Modular synthesis, host-guest complexation and solvation-controlled relaxation of nanohoops with donor-acceptor structures

Carbon nanohoops with donor-acceptor (D-A) structures are attractive electronic materials and biological fluorophores, but their synthesis is usually challenging. Moreover, the preparation of D-A nanohoop fluorophores exhibiting high fluorescence quantum yields beyond 500 nm remains a key challenge. This study presents a modular synthetic approach based on an efficient metal-free cyclocondensation reaction that readily produced nine congeners with D-A or donor-acceptor-donor ' (D-A-D ') structures, one of which is water-soluble. The tailored molecular design of nanohoops enabled a systematic and detailed study of their host-guest complexation with fullerene, optical properties, and charge transfer (CT) dynamics using X-ray crystallography, fluorescence titration, steady and ultrafast transient absorption spectroscopy, and theoretical calculations. The findings revealed intriguing physical properties associated with D-A motifs, such as tight binding with fullerene, moderate fluorescence quantum yields (37-67%) beyond 540 nm, and unique solvation-controlled CT relaxation of D-A-D ' nanohoops, where two CT states (D-A and A-D ') can be effectively tuned by solvation, resulting in dramatically changed relaxation pathways in different solvents.

Automated summary

This study presents a modular synthetic approach for the preparation of carbon nanohoops with donor-acceptor or donor-acceptor-donor structures. The properties of these nanohoops, including host-guest complexation with fullerene, optical properties, and charge transfer dynamics, were systematically investigated. The findings revealed intriguing physical properties associated with the donor-acceptor motifs.