Electrogenerated chemiluminescence of the lithium salts of 8-hydroxyquinoline and 2-methyl-8-hydroxyquinoline

Abstract

The spectroscopy, electrochemistry, and electrogenerated chemiluminescence (ECL) of [(q)(qH)Li]_x (qH = 8-hydroxyquinolinato) and [(Meq)(MeqH)Li]_x (MeQH = 2-methyl-8-hydroxyquinolinato) have been investigated. In both acetonitrile and aqueous solutions, [(q)(qH)Li]_x and [(Meq)(MeqH)Li]_x have absorption maxima at 320 and 309 nm, respectively. When excited at these wavelengths, the complexes emit around 500 nm (blue–green) in acetonitrile. Photoluminescence efficiencies (ϕ_em) were 0.036 for [(q)(qH)Li]_x and 0.012 for [(Meq)(MeqH)Li]_x when compared to Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine) with ϕ_em = 0.042. No photoluminescence was observed in aqueous media. The complexes show irreversible oxidative electrochemistry and quasi-reversible reductions in acetonitrile. ECL efficiencies (ϕ_ecl) were 0.097 for [(q)(qH)Li]_x and 0.080 for [(Meq)(MeqH)Li]_x when compared to Ru(bpy)₃²⁺ (ϕ_ecl = 1) in aqueous buffered solution and 0.035 for [(q)(qH)Li]_x and 0.028 for [(Meq)(MeqH)Li]_x in acetonitrile (0.05 M tri-n-propylamine (TPrA) as an oxidative–reductive ECL co-reactant). The ECL peaks at a potential corresponding to oxidation of both the TPrA and [(q)(qH)Li]_x or [(Meq)(MeqH)Li]_x. Also, qualitative studies using transmission filters suggest that both complexes emit ECL in approximately the same blue–green region as their photoluminescence, indicating that the same excited state is formed in both experiments.