The metal-ligand complex, [Ru(phen)2(dppz)]2+ (phen=1,10-phenanthroline, dppz=dipyrido[3,2-a:2',3'-c]phenazine) (RuPD), was used as a spectroscopic probe for studying nucleic acid dynamics. The RuPD complex displays a long lifetime and a molecular light switch property upon DNA binding due to shielding of its dppz ligand from water. To show the usefulness of this luminophore (RuPD) for probing nucleic acid dynamics, we compared its intensity and anisotropy decays when intercalated into the tRNAval and pBluescript (pBS) II SK(+) phagemid through a comparison with ethidium bromide (EB), a conventional nucleic acid probe. We used frequency-domain fluorometry with a blue light-emitting diode (LED) as the modulated light source. The mean lifetime for the tRNAval (< τ> = 166.5 ns) was much shorter than that for the pBS II SK(+) phagemid (< τ> = 481.3 ns), suggesting a much more efficient shielding from water by the phagemid. Because of their size difference, the anisotropy decay data showed a much shorter rotational correlation times for the tRNAval (99.9 and 23.6 ns) than for the pBS II SK(+) phagemid (968.7 and 39.5 ns). These results indicate that RuPD can be useful for studying nucleic acid dynamics.