Dissertation

|3.1 Brominated Phenanthrolines - A Gate to new Bridging Ligands|
is independent of possible influences which result from other ligands around the ruthenium ion.
Therefore, the phenBr 2 -localized 3 MLCT-states represent the emitting excited states (Kasha’s
rule) in all complexes with n = 1, 2, 3 because they are about 0.16 eV lower in energy than the tbbpycentered
3 MLCT-states. These findings explain as well the unexpected emission properties in the
case of n = 0 within this series very well. A statement about the oxidation potentials of the excited
states is difficult because reductive quenching mechanism will probably lead to a dehalogenation
reaction upon formation of the reduced species as observed in electrochemical measurements.
If this interpretation is correct, [Ru(tbbpy) 3-n (phenBr m ) n ] 2+ -type complexes would be of limited
value as photoredoxactive centers as they would decompose during electron transfer processes.
Note: A correlation of the stokes shifts within this series is rather difficult because the number of
overlapping 1 MLCT absorption bands gives rise to several hard to separate stokes shifts for each
complex which cover the particular phenanthroline centered shift.
The corresponding correlation for the series [Ru(tbbpy) 2 (phenBr m )] 2+ (m = 0, 1, 2, 4) is depicted
in figure 57 and focuses on the influence of the number bromo substituents at the phenanthroline
backbone on photochemical properties of the complex.
E Red
(MLCT )
LL
E Ox (Ru 2+/3+ )
-1.30
-1.20
-1.10
-1.00
0.60
0.70
0.80
0.90
*
d(t ) 2g
phen
E0-0
2.03 eV
(610 nm)
*
Ru 3+ d(t ) 2g
phenBr
E0-0
1.97 eV
(630 nm)
Ru 3+ d(t ) 2g
Ru 3+ d(t ) 2g
m = 0 m = 1 m = 2 m = 4
*
phenBr 2
*
E0-0
1.97 eV
(631 nm)
phenBr 4
E0-0
1.85 eV
(672 nm)
Ru 3+
Figure 57: Combination of the redox potentials of the 3 MLCT excited states that can be tapped
via oxidative quenching mechanism (E(A + /A*)), the ground state redox potential for the oxidation
of the complexes (E(A + /A)) and the emission energies (E 0-0 ) of the series of ruthenium complexes
[Ru(tbbpy) 2 (phenBr m )] 2+ (m = 0, 1, 2, 4).
Unfortunately, no emission data for Ru(Br 2 phen) in acetonitrile was available. Nevertheless it
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