Highly effective near-infrared activating tripleta-triplet annihilation upconversion for photoredox catalysis

Article abstract of DOI:10.1021/jacs.0c06976

Organic triplet-triplet annihilation upconversion (TTA-UC) materials have considerable promise in areas as broad as biology, solar energy harvesting, and photocatalysis. However, the development of highly efficient near-infrared (NIR) light activatable TTA-UC systems remains extremely challenging. In this work, we report on a method of systematically tailoring an annihilator to attain such outstanding systems. By chemical modifications of a commonly used perylene annihilator, we constructed a family of perylene derivatives that have simultaneously tailored triplet excited state energy (T1) and singlet excited state energy (S1), two key annihilator factors to determine TTAUC performance. Via this method, we were able to tune the TTAUC system from an endothermic type to an exothermic one, thus significantly elevating the upconversion performance of NIR light activatable TTA upconversion systems. In conjunction with the photosensitizer PdTNP (10 μM), the upconversion efficiency using the optimal annihilator (100 μM) identified in this study was measured to be 14.1% under the low-power density of NIR light (100 mW/cm2, 720 nm). Furthermore, using such a low concentration of perylene derivative, we demonstrated that the optimal TTA-UC pair developed in our study can act as a highly effective light wavelength up-shifter to enable NIR light to drive a photoredox catalysis that otherwise requires visible light. We found that such an NIR driven method is highly effective and can even surpass directly visible light driven photoredox catalysis. This method is important for photoredox catalysis as NIR light can penetrate much deeper in colored photoredox catalysis reaction solutions, especially when done in a large-scale manner. Furthermore, this TTA-UC mediated photoredox catalysis reaction is found to be outdoor sunlight operable. Thus, our study provides a solution to enhance NIR activatable organic upconversion and set the stage for a wide array of applications that have previously been limited by the suboptimal efficiency of the existing TTA upconversion materials.

Full text of DOI:10.1021/jacs.0c06976

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Highly Effective Near-Infrared Activating Triplet−Triplet
Annihilation Upconversion for Photoredox Catalysis
Ling Huang, Wenting Wu, Yang Li, Kai Huang, Le Zeng, Wenhai Lin, and Gang Han*
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ABSTRACT: Organic triplet−triplet annihilation upconversion
(TTA-UC) materials have considerable promise in areas as broad
as biology, solar energy harvesting, and photocatalysis. However,
the development of highly efficient near-infrared (NIR) light
activatable TTA-UC systems remains extremely challenging. In this
work, we report on a method of systematically tailoring an
annihilator to attain such outstanding systems. By chemical
modifications of a commonly used perylene annihilator, we
constructed a family of perylene derivatives that have simulta-
neously tailored triplet excited state energy (T1) and singlet excited
state energy (S1), two key annihilator factors to determine TTA-
UC performance. Via this method, we were able to tune the TTA-
UC system from an endothermic type to an exothermic one, thus
significantly elevating the upconversion performance of NIR light activatable TTA upconversion systems. In conjunction with the
photosensitizer PdTNP (10 μM), the upconversion efficiency using the optimal annihilator (100 μM) identified in this study was
measured to be 14.1% under the low-power density of NIR light (100 mW/cm2, 720 nm). Furthermore, using such a low
concentration of perylene derivative, we demonstrated that the optimal TTA-UC pair developed in our study can act as a highly
effective light wavelength up-shifter to enable NIR light to drive a photoredox catalysis that otherwise requires visible light. We found
that such an NIR driven method is highly effective and can even surpass directly visible light driven photoredox catalysis. This
method is important for photoredox catalysis as NIR light can penetrate much deeper in colored photoredox catalysis reaction
solutions, especially when done in a large-scale manner. Furthermore, this TTA-UC mediated photoredox catalysis reaction is found
to be outdoor sunlight operable. Thus, our study provides a solution to enhance NIR activatable organic upconversion and set the
stage for a wide array of applications that have previously been limited by the suboptimal efficiency of the existing TTA upconversion
materials.
