Synthesis and assembly of extended quintulene

Cycloarenes, polycyclic aromatic macrocycles with fully annulated benzene rings enclosing an inner cavity, have been studied to understand π-electron delocalization in aromatic systems. Kekulene, the first synthesized cycloarene, exhibits π-electron delocalization within benzenoid rings, as evidenced by its ¹H-NMR spectrum. Its homologs, including cyclo[d,e,d,e,e,d,e,d,e,e]dekakisbenzene, septulene, octulenes, and extended Kekulenes, further advanced theories on superaromaticity and global aromaticity. The inner cavity's symmetry is crucial; C₂−, C₃−, or C₆−symmetric cavities are molecular cutouts of defect-containing graphene, while pentaradial, heptaradial, or octaradial cavities yield non-graphitic, curved structures like septulene and octulenes. Quintulene, a fivefold-symmetric, bowl-shaped cycloarene with a pentagonal central cavity, remained synthetically challenging due to its high molecular strain. This paper reports the successful synthesis of extended quintulene, addressing this long-standing challenge in cycloarene chemistry. The synthesis and characterization of this molecule offers insights into the behavior of naturally curved aromatic systems and their self-assembly properties, complementing existing knowledge of cylinder-shaped carbon nanobelts or nanohoops. The study will also focus on its dimerization behavior, offering a deeper understanding of the assembly of conical nanocarbons. This research advances the understanding of curved nanocarbon assembly, a field with implications for materials science and nanotechnology.

The synthesis of cycloarenes, beginning with Kekulene in 1978, has been a significant area of research in exploring π-electron delocalization and aromaticity. Studies on Kekulene and its homologues, such as those by Staab and Diederich, Funhoff and Staab, Kumar et al., and Majewski et al., have provided valuable insights into superaromaticity and global aromaticity. The importance of the inner cavity's symmetry in determining the molecular geometry has been highlighted, with cycloarenes exhibiting C2, C3, or C6 symmetry representing defective graphenic structures. Conversely, those with pentaradial, heptaradial, or octaradial symmetry, such as septulene and octulenes, are hyperbolic nanocarbons. Previous attempts to synthesize quintulene, initiated by Staab and Sauer in 1984, were unsuccessful primarily due to the substantial strain associated with its curved structure. Recent work on molecular carbon cones, such as those reported by Zhu et al. and Shoyama and Würthner, have provided advancements in the synthesis of related curved structures, yet a quintulene derivative remained elusive. This present work directly addresses this gap in the literature by successfully synthesizing and characterizing extended quintulene, offering a unique example of a naturally curved aromatic system.

The synthesis of extended quintulene (1) started with 5-cyclo-m-phenylene (5CMP) as a macrocyclic precursor. A pentaradial polyphenylene (2) was constructed by coupling 2-bromo-5-mesityl-1,1′-biphenyl with penta-borylated 5CMP (3), synthesized via Ir-catalyzed direct C−H borylation. The structure of (2) was confirmed by NMR spectroscopy and X-ray crystallography. Oxidative cyclodehydrogenation of (2) using iron(III) chloride at 0 °C yielded (1). The product was purified using silica gel flash column chromatography and high-performance liquid chromatography (HPLC). (1) was characterized by MALDI-TOF mass spectrometry, showing a molecular ion peak at 1701.6 Da consistent with the theoretical isotopic distribution. ¹H-NMR spectroscopy revealed eight singlets with intensity ratios consistent with C₅ᵥ symmetry. The inner cavity hydrogens (H₂) appeared at 11.96 ppm. Density Functional Theory (DFT) calculations confirmed the conical structure of (1) with a depth of 4.4 Å, showing smaller pyramidalization angles compared to a counterpart without the cavity. The calculated NMR spectrum matched the experimental data. NICS and ACID calculations indicated a localized π-electron structure. The dimerization kinetics of (1) was studied via NMR spectroscopy, revealing second-order kinetics and a high activation energy of 74.3 ± 1.7 kJ mol⁻¹ in C₂Cl₄D₂ and 80.2 ± 5.6 kJ mol⁻¹ in benzene. Thermodynamic analysis indicated an entropy-driven process, with comparable binding constants for the dimer (1)₂ in both solvents. The optical properties of (1) and (1)₂ were compared using absorption and photoluminescence (PL) spectroscopy, showing a blue-shifted absorption maximum and reduced low-energy absorption band in (1)₂, characteristic of H-type aggregation. PL quantum yields were determined, showing a defect-enhanced PL for monomer (1) and a reduced yield for the dimer (1)₂. Time-resolved PL spectroscopy indicated similar PL lifetimes for (1) and (1)₂, suggesting an absence of excimers.

