Photoinduced Electron Transfer Coupled to Donor Deprotonation and Acceptor Protonation in a Molecular Triad Mimicking Photosystem II

Article abstract of DOI:10.1021/jacs.7b08761

The first artificial donor-sensitizer-acceptor compound in which photoinduced long-range electron transfer is coupled to donor deprotonation and acceptor protonation is reported. The long-lived photoproduct stores energy in the form of a radical pair state in which the charges of the donor and the acceptor remain unchanged, much in contrast to previously investigated systems that exhibit charge-separated states comprised of electron-hole pairs. This finding is relevant for light-driven accumulation of redox equivalents, because it exemplifies how the buildup of charge can be avoided yet light energy can be stored. Proton-coupled electron transfer (PCET) reactions at a phenol donor and a monoquat acceptor triggered by excitation of a Ru(II) sensitizer enable this form of photochemical energy storage. Our triad emulates photosystem II more closely than previously investigated systems, because tyrosine Z is oxidized and deprotonated, whereas plastoquinone B is reduced and protonated.

Full text of DOI:10.1021/jacs.7b08761

Communication
pubs.acs.org/JACS
Photoinduced Electron Transfer Coupled to Donor Deprotonation
and Acceptor Protonation in a Molecular Triad Mimicking
Photosystem II
Andrea Pannwitz and Oliver S. Wenger*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
S
* Supporting Information
ABSTRACT: The first artificial donor−sensitizer−accept-
or compound in which photoinduced long-range electron
transfer is coupled to donor deprotonation and acceptor
protonation is reported. The long-lived photoproduct
stores energy in the form of a radical pair state in which
the charges of the donor and the acceptor remain
unchanged, much in contrast to previously investigated
systems that exhibit charge-separated states comprised of
electron−hole pairs. This finding is relevant for light-
driven accumulation of redox equivalents, because it
exemplifies how the buildup of charge can be avoided
yet light energy can be stored. Proton-coupled electron
transfer (PCET) reactions at a phenol donor and a
monoquat acceptor triggered by excitation of a Ru(II)
sensitizer enable this form of photochemical energy
storage. Our triad emulates photosystem II more closely
than previously investigated systems, because tyrosine Z is
oxidized and deprotonated, whereas plastoquinone B is
reduced and protonated.
n photosystem II, electrons are transferred over long distances
upon photoexcitation, leading to temporary separation of
I
redox equivalents. Many artificial donor−sensitizer−acceptor
compounds emulated the primary charge separation events, and
in numerous cases long-lived electron−hole pairs were formed.1
However, in photosystem II, there is no simple electron−hole
generation, but instead the resulting reduction and oxidation
productsbothundergoproton-coupled electrontransfer (PCET)
for further stabilization.2 Specifically, tyrosine Z oxidation (YZ in
Figure 1a) is coupled to deprotonation yielding a neutral
phenoxyl radical, and the plastoquinone B acceptor (QB in
Figure 1a) is protonated upon reduction.2,3 We present the first
artificial donor−sensitizer−acceptor compound that is able to
emulate this entire series of events seen inphotosystem II, namely
photoinduced long-range electron transfer coupled to proton
release at the donor in combination with proton uptake at the
electron acceptor. The net result is the formation of a long-lived
radical pair state, instead of the simple electron−hole separation
observed many times before (Figure 1b). While there have been
prior studies in which photoinduced electron transfer was
coupled to proton transfer at either the donor or the acceptor
site,4 to the best of our knowledge, our triad is the first purely
molecular system in which PCET reactivity is possible at both
reactants. Moreover, in many previously investigated systems
phototriggered PCET actually occurred between species in their
Figure 1. (a) Primary and secondary electron transfer events (blue
arrows) in photosystem II after excitation of P680 (green central part).
PCET occurs at tyrosine Z (green, lower left) and at plastoquinone B
(green, upper right).3 (b) Generic donor−acceptor compound
exhibiting simple electron−hole separation. (c) Investigated triad
comprised of a phenol donor (PhOH) resembling tyrosine Z (PCET
1), a Ru(II) sensitizer mimicking the function of P680, and a monoquat
acceptor (MQ+) playing the role of plastoquinone B with regard to
uptake of one electron and one proton (PCET 2).
electronic ground states (e.g., between Ru(III) complexes and
phenols), and there was no intramolecular energy storage.4a,b,f,k
Some of the previously investigated systems exhibited charge-
shift reactions without any charge build-up.
