Stabilization of triple helical structures is extremely important for carrying out their biological functions. Nucleic acid triple helices may be formed with DNA or RNA strands. In contrast to many studies in DNA, little has been reported concerning the recognition of the RNA triplex by transition-metal complexes. In this article, [Ru(phen)(2)(mdpz)](2+) (Rul) is the first metal complex able to enhance the stability of the RNA triplex Poly(U)center dot Poly(A)*Poly(U) and serve as a prominent molecular "light switch" for the RNA triplex.