Excited-state proton-coupled electron transfer within ion pairs

Article abstract of DOI:10.1039/c9sc04941j

The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.

Full text of DOI:10.1039/c9sc04941j

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ARTICLE
EXCITED-STATE PROTON-COUPLED ELECTRON TRANSFER WITHIN
ION PAIRS
Wesley B. Swords,a,b,† Gerald J. Meyer b and Leif Hammarström a*
Received 00th January 20xx,
Accepted 00th January 20xx
The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus
on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction
orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET.
Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an
ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in
acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation
of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized
salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the
first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET
rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a
proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in
acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants
that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET
mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that
could compete in alternate PCET pathways.
DOI: 10.1039/x0xx00000x
electron-transfer quenching by a redox mediator to yield an
oxidized or reduced sensitizer. This oxidized or reduced
sensitizer then initiates a thermal PCET reaction with a
secondary substrate.5 Alternatively, the excited sensitizer can
directly participate in the PCET reaction.6 While excited-state
PCET (ES-PCET) has been less explored than thermal PCET, there
are a growing number of fundamental and application-based
studies in the field.1,6–9
ES-PCET reactions occur through three distinct mechanisms,
Scheme 1,1,10 1) stepwise electron transfer-proton transfer
(ETaPTb), 2) stepwise proton transfer-electron transfer (PTaETb),
and 3) concerted electron-proton transfer (CEPT), in which the
proton and electron are transferred in the same step with a
common transition state. The concerted mechanism is expected
to be more valuable for selective catalysis as it avoids high
energy intermediates11 and may occur with lower reaction
barriers.12 However, the kinetic penalty associated with proton
tunnelling and the need to bring three reagents together may
favor the stepwise pathways.13–15
Introduction
The creation of energy-rich fuels from small molecules is
dependent upon the ability to effectively couple proton and
electron transfer. There is growing interest in the use of solar
energy to drive these proton-coupled electron transfer (PCET)
reactions.1–4 To accomplish this feat, molecular systems that
effectively couple light energy to proton and electron transfer
are needed. Two approaches that have been utilized to couple
light to PCET include the flash-quench technique, where
photoexcitation of
Scheme 1. ES-PCET reaction diagram. ES* is the excited sensitizer, GS– is the reduced
sensitizer, R-OH is the protonated substrate, and B is a base.
a sensitizer is followed by rapid
In this work, we show electrostatic interactions provide a
general means to investigate ES-PCET reactions without the
limitations imposed by covalent or hydrogen bonds. Upon
excitation of ion pairs formed between cationic ruthenium
compounds and anionic salicylate derivatives, quenching of the
ruthenium excited state proceeded through a PCET mechanism.
One-electron oxidation of the ion-paired salicylate by the
photoexcited ruthenium was coupled to proton transfer within
an internal salicylate hydrogen bond. Thus, upon light
a. Department of Chemistry, Ångstrꢀm Laboratories, Uppsala University, Box 523,
SE75120 Uppsala, Sweden.
b. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill
27599, United States.
† Current Address: Department of Chemistry, University of Wisconsin–Madison,
1101 University Avenue, Madison, Wisconsin 53706, United States.
Electronic Supplementary Information (ESI) available: [Rodgers’ ion-paired ET
reanalysis, salicylate derivatives characterization, ruthenium complex square-wave
analysis, ion-pair characterization, and transient spectroscopy]. See
DOI: 10.1039/x0xx00000x
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excitation, the electron and proton transfer occurs entirely ES-PCET mechanism with the solution buffer (seVeiewsuArptipcloe Ortnilninge
DOI: 10.1039/C9SC04941J
within the ion pair, removing any need for reactant diffusion.
Salicylate was chosen as the counteranion in this study
information).
Herein, ion pairing is used to study a fundamental ES-PCET
because it contains an asymmetric internal hydrogen bond reaction between cationic ruthenium sensitizers and anionic
between phenolic-OH and carboxylate functional salicylate derivatives. Pre-association of the ion-paired complex
a
a
group.16–18 It has been well documented under aqueous occurred in acetonitrile solution with equilibrium constants on
conditions that, upon one-electron oxidation, transfer of the the order of 105. Following photoexcitation of the ion-pairs,
localized phenolic-OH proton to the carboxylate forms a unimolecular quenching of the ruthenium excited state
carboxylic acid and phenoxyl radical. In flash-quench studies, corresponded to intra-ion-pair PCET. Importantly, as with prior
PCET proceeded through
a
CEPT mechanism, where covalent and hydrogen-bond systems, these rate constants
intramolecular proton transfer occurred in concert with could be measured without complications from diffusion.
electron transfer to an oxidized ruthenium(III) center.19,20 With Systematic variation of the driving force for PCET allowed
the need for more non-polar, aprotic organic solvents like analysis of the ES-PCET mechanism, which was ascribed to a
acetonitrile or dichloromethane to assist the preorganization of sequential PTaETb reaction. A pre-equilibrium model of this
ion-pairs,21,22 we hoped the use of salicylate would bias our mechanism provided a rare example of rate constants near the
system towards a concerted mechanism while providing a Marcus barrierless region in a PCET reaction, i.e. a region of
scaffold to systematically study salicylate oxidation under driving force where the rate changes only weakly, or not at all.
non-aqueous conditions.
At even higher driving force, the rate decreases with increasing
The use of undirected ion pairs presents a new approach to driving force, in the so-called Marcus inverted region.40 This
the field of ES-PCET. Almost all studies to date of ES-PCET fall behaviour was clearly demonstrated more than 30 years ago for
under three categories. 1) Diffusional pathways, where the ground-state electron transfer (charge shift) by Closs, Miller,
electron and proton transfer components react through a and co-workers,41 and for photochemical charge recombination
diffusional interaction.23–25 2) Covalent bonds, where the by Wasielewski and co-workers.42 For PCET reactions, however,
sensitizer and PCET reactant are bound through an organic inverted region behaviour was shown only recently,9,30 and
framework.9,26–30 3) Hydrogen bonds, where the sensitizer and even near-barrierless PCET reactions are rare.43 Ion pairing was
PCET reactant are brought together through a hydrogen critical to our ability to investigate this mechanism by reducing
bond.3,31–34 The use of covalent and hydrogen bonds in ES-PCET the reaction order for PCET and provides a broad, general
has facilitate large gains in the fundamental understanding of methodology that will be of interest in future application and
ES-PCET. One large convenience afforded through the latter two mechanistic PCET investigations.
methods is that the removal of reactant diffusion allows direct
measurement of the ES-PCET reaction rate constants. However,
these methods also have limitations. The covalent systems are
Experimental
often synthetically difficult to prepare. The hydrogen-bond
systems inherently require specific functionality to form the
hydrogen bond interface, and the association constant is small
Materials
Acetonitrile (spectroscopic grade, Alfa Aesar) was used as
received. All seven salicylate derivatives (> 97%) were
purchased from Sigma Aldrich and used as received. The
ruthenium compounds utilized were all synthesized for prior
studies.44–46 Tetrabutylammonium 30-hydrate (Sigma Aldrich,
> 98%) was used as received.
(K
association constants on the order of K
dielectric solvents (DMSO) and K
104–105 M–1 in lower polarity

103 M–1) in most cases. Salt-bridged systems have achieved
102–103 M–1 in high


solvents (THF, DCM), but the studies focused on variations of
the salt bridge and not on systematic variations of the donor-
acceptor components themselves.31,33–37 Thus, there have been
no systematic ES-PCET studies of donor-acceptor systems
bound by general, non-directional ion pairing. Through removal
of both the synthetic difficulty of covalent systems and required
hydrogen bond functionality, electrostatic ion pairs may
provide a more general methodology toward fundamental and
application-oriented ES-PCET studies.