Our key compound is a molecular triad (Figure 1c) comprised
2+
of a Ru(bpy)3 (bpy = 2,2′-bipyridine) photosensitizer, a
monoquat (MQ+, N-methyl-4,4′-bipyridinium) acceptor, and a
Received: August 17, 2017
© XXXX American Chemical Society
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di-tert-butylphenol (PhOH) donor. Its synthesis including full
characterization is reported in the Supporting Information (pp
S2−S13).InCH3CN,excitationoftheRu(II)sensitizerat532nm
product requires somewhat more effort. The spectral signature of
the anticipated phenoxyl radical (PhO•) was determined by
oxidizing a suitable reference compound under alkaline
conditions (Figure 2c; see SI p S22 for details). The main
absorption bands of PhO• overlap spectrally with those from
MQH•+, and the extinction coefficients of PhO• at the relevant
wavelengthsareaboutanorderofmagnitudeweakerthanthoseof
MQH•+ (Figure 2b/c);5b,c hence, direct observation of PhO• is
hampered.
3
merely leads to MLCT luminescence and no photochemistry
occurs (SI pp S15−S16). Not surprisingly, in order for the
anticipated reductive and oxidative PCET events to become
viable photochemical reaction pathways, acids and bases must
both be simultaneously present. Pyridine (py) turned out to be
optimal as solvent, and through addition of some triflic acid,
pyridinium (pyH+) was formed (0.22 M). The resulting py/pyH+
mixturewasasuitableorganicbufferforourspectroscopic studies.
UV−vis and 1H NMR data indicate that the phenolic unit of the
triad remains largely protonated in this solvent while the MQ+
unit remains unprotonated (SI p S17−S20).
SeverallinesofevidenceneverthelessclearlyindicatethatPhO•
is formed after photoexcitation of the triad. First, the photo-
product observedindeaerated py/pyH+ at25°Chasalifetime(τ)
of 1.9 0.2 μs (see below), compatible with the μs-lifetimes of
long-range electron−hole separation in comparable triarylamine-
Ru(II)-quinone compounds and other related triads.7 By
contrast, in a reference dyad comprised of only the Ru(II)
sensitizer and the MQ+ acceptor no long-lived photoproduct is
formed (τ ≤ 10 ns, SI pp S37−S40), indicating that the phenolic
reaction siteplays akeyroleinstabilizingthe1.9 μs-photoproduct
observed for the triad. Furthermore, according to electrochemical
studies phenols undergo concerted PCET in the presence of
pyridine, leading to a substantial lowering of the oxidation
potential due to coupled proton release.8 It is clear that PhOH
Selective photoexcitation of the Ru(II) sensitizer of the triad at
532 nm in the py/pyH+ mixture leads to the formation of a long-
lived photoproduct exhibiting the transient absorption spectrum
shown as a green trace in Figure 2a. This spectrum is dominated
3
2+
cannot be oxidized to PhOH•+ by MLCT-excited Ru(bpy)3
due tolackof sufficient drivingforce (ΔG°ET =+0.5 eV; SIp S29),
but oxidationtoPhO• isreadily possible (ΔG°PCET =−0.4eV;see
below).4a−m,9 Lastly, in neat pyridine a transient absorption band
appearing at 420 nm (black trace in Figure 2a) signals the
formation of phenolate (PhO−) as an additional photoproduct to
the above-mentioned MQ• species formed under these acid-free
conditions (SI p S32 for further details). The PhO− species
appears as a result of initial formation of PhO• and Ru(bpy)3+ by
3
reductive MLCT quenching and subsequent rapid thermal
reverse electron transfer leading to PhO− and Ru(bpy)3 in a
2+
subset of triads (SI p S31). The net result is simple proton release
at the phenol upon Ru(II) excitation, and such apparent
photoacid behavior has been reported previously for Ru(II)-
phenol and Re(I)-phenol dyads.10 Thus, the PhO− signature at
420 nm indicates the intermediacy of PhO• in neat pyridine.
Given these three lines of evidence (1.9 μs lifetime of the triad
photoproduct compared to sub-10 ns lifetimes of several Ru(II)−
MV2+ dyads(SIppS37−40);11 knownPCETchemistryofPhOH
in the presence of pyridine;8,9 PhO− signature in neat py signaling
the intermediacy of PhO•), it is safe to conclude that the main
photoproduct after excitation of the triad in py/pyH+ is indeed
comprised of PhO• and MQH•+.
