PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst

Article abstract of DOI:10.1021/jacs.1c01701

Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfe

Full text of DOI:10.1021/jacs.1c01701

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Article
PCET-Based Ligand Limits Charge Recombination with an Ir(III)
Photoredox Catalyst
Hannah Sayre,∥ Hunter H. Ripberger,∥ Emmanuel Odella, Anna Zieleniewska, Daniel A. Heredia,
Garry Rumbles, Gregory D. Scholes, Thomas A. Moore, Ana L. Moore,* and Robert R. Knowles*
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ABSTRACT: Upon photoinitiated electron transfer, charge recombina-
tion limits the quantum yield of photoredox reactions for which the rates
for the forward reaction and back electron transfer are competitive.
Taking inspiration from a proton-coupled electron transfer (PCET)
process in Photosystem II, a benzimidazole-phenol (BIP) has been
covalently attached to the 2,2′-bipyridyl ligand of [Ir(dF(CF3)-
ppy)2(bpy)][PF6] (dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-
(trifluoromethyl)pyridine; bpy = 2,2′-bipyridyl). Excitation of the
[Ir(dF(CF3)ppy)2(BIP-bpy)][PF6] photocatalyst results in intramolecu-
lar PCET to form a charge-separated state with oxidized BIP. Subsequent
reduction of methyl viologen dication (MV2+), a substrate surrogate, by
the reducing moiety of the charge separated species demonstrates that
the inclusion of BIP significantly slows the charge recombination rate. The effect of ∼24-fold slower charge recombination in a
photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.
the substrate radical/radical ion. This principle was demon-
strated in a spectroscopic study of a photoredox-catalyzed
hydroamidation reaction.3 In this particular reaction, the joint
action of the excited state of an Ir(III) polypyridyl photo-
sensitizer and Brønsted base homolyzes the strong N−H bond
of an N-aryl amide. Although the radical generation in this
system was highly efficient (quenching yield of ∼90%), the
overall quantum efficiency was limited by a majority of the
amidyl radical intermediate undergoing a charge recombina-
tion process. It was then demonstrated that the addition of a
disulfide additive led to the reversible trapping of the amidyl
radical, disfavoring charge recombination pathways and
resulting in a 4-fold improvement in the reaction quantum
yield. The use of additives is an elegant solution to specific
instances of poor quantum efficiency; however, a more general
approach to increasing the lifetime of the reactive radical/
radical ion would be to introduce processes as part of the
catalyst that would slow charge recombination with the
oxidized/reduced photocatalyst. This strategy could expand
the scope of numerous photocatalytic methods that are
currently limited by charge recombination dynamics.
INTRODUCTION
■
Photosynthesis has transformed the Earth and has the potential
to sustainably contribute to global energy resources. One step
toward a sustainable chemical industry would be to develop
solar energy sources for the myriad chemical processes that are
essential to the global chemical enterprise.1 Photoredox
catalysis offers an opportunity to use solar energy to replace
or augment thermal energy and provide the necessary potential
to drive starting materials along their reaction coordinates to
form the desired products. In a photoredox catalytic cycle,
photoinduced electron transfer between photocatalyst excited
state and the substrate promotes the catalytic generation of
reactive radical intermediates under mild conditions. This has
resulted in the widespread adaptation of photoredox protocols
into a variety of catalytic contexts, enabling chemistry that is
often unattainable using classical conditions and reagents.2
However, many transformations in contemporary photo-
redox catalysis are limited in either scope or quantum
efficiency by charge recombination between the nascent radical
(or radical ion) and the oxidized/reduced form of the
photocatalyst.3,4 Charge recombination (CR) following excited
state electron transfer in bimolecular processes often proceeds
at a rate close to the diffusion limit (Figure 1A).5−7
Accordingly, the rate of the forward reaction must be
sufficiently fast to be kinetically competitive for a substantial
yield of product. One strategy to favor the product-forming
pathway over recombination would be to introduce processes
that slow recombination and therefore increase the lifetime of
Received: February 11, 2021
Published: August 11, 2021
https://doi.org/10.1021/jacs.1c01701
J. Am. Chem. Soc. 2021, 143, 13034−13043
© 2021 American Chemical Society
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Figure 1. (A) Charge recombination (CR) in photoredox catalysis can be competitive with the forward reaction to produce product B from
substrate A. (B and C) Precedent bioinspired constructs bearing a benzimidazole-phenol (BIP) platform. (D) Structural modification of an iridium-
based photocatalyst with BIP can limit charge recombination.
