Ligand Characterization and DNA Intercalation of Ru(II) Polypyridyl Complexes: A Local Vibrational Mode Study

We investigated in this work ruthenium-ligand bonding across the RuN framework in 12 Ru(II) polypyridyl complexes in the gas phase and solution for both singlet and triplet states, in addition to their affinity for DNA binding through pi-pi stacking interactions with DNA nucleobases. As a tool to assess the intrinsic strength of the ruthenium-ligand bonds, we determined local vibrational force constants via our local vibrational mode analysis software. We introduced a novel local force constant that directly accounts for the intrinsic strength of the pi-pi stacking interaction between DNA and the intercalated Ru(II) complex. According to our findings, [Ru(phen)(2)(dppz)](2+) and [Ru(phen)(2)(11-CN-dppz)](2+) provide an intriguing trade-off between photoinduced complex excitation and the strength of the subsequent pi-pi stacking interaction with DNA. [Ru(phen)(2)(dppz)](2+) displays a small singlet-triplet splitting and a strong pi-pi stacking interaction in its singlet state, suggesting a favorable photoexcitation but potentially weaker interaction with DNA in the excited state. Conversely, [Ru(phen)(2)(11-CN-dppz)](2+) exhibits a larger singlet-triplet splitting and a stronger pi-pi stacking interaction with DNA in its triplet state, indicating a less favorable photoinduced transition but a stronger interaction with DNA postexcitation. We hope our study will inspire future experimental and computational work aimed at the design of novel Ru-polypyridyl drug candidates and that our new quantitative measure of pi-pi stacking interactions in DNA will find a general application in the field.