The mechanism for formation and decay of this radical pair
state is elucidated in the following. After excitation of the triad at
532 nm with pulsesof ∼10 ns duration, the 3MLCT luminescence
emitted by the Ru(II) photosensitizer at 630 nm in deaerated py/
pyH+ decays in biexponential fashion (green trace in Figure 3a)
withτ=68 7ns(85%)andτ′=780 80ns(15%). TheMQH•+
related transient absorption signals at 395 and 610 nm rise with τ
= 68 7 ns and then decay with τ″ = 1.9 0.2 μs (green traces in
Figure 3b/c). Thus, 85% of all excited triads undergo photo-
reaction to the PhO• and MQH•+ products, whereas 15% exhibit
ordinary 3MLCT decay like the isolated Ru(bpy)32+ complex. In
py/pyD+ where the phenolic moiety of the triad is deuterated, the
major 3MLCT luminescence decay component and the rise of the
transient absorption signals at 395/610 nm are decelerated to τ′ =
150 15 ns. Thus, photoproduct formation occurs with an H/D
kinetic isotope effect (KIE) of 2.2 0.2, indicating that proton
motion is involved in the rate-determining reaction step. By
Figure 2. (a) Transient absorption spectra after excitation of the triad at
532 nm in pyridine with 0.22 M pyridinium (green, 34 μM triad), and in
neat pyridine (black, 55 μM triad). Detection occurred in a 200 ns time
window 2 μs after excitation with pulses of ∼10 ns duration. (b)
Electrochemically generated spectra of MQH•+ (green) and MQ•
(black) using a reference compound (inset) in CH3CN with 0.1 M
TBAPF6. (c) Spectrum of chemically generated PhO• using a reference
compound (see inset) in 6:1 toluene/pyridine. Extinction coefficients
were estimated based on previously published spectra (SI pp S21−S22).5
(d) Difference spectrum obtained from a solution of the triad in CH3CN
before and after addition of 4 equiv of TBAOH, in order to determine the
spectral contribution of PhO−.
by the spectral signature of MQH•+, i. e., the reduced and
protonated form of the monoquat acceptor resembling the well-
known methylviologen radical monocation spectrum.5a,6 Com-
parison to the spectro-electrochemical data in Figure 2b (green
trace), for which a MQ+ reference compound (see inset) was
protonated and reduced (see SI p S21 for details), corroborates
this assignment. In neat py without pyH+, MQ• is instead formed,
asseenfromcomparison oftheblacktraces inFigure2a(transient
absorption) and Figure 2b (spectro-electrochemistry). The same
observationismadewithotherbases,forexamplewithpyrrolidine
(see SI pp S35−S36).
While the formation of MQH•+ from MQ+ in the py/pyH+
solution is immediately evident, identification of the oxidation
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arrow in Scheme 1, ‘Ru+’ denotes Ru(bpy)3 ), and protonation of
MQ• liberates another 0.2 eV (third dashed downward arrow in
Scheme 1) (SI p S29). The final photoproduct observed in Figure
2a stores ca. 1.2 eV (Scheme 1, top right).
+
Based on the observable ΔOD values and the extinction
coefficients of the photoproducts, we estimate that 85−90% of all
3MLCT-excited triad molecules reach the radical pair state (SI pp
S41−S42), in good agreement with the 3MLCT decay behavior
discussed above (15% unquenched 3MLCT states).
The relevant acidity constants are such that in the py/pyH+
mixture less than 0.1% of all triad molecules are expected to be
deprotonated at the phenolic donor site (SI pp S17−S18), yet in
85−90% of all excited triads PhO• and MQH•+ are formed (see
above); hence, it seems clear that the observable photochemistry
originates largely from triad molecules with their phenol moieties
initially protonated, not from phenolate species. Furthermore, a
dynamic shift of the acid−base equilibrium between PhOH and
py after photoexcitation of the triad can be excluded as a major
contributor, because the shift in equilibrium has only a rate
constant on the order of 10−3 to 10−1 s−1 (SI p S43) whereas the
observable time constant for product formation is several orders
of magnitude faster (τ = 68 7 ns; τ−1 ≈ 1.5 0.2 × 107 s−1).
