Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Rhodium 247
Table 1 Rate constants for charge recombination following electron transfer from DNAbound,
photoexcited donors to ∆-Rh(phi)2(bpy) 3+ a
Donor DNA krec [10 9 s –1 ] –∆G ◦ [V]
∆-Ru(phen)2(dppz) 2+ Calf thymus 9.2 1.66
rac-Ru(bpy)2(dppz) 2+ Calf thymus 7.1 1.69
∆-Ru(dmp)2(dppz) 2+ Calf thymus 11 1.59
∆-Ru(phen)2(F2dppz) 2+ Calf thymus 7.7 1.68
rac-Ru(phen)2(Me2dppz) 2+ Calf thymus 9.2 1.67
∆-Os(phen)2(dppz) 2+ Calf thymus 11 1.21
Λ-Ru(phen)2(dppz) 2+ Calf thymus 4.5
∆-Ru(phen)2(dppz) 2+ Poly-d(AT) 7.4
∆-Ru(phen)2(dppz) 2+ Poly-d(GC) 0.21
a Based on [144]
presence of Ru(III) oxidized donor. The rate constants for charge recombination
process (krec) were obtained from the decay of this signal. The data
for seven donor–acceptor pairs are given in Table 1. Within this series, the
driving force (∆G ◦ ) is comparable but the donors vary with respect to intercalating
ligand, ancillary ligands, chirality, and metal center. Despite such
a range of chemical properties, the rate observed is centered around 10 10 s –1 .
A significant difference in rate is observed, however, when the absolute
configuration of the donor is varied. For the right-handed ∆-Ru(phen)2dppz 2+
the value is 2.5 times higher with respect the left-handed enantiomer indicating
a deeper stacking of this complex into the double helix. This result,
according to the authors, clearly suggests that the electron transfer process
required the intervening aromatic base pairs. The notion of highly efficient
ET through the stack of DNA base is also strongly supported by the finding
that the largest change in electron transfer rate is observed when the sequence
of the DNA bridge is changed: for the same donor and acceptor reactants the
rate with poly d(AT) is 30 times higher than with poly d(GC). This is an importantresultthatindicatesthattheπ-stacked
bases of the DNA provide an
effective pathway for electron transfer reactions. However, the crucial point of
this study that involves an untethered Ru/DNA/Rh system concerns the distances
over which fast ET occurs. The question is: do the donor and acceptor
complexes contact each other, or does electron transfer occur at long range?
Two models were considered by the authors to interpret the experimental results:
i) a cooperative binding model with ET over short D–A distance and
ii) a random binding model with ET over long distances. On the basis of
DNA photocleavage experiments, the first hypothesis was reject in favor of
a rapid long range ET with a shallow distance dependence [160]. On the other
hand, soon thereafter, Barbara [161] reinterpreted the experimental results on
a quantitative basis using computational simulation procedures and demon-