Thermodynamic Locking of Conformational Chirality in a [9]Cycloparaphenylene

Abstract

Herein, we report the thermodynamic locking of conformational chirality in a [9]cycloparaphenylene (CPP) by external appending of (R)- or (S)-BINOL units. Through-space steric interactions induce asymmetric torsion in the CPP backbone, generating a strained nanohoop with the lowest-energy conformer (ΔG = −2.9 kcal mol–1, corresponding to >99% population of the preferred conformer at 293 K). Temperature- and solvent-dependent circular dichroism (CD) spectra exhibit minimal signal variation, in marked contrast to conventional dynamic chiral systems and the monomeric precursor. Variable-temperature NMR reveals rapid interconversion (ΔG ‡ = 12.4 kcal mol–1), confirming fast kinetics that enable equilibration to the thermodynamically favored twisted conformation. Single-crystal X-ray diffraction and time-dependent density-functional theory (TD-DFT) calculations, including model compounds devoid of BINOL chromophores, unambiguously assign the intense Cotton effects to torsionally distorted CPP-centered π–π* transitions.