To our knowledge only one study has potentially measured
ES-PCET in ion-pairs without an intermolecular hydrogen bond.
Auodia and Rodgers reported that electron-transfer rate
constants within ion pairs of a tyrosine terminated anionic
polypeptide chain and a tetracationic porphyrin sensitizer
varied with the pH of the aqueous solution.38,39 While they
assigned these pH dependant rate constants to electron
transfer, a re-analysis of the data supports that the reaction
Electrochemistry
Square-wave and cyclic voltammetery were collected on a
autolab potentiostat in a standard 3-electrode set-up. A
platinum disk was used as the working electrode, a platinum rod
as the counter electrode, and a Ag/AgNO3 electrode was used
as a pseudo reference electrode. An inert electrolyte composed
of 100 mM TBAClO4/CH3CN was used. All potentials were
externally referenced to Fc/Fc+ (630 mV vs. NHE).47
UV-Visible Spectroscopy
UV-vis absorption spectra were acquired on a Varian Cary 50
UV−vis spectrophotometer in
spectrophotometric quartz cuvettes. Resolution of 1 nm was
used.
Time-Resolved Photoluminescence and Nanosecond Transient
Absorption Spectroscopy
1
cm path length
most
likely
occurred
through
an
Time-resolved photoluminescence and nanosecond transient
2 | J. Name., 2012, 00, 1-3
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Results and Discussion
View Article Online
DOI: 10.1039/C9SC04941J
Scheme 2. A) Salicylate derivatives and B) tetracationic ruthenium sensitizers.
Characterization of Salicylate Anions
The seven salicylic acids (R-HSA), where R is the functional group
para- to the phenol (OH-, OMe-, Me-, H-, F-, Cl-, acetyl-), were readily
deprotonated in acetonitrile (CH3CN) through the in situ addition of
tetrabutylammonium hydroxide 30-hydrate (TBAOH) to form the
salicylate anions (R-SA–), Scheme 2. The deprotonation was
monitored by UV-vis spectroscopy. Hypsochromic shifts of 10–20 nm
(~0.1–0.18 eV) were accompanied by a slight decrease in the
absorption intensity. This change in absorption was linear with
respect to the TBAOH concentration up to one equivalent, upon
which the spectral changes saturated. A set of isosbestic points was
maintained throughout the titration indicative of clean conversion to
the deprotonated anion. Figure 1A-C shows the deprotonation for H-
HSA to H-SA–, along with the extinction coefficient spectra of each
entity. Acetyl-HSA showed different spectroscopic changes than the
other six R-HSA derivatives. Upon the addition of TBAOH, a
significant increase in the low-energy absorption intensity occurred,
Figure 1D.
absorption (TA) single wavelength kinetic data were collected
on an Applied Photophysics spectrometer. Optical excitation
was afforded by an OPO (opotek) pumped by a Q-switched,
frequency tripled (355 nm) Nd:YAG laser (Quantel, Brilliant B).
Pulses had an ~ 7 ns FWHM at 460 nm (c.a. 10 mJ/pulse). A
pulsed Xenon lamp of an Applied Photophysics LKS60 setup
provided probe light that was passed through 1 cm2 quartz
cuvette 90° to the laser and through a monochromator before
hitting the P928 photomultiplier. For photoluminescence
measurements the probe lamp was not used, and
photoluminescence was detected at 90° to the incident laser
through the same detection system. The PMT signal was
converted and digitized using a HP Infinitum S5 digital
oscilloscope (2G samples/s). Transient absorption traces were
generated from the raw data using the LKS60 software.
Full transient absorption spectra were acquired on a Spectra
Physics Quanta-Ray system with a frequency doubled (532 nm),
Q-switch Nd:YAG laser. The pulse laser was connected to a
transient absorption detection system (Edinburgh Instruments),
equipped with a monochromator and a pulsed Xe arc lamp. A
transient absorption spectrum of the sample was collected at
90o to the incident laser by a Tektronix 500MHz digital
oscilloscope coupled to a CCD camera. The output was
processed with Edinburgh Instruments’ L900 software. All data
analysis was performed on OriginPro2016 and 2017 software.
Titrations
All samples were purged with nitrogen for 5-10 minutes prior to
measurements and a flow of nitrogen was maintained over the
samples during data collection. Ruthenium concentrations were
held around 20–30 µM. Stock solutions of the salicylate
derivatives were prepared at 5–10 mM and were titrated into
the ruthenium samples in 10–100 µL amounts.
Figure 1. A) UV-vis absorption spectral changes upon the addition of
tetrabutylammonium hydroxide to an ~80 µM solution of H-HSA in CH3CN. B)
Normalized change in absorbance monitored at the absorbance maxima for H-HSA
and H-SA–. Dotted line is a linear fit through the first five data points. C/D) Extinction
coefficient spectrum for R-HSA and R-SA– when C) R = H or D) R = acetyl. Asterisk
marks the tautomer absorbance.
Cage Escape Quantum Yields
Cage escape quantum yields were determined from the
nanosecond transient absorption experiments through eqn 1.
Ru(bpy)32+ was utilized as an actinometer assuming a unity yield
of intersystem crossing. The Δε450 between the ground-state
2+
Ru(bpy)3 and the excited-state Ru(bpy)32+* was −1.5 × 104
M−1 cm−1,48 and Δε510 between the ground-state Ru-Bpz4+ and
the reduced Ru-Bpz3+ was 1.05 × 104 M−1 cm−1. Salicylate
concentrations of ~75 µM were utilized as at this concentration
> 98% of the photoluminescence had been quenched.
퐴
휆1
(1−10−퐴
)
푅푢 푏푝푦
(
)
460
휀
(
)
3
휆1
푅푢−퐿퐿
∅
퐶퐸
=
(1)
퐴
휀
−퐴
460
휆2
(
)
푅푢−퐿퐿
1−10
(
)
휆2
푅푢(푏푝푦)
3
Figure 2. A) Salicylate proton-transfer tautomer. The normal and tautomeric forms
are labelled. B) Absorption spectral changes upon the addition of increasing amounts
of H2O to CH3CN solutions of the indicated salicylate derivatives, ~ 250 μM. Asterisks
mark the tautomer absorbance.
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J. Name., 2013, 00, 1-3 | 3
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Table 1. Redox potentials, photophysical properties, pKas, and tautomer equilibrium constants for R-SA–
.
R
Eapp(R-SA–OH/R-SA•
)
λmax,R-HSA (nm),
εR-HSA (M–1 s–1)
335, 4200
333, 4200
314, 3800
304, 3700
314, 4400
316, 3500
305, 3100
λmax,R-SA– (nm),
εR-SA- (M–1 s–1)
320, 4100
318, 4100
304, 3600
295, 3400
305, 4100
307, 3200
292, 13200
pKa
KEQ,Taut
O
(V vs. NHE)
0.56a
0.79
0.97
1.10
OH
OMe
Me
H
16.9
16.6
16.9
16.7b
15.8
15.6
15.4
0.03
0.04
0.07
0.10
0.07
0.11
1.2
F
Cl
1.11
1.16
acetyl
1.30
aOxidation of OH-SA– was quasi-reversible. The value reported is the E1/2. bAligns with literature value of 16.7.50
1.32
acetyl-SA–
The addition of water to a CH3CN solution of salicylate
m = 30
m = 26
1.28
1.16
(
H-SA–) was shown to shift the equilibrium towards the normal
Cl-SA–
m = 30
isomer (protonated phenol).16 Therefore, to show that the low
intensity absorbance measured for the expanded series of
R-SA– derivatives was due to this tautomer, titrations with
deionized water were performed, Figure 2. A loss of the
tautomer absorbance was correlated to a growth of the normal
isomer absorbance and confirmed our assignment of the
low-energy absorbance to the tautomer.
