A novel ruthenium(II) complex of [Ru(bpy)(2)(Hbopip)](ClO4)(2) (in which bpy = 2,2'-bipyridine, Hbopip = 2-(4-benzoxazolyl)phenylimidazo[4,5-f][1,10]phenanthroline) was synthesized and characterized. The spectrophotometric pH titrations of the complex showed that it acted as a pH-induced luminescence "off-on-off' switch: a luminescence off-on switch with a luminescence enhancement factor of I-pH=3.0/I-pH=1.0 = 20 occurring over a narrow pH range of 1.00-3.00 plus a luminescence on-off switch with a luminescence enhancement factor of 3 over a pH range of 3.20-9.40. The excited-state ionization constant of the complex ;derived, pK(a1)* = 3.06, is 1.36 pK(a) units greater than the ground-state pK(a1) = 1.70, and pK(a2)* = 5.01 and pK(a3)* = 8.22 are comparable to the ground-state pK(a2) = 5.23 and pK(a3) = 8.22, respectively. The complex avidly bound to calf thymus DNA with a large binding constant of (1.2 +/- 0.3) x 10(7) M-1 in buffered 50 mM NaCl, as evidenced by UV-vis and luminescence titrations, steady-state emission quenching by [Fe(CN)(6)](4-), DNA competitive binding with ethidium bromide, viscosity measurements, and DNA melting experiments.