Investigations on the electronic effects of the peripheral 4′-group on 5-(4′-substituted)phenylazo-8-hydroxyquinoline ligands: zinc and aluminium complexes

Abstract

A new series of 5-(4′-substituted)phenylazo-8-hydroxyquinolines (H[L–R]; R = N(CH₃)₂, I, C₂H₅, II, n-C₄H₉, III, C(CH₃)₃, IV, H, V and F, VI) has been prepared and the corresponding Zn[L–R]₂ (1a–6a) and Al[L–R]₃ (1b–6b) complexes successfully synthesized. These compounds have been studied in order to design new molecular materials with enhanced electron transport properties. The obtained species have been extensively characterized by absorption and emission spectra and by cyclic voltammetric measurements. Experimental and computational results show that the Zn[L–N(CH₃)₂]·2H₂O (1a) and Al[L–N(CH₃)₂] (1b) complexes only feature luminescence (at 620 and 600 nm), respectively. The unique effects, which are induced by the NN–C₆H₄–N(CH₃)₂ group, are further proved by a reversible electron transfer process detected by cyclic voltammetry. These outcomes, discussed on the basis of theoretical calculations performed on the {H[L–N(CH₃)₂]}⁻, H[L–N(CH₃)₂] and {H[L–N(CH₃)₂]}⁺ species, suggest that metal complexes formed by 5-(4′-N,N-dimethylamino)phenylazo-8-hydroxyquinoline should be considered as electron transport materials suitable for applications in photonic devices.