Controlling the Flipping Motion of Cycloparaphenylene Dimers

Inhibiting both the rotating and flipping processes is a prerequisite for constructing cycloparaphenylene (CPP) dimers with well-defined conformation and chirality. Configurationally stable CPP dimers 1 and 2 are synthesized using cyclocondensation as a key step, from a newly designed diketone macrocyclic precursor. Variable-temperature nuclear magnetic resonance analyses and theoretical calculations confirmed that the flipping process of both CPP rings of 2 was inhibited, while only one CPP ring was allowed for flipping for 1. This control over flipping dynamics is achieved by the introduction of sterically demanding substituents. Steady-state fluorescence spectroscopy and theoretical calculations revealed solvatochromic behavior in the emission for 1 and solvent-dependent anti-Kasha emission for 2. A 1:2 host-guest complex of dimer 2 and C70 was established, showing negative cooperativity.