Long-range proton-coupled electron transfer in phenol-Ru(2,2′- bipyrazine)32+ dyads

Article abstract of DOI:10.1039/c3cp55071k

Two dyads in which either 4-cyanophenol or un-substituted phenol is connected via a p-xylene spacer to a Ru(bpz)₃²⁺ (bpz = 2,2′-bipyrazine) complex were synthesized and investigated. Selective photo-excitation of Ru(bpz)₃²⁺ at 532 nm in a CH ₃CN-H₂O mixture leads to the formation of 4-cyanophenolate or phenolate along with Ru(bpz)₃²⁺ in its electronic ground state. This apparent photoacid behavior can be understood on the basis of a reaction sequence comprised of an initial photoinduced proton-coupled electron transfer (PCET) during which 4-cyanophenol or phenol is oxidized and deprotonated, followed by a thermal electron transfer event in the course of which 4-cyanophenoxyl or phenoxyl is reduced by Ru(bpz)₃⁺ to 4-cyanophenolate or phenolate. Conceptually, this reaction sequence is identical to a sequence of photoinduced charge-separation and thermal charge-recombination events as observed previously for many electron transfer dyads, with the important difference that the initial photoinduced electron transfer process is proton-coupled. The dyad containing 4-cyanophenol reacts via concerted-proton electron transfer (CPET) whereas the dyad containing un-substituted phenol appears to react predominantly via a stepwise PCET mechanism. Long-range PCET is a key reaction in photosystem II. Understanding the factors that govern the kinetics of long-range PCET is desirable in the broader context of light-to-energy conversion by means of proton-electron separation across natural or artificial membranes.

Full text of DOI:10.1039/c3cp55071k

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Long-range proton-coupled electron transfer in
2+
phenol–Ru(2,20-bipyrazine)3 dyads†
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 3617
Catherine Bronner and Oliver S. Wenger*
Two dyads in which either 4-cyanophenol or un-substituted phenol is connected via a p-xylene spacer
2+
to a Ru(bpz)3 (bpz = 2,20-bipyrazine) complex were synthesized and investigated. Selective photo-
2+
excitation of Ru(bpz)3 at 532 nm in a CH3CN–H2O mixture leads to the formation of 4-cyanophenolate
2+
or phenolate along with Ru(bpz)3 in its electronic ground state. This apparent photoacid behavior can be
understood on the basis of a reaction sequence comprised of an initial photoinduced proton-coupled
electron transfer (PCET) during which 4-cyanophenol or phenol is oxidized and deprotonated, followed by
a thermal electron transfer event in the course of which 4-cyanophenoxyl or phenoxyl is reduced by
+
Ru(bpz)3 to 4-cyanophenolate or phenolate. Conceptually, this reaction sequence is identical to a
sequence of photoinduced charge-separation and thermal charge-recombination events as observed
previously for many electron transfer dyads, with the important difference that the initial photoinduced
electron transfer process is proton-coupled. The dyad containing 4-cyanophenol reacts via concerted-
proton electron transfer (CPET) whereas the dyad containing un-substituted phenol appears to react
predominantly via a stepwise PCET mechanism. Long-range PCET is a key reaction in photosystem II.
Understanding the factors that govern the kinetics of long-range PCET is desirable in the broader context of
light-to-energy conversion by means of proton–electron separation across natural or artificial membranes.
