A new internal charge transfer probe for the highly selective detection of Zn(II) by means of dual colorimetric and fluorescent turn-on responses

A new zinc selective probe, Naphtha-1 has been designed and synthesized by incorporating 4-[4′-(dimethylamino)]stilbazole chromophore on the photoemittive 1,8-naphthalimide core and investigated for its metal ion sensing potential by optical spectral techniques. The signaling mechanism operates on ground of enhanced internal charge transfer (ICT) interaction from the conjugated dimethylanilino donor to the metal ion bound pyridyl ring, acting as one of the ligands. Of the metal ions, Na+, Li+, K+, Ca2+, Ba2+, Mg2+, Co2+, Ni2+, Cu2+, Cd2+, Zn2+, and Ag+ examined, only Zn2+ induced dramatic absorbance red shift by 120 nm (yellow to red), and the concentration dependent linear increases in fluorescence intensity by 44-fold. The stability constants, log Ks follow the order Zn2+ ≫ Mg2+ > Cd2+ > Cu2+ > Ag+ ≅ Co2+ ≅ Ni2+ ≅ Ca2+ ≅ Ba2+ ≅ Na+ ≅ K+ ≅ Li+. The superior interaction of Zn2+ is evident from its 3 order of magnitude higher log K compared to the next competing Mg2+. These findings reveal that Naphtha-1 has potential to efficiently discriminate Zn2+ over several other cations by dual naked eye detection and high fluorescence switch-on response.