Protonation-Induced Antiaromaticity in Octaaza[8]circulenes: Cyclooctatetraene Scaffolds Constrained with Four Amidine Moieties.

The switching of cyclic π-conjugation pathways using external stimuli is an attractive research topic in the field of organic chemistry. Here, we synthesized C4h -symmetric octaaza[8]circulenes with four peripherally arranged amidine moieties that exhibit enhanced antiaromaticity upon protonation. Titration experiments with methanesulfonic acid revealed the formation of the tetraprotonated forms of the octaaza[8]circulenes in solution. Single-crystal X-ray diffraction analyses and theoretical calculations indicated that the contribution of the 8π antiaromatic character of the octaaza[8]circulenes is enhanced by the delocalization of charge through the protonation of the pyridine rings.

Quadruply BN-Fused Tetrathia[8]circulenes with Flexible Frameworks: Synthesis, Structures and Properties.

Quadruply BN-fused tetrathia[8]circulenes were synthesized through four-fold electrophilic borylation. The single-crystal X-ray diffraction analysis revealed that the BN-fused tetrathia[8]circulene with peripheral phenyl groups exhibits crystal polymorphism, in which the circulene core adopts both planar and saddle conformations in the solid state. The experimental and theoretical studies revealed that the weaker aromaticity of azaborine compared with benzene renders the flexibility of the BN-fused tetrathia[8]circulenes.

Enthalpically and Entropically Favorable Self-Assembly: Synthesis of C4h -Symmetric Tetraazatetrathia[8]circulenes by Regioselective Introduction of Pyridine Rings.

Self-assembly of π-conjugated molecules in solution generally occurs owing to either an enthalpic or an entropic gain; however, designing π-conjugated systems that simultaneously exhibit enthalpically and entropically favorable self-assembly behavior is challenging. Herein, the self-assembly behavior of tetraazatetrathia[8]circulenes is disclosed, which is driven by both enthalpy and entropy. Single-crystal X-ray diffraction analysis demonstrated that molecules of these tetraazatetrathia[8]circulenes form face-to-face stacked dimers with a 1D columnar structure owing to the circularly arranged dipole moments. Importantly, concentration- and temperature-dependent 1 H NMR spectra revealed that the formation of self-assemblies of tetraazatetrathia[8]circulenes in chloroform and methanol is favored by both enthalpic and entropic factors. The unique association behavior is due to the presence of sp2 -hybridized nitrogen atoms, which weakly coordinate to the hydrogen atoms of these solvents and reduce the π-electron density of the circulene cores.

Aggregation-Induced Emission in Tetrathia[8]circulene Octaoxides via Restriction of the Dynamic Motion of their Negatively Curved π-Frameworks.

Curved polycyclic aromatic hydrocarbons exhibit intriguing properties, but their flexible and dynamic nature is often considered to be unfavorable for achieving excellent emission properties. In this study, we prepared tetrathia[8]circulene octaoxides that adopt a negatively curved conformation. We found that these molecules exhibit aggregation-induced emission (AIE) that originates from the restriction of the dynamic motion of their saddle-shaped π-frameworks in the solid and aggregated states.

Systematic Synthesis of Tetrathia[8]circulenes: The Influence of Peripheral Substituents on the Structures and Properties in Solution and Solid States.

We developed the diversity-oriented approach for the synthesis of tetrathia[8]circulenes with a variety of peripheral substituents. Iridium-catalyzed direct C-H borylation of tetrathienylene provided 1,4,7,10-tetraboryltetrathienylene as a major product. 1,4,7,10-Tetraboryltetrathienylene served as an a key intermediate to achieve the selective synthesis of octasubstituted or tetrasubstituted tetrathia[8]circulenes via rhodium-catalyzed annulation with symmetric internal alkynes or sequential Sonogashira-Hagihara coupling and base-promoted intramolecular cyclization. A variety of substituents were installed at the peripheral positions of tetrathia[8]circulenes systematically. The self-assembling behavior of tetrathia[8]circulenes was investigated using 1H NMR and AFM measurements. The number and the chain length of alkyl groups exerted a significant influence on the aggregation ability and the crystal packing structures of tetrathia[8]circulenes in both solution and solid states. We also found that the molecular arrangement of the self-assembled tetrathia[8]circulene molecules affected the hole mobility assessed by the FP-TRMC method.

Synthesis and Photodynamics of Tetragermatetrathia[8]circulene.

Tetragermatetrathia[8]circulene has successfully been synthesized from tetraiodotetrathienylene through palladium-catalyzed germylation and rhodium-catalyzed intramolecular dehydrogenative cyclization. A single-crystal X-ray diffraction analysis elucidated that tetragermatetrathia[8]circulene has a highly symmetric, planar, and rigid structure. DFT calculations suggested that LUMO of tetragermatetrathia[8]circulene is stabilized due to σ*-π* interactions between σ* orbitals of the Ge-C bonds and π* orbitals of the thiophene rings. The excited dynamics of tetragermatetrathia[8]circulene have also been found to be significantly affected by the heavy-atom effect of the germanium atom.

Synthesis of Tetrasilatetrathia[8]circulenes by a Fourfold Intramolecular Dehydrogenative Silylation of C-H Bonds.

The first two examples of a tetrasilatetrathia[8]circulene were synthesized in two steps from a tetraiodotetrathienylene by using a fourfold intramolecular dehydrogenative silylation of C-H bonds as the key step. A single-crystal X-ray diffraction analysis revealed that tetrasilatetrathia[8]circulene contains a perfectly planar [8]radialene skeleton. The excited-state dynamics of tetrasilatetrathia[8]circulene were evaluated by steady-state and time-resolved spectroscopic measurements, revealing an efficient intersystem crossing in the excited state.