Photocatalytic Chemoselective C-C Bond Cleavage at Room Temperature in Dye-Sensitized Photoelectrochemical Cells

Article abstract of DOI:10.1021/acscatal.1c00198

Selective cleavage of C-C bonds can be a valuable tool for various applications including polymer degradation and biomass utilization. Performing chemical transformations involving C-C bond cleavage steps under mild conditions and ambient temperature remains challenging due to the high dissociation energies of the C-C bond. This fundamental challenge can be solved by coupling a dye-sensitized photoelectrochemical cell (DSPEC) system, that generally targets the water splitting reaction, with a hydrogen atom transfer (HAT) mediator (HAT-DSPEC). Here, we report the solar-driven selective cleavage of the C(aryl)-C(alkyl) σ-bond in lignin at ambient temperature using an HAT-DSPEC under redox-neutral conditions. The photocatalyst (bis-2,2′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid)Ru(II) (RuC) adsorbed onto a TiO2 nanorod array with the length of ～1.6 μm and a rod diameter of 100 nm atop fluorine-doped tin oxide (FTO|TiO2 NRAs|RuC) film was prepared and investigated with an HAT mediator, 4-acetamido 2,2,6,6-tetramethylpiperidine-1-oxyl (ACT), in solution. Photophysical and electrochemical studies of RuC and ACT with a lignin model compound, 1-(4-hydroxy-3,5-dimethoxyphenyl)-2-(2-methoxyphenoxy) propane-1,3-diol (LMC) reveal that the metal-to-ligand charge transfer (MLCT) excited states from the RuC are efficiently quenched in the presence of ACT with LMC. The HAT-DSPEC photoanode, containing the surface-bound photocatalyst RuC at the photoanode with ACT and LMC in solution, sustained an excellent photocurrent density, significantly outperforming that with the photocatalyst RuC alone. Moreover, the chemoselective cleavage of the C(aryl)-C(alkyl) bond in the LMC at the ambient temperature was demonstrated in the HAT-DSPEC system with a remarkable photocatalytic turnover number (>3000) leading to excellent selectivity (>90%) of C-C bond cleavage under AM1.5G irradiation (1 sun, 100 mW cm-2). These results were obtained over short reaction times and mild, redox-neutral reaction conditions without the need for extended reaction time (e.g., >24 h) or high temperature that is typical of homogeneous catalytic systems. This is the first report to demonstrate that an HAT-DSPEC can serve as a viable method for performing visible-light-driven selective C-C bond cleavage at ambient temperature.

Full text of DOI:10.1021/acscatal.1c00198

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Research Article
Photocatalytic Chemoselective C−C Bond Cleavage at Room
Temperature in Dye-Sensitized Photoelectrochemical Cells
Shuya Li, Saerona Kim, Andrew H. Davis, Jingshun Zhuang, Eric Wolfgang Shuler, Debora Willinger,
Jae-Joon Lee, Weiwei Zheng, Benjamin D. Sherman, Chang Geun Yoo, and Gyu Leem*
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ABSTRACT: Selective cleavage of C−C bonds can be a valuable
tool for various applications including polymer degradation and
biomass utilization. Performing chemical transformations involving
C−C bond cleavage steps under mild conditions and ambient
temperature remains challenging due to the high dissociation
energies of the C−C bond. This fundamental challenge can be
solved by coupling a dye-sensitized photoelectrochemical cell
(DSPEC) system, that generally targets the water splitting reaction,
with a hydrogen atom transfer (HAT) mediator (HAT-DSPEC).
Here, we report the solar-driven selective cleavage of the C(aryl)−
C(alkyl) σ-bond in lignin at ambient temperature using an HAT-
DSPEC under redox-neutral conditions. The photocatalyst (bis-
2,2′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid)Ru(II)
(RuC) adsorbed onto a TiO2 nanorod array with the length of ∼1.6 μm and a rod diameter of 100 nm atop fluorine-doped tin
oxide (FTO|TiO2 NRAs|RuC) film was prepared and investigated with an HAT mediator, 4-acetamido 2,2,6,6-
tetramethylpiperidine-1-oxyl (ACT), in solution. Photophysical and electrochemical studies of RuC and ACT with a lignin
model compound, 1-(4-hydroxy-3,5-dimethoxyphenyl)-2-(2-methoxyphenoxy) propane-1,3-diol (LMC) reveal that the metal-to-
ligand charge transfer (MLCT) excited states from the RuC are efficiently quenched in the presence of ACT with LMC. The HAT-
DSPEC photoanode, containing the surface-bound photocatalyst RuC at the photoanode with ACT and LMC in solution, sustained
an excellent photocurrent density, significantly outperforming that with the photocatalyst RuC alone. Moreover, the chemoselective
cleavage of the C(aryl)−C(alkyl) bond in the LMC at the ambient temperature was demonstrated in the HAT-DSPEC system with
a remarkable photocatalytic turnover number (>3000) leading to excellent selectivity (>90%) of C−C bond cleavage under AM1.5G
irradiation (1 sun, 100 mW cm−2). These results were obtained over short reaction times and mild, redox-neutral reaction conditions
without the need for extended reaction time (e.g., >24 h) or high temperature that is typical of homogeneous catalytic systems. This
is the first report to demonstrate that an HAT-DSPEC can serve as a viable method for performing visible-light-driven selective C−C
bond cleavage at ambient temperature.
