Sequential halochromic/mechanochromic luminescence of pyridyl-substituted solid-state emissive dyes: thermally controlled stepwise recovery of the original emission color†
Stimuli-responsive organic dyes that switch the color of their solid-state emission upon exposure to external stimuli represent an important class of materials due to their potential applications in various areas of sensing technology. Although an increasing number of multi-stimuli-responsive organic dyes have been reported in recent years, only a few solid-state fluorophores are known to respond to two kinds of stimuli in a stepwise and reversible manner. Herein, we report the sequential halochromic (acidochromic)/mechanochromic luminescence behavior of a pyridyl-substituted solid-state fluorophore. The solid-state emission color of this pyridyl-substituted dye was systematically shifted in the bathochromic direction by forming cocrystals with several benzoic acid derivatives that exhibit different pKa values. Upon grinding these cocrystals with a spatula, further bathochromic shifts of their maximum emission wavelengths were observed upon amorphization. Conversely, the emission color and crystallinity can be recovered upon heating the ground samples of the cocrystals to their cold-crystallization transition temperature (Tc ∼ 100 °C). Heating the ground cocrystals further (185–260 °C) removes the benzoic acid derivatives, which restores the original emission color of the pyridyl-substituted dye. In other words, we have developed a new system that exhibits a two-step emission color recovery in response to temperature by combining halochromic and mechanochromic luminescence in series.