Reversible photochromic system based on rhodamine B salicylaldehyde hydrazone metal complex

Article abstract of DOI:10.1021/ja411689w

Photochromic molecules are widely applied in chemistry, physics, biology, and materials science. Although a few photochromic systems have been developed before, their applications are still limited by complicated synthesis, low fatigue resistance, or incomplete light conversion. Rhodamine is a class of dyes with excellent optical properties including long-wavelength absorption, large absorption coefficient, and high photostability in its ring-open form. It is an ideal chromophore for the development of new photochromic systems. However, known photochromic rhodamine derivatives, such as amides, exhibit only millisecond lifetimes in their colored ring-open forms, making their application very limited and difficult. In this work, rhodamine B salicylaldehyde hydrazone metal complex was found to undergo intramolecular ring-open reactions upon UV irradiation, which led to a distinct color and fluorescence change both in solution and in solid matrix. The complex showed good fatigue resistance for the reversible photochromism and long lifetime for the ring-open state. Interestingly, the thermal bleaching rate was tunable by using different metal ions, temperatures, solvents, and chemical substitutions. It was proposed that UV light promoted isomerization of the rhodamine B derivative from enol-form to keto-form, which induced ring-opening of the rhodamine spirolactam in the complex to generate color. The photochromic system was successfully applied for photoprinting and UV strength measurement in the solid state. As compared to other reported photochromic molecules, the system in this study has its advantages of facile synthesis and tunable thermal bleaching rate, and also provides new insights into the development of photochromic materials based on metal complex and spirolactam-containing dyes.

Full text of DOI:10.1021/ja411689w

Article
pubs.acs.org/JACS
Reversible Photochromic System Based on Rhodamine B
Salicylaldehyde Hydrazone Metal Complex
Kai Li,† Yu Xiang,† Xiaoyan Wang,† Ji Li,† Rongrong Hu,‡ Aijun Tong,*,† and Ben Zhong Tang*,‡
†Department of Chemistry, Beijing Key Laboratory for Microanalytical Methods and Instrumentation, Key Laboratory of Bioorganic
Phosphorus Chemistry and Chemical Biology (Ministry of Education), Tsinghua University, Beijing 100084, China
‡Department of Chemistry, State Key Laboratory of Molecular Neuroscience, Institute of Molecular Functional Materials, The Hong
Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
S
* Supporting Information
ABSTRACT: Photochromic molecules are widely applied in
chemistry, physics, biology, and materials science. Although a
few photochromic systems have been developed before, their
applications are still limited by complicated synthesis, low
fatigue resistance, or incomplete light conversion. Rhodamine
is a class of dyes with excellent optical properties including
long-wavelength absorption, large absorption coefficient, and
high photostability in its ring-open form. It is an ideal
chromophore for the development of new photochromic
systems. However, known photochromic rhodamine deriva-
tives, such as amides, exhibit only millisecond lifetimes in their colored ring-open forms, making their application very limited
and difficult. In this work, rhodamine B salicylaldehyde hydrazone metal complex was found to undergo intramolecular ring-open
reactions upon UV irradiation, which led to a distinct color and fluorescence change both in solution and in solid matrix. The
complex showed good fatigue resistance for the reversible photochromism and long lifetime for the ring-open state. Interestingly,
the thermal bleaching rate was tunable by using different metal ions, temperatures, solvents, and chemical substitutions. It was
proposed that UV light promoted isomerization of the rhodamine B derivative from enol-form to keto-form, which induced ring-
opening of the rhodamine spirolactam in the complex to generate color. The photochromic system was successfully applied for
photoprinting and UV strength measurement in the solid state. As compared to other reported photochromic molecules, the
system in this study has its advantages of facile synthesis and tunable thermal bleaching rate, and also provides new insights into
the development of photochromic materials based on metal complex and spirolactam-containing dyes.
application in the development of electrochemical
devices.27−30 Unfortunately, the fatigue resistance of spiropyran
is still moderate, especially under strong irradiation.14,31 On the
other hand, dithienylethene derivatives have attracted much
attention because of their excellent fatigue resistance and
thermally irreversible photochromic properties. The response
of dithienylethene to UV irradiation is fast both in solution and
in the solid state.32−34 Nevertheless, the unique structure of
dithienylethene usually makes the synthesis of its derivatives
multiple-step and high-cost.35−37 Therefore, the development
of a facilely synthesized photochromic system with good
photochromic properties is still highly demanded and
challenging.38−41 Moreover, for most of the known photo-
chromic molecules, the available methods to modulate thermal
bleaching rate are still limited. Besides environmental factors
such as temperature and solvent, changing substitution groups
is a common strategy, but it usually requires intense synthesis
works.42,43 Thus, simpler methods to tune thermal bleaching
rate are also needed.
