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

Nanothreads are emerging one-dimensional sp3-hybridized materials with high predicted tensile strength and a tunable band gap. They can be synthesized by compressing aromatic or nonaromatic small molecules to pressures ranging from 15-30 GPa. Recently, new avenues are being sought that reduce the pressure required to afford nanothreads; the 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 to 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 for 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.

Scalable Synthesis of Crystalline One-Dimensional Carbon Nanothreads through Modest-Pressure Polymerization of Furan.

Carbon nanothreads, which are one-dimensional sp3-rich polymers, combine high tensile strength with flexibility owing to subnanometer widths and diamond-like cores. These extended carbon solids are constructed through pressure-induced polymerization of sp2 molecules such as benzene. Whereas a few examples of carbon nanothreads have been reported, the need for high onset pressures (≥17 GPa) to synthesize them precludes scalability and limits scope. Herein, we report the scalable synthesis of carbon nanothreads based on molecular furan, which can be achieved through ambient temperature pressure-induced polymerization with an onset reaction pressure of only 10 GPa due to its lessened aromaticity relative to other molecular precursors. When slowly compressed to 15 GPa and gradually decompressed to 1.5 GPa, a sharp 6-fold diffraction pattern is observed in situ, indicating a well-ordered crystalline material formed from liquid furan. Single-crystal X-ray diffraction (XRD) of the reaction product exhibits three distinct d-spacings from 4.75 to 4.9 Å, whose size, angular spacing, and degree of anisotropy are consistent with our atomistic simulations for crystals of furan nanothreads. Further evidence for polymerization was obtained by powder XRD, Raman/IR spectroscopy, and mass spectrometry. Comparison of the IR spectra with computed vibrational modes provides provisional identification of spectral features characteristic of specific nanothread structures, namely syn, anti, and syn/anti configurations. Mass spectrometry suggests that molecular weights of at least 6 kDa are possible. Furan therefore presents a strategic entry toward scalable carbon nanothreads.

'Sacrificial' supramolecular assembly and pressure-induced polymerization: toward sequence-defined functionalized nanothreads.

Limited supramolecular strategies have been utilized to synthesize sequence-defined polymers, despite the prominence of noncovalent interactions in materials design. Herein, we illustrate the utility of 'sacrificial' aryl-perfluoroaryl supramolecular synthons to synthesize sp3-hybridized nanothreads from sp2-enriched reactants. Our strategy features A-B reactant pairs in the form of a phenol:pentafluorophenol co-crystal that is preorganized for an electronically-biased and sequence-defined polymerization. The polymerization, initiated at 12 GPa, affords an alternating copolymer featuring exogenous -OH functionalities. The external substitution is confirmed through IR spectroscopy. Importantly, the inclusion of the functional unit provides the first experimental glimpse at reaction mechanism: keto-enol tautomerization that can only occur during cycloaddition is observed through IR spectroscopy. Our approach realizes the first example of a functionalized nanothread and attains sequence definition through sacrificial supramolecular preorganization and presents a further approach for de novo design of complex nanothreads.