Solid-state fluorescence properties and mechanofluorochromism of D–π-A pyridinium dyes bearing various counter anions

A pronounced bathochromic shift-type mechanofluorochromism (MFC) is found for D–π-A pyridinium dyes OD1–OD3 having diphenylamino group as an electron donor and pyridinium ring as an electron acceptor linked by carbazole as a π-conjugated bridge, and bearing various counter anions (X−=Cl−, Br−, or I−). Interestingly, both absorption and fluorescence peaks of as-recrystallized dyes show bathochromic shift by grinding, and the degrees of the bathochromic shift depend on the type of counter anion of OD1–OD3. We propose that the use of various counter anions for D–π-A pyridinium dyes can be one of the most effective strategies, not only to express a pronounced MFC, but also to control changes of fluorescence properties by the grinding and heating, resulting in fine-tuning of fluorescent color change by the MFC.