Thus, we conclude that photoproduct formation proceeds
according to the sequence of elementary steps illustrated in
Scheme 1. Initial oxidation of PhOH by 3MLCT-excited Ru(II)
sensitizer occurs in concert with proton release to py and is rate-
Figure 3. (a) Temporal evolution of luminescence recorded at 630 nm.
(b, c) Temporal evolution of transient absorption signals at 395 and 610
nm. Excitation of 34 μM triad in deaerated py/pyH+ (green) or py/pyD+
(gray) occurred at 532 nm with laser pulses of ∼10 ns duration.
contrast, the decay of the photoproduct exhibits no significant
KIE (0.9 0.2).
3
Oxidative quenching of MLCT-excited photosensitizer by
MQ+ isendergonic by0.2eV(SIp S29), and even whenoccurring
in concert with proton uptake at the acceptor there is no
significant driving force for this reaction step. The observation of
essentially unquenched MLCT emission in the Ru(II)−MQ+
3
reference dyad in py/pyH+ compared to pyridine solution
corroborates this interpretation. (SI pp S37−S40). Reductive
3MLCT quenching by PhOH is endergonic by 0.5 eV, but
concerted PCET yields a driving force of ca. −0.4 eV (SI p S29).
Thus, it is plausible that reductive quenching by PhOH with
concerted proton release to py is the rate-determining reaction
step (uppermost dashed downward arrow in Scheme 1, labeled
RDS),inlinewiththeH/DKIEof2.2 0.2andinagreementwith
complementary studies in neat pyridine (SI pp S33−S34).
Subsequent electron transfer from reduced photosensitizer to
MQ•+ is then exergonic by 0.3 eV (second dashed downward
+
determining. Subsequent electron transfer from Ru(bpy)3 to
MQ+ is followed by protonation of MQ• by pyH+. The overall
PCET chemistry at the monoquat acceptor does not take place in
concerted fashion because the initial step of the electron transfer,
proton transfer sequence is sufficiently exergonic. The long-lived
photoproduct stores ca. 1.2 eV and decays via a rate-determining
intramolecular reverse electron transfer from MQH•+ to PhO•,
leading to the formation of MQH2+ and PhO−, in line with the
absence of a H/D KIE for photoproduct decay. In the py/pyH+
mixture, deprotonation of MQH2+ and protonation of PhO− are
Scheme 1. Energy Level Scheme Illustrating the Key Elementary Steps Leading to the Formation of the Long-Lived PhO•/MQH•+
a
Radical Pair and Its Subsequent Decay
a
See SI pp S21−S32 for details. CPET = concerted proton-electron transfer, ET = electron transfer, PT = proton transfer, RDS = rate determining
step.
C
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thephenolicdonorandthemonoquatacceptor, andasequenceof
concerted and stepwise PCET processes is involved. The
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ASSOCIATED CONTENT
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/jacs.7b08761.
■
S
Detailed synthetic protocols and characterization data,
description of equipment and methods, supplementary
electrochemical and spectroscopic data, and detailed
thermochemical discussion (PDF)
(c) Costentin, C.; Robert, M.; Savea
4552. (d) Costentin, C.; Robert, M.; Savea
43, 1019.
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AUTHOR INFORMATION
Corresponding Author
*oliver.wenger@unibas.ch
■
(9) (a) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110,
6961. (b) Huynh, M. T.; Mora, S. J.; Villalba, M.; Tejeda-Ferrari, M. E.;
Liddell, P. A.; Cherry, B. R.; Teillout, A. L.; Machan, C. W.; Kubiak, C. P.;
Gust, D.; Moore, T. A.; Hammes-Schiffer, S.; Moore, A. L. ACS Cent. Sci.
2017, 3, 372.
(10) (a) Kuss-Petermann, M.; Wolf, H.; Stalke, D.; Wenger, O. S. J. Am.
Chem. Soc. 2012, 134, 12844. (b) Bronner, C.; Wenger, O. S. Phys. Chem.
Chem. Phys. 2014, 16, 3617.
ORCID
Oliver S. Wenger: 0000-0002-0739-0553
Notes
The authors declare no competing financial interest.
(11) (a) Lomoth, R.; Haupl, T.; Johansson, O.; Hammarstrom, L.
̈
ACKNOWLEDGMENTS
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■
This work was funded by the Swiss National Science Foundation
through Grant No. 200021_146231/1 and the NCCR Molecular
Systems Engineering.
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