Donor-Acceptor Fluorophores for Energy-Transfer-Mediated Photocatalysis

Article abstract of DOI:10.1021/jacs.8b07271

Triplet-triplet energy transfer (EnT) is a fundamental activation pathway in photocatalysis. In this work, we report the mechanistic origins of the triplet excited state of carbazole-cyanobenzene donor-acceptor (D-A) fluorophores in EnT-based photocatalytic reactions and demonstrate the key factors that control the accessibility of the ³LE (locally excited triplet state) and ³CT (charge-transfer triplet state) via a combined photochemical and transient absorption spectroscopic study. We found that the energy order between ¹CT (charge transfer singlet state) and ³LE dictates the accessibility of ³LE/³CT for EnT, which can be effectively engineered by varying solvent polarity and D-A character to depopulate ³LE and facilitate EnT from the chemically more tunable ³CT state for photosensitization. Following the above design principle, a new D-A fluorophore with strong D-A character and weak redox potential is identified, which exhibits high efficiency for Ni(II)-catalyzed cross-coupling of carboxylic acids and aryl halides with a wide substrate scope and high selectivity. Our results not only provide key fundamental insight on the EnT mechanism of D-A fluorophores but also establish its wide utility in EnT-mediated photocatalytic reactions.

Full text of DOI:10.1021/jacs.8b07271

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Article
Donor-Acceptor Fluorophores for Energy Transfer Mediated Photocatalysis
Jingzhi Lu, Brian Pattengale, Qiuhua Liu, Sizhuo Yang,
Wenxiong Shi, Shuzhou Li, Jier Huang, and Jian Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07271 • Publication Date (Web): 02 Oct 2018
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Donor-Acceptor Fluorophores for Energy Transfer Mediated Photoca-
talysis
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Jingzhi Lu,† Brian Pattengale,‡ Qiuhua Liu,†,§ Sizhuo Yang,‡ Wenxiong Shi,#,¤ Shuzhou Li,*,# Jier
Huang,*,‡ and Jian Zhang*,†,⊥
†Department of Chemistry, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States
‡Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201, United States
§School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
#School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798
¤State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Key Lab of Fiber Modification & Functional
Fiber of Tianjin, Tianjin Polytechnic University, Tianjin, 300387, China
⊥Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
ABSTRACT: Triplet-triplet energy transfer (EnT) is a fundamental activation pathway in photocatalysis. In this work, we
report the mechanistic origins of triplet excited state of carbazole-cyanobenzene donor-acceptor (D-A) fluorophores in EnT
3
based photocatalytic reactions and demonstrate the key factors that control the accessibility of LE (locally-excited triplet
3
state) and CT (charge transfer triplet state) via a combined photochemical and transient absorption spectroscopic study.
1
3
We found that the energy order between CT (charge transfer singlet state) and 3LE dictates the accessibility of LE/3CT for
3
EnT, which can be effectively engineered by varying solvent polarity and D-A character to depopulate LE and facilitate EnT
from the chemically more tunable CT state for photosensitization. Following the above design principle, a new D-A fluoro-
3
phore with strong D-A character and weak redox potential is identified, which exhibits high efficiency for Ni(II)-catalyzed
cross-coupling of carboxylic acids and aryl halides with a wide substrate scope and high selectivity. Our results not only
provide key fundamental insight on the EnT mechanism of D-A fluorophores but also establish its wide utility in EnT-
mediated photocatalytic reactions.
INTRODUCTION
incorporated in PCs,10 this strategy induces concerns of
potential metal contamination. As a result, metal-free or-
ganic PCs that can achieve efficient ISC have been become
the most attractive alternatives.11 Organic PCs with high
ΦISC have been reported after incorporating carbonyls and
heavy halogens,12 and have been used in synthetic photo-
chemistry for EnT-based reactions.13 Very recently, we
have also developed metal-free PCs based on donor-
acceptor (D-A) fluorophores for organic synthesis.14 The
rationale is inspired by the common design principle in
thermally-assisted delayed fluorescence (TADF), where a
sterically hindered D-A dyad usually exhibits a small sin-
glet(1S)-triplet(3T) energy gap (ΔEST) that increases both
down-conversion and up-conversion ISC.15
The past decade has witnessed a renaissance of photoca-
talysis in synthetic organic chemistry.1 Through visible
light sensitization, small molecules are activated under
mild conditions with high tolerance to functional groups.
