Transformational Modulation of Fluorescence to Room Temperature Phosphorescence in Metal–Organic Frameworks with Flexible C–S–C Bonds

Photoluminescent
metal–organic frameworks (MOFs) have been
a subject of considerable interest for many years. However, the regulation
of excited states of MOFs at the single crystal level remains restricted
due to a lack of control methods. The singlet–triplet emissive
property can be significantly influenced by crystal conformational
distortions. This review introduces an intelligent responsive MOF
material, denoted as LIFM-SHL-3a, characterized by flexible
C–S–C bonds. LIFM-SHL-3a integrates elastic
structural dynamics with fluorescence and room temperature phosphorescence
(RTP) modulation under heating conditions. The deformable carbon–sulfur
bond essentially propels the distortion of molecular conformation
and alters the stacking mode, as illustrated by single-crystal-to-single-crystal
transformation detection. The deformation of flexible C–S–C
bonds leads to different noncovalent interactions in the crystal system,
thereby achieving modulation of the fluorescence (F) and RTP bands.
In the final state structure, the ratio of fluorescence is 66.7%,
and the ratio of RTP is 32.6%. This stands as a successful demonstration
of modulating F/RTP within the dynamic MOF, unlocking potential applications
in optical sensing and beyond. Especially, a PL thermometer with a
relative sensitivity of 0.096–0.104%·K–1 in the range of 300–380 K and a H2S probe with
a remarkably low LOD of 125.80 nM can be obtained using this responsive
MOF material of LIFM-SHL-3a.