Received 2nd December 2013,
Accepted 19th December 2013
DOI: 10.1039/c3cp55071k
www.rsc.org/pccp
which is not coupled to proton transfer is of course rather well
understood.35,36
Introduction
Many important redox reactions including water oxidation
Our prior studies demonstrated that bimolecular PCET
2+
and carbon dioxide reduction must be coupled to acid/base between various phenols and photoexcited Ru(bpz)3 (bpz =
chemistry in order to proceed efficiently.1 Thus, there is signi- 2,20-bipyrazine) occurs via a concerted proton–electron transfer
ficant interest in understanding proton-coupled electron transfer (CPET) mechanism which takes place when the reactants
(PCET) reactions at the most fundamental level both in artificial form weak hydrogen-bonded encounter adducts in solution
and biological systems.2–8 Phenol molecules have played a parti- (Scheme 1a).37,38 In the case of 4-cyanophenol (R = CN) the
cularly prominent role in mechanistic PCET studies because their CPET process was associated with an H/D kinetic isotope effect
redox and acid/base chemistry are strongly interrelated.9–12 Much
research focused on bimolecular PCET in which phenols form
hydrogen-bonded encounter complexes with reaction partners
that can act as electron and/or proton acceptors.13–19 In addition,
there has been significant work on dyads in which phenols are
connected covalently to an electron-accepting moiety and where
the solvent (or added base) acts as a proton acceptor.20–27 The
importance of the distance between electron/proton donating
and accepting sites in covalent PCET dyads has received
increasing attention recently.28–34 Long-range electron transfer
Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel,
Switzerland. E-mail: oliver.wenger@unibas.ch
2+
† Electronic supplementary information (ESI) available: Synthetic protocols and
product characterization data, additional luminescence and transient absorption
data, and crystallographic data for ligand 11 (CIF). CCDC 951403. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cp55071k
Scheme 1 (a) Phenol–Ru(bpz)3 reaction pairs which were previously
investigated in the context of hydrogen-atom transfer (HAT)-like PCET.37
(b) Dyads investigated in this work. ET = electron transfer, PT = proton
transfer, KIE = H/D kinetic isotope effect.
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(KIE) of 10.2 ꢀ 0.6, while for phenol (R = H) we found KIE =
3.4 ꢀ 0.2.37 In this paper we report on the two new covalent
dyads shown in Scheme 1b in which 4-cyanophenol or phenol is
2+
connected covalently to Ru(bpz)3 via a p-xylene spacer.
We aimed to explore how driving-force changes (brought about
by the change from 4-cyanophenol to un-substituted phenol) affect
the PCET mechanisms and rates in the two different settings
shown in Scheme 1. This is interesting because in both settings
the same reactants are involved, but in one case the PCET reaction
resembles hydrogen atom transfer (Scheme 1a) whereas in the
other case a multi-site PCET is operative (Scheme 1b). In other
words, the electron and proton transfer directions are the same in
Scheme 1a but different in Scheme 1b.
Fig. 1 Crystallographic structure of ligand 11. Anisotropic displacement
parameters are drawn at the 50% probability level.
protocols including product characterization data are given in
the ESI.†
2+
The result of an X-ray structure analysis of 11 is shown in
Fig. 1. This ligand crystallizes in the monoclinic space group
P21 with a single molecule in the asymmetric unit. Molecule 11
arranges in sheets along the crystallographic a- and c-axes with
hydrogen bonds between the phenol and a nitrogen atom of
bpz (the closest intermolecular O1–N2 distance is 2.798(2) Å)
but there is no evidence for p–p interactions. The two pyrazine
units of a given bpz ligand are oriented in a parallel fashion, and
they are nearly coplanar with a torsion angle (N3–C5–C4–C3) of
only 12.5(2)1 between them. The dihedral angle between the
xylene unit and the neighboring pyrazine (C6–C7–C9–C14) is
37.0(2)1 and the dihedral angle between the phenol and the
xylene (C13–C12–C17–C18) is 72.5(2)1, in line with the equili-
brium torsion angles expected for oligo-p-xylenes.45,46
Our PCET study involving photoexcited Ru(bpz)3 as a key
reactant in the two dyads is possible because this particular
complex is a substantially stronger photooxidant than the more
2+
commonly used Ru(2,20-bipyridine)3 complex.
Results and discussion
Syntheses and structural aspects
The more common 2,20-bipyridine ligand has very frequently
been used in electron or energy transfer dyads39,40 but for 2,20-
bipyrazine (bpz) this is not the case, and the covalent attach-
ment of rigid rod-like bridges (including phenol units) to bpz
had to be newly developed (Scheme 2). Commercially available
2-amino-pyrazine (1) was reacted with N-bromosuccinimide to
afford 2-amino-5-bromo-pyrazine (2).41 Treatment of molecule 2
with HI, I2, and NaNO2 gave 2-bromo-5-iodo-pyrazine (3)42 which
was reacted with 4-trimethylsilyl-2,5-dimethylphenylboronic acid
(4)43 to afford the Suzuki coupling product 5. Subsequent
Stille coupling to tri-(n-butyl)stannylpyrazine (6)44 led to the
bipyrazine ligand with one attached p-xylene unit (7). Depro-
tection of the TMS group with Br2 gave molecule 8 which
was reacted with boronic acid derivatives of phenol and
4-cyanophenol. In the case of the simple phenol commercial
2-hydroxyphenylboronic acid (9) could be used directly,
whereas in the case of 4-cyanophenol an appropriate boronic
acid coupling partner (10) with the phenolic group protected by
a methoxymethyl ether had to be prepared. The final ligands
(11, 12) were reacted with Ru(bpz)2Cl2. Detailed synthetic
Photochemistry of the dyads in pure CH3CN and in
CH3CN–H2O mixtures
In pure (aerated) CH3CN photoexcitation of the ruthenium unit
of the two dyads simply produces 3MLCT luminescence but
there is no photochemistry. The luminescence spectra and the
excited-state lifetimes (t = 474 ns for CN–PhOH–Ru2+; t = 503 ns
for PhOH–Ru2+) are virtually identical to those of the Ru(bpz)3
2+
reference complex (t = 520 ns) under the same conditions
(Fig. S1a and b, ESI†). The transient absorption spectra of the
dyads resemble those of the reference complex (Fig. S2, ESI†).