KEYWORDS: heterogeneous photocatalysis, bond cleavage, solar energy, lignin, dye-sensitized photoelectrochemical cell,
hydrogen atom transfer
source for the production of low-molecular-weight aromatic
chemicals (e.g., catechol) that serve as feedstocks for the
chemical industry.14 Recent research for lignin valorization is
focused on chemical strategies for depolymerization affording
high selectivity and productivity under mild reaction
conditions, to enable the formation of well-defined and
targeted aromatic chemical building blocks. Several electro-
INTRODUCTION
■
Selective cleavage of C−C bonds is a useful chemical
transformation in organic synthesis and the chemical
industry.1−3 This transformation is integral for the depolyme-
rization of lignin, a sustainable and renewable feedstock.1,4−6
Lignin is the largest noncarbohydrate component in
lignocellulosic biomass and features aromatic functional groups
distributed within a highly branched macromolecule structure,
covalently linked by aliphatic ether bonds.7 The carbohydrate
and/or aromatic-rich fraction is converted to biofuel through
fermentation.8−10 Thus, the conversion of lignin into valuable
low-molecular-weight chemicals has been considered as a
marketable application in the pulping and paper industry.11−13
It also represents a renewable alternative to petroleum as a
Received: January 14, 2021
Revised: February 27, 2021
Published: March 11, 2021
https://dx.doi.org/10.1021/acscatal.1c00198
© 2021 American Chemical Society
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Figure 1. Illustration of a mesoporous structured TiO2 type DSPEC containing a Ru-based chromophore−water oxidation catalyst assembly system
for solar-driven water oxidation (previous studies) and a one-dimensional TiO2 nanorod type dye-sensitized photoelectrochemical cell incorporated
with a hydrogen atom transfer mediator (HAT-DSPEC, current approach). The TiO2 nanorods (TiO2 NRAs) grown on fluorine-doped tin oxide
(FTO) glass substrate (FTO|TiO2 NRAs) followed by the immobilization of photocatalyst RuC were used as the photoanode. The HAT-DSPEC is
a single compartment three-electrode cell with the photoanode, platinum wire, and nonaqueous Ag/Ag+ electrode incorporated with the soluble
hydrogen atom transfer mediator in nonaqueous solvent. The β-aryl ether linkage containing a phenolic group is shown in a representative lignin
fragment. In the conventional route a, Caryl−Cα bond cleavage proceeds under high temperature and/or extended reaction time using a
homogeneous metal catalyst.
chemical and photochemical reactions have been performed
for lignin depolymerization that proceed under mild
conditions.15−20 For example, the Stephenson group has
reported a highly selective two-step (alcohol oxidation
followed by reductive cleavage) depolymerization of native
lignin at ambient temperature via electrochemical and
photochemical approaches under mild conditions.16 In 2020,
we reported a photochemical system for the initial oxidation of
the secondary benzylic alcohols in lignin model compounds
and real lignin as part of a selective two-step process prior to
C−O bond cleavage in the lignin backbone.21
Environmentally sustainable and renewable energy tech-
nologies require more widespread use of solar energy. Dye-
sensitized photoelectrochemical cells (DSPECs) offer a
method of using renewable solar energy to drive energetically
demanding chemical conversions.22,23 Meyer, Schanze, and co-
workers have developed a DSPEC for solar-driven water
oxidation or hydrogen production for the generation of solar
fuels.22,24−27 DSPECs utilize n- or p-type mesoporous
structured semiconductor oxides with surface-bound molecular
chromophore−catalyst assemblies to carry out water oxidation
or water/proton reduction. Though significant progress has
been made in the past decade, light-driven water splitting
remains a major challenge because of the short-term stability of
molecular light-harvesting materials and the thermodynamic
and/or kinetic barriers for the required chemical reactions in
aqueous media under solar light illumination.28 Thus, in light
of the challenges facing DSPECs for solar fuels, our previous
study explored applying a conventional mesoporous TiO2 type
DSPEC with a hydrogen atom transfer (HAT) mediator in the
presence of a base in nonaqueous media for selective oxidation
of 2° benzyl alcohol moieties in lignins.21 This approach
requires a second step to complete the C−O/C−C bond
cleavage with additional photosensitizers. Moreover, the C−C
bonds cannot be fully oxidized electrochemically or photo-
chemically at room temperature.29 Specifically, examples of the
direct bond cleavage of Caryl−Cα at room temperature are rare
and require elevated temperature (e.g., 80 °C) and/or
extended reaction periods (>40 h), and the presence of a
catalyst.30,31 However, despite this progress, no heterogeneous
photocatalytic systems for the cleavage of the aryl α-carbon
bond in lignin substrates have been reported using a DSPEC
system at ambient temperature without the need of base or
acid.
This report presents a heterogeneous photocatalytic process
to cleave the Caryl−Cα bond in a phenolic lignin model
compound using a one-dimensional TiO2 nanorod array type
DSPEC coupled with a HAT mediator (HAT-DSPEC, Figure
1). This system operates under solar light irradiation, at
ambient temperature, and in an open-air environment without
the need of additional base or acid. Previously, mesoporous
surfaces prepared from TiO2 nanosphere sol−gels are most
commonly used for dye-sensitized photoanode (Figure 1,
previous studies);32 however, TiO2 nanorod array electrodes
were selected for this study because the one-dimensional
morphology with a high aspect ratio of the crystalline TiO2
nanorod arrays (TiO2 NRAs) can facilitate charge transport
through a direct electrical path to the conductive fluorine-
doped tin oxide (FTO) surface.33 The HAT-DSPEC
incorporates TiO2 nanorod arrays on FTO (FTO|TiO2
NRAs), a polypyridyl Ru photocatalyst containing dicarboxylic
acid anchoring groups [RuC, (bis-2,2′-bipyridine)(2,2′-bipyr-
idine-4,4′-dicarboxylic acid)Ru(II)], and 4-acetamido-2,2,6,6-
tetramethyl-1-peperidine-N-oxyl (ACT) as an HAT mediator.
