which suggest complex formation between 1 and Ni2+ ion. In addition, the new absorption band centered at 540 nm appeared with increasing intensity with increasing Ni2+ ion concentration. Indeed, sensor 1 displayed a remarkable enhancement in absorption ratios (A541/A341) from 0 to 15.3 upon one equiv of Ni2+ ion treatment. As shown in the inset of Fig. 2a, the absorption bands at 341 nm and 540 nm decreased and increased linearly up to 1 equiv of Ni2+ ion, respectively, indicating the formation of a 1:1 complex