Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 133
(c) To oxidize [Ru(bpy)3] 2+ in the presence of a strong reductant (oxidative
reduction). For example, light is generated upon continuous oxidation
of [Ru(bpy)3] 2+ at a working electrode in the presence of C2O4 2– [129, 130].
This reductant in a first one-electron reaction generates the strongly reducing
CO2 – radical that can reduce [Ru(bpy)3] 3+ to the excited ∗ [Ru(bpy)3] 2+
[Ru(bpy)3] 2+ –e – → [Ru(bpy)3] 3+ (15)
[Ru(bpy)3] 3+ +C2O4 2– → [Ru(bpy)3] 2+ +CO2 +CO2 – (20)
[Ru(bpy)3] 3+ +CO2 – → ∗ [Ru(bpy)3] 2+ +CO2 . (21)
These chemiluminescent electron transfer reactions are quite interesting
from an applicative [134–136] as well as from a theoretical viewpoint. Actually,
method a is at the basis of electroluminescent materials, such as organic
light-emitting diodes (OLEDs) and similar devices, which are receiving increasing
interest for practical applications [137–141].
4
Some Important Features of Ru(II) Polypyridine Complexes
4.1
Nonradiative Decay Rate Constants and Emission Spectral Profiles
of Ru(II) Polypyridine Complexes
Radiationless decay from MLCT states of metal polypyridine complexes occurs
with energy release into medium-frequency (polypyridyl-based) modes
and, to a lower degree, low-frequency modes and solvent [4, 142–149]. Averaging
the medium-frequency modes which mainly promote the transition
and combining low-frequency modes, including solvent, into a single mode,
treated classically, the rate constant for radiationless decay knr is predicted to
follow the so-called energy gap law [150–154]. Most of the work to define this
topic has been made by using Ru(II) polypyridine complexes as models; however,
the approach also applies to any MLCT emitter, as largely demonstrated
for Os(II) [146, 147, 155] and Re(I) polypyridine [147, 149, 156] complexes.
Actually, the energy gap law can be expressed by Eq. 22, where β0 includes the
vibrationally induced electronic matrix element and F(calc) is the vibrational
overlap factor (the quantity 1s in Eq. 22 is used to give unitless expression):
ln(knr · 1s)=lnβ0 +ln[F(calc)] . (22)
In a simplified version, F(calc) can be expressed as in Eq. 23 [157]:
� �
– γ E0
F(calc) ∝
�ω
� �
E0
γ =ln –1.
SM�ω
(23)