Polypyridyl Ru(II) complexes have been extensively used as
light-harvesting units for dye-sensitized solar cells, in artificial
photosynthesis for solar fuels, as well as for in vitro cellular and
in vivo applications due to their excellent photophysical
properties such as long excited-state lifetimes and high
absorptivity in the visible spectrum from a strong metal-to-
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Figure 2. (a) Absorption and emission spectra of RuC (black lines) and RuC/ACT (red lines) in acetonitrile (ACN). Absorption and emission
spectra were normalized to the λmax value. Emission spectra were acquired with excitation at λex = 410 nm. (b) Emission quenching for RuC with
ACT (c = 0−1.2 mM) in ACN solution. (c) Comparison of emission lifetimes of RuC, with ACT and both ACT and LMC in solution. (Inset: the
time-resolved emission traces are shown.) (d) Stern−Volmer plots (I0/I vs [Q], Q: ACT, LMC, or ACT and LMC) for emission quenching of
RuC with LMC (black triangle), ACT (red square), and both ACT and LMC (blue dots) by monitoring the emission intensity at 667 nm (λex
=
410 nm).
ligand charge transfer (MLCT) transition.34−38 ACT, an
organic nitroxyl radical, is used as the mediator for oxidation of
benzylic alcohol functional groups in lignin due to its high
nitroxyl/oxoammonium redox potential and chemical stabil-
ity.20 Figure 1 represents the chemoselective Caryl−Cα bond
cleavage in a β-aryl ether dimeric lignin model compound
(LMC) by the HAT-DSPEC with solar light illumination at
ambient temperature under redox-neutral conditions. This
LMC structure containing the most abundant interunit linkage
in native lignin is used as a representative lignin model
compound.
lifetime measurements were carried out in the presence of an
increasing concentration of ACT (ranging from 0 to 1.2 mM)
in ACN solution at room temperature. Figure 2b shows that
emission from RuC* was efficiently quenched by the presence
of ACT revealing a Stern−Volmer quenching constant value
(Ksv) ∼1.56 × 103 M−1. The Stern−Volmer plot for emission
quenching of RuC with ACT is shown in Figure 2d. Our
previous work has demonstrated the quenching of RuC in the
presence of both hydrogen transfer agent N-hydroxyphthali-
mide (NHPI), a related nitroxyl HAT species, and the base
2,6-lutidine.21 It is interesting to note that the quenching of
RuC* emission with ACT shows a ∼3-fold amplification in
quenching efficiency without the need for a base compared to
that of the NHPI/base pair. This enhanced quenching effect of
ACT with photoexcited RuC* can lead to an excellent
photocatalytic performance in the HAT-DSPEC system under
visible light illumination. Moreover, as shown in Figure 2c, the
average emission lifetimes for RuC* in the absence and
presence of ACT were 1143 and 431 ns, respectively, in ACN
solution. The emission lifetime of RuC* in the presence of
ACT decreased significantly which is consistent with the above
quenching experiments. These observations present strong
evidence for electron transfer between photoexcited RuC* and
ACT under visible light illumination, resulting in the formation
of RuC(III) or/and oxidized ACT.
RESULTS AND DISCUSSION
■
The initial study focused on examining the photophysical
properties of RuC with ACT in acetonitrile (ACN) solution
and at the interface of an FTO|TiO2 NRAs electrode. The
UV−vis absorption and photoluminescence emission spectra
of RuC indicate a strong MLCT absorption band with λmax
∼460 nm and a distinguished emission with λmax ∼656 nm
3
arising from the MLCT excited state (black solid lines) in
Figure 2a. ACT displays a similar absorption band with λmax
∼460 nm and an overlapped emission band with λmax ∼646 nm
in Figure S1. After the addition of ACT to the RuC solution,
the absorption near 460 nm slightly increases due to the
additive contribution of ACT in this spectral range; however,
the emission intensity from the excited-state RuC* decreases
substantially in the presence of ACT (red solid line in Figure
2a). This observation suggests efficient charge transfer from
photoexcited RuC* to ACT. To demonstrate the effect of
ACT on the RuC* emission, quenching and fluorescence
Encouraged by these results, we next investigated the
photophysical properties of RuC and ACT (RuC/ACT) in the
presence of the LMC. Upon the addition of LMC, the
emission lifetime of the RuC/ACT decreases from 431 to 354
ns as shown in Figure 2c. The observed behavior can be
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Figure 3. (a) Incident photon to charge carrier efficiency (IPCE) tests for an FTO|TiO2 NRAs electrode (gray dots) and TiO2 NRAs coated with
RuC (FTO|TiO2 NRAs|RuC) (red dots). The pictures of FTO|TiO2 NRAs electrodes without and with RuC are shown in the inset. (b) CVs of
FTO|TiO2 NRAs|RuC (black), the same with 3 mM ACT (red), and both 3 mM ACT and 8 mM LMC (blue).
explained by the intermolecular energy/electron transfer
process between RuC/ACT and LMC that can drive
photocatalytic oxidation of LMC under visible light illumina-
tion. Figure 2d illustrates the Stern−Volmer plots and lists the
Ksv of RuC/LMC, RuC/ACT, and RuC/ACT/LMC. On the
basis of the lifetime measurement above, the persistence of
RuC* was largely unaffected by the presence of LMC in the
absence of ACT. Increasing the concentration of LMC from 0
to 1.2 mM in the RuC acetonitrile solution, with no ACT
present, showed slight quenching with Ksv ∼0.19 × 103 M−1.
