Phosphorescence "Turn-On" Sensing of Anions by Rhenium(I) Schiff-Base Complexes

Monometallic rhenium(I) polypyridine complexes (1-4) having imine functionalized phenol moieties based on 2,2'-bipyridine ligands L1-L4 have been used as phosphorescent turn-on sensors for anions. These complexes showed various degrees of response to F-, CN-, CH3COO- and H2PO4- anions in the UV-vis absorption spectra and displayed a phosphorescence turn-on sensor for F-, CN-, CH3COO- and H2PO4- anions with different sensitivities. There is a substantial increase in emission intensity, quantum yield and luminescence lifetime is observed through deprotonation of the phenolic OH proton in 1-4 upon the addition of these anions. The signaling mechanism relies on the formation of acid-base equilibrium through a set of well-defined isosbestic points between 1-4 and anions, giving high equilibrium constants in the order F->CN->CH3COO->H2PO4- with a 1:1 binding stoichiometry. The selectivity and sensitivity towards F- were explained in terms of H-bonding interaction between 1-4 and F-, then deprotonation of 1-4 by (HNMR)-H-1 titration analysis. The excellent selectivity of 1-4 for F- is ascribed to the fitness in the acidity of its phenolic OH-group, which is able to discern the subtle difference in the affinity of F-, CN-, CH3COO- and H(2)PO4(-)ions to OH proton of phenolic group.