INTRODUCTION
■
Photochromic materials have attracted increasing interest
because of their potential application in developing molecular
switches,1,2 molecular logic gates,3 optical data storage,4
optoelectronic devices,4−8 photoresponsive supramolecular
self-assembly,9−11 photocontrollable drug or ion delivery,12−15
photocontrollable biological processes16,17 and molecular
machines.18−20 Among them, organic photochromic molecules,
including azobenzene, spiropyran, dithienylethene, spirooxa-
zine, fulgide, anil, viologen, etc., undergo color changes upon
photo irradiation, and have shown good performance in their
chemical, biological, physical, and nanotechnology applica-
tions.13,21 Azobenzene is one classic photochromic material
with its trans and cis isomers interchanged by light irradiation
and heat,22,23 making it a very useful platform for photo-
triggered biological systems.17,23,24 However, the incomplete
reverse between trans and cis isomers and moderate fatigue
resistance limit its scope of practical application.25 Spiropyran is
another well-known photochromic compound, which has a
dipole moment enhancement and a large absorption change
upon UV irradiation.26 The phenol group in the ring-open form
of spiropyran can chelate metal ions, which enables its
Received: November 28, 2013
Published: January 7, 2014
© 2014 American Chemical Society
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Rhodamine is a class of dyes with long-wavelength
absorption and emission, high absorption coefficient and
quantum efficiency, and good photostability. It is widely
available and used in industrial coloration, biomarkers, and
fluorometric probes.44−46 However, the report of using
rhodamine derivatives as long-lifetime photochromic molecules
is still very scarce. As shown in Scheme 1, rhodamine amide
Scheme 1. Short-Lifetime Photochromism of a Rhodamine
Amide
Figure 1. (a) Photographic images of 1-Zn in THF upon irradiation,
and the recovery in dark at 25 °C. (b) Absorption and (c) fluorescence
spectra of 1-Zn in THF before and after UV irradiation. [1] = 10
μmol/L, [Zn(II)] = 100 μmol/L.
was reported to exhibit photochromic features in 1977.47
Unfortunately, the lifetime of the ring-open state is very short,
with only a few milliseconds in polar solvents.48 Because of the
short lifetime and low quantum efficiency of the photoinduced
reaction, the photochromic properties of rhodamine amides
were almost disregarded in the next 30 years.49−52 New
photochromic molecules can be developed by taking
advantages of rhodamine’s excellent optical properties if the
lifetime of the ring-open state is dramatically improved.
In this work, rhodamine B salicylaldehyde hydrazone, which
contains a rhodamine amide moiety and a photosensitive
salicylaldehyde Schiff bases part,53,54 was found to exhibit
photochromic properties with a long lifetime when forming
metal complex. This photochromic system exhibited good
fatigue resistance, and the thermal bleaching rate was tunable
by using different metal ions to form the complex. Moreover,
the photochromic system was successfully applied for photo-
printing and UV strength measurement in the solid state.
gradually turned purple. When the UV light was removed, the
solution restored to its original state in 10 min, suggesting a
long lifetime of the purple form under ambient condition
(Supporting Information video 1). Before UV irradiation, no
absorption band above 500 nm was observed for 1-Zn.
However, upon irradiation at 365 nm, a strong band centered
at 554 nm emerged (Figure 1b). The molar absorption
coefficient at 554 nm was as high as 6.7 × 104 cm−1 mol−1 L for
1-Zn after UV irradiation, which was higher than most reported
photochromic systems with similar absorption ranges.6,10,35−39
In contrast, the fluorescence emission of 1-Zn at 527 nm was
intense before UV irradiation, while it decreased and
transformed into two weaker peaks at 513 and 582 nm upon
UV irradiation (Figure 1c). When in the dark, the fluorescence
of UV-irradiated 1-Zn also gradually recovered.
