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

The sexidentate ligand (H2NCH2CH2NHC:O)2CH(CH2R) (L; R = Ph) offers two distinct adjacent quadridentate coordinating compartments: a compartment B, consisting of two amine N atoms and two quinoline N atoms, a poorly coordinating compartment AH2, consisting of two amine N atoms and two amide N atoms, which, on deprotonation of the two amide groups, give the strongly coordinating donor set A2-. Potentiometric and spectroscopic studies showed that at pH = 7.5 the NiII ion stays in the B compartment (high-spin state, octahedral stereochem.) and at pH ≥ 9.5, NiII is located in the adjacent A2- compartment, as a low-spin center, in a square stereochem. Thus, the NiII center 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 to give L (R = 9-anthracenyl(system 2)), the translocation of the NiII 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 NiII-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 obsd. for system 1.