F-SA–
H-SA–
1.12
1.08
m = 27
Me-SA–
m = 27
m = 22
m = 38
0.96
0.80
OMe-SA–
OH-SA–
0.56
0.52
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
The absorption spectra of the salicylate derivatives in CH3CN
were fit with the sum of two Gaussian functions to approximate
the absorbance of the tautomer. It was assumed that the
extinction coefficient of the two species at the maximum
absorbance was identical, as was previously done for similar
phenol/carboxylate tautomers.51 Therefore, the ratio between
the maxima absorbances gave the equilibrium constant for the
tautomerization (KEQ,Taut), Table 1. A measurable increase in the
tautomer equilibrium constant coincided with increased
electron withdrawing character of the functional group para- to
the phenolic-OH. This substitution para- to the phenolic-OH
group is known decrease the intrinsic pKa of the phenolic-OH.18
This decreases the difference in pKa between the carboxylate
and phenolic-OH, thus lowering the driving force for proton
transfer and increasing the concentration of tautomer in
solution.
Log()
Figure 3. Peak potential from irreversible cyclic voltammograms of the indicated R-
SA–s vs. the log of the scan rate (). The indicated slopes (m) are given in mV/decade.
Note the peak potential axis is broken in multiple places for clarity. Error bars are
included and in most cases are similar in size to the symbol.
The pKa associated with this deprotonation in CH3CN was
determined through the spectrophotometric titration of
2,4-bis(tetramethylphenyl)-7-(dimethylamino)quinoline (pKa
15.2 in CH3CN).49 The measured pKa of H-HSA (16.7) aligned well
with that previously reported as 16.7.50 For OH-
OMe-, and Me-
HSA, the pKas were found in the range of 16.9–16.6. Whereas
F- Cl-, and acetyl-HSA were more acidic, in the range 15.8–15.4.
The pKa values are presented in Table 1.
Upon deprotonation, small red-absorbing shoulder
=
,
,
a
appeared at ~340 nm for all the R-SA– compounds, except
acetyl-SA–. This absorption was previously identified for H-SA–
in acetonitrile and ethanol as a proton-transfer tautomer,
Figure 2A.16,17 In this tautomeric form, the proton is localized on
the carboxylate functional group instead of the phenolic
oxygen. This tautomeric form was not expected at the outset of
this study as previous PCET experiments in solely H2O did not
observe this tautomer.19 However, in non-polar organic solvents
this tautomer has been characterized in both intra- and
intermolecular phenol-carboxylate systems.16,17,51 In the
intermolecular studies, non-polar solvents were suggested to
better stabilize larger, delocalized anions, such as expected for
a phenolate, and the equilibrium between the normal and
tautomeric form was found to shift towards the tautomer as
solvent polarity decreased.51 As acetonitrile is a reasonably
polar organic solvent, only a small amount of tautomer
absorbance was found for the salicylate derivatives studied,
The apparent reduction potentials of the salicylate
derivatives were determined through cyclic voltammetry,
Eapp(
R-SA•O/R-SA–OH) where R-SA– is the salicylate derivative
OH
before oxidation and R-SA• is the oxidized R-SA– that has
O
undergone an intermolecular proton transfer to the carboxylate
functional group. The oxidation of the R-SA– compounds was
completely irreversible as expected for phenolic compounds
that undergo an irreversible dimerization after oxidation.52
Also, because the proton transfer is coupled with electron
transfer in the oxidation, a true one-electron reduction
potential (Eo(R-SAOH/R-SA–OH) for R-SA– could not be measured.
However, the apparent reduction potential for the PCET
reaction was estimated through a scan rate (
) dependence,
Eapp ) versus the oxidative
(
R-SA•O/R-SA–OH).52–54 A plot of log(

peak potential was linear with slopes of 20–30 mV/decade,
within a reasonable deviation from the theoretical 19.7
barring acetyl-SA–
.
4 | J. Name., 2012, 00, 1-3
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(OMe), 4,4’-di-tert-butyl-2,2’-bipyridine (Dtb), 2,2’-bipyridine (Bpy),
View Article Online
DOI: 10.1039/C9SC04941J
and 2,2’-bipyrazine (Bpz). Herein, the compounds will be denoted
Ru-LL where LL is the short name of the derivatized bipyridine or
bipyrazine ligand (Scheme 2B).
The electrochemical properties of the Ru-LL compounds, except
Ru-Bpz which was previously reported,58 were determined through
square-wave voltammetry in 0.1 M TBAClO4/CH3CN solution. The
one-electron oxidation of Ru-LL (Ru-LL5+/4+) was formally a ruthenium
centered oxidation, RuIII/II. These potentials were measured to be
between 1.39 and 2.10 V, with a negative shift of the potential in the
order Ru-OMe < Ru-Dtb < Ru-Bpy < Ru-Bpz. This trend followed the
electron donating ability of the ancillary ligands. The first one-
electron reduction of ruthenium polypyridyl compounds, Ru-LL4+/3+
has been shown to occur at the ligand that is the best π-acceptor.59,60
For Ru-Bpz this was the Bpz ligand, whereas for Ru-OMe, Ru-Dtb,
and Ru-Bpy, the reduction was localized to a tmam ligand. The
reduction of Ru-Bpz was reversible and ~ 300 mV more positive than
the reduction of the other three compounds confirming that Bpz was
the ligand reduced. The other three compounds showed irreversible
reductions associated with the tmam ligand. The irreversible nature
of this reduction has been shown for similar compounds and
ligands.22,61 The reduction potential was therefore estimated from
the peak cathodic current of the square-wave voltammogram. The
Figure 4. UV-visible absorption spectra of A) Ru-Dtb and B) Ru-Bpz (~25 µM) upon
the addition of 0 to 740 µM Cl-SA–. Difference spectra calculated by subtracting the
absorption spectra at no Cl-SA– from the spectra with Cl-SA– present for C) Ru-Dtb
and D) Ru-Bpz. Arrows indicate the spectral changes upon Cl-SA– addition.
mV/decade expected for a PCET reaction, Figure 3.52 The y-
intercept was corrected for scan rate independent variables and
the apparent PCET reduction potentials (Eapp) are presented in
excited-state reduction potential (Ru-LL4+
*
) was estimated
Table 1.
/3+
Table 2. Spectroscopic and Thermodynamic Properties of the Ru-LL Compounds.
through Equation 2, where ΔG0 is the Gibbs free energy change
ES
from the ground state to the excited state, which was reported
previously through
Ru-LL
Ru-
Ru-LL4+/3+ ΔG0
Ru-LL4+*/3+
(V)
0
(µs)
ES
(eV)a
a
Frank–Condon line-shape analysis.45 All
LL5+/4+ (V)
(V)
electrochemical values are included in Table 2.
퐸표(Ru-LL4+∗/3+) = 퐸표(Ru-LL4+/3+) + ∆퐺ES
Table 3. First ion-pairing equilibrium constants.
Ru-Bpz
Ru-Bpy
Ru-Dtb
2.10b
1.57
1.51
–0.50b
2.09
2.02
1.97
1.88
1.59
1.23
1.18
1.02
1.78
0.61
0.36
0.17
(2)
–0.79
–0.80
–0.86
Ru-OMe 1.39
aData from reference 45. bData taken from reference 44
KEQ,1 (x 105)
.