lanthanide ions, concomitant suboptimal upconversion effi-
ciency, and typically require laser light with a relatively high
excitation power intensity.16,17
INTRODUCTION
■
Photon upconversion provides the possibility to convert low-
energy near-infrared photons (650−1300 nm) under low-
irradiation conditions to high-energy light emission.1,2 In
particular, materials that can upconvert near-infrared light
(NIR) to visible emissions have considerable promise in wide-
ranging applications in solar energy harvesting,3 photo-
catalysis,4 and biology5−7 For example, such upconversion
systems are able to enhance the utilization of near-infrared
photons from the solar energy spectrum in order to increase
the efficiency of solar energy harvesting.8−10 Moreover, due to
their deep tissue penetration ability, reduced photon damage,
and decreased light scattering, NIR absorbing upconversion
materials have great potential for the exploration of biological
applications.5−7 In particular, NIR absorbing lanthanide dope
upconversion materials have been reported on in numerous
areas, such as photodynamic therapy,6 biological imaging,11,12
chemical sensing,13 and optogenetics.14,15 Despite holding
such promise, inorganic upconversion materials are susceptible
to certain inherent challenges, such as the low absorption of
To overcome these key and urgent challenges, organic
triplet−triplet annihilation photon upconversion (TTA-UC)
materials have emerged as a highly desirable next generation
upconversion system.18−30 Compared to their inorganic
counterparts, such an organic-based upconversion system
generally requires light with a lower power intensity and offers
potentially higher quantum efficiency. In general, TTA-UC
consists of a two-part system, including a photosensitizer (Sen)
molecule and an annihilator molecule (An). As outlined in
Received: June 29, 2020
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A

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Figure 1. (a) Schematic illustration of the design for our NIR exciting TTA-UC system via the optimization of triplet excited state of an annihilator
(Py0−Py5). (b) Molecular structures of the photosensitizer (PdTNP) and the group of annihilators (Py0, Py1, Py2, Py3, Py4, and Py5) in this
study. (c) Upper: the general rules of design annihilators in TTA-UC. Bottom: the singlet excited state and triplet excited state of PdTNP and the
annihilators of Py0−Py5, and the respective TTA-UC efficiencies, 100 mW/cm2 under 653 nm laser or 720 nm LED. (d) Photographs of TTA-UC
with Py0−Py5 as annihilators, respectively, taken under 100 mW/cm2 of 653 nm light.
Figure 1a, in a typical TTA-UC process, the photosensitizer is
first excited to its singlet excited state 1[Sen]* by long
wavelength low-energy photons; this is followed by an
intersystem crossing (ISC) in order to reach its triplet state
3[Sen]*. The photosensitizer subsequently transfers its triplet
excited energy to the triplet excited state of the annihilator
3[An]* through triplet−triplet energy transfer (TTET).
Afterward, when the collision of the annihilators in the triplet
state occurs, an annihilator can reach an excited-singlet state
(1[An]*), which ultimately leads to an upconverted photon
outcome when the annihilator decays back to the ground
state.18−30
Despite advances in short wavelength visible light activatable
TTA-UC systems,31−34 the development of an effective NIR
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light activated TTA-UC has been challenging.4,35−49 The
current NIR light activated TTA-UC typically relied on the
photosensitizer optimization, and the respective systems are
summarized in Table S1.4,35−49 For example, PtTPTNP
(TPTNP = tetraphenyltetranaphtho [2,3] porphyrin) was
coupled with rubrene or perylenediimide (PDI) to upconvert
690 nm incident photons into yellow fluorescence. The TTA-
UC efficiency of these TTA-UC pairs have been modest, and
the TTA-UC efficiency of PtTPTNP/rubrene and PtTPTNP/
has been restricted by the suboptimal NIR activated TTA-UC
efficiency. To date, TTA-UC dye pairs that contain the
perylene annihilator and platinum(II) tetraphenyltetranaph-
thoporphyrin (PtTNP) as the sensitizer as well as a TTA-UC
pair of furanyldiketopyrrolopyrrole (FDPP) annihilator and
palladium(II) octabutoxyphthalocyanine (PdPc) sensitizer
have been attempted.4 Nevertheless, the TTET process from
PtTNP to perylene is an unfavorable endothermic process, as
the T1 state of perylene (1.53 eV)61 is higher laying than the
T1 state of PtTNP (1.39 eV).36 As a consequence, the TTA-
UC efficiency was low, at just 2%, even with high
concentrations of annihilators perylene (31.3 mM) in
conjunction with photosensitizer PtTNP (10 μM).4 Therefore,
the development of highly effective exothermic NIR activated
TTA-UC is urgently important in regard to light driven
photoredox catalysis.
PDI have been reported to be 6.6
0.4% and 6.0
0.5%,
respectively.36 In addition, the photosensitizers of supra-
molecular ruthenium complexes presented the NIR absorption
at 750 nm but their TTA upconversion efficiency was also
found to be extremely poor (0.75%) with the annihilator PDI
in degassed 2-methyltetrahydrofuran.37 Moreover, a rather low
(0.43%) TTA-UC efficiency was observed in solid state film
where a lipophilic NIR absorbing osmium complex was used as
the sensitizer and rubrene was used as the annihilator.38 The
value of this TTA-UC pair in dichloromethane is 0.0047% and
in terms of its quantum yield it is 0.00235%. For other osmium
complex systems, the TTA-UC efficiencies were also recorded
RESULTS AND DISCUSSION
■
Compared to the above photosensitizer optimization methods,
systematic chemically modification of the annihilators may lead
to tunable properties of the S1 and T1 states, as well as
subsequent improvement of the NIR light activated TTA
upconversion performance. Herein, in this work, we attained
highly efficient NIR exciting TTA-UC via optimizing the
annihilator. In particular, we systematically synthesized a series
of perylene derivative compounds and studied their triplet
excited states. By doing so, we are able to tune the TTA-UC
pair from the endothermic to the exothermic type, as well as
improve the ultimate TTA-UC performance. This molecular
structural modification was found to be able to simultaneously
tune the two key photophysical properties of the Py
annihilators in relation to TTA-UC enhancement (i.e., the
singlet excited states (1An*) and the triplet excited states
(3An*)). As illustrated in Figure 1a, both the energy levels of
2+
to be modest: 2.7% for the Os(bptpy)2 sensitizer and
2,5,8,11-tetra-tert-butylperylene (TTBP) annihilator39 and
2+
5.9% in the pair of Os(peptpy)2 and TTBP. The latter of
TTA-UC pair-based hydrogel was explored in regard to in vitro
optogenetic genome engineering in hippocampal neuron cell
culture.40
Moreover, recently, the silyl-substituted anthracene violet
annihilator that was coupled with a NIR absorbing osmium
complex presented a high TTA-UC efficiency (11%) and large
anti-Stokes shift (1.28 eV).41 The TTA-UC efficiency of NIR
absorbing phthalocyanine (e.g., palladium phthalocyanine)
photosensitizer together with the diketopyrrolopyrrole deriv-
atives annihilator was 3.2% in degassed toluene.42 Meanwhile,
under NIR light irradiation, the TTA-UC efficiency (11.2%)
was observed in the presence of Pd-phthalocyanine and
rubrene.43 In another example, in regard to the TTA-UC pair
of Pt(II) meso-tetraphenyltetrabenzoporphine (PtTPBP)
sensitizer and 9,10-bis[((triisopropyl)silyl)ethynyl]anthracene
(TIPS-Ac) annihilator, direct NIR light excitation at 785 nm
(the S0 to T1 transition of PtTPBP) led to 2.1% TTA-UC
efficiency.44 Nevertheless, these above important progresses
typically require high annihilator concentrations (>1.0 mM)
and relatively high power excitation light density.
