Synthesis and assembly of extended quintulene

Cycloarenes are polycyclic aromatic macrocycles composed of fully annulated benzene rings surrounding an inner cavity with inward-pointing C–H bonds. Historically, their synthesis has enabled tests of π-electron delocalization, with Kekulene serving as a benchmark system whose NMR data supported localized benzenoid sextets over global delocalization. Related cycloarenes (e.g., cyclo[d,e,d,e,e,d,e,d,e,e]dekakisbenzene, septulene, octulenes, and extended Kekulenes) have informed concepts of superaromaticity and global aromaticity. The symmetry and size of the inner cavity dictate whether a cycloarene resembles graphenic cutouts (C2, C3, or C6 symmetry) or adopts a non-graphitic curved geometry (pentaradial, heptaradial, octaradial). A fivefold-symmetric pentagonal central cavity yields a bowl-shaped cycloarene termed quintulene, whose synthesis had failed since early attempts in 1984 due to high strain. The present work addresses this long-standing synthetic challenge by constructing an extended quintulene via π-extension of a pentaradial macrocyclic precursor, aiming to establish its structure, assess its self-assembly behavior in solution, quantify the kinetics and thermodynamics of dimerization, and elucidate interlayer electronic coupling through optical characterization.

Prior studies established cycloarenes as platforms to probe aromaticity, with Kekulene’s NMR demonstrating localized benzenoid sextets. Subsequent homologs and extended coronoids further supported localized Clar sextets and challenged notions of superaromaticity. Cycloarenes with hexagonal inner cavities relate to porous graphene subunits, whereas those with non-hexagonal cavities (septulene, octulene) exhibit curved, hyperbolic nanocarbon structures. Quintulene, featuring a pentagonal inner cavity and fivefold symmetry, remained elusive due to large strain in a curved lattice despite attempts in 1984. Synthetic advances in oxidative cyclodehydrogenation typically access planar nanographenes, while curved bowl-shaped PAHs remain challenging. Recent reports of molecular carbon cones and defect-containing bilayer graphene highlight emerging curved nanocarbon architectures and aggregation phenomena, motivating the present effort to realize and study quintulene analogs.

• Design and precursors: Selected 5-cyclo-m-phenylene (5CMP) as a macrocyclic precursor with a pentaradial inner cavity. Prepared penta-borylated 5CMP (compound 3) via Ir-catalyzed direct C–H borylation using bis(pinacolato)diboron and [Ir(cod)(OMe)]2/dtbpy in THF at 65 °C for 24 h.
• Construction of pentaradial polyphenylene (compound 2): Coupled 2-bromo-5-mesityl-1,1′-biphenyl with penta-borylated 5CMP (3) under Suzuki–Miyaura conditions (Pd2(dba)3, SPhos, Cs2CO3, toluene/H2O, 100 °C, 24 h). Purified by silica gel chromatography; structure confirmed by NMR and single-crystal X-ray diffraction.
• Ring-closing cyclodehydrogenation to extended quintulene (compound 1): Oxidative cyclodehydrogenation of 2 with FeCl3 (3.0 equivalents per hydrogen) in dichloromethane at 0 °C for 10 h. Purified by flash chromatography and HPLC (5PBB column; toluene mobile phase; retention time 73.5 min). Yield: ~5% (limited by chlorination or incomplete ring closures).
• Mass spectrometry: MALDI-TOF confirmed 1 with a molecular ion at m/z 1701.6 Da and isotopic distribution matching formula [C90H25(C9H11)5].
• NMR spectroscopy and DFT: 1H NMR (C2Cl4D2) exhibited eight singlets consistent with C5v symmetry; inner-cavity protons at 11.96 ppm. DFT (B3LYP/6-31G(d,p)) optimized a conical structure (depth ~4.4 Å). Calculated NMR matched experiment. Aromaticity assessed via NICS (B3LYP/6-31G(d,p)) and ACID plots, indicating localized benzenoid sextets (Clar’s rule) and a positive NICS in the central cavity.
• Dimerization studies: Monitored time-dependent 1H NMR in C2Cl4D2 at 30 °C, observing conversion of monomer 1 to a new species assigned to dimer (1)2. Equilibrium reached after ~34.4 h. MALDI-TOF of equilibrated sample showed a new peak at m/z 3403.3 Da matching (1)2; isolated monomer and dimer by HPLC (5PBB; monomer/dimer separated; dimer collected at 23.8 min). NOE NMR revealed interlayer proton proximities (shortest Ha–Ha′ distance 3.6–3.8 Å). DFT modeling supported a bilayer π–π stacked conical dimer and reproduced NMR features (two distinct inner-proton shifts at 12.74 and 7.86 ppm).
• Kinetics and thermodynamics: Quantified concentrations by NMR integrals using 2Cdimer + Cmonomer = C0. Determined second-order kinetics; obtained reaction rate constants at different temperatures; Arrhenius analysis yielded activation energies. Equilibrium binding constants (K2) from equilibrated mixtures; van’t Hoff analysis provided ΔH and ΔS. Solvent dependence studied in C2Cl4D2 and C6D6.
• Optical characterization: UV–vis absorption and photoluminescence (PL) spectra recorded for isolated 1 and (1)2. PL quantum yields measured by absolute method with integrating sphere. Time-resolved PL decay collected to extract lifetimes; radiative (kr) and nonradiative (knr) rate constants calculated from PLQY and lifetime.