Compounds Ru-Bpz
Ru-Bpy
Ru-Dtb
Ru-OMe
OH-SA–
OMe-SA–
Me-SA–
H-SA–
2.3 ± 0.8 1.4 ± 0.3
1.5 ± 0.2 0.8 ± 0.1
2.4 ± 0.5 1.7 ± 0.2
1.2 ± 0.2 1.2 ± 0.1
0.9 ± 0.3 0.50 ± 0.04 1.1 ± 0.1 1.2 ± 0.1
1.2 ± 0.2 0.55 ± 0.02 0.9 ± 0.1 1.0 ± 0.1
1.4 ± 0.2 0.83 ± 0.07 2.0 ± 0.7 1.2 ± 0.1
2.0 ± 0.2 1.8 ± 0.5
1.1 ± 0.1 2.5 ± 0.6
1.8 ± 0.3 2.4 ± 0.7
1.7 ± 0.2 3.0 ± 0.6
Ion Pair Formation
Typical ruthenium tris-bipyridyl complexes have a 2+ cationic
charge associated with the d6 ruthenium center. The dicationic 4,4’-
bis(trimethylaminomethyl)-2,2’-bipyridine (tmam) ligand has been
used to increase the charge of the polypyridyl compounds by 2+ per
ligand. This has been shown to enhance the formation of ion pairs
between cationic ruthenium polypyridyl compounds and anions in
acetonitrile (CH3CN).23,44,55–57 The four ruthenium compounds
utilized in this study follow the common structure of
[Ru(tmam)(LL)2](PF6)4, where LL was 4,4’-dimethoxy-2,2’-bipyridine
F-SA–
Cl-SA–
acetyl-SA–
All four Ru-LL compounds exhibited UV-vis absorption spectra
with transitions between 200–650 nm. The low energy absorption
bands centered around 460 nm were assigned as metal-to-ligand
charge-transfer (MLCT) transitions.62 Absorption features in the UV
were due to ligand centered transitions. The addition of the R-SA–
derivatives to CH3CN solutions of Ru-LL induced changes in the UV-
vis absorption spectra. Figure 4 shows a representative example of
Cl-SA– with Ru-Dtb and Ru-Bpz. For Ru-Bpz, a bathochromic shift in
the low-energy MLCT and decrease in the MLCT intensity occurred.
For Ru-Dtb, Ru-OMe, and Ru-Bpy a hypsochromic shift of the MLCT
was accompanied by an increase in MLCT intensity. Isosbestic points
were maintained up to 5 equivalents of added R-SA–. R-SA–
derivatives to CH3CN solutions of Ru-LL induced changes in the UV-
vis absorption spectra. Figure 4 shows a representative example of
Cl-SA– with Ru-Dtb and Ru-Bpz. For Ru-Bpz, a bathochromic shift in
Figure 5. Change in absorbance at the indicated wavelengths from the titration of A)
Ru-Dtb and B) Ru-Bpz (25 µM) with Cl-SA–. Wavelengths were chosen as they are the
isosbestic points for the formation of the doubly ion-paired species and therefore the
changes are only due to the first ion-pair formation. The blue dotted line is a fit to a
1:1 binding model.
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Table 4. Excited-state Reduction Potentials, Thermodynamic Driving Forces, Diffusional Quenching Rate Constants, and Ion-Paired ES-PCET Lifetimes and Rate Constants.
DOI: 10.1039/C9SC04941J
Ru-LL
R-SA–
ΔG0
(eV)
ΔG0
(eV)
KSV
kq
τPCET
kPCET
kET
,b
PCET
ET
,
b
1
1
1
1
(x 105 M– )
(x 1010 M–1 s– ) (x 10–8 s) (x 107 s– ) (x 107 s– )
Ru-Bpz
OH
OMe
Me
H
F
Cl
Acetyl
OH
OMe
Me
OH
OMe
–1.03
–0.80
–0.62
–0.49
–0.48
–0.44
–0.29
–0.67
–0.44
–0.26
–0.62
–0.39
–0.46
–1.12
–0.88
–0.69
–0.55
–0.55
–0.50
–0.29
–0.76
–0.52
–0.33
–0.71
–0.47
–0.55
1.15 ± 0.05
1.30 ± 0.04
1.06 ± 0.06
1.11 ± 0.05
1.11 ± 0.03
1.20 ± 0.02
1.20 ± 0.04
0.07 ± 0.01
6.4
7.3
5.9
6.2
6.2
6.7
6.7
1.1
0.8
0.9
1.4
2.1
2.4
2.6
4.4
1.9
2.9
8.6
2.1
4.5
1.6
13.
11.
7.1
4.8
4.2
3.8
2.3
5.3
3.4
1.2
4.7
2.2
6.3
43.
28.
10.
4.8
6.0
3.5
0.2
18.
8.6
1.7
15.
5.6
21.
Ru-Bpy
Ru-Dtb
0.10 ± 0.01
a
1.6
a
a
a
a
a
a
a
Ru-OMe OH
aDynamic quenching not observed over the range of concentrations studied.
The isosbestic points shifted slightly, by < 5 nm, upon R-SA–
additions larger than 5 equivalents for many of the combinations.
This shift indicated that a second R-SA– bound to the ion pair, [Ru-
LL4+,2R-SA–]2+. A 1:1 binding model was used to fit the changes in the
absorbance monitored at the first set of isosbestic points. This
allowed the changes in absorbance associated with the second ion
pairing to be estimated. This analysis yielded second equilibrium
constants (KEQ,2) of 1–5 x 103; about two orders of magnitude lower
than the first ion pair. The doubly ion-paired species will not be
discussed further as the concentrations utilized to study the ES-PCET
reaction were not large enough for an appreciable amount to form.
Selected ion pairs were characterized by 1H nuclear magnetic
resonance (NMR). The addition of 0.5, 1, and 3 eq of Me-SA– to
CD3CN solutions of Ru-Bpz and Ru-Dtb led to measurable shifts in the
resonances of the protons on the polypyridyl ligands, Figure 6. Large
downfield shifts were observed only in the resonances of the
3,3’- and CH2-protons on the dicationic tmam ligand, Figure 6C. No
significant shifts were detected for the proton resonances of the Dtb
or Bpz ligands. This confirmed that the ion pair was formed through
interaction of the R-SA– with the dicationic tmam ligand. Small
upfield shifts were also quantified for the Me-SA– aromatic protons.
Proton-Coupled Electron Transfer Excited-State Quenching
Pulsed laser excitation of the Ru-LL compounds in CH3CN led
to room temperature, visible photoluminescence from the
3MLCT excited state (Ru-LL4+*). With no R-SA– present,
first-order kinetic decays could be fit by a single-exponential
equation. The excited-state lifetimes (τ) decreased with the
electron donating ability of the ancillary bipyridine ligands,
Figure 6. 1H Nuclear magnetic resonance spectra recorded in CD3CN for Ru-Bpz upon
the addition of up to 3 eq. of Me-SA–. A) Shows the aromatic region, asterisks mark
the proton resonances of Me-SA–. B) Shows the methylene resonances on the tmam
ligand. Arrows show the downfield shift of the A) 3,3’-tmam protons and B) the
methylene tmam protons. C) The total change in chemical shift (ppm) between 0
and 3 eq. of Me-SA– shown for all proton resonances on Ru-Bpz and Me-SA–.
the low-energy MLCT and decrease in the MLCT intensity occurred.