Obviously, under these conditions electron transfer (ET) from
2+
4-cyanophenol and phenol to *Ru(bpz)3 is highly inefficient.
This makes sense because 4-cyanophenol and phenol are
oxidized at potentials above 1.2 V vs. Fc+/Fc in CH3CN47
whereas *Ru(bpz)3 is reduced at ca. 1.0 V vs. Fc+/Fc48 hence
2+
simple ET is endergonic by Z0.2 eV in both dyads (the standard
oxidation potentials of 4-cyanophenol and phenol in CH3CN
are 1.40 V and 1.25 V vs. Fc+/Fc, respectively.).47
In CH3CN–H2O mixtures the 3MLCT luminescence of the
dyads is quenched, and the excited-state lifetimes shorten to
148 ns for CN–PhOH–Ru2+ and B20 ns for PhOH–Ru2+ while
that of the reference complex is 772 ns (Fig. S3, ESI†). The black
traces in Fig. 2a and b are transient absorption spectra obtained
for B10ꢁ5 M solutions of CN–PhOH–Ru2+ and PhOH–Ru2+ in
CH3CN–H2O (4 : 1 (v : v) for CN–PhOH–Ru2+ in Fig. 2a and 1 : 1
(v : v) for PhOH–Ru2+ in Fig. 2b). Selective Ru(bpz)3 excitation
2+
Scheme 2 Synthesis of the ligands for the two dyads. (a) NBS, CH2Cl2;
(b) aq. HI, I2, NaNO2; (c) Pd(PPh3)4, Na2CO3, EtOH, toluene; (d) Pd(PPh3)4,
m-xylene; (e) Br2, NaOAc, THF; (f) HCl, dioxane. See ESI† for details.
occurred at 532 nm with laser pulses of B10 ns duration. In the
case of CN–PhOH–Ru2+ detection took place with a time delay
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Fig. 2 (a, b) Black traces: transient absorption spectra of the CN–PhOH–
Ru2+ (a) and PhOH–Ru2+ (b) dyads in CH3CN–H2O recorded by time-
averaging over 200 ns after excitation at 532 nm (time delay of 2 ms in (a),
no time delay in (b)). The red traces in (a, b) were obtained by subtracting
the blue traces from the green traces in (c, d). (c, d) UV-Vis spectra of
CN–PhOH–Ru2+ (c) and PhOH–Ru2+ (d) in CH3CN–H2O at pH 7 (blue
traces) and at pH 11 (green traces).
of 2 ms and subsequent time-averaging over 200 ns whereas in
the case of PhOH–Ru2+ no time delay was necessary because the
photoproducts form more rapidly in this dyad (see below).
For CN–PhOH–Ru2+ one observes bleaching at B375 nm and
absorption bands at B420 and B440 nm (black trace in Fig. 2a).
For PhOH–Ru2+ one detects the same three bands but at slightly
longer wavelengths (black trace in Fig. 2b).
Identification of the photoproducts formed in CN–PhOH–
Ru2+ and PhOH–Ru2+ is straightforward when considering the
UV-Vis spectra of the dyads shown in Fig. 2c and d (blue traces)
and the spectra of their deprotonated forms (green traces).
