Cyclobutane-linked Nanothreads through Thermal and Photochemically Mediated Polymerization of Cyclohexadiene

Carbon nanothreads are a rapidly growing class of 1D nanomaterials with sp3-hybridized diamond-like backbones. Most nanothreads are synthesized through the pressure-induced polymerization of aromatics, resulting in diverse structures and functionalities. Aside from precursor selection, there are currently limited means to control nanothread reaction pathway or polymerization outcome. Analogous to selection rules that govern outcomes in small molecule chemistry, we investigated both thermally and photochemically mediated polymerizations of skipped and conjugated dienes (1,3- and 1,4-cyclohexadiene) under high pressure and explored the resultant product selectivity. For 1,3-cyclohexadiene, both approaches yield largely amorphous products owing to competing reaction pathways. Thermally mediated polymerization of 1,4-cyclohexadiene yields a crystalline product; however, the identification of the backbone composition is consistent with multiple different reaction pathways being accessible. While support for cyclobutane structures is present, comparison to the simulated structures suggests multiple products are obtained from the thermal polymerization of 1,4-cyclohexadiene. In contrast, the recovered product obtained from photochemically mediated polymerization of the skipped diene has different d-spacings and is consistent with simulations that support a single reaction pathway toward cyclobutane-linked nanothreads. These results suggest that photochemical activation can enable product selectivity in nanothread synthesis.