Synthesis and solid-state dynamics of molecular dirotors

We report the design, synthesis, thermal properties, and dynamic NMR characterization of four new molecular dirotors, each containing two rotary 1,4-diethynyl phenylene units, a bridging 1,3-bis(diphenylmethyl) benzene stator, and two capping triphenylmethyl groups. These compounds are variations of the well-known wheel and axle structures. Ambient temperature quadrupolar echo 2H NMR spectra of phenylene-deuterated samples revealed line shapes consistent with dynamic processes having exchange rates >108 s−1, and more than the two minima typically observed for most phenylene rotators. Reasonable line-shape simulations were obtained with a model that involves four minima related by angular displacements of 0°, 30°, 180°, and 230°. Cooling the samples to 260 K revealed spectra consistent with samples that contain two or more components. These results indicate that novel wheel and axle structures do create a certain amount of free volume that allows for the desired mobility of the rotating units.