On the other hand, visible light driven photoredox catalysis
has been used in the synthesis of small molecules, polymers as
well as organic nanomaterials.50−53 However, there are a series
of intrinsic limitations for the use of such visible light
illumination in photoredox catalysis, especially for large-scale
setup.4 More specifically, both the substrates and photo-
catalysts typically strongly absorb visible light. Thus, substrates
compete with photocatalysts for the absorption of incident
visible light, leading to suboptimal reaction efficiency. More-
over, the penetration depth of visible light is very shallow
through the colored reaction solution, leading to challenges in
large-scale reactions. In contrast, due to minimal overlap with
the absorption of substrates and photocatalysts, NIR light
(>700 nm) has a much deeper penetration depth in these
colored reaction mediums.4,54,55 However, directly using the
low energy of NIR light has been challenging for driving
photoredox catalysis.56−58 In this regard, TTA-UC has held
great promise as the phototransducer to drive photoredox
catalysis under NIR light illumination.4,59,60 Yet, such promise
3
1An* and An* are essential for effective TTA-UC and there
are two general rules for designing TTA-UC annihilators: (1)
the annihilator should have a lower laying triplet excited state
than that of the triplet excited state of the photosensitizer
3
(3[An]* < [Sen]*), allowing an efficient TTET process to
occur from photosensitizer to annihilator; (2) the doubled
energy of the triplet excited state of the annihilator should be
higher than that of the singlet excited state of the annihilator (2
3
1
× [An]* > [An]*), enabling the ultimate effective TTA-
upconversion to take place. Following these rules, we
developed a family of the new Py annihilators by altering the
aromatic groups appended to Py (Figure 1b). We found that
these respective Py derivatives have simultaneously fine-tuned
3[An]* and 1[An]*. In this study, we used a typically used NIR
absorbing photosensitizer palladium(II) tetraphenyltetranaph-
thoporphyrin (PdTNP), which has intense absorption in the
NIR region (λex = 702 nm, ε = 1.64 × 105 M−1 cm −1) and a
long triplet excited lifetime (τT = 65 μs) (Table S2).62,63
Accordingly, in conjunction with PdTNP, we found that such
an annihilator modification strategy was able to systematically
enhance the TTA-UC performance by switching the TTA-UC
system from the undesired endothermic type (3[An]* >
3[Sen]*) to a favorable exothermic one (3[An]* < 3[Sen]*). In
addition, since the singlet excited energy 1[Py]* can be
simultaneously tuned along with the chemical modification of
Py, the ultimate upconverted emission color was found to be
able to be adjusted from 490 to 580 nm. In particular, the
highest recorded TTA-UC efficiency (16.7%, 0.1 W/cm2, 653
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nm laser and 14.1%, 0.1 W/cm2, 720 nm LED) was found in
our optimal TTA-UC system (Figure 1c). Moreover, as a
proof-of-principle, we demonstrated that the highly effective
exothermic TTA-UC pair identified in our study enables highly
effective NIR driven photocatalysis of the conventional visible
light photocatalyst of Eosin Y to occur for both small-scale and
large-scale reactions.
