In order to understand the coordination behavior between 1 and Ni2+ ion, the UV–vis absorption spectral titration experiment of 1 (20 μM) with Ni2+ ion (0 to 26 μM) in ethanol was carried out. As shown in Fig. 2a, the addition of Ni2+ ion to the ethanol solution of 1 modulated its absorption bands in terms of λmax and εmax both. 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.