Ion-specific bathochromic shifts: Simultaneous detection of multiple heavy metal pollutants via charge transfer interactions

An anthraimidazoledione-based colorimetric probe (1) has been developed to detect heavy metal pollutants in acetonitrile medium. Adding metal ions, such as Zn2+, Ni2+ and Co2+ induce ion-dependent redshifts in the metal-to-ligand charge transfer (MLCT) band. As a result, the almost colorless solution of 1 turned into green, yellow and red colors upon adding Zn2+, Ni2+, and Co2+ ions. On the other hand, the blue fluorescence of 1 is slightly quenched in the presence of Zn2+ ions, while the diminution is significant for Ni2+ ions. On the other hand, Co2+ ion results in the red-shift of emission maxima and quenching. Thus, we could observe faint cyan-colored fluorescence with Co2+ ions. The binding affinities for metal ions follow Irving–William’s order. At the same time, the colorimetric responses depend on the ionic radius, hydration enthalpy, coordination number, crystal field stabilization energies of the metal ions, etc. However, in the mixed aqueous environment, the ratiometric response was noticed only with Co2+ ions. Along with ratiometric color change, additions of metal ions result in an increase in both anodic and cathodic peak current along with shifts in the redox potential values. Further, the presence of residual Co2+ was determined in natural water samples.