lower than 1.55 eV (Figures S19−S24 and Table S6). By
contrast, Py0 did not quench the phosphorescence of PdTNP
(Figure S25), suggesting that the range of the T1 state for Py0
is from 1.55 to 1.33 eV. We observed that the phosphorescence
of PdTNP is quenched within high concentrations of Py1−Py3
(Figures S26−S28), suggesting that the T1 of Py1−Py3 is close
to the T1 of PdTNP. Compared to other annihilators, Py4 and
Py5 effectively quench the phosphorescence of PdTNP
(Figures S29 and S30), suggesting that the T1 states of Py4
and Py5 are lower than 1.33 eV. In order to further
quantitatively analyze the T1 of these annihilators of Py4 and
Py5, we first measured the phosphorescence of Py4 and Py5 at
77 K to determine the T1 states of Py4 and Py5. However, we
still did not observe the phosphorescence emissions of Py4 and
Py5 (Figure S31). Therefore, we performed time-dependent
density functional theory (TD-DFT) calculations. As can be
seen in Table S3, we found that, when the Py core was
appended by aryl-alkynyl, the T1 state was calculated to
steadily decrease. In particular, the triplet excited state of Py0 is
1.53 eV. This value drops to 1.38 eV for the monoaryl-alkynyl
Pys (Py1, Py2, and Py3) and further decreases to 1.23 eV for
the diaryl-alkynyl Pys (Py4 and Py5). The results of these
calculations are consistent with our estimated T1 state of Py0−
Py5 via the TTET method. Moreover, the doubled triplet
In particular, the rich and versatile chemistry of Py makes it
possible for the rapid, divergent synthesis of the series of Py
annihilators. Herein, via a Sonogashira cross-coupling reaction,
a group of new annihilators (Py1, Py2, Py3, Py4, and Py5) with
an appended aryl-alkynyl to the Py core were able to be
synthesized and characterized (Scheme S1). Due to the large
and efficient π-conjugation, the absorption and fluorescence
wavelengths were found to be red-shifted. The UV−vis
absorption spectra of the annihilators cover a wide range of
electromagnetic spectra, from 400 to 550 nm (Figure S2).
Moreover, the fluorescence of the annihilators occurs at 470−
600 nm (Figure S3), and the respective singlet excited energy
of the annihilators (1[An]*) systematically decreases from
2.78, 2.60, 2.57, 2.56, and 2.44 to 2.39 eV (Table S4). The
fluorescence quantum yields (ΦF) are determined to be 83%,
70%, 72%, 68%, 74%, and 76% for Py0, Py1, Py2, Py3, Py4,
and Py5, respectively (Table S2). In addition, the solvent
dependency of the UV−vis absorption and the fluorescence
emission spectra of Py0−Py5 were measured. All of the
annihilators presented similar UV−vis absorption in solvents of
different polarities (toluene, DCM, THF, and DMF),
suggesting that the ground state of annihilators is not sensitive
for the polarity of solvents (Figure S4).64 Moreover, the
annihilators have similar fluorescence intensity and profiles in
solvents of different polarities, indicating that the S1 state of the
annihilators is not affected by solvents’ polarity (Figure S5).65
The cyclic voltammograms of annihilators Py0−Py5 were
measured (Figures S6−S11 and Table S5). We only observed
reversible oxidation with E1/2 values of 1.02, 0.99, 0.99, 0.84,
0.94, and 0.98 V vs Ag/AgCl, respectively. Compared to other
annihilators, due to the electron-donating carbazole substituted
perylene, Py3 presented a lower oxidation potential of 0.84 V.
To better understand the electronic properties of these
perylene derivatives Py0−Py5, their ground state geometry was
optimized at the B3LYP/6-31+G(d) level of density functional
theory (DFT) (Figures S12−S17). Their HOMOs and
LUMOs are well-distributed over perylene cores, but only
partial electron density is observed for their phenylethynyl
moieties. Their energy band gaps were estimated to be 2.87,
2.58, 2.51, 2.52, 2.24, and 2.22 eV for Py0−Py5, respectively,
which is consistent with the results from the experiments
(Table S4).
3
excited energy of Py (2 × [An]*) is higher than the singlet
excited state energy (1[An]*) in all of the Py derivatives (Py0,
Py1, Py2, Py3, Py4, and Py5) (Table S4), indicating that these
perylene derivatives satisfied the condition of being effective
annihilators.
For TTA-UC to be efficient, 3[Sen]* is required to be above
3[An]*. Thus, favorable exothermic TTA-UC can take place.
The triplet excited state of our photosensitizer PdTNP is
characterized to be located at 1.33 eV (Figure S1). This energy
level is lower than the triplet excited state of the original core
perylene molecule (Py0), similar to those of monoaryl-alkynyl
Pys (Py1, Py2, and Py3) but surpassed those of diaryl-alkynyl
Pys (Py4 and Py5). Thus, accordingly, we anticipated that we
can tailor the TTA-UC system from the undesired
endothermic type to a favorable exothermic one. As such,
the upconversion performance would be divided into three
distinctive levels (Py4, Py5 > Py1, Py2, Py3 > Py0).
In order to validate this hypothesis, we conducted
experiments to examine the TTA-upconversion properties of
this family of Py annihilators in conjunction with the
photosensitizer PdTNP. In our study, we found that a high
concentration of perylene derivatives will significantly quench
the fluorescence intensity of annihilators via inner filter effects
(Figures S32−S37).69 In addition, the two crucial steps in
TTA upconversion, including TTET and TTA, are highly
dependent on the concentration of the annihilators.70
Therefore, we further explored the optimal concentration of
the annihilators in TTA-UC. We found that, by increasing the
concentration of annihilators, the TTA-UC first gradually
increases. Then, when the concentration of the annihilators
exceeds the optimal concentration, the intensity of TTA-UC
did not further increase (Figures S38−S42). By doing so, we
determined that the optimized concentrations of the
annihilators (Py1, Py2, Py3, Py4, and Py5) were to be 600,
1000, 1000, 300, and 100 μM in the presence of PdTNP (10
μM), respectively.
