Synthesis and photophysical characterisation of luminescent zinc complexes with 5-substituted-8-hydroxyquinolines

Abstract

New 5-substituted-8-hydroxyquinoline ligands HL_n (1–3), in which the quinoline and the p-N,N-dimethylphenyl fragments are connected through the –NN– (1), –CHN– (2) or –CH₂–NH– (3) group, were designed and synthesised. Their reaction with a Zn(II) salt gave the pertinent Zn(L_n)₂ complexes (4–6). The compounds 1–4 and 6 are stable in solution and their photophysical properties were explored. Zinc complexes 4 and 6 are both luminescent (4, λ_em: 620 nm, Φ < 1 × 10⁻⁴; 6, λ_em: 475 nm, Φ = 7 × 10⁻³), although the states responsible for the radiative deactivation processes were different: CT in nature for 4 and from a π–π* transition localised on the quinoline fragment for 6. The roles exerted by both the metal and bridge on the photophysical properties are discussed.