  Mechanofluorochromic materials are of great significance for the fabrication
of innovative sensors and optoelectronics. However, efficient
mechanofluorochromic materials are rarely explored due to the deficiency of
existing design strategies. Here, we demonstrate the incarceration of
aggregation-induced emission (AIE) materials within metal-organic framework
(MOF) single crystals to construct a composite system with turn-on
mechanofluorochromism. A new type of AIE@MOF material was designed: integrating
a zeolitic MOF (ZIF-71) and tetraphenylethylene (TPE, a topical AIE material)
to generate a TPE@ZIF-71 system with exceptional turn-on type
mechanofluorochromism. Using terahertz vibrational spectroscopy, we show the
unique fluorochromism emanates from the enhanced nanoconfinement effect exerted
by ZIF-71 host on TPE guest under pressure and its permanent fluorescence after
stress release. Compared with pure TPE, we demonstrate the nanoconfinement in
AIE@MOF not only changes the TPE's turn-off type sensing behavior to a turn-on
type, but boosts the original sensitivity markedly by tenfold. Significantly,
because ZIF-71 prevents the spontaneous recrystallization of TPE upon
unloading, this allows TPE@ZIF-71 to record the stress history. This is the
first demonstration of the Guest@MOF system combining the concepts of AIE and
MOF; its promising properties and potential engineering applications will
stimulate new directions pertaining to luminescent stress sensors and smart
optics.