Our design strategy is inspired by photosynthesis, where
nature employs a series of redox relays to transfer the oxidizing
(h+) and reducing (e−) equivalents generated upon photo-
excitation and charge separation to their respective catalytic
sites. These redox relays are the essential components of a
series of short-distance, fast redox-equivalent transfer steps that
effectively compete against charge recombination. They
promote the efficient (high-yield) transport of redox
equivalents over nanoscale distances by optimizing both the
Marcus-type electronic coupling and thermodynamic factors so
that the forward transfer of redox equivalents is favored over
recombination at each step.8−10 An illustrative example of such
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a redox relay is the hydrogen-bonded tyrosine-histidine (Tyrz-
H190) pair located in Photosystem II (PSII).11 Sunlight
generates the excited state of the reaction-center (P680*) in
PSII, which then reduces a nearby chlorophyll (Chl), forming
an intermolecular charge-separated state, P680•+-Chl•−. This
high energy state is responsible for the light-driven electron
transfer reactions needed for water splitting, but it is
simultaneously susceptible to charge recombination.12,13 To
avoid recombination and to move the oxidizing equivalent
closer to the site of water oxidation, Tyrz, with its hydrogen-
•+
bonded partner H190, rapidly fills the hole on P680 using a
proton-coupled electron transfer (PCET) process.
Figure 2. Chemical structures of cyclometalated Ir(III) photocatalysts
in this study.
Inspired specifically by the function of the Tyrz-H190 pair in
PSII, model systems have been developed for charge transport,
utilizing charge-separated states.14,15 These systems include a
benzimidazole-phenol (BIP, see Figure 1B) platform which
features PCET mimicking the Tyrz-H190 pair.16−18 By
incorporating the BIP as an electron transfer mediator, Zhao
et al. have shown an improvement in the quantum yield for
water splitting using dye-based systems, which is generally low
because the charge recombination reaction is faster than the
catalytic four-electron oxidation (Figure 1B).19
recombination in the intermolecular charge separated state of
oxidized photocatalyst 2+ and MV•+, when compared to an
Ir(III) photocatalyst without an appended BIP (photocatalyst
1). These results are then extended into preparative-scale
photoredox catalysis, where quantum yield enhancement is
observed with photocatalyst 2 in the reduction of redox-active
N-hydroxyphthalimide esters (NHPI esters).
RESULTS AND DISCUSSION
■
In the design of PSII mimics, the incorporation of a
hydrogen-bonded, phenol-imidazole pair into the ligand
framework of a heteroleptic Ru(II) photosensitizer was
shown to similarly impede charge recombination (Figure
Synthesis. The Ir(III) photocatalysts 1 and 2 were
synthesized in accordance with previous reports.23,24 We
hypothesized that the electron-withdrawing ppy ligands were
necessary for efficient oxidation of the BIP. The complete
synthetic route, conditions, and structural characterization of
the BIP-bipyridine ligand (BIP-bpy L) and photocatalysts 1
and 2 are described in the Supporting Information (SI 1.2).
Steady-State Absorption and Emission. The photo-
physics of photocatalyst 2 are intriguingly distinct from those
of the reference photocatalyst 1. The electronic absorption and
emission properties of photocatalysts 1 and 2 are summarized
in Table 1. The absorption spectra of 1 and 2 feature ππ*
1C).20 Emission from the MLCT state of Ru(II) complexes
3
with coordinated imidazole-phenol pairs occurs at identical
energies and with the same quantum yields as control
complexes lacking the PCET ligand, indicating PCET does
not occur in the excited state.20 However, upon addition of
methyl viologen dication (MV2+), electron transfer from the
3MLCT excited state of a similar Ru(II) complex to MV2+
generates an oxidized Ru(III) species.21 Rapid PCET from the
coordinated imidazole-phenol pair fills the hole on Ru,
effectively slowing charge recombination between Ru(III)
and MV•+.