For Ru-Dtb, Ru-OMe, and Ru-Bpy a hypsochromic shift of the MLCT
was accompanied by an increase in MLCT intensity. Isosbestic points
were maintained up to 5 equivalents of added R-SA–. These changes
were assigned to the formation of the ground-state ion pair, [Ru-
LL4+,R-SA–]3+. These changes could be well described by a 1:1 binding
model,63 Figure 5, which provided the equilibrium constant (KEQ,1),
for ion-pair formation, [Ru-LL4+,R-SA–]3+.63 KEQ,1 ranged from 0.5–3 x
105 for all 28 combinations of Ru-LL and R-SA–, Table 3. The
association constants measured for these exclusively electrostatic
ion pairs, formed in the relatively polar CH3CN, are significantly
Ru-OMe < Ru-Dtb < Ru-Bpy < Ru-Bpz, Table 2.
The addition of salicylate anions to solutions of Ru-LL
quenched the excited-state photoluminescence in 13 of the 28
[Ru-LL4+,R-SA–] combinations. These are listed in Table 4. The
other combinations did not show excited-state quenching due
larger than those generally measured in hydrogen bond systems (
K
to unfavorable driving forces (
G0 > 0) and short-lived

103). These electrostatic ion pairs are also orders of magnitude
excited-state lifetimes. For Ru-Bpz, excited-state quenching was
observed with all seven R-SA– derivatives, Figure 7. Upon the
addition of R-SA–, the time-resolved photoluminescence decays
larger than salt-bridged systems in polar organic solvents (K ≈ 102–
103 in DMSO) and on the higher end of systems reported in nonpolar
organic solvents (K ≈ 104–105 in DCM). 31,33–37
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DOI: 10.1039/C9SC04941J
Figure 8. A) Excited-state transient absorption spectrum of Ru-Bpz4+* obtained 1 μs
after laser excitation. The negative change in absorbance at wavelengths longer than
570 nm is from uncorrected emission. B) Transient absorption spectra collected 1 and
5 μs after laser excitation of Ru-Bpz4+* (25 M) in the presence of Cl-SA– (75 M).
Overlaid is the normalized Ru-Bpz3+ delta absorbance spectra. Deviation from the Ru3+
Figure 7. A-C) Time-resolved photoluminescence decays for Ru-Bpz (25 μM) upon the
addition of up to 5 equivalents of A/D) OH-SA–, B/E) Me-SA–, C/F) acetyl-SA–. Black
dotted lines are the instrument response function based on a scattered laser pulse
with no sample present. D-F) Dynamic Stern–Volmer plots. Blue dashed line is a linear
fit to the data.
spectra is due to the absorbance of R-SA•
.
O
indicated that the photoluminescence quenching occurred
within the ion pair, and not from a diffusional reaction. This
could not be modelled as first-order decays. Instead, a sum of
two exponential decays (biexponential) was needed, Equation
3. The longer lifetime, τd, decreased with increased
R-SA– concentration. A Stern–Volmer analysis,64,65 Figure 7, of
the lifetimes (τo/τd) was linear vs. the free concentration of
R-SA– and gave a Stern–Volmer constant (KSV) of around
1.1 x 105 M–1 for Ru-Bpz, Equation 4. KSV is related to the
bimolecular quenching rate constant (kq) by the lifetime (τo) of
lifetime was assigned to ES-PCET within the ion pair, (
where 1/ PCET provided the rate constant for ES-PCET (kPCET).
Due to the shorter intrinsic lifetimes of Ru-Dtb and Ru-OMe
diffusional quenching was too slow to be detected at the
concentrations utilized. A biexponential was still needed to
model the excited-state relaxation in the presence of OH-SA–
The short lifetime still corresponded to the PCET lifetime, and
the longer lifetime could be fixed to the lifetime of the complex
without quencher.
PCET),

,
.
the excited state in the absence of quencher (τd,0): KSV = kqτd,0
.
The 3MLCT excited state of Ru-Bpz was produced for
nanosecond transient absorption spectroscopy through pulsed
laser excitation. In ruthenium polypyridyl excited states the
electron resides on the most electron withdrawing ligand.68
Therefore, for Ru-Bpz, the excited state can formally be
described as an oxidized RuIII metal center with a reduced Bpz
ligand, [RuIII(tmam)(Bpz)(Bpz–)]4+*, Ru-Bpz4+*. The appearance
of absorption features that correspond to the reduced ligand, a
positive delta absorbance at ~380 nm, and the loss of the
ground-state MLCT, a negative delta absorbance at 450 nm,
were indicative of the excited state, Figure 8A. These features
decayed to the ground-state with an identical lifetime to that of
The quenching rate constant was thus calculated to be
kq ~ 6.2 x 1010 M–1 s–1 for all seven R-SA– derivatives, Table 4.
This is close to the diffusion-limited rate constant for electron
transfer quenching of similar ruthenium complexes by iodide,
6.6 x 1010 M–1 s–1.66,67 Therefore, τd was assigned as the lifetime
of the diffusional quenching reaction between non-ion-paired
Ru-Bpz and R-SA–
푃퐿퐼푡 = 푃퐿퐼1푒−푡/휏
.
−푡/휏푑
푃퐶퐸푇
+ 푃퐿퐼2푒
(3)
(4)
휏푑,0
휏푑
[
푄
]
[ ]
= 1 + 푘푞휏0 푄
푓푟푒푒
= 1 + 퐾푆푉
푓푟푒푒
The shorter lifetime of the biexponential model was
independent of the concentration of R-SA– and could be fixed
throughout the titration. This lifetime varied from 8 ns for
Ru-Bpz with OH-SA– to 86 ns for Ru-Bpy with Me-SA–, Table 4.
The concentration independence of the rate constants
the time-resolved photoluminescence. In the case of Ru-Bpy
,
Ru-Dtb, and Ru-OMe, the localization of the electron in the
MLCT excited state is expected to localize on the quaternary
amine ligand (tmam) and thus the excited state can be formally
described as [RuIII(tmam–)(LL)2]4+*, where LL is Bpy, Dtb, or
OMe. This change in excited-state electron localization has been
previously proposed in these and similar compounds.44,45
Excitation of a solution of Ru-LL in the presence of R-SA–
gave significant differences in the nanosecond transient
absorption spectra. Figure 8B shows representative spectra
Scheme 3. Generic mechanism for the ES-PCET reaction within the
[Ru-LL4+*,R-SA–]3+* ion pair, A) Ru-Bpz and B) Ru-Bpy. Green arrow shows the proton
transfer and red arrow shows the electron transfer.
3+
collected 1 and 5 μs after excitation of the [Ru-Bpz4+ Cl-SA–
, ]
ion pair. The appearance of an absorption band centered
around 510 nm was consistent with the formation of the
reduced ruthenium complex ([RuII(tmam)(bpz)(bpz–)]3+,
Ru-Bpz3+). To confirm this, the Ru-Bpz3+ delta absorption
spectrum was generated through reductive excited-state
quenching by tri-p-tolylamine. This spectrum could be
normalized to the spectra of the reduced ion pair at lower
energy wavelengths (> 450 nm). A positive deviation from the
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(< 10%)66,72 and larger than ion-pairs formed between iodide
View Article Online
73
and ruthenium polypyridyl complexes witDhOcIa: 1p0e.1y0i3e9l/dCs9oSCf03449%41J
and 25–50%.21 For PCET in hydrogen-bonded systems,
∅ is
CE
often not measured, or irrelevant because the products never
dissociate. In cases where measured, CE has been much lower
∅
than here, see e.g. ref 74. The larger cage escape quantum yields
measured here can be rationalized based on changes in
coulombic forces in tandem with the proton motion coupled
with electron transfer. Upon oxidation, the salicylate
derivatives become neutral, which removes the coulombic
attraction of the ion-pair. Secondly, unlike simple
single-electron-transfer systems, the oxidation of salicylate
involves the movement of a proton from the phenolic-OH to the
carboxylate functional group. Thus, recombination must also
involve the movement of both the proton and electron. The
driving force for this proton-coupled back-electron transfer
would necessitate considerations of the driving force for both
the electron transfer and proton transfer, which may slow the
back-reaction rate constant allowing cage escape to compete
with and surpass recombination within the solvent cage.
