Effects of anion coordination on the fluorescence of a photo-induced electron transfer (PET) sensor complexed with metal ions

Abstract

Whether π contacts between the fluorophore of the photoinduced electron transfer (PET) sensor adpa ((N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine) and complexed metal ions or coordinated halide ions control the fluorescence intensity of adpa is further investigated via fluorescence, absorbance and density functional theory (DFT) studies. Three metal-adpa complexes were studied and the crystal structures of [Cd(adpa)I2] (1), [Hg(adpa)I2] (2), [Zn(adpa)Cl2](H2O)2 (3) and [Cd(adpa)S2O3]2 (4) are reported. Unlike Cl−, Br−, SCN− and S2O32−, which cause an increase in fluorescence intensity upon binding to CdII(adpa) due to the disruption of metal-fluorophore π contacts, coordination of I− to the CdII(adpa) complex strongly decreases fluorescence intensity. The same trend was found with the addition of I− to the HgII(adpa) and ZnII(adpa) complexes. However, the causes of fluorescence quenching associated with three metal complexes are not necessarily the same. The structure of 3 shows that the anthracenyl fluorophore does not form a π contact with the Zn, relating to the strong CHEF (chelation enhanced fluorescence) effect ZnII shows with adpa. The absorbance study suggests that the fluorescence quenching of the ZnII(adpa) complex with I− addition is due to collisional quenching. Fluorescence quenching in the Hg(adpa)I2 complex is attributed to the long Hg–N bond of 2.788Å to the central N donor of the adpa in 2, which causes quenching by a PET effect from the weakly coordinated N donor. The quenching mechanism associated with the CdII(adpa)I2 complex seems to be more subtle. Structure 1 shows the nearest Cd⋯C distance to the anthracenyl fluorophore of adpa of 3.554Å, where such long distances are usually associated with increases in fluorescence intensity, rather than quenching. Density functional theory (DFT) studies showed that there is structural change in the excited state of the CdII(adpa)I2 complex, which triggers a reordering of frontier molecular orbitals. This leads to the involvement of charge transfer from the fluorophore to the iodine in the S1→S0 transition, which is forbidden.