From closed-shell edge-extended kekulenes to open-shell carbonylated cycloarene diradicaloid

Facile Synthesis of Donor-Acceptor Heterocycloarenes Based on Pyrazine Derivatives Possessing Intriguing Iodide Ion Capture Properties

Donor-acceptor (D-A) conjugated systems have been extensively investigated and play important roles in organic electronics. Incorporating D-A structures into (hetero)cycloarenes endows them tunable electronic properties, while the well-defined cavity remains. However, the synthetic complexity of introducing electron-acceptor moieties into (hetero)cycloarenes limits their development and applications. In this paper, the first family of electronically tunable D-A heterocycloarenes (DAHCn, n = 1-5) based on pyrazine derivatives was facilely synthesized through cyclocondensation reaction from a tetraketone-functionalized heterocycloarene precursor prepared using the ketal-protection strategy. The effect of expanded conjugation and the inserted electron-withdrawing group on the electronic structures of the D-A heterocycloarenes was studied systematically by X-ray crystallographic analysis, various spectroscopic measurements, and theoretical calculations. Interestingly, the presence of an electron-withdrawing group polarizes the inner C(sp2)-H and significantly increases the binding affinities of D-A heterocycloarenes to the iodide anion. Meanwhile, the anion affinity can be further modulated by the type of attached substituents and the distance of polarization. More importantly, the dicyanopyrazine derivative DAHC3 shows the highest binding strength to the iodide ion as a 2:1 sandwich complex (log β2 = 12.3 and ΔG = -69.1 kJ mol-1), which is the strongest iodide receptor using C(sp2)-H hydrogen bonding interactions reported to date. Our finding provides a new strategy to design and synthesize D-A heterocycloarenes and strong anion receptors.

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.

Tuning Molecular Packing and Boosting Self-Assembling Properties via Ring Fusion Strategy in Naphthalimide-Based A-D-A Conjugated Systems

Two fully fused acceptor-donor-acceptor (A-D-A) architecture conjugated derivatives (NPF and NCF) comprising an electron-withdrawing naphthalimide (NMI) and two different electron-donating cores, phenanthrene and carbazole, respectively, were conveniently synthesized by bismuth(III)-catalyzed selective cyclization of vinyl ethers. Compared with their corresponding single bond-linked A-D-A molecules NPS and NCS, both having a moderately twisted aromatic configuration, the ring fusion strategy leads to fully coplanar conjugated skeletons and greatly changes the electronic structures, photophysical properties, self-assembling behaviors, and molecular packing motifs. In particular, the naphthalimide/carbazole derivative NCF exhibits intriguing 2D brickwork packing and significantly enhanced self-assembling properties.

Bowl-Shaped Kekulene Analogues: Cycloarenes with two Five-Membered Rings

Kekulene, a cycloarene composed of 12 fused benzene rings in a circular arrangement, exhibits a highly planar and robust structure. Kekulene has been the subject of investigation into its aromaticity and electronic structure, particularly in relation to the cyclic benzenoid. We have successfully synthesized novel bowl-shaped kekulene analogues with five-membered rings incorporated into the kekulene structure. The results of DFT calculations and VT-NMR spectra indicate that inversion of their concave-convex structures occurs at room temperature. The NICS and AICD plots predict that the Clar's type resonance structure is found in a manner analogous to the pristine kekulene, albeit with the interruption of the π-conjugation on the sp3 carbons at the five-membered rings. Despite the presence of the Clar's resonance structure, the Diels-Alder reaction proceeded smoothly with a dienophile, in contrast to the behavior of planar kekulene derivatives. This study will lead to the creation of novel bowl-shaped compounds and development of reactivity in aromatic compounds.

Generalized kekulenes and clarenes as novel families of cycloarenes: structures, stability, and spectroscopic properties

Cycloarenes constitute a captivating class of polycyclic aromatic hydrocarbons with unique structures and properties, but their synthesis represents a challenging task in organic chemistry. Kekulenes and edge-extended kekulenes as classic types of cycloarenes play an important role in the comprehension of π electron distribution, but their sparse molecular diversity considerably limits their further development and application. In this work, we propose two novel classes of cycloarenes, the generalized kekulenes and the clarenes. Using density functional theory, we carry out a comprehensive study of all possible isomers of the generalized kekulenes and clarenes with different sizes. By applying a simple Hückel model, we show that π delocalization plays a crucial role in determining the relative stability of isomers. We also discover that π-π stacking is commonly present in certain larger clarenes and provides a considerable additional stabilization effect, making the corresponding isomers the lowest-energy ones. Among all considered typical looped polyarenes, generalized kekulenes and/or clarenes are revealed to be the energetically most stable forms, suggesting that these novel cycloarenes proposed here would be viable targets for future synthetic work. The simulated 1H NMR spectra and UV-vis absorption spectra provide valuable information about the electronic and optoelectronic properties for the most stable generalized kekulene and clarene species and may support their identification in future synthesis and experimental characterization.