This research successfully synthesized extended quintulene (1), a previously elusive, fivefold-symmetric, bowl-shaped cycloarene. The molecule was fully characterized using mass spectrometry (MALDI-TOF, peak at 1701.6 Da), NMR spectroscopy (eight singlets consistent with C5v symmetry, inner cavity protons at 11.96 ppm), and DFT calculations, which confirmed its conical structure with a depth of 4.4 Å. Crucially, extended quintulene was found to undergo a relatively slow dimerization reaction in solution, forming a stable bilayer complex ((1)₂) via π-π stacking. This dimerization process was thoroughly investigated. The kinetic analysis revealed it to be a second-order reaction with a remarkably high activation energy (74.3 ± 1.7 kJ mol⁻¹ in C₂Cl₄D₂ and 80.2 ± 5.6 kJ mol⁻¹ in benzene), suggesting a significant energy barrier to dimer formation. Thermodynamic analysis further demonstrated that the dimerization is entropy-driven, driven by the increased disorder resulting from the complex formation. The optical properties of both the monomer (1) and dimer ((1)₂) were investigated. The absorption and photoluminescence (PL) spectroscopy revealed that the dimer exhibited a blue-shifted absorption maximum and reduced low-energy absorption compared to the monomer, characteristic of H-type aggregation. Additionally, the PL quantum yield was determined for both species (11.7% for (1) and 7.8% for (1)₂), highlighting the effect of dimerization on the luminescence properties. The similar PL lifetimes suggest an absence of excimers. The slower than typical dimerization of extended quintulene permitted both monomer and dimer to be isolated via HPLC, a novel aspect of this study. The structure of the dimer was confirmed by MALDI-TOF (peak at 3403.3 Da), NMR, and NOE spectroscopy.

The successful synthesis of extended quintulene fills a significant gap in cycloarene chemistry, demonstrating the feasibility of synthesizing highly strained, curved aromatic molecules. The observation of slow, entropy-driven dimerization via π-π stacking provides valuable insights into the self-assembly behavior of conical nanocarbons. The high activation energy and thermodynamic parameters shed light on the interplay between energy and entropy in driving this self-assembly process. The optical properties of the monomer and dimer provide evidence for H-type interlayer coupling in the dimer, further enhancing understanding of the electronic interactions within these stacked structures. The ability to isolate and characterize both the monomer and dimer represents a unique advancement in the study of curved aromatic molecules. This study opens new avenues for exploring the synthesis of larger and more complex curved nanocarbons and investigating their potential applications in areas such as materials science and nanotechnology.

This research successfully synthesized and characterized extended quintulene, a novel curved aromatic molecule. Its unique dimerization behavior, characterized by high activation energy and entropy-driven kinetics, provides important insights into the self-assembly of conical nanocarbons. The observed H-type coupling in the dimer contributes to our understanding of electronic interactions in π-stacked systems. Future research directions include exploring the synthesis of larger extended quintulene analogs and studying their charge-carrier transport properties to potentially develop new semiconductor materials.

The low yield (5%) of extended quintulene synthesis limits the scalability of the process. Further optimization of the cyclodehydrogenation reaction is needed to improve the yield and efficiency of the synthesis. The study primarily focuses on solution-phase dimerization; investigating the solid-state packing and properties of extended quintulene would provide a more complete picture of its behavior.