One popular activation strategy is photoredox catalysis, by
which the photocatalyst (PC, also referred as photosensi-
tizer), upon visible light excitation, initializes two sequen-
tial single electron transfer (ET) processes to turnover and
promote the transformation of reactant(s) to product(s).2
A second fundamental activation pathway that does not
involve any charge separation is energy transfer (EnT),
which occurs directly from the electronically excited state
(ES) of PC (*PC) to substrate and initializes its subsequent
transformation.3 Building on the early studies,4 the reac-
tion scope of EnT-mediated photocatalysis has been re-
cently expanded to cycloaddition,5 isomerization,6 and
cross-coupling,7 to name a few. In several cases, the com-
bination of EnT with other activation modes such as Lewis
acid8 and hydrogen-bond9 catalysis can also realize excel-
lent enantioselectivity.
While these examples demonstrate the large potentials
of using D-A fluorophores as metal-free PCs for EnT-
mediated organic synthesis, the mechanistic origins of EnT
process remain unclear. Typically, the triplet state (3T) of
organic D-A fluorophores has two main characters, name-
ly, local excitation (3LE) within a particular molecular sub-
unit (either D or A) and charge-transfer (3CT) transition
from donor-based HOMO to acceptor-based LUMO. Recent
experimental16 and theoretical17 studies have revealed that
3LE mediates the efficient ISC from 1CT to 3CT via spin-
orbit18 as well as spin-vibronic coupling.19 It then raises a
question whether both 3CT and 3LE can be utilized for EnT.
One important character for a good PC, regardless the
activation pathways, is a sufficiently long-lived ES, which is
usually enabled by spin interconversion. While a near uni-
ty of ΦISC (ISC = intersystem crossing) can be often
achieved when heavy transition metals (e.g. Ru or Ir) are
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If so, how can one modulate their accessibility for the syn-
Page 2 of 8
ent triplet-triplet EnT efficiency of PC to the two isomers.
In general, if the triplet energy (ET) of PC is smaller than
that of the Z-isomer but larger than that of the E-isomer,
EnT is more thermodynamically favorable to the E-isomer,
resulting in accumulation of Z-isomer and a higher E/Z
isomerization efficiency (defined by Z/E ratio). Provided
that the “photostationary state” is reached by sufficient
light irradiation, the other photophysical properties of PCs
such as molar absorptivity and quantum yield become in-
significant. As such, it is convenient to benchmark the pho-
tocatalytic efficiencies solely on the basis of ET.
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thetic purpose?
Herein, we report a systematic study of the photocatalyt-
ic applications of Cz-CN (Cz = carbazole, CN = cyanoben-
zene) based D-A fluorophores. Using the well-known Z/E
isomerization of stilbene as a prototypic reaction and opti-
cal femtosecond (fs-TA) and nanosecond (ns-TA) transient
absorption spectroscopy, we found that indeed both 3LE
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and CT are amenable for the EnT-based reactions and the
energy order between 1CT and 3LE dictates their accessibil-
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ity. Since the energy level of LE is relatively fixed and not
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significantly affected by reaction media, the tunability of D-
A fluorophores is best realized via adjusting the energy
level of 3CT, and its accessibility can be achieved by de-
creasing solvent polarity or increasing the D-A CT charac-
ter of the fluorophores. Finally, the validity of this design
principle was further tested and applied in the identifica-
tion of new D-A PCs for Ni(II)-catalyzed cross-coupling of
carboxylic acids with aryl halides.
