bicyclo[2.2.2]octene units was synthesized and oxidized to stable radical cation salt 2(*+)SbCl(6)(-), whose structure was determined by X-ray crystallography. Although the intrinsic electronic structure of 2(*+) is predicted to be close to structure A, an interaction with the counteranion makes structure B contribute significantly. A part of the salt 2(*+)SbCl(6)(-) underwent rearrangement to arenium ion 6(+), whose structure was also clarified by X-ray crystallography.

Novel Cyclooctatetraene Radical Cation Planarized by Full Annelation with Bicyclo[2.1.1]hexene Units

A novel cyclooctatetraene (COT) radical cation fully annelated with bicyclo[2.1.1]hexene units was prepared as SbCl(6)(-) salt, and planarity of the octagonal ring was clarified by ESR and theoretical calculations. Its longest wavelength absorption (630 nm) is blue-shifted from that (745 nm) of COT radical cation annelated with bicyclo[2.2.2]octene units due to the widening of the HOMO-SOMO gap accompanying the flattening of the COT ring. [structure: see text]

Crystal Structures and Spectroscopic Characterization of Radical Cations and Dications of Oligothiophenes Stabilized by Annelation with Bicyclo[2.2.2]octene Units: Sterically Segregated Cationic Oligothiophenes

The remarkably stable SbF(6)(-) salts of the radical cations of bithiophene 1(2T) and terthiophene 1(3T), completely surrounded by bicyclo[2.2.2]octene (BCO) units, were obtained by one-electron oxidation of the neutral precursors with NO(+)SbF(6)(-), and their solid-state structures were determined by X-ray crystallography. In these radical cations, the presence of quinoidal character was apparent, as shown by the increased planarity and by comparison of the bond lengths with those of the neutral precursors. The shortest intermolecular pi-pi distances in the crystal structure of 1(2T)(*)(+)SbF(6)(-) (distance between two sp(2) carbon atoms, 4.89 A) and 1(3T)(*)(+)SbF(6)(-) (distance between an sp(2) carbon and a sulfur atom, 3.58 A) were found to be longer than the sums of the van der Waals radii of the corresponding atoms. In accord with this, no apparent change was observed in ESR and UV-vis-NIR spectra of solutions of 1(2T)(*)(+) and 1(3T)(*)(+) upon lowering the temperature, indicating that the pi- (or sigma-) dimer formation is inhibited in solution as well as in the solid state. The dications 1(2T)(2+) and 1(3T)(2+) were generated with the stronger oxidant SbF(5) in CH(2)Cl(2) at -40 degrees C and characterized by NMR spectroscopy. In the (1)H NMR spectra, two conformers were observed for each dication of both 1(2T)(2+) (transoid (t) and cisoid (c)) and 1(3T)(2+) (t,t and c,t) at room temperature due to the high rotational barrier around the inter-ring bond(s) between thiophene rings, which was caused by the enhanced double bond character of these bonds following two-electron oxidation. This is supported by DFT calculations (B3LYP/6-31G(d)), which predicted the rotational barriers in the dications of unsubstituted bithiophene and terthiophene to be 27.6 and 22.9 kcal mol(-)(1), respectively. In the case of quaterthiophene and sexithiophene surrounded by BCO frameworks 1(4T) and 1(6T), oxidation with even one molar equivalent of NO(+)SbF(6)(-) afforded the dication salts 1(4T)(2+)2SbF(6)(-) and 1(6T)(2+)2SbF(6)(-), which were isolated as stable single crystals and allowed the X-ray crystallography. In their crystal structures, the cationic pi-systems became planar again due to the great contribution of quinoidal resonance structures, and the pi-systems, which were arrayed in a parallel geometry, were also segregated by the steric effect of BCO units. The degree and tendency of changes in the bond lengths of thiophene rings of 1(4T)(2+) and 1(6T)(2+) as compared with neutral precursors were similar to those of 1(2T)(*)(+)SbF(6)(-) and 1(3T)(*)(+)SbF(6)(-), respectively, implying that the contribution of quinoidal character is modulated by the amount of positive charge per thiophene unit.

Synthesis and Properties of Novel Oligothiophenes Surrounded by Bicyclo[2.2.2]octene Frameworks

Novel oligothiophenes surrounded by bicyclo[2.2.2]octene (abbreviated as BCO) frameworks ranging from dimer to hexamer, 1(nT) (n = 2, 3, 4, 6), were prepared, and their structures and electronic properties were investigated. Dimer 1(2T) was synthesized by oxidative coupling of the 2-lithiated monomer generated from 4,5-BCO-annelated 2-bromothiophene 8 with CuCl2 in 76% yield. Trimer 1(3T) and tetramer 1(4T) were synthesized by Stille coupling of 2,5-dibromo-3,4-BCO-annelated thiophene 4 and of the 5,5'-dibromo derivative of bis(3,4-BCO-thiophen-2-yl) 10 with 2-stannylated 4,5-BCO-annelated thiophene 9 in 41% and 46% yield, respectively. Hexamer 1(6T) was synthesized by oxidative coupling of terthiophene 12, tris-annelated with BCO units, in 81% yield. X-ray crystallographic studies showed that the thiophene rings in 1(2T) and 1(3T) are rotated around the inter-ring C-C bond(s) with the C=C-C=C dihedral angles of -174.3(5) degrees for 1(2T) and -149.7(3) degrees and 34.4(3) degrees for 1(3T). In the crystal structures of 1(2T) and 1(3T), no pi-stacking was observed as expected from the steric effect of the BCO units. Theoretical calculations for 1(2T) and 1(3T) at the B3LYP/6-31G(d) level indicated that the annelation with BCO units either at the 2,3- or 3,4-positions of thiophene rings raises both the KS HOMO and LUMO levels. In the electronic absorption spectra of 1, the longest wavelength absorption band corresponding to the pi-pi transition is bathochromically shifted with the increase in absorption intensity as the number of thiophene rings increases, and the absorption of the polythiophene 1 with infinite length was predicted to be 419 nm. The cyclic voltammetry of 1 in CH2Cl2 at -78 degrees C (2T) or at room temperature (3T, 4T, 6T) showed two reversible oxidation waves, indicating that the radical cation and dication of 1 are stable under these conditions.