Photochemistry of cis-[Ru(bpy)2(NH3)2]2+ complex, a promising light-activated anticancer agent

Complex cis-[Ru(bpy)2(NH3)2]2+ (1) is a promising photo-activated chemotherapy (PACT) agent; however, its photophysical and photochemical properties have been poorly studied. In this work photoreactions of complex 1 in aqueous solutions were studied by means of steady-state photolysis, nanosecond laser flash photolysis, and ultrafast transient absorption spectroscopy. Excitation of 1 in the wavelength range of 254–505 nm results in substitution of two ammonia ligands by water molecules followed by partial dissociation with the formation of complexes cis-[Ru(bpy)2(H2O)2]2+ (2) and cis-[Ru(bpy)2(H2O)(OH)]+ (3), which exist in equilibrium. The quantum yield of photo-aquation is rather low (about 1%) and decreases on going from the UV to visible spectral range. Excitation of complex 1 in the region of the intra-ligand transition (350 nm) results in the formation of vibrationally hot, triplet excited metal-to-ligand charge transfer (3MLCT) state with a characteristic time shorter than 100 fs. The vibrational cooling time was about 3 ps. The thermally equilibrated 3MLCT state decays within 120 ns in two parallel processes, viz., intersystem crossing to the ground state and exchange of NH3 ligand with a water molecule to give the complex cis-[Ru(bpy)2(NH3)(H2O)]2+ (4). Possible reactions of complex 4 include the inverse substitution of water molecule by the ammonia ligand with the restoration of the initial structure of the complex and the exchange of the second NH3 ligand with a water molecule with the formation of reaction products, viz., complexes 2 and 3. The photochemical properties of complex 1 are independent of the presence of dissolved oxygen in solution, which is favorable for the PACT applications.