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 π–π 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 π–π 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
π–π stacking interaction with DNA. [Ru(phen)2(dppz)]2+ displays a small singlet–triplet
splitting and a strong π–π 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 π–π 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 π–π stacking
interactions in DNA will find a general application in the field.