The Stern−Volmer constants were calculated according to eq
1 where I0 is the initial emission intensity of the RuC solution,
I is the emission intensity of RuC in the presence of Q (Q =
ACT, LMC, or ACT and LMC), and [Q] is the quencher
concentration. The steady-state emission spectra of RuC with
increasing ACT/LMC pair concentrations (c = 0−1.2 mM in
ACN solution) are shown in Figure S2. Consistent with the
emission lifetime measurements above, RuC was efficiently
quenched by the ACT/LMC pair, revealing a Ksv ∼2.38 × 103
M−1, in comparison to the quenching of RuC/ACT without
LMC (Ksv ∼1.56 × 103 M−1). From these results, it is
concluded that the presence of LMC regenerates ACT from
oxoammonium ACT+ formed following the photoexcited
energy/electron transfer between RuC(II)* and/or RuC(III)
and ACT.
NRAs’ surface by the interaction of the polar carboxylate
moieties of RuC (FTO|TiO2 NRAs|RuC). RuC was adsorbed
onto an FTO|TiO2 NRAs semiconductor electrode (1.16 mM
RuC in acetonitrile solution for 12 h) as previously
reported.37,41 The MLCT absorption and steady-state emission
of FTO|TiO2 NRAs|RuC film corresponded to a blue-shifted
absorption and emission bands with λmax ∼454 and 643 nm,
respectively, compared to that of RuC in ACN solution
(Figure S4). The surface coverage of RuC on the TiO2 NRAs
was quantified by calculating the molar surface concentration
of RuC according to the previous equation.42 The calculated
surface coverage for RuC onto the TiO2 NRA film with a
thickness of ∼1.6 μm was ∼5.29 × 10−9 mol cm−2 (Table S1).
This surface coverage is consistent with that of polymer-based
Ru(II) chromophores containing carboxylic acid moieties on
mesoporous TiO2 film from our previous study.41
To demonstrate electron injection of photoexcited RuC*
into one-dimensional TiO2 NRAs, incident photon-to-current
efficiency (IPCE) measurements were conducted with the
FTO|TiO2 NRAs electrode before and after the immobilization
of RuC as shown in Figure 3a. The IPCE spectrum showed a
maximum value of ∼10% at ∼460 nm that corresponds to the
MLCT absorption band maximum of the RuC in an FTO|
TiO2 NRAs|RuC film, while a bare FTO|TiO2 NRAs substrate
did not show photocurrent in the visible spectrum. Cyclic
voltammetry (CV) was carried out in acetonitrile solution
containing 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6) to study the electrochemical response of the FTO|
TiO2 NRAs|RuC surface in the dark in the presence or absence
of ACT and LMC with representative voltammograms shown
in Figure 3b. The FTO|TiO2 NRAs|RuC in a neat electrolyte
(black voltammogram in Figure 3b) shows a characteristic
redox couple for the surface-bound RuCIII/II with E1/2 = 1.09 V
vs Ag/Ag+ (Figure 3b, inset). Introduction of ACT to the
solution (red voltammogram in Figure 3b) brought on a new
wave with a more cathodic onset compared to the RuCIII/II
couple beginning around 0.3 V vs Ag/Ag+. This wave shows
diffusional character, and the steady-state current at potentials
>0.7 V vs Ag/Ag+ is dependent on the concentration of ACT
added to the solution. This wave is attributed to the oxidation
of ACT to the oxoammonium species ACT+. A similar wave is
observed with an FTO|TiO2 NRAs surface (no bound RuC),
and therefore, RuC is not implicated in mediating this process
(Figure S5). This result does indicate that applied biases higher
I0
I
= 1 + Ksv[Q]
(1)
Observation of enhanced RuC* emission quenching in the
presence of ACT and LMC in solution indicated the viability
of developing a heterogeneous HAT-DSPEC system with the
overall goal of facilitating light-driven oxidative Caryl−Cα bond
cleavage in lignin at ambient temperature. The FTO|TiO2
NRAs films were prepared by a hydrothermal reaction and
annealed at 400 °C for 3 h according to the previous study.39,40
The top-down and cross-sectional morphologies of FTO|TiO2
NRAs were characterized by scanning electron microscopy
(SEM) (Figure S3). The TiO2 NRAs are perpendicular to the
surface of the FTO substrate. The corresponding average
diameter and length of the TiO2 nanorods were approximately
100 nm and 1.6 μm, respectively. The thickness of the TiO2
NRAs by SEM is consistent with the film height, 1.5 μm,
measured by profilometry (data not shown here). In Figure 1,
RuC is shown covalently immobilized onto the FTO|TiO2
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Figure 4. (a) Photocurrent−time traces measured with the FTO|TiO2 NRAs|RuC electrode (black), with 1 mM (red), 2 mM (blue), and 3 mM
(green) ACT in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile solution with an applied bias of 0.1 V vs Ag/Ag+ under
illumination (AM 1.5G, 1 sun, 100 mW cm−2) with 1 min on/off cycles. (b) Plot of photocurrent vs [ACT]. (c) Photocurrent−time traces of FTO|
TiO2 NRAs|RuC electrodes with 3 mM ACT and increasing LMC concentrations from 0 to 1.5 mM (0.5 mM increments). (d)
Chronoamperograms of the FTO|TiO2 NRAs|RuC electrode with 3 mM ACT in the presence of LMC from 0 to 1 min. The FTO|TiO2 NRAs|
RuC active cell area was 1.14 cm2 for measurements of photocurrent−time traces.
than 0.3 V vs Ag/Ag+ will cause substantial background
current. The addition of LMC with ACT in solution (blue
voltammogram in Figure 3b) introduces a catalytic wave
coinciding with the formation of the oxoammonium species.
