Formation of a Macrocycles-in-a-Macrocycle Superstructure with All-gauche Conformation by Reversible Radical Association

Self-Assembly of Triphenylamine Macrocycles and Co-assembly with Guest Molecules at the Liquid-Solid Interface Studied by STM: Influence of Different Side Chains on Host-Guest Interaction

The side chains of macrocyclic molecules have a non-negligible effect on the two-dimensional (2D) supramolecular networks at the liquid-solid interface. In this study, we investigate the self-assembly behaviors of two conjugated triphenylamine macrocycles modified with different alkyl chains and construct the host-guest supramolecular nanopatterns on the highly oriented pyrolytic graphite with a scanning tunneling microscope. In combination with density functional theory calculations, how different side chains affect the host-guest interaction is discussed. This work provides insights into constructing a 2D host-guest dynamic co-assembly on the surface.

Steric Hindrance Favors σ Dimerization over π Dimerization for Julolidine Dicyanomethyl Radicals

Metastable radicals exist in a steady-state equilibrium in solution with dimers, which can be either σ dimers or π dimers. Here, we show that steric hindrance at the para position causes julolidine-derived dicyanomethyl radicals to form σ dimers rather than π dimers, the opposite behavior as seen in other carbon-centered radicals, where steric hindrance typically favors pimerization. The change in dimerization mode can be attributed to weaker London dispersion forces and a decreased orbital overlap in the sterically hindered dicyanomethyl radical π dimers, while the bulky groups exert relatively little effect on the energy of the σ dimer.

A dicyanomethyl radical stabilized by ferrocene: a new building block for radical-based dynamic covalent chemistry with redox activity

A novel dicyanomethyl radical conjugated with ferrocene as an electron donating unit through a benzene linker is firstly prepared. This radical is in equilibrium with the σ-dimer in solution, and...

An Open-Shell Coronoid with Hybrid Chichibabin-Schlenk Conjugation

A hexaradicaloid molecule with alternating Kekulé and non-Kekulé connectivities between adjacent spin centers was obtained by fusing two classic conjugation motifs, found respectively in the Chichibabin and Schlenk hydrocarbons, into a coronoid structure. 1 H NMR, ESR and SQUID experiments, combined with computational analyses reveal that the system has a singlet ground state, characterized by a significant hexaradicaloid character ( γ 0 = 0.826, γ 1 = γ 2 = 0.773). It possesses multiple thermally accessible high-spin states (up to the septet), with uniform energy gaps of ca 1.0 kcal/mol between consecutive multiplicities. In line with its open-shell character, the coronoid has a small electronic bandgap of 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, provides an example of a design strategy for multistate spin switches and redox-amphoteric NIR dyes.

Unveiling the Hidden σ-Dimerization of a Kinetically Protected Olympicenyl Radical

The σ-dimer of a kinetically protected olympicenyl radical, which evaded the experimental detection, was revealed by conversion into biolympicenylidene with E-configuration in a regioselective manner. The complicated stereochemistry and energetics of the σ-dimers derived from C_2v symmetry and uneven spin distribution of the olympicenyl radical were revealed by the theoretical calculations, and the energetic preference of π-dimer over σ-dimer by a minute gap was disclosed. The E-biolympicenylidene, a polycyclic ene structure previously considered as reactive intermediate in the phenalenyl radical system, exhibited exceptional stability, which allowed for a detailed investigation on its singlet diradical character and physical properties by means of X-ray crystallography, UV-vis-NIR absorption/emission spectroscopy and cyclic voltammetry, and assisted by theoretical calculations. The E-biolympicenylidene showed high resistance towards both thermal and photochemical ring-cyclization reactions, which was attributed to high activation energies for the rate-determining electrocyclization operated on both disrotatory and conrotatory mode, as well as a small spin density at the bonding sites for the radical-radical coupling process.