Table 1. Electronic Absorption and Emission of 1 and 2
λmaxabs/nm
However, these concepts and their attendant advantages are
less explored in general photocatalytic organic synthesis, and
we questioned whether BIP appended to an Ir(III) photo-
sensitizer could play a similar role in h+ transfer, effectively
delaying charge recombination and extending the lifetime of
the substrate radical (Figure 1D). When compared to
ruthenium photocatalysts, iridium photocatalysts offer greater
stability and tunability of photophysical properties, due to
larger ligand field splitting and the ease with which they
support cyclometallating C−N ligands (such as 2-phenyl-
pyridine, ppy).22 There are two possible mechanisms by which
the appended BIP can slow charge recombination. If BIP
oxidation is thermodynamically capable of quenching the Ir-
a
a,
b
a
,
b
,
c
d
photocatalyst
(ε/M−1 cm−1
)
λ
em/nm
Φem
0.90
0.0060
λ
em/nm
1
2
380 (5100)
370 (18000)
498
515
460
570
a
b
c
In CH3CN at room temperature. In deoxygenated solvent. Relative
d
to Ir(ppy)3. In 4:1 MeOH:EtOH glassy matrix at 77 K.
transitions in the UV and a lowest energy transition in the
near-UV assigned as a mixed ligand centered (LC) (ppy π →
π*) and Ir(d) → ppy(π*) transition (Figure S9).25 The
electronic absorption and emission spectra of 1 are consistent
with literature reports.23,26 Emission quantum yields were
determined relative to Ir(ppy)3 (Φ = 0.97 in MeTHF).27 The
emission quantum yield of 2 is approximately 2 orders of
magnitude lower than that of 1, indicating quenching of the
emissive excited state (Figure S10). Addition of 10 mM
phenylphosphoric acid to 2 increases the emission quantum
yield to 0.70 (Figure S11). Protonation of the imidazole
proton acceptor in acidic solution prevents intramolecular
PCET. The emission recovery upon acid addition suggests
PCET quenches the emission of 2.
3
complex MLCT state, intramolecular PCET will result in a
charge-separated state (3CS), leaving a positive charge
localized on BIP. On the other hand, BIP can be oxidized by
Ir(IV), formed by electron transfer to the substrate from the
3MLCT, as is the case in the aforementioned Ru(II)
complexes.
We report herein an Ir(III) complex (Figure 2, photocatalyst
2) inspired by the Tyrz-H190 redox relay of PSII. Electro-
chemical, spectroscopic, and computational characterization
demonstrates that excitation of 2 with blue light rapidly
produces an intramolecular, triplet charge separated state
(Figure 1D, middle). Characterization of charge recombination
kinetics with MV2+ quencher illustrates that 2 slows charge
Electrochemistry. Cyclic voltammetry experiments were
performed in degassed acetonitrile solution to gain insight into
the electrochemical parameters of 2 compared with 1. Figure 3
displays cyclic voltammograms of 1 and 2 and Table 2
summarizes the most relevant electrochemical parameters. For
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absorption experiments were performed in deoxygenated
acetonitrile with λirr = 400 nm, and data were analyzed using
Glotaran analysis.30 The visible spectroelectrochemistry
spectra of 1 and 2 are provided in Figure S15.
Excitation of 1 produces an excited state species with
absorption at 476 nm and two decay components. The first
component has a 1.5 ps lifetime, assigned as vibronic relaxation
within the 3MLCTppy excited state (Figure 4A). A ppy(π*) →
bpy(π*) ligand-to-ligand charge transfer (LLCT) transition on
the picosecond time scale has been reported for several
cyclometalated Ir(III) complexes.31−33 However, a spectro-
scopic shift that would indicate a LLCT transition was not
observed in the excited state absorption of 1. Transient
absorption spectroscopy with the homoleptic complex Ir-
(dFppy)3 (dFppy = 2,4-difluorophenyl-2-pyridyl) reveals
similar excited state absorption to that of 1 (Figure S25).
The lowest energy excited state of 1 is thus assigned as the
Figure 3. Cyclic voltammograms of 2 (green trace) and the reference
photocatalyst 1 (blue trace). Experimental conditions: 1 mM of the
photocatalyst, 0.1 M nBu4NPF6 supporting electrolyte in dry CH3CN.
