Photochemistry and Photophysics of Coordination Compounds

58 N.A.P. Kane-Maguire
cells are hypoxic [111], the increase in photodamage at lower O2 levels may
provide a selectivity advantage for these [Cr(diimine)3] 3+ reagents in terms
of their future potential as phototherapeutic agents.
In another aspect of this study [118], a mathematical analysis of the emission
quenching data was undertaken. Representative steady-state intensity
and lifetime Stern–Volmer (SV) plots for quenching of [Cr(phen)3] 3+ emissionbycalfthymusB-DNAareshowninFig.18.Fromthelifetimedata,
a bimolecular quenching rate constant of 1.1 × 10 8 M –1 s –1 was extracted
(a value close to that anticipated for a diffusion controlled process). In contrast,
the steady-state SV plot showed strong upward curvature at higher DNA
concentrations. This observation was attributed to the formation of a nonluminescent
[Cr(phen)3] 3+ /DNA ion pair, and allowed an estimation to be
made for the binding constant with DNA (KDNA ≈ 4000 M –1 ).
Fig. 18 Stern–Volmer plots for [Cr(phen)3] 3+ emission quenching in air-saturated 50 mM
Tris-HCl buffer (pH 7.4) by calf thymus B-DNA at 22 ◦ C: • steady-state data, � lifetime
data
A limitation of this initial work with [Cr(bpy)3] 3+ and [Cr(phen)3] 3+
is the relatively small binding constants of these compounds with DNA.
In a subsequent study [119], the photoredox behavior of the complex
[Cr(phen)2(DPPZ)] 3+ with DNA was investigated, where the third diimine
ligand is dipyridophenazine, DPPZ. The value of KDNA increased by two
orders of magnitude, consistent with the known ability of the DPPZ ligand
to intercalate into DNA base stacks [113]. Perhaps more importantly, the
complex was found to have an E o ( ∗ Cr 3+ /Cr 2+ )value80 mV more positive
than that for [Cr(phen)3] 3+ , which placed it in the thermodynamic threshold
range required for direct oxidation of the nucleobase adenine [115]. In accord
with this thermodynamic argument, SV plots of the quenching of the emission
lifetime of [Cr(phen)2(DPPZ)] 3+ in the presence of deoxyguanosine-5 ′ -
monophosphate and deoxyadenosine-5 ′ -monophosphate yielded quenching
rate constants of 2.4 × 10 9 M –1 s –1 and 1.8 × 10 7 M –1 s –1 , respectively [119].
More recently [120], a report by Vaidyanathan and Nair describes nucleobase
photooxidation by the terpyridine Cr(III) derivatives [Cr(ttpy)2] 3+