Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 131
A carefully studied example of reductive electron transfer quenching (Eq. 9)
is that involving Eu2+ aq as a quencher [114, 115]:
∗ [Ru(bpy)3] 2+ +Euaq 2+ k=2.8×107 M –1 s –1
–––––––––––––––––––→[Ru(bpy)3] + +Euaq 3+ . (9)
The difference spectrum obtained by flash photolysis after a 30-ns light
pulse shows a bleaching in the region around 430 nm due to depletion of
[Ru(bpy)3] 2+ and an increased absorption around 500 nm due to the forma-
tion of [Ru(bpy)3] + (note that both Eu 2+
aq
and Eu3+ aq
are transparent in this
spectral region). Clear kinetic evidence for reductive quenching comes from
the observation that the growth of the absorption at 500 nm occurs at a rate
equal to the rate of decay of the luminescence emission of ∗ [Ru(bpy)3] 2+ .As
it may happen in excited-state reactions, the products of Eq. 9 have a high
energy content and thus they give rise to a back electron transfer reaction
[Ru(bpy)3] + +Euaq 3+ k=2.7×107 M –1 s –1
–––––––––––––––––––→[Ru(bpy)3] 2+ +Euaq 2+ , (10)
which can be monitored (on a longer timescale) through the recovery of the
430-nm absorption or the disappearance of the 500-nm absorption.
In several cases direct evidence for energy transfer quenching (i.e., sensitized
luminescence or absorption spectrum of the excited acceptor) or
electron transfer quenching (i.e., absorption spectrum of redox products) is
difficult or even impossible to obtain for bimolecular processes. In such cases,
free energy correlations of rate constants are quite useful to elucidate the reaction
mechanism [108, 116–118]. As we will see later, photoinduced energy
and electron transfer processes can take place very easily in suitably organized
supramolecular systems.
3.6
Chemiluminescence and Electrochemiluminescence Processes
As mentioned in the introductory chapter (Balzani et al. 2007, in this volume)
[119], excited states can be generated in very exergonic electron transfer
reactions. Formation of excited states can be easily demonstrated when
the excited states are luminescent species. Because of its stability in the reduced
and oxidized forms and the strong luminescence of its excited state,
[Ru(bpy)3] 2+ is an extremely versatile reactant for a variety of chemiluminescent
processes [32, 120–124].
In principle, there are two ways to generate the luminescent ∗ [Ru(bpy)3] 2+
excited state in chemical reactions. One way (Eq. 11) is to oxidize [Ru(bpy)3] +
with a species X having reduction potential E 0 (X/X – )morepositivethan
0.84 V, and another way (Eq. 12) is to reduce [Ru(bpy)3] 3+ with a species Y –