A novel fluorescent chemosensor for Cu2+ ion based on a new hexadentate ligand receptor: X-ray single crystal of the perchlorate salt of the ligand, ion selectivity assays and TD-DFT study

New hexadentate chelates, H2L1 and L-1, were synthesized. The Schiff base imine ligand, L-1, was prepared by reaction of 2,2'-[ethane-1,2-diylbis(oxy)]dibenzaldehyde and propanolamine, and characterized by infrared (IR), H-1 NMR and C-13 NMR spectroscopies. The amine ligand, H2L1, was prepared in the form of perchlorate salt ([H4L1](ClO4)(2),) by reduction of L-1 with NaBH4, and was characterized by IR (in KBr pellet), H-1 NMR, C-13 NMR and 2D NMR correlation spectroscopy (COSY) (in solution). The X-ray crystal structure of [H4L1](ClO4)(2) was also determined and discussed. Fluorescence studies showed that H2L1 is a highly selective novel fluorescence chemosensor for the Cu2+ cation. The sensor shows selectivity for Cu2+ over 12 other metal cations. In presence of Cu2+ (5 equivalents in the mu M range) the fluorescence emission of the sensor, at 309 nm, is quenched by more than 98%. TD-DFT calculations show that ligand to metal charge transfer transitions in the complex formed between Cu2+ and the amine ligand contribute to the quenching of the fluorescence. An explanation based on TD-DFT results is also proposed for the weak fluorescence quenching in the presence of Zn2+.