Four-component assembly in the crystalline state driven by amidinium–carboxylate salt bridge formation from an aqueous solution

A series of diamidine dihydrochlorides was prepared utilizing a spacer unit to control the distance between the two amidinium groups. The introduction of two amidinium groups to the 1,8-position of each spacer unit (i.e., 9,10-dihydroanthracene, anthracene, biphenylene) can control the direction of formation of a self-assembled structure. The fine-tuning of the distances between the two amidinium groups in the spacer units can help control the stabilizing interactions of two carboxylic acid units (intermolecular attraction) after the four-component assembly (see Scheme 1). Based on this concept, we succeeded in the formation of a four-component box-like assembled structure using amidinium–carboxylate salt bridge formation in the crystalline state from aqueous solutions.