Fluorescence modulation of pyridinium betaines: a mechanofluorochromic investigation

A reversible change in a material's fluorescence spectrum on the application of force is known as mechanofluorochromism (MFC) and is a well-established field of study. However, the mechanism(s) responsible for the chromism may be different for each new material and it is important to elucidate these for many reasons, including the rational design of new analogues with targeted properties. Herein, the photophysical properties and mechanistic understanding of two MFC pyridinium betaines are reported. The emission sensitivity is explained by the coexistence of crystalline and amorphous phases after the application of mechanical force, with increased conformational flexibility in the amorphous phase facilitating red-shifts in emission. This explanation is supported by evidence from a range of spectroscopic techniques, including electron diffraction (ED) and fluorescence lifetime imaging microscopy (FLIM) mapping, two techniques that have, to the best of our knowledge, not been applied in the field of MFC to mechanically ground particles. For one of the compounds, ED on ground microcrystallites shows unambiguously that the same crystalline phase is retained after grinding, along with an amorphous contribution, providing direct evidence for the crystalline-amorphous mechanism, and the presence of these two phases is further supported by FLIM mapping. We envision these techniques will be highly instructive for the analysis of similar materials.