Comparison of shearing force and hydrostatic pressure on molecular structures of triphenylamine by fluorescence and Raman spectroscopies.

Luminescent mechanochromism (e.g., shearing force and hydrostatic pressure) has been intensively studied in recent years. However, there are few reported studies on the difference of the molecular configuration changes induced by these stresses. In this study, we chose triphenylamine, C18H6N (TPA), as a model molecule to study different molecular configuration changes under shearing force and hydrostatic pressure. Triphenylamine is an organic optoelectric functional molecule with a propeller-shaped configuration, a large conjugate structure, and a single molecular fluorescence material. Fluorescence and Raman spectra of TPA were recorded in situ under different pressures (0-1.9 GPa) produced by the mechanical grinding or using a diamond anvil cell (DAC). Our results show that the crystal phase of TPA transformed to the amorphous phase by grinding, whereas no obvious phase transition was observed under hydrostatic pressure up to 1.9 GPa, indicating the stability of TPA. Hydrostatic pressure by DAC induces molecular conformation changes, and the pressure-induced emission enhancement phenomenon of TPA is observed. By analyzing the Raman spectra at high pressure, we suggest that the molecular conformation changes under pressure are caused by the twisted dihedral angle between the benzene and the nitrogen atom, which is different from the phase transformation induced by the shearing force of grinding.