Effect of Organic Bases on Fluorescence Quenching Behaviors and Acid–Base Responsivity of Coumarin‐Based π‐Conjugated Polymers Compared With 3,7‐Dibromo‐4‐Methylcoumarin and 3,7‐Bis(Phenylethynyl)‐4‐Methylcoumarin

ABSTRACT π‐Conjugated polymers containing coumarin backbone in the main chain show acid–base responsive fluorescence. The α‐pyrone ring in the coumarin unit undergoes hydrolysis by inorganic bases and triggers the change of the expanded π‐conjugated system, resulting in the reversible fluorescence on/off switching, dual‐colored halochromic fluorescence, and alkali metal ion recognition. We assume that the coumarin polymers form complexes with alkali metal ions during hydrolysis. In this study, we investigated the fluorescence quenching behaviors and acid–base responsivity of 3,7‐dibromo‐4‐methylcoumarin, 3,7‐bis(phenylethynyl)‐4‐methylcoumarin (model compound), and two types of coumarin‐based π‐conjugated polymers by the addition of organic bases for comparison. Consequently, the model compound demonstrated remarkably reversible on/off switching behaviors, highlighting the significant role of the metal‐free organic bases and HCl aqueous solution in facilitating the ring‐opening and closing properties. The coumarin‐based polymers demonstrated notable reversibility specifically in PL intensity. These results are expected to enhance our understanding of how alkali metal ions are recognized by coumarin π‐conjugated polymers.