(Deprotonation of the phenolic units occurred by addition of
excess NaOH to the CH3CN–H2O mixtures.) When subtracting
the blue traces from the green traces in Fig. 2c and d one
obtains the difference spectra shown as red traces in Fig. 2a
and b. The agreement between these derived difference spectra
and the experimental transient absorption spectra (black traces
in Fig. 2a and b) is nearly perfect, indicating that the photo-
products in the dyads are the phenolates and Ru(bpz)32+ in the
electronic ground state. It is as if the two dyads merely acted as
photoacids.
Scheme 3 Reaction sequence explaining the apparent photoacid behavior
of the two dyads shown in Scheme 1b.
separate question that we address later. The key point here is
that phenol oxidation is coupled to release of the phenolic
proton (by whichever mechanism), as is commonly observed
when phenols are oxidized.9–27 This PCET process produces
H3O+, neutral 4-cyanophenoxyl or phenoxyl radicals, and
+
Ru(bpz)3 (step 2 in Scheme 3). Thermal electron transfer in the
+
reverse direction from Ru(bpz)3 to 4-cyanophenoxyl or phenoxyl
can then produce the spectroscopically observed photoproducts
comprised of 4-cyanophenolate or phenolate in addition to
Ru(bpz)32+ (step 3 in Scheme 3). In principle, this reaction sequence
is completely analogous to the sequence of photoinduced charge-
separation followed by thermal charge-recombination events
previously observed in many electron transfer dyads. The only
difference in our dyads is that the initial photoinduced electron
transfer step is proton-coupled. Thermal back electron transfer
is rapid because the electron transfer distance is short and
because the process is highly exergonic; thermal re-protonation
of 4-cyanophenolate or phenolate is comparatively slow because
this is a bimolecular process (step 4 in Scheme 3).
Photochemical reaction pathways in CH3CN–H2O
The apparent photoacid behavior of the two dyads shown in
It is tempting to make free energy estimates for the individual
Scheme 1b is unlikely to be the result of direct phenol depro- reaction steps shown in Scheme 3 based on electrochemical
2+
tonation after excitation of the Ru(bpz)3 complex. Phenol potentials and acidity constants. However, for two reasons we
excitation at 532 nm is highly inefficient, and it is not obvious refrain from implementing this idea: (i) the pKa values of the
why the phenols should become significantly more acidic when 4-cyanophenoxyl and phenoxyl radical cations (CN–PhOH+/
the metal complex is excited, particularly in view of the fact that PhOH+) would be of key importance for this purpose but these
the two moieties are kept apart by a p-xylene spacer.
acidity constants are not known for CH3CN–H2O mixtures, and
If direct phenol deprotonation after selective ruthenium they are experimentally very tricky to determine. (ii) Phenol
excitation is impossible, then how else can the photochemical oxidation in CH3CN–H2O mixtures is irreversible (either due to
formation of 4-cyanophenolate and phenolate in combination loss of the phenolic proton or fast dimerization of the phenoxyl
2+
with Ru(bpz)3 in its electronic ground state be explained? radicals) hence accurate determination of standard potentials
The reaction sequence illustrated by Scheme 3 provides a is not possible under these conditions.
plausible explanation. Selective excitation of the metal complex
The reaction sequence in Scheme 3 provides the only viable
at 532 nm triggers electron transfer (ET) from CN–PhOH or explanation for the occurrence of 4-cyanophenolate or phenolate
2+
2+
PhOH to Ru(bpz)3 coupled to release of the phenolic proton following selective Ru(bpz)3 excitation at 532 nm. There is no
(PT) to water (step 1 in Scheme 3). Whether this PCET process spectroscopic evidence for 4-cyanophenoxyl or phenoxyl radicals
occurs in a concerted or consecutive (stepwise) fashion is a in transient absorption spectra (in particular the 4-cyanophenoxyl
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radical has a rather diagnostic spectral signature that we and others
have detected many times before).37,49,50 From this we conclude
that the initial PCET event (step 1 in Scheme 3) is rate-determining
while the thermal back-ET process (step 2 in Scheme 3) is rapid.
In this scenario a significant population of 4-cyanophenoxyl or
phenoxyl radical intermediates never builds up.
The key question then is whether the initial rate-determining
PCET step is a concerted or a stepwise reaction. As noted above,
the phenols do not become more acidic upon excitation of the
Ru(bpz)32+ complex hence direct photoacid behavior is excluded.
However, in our CH3CN–H2O solutions the phenols are in
equilibrium with their phenolate forms (eqn (1)).
