MODULE 28_03
Electron Transfer in the Photosciences
Advantages of photoexcitation
1 No transfer occurs until the photons are absorbed. Thus systems can be manipulated
(synthesized, mixed, etc.) in their ground states.
2 Wavelength specificity allows preferential excitation of one component.
3 Photon absorption is virtually instantaneous and thus experimental "dead" time is nonexistent.
4 Two rate constants can usually be determined for each D/A couple (Figure 28.1).
5 Electronically excited states have > 1 eV more energy than the parent ground states and
are therefore better oxidants and reductants.
M* + A
kET
h
M+ + A-
k-ET
M+A
FIG. 28.1
Experimental Observations:
Measurements of bimolecular electron transfer rate constants permeate the literature, but they are
of little use in testing the theoretical picture since
(1) Distance and orientation effects are averaged over the population of reactants.
0
(2) Diffusion effects tend to obscure the details of the log k vs. G curve at high driving
force (Figure 28.2).
1
diffusion
limit
log k
FIG. 28.2
increasing driving force
G0
0
Referring to Figure 28.2, we see that as the driving force increases, so does the bimolecular rate
constant for the reaction (the normal region). However, at some point, even though the driving
force continues to increase, the rate constant levels off because the rate-limiting step is now the
diffusion of the reactants together. Such plots are termed Rehm-Weller plots and have been
generated by several investigators of bimolecular solution phase electron transfer reactions (e.g.,
Weller, Balzani, Wilkinson...)
Real progress began to be made when experimentalists confined D and A and prevented their
mutual diffusion. There are three basic approaches to this:
(1) Retain D and A as discrete molecular species, but disperse them in rigid glasses (J.R.
Miller, 1975; G. McLendon, 1983).
This is a relatively crude approach since a
distribution of RDA values are generated. However, from an analysis it appeared that the
relationship kET  exp( RDA ) was followed with ~ 0.01 pm-1.
(2) Link D to A via a rigid spacer entity, using covalent bonding, e.g.
D
SPACER
2
A
Variations in the length of the spacer allow RDA changes to be effected, and changing the
reduction potentials of D and A at constant RDA allows G0 effects to be investigated.
Closs and Miller adopted this approach in their seminal paper in 1983. They employed steroids,
decalins, and cyclohexanes as rigid spacers. Others followed Closs and Miller ussing somewhat
different approaches. Thus Verhoeven and Padden- Row used a series of linked norbornanes,
Isied, and independently Klapper employed oligoprolines of variable lengths, and Gray et al.
covalently attached Ru(II) complexes to histidine residues on the surface of cytochrome-c.
Figure 28.3 shows some of the Closs and Miller results. The inverted region is clearly apparent
in all solvents and the  value shifts to lower energies as the polarity of the solvent is reduced
(because the inner sphere contribution is lessening). 
Distance effects:
Closs and Miller used rigid spacers of different lengths (Closs, et al. J. Phys. Chem. (1986), 90,
3673), as detailed in the energy transfer module.
The systems were irradiated by an electron beam in a pulse radiolysis experiment, both entities
pick up electrons (only one per molecule) and then the Np radical anion transfers the electron to
the benzophenone moiety. The measured rate constants dramatically increase as the throughbond distance decreases.
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Use of Oligoprolines
Oligoprolines form rigid bridges. Klapper et al linked tyrosine
to one end and tryptophan to other and generated a trp radical
and measured the rate parameter as this decayed to generate the
tyr radical.
Carrying out the experiment as function of the
number of prolines in the linker led to an evaluation of .
Isied et al. linked different transition metal complexes to each end of oligopro bridges of
different length and obtained distance effects. Schanze & Sauer linked Ru(II) polypyridyl
complexes at one end and quinones at the other, again finding distance effects.
3) Protein-based systems
Two approaches have been employed. As indicated above, proteins can be modified at surface
sites. The Isied and the Gray groups did this in 1982. Both groups found that
kET  f (G 0 , R)
The other approach has been to employ electrostatically joined redox protein pairs. The Hoffman
(1983) and McLendon (1984) groups both did this and showed that electron transfer can occur
over distances of up to 2 nm.