Mechanistic Discussion
Figure 9. Single wavelength transient absorption kinetics monitored at 405 nm, red,
and 510 nm, green, for [Ru-LL4+,Cl-SA–]3+. The purple trace is the excited-state decay.
Overlaid in black are biexponential fits with the two lifetimes shared between the
three traces. The negative signal is due to small amounts of excited-state bleach still
present of the monitored wavelengths.
reduced complex spectra was present at higher energies,
around 430 nm, Figure 8B. It has been reported that an oxidized
phenoxyl radical (PhO•) absorbs light in this region, e.g. the
tyrosine phenoxyl radical has an absorption at 410 nm.69–71
Therefore, this absorption was attributed to the absorption of
the oxidized salicylate, in which the proton has transferred to
the carboxylate group, R-SA•O, Scheme 3. This provided a clear
indication that the excited state was quenched by an ES-PCET
reaction. Similar spectral features were obtained for all
[Ru-LL4+,R-SA–]3+ ion pairs that showed photoluminescence
quenching.
The oxidation of salicylate by photooxidized ruthenium
complexes is known to occur through PCET reaction in
water.19,75 However, to the best of our knowledge, no studies to
date have investigated the oxidation mechanism of salicylate in
organic solvents. Above, it was shown that the excited state of
cationic Ru-LL compounds could oxidize salicylate in CH3CN.
Nanosecond transient absorption spectroscopy confirmed that
the reaction proceeded through ES-PCET. Time-resolved
photoluminescence experiments showed that the ES-PCET
reaction occurred through both a diffusional reaction between
non-associated pairs and within the photoexcited ion pairs.
While the diffusional reaction occurred near the diffusion limit,
6 x 1010 M–1 s–1, preventing mechanistic analysis, within the
pre-formed ion pairs the first-order ES-PCET rate constants
were measured directly. Below, we analyse these rate constants
along with the above results to probe the ES-PCET mechanism.
As stated in the introduction, this ES-PCET mechanism could
proceed through either 1) electron transfer-proton transfer,
ETaPTb, 2) proton transfer-electron transfer, PTaETb, or
3) concerted electron-proton transfer, CEPT. Previously, the
concerted mechanism was identified for the oxidation of
salicylate in water through flash-quench transient absorption
spectroscopy and was initially expected to be the active
mechanism in CH3CN.19,75
Single wavelength kinetic analysis at wavelengths near the
isosbestic points between the ground and excited state of
Ru-Bpz (405 nm and 510 nm) in the presence of R-SA– allowed
the formation of Ru-LL3+ (510 nm) and R-SA• (405 nm) to be
O
monitored independent of the excited-state decay, Figure 9.
The emission decay was also monitored to directly compare the
rates of formation with the excited-state decay. The absorption
changes both at 405 nm and 510 nm showed a biexponential
signal rise and yielded lifetimes that agreed with the time-
resolved photoluminescence titrations, Figure 9. In fact, the
ratio of the pre-exponential factors for the two lifetimes,
APCET/Ad, aligned with those of the excited-state decay. This
ratio also aligned with the expected ratio of free ruthenium
complex to ion-paired ruthenium, Ru-LLfree
:
[Ru-LL4+,R-SA–]3+
,
.
based on the equilibrium constant for ion-pair formation, KEQ,1
This indicated that quenching through both the diffusional
reaction and from the pre-formed ion pair occurred through ES-
PCET. Both the oxidized salicylate radical and the reduced
ruthenium complex could be identified as primary
photoproducts, as both R-SA• and Ru-LL3+ had identical
O
The appearance of
a ground-state tautomer in the
formation rate constants that aligned with excited-state decay.
absorption spectra of R-SA– suggested, however, that the
salicylate oxidation mechanism in organic solvents may differ
from that in water. In this case, a stepwise proton-transfer,
electron-transfer mechanism could potentially be favored. The
ground-state proton transfer would allow electron transfer to
occur through the phenolate (R-O–), which for H-SA– is known
to have a more negative redox potential (0.77 V vs. NHE in
water) than the protonated phenol (1.48 V in water).19 Hence,
there is a significantly larger driving force for electron transfer
through the phenolate. This also suggests that a stepwise
Cage escape quantum yields (
∅CE) of the photoproducts
were estimated at 3 eq. of the respective R-SA– with Ru-Bpz
.
The change in absorbance at 510 nm, which corresponds solely
to the reduced ruthenium complex, was used to estimate the
concentration of cage escaped products and the unity internal
2+
conversion efficiency of Ru(bpy)3 used as an actinometer.
Quantum yields of 60–70% were calculated for all R-SA–
derivatives. These cage escape yields are significantly larger
than those found in the diffusional excited-state electron-
transfer oxidation of iodide by ruthenium excited states
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DOI: 10.1039/C9SC04941J
0
•
–
0
∆퐺푃퐶퐸푇 = ℱ[퐸푎푝푝 푅– 푆퐴0 푅– 푆퐴푂퐻 − 퐸 (푅푢– 퐿퐿4+∗/3+)]
(5)
(
⁄
)
)
∆퐺푃0푇 = − ln(퐾
(6)
(7)
퐸푄,푇푎푢푡
푎
푅푇
∆퐺푃0퐶퐸푇 = ∆퐺푃0푇 + ∆퐺퐸0푇
푎
푏
ln(퐾퐸푄,푇푎푢푡
)
∆퐺퐸0푇 = ∆퐺푃0퐶퐸푇
−
(8)
(9)
푏
푅푇
3+
푘푃푇 푘퐸푇
푎
[
푑 푅푢
-
퐿퐿3+,푅
푑푡
푆퐴•푂
]
-
푏
−
−
퐿퐿4+∗, 푅
-
-
퐿퐿4+∗, 푅
-
[
] [
푆퐴푂퐻 = 퐾퐸푄,푇푎푢푡푘퐸푇 푅푢
]
푆퐴푂퐻
푅푎푡푒 =
=
푅푢
-
푏
푘−푃푇
푎
2
)
0
푖
−(휆 +∆퐺
푖
4휋3
ℎ2휆푖푘퐵
푇 |퐻푎푏,ꢀ|2푒
(푖 = 퐸푇푏 표푟 푃퐶퐸푇)
(10)
4휆 푘
푇
퐵
푖
푘ꢀ = √
+•
+ 휔 퐷+•퐴−• − 휔 퐷퐴 − ∆퐺퐸푆
)}
(11)
(12)
{ [
( ) (
∆퐺° = 푁퐴 푒 퐸° 퐷 /퐷 − 퐸° 퐴/퐴
−•)]
(
)
(
2
2
[ ( ) ( ) ]
푧 퐴 −푧 퐷 −1 푒
[
]
∆푧−1 푒
휔 =
=
4휋휀0휀푟푎
4휋휀0휀푟푎
Scheme 4. Proposed PTET reaction mechanism within the photoexcited ion pairs.
electron-transfer, proton-transfer reaction is unfavorable,
because the driving force for electron transfer from the
protonated phenol would be uphill or have a small favorable
driving force in the case of Ru-Bpz. Therefore, the possibility
that the ES-PCET, mechanism follows a stepwise proton
transfer, electron transfer mechanism was investigated,
depicted in Scheme 4.