WE: glassy carbon. Pseudo RE: Ag wire (ferrocene as internal
reference). CE: Pt wire. Scan rate, 100 mV s−1.
3
3MLCTppy state (Figure 4C). The MLCTppy state of 1 has a
2.3 μs lifetime in acetonitrile, coinciding with relaxation back
to the ground state in close agreement with literature
values.34,35
Distinct excited state absorption features are observed upon
excitation of photocatalyst 2. Excited state absorption features
with maxima at 522 and 553 nm and broad absorption
between 600 and 740 nm rise with τ1 = 1.8 ps (Figure 4B) and
decay to the ground state with τ2 = 220 ns (Figure S27). The
isosbestic point at 495 nm indicates a transition between two
excited states and is assigned to the LLCT transition (Figure
4C). A similar spectrum with maximum absorption at 499 and
530 nm was reported upon photoreduction of 1 by
benzothiophene in acetonitrile.34 The red-shift between the
excited state absorption of 2 and reduced 1 is expected with
the increased aromatic conjugation provided by BIP covalently
linked to bipyridine. Similar red shifts are observed upon
increased aromatic conjugation of the polypyridyl ligand in
other [Ir(ppy)2(NN)]+ complexes.33 The similarity of the
transient absorption spectra of 2 with the spectrum of reduced
1 indicates that the negative charge resides in the bpy(π*)
LUMO. A LLCT transition was detected in the complex
[Ir(dF(CF3)ppy)2(deeb)][PF6] (deeb = 4,4′-diethylester-
2,2′-bipyridyl) using time-resolved IR spectroscopy.28 The
extended conjugation of the bipyridyl ligand on 2 likely
facilitates the LLCT. The excited state dynamics of 2 are
summarized in Figure 4D.
Table 2. Electrochemical Potentials of Photocatalysts 1 and
2 and Uncoordinated BIP-bpy L
a
a
a
E
1/2[IrIV/IrIII]
E
1/2[BIP•+/BIP]
E
1/2[bpy0/bpy−]
photocatalyst
[ΔE]
[ΔE]
[ΔE]
1
2
1.35 [90 mV]
1.24
n/a
n/a
0.72 [60 mV]
0.71 [110 mV]
−1.64 [60 mV]
−1.62 [60 mV]
−1.99 [380 mV]
b
c
BIP-bpy L
a
b
Reported in V vs Fc+/Fc. Reported potential for the anodic peak
current of an irreversible oxidation. Cyclic voltammogram shown in
Figure S12.
c
2, a reversible one-electron reduction wave is observed (E1/2
=
−1.62 V vs Fc+/Fc, ΔEp = 60 mV), which corresponds to
adding an electron to a bipyridyl π* orbital.26,28 This value is
20 mV more positive than 1 (E1/2 = −1.64 V vs Fc+/Fc, ΔEp =
60 mV), suggesting that the covalent linkage of the BIP moiety
on the periphery of the photocatalyst does not significantly
affect the ligand-based bpy0/− redox couple. On the other
hand, a reversible one-electron oxidation wave at 0.72 V vs
Fc+/Fc (ΔEp = 60 mV) is observed for 2 and is assigned as the
phenoxyl radical/phenol couple of the attached BIP. This
anodic couple of 2 occurs at a less positive potential than the
value expected for the one-electron Ir(IV/III) couple (removal of
an electron from the Ir-based HOMO with mixed molecular
orbital contributions from phenyl-pyridine π orbitals),26,28
which is observed for 1 at 1.35 V vs Fc+/Fc (ΔEp = 90 mV). In
the case of 2, the phenol oxidation is coupled with proton
transfer to the proximal nitrogen of the benzimidazole moiety
through an intramolecular PCET process. The observed E1/2
value for the phenoxyl radical/phenol couple in 2 is anodically
shifted relative to the E1/2 for the BIP model not covalently
linked to a bipyridine (E1/2 = 0.59 V vs Fc+/Fc),29 consistent
with the electron withdrawing effect of bipyridine which makes
the phenol moiety harder to oxidize. Moreover, an irreversible
feature at ∼1.24 V vs Fc+/Fc was found (Figure S14); this
oxidation process is assigned most likely to the metal center
(Ir(IV/III) couple).
