Mechanism of Turn-on Fluorescent Sensors for Mercury(II) in Solution and Its Implications for Ligand Design

The tendency of a Hg-II ion to strongly quench fluorescence of potential fluorescent sensors is explored. Fluorescence measurements show the expected order of the chelation-enhanced fluorescence (CHEF) effect of Zn-II > Cd-II >> Hg-II similar to Cu-II, which is interpreted as (1) unpaired electrons causing the weak CHEF effect for Cu-II and (2) the order Zn-II > Cd-II >> Hg-II reflecting the heavy atom effect, which may be due to increasing spin-orbit coupling constants (zeta) for Zn-II < Cd-II << Hg-II. The structures of mercury(II) complexes of N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine (ADPA) are reported. [Hg(ADPA)Cl2HgCl2] (1) has one Hg-II held by two bridging chlorides, while the other Hg-II is coordinated to the ADPA ligand. The latter Hg-II has a nearest pi contact of 3.215 angstrom with a C atom from the anthracenyl group, which falls in the range of reported Hg-C pi contacts with aromatic groups. This contact may be important in quenching the fluorescence of the He-II/ADPA complex. A density functional theory study shows that the Hg-C interaction is strong enough to prevent a simple HOMO -> LUMO transition of the fluorophore. In fact, the S-0 -> S-1 and S-2 transitions in the H-II/ADPA complex have significant charge-transfer character to mercury. An important aspect of the coordination geometry of Hg-II is illustrated by 1, where Hg-II tends to form a few (often only two) short bonds to the more covalently binding donor atoms present, with much longer bonds to other donor atoms. The Hg-N bonds to the two pyridyl N-donor atoms of ADPA in 1 are relatively short at 2.212(8) and 2.224(8) angstrom, while that to the central saturated N-donor atom of ADPA is long at 2.603(8) angstrom. The latter long Hg-N bond may allow a photoinduced electron-transfer (PET) effect, quenching the fluorescence of the anthracenyl fluorophore. The structure of [Hg(ADPA)Br-2] (2) reflects the more covalent binding of the two bromine ligands compared to the clorine ligands of 1, with much longer Hg-C contacts with the anthracenyl fluorophore and a Hg-N contact with the saturated N atom of ADPA of 2.917 angstrom. The latter long Hg-N contact is related to the nearly negligible fluorescence of the ADPA complex in the presence of added Br-. The addition of extra ligands to the Hg-II/ADPA complex produces a weak increase in the fluorescence intensity for OH- similar to Cl- >> Br- > 1(-), which is discussed in terms of an increasing PET effect, and to collisional quenching. The ligand design principles for generating turn-on sensors for mercury suggested by this work are discussed.