To start, the Gibbs free energy change for the overall PCET
reaction within the ion pair, ΔG0PCET, was estimated through
Equation 5,19 Eapp is the apparent reduction potential for the
PCET and encompasses the driving force for proton transfer and
the salicylate reduction potential, and is reported in Table 4.
This overall driving force can be broken into a sum of the driving
force for the initial PTa between the phenolic-OH and
carboxylate, ΔG0PT,a, and secondary ETb from the phenolate to
estimated as 2.5 x 10–4 eV (2 cm–1), indicative of a non-adiabatic
electron transfer. For comparison, a fit to kPCET vs. –ΔG0PCET using
equation 10 (i = PCET) resulted in a very poor fit (Figure 10B); a
clear indication that
a concerted mechanism was not
operational under our conditions. No turnover of the kinetics to
a Marcus inverted barrier was discernible. Note that the data
are first-order rate constants within the ion pairs that are not
limited by diffusion. Nevertheless, the minimal curvature in the
plots reduces the accuracy of the fit. This may impact the true
values of
 and Hab, but their magnitudes should be reasonable
approximations.
The other three ruthenium complexes were also quenched
through the same mechanism with the most electron donating
R-SA– derivatives, OH-
, OMe-, and Me-, Figure 10A. However,
the estimated kET,b deviated from the Marcus curve defined by
Ru-Bpz, with most of the rate constants larger than would be
expected. The reason for this deviation is unclear, however we
believe it comes about due to the difference in the localization
of the excited electron in the photoexcited ruthenium
complexes. As shown in Scheme 3, the excited state of Ru-Bpz
localizes the excited electron on the ancillary Bpz ligand away
the ruthenium excited state, ΔG0
.
The estimated
tautomerization equilibrium constant, KEQ,Taut, Table 1, was
related to ΔG0 through Equation 6. Subtraction of ΔG0
ET,b
PT,a
PT,a
from ΔG0
provided the driving force for the excited-state
PCET
electron transfer (ΔG0ET,b), Equations 7 and 8. To elucidate the
electron-transfer rate constants, the pre-equilibrium
approximation was used to develop a rate law based on Scheme
4, Equation 9. Through this assumption, the kPCET measured via
time-resolved photoluminescence is equal to KEQ,Taut
thus simple division provided kET,b
 kET,b, and
.
from the direction of charge transfer. Whereas, for Ru-Bpy
,
A plot of kET,b vs. ΔGET in Figure 10A showed the increase in
rate constant with increasing driving force for electron transfer
in the normal to near barrierless region as described by Marcus
theory.40,76 Ru-Bpz provided the most complete data set (7
points) and these data provided a reasonable fit to Marcus
theory, Equation 10.40,43 The fit, dashed line in Figure 10A,
allowed the two variable parameters of the Marcus equation,

and Hab, to float. All constants were fixed to their known values
and the temperature was fixed as 298 K (room temperature).
Thus, the fit is rather well-defined by the curvature and the
approximate rate maximum. From this fit a reorganization
energy, λ, of 1.0 eV (8100 cm–1) was approximated. This is close
to the expected ~ 1.0 eV for electron transfer with ruthenium
polypyridyl compounds.77 The electronic coupling, Hab, was
Figure 10. A) Relationship between the rate constants for ETb within the ion pair and the
driving force for electron transfer. B) Relationship between the observed PCET rate
constant and driving force for PCET (note the Y-axis spans a much smaller range than in
10A). Dashed lines are best fits to the Marcus equation with a Y2 weighting restricted to
the data points for Ru-Bpz (purple, squares). Note, the fit is significantly better for ET
over PCET.
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Ru-Dtb, and Ru-OMe the electron is localized on the cationic constant (VCPET2) is approximately equal to the coVmiewbiAnrtaictleioOnnlionef
DOI: 10.1039/C9SC04941J
tmam ligand directly between the bound salicylate and the the electronic coupling and overlap between the proton
ruthenium. At a first glance this difference in localization should vibrational wavefunctions, VCPET2 ≈ VET2 × SPT.15 The present case
slow the electron transfer as the electron localized on the tmam is extreme to which the electron transfer can be thought of as
ligand would repel the anionic salycilate,78 lengthening the gated by internal proton transfer within salicylate. Thus, we
electron transfer distance. It may also be expected that electron were able to deconvolute the electron-transfer rate constants,
transfer across a reduced bipyridine ligand (by super-exchange and in turn estimate the electronic coupling constant for
or hopping)79 would be less favorable, due to the high energy of electron transfer (VET, also referred to as HAB). This
the (virtual) intermediate with two electrons added to the non-adiabaticity of the electron transfer does not indicate that
ligand. However, facile charge transfer across reduced the coupling between the electron and proton is weak.83–85 This
bipyridine ligands has been observed in the case of iridium31 coupling is intrinsic to the coupling of electron transfer to
complexes and therefore, we suggest that the orientation of the proton transfer within the salicylate. In studies of the PCET
acceptor Ruthenium d-orbitals relative to the salicylate, which between TEMPOH and carboxylates covalently attached to
should not depend on the 3MLCT localization, are more likely to ruthenium, a CEPT was found even when the electron acceptor
matter than the charge on the bipyridine ligand.
and proton acceptor were separated by > 10 Å, i.e. a very small
2 84,85
Another factor that this difference in electron localization VeT
.
The proton transfer is strongly coupled to electron
would affect is the electrostatic work terms, ω, for the electron- transfer within TEMPOH and thus, even with little coupling
transfer reaction. These work terms account for the free energy between the carboxylate proton acceptor and ruthenium
needed to bring the reactants together and to separate the electron acceptor, the mechanism followed a CEPT. A similar
products and should be included in the approximation of the case can be made for the PTaETb mechanism determined for the
driving force for electron transfer, and through extrapolation a oxidation of salicylate in acetonitrile. The electron transfer from
PCET, Equation 11, where D and A are the electron donor and salicylate is dependent upon the internal proton transfer that
acceptor reactants, NA is the fundamental charge, and e is the reveals a strongly reducing phenolate. Therefore, while the
elementary charge.56,80 These work terms can be simplified to outer-sphere electron transfer to ruthenium is non-adiabatic,
equation 12, where z is the charge of the reactant, ε0 is the the electron transfer is strongly coupled to the internal proton
vacuum permittivity, εr is the solvent dielectric, and a is the transfer. This is reflected in the large shift of pKa upon oxidation
distance between the donor and acceptor. In this study, the of the phenol, and conversely a large shift of E0 upon
difference in charge is 3+, which means the work term is deprotonation. Thus, the driving force for CEPT is larger than for
positive, decreasing the overall driving force for the PCET the initial steps of ETa and PTa, which tends to favor CEPT. On
reaction. The variation between Ru-Bpz and the other the other hand, CEPT requires tunneling of both electron and
complexes may come from the difference in charge distribution proton in the transition state that may have a lower probability
in the excited state. For Ru-Bpy
decrease in positive charge near the salicylate and along the
,
Ru-Dtb, and Ru-OMe, the than single tunneling of either electron or proton.
