Anion-π interactions in new electron-deficient π systems: the relevance to solid phosphorescent colors

We herein report a new electron-deficient p system, namely carboxycarbonyl substituted imidazo[1,2-a]pyridinium, which exhibits various noncovalent interactions (hydrogen bonding, anion-pi and eta(1)-type anion-pi) with chloride or perchlorate anion. Crystallographic results demonstrate that the interaction types of chloride anion and pi receptor systems can be tuned: from anion-pi interactions (with guest methanol molecule in crystal) to eta(1)-type anion-pi interactions (without guest methanol molecule in crystal). These carboxycarbonyl substituted imidazo [1,2-a]pyridinium salts also exhibit different phosphorescent colors in the solid state induced by crystal stacking structures or anion-pi interactions that can influence the charge transfer to electron-deficient carboxycarbonyl substituted imidazo[1,2-a]pyridinium. Especially, the solid phosphorescent color changes induced by anion-pi interactions are very interesting. The high solubility in water makes these purely organic phosphorescent materials promising candidates for potential applications.