The isomerization reaction was conducted in a DMF so-
lution of E-stilbene (0.2 M) and PC (0.7 mol%) using white
light irradiation (26 W compact fluorescence lamp, CFL)
under argon atmosphere at room temperature (Figure 1a).
Figure 1b illustrates the correlation between the Z/E ratio
and the energy of the lowest triplet state (ET, see Support-
ing Information Table S3 for details). In general, D-A fluor-
ophores with a large ET (> 2.7 eV) result in lower Z/E rati-
os. Specifically, with an ET of 2.34 eV that is larger than ET
of E-stilbene (2.2 eV22) but smaller than ET of Z-stilbene
(2.5 eV22), 4CzTPN shows the highest isomerization effi-
ciency (Z/E = 8.5), which is followed by 4CzPN (ET = 2.45
eV, Z/E = 8.1), 4CzIPN (ET = 2.53 eV, Z/E = 6.7), and 5CzBN
(ET = 2.68 eV, Z/E = 4.0). However, 2CzPN (ET = 2.63 eV)
and 2CzTPN (ET = 2.48 eV) do not follow the trend and give
an essentially the same efficiency (Z/E ≈ 2) as those with
a large ET (Figure 1b).
Scheme 1. Chemical Structures of Cz-CN Donor-
Acceptor (D-A) Fluorophores Used in This Study
RESULTS AND DISCUSSION
We chose eleven Cz-CN based D-A fluorophores of which
the number and position of donor and acceptor are sys-
tematically changed, including five benzonitrile (BN) de-
rivatives where the BN core is coupled with two, three
(×2), four, and five Cz, as well as six dicyanobenzene deriv-
atives where two and four Cz are coupled with terephtha-
lonitrile (TPN), phthalonitrile (PN), and isophthalonitrile
(IPN), respectively (Scheme 1). Except 3,5-2CzBN, most Cz-
CN fluorophores exhibit appreciable visible light absorp-
tion (> 380 nm, see Supporting Information, Figure S1),
indicating the generality of D-A fluorophores for visible
light photocatalysis. The CT nature of ES of D-A fluoro-
phores was clearly indicated by the sensitive solvato-
chromism in fluorescence spectra (Figure S2): the more
polar solvent DMF results in a larger Stokes shift than the
less polar solvent toluene due to the stronger interaction
between the CT nature of PC and the dipole of polar sol-
vents, consistent with previous observations.20 Moreover,
as the D-A interaction increases by increasing the number
of either Cz or CN, both absorption and emission peaks are
red-shifted (Figure S2).
Figure 1. (a) Photosensitized E/Z isomerization of stilbene.
(b) Z/E ratio at the photostationary state vs ET of D-A fluoro-
phores.
This deviation prompts us to hypothesize that, in certain
D-A fluorophores, instead of the lowest triplet state (3T1,
presumably 3CT), triplet state(s) with a larger energy (3Tn),
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likely with a strong LE character, are involved in EnT. In-
deed, TD-DFT calculations with UB3LYP functional and 6-
3
31g(d) basis functions have revealed the strong LE char-
acter of the triplet states of 2CzPN (3T3-4, 3.00-3.05 eV) and
2CzTPN (3T5-7, 3.01-3.18 eV) (see Supporting Information
S-7 for detailed computational procedure and results). This
hypothesis is also consistent with the pivotal role of 3Tn
with 3LE character in the TADF process by inducing an
Photosensitized E/Z isomerization of stilbene was used
to evaluate the performance of D-A fluorophores (Figure
1a).21 This energetically uphill reaction is based on differ-
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efficient ISC via spin-orbit coupling.18 Nevertheless, the
EnT from 3LE needs to be experimentally confirmed.
