Cycloparaazine, a full-azine carbon nanoring

Abstract

Cycloparaphenylenes (CPPs) and related carbon nanorings (CNRs) represent iconic molecular entities in molecular nanocarbon science. While theoretical studies predict that the introduction of nitrogen atoms (N-doping) onto CPP frameworks would add a number of fascinating properties, only a handful of partially N-doped carbon nanorings have been synthesized. We herein report the synthesis of a long-awaited cycloparaazine (CPA), where every para-connected aromatic moiety consists of a N-heterocycle, and two other highly N-doped CNRs. The evaluation of optoelectronic and structural properties coupled with theoretical studies uncovered the impact of both the amount and positioning of N-doping onto the nanorings properties; far less ring strain, red-shifted UV-vis absorption and fluorescence, smaller HOMO-LUMO gaps and both higher reduction and oxidation potentials than pristine CPPs. Ultimately, new potential applications of highly N-doped nanorings were examined in non-covalent supramolecular property engineering with Lewis acids and as energy storage materials.

Fig. 1. Nitrogen-containing carbon nanotubes and nanorings.

a State-of-the-art of nitrogen-incorporation in carbon nanotubes and rings. b Synthesized N-doped carbon nanorings and the use of their nitrogen lone pairs. c Our approach of synthesizing a cycloparaazine.

Fig. 2. Synthesis of CPPs and CPAs.

a State-of-the-art of CPP syntheses. b Retrosynthetic considerations for CPA synthesis.

Fig. 3. Synthetic efforts towards CPA and other N-doped derivatives.

a Au-mediated 3,3’-bipyridine synthesis through 16. b Unsuccessful synthesis of [6]CBPy (21). c Successful synthesis of [9]CPA (24), [9]CPP-Py (25) and [9]CPP-Pyr (26). The conformation of the Au-macrocycles 20, 23, 28 and 30 were assigned tentatively based on the reported all-carbon congeners (see SI 6.6 for full details)^,.

Fig. 4. Experimental and theoretical structural examination of N-doped carbon nanorings.

a X-ray crystal structures of [9]CPA (24), [9]CPP-Py (25) and [9]CPP-Pyr (26), two-dimensional packing and NCI plot (nitrogen atoms are highlighted in blue). Solvent molecules and hydrogen atoms are omitted for clarity. b Observed and calculated structural characteristics of N-doped carbon nanorings 24–26. Calculations were performed at the B3LYP-D3/6-31 G(d) level of theory. Standard deviation is given in parenthesis. Parameters of [9]CPP were measured from a published crystal structure. Source data are provided as a Source Data file.

Fig. 5. Photophysical properties of nine-membered carbon nanorings.

a UV–vis (plain lines) and fluorescence (dashed lines) spectra recorded of approx. 10 µm solutions in CH₂Cl₂. b Macroscopic appearance of all four analyzed CNRs in solid and liquid state. Excitation was performed with a Kessil lamp (λ_max = 370 nm, 8 W). c Overview of measured photophysical data and calculated HOMO–LUMO gaps. d Frontier molecular orbitals of N-doped carbon nanorings 24–26 and results of time-dependent DFT calculations. Calculations were performed at the B3LYP-D3/6-31 G(d) level of theory. The main transitions with their major contributions (coefficients >0.4) are shown. See SI 5.3., Tables S3−S5 for all transitions. Source data are provided as a Source Data file.

Fig. 6. Electrochemical properties of nine-membered carbon nanorings.

a Cyclic voltammograms (CV) of N-doped nanorings 24–26 and [9]CPP in THF (reduction) and CH₂Cl₂ (oxidation). Initial scan directions are indicated by arrows and irreversible reduction potentials are determined on inflection point. Details of electrochemical measurements are described in the SI, chapter 8. b Evaluation of the number of transmitted electrons via the peak separation of reversible reductions of ferrocene and [9]CPP-Py (25). c Overview of measured CV data in THF and CH₂Cl₂, respectively. d Overview of measured CV data in CH₃CN. Source data are provided as a Source Data file. rev.: reversible.

Fig. 7. Potential application of N-doped carbon nanorings in non-covalent supramolecular property engineering and long-term performance for energy storage.

a Supramolecular [9]CPP-Py BCF adduct formation. b ¹H NMR titration study of [9]CPP-Py (25) with BCF in CD₂Cl₂ (600 MHz, 293 K). c UV–vis (plain lines) and fluorescence (dashed lines) titration study of [9]CPP-Py (25) with BCF recorded of approx. 10 µm solutions CH₂Cl₂. d Emission of [9]CPP-Py BCF adducts in CH₂Cl₂ (10 µm) under irradiation with a Kessil lamp (λ_max = 370 nm, 8 W). e H-cell cycling performance of [9]CPP-Py (25), [9]CPP-Pyr (26) and [9]CPP against the ferrocene/ferrocenium couple in CH₃CN (0.25 mm). f Electron transmissions during charging and discharging events of [9]CPP-Py (25) and CPP. g Spectroelectrochemical analysis of the reduction of [9]CPP-Py (25) in CH₃CN (0.25 mm). h ¹H NMR study of electrochemical products in d₈-THF. i Back-oxidation of [9]CPPy^•– radical anion (25^•–) in CH₃CN. j Proposed electrochemical processes during charging and discharging, energy calculations and NICS_iso values. The calculations were performed on the PW6B95/def2-QZVP (energies) and HF/6-31 + (d,p) (NICS) levels of theory. Source data are provided as a Source Data file.