The DNA duplex binding properties of previously reported dinuclear RuII complexes based on the ditopic ligands tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′,3′′-j]phenazine (tppz) and tetraazatetrapyrido[3,2-a:2′3′-c:3′′,2′′-l:2′′′,3′′′-n]pentacene (tatpp) are reported. Photophysical and biophysical studies indicate that, even at high ionic strengths, these complexes bind to duplex DNA, through intercalation, with affinities that are higher than any other monointercalating complex and are only equalled by DNA-threaded bisintercalating complexes. Additional studies at high ionic strengths using the 22-mer d(AG3[T2AG3]3) [G3] human telomeric sequence reveal that the dinuclear tppz-based systems also bind with high affinity to quadruplex DNA. Furthermore, for these complexes, quadruplex binding is accompanied by a distinctive blue-shifted “light-switch” effect, characterized by higher emission enhancements than those observed in the analogous duplex effect. Calorimetry studies reveal that the thermodynamics of duplex and quadruplex binding is distinctly different, with the former being entirely entropically driven and the latter being both enthalpically and entropically favored.