Crystal phases and phase transitions in a highly polymorphogenic solid-state molecular gyroscope with meta-methoxytrityl frames

The energy landscape of molecular gyroscopes with meta-methoxy substituted trityl groups may include many stereoisomers due to the helicity of the two triarylmethyl propeller moieties and the stereochemical label provided by the metamethoxy group. While low equilibration barriers and rapid equilibration in solution prevent isolation and observation by relatively slow techniques such as H-1 and C-13 NMR, their many structural options may offer opportunities to prepare several crystal forms, or conformational polymorphs. In this paper, we report the synthesis and characterization of 1,4-bis[tri-(meta-methoxyphenyl)propynyl] benzene (2), a crystalline molecular gyroscope with a phenylene rotator, a dialkyne axle, and two tri-meta-methoxysubstituted trityl frames. Using a combination of single-crystal X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), high resolution C-13 cross-polarization magnetic-angle spinning (CPMAS) NMR, and polarized microscopy, we obtained experimental evidence for seven polymorphs labeled A-G and up to six phase transitions that relate them.