Next, we studied the T1 of the annihilators. We estimate the
T1 state of annihilators by the triplet−triplet energy transfer
(TTET) method.66−68 In the experiment, we selected meso-
tetraphenyl-tetrabenzoporphine palladium complex (PdTPBP)
and PdTNP as photosensitizers, which have different wave-
lengths of phosphorescence emission. According to phosphor-
escence spectra of PdTPBP and PdTNP, we calculated the T1
values as 1.55 and 1.33 eV, respectively. When the annihilators
were titrated into the solution of PdTPBP or PdTNP, the
phosphorescence were observed to be significantly quenched,
indicating that the T1 of annihilators is lower than the T1 of the
photosensitizers (Figure S18). In particular, we observed that
the phosphorescence of PdTPBP was clearly quenched for all
of the annihilators, suggesting that the T1 state of Py0−Py5 is
Our experimental results agree quite well with our
hypothesis. Under the optimal conditions, Py0 was not
observed to produce TTA-UC; the monoaryl-alkynyl annihi-
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Figure 2. (a) Normalized upconversion emission spectra of Py1−Py5 in toluene, λex = 653 nm (100 mW/cm2). (b) CIE diagram representing the
adjustable upconversion emission colors. (c) Upconverison pictures of Py0−Py5 with PdTNP. For Py0−Py3 with PdTNP, the excitation power is
1.0 W/cm2; for Py4 and Py5 with PdTNP, the excitation power is 0.1 W/cm2, λex = 653 nm. (d) Threshold intensity and upconversion efficiency of
Py1−Py5 in conjunction with PdTNP. (e) Upconversion emission spectra of the TTA-UC system (Py5 (100 μM) as the annihilator and PdTNP
(10 μM) as the photosensitizer) under argon with various excitation intensities (λex = 653 nm; 5.8, 7.5, 9.4, 15.8, 27.4, 51.3, 60.8, 77.3, 105.9, 146.5,
178.3, 216.6, 263.7, 293, 388.5, 454.8, 532.5, and 636.9 mW/cm2). (f) Dependence of the TTA-UC intensity of the solution of PdTNP and Py5 on
the various incident power densities. The results fit the lines that have slopes of 2.06 (black, below) and 1.24 (red, above) in the low- and high-
power regions, respectively; Ith is 40.7 mW/cm2.
lators (Py1, Py2, and Py3) showed modest TTA upconversion
signals. In stark contrast, the diaryl-alkynyl annihilators (Py4
and Py5) show intense TTA upconversion emissions (Figure
2c). Moreover, since the systematic chemical modification of
Py also shifted the singlet excited state energy of the
annihilators, the respective TTA-UC emission colors were
found to be able to be simultaneously tuned from 490 to 580
nm. The monoaryl-alkynyl perylene as annihilators (Py1, Py2,
and Py3) show that the upconversion luminescences are cyan
(Py1) and green (Py2 and Py3). Along with the increase in π
conjugation, the TTA-UC emission peaks of Py4 and Py5
further red-shifted and showed yellow and orange colors,
respectively (Figure 2c). We calculated their color coordinates
in the CIE diagram, which are (0.24, 0.64), (0.30, 0.66), (0.27,
0.67), (0.42, 0.57), and (0.47, 0.53) for Py1, Py2, Py3, Py4,
and Py5, respectively (Figure 2b). Moreover, an important
parameter for upconversion systems is the excitation rate
threshold (Ith), wherein the upconverted light intensity
switches from a quadratic to a linear dependence on incident
power. We find that, for Py1, Py2, and Py3, these thresholds
are 347.0, 65.3, and 53.7 mW/cm2, respectively (Figures S43−
S45). For Py4 (Figure S46) and Py5 (Figure 2e,f), the
thresholds fall at 42.9 and 40.7 mW/cm2, respectively.
as this is an established standard method for TTA-UC
efficiency measurement. More specifically, TTA-UC emissions
for monoaryl-alkynyl appended Pys (Py1, Py2, and Py3) have
peaks at 518, 526, and 525 nm, respectively, and their
upconversion efficiencies were detected to be 0.062%, 0.25%,
and 0.24% in toluene. The TTA-UC efficiencies of diaryl-
alkynyl appended Py annihilators (Py4 and Py5) were also
measured. When Py4 was used as the annihilator, green
emissions peaking at 550 nm were observed and the
upconversion efficiency went up to 7.6%. When the Py5 was
used as the annihilator, the record highest TTA upconversion
efficiency (16.7%) was found.
