Photoacid-Spiropyran Exhibits Different Mechanofluorochromism before and after Modification of Tetraphenylethene under Grinding and Hydrostatic Pressure

Mechanofluorochromic properties of powder crystals and single crystals of photoacid-spiropyran (PASP) and tetraphenylethene-decorated Photoacid-spiropyran (PASPTPE) under ambient pressure grinding and hydrostatic pressure compression were investigated, respectively. Both PASP and PASPTPE show on/off mechanofluorochromic properties upon anisotropic grinding at ambient pressure. While under isotropic high hydrostatic pressure, the emission intensity of PASP increased at the first stage and then decreased, whereas the emission intensity of PASPTPE decreased continuously. Single-crystal structure analysis indicate that both PASP and PASPTPE molecules are packing in the antiparallel mode, and various intermolecular and intramolecular interactions (C-H center dot center dot center dot pi, C-H center dot center dot center dot O and O-H center dot center dot center dot O) except for pi-pi pi interaction were existing in the initial crystals before applying pressure. The significant difference between the two molecules is the dihedral angle between the indole and phenol ring in PASP is much larger than it is in PASPTPE. In situ absorption and Raman spectra as well as theoretical calculation were approved that the intermolecular distance would be shortened when pressure was acted on the crystal. In PASPTPE, pi-pi interactions would be formed more easily as the pi plane of it is much flatter. While in PASP, pi-pi interactions formed till the pressure reached a high level. Before that, intensification of intermolecular interactions resulted in the emission enhancement.