UCLA

A new derivative of a crystalline linearly conjugated phenyleneethynylene molecular dirotor was synthesized for its potential in optoelectronic applications. Previously developed conjugated molecular dirotors contained a central pentiptycene stator flanked by two outer trityl stators to promote free rotation of the dual phenylene rotors. A new outer stator, the meta-terphenyl moiety, was utilized instead of the smaller trityl group in attempt to increase the free volume within the crystal of the target compound 9 and its deuterated isotopologue 11. The increase in free volume was expected to result in decreased rotational barrier towards a freely rotating dirotor, while preserving beneficial photophysical properties within the central axle and rotor framework.

The synthesis was carried out and resulted in relatively low overall yields of compounds 9 and 11. The low yields resulted due to the lengthy nine-step synthesis required to achieve the final product, but was mostly due to the final two Sonogashira coupling reactions. These two reactions required the reaction of high-molecular-weight hydrocarbons that could not easily dissolve in the reaction mixture and thus contributing to unsatisfactory yields. This inability to dissolve also decreased the efficiency of purification techniques, leading to poor separation via column chromatography using silica gel.

The target compound 9 was crystallized in DCM/toluene and obtaining a crystal in the space group P1-bar. Photophysical measurements were carried out on the target compound 9 and two major spectral bands were present, indicative of the presence of the meta-terphenyl and pentiptycene chromophores. The crystalline powder was also measured at 298K and 77K, yielding similar spectra with an additional band present. This band was attributed to the increased population of non-coplanar phenylene rotors. Attempts were made towards measuring fluorescence lifetime, but the lifetime was too short to achieve significant data using our instrument.

Rotational dynamics were measured using variable temperature solid-state nuclear magnetic resonance. The spectra indicated four-fold rotational dynamics in the fast regime at high temperatures, which progressed to two-fold rotational dynamics at low temperatures. Attempts were made to simulate the set of spectra and determine the kinetics and activation energy, but the suitable fitting of the spectra was unsuccessful.