The blue voltammogram does not reach a diffusion-controlled
steady state due to the catalytic regeneration of reduced N-oxyl
(R2NO• or N-hydroxyl R2NOH) after reaction with LMC and
subsequent reformation of the oxoammonium ACT+ at the
electrode surface. The FTO|TiO2 NRAs|RuC surface in the
presence of just LMC in solution only shows increased anodic
currents at a higher applied bias (Figure S5) highlighting the
importance of ACT as a hydrogen atom transfer mediator to
activate the oxidation of LMC. While carried out under dark
conditions, these results show that (1) RuCIII is electrochemi-
cally competent to form the active oxoammonium ACT+
species, and (2) the direct electrochemical activation of this
process occurs only at higher applied bias, namely, above 0.4 V
vs Ag/Ag+. An increase in the current at more negative applied
bias under illumination with visible light then indicates
effective photochemical activation of the system.
The photochemical behavior of the HAT-DSPEC photo-
anode in the presence of the nitroxyl radical ACT and ACT
plus the phenolic lignin model compound LMC is shown in
Figure 4. The photocurrent response of the FTO|TiO2 NRAs|
RuC electrode in a nonaqueous electrolyte was examined with
increasing ACT concentration during light on/off cycles under
AM 1.5G illumination (100 mW cm−2), with an applied bias
0.1 V vs Ag/Ag+. As shown in Figure 4a, the FTO|TiO2 NRAs|
RuC with ACT showed a significant increase in maximum
photocurrent up to approximately 190 μA cm−2. This aminoxyl
radical (or the corresponding hydroxylamine) from ACT can
serve as a source of electrons, regenerating the ground-state
Ru(II) photocatalyst following charge injection of RuCII* to
TiO2 and subsequent formation of Ru(III) as the first electron
transfer step in the process.43 Increasing [ACT] from 0 to 2
mM results in a pronounced increase linearly from ∼11 to 160
μA cm−2, and then the addition of 3 mM ACT reaches a
saturation level of photocurrent 190 μA cm−2 shown in Figure
4b. Next, the photocurrent transients were measured in the
presence of LMC under the same photochemical conditions
used for the photocurrents with [ACT] = 3 mM in Figure 4c.
The addition of LMC (from 0 to 1.5 mM) to solution leads to
a remarkable increase in the photocurrent from ∼190 to 690
μA cm−2 measured at the FTO|TiO2 NRAs|RuC photoanode.
The integration of the resulting time/photocurrent traces gives
the total charge passed during the photocurrent measurements.
The photocurrent transients at the FTO|TiO2 NRAs|RuC
photoanode coupled with 3 mM ACT revealed the total
consumed charges of 14, 30, 41, and 49 mC for 0.0, 0.5, 1.0,
and 1.5 mM LMC, respectively (Figure 4d). Compared to the
absence of LMC, this total consumed charge increased by
approximately 250% in the presence of LMC (1.5 mM), is
attributed to effective LMC oxidation in the redox reactions,
and provides the basis of a high photocatalytic turnover
number (PTON) observed in this HAT-DSPEC system. This
observation in the presence of LMC supports the emission
quenching experiment, in which the addition of LMC showed
the highly efficient quenching with RuC/ACT. We assume
that LMC plays an important role of a sacrificial mediator in an
HAT-DSPEC system. Sequentially, this behavior is believed to
proceed by (1) light absorption and e− injection from RuC to
the TiO2 NRA surface, (2) oxidation of ACT to form ACT+ by
RuC(III) reforming the ground-state RuC(II), and (3) HAT
oxidation of LMC by ACT+ regenerating the hydroxylamine
radical R2NO• (or the reduced aminoxyl radical R2NOH).
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Figure 5. (a) Photocatalytic C−C bond cleavage reaction of a phenolic lignin model compound LMC in an HAT-DSPEC at room temperature
(right). Yields and selectivity were determined by 1H NMR analysis of the crude reaction mixtures relative to dimethyl sulfoxide (DMSO) internal
standard. Selective 2° benzyl alcohol oxidation of nonphenolic compounds using a heterogeneous HAT-DSPEC and thermal catalytic method
(left). (footnote a) The published value, 55%, from ref 17 was used for the product yield of the nonphenolic lignin model compound 5. (b)
Aliphatic and aromatic regions of 2D HSQC NMR spectra of the LMC before and after photocatalytic reactions under the standard conditions in
the HAT-DSPEC.
