Marcin A. Majewski

43,44,47,48-Tetrathiaoctaphyrin(2.1.1.1.2.1.1.1), obtained in a concise synthesis, is found to adopt a C₂-symmetric figure-eight conformation in solution and in the solid state. The macrocycle is globally nonaromatic but exhibits strong absorption in the visible and near-infrared regions. Metalation of the octaphyrin with [Rh^I(CO)₂Cl]₂ produced an unprecedented dinuclear complex featuring two octahedral Rh^III centers and four Rh–C bonds. The latter species is globally aromatic and has a distinct, quasi-rectangular shape. Using an in-depth NMR and computational analysis, the Rh₂ complex is found to exhibit an unusual case of isomerism resulting from the combined effect of local helicity, conformationally locked trans-vinylene bridges, and polar axial ligation.

Two families of difluorenoheterole diradicaloids were synthesized, featuring isomeric ring systems with distinct conjugation topologies. The two types of difluorenoheteroles contain, respectively, a Chichibabin-like motif (CH) and a newly introduced heteroatom-linked triphenylmethyl dyad (TD-X). Combined experimental and theoretical investigations show that the TD-X systems have reduced quinoidal character but the interaction between formal spin centers is sufficiently strong to ensure a singlet ground state. The singlet–triplet energy gaps in the TD-X difluorenoheteroles are strongly affected by the heterocyclic ring, with values of −4.3 and −0.7 kcal mol⁻¹ determined for the pyrrole- and thiophene-containing analogues, respectively. In cyclic voltammetry experiments, the TD-X systems show diminished energy gaps and superior reversibility in comparison with their CH counterparts. The radical anions and cations obtained from these diradicaloids show extremely red-shifted bands, occasionally with λ_max > 3500 nm. Computational studies show that some of these ions adopt distonic structures and may be characterized as class-II mixed-valence species.

The composition of hemicelluloses differ considerably in extant hardwood and softwood. In this study, we demonstrate that there are also significant differences between the hemicellulose composition of angiosperm and gymnosperm fossil wood (Eocene to Miocene). Because of the excellent preservation and high hemicellulose contents, Fourier-transform infrared (FT-IR) spectroscopy of xylites shows bands that are characteristic of various hemicelluloses and pectin (despite the overlapping of cellulose and lignin IR bands). In particular, fossil softwood samples show IR bands typical of mannose-containing hemicellulose. The composition of hemicellulose is determined through methanolysis gas chromatography–mass spectrometry (GC–MS). The results correspond with those of extant softwood and hardwood hemicelluloses because of the excellent preservation of 13–35 Myr fossil wood deposited under reducing marine conditions. The main saccharide building blocks found in angiosperm Eocene and Miocene wood are glucose and xylose. In contrast, mannose, galactose, and glucose are dominant in gymnosperms. The relationship among xylose, mannose, and rhamnose suitably differentiates both fossil and extant woods. Moreover, the relatively high contents of rhamnose and galacturonic acid identified mainly in angiosperms indicate the presence of preserved pectin, which has not been previously documented in fossil organic matter. Nuclear magnetic resonance (NMR) spectroscopy reveals the presence of xylogalacturonan fragments in a separate fraction of the angiosperm Eocene wood. The presence of pectin, a significantly labile carbohydrate polymer, confirms the perfect preservation of fossil wood in marine deposits. This study sheds new light on the identification of xylite affinities based on their hemicellulose composition and promotes research toward the palaeochemotaxonomy of fossil plants.

A hexaradicaloid molecule with alternating Kekulé and non-Kekulé connectivities between adjacent spin centers was obtained by fusing two conjugation motifs in Chichibabin and Schlenk hydrocarbons into a coronoid structure. ¹H NMR, ESR, and SQUID experiments and computational analyses show that the system has a singlet ground state with a significant hexaradicaloid character (γ0=0.826, γ1=γ2=0.773). It has multiple thermally accessible high-spin states (up to the septet), with uniform energy gaps of ca 1.0 kcal mol⁻¹ between consecutive multiplicities. In line with its open-shell character, the coronoid has a small electronic band gap (ca. 0.8 eV) and undergoes two consecutive one-electron oxidations at low potentials, yielding cationic forms with extended near-infrared absorption. The hexaradicaloid, which combines open-shell and macrocyclic contributions to its π conjugation, is an example of a design strategy for multistate spin switches and redox-amphoteric NIR dyes.