We then studied the TTA-UC performance of the optimal
Py annihilators (Py4 and Py5) under the illumination of 720
nm NIR LED. The transition threshold (Ith) between the
quadratic and the linear regime, respectively, occurs near 46.4
and 46.0 mW/cm2 (Figure 3b,e), which is comparable to the
value under 653 nm light (Figure 2d). As shown in Figure 3c,f,
the upconversion efficiencies of Py4 and Py5 increased and
reached a plateau level at 6.4% and 14.1% beyond their
respective Ith values (46.4 and 46.0 mW cm−2) using
indocyanine green (ICG, Φf = 12% in DMSO) as a reference
(Table S11). These upconversion efficiencies are similar when
we used the calculated ΦUC (653 nm) to estimate the ΦUC
(720 nm) (Table S10). In addition, the anti-Stokes shifts of
PdTNP/Py4 and PdTNP/Py5 are 0.66 and 0.45 eV,
respectively. More importantly, such upconversion lumines-
The respective upconversion efficiencies were first evaluated
under 100 mW/cm2 and 653 nm light excitation (Figure S47
and Table S9). In this study, the fluorescence quantum yield of
zinc phthalocyanine (ZnPc) in DMF was used as the reference,
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Figure 3. (a) Full UC spectra profile of Py4 as annihilators and PdTNP as photosensitizers under Ar with various excitation intensities at 720 nm
(7.5−160 mW/cm2). A 690 nm long-pass filter was used to block potential shorter wavelength light interference from the LED. (b) Dependence of
the TTA-UC intensity of the solution of PdTNP and Py4 on the various incident power densities. The results fit the lines that have slopes of 2.02
(black, below) and 0.98 (red, above) in the low- and high-power regions, respectively; the Ith is 46.4 mW/cm2. (c) Upconversion efficiency of the
mixture of PdTNP and Py4 measured as a function of 720 nm incident power density; inset photographs are TTA-UC emission for Py4 under 720
nm LED irradiation (10, 20, and 30 mW/cm2). (d) Full UC spectra profile of Py5 as annihilators and PdTNP as photosensitizers under Ar with
various excitation intensities (λex = 720 nm, 7.5−160 mW/cm2). A 690 nm long-pass filter was used to block potential shorter wavelength light
interference from the LED. (e) Dependence of the TTA-UC intensity of the solution of PdTNP and Py5 on the various incident power densities.
The results fit the lines that have slopes of 1.96 (black, below) and 1.07 (red, above) in the low- and high-power regions, respectively; the Ith is 46.0
mW/cm2. (f) Upconversion efficiency of the mixture of PdTNP and Py5 measured as a function of 720 nm incident power density; inset
photographs are TTA-UC emission for Py5 under 720 nm LED irradiation (10, 20, and 30 mW/cm2).
cence that uses Py4 and Py5 as annihilators is strong enough to
be clearly observed by the naked eye under 720 nm NIR LED,
with a power density as low as 10 mW/cm2 (Figure 3c,f).
In order to explore the reasons for the outcome of the ΦUC
for the different TTA-UC pairs, we measured the phosphor-
escence quenching within different annihilators and then
calculated the Stern−Volmer quenching constant (ksv) and the
bimolecular quenching constant (kq) (Table S7).71−73 Py4 and
Py5 presented higher ksv values than those of Py1−Py3,
indicating that a lower level of the T1 state of annihilators than
PdTNP is beneficial for TTA upconversion. Moreover, we
calculated the TTET (ΦTTET) and normalized TTA efficiency
(ηTTA) of TTA-UC pairs according to a well-established
method in the literature (Table S8).74−76 Due to the higher T1
state, the ΦTTET is as low as 1.87% for Py0. For the Py1−Py3,
the ΦTTET values are 5.54%, 17.1%, and 5.97% due to the small
energy gap between PdTNP and annihilators, respectively
(Figure S48). In contrast, the higher ΦTTET values for Py4
(33.7%) and Py5 (41.1%) were observed in the presence of
PdTNP. Additionally, only Py4 and Py5 show high ηTTA values,
those of 30.4% and 53.2%, respectively. We further explored
the microscopic structures of our systems. In particular, we
conducted transmission electron microscope (TEM) imaging
for the drop-casted PdTNP/Py4 and PdTNP/Py5, and these
samples were negatively stained via 5% sodium phosphotung-
state (Figure S54). Interestingly, in the TEM imaging, we
observed that the more effective UC pair (PdTNP/Py5) can
form nanoparticles. This self-assembly is also likely to
contribute to the promotion of the TTET process, thereby
improving the ultimate TTA-UC efficiency.
Upon identifying the best performing TTA-UC pairs, as a
proof-of-principle, we used such TTA-UC pairs as photo-
transducers in order to study NIR light driven photoredox
catalysis in conjunction with conventionally green absorbing
photocatalyst of Eosin Y. Photoredox catalysis is a typically
complex photochemical process.50−53 The product yield is
dependent on a series of factors, including the intensity of the
incident light, the reaction device setup, and the specific nature
of the reactions.50−53 Although in previous studies NIR-
activated photoredox catalysis has been observed with high
yields,4 the specific power density and the relationship between
TTA-UC efficiency and the outcome reaction yields appears to
be absent. In addition, in view of the specific nature of different
photoredox catalytic reactions, the relationship between the
photoredox process and TTA-UC may vary and needs to be
investigated on a case-by-case basis. In our work, we chose the
photooxidation of aryl boronic acid to phenol as an example of
a reaction. Due to mild reaction conditions, high product of
yield and good selectivity, such synthesis of the phenols from
boronic acids, is much more plausible as compared to the
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Figure 4. NIR light activation photooxidation arylphenylboronic acid. (a) Different TTA-UC pairs coupling with Eosin Y mediated
photooxidiation under NIR light illumination. (b) Product yields under different reaction volumes; the diameter of the reactor is 3.9 cm, the
diameter of the NIR LED beam is 3.1 cm, the rate of magnetic stirring is 300 rpm. (c) Other arylphenylboronic acid photooxidation results.
