Photochemically-Mediated Polymerization of Molecular Furan and Pyridine: Synthesis of Nanothreads at Reduced Pressures

Abstract

Nanothreads are an emerging one-dimensional sp3-hybridized material with high predicted tensile strength and a tunable band gap. They can be synthesized by compressing aromatic, or non-aromatic small molecules under 15-30 GPa of pressure. Recently, new avenues are being sought that reduce the pressure required to afford nanothreads; focus has been placed on the polymerization of molecules with reduced aromaticity, favorable stacking, and/or the use of higher reaction temperatures. Herein, we report the photochemically-mediated polymerization of pyridine and furan aromatic precursors, which achieves nanothread formation at reduced pressures. In the case of pyridine, it was found that a combination of slow compression/decompression with broadband UV light exposure yielded a crystalline product featuring a six-fold diffraction pattern with similar interplanar spacings of previously synthesized pyridine-derived nanothreads at a reduced pressure. When furan is compressed to 8 GPa and exposed to broadband UV light, a crystalline solid is recovered that similarly demonstrates X-ray diffraction with an interplanar spacing akin to that of the high-pressure synthesized furan-derived nanothreads. Our method realizes a 1.9-fold reduction in the maximum pressure required to afford furan-derived nanothreads and a 1.4-fold reduction in pressure required pyridine-derived nanothreads. Density functional theory and multiconfigurational wavefunction-based computations were used to understand the photochemical activation of furan and subsequent cascade thermal cycloadditions. The reduction of the onset pressure is caused by an initial [4+4]-cycloaddition followed by increasingly facile thermal [4+2]-cycloadditions during polymerization. Density functional theory and multiconfigurational wavefunction-based computations were used to understand the photochemical activation of furan and subsequent cascade thermal cycloadditions.