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Femtosecond and Nanosecond Transient Absorption
Spectroscopy. In order to seek further support for the
proposed EnT mechanisms, we turn to fs-TA and ns-TA
spectroscopy to investigate the ES dynamics of two model
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D-A fluorophores, 2CzPN and 4CzIPN, since their CT and
3LE states have distinct spectral features.23 Upon the exci-
tation at 400 nm, the fs-TA spectra of both compounds
show a prominent absorption band at 480 nm and a broad
absorption > 600 nm (Figure 2a and 2b) that can be as-
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signed to CT absorption. At early time (< 2 ps) (insets of
Figure 2a and 2b), spectral evolution in the whole spectral
window involves the rising of 480 nm band and decay of
the > 600 nm absorption feature, which can be attributed
to the vibrational cooling from a Franck-Condon (FC) state
to the relaxed S1 state. At later times (> 2 ps), distinct
growth in the ~525-700 nm and simultaneous decay at
480 nm and > 700 nm can be attributed to the ISC process
from 1CT to the triplet states (3CT/3LE). This evolution
agrees well with the ns-TA spectra (Figure 2c and 2d),
where ISC process continues to evolve until ~150 ns. Fol-
lowing the ISC process, the triplet states eventually return
to the ground state with a slow time constant on a micro-
second time scale. The ISC process is further supported by
the ns-TA spectra at NIR region (Figure 2c and 2d), where
the decay of absorption feature < 1000 nm corresponding
Figure 2. Femtosecond TA spectra of 2CzPN (a) and 4CzIPN
(b) in visible region following 400 nm excitation. The inset
shows the corresponding early time spectra. Nanosecond TA
spectra of 2CzPN (c) and 4CzIPN (d) in both visible and NIR
regions following 400 nm excitation. The break denotes the
spectral range not covered by ns-TA experiments. The extinc-
tion coefficients obtained from global fitting model for 2CzPN
(e) and 4CzIPN (f).
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to CT is accompanied by the growth of the feature > 1000
nm that represents triplet state. According to previous
report, we can further assign the spectral features at 525-
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700 nm and 900-1200 nm to 3LE and CT, respectively.23
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These results together suggest that 3LE and CT are both
present in 2CzPN and 4CzIPN, and the proposed triplet
state (T1) is likely associated with the equilibrium between
3LE and 3CT.
We next probe the kinetics of the spectral features of 3LE
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and CT to evaluate the EnT process in the presence of E-
stilbene as the energy acceptor (quencher). Figure 3 com-
pares the kinetic traces of 2CzPN and 4CzIPN at 660 nm
(3LE) and 1000 nm (3CT) in the presence of different con-
centrations of E-stilbene (see Figure S8-S10 in Supporting
Information for TA spectra). The significantly enhanced
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decay observed for both samples at 3LE and CT spectral
regions in the presence of E-stilbene suggests that both
triplet states are responsible for EnT process, supporting
triple state EnT mechanism, although their relative contri-
bution might be different between 2CzPN and 4CzIPN.
Figure 3. Kinetic traces of ns-TA spectra in the presence of
different concentrations of E-stilbene (quencher) for 2CzPN at
660 nm (a) and 1000 nm (c) and for 4CzIPN at 660 nm (b) and
1000 nm (d).
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Table 1. Fitting Parameters for 2CzPN and 4CzIPN Obtained from Global Fitting
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sample
[E-stilbene]
0 mM
τ1 (ps)
τ2 (ns)
τ3 or τ3’ (μs)
1.8
τEnT, 3CT (ns)
τEnT, 3LE (ns)
NA
NA
5
6
7
8
4 mM
0.132, 0.127
0.025
142
137
2CzPN
1.7
25
40 mM
400 mM
0 mM
not measured
25
not measured
NA
0.081
85
1.4
NA
687
9
4 mM
0.461, 0.600
0.076
1050
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4CzIPN
5.7
47
40 mM
400 mM
not measured
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80
0.049
not measured
To quantitatively evaluate EnT dynamics in these sys-
tems, global fitting is used to simultaneously fit the kinetic
traces of fs- and ns-TA spectra at different wavelengths by
using the same set of rate constants while varying the ex-
tinction coefficients (see Supporting Information S-8 for
details). We first fit the kinetic traces of the samples in the
absence of E-stilbene to obtain the time constants for in-
trinsic ES relaxation dynamics of 2CzPN and 4CzIPN. The
best fits to these kinetic traces are shown in Figure S11-12.