conventional transformations using aryl halides and aryl
diazonium salts as substrates.77−79 The PdTNP/Py5 as the
brightest TTA-UC pair was found to be effective in
conjunction with Eosin Y and the oxidant of PhI(OAc)2
(Figures S49 and S50) for such a NIR light driven reaction.
In contrast, we did not observe good product yields for other
suboptimal TTA-UC pairs (Py0−Py4 and PdTNP) (Figure
4a). This result not only clearly shows that an increase in the
TTA-UC efficiency can lead to improved reaction yields but
also suggests that enhancement of TTA-UC efficiency via the
rational design of the annihilators is essential to increasing the
product yield. Moreover, it is exciting that we further found
that such a NIR driven method can even surpass directly
visible light driven photoredox catalysis. This is the case
because of the reduced photobleaching for the Eosin Y with
NIR light. For example, in the absence of the TTA-UC pair,
the photocatalytic reaction with green light (530 nm, 20 mW/
cm2) leads to a 76% product yield. (Figure S49, entry 1). In
contrast, the combination of the pair of PdTNP and Py5 is able
to effectively improve the product yield to 78.2% under an NIR
LED (20 mW/cm2, 720 nm) (Figure S49, entry 10).
Meanwhile, due to the deep penetration of the colored
reaction solution for NIR light, we were able to carry out
photooxidation in a large-scale reaction. More specifically, for
directly visible light driven photooxidation, when we increased
the reaction volume from 2 to 20 mL, the product yield
significantly dropped from 76% to 28%. In contrast, we
observed a quite insignificant decrease from 78.2% to 60% in
the yield of the photooxidation product in the presence of
TTA-UC under NIR light (Figure 4b and Figure S51). In
addition, we found similar high product yields with a series of
other electron-rich, electron-deficient, and electron-neutral
arylboronic acids with such a TTA-UC pair (Figure 4c).
Interestingly, since only low-power NIR light can activate
PdTNP/Py5 to produce bright visible upconversion light, we
explored whether outdoor sunlight can drive the photo-
oxidation of aryl boronic acid in order to generate aryl phenol.
As a result, in the presence of a 690 nm long pass filter, which
can minimize the interference of the shorter wavelength light,
the product yield was observed to be 21% in the presence of
PdTNP/Py5 and Eosin Y. In contrast, in our control
experiment, in the absence of PdTNP/Py5, the product yield
of Eosin Y alone was only ∼10% (Figure S52). Compared to
our results with the Eosin Y alone in the dark (5.4%), the
difference is quite insignificant. We would like to note that the
sunlight driven reaction yield should be further increased, as
our proof-of-concept sunlight reaction setup can be further
optimized as well as be done with stirring. These results
suggest that, via coupling with a visible light absorbing
photocatalyst, the highly effective exothermic NIR activation
TTA-UC that we developed as a phototransducer acts as a
robust, general photontransducer platform that overcomes the
shortcomings of existing visible light driven photoredox
catalysis, especially in their large-scale manner. It is also
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Article
worth noting that the concentration of our exothermic
annihilator is ∼300 times less than the reported endothermic
one in the literature.4
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
Finally, we attempted to study the mechanism of photo-
oxidation of aryl boronic acid to produce aryl phenol (Figure
S56). In this reaction, the *[Eosin Y]1 is essential for the
catalysis of the photooxidation of aryl boronic acid. Under the
653 nm laser, in regard to the Eosin Y only solution, we did not
observe the fluorescence emission of Eosin Y at 575 nm,
suggesting that a long wavelength light (>650 nm) cannot
excite the Eosin Y (Figure S53b). Similarly, under 653 nm
light, in the presence of PdTNP/Eosin Y, we did not observe
the TTA-UC at 575 nm, indicating that PdTNP is not able to
https://pubs.acs.org/doi/10.1021/jacs.0c06976.
