A theoretical study on the substituent effect of DNA-photocleavage by [Ru(phen)2(6-R-dppz)]2+ (R = H, OH, and NO2)

A theoretical study was conducted on the effects of three substituents in the Ru(II) polypyridyl complex, [Ru(phen)₂(6-R-dppz)]²⁺, (R = H for complex 1, OH for complex 2, and NO₂ for complex 3), on the photocleavage of DNA. The geometric and electronic structures of these complexes in the ground (S₀), first singlet (S₁), and triplet (T₁) excited states, were calculated using the density functional theory (DFT), Hartree–Fock (HF), and configuration interaction singles (CIS) methods. The excited reduction potentials of the Ru(II) complexes in aqueous solution, which appear to be primarily responsible for the DNA-photocleavage behavior, were calculated by thermodynamic cycle, and determined to be higher than the oxidation potentials of some DNA-bases. The order of the reduction potentials changes from 3 > 2 > 1 in the ground state to 2 > 1 > 3 in the excited state, suggesting that the substituent on the main ligand has a significant effect on the electrochemical properties of the excited Ru(II) polypyridyl complexes. In addition, these theoretical results can explain two experimentally-observed phenomena: (a) the cleavage of DNA by complex 3 in the absence of light irradiation; (b) the inability of complex 3 to emit luminescence in its excited triplet state in aqueous solution.