Infrared Spectroelectrochemistry and Time-Resolved
Infrared Spectroscopy. The previous section underlined the
effect of linking the BIP moiety to the photocatalyst: ultrafast
3
quenching of the MLCTppy excited state by BIP oxidation,
resulting in a charge-separated state. Spectroscopic evidence of
this charge separation, which ultimately leads to the
benzimidazole protonation via intramolecular PCET, is also
shown by time-resolved infrared (TRIR) spectroscopy and
supported by infrared spectroelectrochemistry (IRSEC). Infra-
red spectroelectrochemistry (IRSEC) is valuable to follow the
changes in the vibrational modes associated with both the
reduced and oxidized forms of transition metal photocatalysts.
The difference IR spectra taken under oxidative and reductive
polarization as well as the recorded IR spectra collected
stepwise in both electrochemical conditions for complexes 1
and 2 are shown in Figures S16 and S17, respectively. The
difference IR spectra of oxidized and reduced forms are
instrumental to the interpretation of the time-resolved infrared
(TRIR) spectra (Figure 5); similar pattern in the band
Visible Transient Absorption Spectroscopy. The
photophysics of 1 and 2 were examined with visible and
mid-IR transient absorption spectroscopy. All transient
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Figure 4. (A) Evolution associated difference spectra obtained from global analysis fits of visible transient absorption spectra of 1 and (B) 2 in
CH3CN. λirr = 400 nm, 130 μJ/cm2. (C) Jablonski diagrams depicting excited state dynamics of 1 and 2. (D) Summary of photophysical steps
3
involved in the formation of the CSbpy state of 2.
evolution in the TRIR spectra is observed. Small differences
between TRIR and IRSEC experiments are expected in the
frequency and intensity of the bands, since certain transitions
associated with the excited states cannot be replicated through
electrochemical means.36 Control IRSEC experiments per-
formed with 1, without the pendant BIP moiety, show
practically no changes in the region of interest (1550−1500
cm−1, Figure 5A), in concordance with the featureless broad
absorption observed in the TRIR spectra (Figure 5C). The
ground state IR spectrum of 2 in CD3CN (Figure S17A, black
trace) resembles the ground state IR spectrum reported for an
analogous Ir(III) complex, in which the only structural
difference is the presence of ethyl ester groups at 4 and 4′
positions of the bipyridine ligand instead of BIP.28 In
photocatalyst 2, distinguishing bands are present at the ring
stretching region; the experimental frequencies together with
their assignments are detailed in Table S1.
Upon electro-oxidation of 2, new bands appear consistent
with formation of phenoxyl radical and protonation of
benzimidazole nitrogen through intramolecular PCET.
Among them, the band at 1556 cm−1 is associated with the
NH in-plane bending mode of the benzimidazolium ion, and
the slight shift of the ν(CN) of the benzimidazole to lower
energy is expected upon protonation.37 The bands located at
1622 and 1620 cm−1 in the IRSEC difference spectra and
TRIR spectra, respectively, are assigned to the imidazole CN
stretching mode and suggest proton transfer from the phenol.
The phenoxyl radical is characterized by the ν7a(C−O) mode
located at 1515 cm−1, and it occurs at a similar frequency as
that observed for other substituted phenoxyl radicals.38,39 In
the TRIR spectra of 2 (Figure 5D), the frequency of this mode
is found at ∼1521 cm−1 and the upshift suggests a weaker
3
hydrogen bond in the CS state. The bands at 1620 and 1521
cm−1 signaling proton transfer and electron transfer,
respectively, are evident in the TRIR spectra within 400 fs
following excitation with a pulse centered at 400 nm. This
spectrum is assigned to the radical cation of the BIP formed by
an ultrafast PCET process (<400 fs) in which the transfer of an
electron from the phenol to reduce Ir(IV) to Ir(III) is coupled
to the transfer of the phenolic proton to yield the
benzimidazolium cation. Intramolecular PCET quenches the
3
3MLCTppy excited state in less than 400 fs to yield the CSppy
species observed in the TRIR spectrum at 400 fs (Figure 5D).
