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

The sexidentate ligand 1 offers two distinct adjacent quadridentate coordin ating compartments: a compartment B, consisting of two amine nitrogen atoms and two quinoline nitrogen atoms, and a poorly coordinating compartment AH (2), consisting of two amine nitrogen atoms and two amide nitrogen atoms, w hich, on deprotonation of the two amide groups, give the strongly coordinat ing donor set A(2-). Potentiometric and spectroscopic studies have shown th at at pH=7.5 the Ni-II ion stays in the B compartment (high-spin state, oct ahedral stereochemistry) and at pH greater than or equal to 9.5, Ni-II is l ocated in the adjacent A(2-) compartment, as a low-spin centre, in a square stereochemistry. Thus, the Ni-II centre can be translocated from one compa rtment to the other by varying the pH from 7.5 to 9.5 and vice versa. The B -to-AH(2) translocation (tau=0.25 +/- 0.01 s) is faster than the reverse A( 2-)-to-B process (tau=2.2 +/- 0.1 s). When an anthracene (An) fragment is c ovalently linked to the AH(2) moiety (system 2), the translocation of the N i-II ion switches ON/OFF the An fluorescent emission, depending on whether the metal is positioned in the A(2-) (OFF) or B compartment (ON). Quenching is due to a Ni-II-to-An* electron transfer process. Owing to steric hindra nce of the bulky An substituent, both direct (B-to-AH(2), tau=12 +/- 1 s) a nd reverse (A(2-)-to-B, tau=66 +/- 12 s) processes are distinctly slower th an observed for system 1.