Three different fluorescent responses to transition metal ions using receptors based on 1,2-bis- and 1,2,4,5-tetrakis-(8-hydroxyquinolinoxymethyl)benzene

Abstract

The dipod 1,2-bis(8-hydroxyquinolinoxymethyl)benzene (3) and tetrapod 1,2,4,5-tetrakis(8-hydroxyquinolinoxymethyl)benzene (5) have been synthesized through nucleophilic substitution of respective 1,2-bis(bromomethyl)benzene (2) and 1,2,4,5-tetra(bromomethyl)benzene (4) with 8-hydroxyquinoline (1). For comparison, 1,3,5-tris(8-hydroxyquinolinoxymethyl)benzene derivatives (7a and 7b) have been obtained. The complexation behavior of these podands towards Ag+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ metal ions has been investigated in acetonitrile by fluorescence spectroscopy. The sterically crowded 1,2,4,5-tetrapod 5 displays unique fluorescence ‘ON–OFF–ON’ switching through fluorescence quenching (λmax 395 nm, switch OFF) with <1.0 equiv of Ag+ and fluorescence enhancement (λmax 495 nm, switch ON) with >3 equiv Ag+ and can be used for estimation of two different concentrations of Ag+ at two different wavelengths. The addition of Cu2+, Ni2+, and Co2+ metal ions to tetrapod 5 causes fluorescence quenching, i.e., ‘ON–OFF’ phenomena at λmax 395 nm for <10 μM (1 equiv) of these ions but addition of Zn2+ and Cd2+ to tetrapod 5 results in fluorescence enhancement with a gradual shift of λem from 395 to 432 and 418 nm, respectively. Similarly, dipod 3 behaves as an ‘ON–OFF–ON’ switch with Ag+, an ‘ON–OFF’ switch with Cu2+, and an ‘OFF–ON’ switch with Zn2+. The placement of quinolinoxymethyl groups at the 1,3,5-positions of benzene ring in tripod 7a–b leads to simultaneous fluorescence quenching at λmax 380 nm and enhancement at λmax 490 nm with both Ag+ and Cu2+. This behavior is in parallel with 8-methoxyquinoline 8. The rationalization of these results in terms of metal ion coordination and protonation of podands shows that 1,2 placement of quinoline units in tetrapod 5 and dipod 3 causes three different fluorescent responses, i.e., ‘ON–OFF–ON’, ‘ON–OFF’, and ‘OFF–ON’ due to metal ion coordination of different transition metal ions and 1, 3, and 5 placement of three quinolines in tripod 7, the protonation of quinolines is preferred over metal ion coordination. In general, the greater number of quinoline units coordinated per metal ion in 5 compared with the other podands points to organization of the four quinoline moieties around metal ions in the case of 5.