Photophysics and Excited State Reactions of [Ru(bpy)₂dppn]²⁺: A Computational Study

In this work, we used DFT and TD-DFT in the investigation of the structural parameters and photophysics of the complex [Ru(bpy)₂dppn]²⁺ (dppn=benzo[i]-dipyrido[3,2-a:2',3'-c]phenazine) in water, and its suitability as a photosensitizer (PS) in photodynamic therapy (PDT). For that, the thermodynamics of electron transfer (ET) and energy transfer (ENT) reactions in the excited state with molecular oxygen and guanosine-5'-monophosphate (GMP) were investigated. The overall intersystem crossing (ISC) rate constant was approximately 10¹² s^-1, indicating that this process is highly favorable, and the triplet excited states are populated. The triplet excited states are known to lead to photoreactions between the PS and species of the medium or directly with nucleobases. Here, we show that the Ru-dppn complex can react favorably to oxidize the GMP and generate singlet oxygen. Furthermore, this complex can also act as an intercalator between DNA base pairs and undergo dual-channel reactions. It has been proposed that the T₂ excited state is responsible for oxidizing the GMP, but we show that T₁ is thermodynamically capable of undergoing the same reaction. In this sense, docking simulations were carried out to investigate further the interactions of the Ru-dppn complex with a DNA fragment.