Following the LLCT transition, the TRIR spectrum is similar
to that observed by IRSEC (Figure 5B) and is assigned to the
3CSbpy state. This species further cools to yield the spectrum
taken at 7 ns.
Density Functional Theory Calculations. To gain
further qualitative insight into the nature of the excited states
of 1 and 2, density functional theory calculations were carried
out using the Gaussian 16 software package.40 Calculations
were performed with either the unrestricted B3LYP, CAM-
B3LYP, or wB97XD functional and with the 6-311G+(d,p)/
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Figure 5. (A) Sum of IRSEC difference spectra upon positive and negative polarization of 1, (B) sum of IRSEC difference spectra upon positive
and negative polarization of 2, (C) TRIR spectra of photocatalyst 1, and (D) TRIR spectra of photocatalyst 2 in CH3CN, λirr = 400 nm, 3 μJ.
LanL2DZ (Ir) split basis set, in an acetonitrile polarizable
continuum solvent model. The full details of these methods
can be found in the SI. Across all three functionals,
optimization of the lowest energy triplet stationary point of
2 shows that the photocatalyst has undergone intramolecular
PCET, consistent with the 3CSbpy state (Figure S52).
Visualization of the spin density and comparison of the change
in charge on each atom between the two states using natural
population analysis supports a structure where the phenol has
been oxidized and the bipyridyl has been reduced. In contrast,
calculations show that the lowest energy triplet stationary point
of 1 has significant spin density on the ppy ligand (Figure
S49), consistent with our assignment of the lowest energy
recombination, kCR, was determined to be 2.2 × 1010 M−1
s−1 with photocatalyst 1 and 2.9 × 109 M−1 s−1 with
photocatalyst 2, indicative of a 7.6-fold decrease (Figure 6).
3
excited state of 1 as the MLCTppy state.
Limited Charge Recombination. The ability of the BIP
ligand to limit charge recombination in 2 upon photoinitiated
electron transfer to methyl viologen dication (MV2+) was
investigated with transient absorption spectroscopy. Solutions
of 1 and 2 were absorbance-matched in a 3.3 mM stock
solution of MV2+. Photocatalysts 1 and 2 both undergo
photoinitiated electron transfer to MV2+ upon λirr = 400 nm in
CH3CN. Formation of the characteristic MV•+ spectrum was
observed on the nanosecond time scale (Figures S35 and S36).
Charge recombination between the methyl viologen radical
cation and the oxidized catalyst (1+ or 2+) was monitored as a
function of 607 nm absorption (ε = 13 900 M−1 cm−1),41
beginning at 1 μs, after the decay of the excited state
absorption. The second order rate constant for charge
Figure 6. Second order kinetic plots with the inverse methyl viologen
radical cation concentration determined from the decay of absorption
at 607 nm with 1 or 2 absorbance-matched and 3.3 mM MV2+ in
CH3CN. λirr = 400 nm, 500 μJ/cm2.
For photocatalyst 1+, the 1.80 V driving force for
recombination is significantly greater than the 1.17 V driving
force for charge recombination to 2+. The approximately order
of magnitude difference in kCR is likely due to the difference in
driving force for charge recombination from MV•+ (MV2+/•+
=
−0.85 V vs Fc+/Fc)42 to the oxidized photocatalyst and to the
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Figure 7. (A) Photoinduced electron transfer to N−hydroxyphthalimide ester F generates radical anion F•− that can either fragment to form
product or undergo charge recombination with oxidized photocatalyst. (B) Comparison of reaction rate and quantum yield in the reduction of F,
catalyzed by either photocatalyst 1 or 2. Conditions: 0.3 mmol F (1.0 equiv, 0.1 M THF); photocatalyst OD440 nm = 1.1; 1.0 equiv. thiophenol; 40
W, 440 nm Kessil lamp with attached 440 nm bandpass interference filter, fwhm of 10 nm; 25 °C. Product yield determined by GC-FID with an
internal standard. Quantum yield calculated by average of two trials.
different recombination mechanisms, requiring ET with 1 and
PCET with 2.
difference in extinction coefficients between the two photo-
catalysts (see SI for details).
