A novel dinuclear Ru(II) complex, [(bpy)(2)Ru(ebipcH(2))Ru(bpy)(2)](ClO4)(4), where bpy = 2,2'-bipyridine and ebipcH(2) = N-ethyl-4,7-bis([1,10]-phenanthroline[5,6-f]imidazol-2-yl)carbazole, has been newly synthesized. The pH effects on UV-vis absorption and emission spectra of the complex are studied, and ground- and excited-state ionization constants of the complex are derived. The binding of the complex to calf thymus (ct) DNA is investigated with absorption and luminescence titrations, steady-state emission quenching, and viscosity measurements. The complex acts as a pH-induced "on-off" emission switch between pH 8.0 and pH 10.0 with a maximum on-off ratio of similar to100 which is favorably compared with the other imidazole-containing Ru(II) complex congeners, and a strong ct-DNA intercalator with an intrinsic binding constant of 1.31(+/-0.08) x 10(6) M-1 in buffered 50 mM NaCl.