Macrocyclic Molecular Rotors with Bridged Steroidal Frameworks

In this work, we describe the synthesis and solid-state
dynamics
of isomeric molecular rotors 7E and 7Z, consisting of
two androstane steroidal frameworks linked by the D rings by triple
bonds at their C17 positions to a 1,4-phenylene rotator. They are
also linked by the A rings by an alkenyl diester bridge to restrict
the conformational flexibility of the molecules and reduce the number
of potential crystalline arrays. The analysis of the resulting molecular
structures and packing motifs offered insights of the internal dynamics
that were later elucidated by means of line shape analyses of the
spectral features obtained through variable-temperature solid-state 13C NMR; such analysis revealed rotations in the solid state
occurring at kilohertz frequency at room temperature.