Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 55
emission energies in eV units and the ground state standard reduction potentials,
E o (Cr 3+ /Cr 2+ ), obtained from cyclic voltammetry (CV) measurements.
A representative illustration of the relevant energetics is shown in Fig. 15
for the case of [Cr(bpy)3] 3+ ,fromwhichE o ( ∗ Cr 3+ /Cr 2+ ) is determined to be
1.44 V versus NHE in aqueous solution [103].
Fig. 15 Energetics associated with 2 Eg excited state oxidizing power
In contrast, the corresponding E o ( ∗ Ru 2+ /Ru + ) value for [Ru(bpy)3] 2+
(where ∗ Ru 2+ is the 3 MLCT excited state) is reported as 0.84 V [107]. The
primary reason for these differences is the relatively minor energetic cost
of ground state M n+ → M (n–1)+ reduction in the Cr(III) case, which leaves
approximately 85% of the free energy of the 2 Eg excited state available for
photoredox (as opposed to 40% for the Ru(II) analog).
Another important observation is that the 2 Eg → 4 A2g emission signal
of [Cr(diimine)3] 3+ complexes in ambient solution is significantly quenched
by the presence of dissolved oxygen, 3 O2, as the result of an energy transfer
process generating excited state singlet oxygen ( 1 O2) [103, 105, 108]:
( 2 Eg)Cr 3+ + 3 O2 → ( 4 A2g)Cr 3+ + 1 O2 . (2)
SingletoxygenproductioninEq.2thenprovidesanalternativemethodfor
substrate oxidation, where the Cr(III) 2 Eg excited state is functioning as
a photocatalyst. During the present review period, Pagliero and Argüello
examined the role of O2 in the photooxidation of phenols in aqueous solution,
employing [Cr(phen)3] 3+ as the photocatalyst [109]. Although direct
phenol oxidation according to Eq. 1 is thermodynamically feasible, under airsaturated
conditions the net photochemistry is dominated by a singlet oxygen
mediated pathway leading to benzoquinone as the sole organic product. The
results confirm and amplify the observations from an earlier study [110], and
have practical relevance to the emerging field of photoremediation of waste
waters [111].
Despite the large number of molecules that have been shown to quench
the 2 Eg excited state of [Cr(diimine)3] 3+ complexes via Eq. 1, biological substrates
have very rarely been employed in this role. Several recent studies