1H−13C heteronuclear single quantum coherence (2D HSQC
The above experiments give a strong indication that an
HAT-DSPEC can serve as a heterogeneous photocatalytic
system under mild aerobic conditions for the selective C−C
bond cleavage of LMC which structurally mimics the abundant
phenolic groups present in lignin.44 To perform the photo-
catalytic conversion of LMC in an HAT-DSPEC, LMC (15.0
mg, 0.044 mmol) and ACT (6.0 mg, 0.028 mmol) were
dissolved in 5.0 mL of acetonitrile containing TBAPF6 (0.2 g,
0.5 mmol). A 5 h continuous illumination experiment was
performed with the photoanode active cell area ∼2.1 cm2
under an applied bias of 0.1 V vs Ag/Ag+ with an AM1.5G
(100 mW cm−2) solar simulator using the aforementioned
NMR) spectra of the starting LMC (Hα at δH 4.72 and δC
72.34 ppm, Hβ peaks at δH 4.33 and δC 84.06 ppm in the
aliphatic region) completely disappeared following the
illumination period (Figure 5b, left). This disappearance can
be attributed to CC and/or CO bond cleavages in LMC.
Moreover, 2D HSQC NMR signals were observed for new
chemical shifts, δH 9.47 and δC 186.18 ppm (Figure 5b, right,
inset), suggesting the formation of aldehyde-containing
monomeric products from LMC. In the aromatic region, the
characteristic signals of 2 and 3 are attributed to CC bond
scission.
To gain more insight into the influence of the RuC
photocatalyst and the need for applied illumination to support
the conversion of LMC to 2 and 3, several control experiments
with product analysis by 2D HSQC NMR spectroscopy were
performed as shown in Figure S7. In these experiments, it was
observed that only ∼10% of the secondary alcohol CαOH
converted to the ketone CαO without light illumination or
RuC present. The observation of a slight production of the
ketone indicates that electrocatalytic oxidation of the 2° benzyl
alcohol CαOH might occur due to the applied bias of 0.1 V
vs Ag/Ag+ used in the photochemical experiments and the
presence of LMC. It is interesting to note that the current of
FTO/TiO2 NRAs with ACT and LMC is increased to 12 μA
compared to that of FTO/TiO2 NRAs with ACT without
LMC in Figure S5. LMC can be a sacrificial mediator and/or
1
conditions. By examination of H and 13C NMR, LMC was
completely consumed, and cleavage products 2 and 3 were
observed after 5 h at room temperature (Figure S6). The
oxidative reaction in an HAT-DSPEC afforded the chemo-
selective CarylCα bond cleavage products 2 (95%) and 2,6-
dimethoxybenzoquinone 3 (87%) (Figure 5a). The yield was
1
determined by H NMR with DMSO as an internal standard.
Previously, we investigated the selective oxidation of the
secondary alcohol CαOH to ketone CαO in the
nonphenolic group of the lignin model compound in a
DSPEC with NHPI/2,6-lutidine as an HAT cocatalyst.8 On
the basis of our previous study, the oxidation of the secondary
alcohol results in the disappearance of Hα and a new chemical
shift due to the conversion of CαOH to CαO in the β-O-4
aryl ether linkages. Very interestingly, the two-dimensional
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Figure 6. (a) Conversion of LMC to major products 2 and 3 monitored by GC-FID with and without 1 sun light illumination over 72 h. (b)
PTON plot as a function of time. (c) Long-term photocurrent density of an HAT-DSPEC with (red shade) and without (gray shade) LMC in 0.1
M TBAPF6 with 3 mM HAT under an applied bias of 0.1 V vs Ag/Ag+ at ambient temperature in an aerobic condition. The active photoanode was
irradiated by an AM 1.5G solar simulator (1 sun, 100 mW cm−2). Illustration (right) comparing the HAT-DSPEC with and without LMC using a
three-electrode cell [working electrode (WE), FTO|TiO2 NRAs|RuC; counter electrode (CE), Pt; reference electrode (RE), nonaqueous Ag/Ag+].
oxidizing agent leading to oxidation of the secondary alcohol
CαOH despite being under a low bias, 0.1 V vs Ag/Ag+.
These control experiments prove that the presence of RuC and
light is essential, and any electrochemical oxidation does not
result in bond cleavage as the Hα and Hβ peaks in the aliphatic
region remain after the illumination period in the control.
Taken together, the examination of photocatalytic LMC in the
HAT-DSPEC system under 1 sun illumination at room
temperature revealed the evidence for cleavage of the CC
bond of aryl ether β-O-4 linkages in LMC. We hypothesized
that the pendant 4-phenolic group in LMC may contribute to
retention times of 14.92 min for 2 and 12.59 min for 3
(Figure S9). DMSO was injected into the mixture solution as
an internal standard. The data show that direct cleavage of the
C−C bond was >95% after the 4 h experiment (Figure 6a).
Interestingly, while product 3 was kept as 93% yield after the 5
h continuous illumination, the product yield of 2 decreased
slightly after 4 h. We assumed that the monomeric
benzoquinone 2 can be further oxidized to a dimeric
benzoquinone by the strong oxidized mediate HAT+ without
the light illumination. However, a 5 to 72 h dark period after
illumination resulted in no net change in the amount of
product 2 and 3 formed in an HAT-DSPEC system, thus
indicating that the reaction selectivity was dependent on the
reaction time and the presence of illumination.
According to the above experiments, the RuC photocatalyst
adsorbed to the TiO2 NRAs plays an important role of photon
absorption and activates the HAT to catalyze the oxidative
cleavage reaction. The photocatalytic activity of RuC, PTON,
which represents the effectiveness of the photocatalyst in this
reaction system, was evaluated by GC-FID. The number of
moles of selective bond cleavage products of 2 and 3 from
LMC was determined and quantified by GC-FID over 5 h.
