Synthesis of a Möbius carbon nanobelt

Technologies for the creation of topological carbon nanostructures have greatly advanced synthetic organic chemistry and materials science. Although simple molecular nanocarbons with a belt topology have been constructed, analogous carbon nanobelts with a twist—more specifically, Möbius carbon nanobelts (MCNBs)—have not yet been synthesized owing to their high intrinsic strain. Here we report the synthesis, isolation and characterization of a MCNB. Calculations of strain energies suggest that large MCNBs are synthetically accessible. Designing a macrocyclic precursor with an odd number of repeat units led to a successful synthetic route via Z-selective Wittig reactions and nickel-mediated intramolecular homocoupling reactions, which yielded (25,25)MCNB over 14 steps. NMR spectroscopy and theoretical calculations reveal that the twist moiety of the Möbius band moves quickly around the MCNB molecule in solution. The topological chirality that originates from the Möbius structure was confirmed experimentally using chiral HPLC separation and circular dichroism spectroscopy. Strategies for the creation of topological carbon nanostructures have greatly advanced synthetic organic chemistry and materials science. Now, the synthesis of a Möbius carbon nanobelt, a molecule with a twist on belt-shaped aromatic hydrocarbons, is reported.


Results and discussion
Here we report the synthesis, isolation and optical analysis of a Möbius carbon nanobelt (MCNB), that is, a fully fused CNB with a twist. The key to the synthesis of such MCNBs is a modification of our previously reported synthetic strategy for CNBs 19,39 . As shown in Fig. 1c, (n,n)CNBs (n = 6, 8 and 12, where (n,n) is the chiral index of the corresponding carbon nanotubes) were synthesized via a reductive homocoupling reaction using cyclic molecules that consisted of dibromoparaphenylene and cis-ethenylene precursors 19,39 . The important feature of this method is that a CNB can be generated when the number of repeat units is even, whereas an MCNB can be obtained when the number is odd. This is a simple but powerful method for the synthesis of a complex Möbius topology from highly symmetric precursors.
Strain energy calculation. The target size of the MCNB was determined using density functional theory (DFT) calculations. We found that MCNBs have a higher strain energy than CNBs of the same size (for details, see Supplementary Fig. 1), and that the strain of the MCNBs is mainly induced during the final bond-formation step. Figure 2a,b shows the hypothetical homodesmotic reactions using (n,n)MCNBs, (n,n)CNBs and their corresponding precursors (pre(n,n)MCNBs and pre(n,n)CNBs), based on which the strain induced in the final bond-formation step (ΔH FBF (kcal mol -1 )) was estimated. Cis-stilbene and phenanthrene were used as reference molecules. For belts of a similar size, the ΔH FBF of the MCNB was much higher than that of the CNB (for example, (6,6)CNB, ΔH FBF = 40.2 kcal mol -1 ; (7,7)MCNB, ΔH FBF = 121.1 kcal mol -1 ). As (6,6)CNB was successfully synthesized using a nickel-mediated homocoupling reaction, the strain energy allowed by this synthetic method was estimated to be approximately 40 kcal mol -1 . Based on these considerations, the synthetic pathway and the symmetry of the product, (15,15)MCNB (ΔH FBF = 51.1 kcal mol -1 ) and (25,25) MCNB (ΔH FBF = 29.6 kcal mol -1 ) were selected as the targets. The strain energies of the molecules were overall 85.7 and 49.4 kcal mol -1 , respectively, which indicates that the strain decreases with increasing size of the MCNB (for details, see Supplementary Fig. 1).