R–PhOH–Ru2+ + H2O # R–PhOꢁ–Ru2+ + H3O+
(1)
In principle it is conceivable that phenol deprotonation first
Fig. 3 (a) Temporal evolution of the transient absorption signals at
380 nm (black), 420 nm (blue), and 475 nm (green) after 532 nm excitation
of CN–PhOH–Ru2+ in 4 : 1 (v : v) CH3CN–H2O. (b) The same experiments
for CN–PhOD–Ru2+in 4 : 1 (v : v) CH3CN–D2O. (c) The same experiments
for PhOH–Ru2+ in 1 : 1 (v : v) CH3CN–H2O. (d) The same experiments for
PhOD–Ru2+ in 1 : 1 (v : v) CH3CN–D2O. The excitation pulse width was
B10 ns in all cases.
has to occur via this chemical equilibrium before photoexcited
2+
Ru(bpz)3 oxidizes the phenolate. In other words, there could
be a rate-determining phenol deprotonation step followed by a
rapid (intramolecular and highly exergonic) ET event (which
would then continuously induce a shift in chemical equilibrium
from phenol to phenolate). In this scenario the overall reaction
rate cannot be faster than the rate of phenol deprotonation
(kꢁH+). 4-Cyanophenol has pKa E 8 in H2O.12 It follows that Ka = PCET reaction in Scheme 3 (step 1) is indeed a concerted proton–
(kꢁH+/k+H+) = 10ꢁ8 M, where k
and k+H+ are the rate constants electron transfer (CPET) event; as noted above, a PT–ET sequence
ꢁH+
of 4-cyanophenol deprotonation and 4-cyanophenolate protona- is impossible after selective Ru(bpz)32+ excitation at 532 nm.
tion, respectively. Even under the assumption that protonation
Analogous kinetic experiments were performed with the
of 4-cyanophenolate occurs at a diffusion-controlled rate of PhOH–Ru2+ dyad in 1 : 1 (v : v) CH3CN–H2O (Fig. 3c) and with
k+H+ = 1011 Mꢁ1 ꢁ1, the rate of 4-cyanophenolate deprotona- its deuterated analog in 1 : 1 CH3CN–D2O (Fig. 3d). The reaction
tion will be limited to k
= 103 ꢁ1. For deprotonation of kinetics are markedly faster in this case. The time constants of
un-substituted phenol (pKa E 10 in H2O)12 an upper limit of product formation are 17 ns (PhOH–Ru2+) and 18 ns (PhOD–Ru2+).
= 101 sꢁ1 can be estimated. These estimated maximal rate Evidently, there is no significant H/D KIE in this case, and thus a
s
s
ꢁH+
k
ꢁH+
constants of 4-cyanophenol and phenol deprotonation cannot distinction between concerted and stepwise PCET mechanisms is
be reconciled with the nanosecond kinetics reported in the next not possible on the basis of the kinetic data. We note that the
section. Consequently, a PT–ET mechanism based on the absence of an H/D KIE is not an argument against CPET
chemical equilibrium in eqn (1) is unlikely.
because several other examples are known in which concerted
We are thus left with the mechanistic possibilities of (i) a proton–electron transfers were unambiguously identified as
rate-determining CPET (concerted proton–electron transfer) or rate-determining reaction steps, yet no significant KIEs could
(ii) a rate-determining ET process leading to phenol oxidation be detected.30
followed by subsequent release of the phenolic proton (before
back-ET produces the observable phenolate and Ru(bpz)3
One piece of evidence points towards CPET as a rate-
determining reaction step in PhOH–Ru2+: in anhydrous CH3CN
2+
photoproducts). This mechanistic issue will be addressed no photochemistry occurs (see above). Water must be present in
further in the following section.