Scheme of synthetic pathways, discussions of chemicals
and characterization methods used, measurement of
fluorescence quantum yield, electrochemical measure-
ments, calculation of standard oxidation/reduction
potential, TTA upconversion characterization, measure-
ment of the upconversion efficiency, Stern−Volmer
quenching plot experiment, triplet−triplet energy trans-
fer efficiency measurements, normalized triplet−triplet
annihilation efficiency calculation, TEM imaging, visible
light and sunlight driven photooxidation arylboronic
acid to phenol, and large volume NIR light driven
photooxidation aryl boronic acid to phenol, tables of
summarized reported NIR to visible light TTA
upconversion system, photophysical parameters, DFT
calculation for the annihilators, summary of the singlet
excited/triplet excited state energy levels, electro-
chemical properties, Stern−Volmer quenching constant
and bimolecular quenching constant, TTET quantum
efficiency and normalized TTA efficiency, calculation
summary of TTA-UC efficiency, estimated TTA
upconversion efficiency, and calculation summary of
TTA-UC efficiency, and figures of UV−vis absorption
spectra, fluorescence spectra, cyclic voltammograms,
electron density maps, simplified Jablonski diagram
illustration, phosphorescence intensity decays, Stern−
Volmer plots, normalized fluorescence emission spectra,
quantitative analysis of the relationship between
fluorescence and upconversion intensity and concen-
tration, double-logarithmic plot, TTA-UC spectra,
optimization and control experiments, setup of photo-
oxidation of aryl boronic acid to aryl phenol reaction,
NIR activated photooxidation of arylboronic acid to
arylphenol, reaction setup for the reaction, chemical
equations, illustration reaction setups, TEM images,
proposed mechanism of NIR driven TTA-UC mediated
photooxidation of aryl boronic acid, NMR spectra, and
ESI-MS spectra (PDF)
1
sensitize the Eosin Y to generate the *[Eosin Y] (Figure
S53b). In contrast, in the presence of PdTNP/Py5 and Eosin
Y, the TTA-UC peaks at 550 and 570 nm were observed to be
significantly declined at the same the emission that Eosin Y at
575 nm was found to emerge. This indicates that the ground
state of Eosin Y can absorb the TTA-UC from PdTNP/Py5 to
produce *[Eosin Y]1 (Figure S53c). Meanwhile, significant
spectra overlap was observed between Py5 emission and Eosin
Y absorption, suggesting that the NIR light driven photo-
oxidation of aryl boronic acid is likely to occur via the TTA-
UC emission and Eosin Y reabsorption (Figure S53d).
Moreover, we compared the product yield in our system
under control conditions. As a result, under control conditions,
only in the presence of Eosin Y or PdTNP were the product
yields very low (7.2% and 4.9%) respectively, under NIR light
illumination (Figure S49, entries 3 and 5). Similarly, in the
presence of PdTNP/Eosin Y or PdTNP/Py5, the product
yields were also only 5.3% and 5.4%, respectively (Figure S49,
entries 6 and 7), which is close to the product yield in the dark
(5.7%) (Figure S49, entry 2). In contrast, in our system
(PdTNP/Py5 and Eosin Y), the product yield was significantly
higher (78.2%). These results further confirmed that both
Eosin Y and TTA-UC are essential, as illustrated in our
proposed mechanism, where Eosin Y absorbs the TTA-UC
1
from PdTNP/Py5 and is then converted to [Eosin Y] * in
order to enable the photooxidation of arylboronic acid.
CONCLUSION
■
In summary, we have demonstrated that the performance of
NIR light mediated TTA upconversion is able to be
significantly improved by optimizing the chemical structures
of Py annihilators and their respective triplet energy levels. In
view of this theory, we obtained the highest recorded NIR to
green TTA-UC pair (ΦUC = 16.7% (653 nm) and 14.1% (720
nm), PdTNP and Py5) with a low-concentration annihilator
(100 μM), as well as a low-power density of NIR light.
Moreover, via coupling with a visible light absorbing
photocatalyst (Eosin Y), we demonstrated that our exothermic
NIR activating TTA-UC system can act as an effective
phototransducer to boost the photocatalytic yield, especially
in large-scale reactions. This system was found to be effective
enough to even be operated under ambient sunlight
conditions. Together, this study provides a versatile and
straightforward approach toward the development of highly
efficient TTA-UC pairs that can operate under low-power NIR
light. This is particularly valuable for photocatalysts and a
variety of biological and solar energy harvesting applications.
AUTHOR INFORMATION
■
Corresponding Author
Gang Han − Department of Biochemistry and Molecular
Pharmacology, University of Massachusetts Medical School,
Worcester, Massachusetts 01605, United States; orcid.org/
0000-0002-2300-5862; Email: Gang.Han@umassmed.edu
Authors
Ling Huang − Department of Biochemistry and Molecular
Pharmacology, University of Massachusetts Medical School,
Worcester, Massachusetts 01605, United States
Wenting Wu − State Key Laboratory of Heavy Oil Processing
School of Chemical Engineering, China University of Petroleum,
Qingdao 266580, P. R. China; orcid.org/0000-0002-8380-
7904
Yang Li − Department of Biochemistry and Molecular
Pharmacology, University of Massachusetts Medical School,
Worcester, Massachusetts 01605, United States; orcid.org/
0000-0002-9544-5840
H
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Kai Huang − Department of Biochemistry and Molecular
Pharmacology, University of Massachusetts Medical School,
Worcester, Massachusetts 01605, United States
Le Zeng − Department of Biochemistry and Molecular
Pharmacology, University of Massachusetts Medical School,
Worcester, Massachusetts 01605, United States
Wenhai Lin − Department of Biochemistry and Molecular
Pharmacology, University of Massachusetts Medical School,
Worcester, Massachusetts 01605, United States
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.H. would like acknowledge thanks for the support from the
start-up funding from University of Massachusetts-Medical
School.
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