DNA Sequence and Ancillary Ligand Modulate the Biexponential Emission Decay of Intercalated [Ru(L)2dppz]2+ Enantiomers

The bi‐exponential emission decay of [Ru(L)₂dppz]²⁺ (L=N,N′‐diimine ligand) bound to DNA has been studied as a function of polynucleotide sequence, enantiomer, and nature of L (phenanthroline vs. bipyridine). The lifetimes (τ_i) and pre‐exponential factors (α_i) depend on all three parameters. With [poly(dA‐dT)]₂, the variation of α_i with [Nu]/[Ru] has little dependence on L for Λ‐[Ru(L)₂dppz]²⁺ but a substantial dependence for Δ‐[Ru(L)₂dppz]²⁺. With [poly(dG‐dC)]₂, by contrast, the Λ‐enantiomer α_i values depend strongly on the nature of L, whereas those of the Δ‐enantiomer are relatively unaffected. DNA‐bound linked dimers show similar photophysical behaviour. The lifetimes are identified with two geometries of minor‐groove intercalated [Ru(L)₂dppz]²⁺, resulting in differential water access to the phenazine nitrogen atoms. Interplay of cooperative and anti‐cooperative binding resulting from complex–complex and complex–DNA interactions is responsible for the observed variations of α_i with binding ratio. [Ru(phen)₂dppz]²⁺ emission is quenched by guanosine in DMF, which may further rationalise the shorter lifetimes observed with guanine‐rich DNA.