Dynamic Covalent Properties of a Novel Indolo[3,2-b]carbazole Diradical

This work describes the synthesis and properties of a dicyanomethylene-substituted indolo[3,2-b]carbazole diradical ICz-CN. This quinoidal system dimerises almost completely to (ICz-CN)₂ , which contains two long C(sp³ )-C(sp³ ) σ-bonds between the dicyanomethylene units. The minor open-shell ICz-CN component in the solid-state mixture was identified by EPR spectroscopy. Cyclic voltammetry and UV-visible spectroelectrochemical data, as well as comparison with reference monomer ICz-Br reveal that the nature of the one-electron oxidation of (ICz-CN)₂ at ambient temperature and ICz-CN at elevated temperature is very similar in all these compounds due to the prevailing localization of their HOMO on the ICz backbone. The peculiar cathodic behaviour reflects the co-existence of (ICz-CN)₂ and ICz-CN. The involvement of the dicyanomethylene groups stabilizes the close-lying LUMO and LUMO+1 of (ICz-CN)₂ and especially ICz-CN compared to ICz-Br, resulting in a distinctive cathodic response at low overpotentials. Differently from neutral ICz-CN, its radical anion and dianion are remarkably stable under ambient conditions. The UV/Vis(-NIR) electronic transitions in parent (ICz-CN)₂ and ICz-CN and their different redox forms have been assigned convincingly with the aid of TD-DFT calculations. The σ-bond in neutral (ICz-CN)₂ is cleaved in solution and in the solid-state upon soft external stimuli (temperature, pressure), showing a strong chromism from light yellow to blue-green. Notably, in the solid state, the monomeric diradical species is predominantly formed under high hydrostatic pressure (>1 GPa).

Cyclophane-Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18-Porphyrinyl Dicyanomethyl Diradicals

2,18-Bis(dicyanomethyl)-substituted Ni^II porphyrin 8 and Zn^II porphyrin 11 were prepared and subjected to oxidation with PbO₂ in CH₂ Cl₂ at 298 K to give cyclophane-type chlorin dimers (9)₂ and (12)₂ as a consequence of double recombination of biradicals 9 and 12, respectively. Dimer (9)₂ takes a syn-conformation of two distorted Ni^II chlorins but (12)₂ takes an anti-conformation of relatively planar Zn^II chlorins. At 298 K, dimer (9)₂ is stable and its ¹ H NMR spectrum is sharp but becomes broad at high temperature, while the ¹ H NMR spectrum of (12)₂ is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of (12)₂ is much less than that of (9)₂ . The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer (16)₂ to give homo dimers (9)₂ and (15)₂ .

Aromatic Nanosandwich Obtained by σ-Dimerization of a Nanographenoid π-Radical

A 139-π-electron nanographenoid radical was obtained by expanding the periphery of a naphthalimide–azacoronene hybrid with a methine bridge. The radical was isolated in the form of its σ-dimer, which was shown to possess a conformationally restricted two-layer structure both in the solid state and in solution. The dimer is cleaved into its parent radicals when exposed to ultraviolet or visible radiation in toluene solutions but is resistant to thermally induced dissociation. Under inert conditions, the radicals recombine quantitatively into the σ-dimer with observable kinetics, but they are oxidized into a ketone derivative in the presence of atmospheric oxygen. Combined structural, spectroscopic, and theoretical evidence shows that the σ-dimer contains a weak C(sp³)–C(sp³) bond, but is stabilized against thermal dissociation by a very strong dispersive interaction between the overlapping π surfaces.

Formation and trapping of the thermodynamically unfavoured inverted-hemicucurbit[6]uril

Formation of inverted-cis-cyclohexanohemicucurbit[6]uril (i-cis-cycHC[6]), with up to 33% isolated yield, can be induced at the expense of thermodynamically favoured cis-cycHC[6]. Reaction selectivity is governed by the solution-based template-aided dynamic combinatorial chemistry and continuous precipitation of the formed macrocycles. Different binding affinities of three diastereomeric cycHC[6]s with trifluoroacetic acid is demonstrated.

Stable radical versus reversible σ-bond formation of (porphyrinyl)dicyanomethyl radicals

(Porphyrinyl)dicyanomethyl radicals were produced by oxidation of dicyanomethyl-substituted porphyrins with PbO2. These radicals constitute a rare example displaying stable radical versus dynamic covalent chemistry (DCC) depending upon the substitution position of the dicyanomethyl radical. meso-Dicyanomethyl-substituted radicals exist as stable monomeric species and do not undergo any dimerization processes either in the solid state or in solution. In contrast, β-dicyanomethyl-substituted radicals are isolated as σ-dimers that are stable in the solid-state but display reversible σ-dimerization behavior in solution; monomeric radical species exist predominantly at high temperatures, while σ-dimerization is favoured at low temperatures. This dynamic behaviour has been confirmed by variable-temperature 1H NMR, UV-vis and EPR measurements. The structures of the stable radical and σ-dimer have been revealed by single-crystal X-ray diffraction analysis. The observed different reactivities of the two (porphyrinyl)dicyanomethyl radicals have been rationalized in terms of their spin delocalization behaviours.