With the stepwise addition of Ni2+ ion, the original absorption bands centered at 341 nm gradually decreased with concomitant appearance of a red shifted peak around 540 nm. Two well-defined isosbestic points were observed at 303 nm and 380 nm, 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.