Beilstein Journal of Organic Chemistry

A diindenophenanthrene biradicaloid, formally derived from Chichibabin's hydrocarbon, is obtained in a short, scalable synthesis. The present system is electron-rich and devoid of conjugated substituents, and still exhibits very good stability under ambient conditions. The introduction of the diindeno[1,2-a:2′,1′-i] phenanthrene ring framework results in a singlet biradicaloid system with an easily accessible triplet state (ΔES–T = −1.30 kcal mol⁻¹) and a small electronic bandgap (1.39 V). The stability limits of the title hydrocarbon were explored systematically in the solid state, to reveal an unusual thermally initiated hydrogen-scrambling oligomerization process.

[4]Chrysaorene, a fully conjugated carbocyclic coronoid, is shown to be a low-bandgap π-conjugated system with a distinct open-shell character. The system shows good chemical stability and can be oxidized to well-defined radical cation and dication states. The cavity of [4]chrysaorene acts as an anion receptor toward halide ions with particular selectivity toward iodides (K_a = 207 ± 6 M⁻¹). The interplay between anion binding and redox chemistry is demonstrated using ¹H NMR analysis in solution. In particular, a well-resolved, paramagnetically shifted spectrum of the [4]chrysaorene radical cation is observed, providing evidence for inner iodide binding. The radical cation–iodide adduct can be generated in thin solid films of [4]chrysaorene by simple exposure to diiodine vapor.

An octaphyrin(1.2.1.1.1.2.1.1) containing two conjugated ethynylene bridges has been synthesized and characterized. The macrocycle reveals complex conformational dynamics dependent on its protonation and oxidation state. The [40]annulenoid macrocycle and its [38]annulenoid oxidized form display residual macrocyclic ring currents. In spite of its low apparent aromaticity the new octaphyrin is a potent chromophore with a vis-NIR absorption profile strongly influenced by the redox and acid–base chemistry.

Two hitherto unknown core modified hexaphyrin analogues have been synthesized and characterized where the conformational dynamics of these macrocycles in the free base form is achieved by the rotation of thienothiophene units. Further unique property of these macrocycles is the Hückel-Möbius topological switching. The thermodynamic equilibrium and kinetics of the interconversion leading to Hückel-Möbius switches have been triggered by external stimuli, such as protonation and/or temperature. We have provided a thorough solution-state spectroscopic characterization, solid-state structural evidence combined with in-depth theoretical calculations to investigate the crucial factors involved in such interconversion between Hückel and Möbius topologies for these hexaphyrins which will be useful in designing future new members to expanded porphyrin chemistry.

Octulene, the higher homologue of kekulene and septulene, was synthesized using the fold-in method. This new hydrocarbon macrocycle contains a large 24-membered inner circuit, which is peripherally fused to 24 benzene rings. Such an arrangement produces considerable hyperbolic distortion of the π-conjugated surface. The consequences of distortion in octulene were explored using photophysical methods, which revealed a reduced electronic band gap and greater flexibility of the π system. Octulene contains a functional cavity with a diameter larger than 5.5 Å that is capable of efficiently binding the chloride anion in a nonpolar solvent (K_a = 2.2(4) × 10⁴ M⁻¹, 1% dichloromethane (DCM) in benzene). The octulene–chloride interaction is stabilized by eight weak C(sp²)H⋅⋅⋅Cl bonds, providing the first example of a hydrocarbon-based anion receptor.

Two coronoid hydrocarbons, [3]- and [4]chrysaorene were synthesized from fluorenophane precursors using the fold-in strategy. [3]Chrysaorene is a bowl-shaped fragment of the C₂₄₀-I_h fullerene, whereas [4]chrysaorene is planar and contains a unique large 24-membered internal ring. The chrysaorenes show geometry-dependent magnetic properties and are strongly fluorescent.