1,4-Naphthalenediyl-Bridged Molecular Gyrotops: Rotation of the Rotor and Fluorescence in Solution

Macrocage molecules with a bridged
π-electron system have
been reported as molecular gyrotops in which the π-electron
system can rotate within the cage. We recently reported the dynamics
of the rotor in solution using 1,4-naphthalenediyl-bridged molecular
gyrotops, which consist of cages formed of three C14, C16, or C18 chains. In this work, we synthesized
novel gyrotops with C15 and C17 chains and systematically
investigated the activation energies for the rotation of the rotor
in solution. The activation energies for rotation in solution were
found to decrease with increasing size of the cage. Therefore, a rotational
barrier can be designed by adjusting the length of the side chains
in these molecular gyrotops. Additionally, these gyrotops were fluorescent
in solution; the quantum yields and lifetimes of the fluorescence
were investigated. However, these properties were not influenced by
the chain length owing to a large difference in time scale between
fluorescence (10–8–10–9 s) and the rotational dynamics inside the cage (10°–10–5 s).