A Fullerene‐Based Molecular Torsion Balance for Investigating Noncovalent Interactions at the C₆₀ Surface

Abstract

To investigate the nature and strength of noncovalent interactions at the fullerene surface, molecular torsion balances consisting of C60 and organic moieties connected through a biphenyl linkage were synthesized. NMR and computational studies show that the unimolecular system remains in equilibrium between well‐defined folded and unfolded conformers owing to restricted rotation around the biphenyl C−C bond. The energy differences between the two conformers depend on the substituents and is ascribed to differences in the intramolecular noncovalent interactions between the organic moieties and the fullerene surface. Fullerenes favor interacting with the π‐faces of benzenes bearing electron‐donating substituents. The correlation between the folding free energies and corresponding Hammett constants of the substituents in the arene‐containing torsion balances reflects the contributions of the electrostatic interactions and dispersion force to face‐to‐face arene–fullerene interactions.

Understanding the Nature and Strength of Noncovalent Face‐to‐Face Arene–Fullerene Interactions

“…In addition, the interaction between C 60 and a phenoxide anion could be regarded as a model for the poorly understood anion–fullerene interactions [11] . As seen in Figure 5, the Δ G fold value of 1 PhOH in CDCl 3 is consistent with the correlation line with the Hammett constant obtained in our previous study [12] . However, in C 6 D 6 , the Δ G fold value of 1 PhOH was significantly far off the linear relationship.…”

“…Figure 5a shows the relationship between the individual Δ G fold values and the Hammett constant for the substituent in the terminal benzene ring. Overlooking the whole, the high linearity of the correlation of Δ G fold with the Hammett constant [21] at the para position ( σ para ) of the corresponding arene molecules (Ar−H) in both CDCl 3 and C 6 D 6 can be explained based on electrostatic interactions, as discussed in our previous paper [12] . The slope of the Hammett plot is steep for the less polar solvent (C 6 D 6 ), while it is flatter for the more polar solvent (CDCl 3 ).…”

“…ΔG fold [a] [kJ mol À 1 ] ΔΗ fold [b] [kJ mol À 1 ] ΤΔS fold [b] [kJ mol [d] 0.36 � 0.12 n/a [e] 5.04 � 0.66 n/a [a] The values in the data are averaged over multiple (� 3) measurements at 298 K. For an error analysis see Refs. [12,13] and the Electronic Supporting Information. [b] The values in the data are averaged over multiple (� 3) measurements at 298 K. Uncertainty was estimated using error propagation equations.…”

“…Therefore, quantitative analysis and evaluation of such weak interactions appearing on the curved π‐electron systems can contribute to various research fields. To quantify the noncovalent interactions at the fullerene surface, we have recently proposed a unimolecular model system that exhibits two types of conformational isomerism [12] . This system was synthesized based on the molecular design of Wilcox's torsion balance [4b, e] .…”

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

“…On this basis, rotational barriers ( ΔG ‡ ) and differences in the free energies of ground-state conformers ( ΔG fold ) are used to assess the thermodynamics of noncovalent interactions. The free binding energy associated with the pilot interaction in the folded state can be quantitatively experimentally determined from the ratio of folded and unfolded states, more frequently by NMR spectroscopy. ,− SAPT analysis was also applied to torsion balance systems . In the last few years, this intramolecular approach has been used in a number of applications, in particular contributing to quantifying noncovalent interactions involving π-clouds. ,− …”