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.

Fullerene Wires Assembled Inside Carbon Nanohoops

Carbon-nanohoop structures featuring one or more round-shaped cavities represent ideal supramolecular hosts for spherical fullerenes, with potential to form host-guest complexes that perform as organic semiconductors in the solid state. Due to the tight complexation between the shape-complementary hosts and guests, carbon nanohoops have the potential to shield fullerenes from water and oxygen, known to perturb the electron-transport process. Many nanohoop receptors have been found to form host-guest complexes with fullerenes. However, there is only a little or no control over the long-range order of encapsulated fullerenes in the solid state. Consequently, the potential of these complexes to perform as organic semiconductors is rarely evaluated. Herein, we present a survey of all known nanohoop-fullerene complexes, for which the solid-state structures were obtained. We discuss and propose instances where the inclusion fullerene guests form discrete supramolecular wires, which might open up possibilities for their use in electronic devices.

Manipulations of Chiroptical Properties in Belt-Persistent Cycloarylenes via Desymmetrization with Heteroatom Doping

A desymmetrization strategy has been devised in the design of molecular cylinders to maximize the dissymmetry factor relevant to circularly polarized light. Although the highest dissymmetry factor of organic molecules was previously achieved with a chiral belt-persistent cycloarylene having magnetic and electric transition dipole moments in parallel, we noticed that an unbalanced magnitude of two moments was detrimental for higher dissymmetry factors. In this study, a molecular cylinder was desymmetrized by arraying doped and undoped panels via stereoselective cross-coupling macrocyclization. The desymmetrization succeeded in balancing two moments by reducing the electric transition moment at the global minimum but failed to maximize the dissymmetry factor. Structural studies revealed that the dissymmetry factor is sensitive to subtle structural fluctuations, while the rotatory strength is not affected. This study is important for the development of chiroptical materials.

Conjugated Molecular Nanotubes

Molecular compounds with permanent tubular architectures displaying radial π-conjugation are exceedingly rare. Their radial and axial delocalization presents them with unique optical and electronic properties, such as remarkable tuning of their Stokes shifts, and redox switching between global and local aromaticity. Although these tubular compounds display large internal void spaces, these attributes have not been extensively explored, thus presenting future opportunities in the development of materials. By using cutting-edge synthetic methodologies to bend aromatic surfaces, large opportunities in synthesis, property discovery, and applications are expected in new members of this family of conjugated molecular nanotubes.

A Macrocyclic Gold(I)-Biphenylene Complex: Triangular Molecular Structure with Twisted Au2 (diphosphine) Corners and Reductive Elimination of [6]Cycloparaphenylene

The digold(I) complex [Au₂ Cl₂ (Cy₂ PCH₂ PCy₂ )] reacts with 4,4'-diphenylene diboronic acid to form a triangular macrocyclic complex with twisted Au-P-C-P-Au groups at the three corners. The synthesis of the complex and its chemical oxidation produced [6]cycloparaphenylene ([6]CPP) in 59 % overall yield.

Propeller-Shaped Semi-fused Porphyrin Trimers: Molecular-Symmetry-Dependent Chiroptical Response

Triple helicene-like semi-fused trimeric Ni^II porphyrins were constructed by alkyne trimerization of an ethynyl-substituted porphyrin and subsequent three-fold Grignard addition to the formyl groups and acid-catalyzed intramolecular cyclization. The presence of stereogenic sp³ carbons in the central bridge leads to small inter-porphyrin conjugative interactions as was revealed by electrochemical and optical properties. Two diastereomers with stable chiral conformations were optically resolved, and the separated enantiomers displayed considerably intense circular dichroism. Importantly, the chiroptical response of C₃ -symmetric helical isomer (|Δϵ|=830 m^-1  cm^-1 ) is 1.8 times amplified from that of C₁ -symmetric one (|Δϵ|=470 m^-1  cm^-1 ). The observed amplification has been interpreted in terms of different spatial arrangements of the three porphyrins.

Selective Synthesis of Conjugated Chiral Macrocycles: Sidewall Segments of (-)/(+)-(12,4) Carbon Nanotubes with Strong Circularly Polarized Luminescence

Carbon nanotubes (CNTs) have unusual physical properties that are valuable for nanotechnology and electronics, but the chemical synthesis of chirality- and diameter-specific CNTs and π-conjugated CNT segments is still a great challenge. Reported here are the selective syntheses, isolations, characterizations, and photophysical properties of two novel chiral conjugated macrocycles ([4]cyclo-2,6-anthracene; [4]CAn_2,6 ), as (-)/(+)-(12,4) carbon nanotube segments. These conjugated macrocyclic molecules were obtained using a bottom-up assembly approach and subsequent reductive elimination reaction. The hoop-shaped molecules can be directly viewed by a STM technique. In addition, chiral enantiomers with (-)/(+) helicity of the [4]CAn_2,6 were successfully isolated by HPLC. The new tubular CNT segments exhibit large absorption and photoluminescence redshifts compared to the monomer unit. The carbon enantiomers are also observed to show strong circularly polarized luminescence (g_lum ≈0.1). The results reported here expand the scope of materials design for bottom-up synthesis of chiral macrocycles and enrich existing knowledge of their optoelectronic properties.