  Mechanoluminescent materials have a wide range of promising nanotechnological
applications, such as photonics based sensors and smart optoelectronics.
Examples of mechanoluminescent metal-organic framework (MOF) materials,
however, are relatively scarce in the literature. Herein, we present a
previously undescribed Guest@MOF system, comprising Perylene@ZIF-8 nanoplates
which will undergo a reversible 442 nm <->502 nm photoemission switching when
subject to a moderate level of mechanically induced pressure (~10s MPa). The
nanoplates were synthesized via a high-concentration reaction (HCR) strategy at
ambient conditions, to yield a crystalline ZIF-8 framework hosting the highly
luminous Perylene guests; the latter confined within the porous sodalite cages
of ZIF-8. Remarkably, we show that in a solid-state condition, it is the
spatial isolation and nano-partitioning of the luminescent guests that bestow a
unique solution-like optical properties measured in the host-guest assembly. We
elucidate how the switchable red- or blue-shifts of the visible emission can be
accomplished by mechanically modifying the nanoscale packing of the underlying
nanoplates. Theoretical calculations suggest that the elasticity of the host
sodalite cage coupled with the intermolecular weak interactions of the confined
guest are underpinning the unique mechanoluminescent behavior observed.