The resulting fitting parameters are listed in Table 1, from
which we obtained the intrinsic time constants for vibra-
tional cooling (τ1 = 1/k1), ISC process (τ2 = 1/k2), and tri-
plet state lifetime (τ3 = 1/k3) for 2CzPN and 4CzIPN. More
importantly, we found that the extinction coefficients of S1
and T1 obtained from global fitting show excellent agree-
ment with the corresponding ns-TA spectra (Figure 2e and
2f), which unambiguously confirms the validity of the fit-
ting model.
fairly constant value of ~3.1 eV,23 two strategies can be
used herein to tune the energy level of CT and modulate
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the 3LE/3CT population. (1) Changing solvent polarity.
Since the absorption of Cz-CN D-A fluorophores usually
red-shift in less polar solvents (Figure S2), changing sol-
vent polarity may serve as an approach to modulate the
3LE/3CT population. Indeed, for 2CzTPN, we observed an
increase of Z/E ratio from 2.7 in DMF to 7.3 in toluene. The
redshift of the UV-vis spectra of 2CzTPN in toluene com-
pared that in DMF (Figure 4a), via which the lowest-energy
absorption band (1CTabs, based on the peak position) be-
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comes lower than LE and causes the depopulation of the
latter. (2) Increasing D-A CT character. An increase of the
donor strength in D-A fluorophores is another strategy to
increase the CT character and decrease the 1CT energy. For
instance, although changing solvent polarity for 2CzPN
only results in a limited difference of Z/E ratio in DMF
1
(1.9) and toluene (2.1), which is consistent with its CTabs
being at the larger energy than 3LE in both solvents; re-
The same model was used to fit the kinetic traces of the
TA results in the presence of E-stilbene (Figure S13-S15).
Because triplet EnT to E-stilbene has negligible effect on
the time constants of vibrational cooling (τ1) and ISC pro-
cess (τ2), these parameters were fixed during the fitting
process. The resulting time constant representing the new
lifetime of triplet state in the presence of different concen-
trations of quencher (τ3’) is also listed in Table 1, from
which the EnT time constant (τEnT) was obtained according
to 1/τ3’ = 1/τEnT + 1/τ3. It is interesting to note that, for
2CzPN, τEnT from 3LE is slightly shorter than from 3CT in
the presence of low concentrations of quencher (4 mM)
but much shorter in the presence of higher concentrations
of quencher (τEnT (3LE) = 25 ns with 40 mM quencher; τEnT
(3CT) = 85 ns with 400 mM quencher). These results sug-
placing Cz with a stronger donor, such as 3,6-di-tert-
butylcarbazoly- (tBuCz, ET = 2.97 eV24), shifts CTabs below
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3LE and increases the Z/E ratio to 4.6 (in toluene) in ac-
cordance with CT energy of 2tBuCzPN (~2.56 eV25) (Fig-
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ure 4b). Overall, both experiments strongly suggest that
tuning the energy of 1CT can effectively modulate the
3LE/3CT population and dramatically vary the photocata-
lytic efficiency of the Z/E isomerization of stilbene. In fact,
upon switching solvent and increasing donor strength, the
new Z/E ratios exhibit a good correlation with the ET val-
ues, suggesting the population of 3CT and corresponding
enhanced EnT (Figure S3).
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gest that EnT process is more efficient from LE than from
3CT in 2CzPN, although both triplet states contribute to
EnT. On the other hand, 4CzIPN shows significantly shorter
τEnT from 3CT than 3LE even at low quencher concentration
(4 mM), which indicates that EnT process is dominated by
3CT in 4CzIPN.
