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···π,
C–H···O and O–H···O) except
for π–π 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, π–π interactions
would be formed more easily as the π plane of it is much flatter.
While in PASP, π–π interactions formed till the
pressure reached a high level. Before that, intensification of intermolecular
interactions resulted in the emission enhancement.