pH-Controlled translocation of NiII within a ditopic receptor bearing an appended anthracene fragment: a mechanical switch of fluorescence

Abstract

The sexidentate ligand 1 offers two distinct adjacent quadridentate coordinating compartments: a compartment B, consisting of two amine nitrogen atoms and two quinoline nitrogen atoms, and a poorly coordinating compartment AH₂, consisting of two amine nitrogen atoms and two amide nitrogen atoms, which, on deprotonation of the two amide groups, give the strongly coordinating donor set A²⁻. Potentiometric and spectroscopic studies have shown that at pH = 7.5 the Ni^II ion stays in the B compartment (high-spin state, octahedral stereochemistry) and at pH ≥ 9.5, Ni^II is located in the adjacent A²⁻ compartment, as a low-spin centre, in a square stereochemistry. Thus, the Ni^II centre can be translocated from one compartment to the other by varying the pH from 7.5 to 9.5 and vice versa. The B-to-AH₂ translocation (τ = 0.25 ± 0.01 s) is faster than the reverse A²⁻-to-B process (τ = 2.2 ± 0.1 s). When an anthracene (An) fragment is covalently linked to the AH₂ moiety (system 2), the translocation of the Ni^II ion switches ON/OFF the An fluorescent emission, depending on whether the metal is positioned in the A²⁻ (OFF) or B compartment (ON). Quenching is due to a Ni^II-to-An* electron transfer process. Owing to steric hindrance of the bulky An substituent, both direct (B-to-AH₂, τ = 12 ± 1 s) and reverse (A²⁻-to-B, τ = 66 ± 12 s) processes are distinctly slower than observed for system 1.