Diketo[n]CPPs as chiral and shape-adaptive fullerene hosts and precursors to DBP[n]CPPs

Abstract

Conjugated nanohoops, such as [n]cycloparaphenylenes ([n]CPPs) and derivatives, exhibit unique structural and optoelectronic properties, making them promising candidates for applications in optoelectronic materials, and as hosts for supramolecular chemistry. Using π-systems unsymmetric to rotation or incorporating chiral units can furnish chiral nanohoops. We herein present the synthesis and characterization of diketo[8]- and diketo[9]CPPs, along with their corresponding dibenzo[a,e]pentalene (DBP) derivatives, DBP[8]- and DBP[9]CPP. Due to the central chirality of the diketone-units, these nanohoops are chiral without the possibility of racemization through rotation and show distinct chiroptical properties. The diketo[n]CPPs possess high fluorescence quantum yields of 87% (n = 8) and 92% (n = 9). The shape-adaptive properties of diketo[n]CPPs, facilitated by the tunable kink angle of the diketo unit, enable efficient host–guest interactions with fullerenes. Fluorescence titration revealed a similar binding constant for both fullerenes C₆₀ and C₇₀ (5 × 10⁴ to 7 × 10⁴ M⁻¹ in toluene), corroborated by DFT calculations that illustrate adaptive changes in nanohoop geometry upon fullerene complexation. ESI-MS is employed to generate ionized [1 : 1] host–guest complexes of diketo[9]CPP and DBP[9]CPP with C₆₀ and C₇₀ as guests. The relative stabilities of these complexes are evaluated in energy-resolved collision experiments.