Synthesis.
Our synthetic route to the MCNBs is shown in Figs. 3 and 4. To improve the solubility of the intermediates and products, n-butoxy groups were introduced to the starting material 2. Thus, (25,25)MCNB with 20 butoxy groups (1) was targeted and synthesized from simple precursors 2 and 5 over 14 steps. First, the unsymmetric functionalization of phenanthrene 2 was investigated to ensure a Z-selective Wittig reaction sequence. During the screening of the Lewis-acid-catalysed formylation of 2, we found that monoformylated 3 was obtained selectively using TiCl 4 and MeOCHCl 2 in a high yield (75%), and that a subsequent chloromethylation with ZrCl 4 and MeOCH 2 Cl smoothly afforded the bifunctional phenanthrene 4a with formyl and chloromethyl groups in an 84% yield. The formyl and chloromethyl groups of 4a were then converted into acetal and phosphonium groups, respectively, to yield 4b. The sequential Wittig reaction of 5 with 4a followed by 4b produced the key intermediate 7c (Fig. 3a). Starting from 7c as the monomer, its dimer (8c), trimer (9c) and pentamer (10c) were synthesized via Wittig reactions (Fig. 3b). In these reactions, the formyl and phosphonium groups reacted selectively, as the chloromethyl and dimethylacetal groups were inert under the reaction conditions. The macrocyclization was performed with 10d, which was derived from 10c and bore formyl and phosphonium groups to yield 11 in a 67% yield. The reductive coupling of 11 with Ni(cod) 2 (cod, 1,8-cyclooctadiene) and 4,4′-methoxycarbonyl-2,2′-bipyridyl gave (BuO) 20 (25,25)MCNB (1) in a 20% yield (Fig. 4a). In contrast, only a trace mass peak corresponding to (BuO) 12  The thus obtained Möbius belt 1 was characterized using high-resolution mass spectrometry and NMR spectroscopy. The high-resolution mass spectrum showed an isotope pattern with its highest peak at 3,944.9449, which is in good agreement with the simulated pattern and mass number (m/z = 3,944.9423) expected for C 280 H 260 O 20 (for details, see Supplementary Fig. 3). The DFToptimized structure of 1 shows a C 2 -symmetry with a long (~38 Å) and a short (~30 Å) axis (Fig. 5a). The broadened aromatic signals in the 1 H NMR spectrum observed at 25 °C converged at 140 °C into seven singlet signals, which can be assigned to a-h (shown in Fig. 5b) as supported by DFT calculations (see Supplementary Fig. 7 for details). These results indicate that the twist moiety of the Möbius belt moves quickly around the belt at a high temperature, as predicted for Möbius cyclacenes 40 . As shown in Fig. 5c, the molecular motion was simulated using a density functional tight binding with molecular dynamics (DFTB-MD) calculation (for details, see Materials and methods in the Supplementary Information). Photophysical properties. The photophysical properties of Möbius CNB 1 were also investigated. As shown in Fig. 6a, absorption maxima at 389 and 409 nm as well as a small absorption peak at 477 nm were observed, and greenish-blue fluorescence with maxima at 480, 513 and 551 nm were observed on excitation at 380 nm. Based on the fluorescence quantum yield (10%) and half-life (14.1 ns), the radiative and non-radiative decay rate constants (k r and k nr ) were estimated to be 7.1 × 10 6 and 6.4 × 10 7 s -1 , respectively. Time-dependent DFT calculations of 1 suggested that, unlike in the D 3h -symmetric (6,6)CNB, the S 0 → S 1 transition (assignable to the small band at 477 nm) is symmetry allowed (f = 0.6239), which reflects the lowered symmetry caused by the Möbius topology ( Supplementary Fig. 4). The topological chirality of 1 was also examined experimentally. Chiral separation of 1 was successfully achieved using chiral HPLC, and the circular dichroism (CD) spectrum of each fraction was collected ( Fig. 6b and Supplementary Figs. 5 and 6). Based on the CD spectra simulated using time-dependent DFT calculations (for details, see Supplementary Fig. 6), the first and second fractions were tentatively assigned to M and P chirality, respectively.

Conclusion
In conclusion, we successfully synthesized a MCNB, that is, a topological molecular nanocarbon with a twist on armchair CNBs. The strategy of using a variant of the previously used CNB precursor, cyclo(dibromoparaphenylene-Z-ethenylene) with an odd number of units led to the discovery of a rational synthetic route to such MCNBs. DFT calculations of the intrinsic strain energies suggested that the synthesis of MCNBs with large sizes would be most promising, and therefore, (25,25)MCNB was selected as the target. The synthesis was carried out via Z-selective Wittig and intramolecular homocoupling reactions with nickel complexes to yield decabutoxylated (25,25)MCNB (1) over 14 steps. NMR spectroscopy and DFTB-MD calculations revealed that the Möbius twist structure moved quickly around the molecule in solution. Photophysical measurements revealed that the synthesized MCNB exhibited a greenish-blue fluorescence with a symmetry-allowed S 0 → S 1 transition caused by the lowered symmetry. Experimentally, chiral HPLC separation and CD spectroscopy revealed that the chirality originates from the Möbius topology. The combination of strain calculations with a rational synthetic strategy can be expected to create a variety of topological molecular nanocarbons, which will promote the progress of materials science in this area.

Methods
For the synthesis of (BuO) 20