order for photochemistry to happen, and the likely role of H2O is
that of a proton acceptor. If an ET–PT sequence with a rate-
determining ET step was operative for PhOH–Ru2+ in CH3CN–H2O
Reaction kinetics
Fig. 3a shows the temporal evolution of the transient absorp- why would the rate-determining ET step not occur in pure CH3CN
tion signals at 380 nm (black trace), 420 nm (blue trace), and also where there is no proton acceptor present? Thus one could
475 nm (green trace) after excitation of the CN–PhOH–Ru2+ argue that phenol oxidation in PhOH–Ru2+ is only possible when
dyad in aerated 4 : 1 CH3CN–H2O at 532 nm. An average time occurring in concert with deprotonation. On the other hand we
constant of 139 ns is extracted from kinetic analysis of the three note that the change from pure CH3CN to CH3CN–H2O entails a
single-exponential transients, in line with the luminescence change in phenol oxidation potentials that might render ET more
lifetime of 148 ns reported above. Fig. 3b shows the same set of data favorable, i.e., even a simple photoinduced ET step could possibly
for the deuterated analog (CN–PhOD–Ru2+ in 4 : 1 CH3CN–D2O) become operative only upon solvent change. As noted above,
yielding an average time constant of 285 ns. Thus, an H/D kinetic phenol oxidation in CH3CN–H2O is irreversible, making the determi-
isotope effect (KIE) of 2.0 ꢀ 0.2 is associated with the formation of nation of accurate standard potentials impossible and hence
the photoproducts, indicating that the rate-determining step we refrain from detailed thermodynamic considerations which
involves proton motion. From this we conclude that the initial are bound to ultimately provide no unambiguous answer either.
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On a qualitative level it seems obvious that one-electron oxida- Notes and references
tion of phenol proceeds at less positive potentials than oxida-
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We conclude that in PhOH–Ru2+ the rate-determining
photochemical reaction step is most likely the ET step of an
ET–PT sequence, but a CPET mechanism cannot be ruled out
completely. In a scenario in which both dyads react via a rate-
determining CPET step, the difference in reaction rates (139 ns vs.
17 ns) could easily be explained by the difference in O–H bond
dissociation free energies between 4-cyanophenol (92.6 kcal molꢁ1
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Summary and conclusions
PCET between 4-cyanophenol or phenol and photoexcited
2+
Ru(bpz)3 has been explored in two different settings, namely
in bimolecular reactions resembling hydrogen atom transfer
(Scheme 1a) and in multi-site PCET reactions with covalent dyads
(Scheme 1b). The bimolecular reactions shown in Scheme 1a are
clear-cut cases of concerted PCET processes. The new dyads
shown in Scheme 1b exhibit apparent photoacid behavior which
can only be explained by a sequence of photoinduced PCET
followed by a thermal electron transfer event in the reverse
direction (Scheme 3). An H/D kinetic isotope effect of 2.0 ꢀ 0.2
for CN–PhOH–Ru2+ indicates that the rate-determining step in
this dyad is a concerted proton–electron release at 4-cyanophenol.
This KIE is markedly lower than that of the bimolecular reaction
between 4-cyanophenol and Ru(bpz)32+ (10.2 ꢀ 0.6), but of course
the proton acceptors in the two different scenarios are not the
same (H2O versus a bpz ligand). For PhOH–Ru2+ no significant
H/D KIE is detected hence the initial PCET reaction is most likely
comprised of a rate-determining electron release followed by
subsequent deprotonation of a phenoxyl radical cation before
16 J. Bonin, C. Costentin, C. Louault, M. Robert and J. M.
´
Saveant, J. Am. Chem. Soc., 2011, 133, 6668.
´
17 J. Bonin, C. Costentin, M. Robert and J. M. Saveant, Org.
Biomol. Chem., 2011, 9, 4064.
+
thermal back electron transfer from Ru(bpz)3 produces the
´
18 C. Costentin, M. Robert and J. M. Saveant, Phys. Chem.
observable phenolate photoproduct. The photochemical beha-
vior of the PhOH–Ru2+ dyad thus appears to be in contrast to
the bimolecular reaction between phenol and photoexcited
Ru(bpz)32+ (Scheme 1a) which involves concerted proton–electron
release at the phenol.
Chem. Phys., 2010, 12, 11179.
19 N. V. Lebedeva, R. D. Schmidt, J. J. Concepcion, M. K.
Brennaman, I. N. Stanton, M. J. Therien, T. J. Meyer and
M. D. E. Forbes, J. Phys. Chem. A, 2011, 115, 3346.
¨
20 A. Magnuson, H. Berglund, P. Korall, L. Hammarstrom,
The photochemical reaction sequence occurring in our dyads is
conceptually identical to a sequence of photoinduced charge-
separation and thermal charge-recombination events as previously
reported for many electron transfer dyads. The key characteristic of
the dyads shown in Scheme 1b is that the initial photoinduced
charge-separation event is proton-coupled, involving either con-
certed proton–electron transfer (CN–PhOH–Ru2+) or a sequence of
ET and PT events (PhOH–Ru2+).
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