Changing Photophysical Properties of PC to Modu-
late the EnT Pathway. The mechanistic study highlights
the importance of the depopulation of LE for using highly
tunable CT of D-A fluorophores in EnT-based photocata-
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lytic reactions. We envision that when CT is lowered, the
Figure 4. Fine-tune the 1CTabs energy level to increase E/Z
isomerization of stilbene by (a) changing solvent polarity and
(b) increasing D-A CT character.
population of 3LE should become less efficient. Since for
most Cz-CN D-A fluorophores the 3LE state usually has a
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Based on the mechanistic understanding of the EnT pro-
by largely diminished product yield under air and triplet
quenchers (Table S19, entries 7-8). It is noteworthy that
4DPAIPN compares favorably to conventional organic pho-
tosensitizers such as benzophenone, benzil, 9-flurenone
and common organic dyes such as eosin Y, rhodamine B,
and porphyrin (Table S17). After further reaction screen-
ing of ligand and Ni(II) salt, an excellent yield (91%) of the
cross-coupling product was achieved when sterically hin-
dered amine N-isopropyl-N-methyl-tert-butylamine, a 1:3
molar ratio between carboxylic acid and aryl halide, and
solvent N, N-dimethylacetamide (DMAc) was used (Table
S20-S23). Our result clearly shows that ET proceeds prior
to EnT when both pathways are energetically favorable.
Therefore, when choosing PC for EnT promoted reactions,
ET has to be suppressed by careful consideration of the
electrochemical potentials of all species involved.
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cess, we next further demonstrate the utility of D-A fluoro-
phores for other photoisomerization reactions that have
different energetic requirements. As for β-methylstyrene
with the ET of Z- and E-isomer of 2.9 and 2.6 eV, respective-
ly,26 5CzBN that has a ET of 2.68 eV would be an optimal PC.
To our delight, 5CzBN indeed gave rise to the highest Z/E
ratio of 7.3 in DMF among the four D-A fluorophores exhib-
3
iting the dominant CT character (Table S7). Such catalytic
activity is comparable to Ir(ppy)3 (ppy = 2-phenylpyridine,
ET = 2.51 eV)6a and much more efficient than conventional
organic photosensitizers including benzophenone, benzil,
and 9-fluorinone (Table S8). Similar results were found in
the Z/E isomerization of diisopropyl fumarate (ET = 2.7 eV)
to maleate (ET = 3.1 eV);27 5CzBN gave an excellent Z/E
ratio of 32 in DMF, outperforming Ir(dF-CF3-
ppy)2(dtbpy)PF6 (Z/E ratio = 7.3, ET = 2.64 eV28) (Table
S9). As expected, 4CzIPN is ineffective due to its lower ET.
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Scheme 2. EnT-Mediated Cross-coupling of Carboxylic
Acids and Aryl Halides
EnT Promoted Cross-coupling of Carboxylic Acids
and Aryl Halides via Nickel Complexes. We anticipate
the design principle of D-A fluorophores can also be ap-
plied for other EnT-mediated reactions where the organ-
ometallic complex is the acceptor. MacMillan and co-
workers recently reported the cross-coupling between
carboxylic acids and aryl halides promoted by Ir(ppy)3 via
EnT to a nickel(II)-based catalyst.7b We predicted that a D-
A fluorophore with an ET larger than that of Ir(ppy)3 (2.33
eV) should provide a sufficient sensitization efficiency.
