Tubularenes

Abstract

We report the synthesis and characterization of conjugated, conformationally rigid, and electroactive carbon-based nanotubes that we term tubularenes. These structures are constructed from a resorcin[n _b]arene base. Cyclization of the conjugated aromatic nanotube is achieved in one-pot eight-fold C-C bond formation via Suzuki-Miyaura cross-coupling. DFT calculations indicate a buildup of strain energy in excess of 90 kcal mol^-1. The resulting architectures contain large internal void spaces >260 ų, are fluorescent, and able to accept up to 4 electrons. This represents the first scaffolding approach that provides conjugated nanotube architectures.

Scheme 1. Synthesis of tubular[n_b,n,m,r]arenes 1 and 2.

Fig. 1. (a) Experimental MALDI-TOF MS molecular ion peaks of 1 (blue trace) and 2 (red trace). Black traces represent the simulated [M + H]⁺ isotopic distributions. (b) ¹H NMR of (top) 1 and (bottom) 2 in CD₂Cl₂ at 20 °C. Proton labels according to Scheme 1. (c) Molecular crystal structure of 1 obtained at 150 K. Thermal ellipsoids are set at 50% probability level. The C, N, and O atoms are coloured grey, blue, and red, respectively. The H atoms are omitted for clarity. Bottom: top view of the sphere packing model (van der Waals radii) of 1. (d) DFT calculated barrier for ring flipping. Rotation of the phenyl moiety was followed by tracking the dihedral angle between the highlighted carbon atoms in orange.

Fig. 2. HOMO and LUMO density plots (±0.02 au) of (a) 1, and (b) 2. (c) UV-Vis absorption (solid trace) and emission spectra (dotted trace) of 1, 2, and 3, collected in CH₂Cl₂ at room temperature. Cyclic (CV) and differential pulse voltammetry (DPV) for (d) 1, and (e) 2 in ortho-dichlorobenzene at room temperature. CV scan rate: (d) 100 mV s⁻¹, and (e) 50 mV s⁻¹. A 0.1 M [n-Bu₄N][PF₆] solution was used as supporting electrolyte. The labels on italics correspond to the oxidation level q represented as: (d) [1]^q, and (e) [2]^q. The E_1/2 potentials were obtained from the DPV data. DPV traces are shifted down for better visualization of the data. (f) Experimentally determined HOMO–LUMO energy levels of 1, 2, and [8]CPP.