Synthesis, characterization and metal ion-sensing properties of two Schiff base derivatives.

Two new Schiff base derivatives {2,2′-diphenyl-N,N′-bis(2-pyridylmethylene)biphenyl-4,4′-diamine} (1) and {2,2′-diphenyl-N,N′-bis(salicylidene)biphenyl-4,4′-diamine} (2) were synthesized and characterized by means of elemental analysis, 1H NMR, FT-IR, and standard spectroscopy techniques. The molecular structure of 2 has been determined by X-ray single crystal analysis. The analyses of fluorescence properties of the compounds revealed that 1 and 2 are both poorly fluorescent and display sensitive fluorescence responses to a panel of 24 monovalent, divalent, and trivalent metal ions in CH3CN–DMSO (9:1, v/v). Results with imine 1 showed that Fe3+, Cu2+, Zn2+, Cd2+, Mn2+, Zr4+, Hg2+, Cr2+, Pb2+, Sn2+, Bi2+, Al3+, Ce3+, La3+, Sm3+, Gd3+, Nd3+, Eu3+ and Dy3+ yields red shifts in emission and increases in intensity. And the greatest spectral changes for imine 2 include enhancements in emission intensity coupled with red shifts (Zr4+, Sn2+, Al3+ and Zn2+) and strong quenching (Fe3+). The fluorescence enhancement mechanism of 1 and 2 for metal ions is based on: (i) CN isomerization; (ii) chelation-enhanced fluorescence (CHEF) effect; and (iii) excited-state intra/intermolecular proton transfer (ESPT).