Poly-NHC-based organometallic assemblies 3-PF6, 3-SbF6 and 3-OTf were obtained and verified by NMR spectroscopy, ESI mass spec-trometry and single-crystal X-ray diffraction analyses. Controllable structural interconversion was observed between the poly-NHC-based organometallic assemblies and their self-aggregated dimers in solution affected by concentration, solvent and metal ion. 1H NMR spectra of assembly 3-PF6 in CD3CN at different concentrations demonstrated controllable structural interconversion, and 19F NMR spectrum of assembly 3-PF6 at high concentration further evidenced the presence of the free hexafluorophosphate anion and en-capsulated hexafluorophosphate anion for its two sets of signals. In addition, single-crystal X-ray diffraction analysis provided clear evi-dence that in the solid state, two assemblies 3-PF6 were vertically stuck, forming a self-aggregated dimer with an encapsulated hex-afluorophosphate anion. Investigating the reversible structural interconversion is beneficial for revealing the intrinsic nature on the atom level and paving the way to design the stimuli-responsive functional system.

Developing smart luminescent materials, especially stimulus-responsive fluorescent materials, is a goal of great importance, as well as a challenge. Mechano-responsive fluorescence is a property whereby the fluorescence characteristics (i.e., emission color, quantum yield, or lifetime) of a species change as a result of mechanical stimulation. In general, the said mechanical stimulation causes a phase transition in the material, and it simultaneously induces a fresh interaction of the luminous, resulting in a change in the color of the photoluminescence emission. Therefore, the transition of a material from crystalline-to-amorphous, from amorphous-to-crystalline, or from a crystalline to another, as well as the phase transformation of liquid crystals, can contribute to changes in fluorescence emission triggered by a mechanic stimulus. This article briefly reviews the development of such mechano-responsive fluorescent compounds, which consist of organic or organometallic molecules, and the emerging trends in this research field.