This approach uses self-assembly of D-A systems through electrostatic interactions, and it offers
a convenient way of preparing D-A couples with minimal synthetic demands. Our own studies
(Zhou and Rodgers, 1990) in this arena have employed this approach. We set up self-assembled
systems composed of an anionic porphyrin and cytochrome-c, which has a cationic surface patch
in the vicinity of the heme pocket (Figures 28.3 and 28.4).
FIG. 28.3
4
heme
FIG. 28.4
The Fe atom in the heme can be replaced by two H atoms (free base) or by metals such as Zn or
Mn. This leads to reduction potential variation at the heme site. Uroporphyrin has a total of
eight carboxylate residues around the periphery (Figure 28.5). These enter into ion association
interactions with the lysine residues around the heme cleft
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FIG. 28.5
Similarly, the free base protons in the octa-anionic uroporphyrin (see diagram) can be replaced
by Zn (II), Fe(II), Mn(II)…
protein
Up
Fe
heme
M
M
Again this leads to reduction potential differences in the porphyrin. The ion pair is envisaged to
be structured as shown in the scheme and computer modeling showed the heme edge to
uroporphyrin center to be 0.8 nm. Thus, the uroporphyrin/cyt-c system offers the possibility of
measuring diffusion-free electron transfer over a fixed distance and with variable driving force,
(G 0 ) . Such a system will then allow us to test the exponential term in the relationship
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kET  vn  el exp([G 0   ]2 / 4 RT )
In aqueous solution at pH = 7.4, and an ionic strength of 4 mM the equilibrium
cyt  c(3)  Up
[cyt  c(3) : Up]
1
was shown to have K a 10 M , such that at [Up] = 35  M and [cyt-c] = 65 M, the
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porphyrin was 96% in form of complex.
Now the reduction potentials for the Up free base and the heme are
0
EUp
 0.86V and EC0 ( III )C ( II )  0.26 V

/ Up
thus the electron transfer reaction in the ground state

[C ( III ):Up]  [C ( II ):Up ]
is energetically uphill by 0.6 V. However by photo-exciting the Up, we set up the scheme
CIII:Up(S1)
CIII:Up(T1)
2.2 eV
kCS
CII:Up+
1.6 eV
0.6 eV
kCR
CIII: Up(S0)
Carrying out laser flash excitation at 532 nm generates the Up S1 state, which subsequently
converts to T1. This is 1.6 eV above the ground state and thus the reduction of the ferricytochrome is now 1 eV downhill, i.e., the photon provides the initiating driving force for the
electron transfer sequence shown below.
7
3
[Up:CIII]*
kCS
[Up+: CII]
kCR
[Up:CIII]
[Up] = [cyt-c] = 35  M; pH = 7.26; ionic strength = 4 mM
Cytochrome-c (III) with ZnUp triplet:
growth and decay of separated radical pair at 550 nm
0.5
This experiment yields
forward and reverse rate
constants
absorbance
0.4
0.3
0.2
0.1
FIG. 28.6
0.0
0
2
4
6
8
10
12
time / s
Both forward and reverse rate constants (Figure 28.6) were shown to be independent of
cytochrome-c concentration as required if the reactions are intracomplex. Semi-log lots of the
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measured rate constants against driving force for a series of donors and acceptors are shown in
Figure 28.7 (the thermal back reaction) and Figure 28.8 (the photo-induced forward reaction).
 = 0.7 eV
FIG. 28.7
10 kR / s
-1
10
-5
1
inverted
region
0.1
0.4
0.6
0.8
1.0
1.2
1.4
E / eV
-5
10 kF / s
-1
10
1
FIG. 28.8
0.1
0.2
0.4
0.6
0.8
E / eV
9
1.0
1.2
1.4
The clear conclusion from these two plots is that the thermal back transfer process shows Marcus
behavior (strong inverted region above 0.7 eV), but the reaction between the excited state and its
reaction partner shows Rehm-Weller behavior. This is most likely because the forward reaction
requires significant local diffusion to occur within the complex before the required configuration
is reached. This process would require significant readjustments of the solvent sheaths of the
carboxylate and ammonium residues at the ion pairing sites, which, in turn would increase the
outer sphere contribution to The reverse reaction does not have this requirement since the
reaction partners are now in their optimal positions and the outer sphere contribution to is
thereby lower.
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