• Successful synthesis and unambiguous characterization of extended quintulene (1), a fivefold-symmetric, conical, fully benzenoid cycloarene.
• Structure and aromaticity: DFT-optimized cone depth 4.4 Å. 1H NMR shows inner-cavity protons at 11.96 ppm. NICS values indicate localized aromaticity consistent with Clar sextets: ring I, III, IV aromatic (−8.5, −9.6, −9.4 ppm), ring II, V nonaromatic (−0.6, −2.1 ppm). Central cavity exhibits positive NICS (+5.1 ppm). ACID plots show diatropic currents localized in benzenoid rings.
• Dimerization behavior: In solution, 1 dimerizes slowly to a metastable, isolable bilayer π–π stacked dimer (1)2. Equilibrium achieved after ~34.4 h at 30 °C (C2Cl4D2). MALDI-TOF of isolated dimer shows only m/z 3403.3 Da. NOE confirms interlayer proximity (Ha–Ha′ 3.6–3.8 Å). Dimer 1H NMR displays two distinct inner-cavity signals at 12.74 and 7.86 ppm.
• Kinetics and thermodynamics: Dimerization follows second-order kinetics with high activation energies: Ea = 74.3 ± 1.7 kJ mol−1 in C2Cl4D2; Ea = 80.2 ± 5.6 kJ mol−1 in C6D6. Equilibrium binding constant K2 ≈ 3.6 × 10^3 at 30 °C (C2Cl4D2). van’t Hoff parameters: in C2Cl4D2, ΔH = 7.3 ± 1.0 kJ mol−1, ΔS = 92.2 ± 3.1 J mol−1 K−1 (entropy-driven); in C6D6, ΔH = 1.2 ± 0.1 kJ mol−1, ΔS = 85.2 ± 0.3 J mol−1 K−1 (also entropy-driven, more favorable enthalpically).
• Optical properties and coupling: Monomer 1 absorption maximum at 420 nm with vibronic bands at 445 and 479 nm. Dimer (1)2 shows blue-shifted absorption maximum and reduced low-energy band, indicating H-type aggregation. PL spectra exhibit fine vibronic structure; (1)2 shows a small red-shift (7–8 nm) versus 1. PL quantum yields: 1 = 11.7%; (1)2 = 7.8%. PL lifetimes nearly identical (14.4 ns for 1; 14.3 ns for (1)2), arguing against excimer formation. Derived rates: kr = 8.1 × 10^6 s−1 (1) and 5.4 × 10^6 s−1 ((1)2); knr = 6.1 × 10^7 s−1 (1) and 6.4 × 10^7 s−1 ((1)2), consistent with H-type interlayer coupling.
• Yield and isolation: Optimized FeCl3 cyclodehydrogenation affords 1 in ~5% yield; both monomer and dimer are separable by preparative HPLC.

The work resolves a decades-old synthetic challenge by realizing an extended quintulene with intrinsic fivefold symmetry and a conical geometry. Detailed spectroscopic and computational analyses confirm localized Clar-type aromaticity with a magnetically distinct inner cavity. In solution, the monomer undergoes controlled, comparatively slow, second-order dimerization into a bilayer π–π stacked complex, enabling direct kinetic and thermodynamic measurements that are often inaccessible for rapidly assembling aromatics. The entropy-driven nature and relatively high activation barriers suggest significant desolvation and configurational ordering costs en route to the dimer, while the favorable entropy gain drives association. Optical comparisons between isolated monomer and dimer reveal blue-shifted absorption and reduced low-energy intensity in the dimer, together with a suppressed radiative rate, establishing H-type interlayer coupling in a curved, conical π–system. These insights deepen understanding of how curvature and cavity topology govern self-assembly and excitonic interactions in nanocarbon architectures and demonstrate a rare, well-defined monomer–dimer equilibrium in PAHs.

Extended quintulene was synthesized via π-extension of a pentaradial macrocyclic precursor and ring-closing cyclodehydrogenation, yielding a fivefold-symmetric conical, fully benzenoid cycloarene. It exhibits slow, entropy-driven, second-order dimerization to a metastable, isolable bilayer π–π stacked complex with high activation energy. Spectroscopic and computational analyses confirm localized aromaticity, and optical measurements establish H-type interlayer coupling in the dimer. This work fills a long-standing gap in the cycloarene family and enables systematic studies of conical nanocarbon self-assembly and photophysics. Future research directions include further π-extension of the quintulene framework, derivatization to control phase behavior and packing, and exploration of semiconductor properties and charge-carrier transport as a function of packing mode.

• The oxidative cyclodehydrogenation to form 1 provides a low isolated yield (~5%), attributed to side reactions such as chlorination or incomplete ring closures, potentially limiting scalability.
• Structural elucidation of 1 and (1)2 relies on NMR, mass spectrometry, NOE, and DFT; aggregation behavior and kinetics were characterized in specific solvents (C2Cl4D2 and C6D6), which may influence thermodynamic parameters and could affect generalizability to other media.
• The dimer is metastable and requires careful handling (e.g., immediate solvent removal after HPLC) to avoid dissociation, which may complicate broader materials processing.