4CzIPN was first elected as the PC due to its appropriate ET
value (2.53 eV) and good compatibility with Ni(II)-
complexes in ET-based reactions.14b The reaction of benzo-
ic acid (1) with 4-bromobenzotrifluoride (2) was conduct-
ed in a mixture consisting 4CzIPN, NiCl2∙DME (DME = di-
methoxyethane), dtbbpy (4,4’-di-tert-butyl-2,2’-dipyridyl),
and diisopropylamine (DIPA) in DMF under Ar and white
CFL irradiation (Scheme 2a). Interestingly, instead of the
desired cross-coupling product (3), the dehalogenation
product trifluorotoluene (4) was obtained in 97% yield
(Scheme 2b). Despite the confirmed essential roles of
4CzIPN, light, Ni(II)-complex, and DIPA (Table S18, entries
2-6), the presence of triplet state quencher such as oxygen
and 1,1-diphenylethylene shows limited effect on the reac-
tion yield, which strongly suggests the EnT pathway in
inactive (Table S18, entries 8-9). However, the oxidation
product of DIPA was detected by GC-MS (Figure S18),
which indicates a photoredox/Ni dual catalytic cycle is
likely to yield the dehalogenation product with DIPA as the
reductant (see Figure S19 for a proposed reaction mecha-
nism).29
4DPAPN gives a wide substrate scope for this reaction
(Figure 5). Benzoic acid derivatives with electron-donating
or electron-withdrawing substituents can efficiently cou-
ple with aryl bromide (3b-3c, 95% yield). Aliphatic prima-
ry, secondary and tertiary carboxylic acids are highly tol-
erated (3d-3j, 79%-93% yield). Remarkably, carboxylic
acids with long aliphatic chain (3g, 93% yield) or bulky
adamantane unit (3h, 90% yield) can be delivered to de-
sired cross-coupling product with excellent yields. Benzyl
carboxylic acid can also yield the corresponding ester in
good efficiency (3d, 79% yield). On the other hand, elec-
tron-deficient aryl bromides were able to couple with car-
boxylic acids smoothly, which is consistent with previous
report.7b The reaction tolerated cyanide, carboxylic ester,
trifluoromethyl, and halide, (3k-3p, 62%-85% yield). In-
terestingly, a good yield was also obtained despite the
presence of an electron-donating group -OMe (3m, 83%
yield). Para- and meta-substitution showed similar cou-
pling efficiency (3c and 3n), while ortho-substitution com-
pletely hindered the reaction. Heterocycle such as pyridine
is also well-tolerated (3q-3r, 65%-83% yield).
Therefore, in order to achieve the expected cross-
coupling product, the oxidation ability of *PC needs to be
weakened to suppress the ET event, which can be realized
by replacing Cz with a stronger donor, such as diphenyla-
mine (DPA). Indeed, when 4DPAIPN was then used as the
ox
PC (*E1/2 = +1.10 V14b), the yield of product 4 decreased
to 57% and the cross-coupling product 3 started to appear
(17% yield) (Scheme 2b). The efficiency further increased
ox
when the even less oxidative 4DPAPN as the PC (*E1/2
=
+0.93 V, Figure S16-S17), which afforded 3 with 51% yield
and only a small amount of 4 (17% yield) (Scheme 2b).30
The EnT-promoted formation of 3 was further supported
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AUTHOR INFORMATION
Page 6 of 8
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3
4
5
6
7
Corresponding Author
*lisz@ntu.edu.sg
*jier.huang@marquette.edu
*jzhang3@unl.edu
Notes
The authors declare no competing financial interest.
8
9
ACKNOWLEDGMENT
J.Z. acknowledges a National Science Foundation CAREER
Award (DMR-1554918) for support of this research. Q.L.
thanks the support from China Scholarship Council
(201709480009). B.P. and J.H. acknowledge the support from
National Science Foundation (DMR-1654140). S.L. and W.X.S.
thank the support from Academic Research Fund Tier 1
(RG2/16). The use of NIR and nanosecond transient absorp-
tion spectroscopy at the Center for Nanoscale Materials in
Argonne National Laboratory was supported by the U. S. De-
partment of Energy, Office of Science, Office of Basic Energy
Sciences, under Award No. DE-AC02-06CH11357. The compu-
tational work was partially performed on resources of the
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National
Supercomputing
Centre,
Singapore
(https://www.nscc.sg).
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