PTONs at various points during the 5 h illumination
experiment were determined by dividing the average mol
amount of 2 and 3 measured by the mol amount of RuC
adsorbed to the electrode surface. This gave PTON values of
1120, 2360, 3020, 3420, and 3140 for time points of 1, 2, 3, 4,
and 5 h, respectively (Figure 6b). These results suggested that
the HAT-DSPEC system incorporating the heterogeneous
photocatalyst RuC yields an excellent PTON for the oxidative
Caryl−Cα bond cleavage reaction in the phenolic LMC.
Furthermore, the sustained photocurrent of the complete
photoanode was monitored in the absence and presence of
LMC during 5 h of continuous illumination (Figure 6c). The
the unusual photocatalytic cleavage observed of the aryl Caryl

Cα bond. According to the literature, the catalytic oxidative
reactions of the nonphenolic lignin model compound (5, R =
OCH3, R′ = H, R″ = CH2OH) and phenolic lignin model
compound (LMC, R = OCH3, R′ = OH, R″ = CH2OH) were
tested with a vanadium catalyst in the presence of pyridine at
80 °C for 48 h (Figure 5a).30 To confirm this, a nonphenolic
lignin model compound, 2-phenoxy-1-phenylethanol (PP-ol, R
= R′ = R″ = H), was tested under the same photochemical
conditions used for LMC (Figure 5a). Interestingly, no
oxidative cleavage of the CC bond in the β-O-4 aryl ether
linkage was observed though oxidative conversion of secondary
benzylic alcohol, CαOH, to the ketone form CαO (4)
occurred within 8.5 h at ambient temperature as monitored by
a flame ionization detector in gas chromatography (GC-FID)
(Figure S8). We conclude that the presence of a phenoxy
group in the lignin model compound is essential for the CC
bond cleavage.
In order to investigate the irreversible LMC oxidative
cleavage reaction more directly, the LMC oxidized species
present in solution was characterized by a GC-FID. The
selective cleavage products of 2 and 3 were monitored by
comparing the chromatogram peak intensities with the
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Figure 7. Proposed mechanism for Caryl−Cα bond cleavage of LMC by the HAT-DSPEC.
anodic current gradually decreased over the prolonged
illumination time, which is in part attributed to the irreversible
consumption of LMC and/or degradation of RuC on the TiO2
NRA surface during the experiment period.20,21,28
cleavage, and this leads us to infer the importance of the radical
resonance form in Int2. The consumption of an additional 1
equiv of ACT+ and 1 equiv of ACT possibly proceeds through
the pentacyclic intermediate Int3 (step 7). Mechanistic studies
are planned and currently underway to explore the details of
the CC bond cleavage observed and will be the focus of
future reports.
As shown in Figure 7, the above measurements reveal a
possible reaction mechanism for the oxidative CarylCα bond
cleavage following a series of photodynamic events: after
photoexcitation of the surface-bound photocatalyst RuC on
the FTO|TiO2 NRAs (step 1), the excited-state RuC* is
formed. Electron and hole generation (TiO2 NRAs (e−) and
RuC(III), respectively) occurs following electron injection
into the conduction band of TiO2 NRAs (step 2). Upon the
hole transfer from Ru(III) to ACT (R2NO•) mediator in
solution, the ground-state RuC(II) regenerates (step 3).
Oxidation of R2NO• after hole transfer generates the
oxoammonium R2N+O (ACT+), which is a strong oxidizing
agent and the active species for alcohol oxidation of LMC
(step 4). The photochemical formation of ACT+, and the
buildup of a pool of ACT+ near the photoanode surface, then
initiates a series of steps which ultimately lead to the formation
of 2 and 3. Given the structure of each product 2 and 3 as
Recently, Meyer, Schanze, and co-workers reported the use
of a mesoporous structured TiO2 (nanoTiO2) type DSPEC
system containing Ru(II)-based chromophore−water oxida-
tion catalyst assemblies for light-driven photochemical
oxidation.24−26,46,47 For example, light-driven phenol
(PhOH) and benzyl alcohol (BnOH) oxidation experiments
were carried out in a nanoTiO2 type DSPEC under 1 sun
illumination.33 With the similar surface coverage of photo-
catalyst (6.90 × 10−9 mol cm−2) compared to that of RuC
(5.29 × 10−9 mol cm−2) used for the current approach, the
maximum photocurrents in the presence of PhOH and BnOH
were observed to be 11.5 and 8.5 μA cm−2, respectively.
Recently, we tested a lignin model compound in a nanoTiO2
type DSPEC in the presence of the NHPI/base pair under
illumination (2 sun) over 20 h for light-driven benzylic alcohol
oxidation.21 The maximum photocurrent with the surface
coverage of RuC (4.5 × 10−8 mol cm−2) was observed around
145 μA cm−2. Surprisingly, compared to these previous studies,
the photoelectrochemical performance with the TiO2 NRA-
based HAT-DSPEC is remarkably improved for visible-light-
driven oxidation reactions. The photocurrent generated from
HAT-DSPEC with LMC is ∼690 μA cm−2 under 1 sun and
with 5.29 × 10−9 mol cm−2 RuC. Moreover, the present HAT-
DSPEC system exhibits excellent selectivity and PTON under
1 sun illumination at room temperature during a 5 h photolysis
compared to the thermocatalytic approach which only gave
∼55% of product yield using a homogeneous catalyst for C−C
bond cleavage at a reaction temperature of 80 °C over 48 h.30
Therefore, the present work reports, for the first time, the use
of a TiO2 NRA type HAT-DSPEC under ambient conditions
1
determined by H and 13C NMR, GC-FID, and 2D HSQC
NMR and a comparison with model compounds of each, the
overall conversion represents a 6 e−, 4 H+ oxidation of LMC.
