Reversible Piezofluorochromic Property and Intrinsic Structure Changes of Tetra(4-methoxyphenyl)ethylene under High Pressure

During the past decade, luminescent mechanochromism has received much attention. Despite the garnered attention, only a few studies have reported the effect of internal molecular structure change on the performance of mechanochromic fluorescence. Here, we chose tetra(4-methoxyphenyl)ethylene (TMOE) as a model molecule to study the correlation between structure and fluorescence property under a hydrostatic pressure produced by a diamond anvil cell (DAC). TMOE is a methoxy-substituted tetraphenylethylene (TPE) derivative and has a nearly centrosymmetric structure and a natural propeller shape. Ultraviolet visible absorption and fluorescence spectra of TMOE and TPE in solution proved that the presence of methoxy groups in TMOE is responsible for the difference in fluorescence emissions of TMOE and TPE. Under a hydrostatic pressure, the in situ fluorescence spectra of TMOE at different concentrations show that the fluorescence intensity gradually weakens, accompanied by an obvious redshift. The Raman peak intensities decrease gradually, and the peaks disappear eventually with the pressure increasing. These spectral changes are attributed to the changes in the intramolecular conformation, that is, the strengthening of the weak C-H...O hydrogen bonds in TMOE molecules, which is caused by the twisted dihedral angle between the benzene ring and the carbon rigid plane of ethylene. Density functional theory simulation further confirms that the decreased dihedral angle could weaken Raman peak intensity, which is consistent with our experimental results.