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. They are typically synthesized through pressure-induced polymerizations of aromatics, resulting in diverse structures and functionalities. Aside from precursor selection, there are limited means to control reaction pathway or polymerization outcome. Analogous to selection rules that govern outcomes in molecular chemistry, we investigated thermally and photochemically mediated pressure-induced polymerizations of 1,4-cyclohexadiene and explored the resultant products. Thermally mediated polymerization of 1,4-cyclohexadiene yields a crystalline product; yet identification of the backbone architecture is complicated by the product's less ordered packing in which only two Friedel pairs are observed. Support for cyclobutane-based structures is present when comparing experimentally-obtained data to computed structures, yet further evidence suggesting eliptical cross-sections consistent with anti-cyclobutanes is obtained when comparing experimental data obtained from a Paris Edinburgh (PE) synthesis. In contrast, the recovered product obtained from the photochemically mediated polymerization exhibits different d-spacings and is consistent with simulations that support a single pathway toward highly elliptical syn-cyclobutane-linked nanothreads. These results suggest that photochemistry can enable reaction selectivity in nanothread synthesis.