The PTaETb oxidation of salicylate reported here comes
electron-transfer vector due to electron localization on the about due to the appearance of the proton transfer tautomer
tmam ligand would lower the magnitude of the work term and of the strong internal hydrogen bond. Many groups have
increase the overall driving force for the reaction, shifting the studied the oxidation of phenols with internal hydrogen
data for those three complexes to more negative ΔG values in bonds.84,86 The ability to tune the structure, proton transfer
Figure 10A. The reverse occurs for Ru-Bpz. Therefore, the distance, and hydrogen bond strength make these systems
ΔG0
for Ru-Bpy, Ru-Dtb, and Ru-OMe is underestimated valuable for fundamental studies. We used salicylate as the
ET,b
relative to Ru-Bpz and the rate constants deviate from the fit. known, strong internal hydrogen bond was hoped to favor H+
Typically, a spherical approximation of the reactants and tunneling (due to a large SPT) and potentially a CEPT mechanism,
products could provide a reasonable estimate for the work as has been proposed by others under aqueous conditions.19,20
terms.55 However, for the Ru-LL compounds studied this would However, the present work shows that these strong hydrogen
not differentiate the two excited states. Due to the complexity bonds may also favor proton transfer and a PTaETb mechanism.
of charge distribution in the ruthenium complexes and The appearance of a low-energy absorption that was assigned
salicylate derivatives, developing
a reasonably accurate to the ground-state proton-transfer tautomer in the salicylate
approximation for the electrostatic work terms is derivatives studied here aligns with a model proposed by
difficult.46,73,81,82 However, within a single ruthenium complex, Limbach and co-workers who investigated the same
such as Ru-Bpz, these work terms are not be expected to vary phenomena in intermolecular hydrogen bonds between
significantly, and as such we chose to model solely the Ru-Bpz phenols and carboxylates. They defined the “localized charge
data as it provided the largest uniform series, Figure 10A.
The weak electronic coupling constant measured for the polarity induces proton transfer in the sense that charge is
electron-transfer step of the PCET reaction implies transferred toward the acceptor less capable of charge
solvation” concept, which states that “an increase in the solvent
a
non-adiabatic electron transfer. This is expected for an delocalization.”51 Furthermore, they state that aprotic,
outer-sphere electron transfer reaction from salicylate to the non-polar solvents are better at stabilizing large, delocalized
excited ruthenium complex. In overall non-adiabatic concerted anions such as a phenolate vs. small, localized charges such as a
proton-coupled electron transfer reactions, the CEPT coupling carboxylate. Stated another way, the proton localization along
10 | J. Name., 2012, 00, 1-3
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the hydrogen bond between the salicylate phenol and underwent concerted forward PCET in the normalVrieewgiAortnicleoOf ntlhinee
DOI: 10.1039/C9SC04941J
carboxylate oxygens is determined by the difference in Marcus parabola and inverted region kinetics for the back
stabilization energy between the two anionic groups in the reaction.9,30 An important feature in these systems was the
solvent of interest. Therefore, while the equilibrium constants necessity of covalently linking all three components of the PCET
measured in acetonitrile were ≤ 1, if salicylate were to be reaction. Ion pairing offers a potential way to remove this
dissolved in an even more non-polar solvent such as DCM, the limitation while maintaining a first-order reaction. This ability of
equilibrium would shift further toward the tautomeric form and ion-pairs to reduce the reaction order and overcome diffusion
the equilibrium constants increase. In the case where no without synthetic difficulty of covalent bonds or need of linked
tautomer was observed, water, it was possible to estimate the hydrogen bonds has implications not only in fundamental
driving force for intramolecular proton transfer through the mechanistic studies, but also in applications toward solar
difference in the pKa between the unsubstituted phenol and fuels1,4,93,94 and photosensitized organic synthesis.8,95
benzoate (ΔGPT,a = +340 mV in water).18–20 In acetonitrile this is
not the case as the pKas measured for the unsubstituted phenol
and benzoate do not account for the free energy change
Conclusions
associated with the competitive stabilization of the phenol vs.
the carboxylate within the conjugated salicylate. However, the
ability to measure the tautomer equilibrium constant provides
an alternative means to access the driving force for proton
transfer which was found to be on average an order of
magnitude lower than that in water (ΔGPT,a +55 mV in
acetonitrile for H-SA–). This order of magnitude decrease in the
proton transfer driving force is a key factor in the disparate
mechanism of salicylate oxidation in acetonitrile vs. water.
Work by the Hammarström group on tungsten hydride
compounds has detailed a similar change in mechanism with
proton transfer driving force. In the study of tungsten hydrides
with an external pyridine base, i.e. an intermolecular hydrogen
bond, the reaction mechanism proceeded through an electron
transfer limiting ETaPTb mechanism. However, when the same
pyridine base was appended to the tungsten hydride to form an
intramolecular hydrogen bond, the mechanism could proceed
as either a CEPT or, in the case of weak oxidants, a
pre-equilibrium PTaETb mechanism.12,87 Taken together with our
present results suggests that while stronger H-bonds may
facilitate PT, this does not just favor the CPET mechanism, and
instead, enhancing PT also promotes a PTaETb mechanism. This
concept is an important principle to fundamental catalyst
design where secondary sphere modifications aimed at
facilitating proton transfer to favor a concerted reaction
pathway must account for the stepwise route.
In summary, we provide a systematic, spectroscopic ES-
PCET mechanistic study that occurs within a photoexcited,
coulombic ion pair. These ion pairs formed readily between
cationic ruthenium complexes and anionic salicylate derivates
in CH3CN solution. The use of ion-pairing to preassociate the
photosensitizer and salicylate reduced the reaction order from
2 to 1, which provided unimolecular rate constants for the ES-
PCET reaction. Kinetic experiments on a series of ruthenium
complexes and salicylate derivatives provided a clear curvature
in a plot of PCET rate constants vs. the driving force for PCET.
Correcting for a pre-equilibrium ground state tautomerization
within the salicylate provided electron-transfer rate constants
near the Marcus barrierless region, one of the few reported
cases where this relation has been found. The ability of ion pairs
to reduce reaction orders for complex multicomponent systems
has applications throughout chemistry and is commonly used in
supramolecular applications. The generality provided by
electrostatic interactions has seen limited use in PCET and this
study provides a clear extension of this methodology towards
fundamental PCET investigations and the ready expansion of
this concept to solar fuel and organic photosynthetic
applications will be of great interest.
Conflicts of interest
There are no conflicts to declare.
This work highlights the breadth with which ion pairs may
facilitate the study of ES-PCET. Without the need to ensure
either the ruthenium complexes or salicylates were covalently
connected or had directing hydrogen bonding functionality 28
combinations of ion pairs could be investigated with 13
providing measurable ES-PCET reactivity. The use of ion pairs
facilitated direct measurement of the PCET rate constants and
evaluation of the PCET mechanism for salicylate oxidation in
organic media. Furthermore, the rate constants for electron
transfer within the PCET reaction were found to fall within in
Acknowledgements
The authors would like to acknowledge Prof Jillian Dempsey and
Dr. Christopher Lennox for the gift of the photometric base.
WBS acknowledges support from the NSF Graduate Research
Fellowship Program (Grant DGE-1650116), the NSF Graduate
Research Opportunities Worldwide (2017), and the Swedish
Research Council (Grant no. 2017-05784, grant holder L.H.).
the
normal
to
near-activation-less region of Marcus’ parabola. Most
systematic studies of PCET mechanisms have reported rate
constants within the linear regime of Marcus theory.88–92 Only
recently has clear evidence for the Marcus inverted region for
concerted PCET been disclosed, in which Mayer and coworkers
reported a series of covalently linked donor-acceptor dyads that
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