Charge recombination from F•− to oxidized photocatalyst is
undoubtedly highly exergonic, with more driving force in the
case of 1, but under standard cyclic voltammetry conditions
the reduction of F is irreversible, making it impossible to assign
a midpoint potential for the F/F•− couple. For the improve-
ment in reaction quantum yield to arise from a comparatively
longer-lived F•− intermediate, the decrease in charge
recombination rate for 2 compared to 1 should result in a
higher fraction of F•− fragmenting to F•. To support this
hypothesis, we ideally need to compare the rate constant of
fragmentation with the two rate constants of charge
recombination for the two photocatalysts. We estimated the
first-order rate constant of F•− fragmentation using voltam-
metric methods (see the SI for details). By measuring the
change in peak cathodic potential as a function of scan rate, the
estimated rate constant of fragmentation was calculated to be
(8 5) × 105 s−1. This range of values results from uncertainty
in the half-wave potential for the reduction of F.
Encouraged by these results with MV2+ substrate surrogate,
we sought to determine whether the slower second-order rate
constants for charge recombination between MV•+ and 2+
relative to 1+ might also lead to improved quantum yields in
preparative photoredox reactions. To this end, we designed a
simple model system involving the reduction of a redox-active
phthalimide ester (F, Figure 7A). N-Hydroxyphthalimide
esters have enjoyed renewed interest in the field of organic
synthesis as convenient precursors for alkyl radicals under
reducing conditions.43−50 As illustrated in Figure 7A, an alkyl
radical F• is generated via the loss of carbon dioxide and
phthalimide anion following single-electron reduction of the
phthalimide moiety. It has previously been suggested that the
addition of a proton donor may accelerate the rate of
fragmentation.43,51 However, without a proton donor in
solution, we hypothesized that fragmentation may be in kinetic
competition with charge recombination, and thus may serve as
a platform to test our key hypothesis. Solutions of F containing
either 1 or 2 in THF were absorbance-matched at 440 nm.
Upon irradiation of these solutions with a 440 nm light source
and in the presence of a thiophenol hydrogen-atom donor, we
observed that the initial rate using photocatalyst 2 was more
than twice as fast as an otherwise identical reaction using
photocatalyst 1 (Figure 7B). The photon flux of the light
source used for these reactions was determined using
potassium ferrioxalate actinometry. The internal quantum
yields are (4.9 0.1) × 10−3 for photocatalyst 2 and (2.3
0.3) × 10−3 for photocatalyst 1. A similar quantum yield
increase was observed when reactions were irradiated with a
456 nm broadband light source, after adjusting for the
Transient absorption experiments with 1 or 2 and F (0.1 M)
were conducted to determine charge recombination rate
constants (Figure S42 and S43). However, recombination
rate constants were not obtained because neither the
photocatalyst ground state bleach nor F•− absorption are
3
discernible in the visible spectrum. The MLCT excited state
lifetime of 1 is 2.2 μs in THF and is quenched to 1.3 μs in the
presence of F (Figure S44). The 3CS state lifetime of 2 is 296
ns in THF and is reduced to 279 ns with F (Figure S44). The
lifetime differences with and without F were used to calculate
observed rate coefficients, kobs, for photoinitiated electron
transfer; 3.15 × 105 s−1 with 1 and 2.06 × 105 s−1 with 2 (SI
page S39). Using the measured quantum yields of final
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Article
product, we estimate a ∼24-fold reduction of the charge
recombination rate with 2 compared to 1 (SI page S84). The
increase in quantum yield of final product in switching from 1
to 2 strongly suggests that a meaningful decrease in the charge
recombination rate occurred, allowing a significantly larger
fraction of F•− to successfully fragment.
Comprehensive information regarding synthesis, electro-
chemistry, spectroscopic studies, computational meth-
ods, and reaction characterization (PDF)
AUTHOR INFORMATION
■
An additional consideration for accurate comparison of the
two reaction quantum yields is that fragmentation of F must be
the turnover-limiting step whether photocatalyst 1 or 2 is used.
