Electron transfer fluorescence quenching of napthalenediols by metal ions

Abstract

Fluorescence quenching of four naphthalenediols (1,5-; 1,7-; 2,7-; 2,3-NDs) by transition metal ions (Cu²⁺, Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺, Pb²⁺) has been investigated in 95% water-ethanol mixture. The dynamic quenching is appreciable with all metal ions except Pb²⁺. Cupric ion (Cu²⁺) is found to be an efficient quencher with all naphthalenediols and the sensitivity of 1,7-ND fluorescence is much higher than other naphthalenediols. The quenching rate constants (<em>k</em>_q) have been analyzed for various possible mechanisms. A good correlation of log <em>k</em>_q values with the reduction potentials of metal ions reveals the electron transfer quenching mechanism via a non-emissive exciplex with electron transfer from excited fluorophore to the metal ions. The anomalous behavior of 1,7-ND is explained by its point group symmetry and polarity.