Photoinduced electron transfer reactions between
photoexcited Ru(phen)2dppz2+ (phen =
1,10-phenanthroline, dppz = dipyridophenazine) and acceptors
Rh(phi)2bpy3+ and
Rh(phen)2phi3+ (phi =
9,10-phenanthrenequinone diimine, bpy = 2,2‘-bipyridine) are compared in micelles and
DNA. Both microheterogeneous environments
contain a negatively charged surface and hydrophobic interior and the
cationic complexes associate strongly with
each. However, reactions between molecules bound to DNA or to
micelles show striking differences which can be
correlated with the unique character of the highly ordered, π-stacked
basepairs in DNA compared to the disordered,
aliphatic chains in the micelles. In DNA,
Rh(phi)2bpy3+ quenches
*Ru(phen)2dppz2+ on a fast time scale
(unimolecular
rate constant ≥ 108 s-1),
whereas no detectable quenching of *Ru(II) emission by
Rh(phen)2phi3+ is observed.
In
contrast, both complexes quench equally well in SDS micelles.
Although static quenching on the nanosecond time
scale is observed for Rh(phi)2bpy3+ in
DNA, reactions in SDS occur dynamically by intramicellar diffusion,
with a
bimolecular rate constant of 1.1 × 108
M-1 s-1 for
Rh(phi)2bpy3+ and 1.2 ×
108 M-1
s-1 for
Rh(phen)2phi3+.
Reactions on DNA are also shown to be DNA-mediated in that no
solvent-isotope effects are observed in the quenching.
In addition, there is enantioselectivity seen in reactions on the
right-handed DNA helix but not in the achiral micelle,
indicating that quenching is sensitive to the geometry of
intercalation. Efficient electron transfer quenching in
DNA
compared to SDS micelles therefore provides evidence against the
cooperative association of molecules on DNA
and for the importance of intercalative stacking of the donor and
acceptor for fast electron transfer through the DNA
π-stack.