A novel series of (5-(4_-derivatives phenyl azo)-8-hydroxy-7-quinoline carboxaldehyde) (AQLn) (n = 1, p-OCH₃; n = 2, R = H; and n = 3; p-NO₂) and their complexes [Cu(AQLn)₂].5H₂O are synthesized and investigated. The optimized bond lengths, bond angles and the calculated quantum chemical parameters for AQL_n are investigated. HOMO–LUMO energy gap, absolute electronegativities, chemical potentials, and absolute hardness are also calculated. The thermal properties, dielectric properties, alternating current conductivity (rac) and conduction mechanism are investigated in the frequency range 0.1–100 kHz and temperature range 293–568 K for AQL_1–3 and 318–693 K for [Cu(AQL_1–3)₂].5H₂O complexes. The thermal properties are of ligands (AQL_n) and their Cu(II) complexes investigated by thermogravimetric analysis (TGA). The temperature and frequency dependence of the real and the imaginary part of the dielectric constant are studied. The values of the thermal activation energy of conduction mechanism for AQL_n and their complexes [Cu(AQL_n)₂].5H₂O under investigation are calculated at different test frequencies. The values of thermal activation energies ΔE₁ and ΔE₂ for AQL_n and [Cu(AQL_n)₂.5H₂O decrease with increasing the values of frequency. The ac conductivity is found to be depending on the chemical structure of the compounds. Different conduction mechanisms have been proposed to explain the obtained experimental data. The small polaron tunneling (SPT) is the dominant conduction mechanism for AQL₁ and its complex [Cu(AQL₁)₂].5H₂O. The quantum mechanical tunneling (QMT) is the dominant conductionmechanism for AQL2 and its complex [Cu(AQL₂)₂]^.5H₂O. The correlated barrier hopping (CBH) is the dominant conduction mechanism for AQL₃ and its complex [Cu(AQL₃)₂].5H₂O, and the values of the maximum barrier height (Wm) are calculated.