The ruthenium(II) complexes [Ru(dmb)₂(LH)][ClO₄]₂ [dmb = 4,4′-dimethyl-2,2′-bipyridine; LH = 2-(4-nitrophenyl)imidazo[4,5-f][1,10]phenanthroline (PNOPH), 2-(3-nitrophenyl)imidazo[4,5-f][1,10]phenanthroline (MNOPH) and 2-(2-nitrophenyl)imidazo[4,5-f][1,10]phenanthroline (ONOPH)] and their deprotonated complexes have been synthesized and characterized. The electrochemical behaviour of these complexes were examined and compared to that of [Ru(dmb)₃]²⁺. Non-linear optical (NLO) properties of the complexes were investigated using Z-scan techniques. All of the complexes exhibit both NLO absorption and self-defocusing effect (n₂ = –0.79 × 10^–17 to –1.44 × 10^–17 m² W^–1, α₂ = 3.00 × 10^–11 to 7.09 × 10^–11 m W^–1, [M] = 5 × 10^–5 mol dm^–3 in acetonitrile solution). The corresponding effective NLO susceptibilities |χ ⁽³⁾| of the complexes are 2.86 × 10^–12 to 5.07 × 10^–12 esu. The results indicate that the substitution and deprotonation of the ligand have an effect on the NLO properties of the complexes. Finally, the crystal structure of [Ru(dmb)₂(PNOP)][ClO₄]·H₂O·0.5C₆H₆ was determined by X-ray diffraction analysis; it contains two twisted dmb ligands and the bidentate ligand PNOP with torsional angles between each dmb ring pair of 3.4 and 8.7°.