The exact mechanistic details are the focus of ongoing study,
and elucidation of the mechanism is outside the scope of the
current work; however, we hypothesize the consumption of 2
equiv of ACT+ and 2 equiv of ACT in the formation of 2 and 3
yielding 4 equiv of R2NOH (ACTOH). One equiv of the
oxoammonium likely oxidizes the γ-hydroxyl group in LMC
(the less sterically hindered primary alcohol), resulting in the
formation of Int1 (step 5).20,45 The phenoxy radical species
(in Int2) in the para position might form following hydrogen
atom abstraction by the R2NO•, and the resulting
semiquinone intermediate is imperative to observe the
products of CC bond cleavage (step 6). Model compounds
that do not contain the para-hydroxyl group do not undergo
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to carry out the photocatalytic selective Caryl−Cα bond
cleavage reactions of a lignin model compound.
Christian University, Fort Worth, Texas 76129, United
States
Jae-Joon Lee − Department of Energy Materials and
Engineering, Research Center for Photoenergy Harvesting &
Conversion Technology (phct), Dongguk University, Seoul
04620, Republic of Korea; orcid.org/0000-0001-8966-
0336
Weiwei Zheng − Department of Chemistry, Syracuse
University, Syracuse, New York 13244, United States;
orcid.org/0000-0003-1394-1806
Benjamin D. Sherman − Department of Chemistry and
Biochemistry, College of Science and Engineering, Texas
Christian University, Fort Worth, Texas 76129, United
States; orcid.org/0000-0001-9571-5065
Chang Geun Yoo − Department of Chemical Engineering,
State University of New York College of Environmental
Science and Forestry, Syracuse, New York 13210, United
States
CONCLUSION
■
An HAT-DSPEC has been fabricated by incorporating a one-
dimensional TiO2 NRA type photoanode containing photo-
catalyst RuC with an HAT mediator. The HAT-DSPEC
system is able to activate oxidative bond cleavage of a model
compound based on a common motif found in lignin and
demonstrates a remarkable photocurrent using 5.29 nmol cm−2
RuC. The system yielded high PTON and excellent selectivity
for Caryl−Cα bond cleavage of LMC under 1 sun illumination,
ambient temperature, and aerobic conditions. Further
optimization of the morphology of TiO2 NRAs and the
structure of photocatalyst RuC and HAT may enable an
increase in photocatalytic activity for light-driven oxidative
cleavage reactions. These studies will be carried out in
upcoming reports. Future work will perform solar-light-driven
degradation of native lignin containing abundant phenolic
groups in a one-dimensional TiO2 NRA type HAT-DSPEC
under mild conditions. This report indicates that this first
approach in a one-dimensional TiO2 NRA type HAT-DSPEC
has expanded the scope of phenolic-containing biomacromo-
lecules or petroleum-based polymer for solar-light-driven
polymer degradation with selective bond cleavage under mild
conditions.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00198
Author Contributions
S.L. conceived the project with G.L. providing overall
supervision. S.K., A.H.D., J.Z., E.W.S., and D.W. contributed
to data acquisition and analysis. J.-J.L., W.Z., B.D.S., and
C.G.Y. added conceptual contributions and edited the
manuscript. All authors contributed to the discussion of the
results presented and editing.
ASSOCIATED CONTENT
■
Notes
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00198.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Details of materials, characterization methods, and
characterization data (PDF)
Financial support from State University of New York, College
of Environmental Science and Forestry (SUNY ESF) is
gratefully acknowledged. W.Z. acknowledges support from
ACS Petroleum Research Fund under Award 59861-DNI5 and
NSF CAREER under Award CHE-1944978. J.-J.L. acknowl-
edge the financial support from the Ministry of Science, ICT &
Future Planning (NRF-2016M1A2A2940912). We also thank
Alexander B. Artyukhin and David Kiemle in the Analytical
&Q Technical Services (A&TS) at SUNY ESF for help with
GC. D.W. thanks the TCU College of Science and Engineering
for financial support of this work through a SERC graduate
research grant. B.D.S. acknowledges start-up funding from
Texas Christian University for support of this work.
AUTHOR INFORMATION
■
Corresponding Author
Gyu Leem − Department of Chemistry, State University of
New York College of Environmental Science and Forestry,
Syracuse, New York 13210, United States; The Michael M.
Szwarc Polymer Research Institute, Syracuse, New York
13210, United States; orcid.org/0000-0003-0169-1096;
Email: gyleem@esf.edu
Authors
Shuya Li − Department of Chemistry, State University of New
York College of Environmental Science and Forestry,
Syracuse, New York 13210, United States
Saerona Kim − Department of Chemistry, State University of
New York College of Environmental Science and Forestry,
Syracuse, New York 13210, United States
Andrew H. Davis − Department of Chemistry, Syracuse
University, Syracuse, New York 13244, United States
Jingshun Zhuang − Department of Chemical Engineering,
State University of New York College of Environmental
Science and Forestry, Syracuse, New York 13210, United
States
Eric Wolfgang Shuler − Department of Chemistry, State
University of New York College of Environmental Science and
Forestry, Syracuse, New York 13210, United States
Debora Willinger − Department of Chemistry and
Biochemistry, College of Science and Engineering, Texas
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ACS Catal. 2021, 11, 3771−3781