In this regard, initial rates for the reactions of both
photocatalysts were found to exhibit a zero-order kinetic
dependence on the concentration of the thiophenol reductant,
suggesting that the HAT and photocatalyst turnover steps are
not kinetically relevant (Figure S67). Furthermore, we
observed an insignificant difference in the measured solvent
kinetic isotope effect between reactions with either 1 or 2
(kTHF/kd8‑THF = 1.2), indicating that the solvent (THF) does
not play a key kinetic role in these reactions (SI page S86).
Finally, the measured reaction rate was found to be linear with
respect to light intensity for both photocatalysts, suggesting
that two-photon processes are not operative in this reaction (SI
page S89). These observations are all consistent with
fragmentation being the turnover-limiting step in the process,
and with the notion that competition between fragmentation
and charge recombination is likely a key determinant in the
overall quantum efficiency. Accordingly, we hypothesize that
the improved performance of 2 is a direct result of its slower
charge recombination kinetics, which leads to more productive
fragmentation from a comparatively long-lived F•− intermedi-
ate.
Corresponding Authors
Ana L. Moore − School of Molecular Sciences, Arizona State
University, Tempe, Arizona 85287, United States;
orcid.org/0000-0002-6653-9506; Email: amoore@
asu.edu
Robert R. Knowles − Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States;
orcid.org/0000-0003-1044-4900; Email: rknowles@
princeton.edu
Authors
Hannah Sayre − Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States;
orcid.org/0000-0002-3214-4899
Hunter H. Ripberger − Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States;
orcid.org/0000-0002-3146-9579
Emmanuel Odella − School of Molecular Sciences, Arizona
State University, Tempe, Arizona 85287, United States;
orcid.org/0000-0002-7021-400X
Anna Zieleniewska − National Renewable Energy Laboratory,
Golden, Colorado 80401, United States; orcid.org/0000-
0001-9987-7520
Daniel A. Heredia − School of Molecular Sciences, Arizona
State University, Tempe, Arizona 85287, United States
Garry Rumbles − National Renewable Energy Laboratory,
Golden, Colorado 80401, United States; orcid.org/0000-
0003-0776-1462
Gregory D. Scholes − Department of Chemistry, Princeton
University, Princeton, New Jersey 08544, United States;
orcid.org/0000-0003-3336-7960
CONCLUSIONS
■
The strategy described here is a general method for the
improvement of photoredox reaction quantum efficiency. By
introducing a bioinspired PCET-based process as an intrinsic
component of photocatalyst 2, the dynamics of the yield-
limiting charge recombination process in the catalytic cycle are
altered via the formation of a well-characterized triplet charge-
separated state. The modified charge recombination process
resulted in a 10-fold reduction in the second-order rate
constant for charge recombination between reduced methyl
viologen and oxidized photocatalyst, as well as doubling the
quantum yield of a preparative-scale photoredox reaction. This
observed improvement in photocatalyst performance is
particularly remarkable, given the shorter lifetime of the triplet
charge-separated state (2*) compared to the standard
photocatalyst (1*), and the charge recombination rate in the
preparative-scale reaction is slowed ∼24-fold using 2. Studies
are ongoing to further understand the nature of the charge
separation and charge recombination processes on photo-
catalyst 2, as well as to further improve the performance of 2
and the BIP-based platform in preparative scale reactions.
These results provide further evidence that PCET is a useful
blueprint for improving intermolecular charge-separated state
lifetimes, and we anticipate that this approach can be extended
to other reactions involving both reductive and oxidative
excited-state quenching, increasing the scope and quantum
efficiency for a broad spectrum of photoredox processes.
Thomas A. Moore − School of Molecular Sciences, Arizona
State University, Tempe, Arizona 85287, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c01701
Author Contributions
∥H.S. and H.H.R. equally contributed to the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to express gratitude to James McCusker,
Felix Castellano, and Obadiah Reid for helpful conversations
and to Daniel Oblinsky for instrument assistance in the
Ultrafast Laser Spectroscopy facility at Princeton University.
This work was supported as part of BioLEC, an Energy
Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Basic Energy Sciences under Award
No. DE-SC0019370. This work was authored, in part, by the
National Renewable Energy Laboratory, operated by Alliance
for Sustainable Energy, LLC, for the U.S. Department of
Energy (DOE) under Contract No. DE-AC36-08GO28308.
The views expressed in the article do not necessarily represent
the views of the DOE or the U.S. Government.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c01701.
■
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