A color-switching molecule: Specific properties of new tetraaza macrocyclezinc complex with a facile hydrogen atom

Unsymmetrical aza macrocycle and its zinc complex, which were designed with the aim pf exhibiting the hybrid properties of planar and nonplanar macroc ycles, were newly synthesized and showed the attractive property of being c olor-switching molecules. The colors of the solutions, which contain these compounds with a facile hydrogen atom, can be controlled easily by changing the solvent or the acid and base system. The color of the solution contain ing the free base macrocycle was orange. Importantly, the color of the solu tion containing the zinc complex of macrocycle depended on the solvent; the solution of zinc complex, which was formed in dichloromethane, showed colo rless, and the solutions of acetonitrile and methanol showed yellow and red , respectively. The color of the solution containing the zinc complex was c hanged by the addition of acids or bases, and the transformation between th e colors of the solutions was reversible. This color-switching phenomenon i s considered to be related to the change of the conjugation system with the positional change of a facile hydrogen atom on the aza-macrocycle. This fa cile hydrogen atom would arise from the unsymmetry and the distorted nonpla narity of the macrocyclic ring. Furthermore, this new macrocycle exhibited a high selectivity for binding of lithium ion among alkali and alkaline ear th metal ions, and the color of its solution changed from orange to colorle ss with the formation of lithium complex.