High-Pressure Structural and Electronic Properties of Bibenzyl (1,2-Diphenylethane) from Synchrotron SC-XRD and Two-Photon-Induced Fluorescence.

The structural and electronic properties of bibenzyl (1,2-diphenylethane) are investigated as a function of pressure using single-crystal X-ray diffraction (SC-XRD) and two-photon-induced fluorescence. Bibenzyl is an appealing pseudostilbene compound for the high-pressure synthesis of mixed carbon nanothreads with tunable optical properties, due to its fully saturated linking group. Crystal structures are refined from ambient pressure up to 37 GPa by synchrotron XRD, and the appearance of pseudosymmetry traits at 13 GPa reveals a phase transition that consists of molecular distortions leading to the tripling of the a axis and asymmetric unit while maintaining the unit cell's point group symmetry. Two-photon-induced fluorescence up to 18 GPa provides insights into the structural changes where the strengthening of π-π interactions at the phase transition stabilizes the electronic ground state and drives the molecular distortions that occur at that pressure. Coupling structural and electronic properties rationalizes the high-pressure reactivity compared to other pseudostilbenes, showing that the molecular evolution from the added torsional degree of freedom and establishment of π-π interactions at the phase transition lead the molecular conformation to an unfavorable state for pressure-induced polymerization.