Because of the presence of both rhodamine B and
salicylaldehyde hydrazone moieties in 1-Zn and the indis-
pensable role of Zn(II) chelation in the photochromism, we
propose the mechanism for the color change of 1-Zn upon UV
irradiation is as shown in Scheme 3. To support the hypothesis,
the control compounds of rhodamine B (Rd) and 2-Zn
(Scheme 3) were investigated. Rd in its ring-open form of Rd-
Open was photostable and constantly showed strong
RESULTS AND DISCUSSION
■
Compound 1 was prepared by a simple reaction between 4-
N,N-diethylaminosalicylaldehyde and rhodamine B hydrazide
in high yield according to a reported procedure.55 As shown in
Scheme 2, 1-Zn complex was prepared by the addition of 10-
Scheme 2. Synthesis of 1-Zn
Scheme 3. Proposed Mechanism for Color Change of 1-Zn
upon UV Irradiation and the Known Tautomerism of the
Control Compounds
fold Zn(II) to 1 in THF to ensure efficient formation of the
complex. The absorption and fluorescence characteristics of the
complex (Supporting Information Figure S1) were in
accordance with a previous study.55
Interestingly, complex 1-Zn, not 1 alone, was found to
display reversible photochromism in THF. As shown in Figure
1a, upon irradiation at 365 nm, the light yellow solution
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absorption at 554 nm and fluorescence at 595 nm regardless of
UV irradiation (Figure 2b and d), unless excess base such as
Figure 3. Absorption spectra of 3 and 3-Zn in THF before and after
UV irradiation. [3] = 10 μmol/L, [Zn(II)] = 100 μmol/L.
fluorescence were observed for 1 after exposure to UV
irradiation (Supporting Information Figure S3), which was
due to the isomerization of the salicylaldehyde hydrazone
moiety. This result suggested the essential role of Zn(II)
chelation for the photochromism. In addition to Zn(II), other
metal ions were also used to form complex with 1 to study their
effects on the photochromism. As shown in Supporting
Information Figure S4, different metal ions were added to the
solution of 1 in THF, and the solutions with Cu(II), Ni(II),
Zn(II), and Cd(II) showed strong color change after UV
irradiation. Interestingly, the thermal bleaching rate of 1 in dark
in the presence of these four metal ions was dependent on the
type of metal ions (Figure 4), with 1-Cu and 1-Cd as the
Figure 2. Absorption (a and b) and fluorescence spectra (c and d) of
different states of 1-Zn and its control compounds in THF. [1] = 10
μmol/L, [2] = 10 μmol/L, [Rd] = 10 μmol/L, [Zn(II)] = 100 μmol/
L; ring-open and closed forms of Rd were controlled by the addition
of HCl and NaOH.
NaOH was added to form the colorless and nonfluorescent
spirolactone of Rd-Close (Figure 2a and c). On the other hand,
2-Zn was sensitive to UV irradiation. UV light promoted the
isomerization of its salicylaldehyde hydrazone from the enol-
form to the keto-form.53,54 As shown in Supporting
Information Figure S2, 2-Zn-Enol (before UV irradiation)
and 2-Zn-Keto (after UV irradiation) had similar but indeed
different absorption and fluorescence spectra. It was found that
1-Zn-Keto-Open had two absorbance bands around 400 and
554 nm, and emission bands around 513 and 582 nm, which
were similar to an overlay of 2-Zn-Keto and Rd-Open (Figure
2b and d). In addition, 1-Zn-Enol-Close had two absorbance
band around 325 and 400 nm, and an emission band at 527 nm,
which were similar to an overlay of 2-Zn-Enol and Rd-Close
(Figure 2a and c). On the basis of these results, 1-Zn could be
considered as the combination of Rd and 2-Zn. The 1-Zn
complex took advantages of both the excellent optical
properties of rhodamine B and the fast photoresponse and
long-lifetime photochromic characteristics of salicylaldehyde
hydrazone.53,54 UV light promoted isomerization of the
salicylaldehyde moiety in 1-Zn from the enol-form (1-Zn-
Enol-Close) to the keto-form (1-Zn-Keto-Open), and this
change that induced ring-opening of the rhodamine B
spirolactam due to the charge separation was preferred in the
excited state, generating the purple-colored 1-Zn-Keto-Open
(Scheme 3).
According to the proposed mechanism, the phenol group
should be an indispensable factor for the photochromism
during the enol-to-keto transformation. To testify it, a control
compound 3 containing a pyridine group for metal ion
chelation but lack of the phenolic hydroxyl group as compared
to 1 was prepared. We found that 3 was able to chelate zinc
ions to form 3-Zn, but the complex had no photochromic
property upon UV irradiation (Figure 3).
The role of metal ion chelation on the photochromism of
complex 1-Zn was also investigated. Without Zn(II), the five-
membered spirolactam ring in 1 was photostable upon UV
irradiation. Only very slight changes of absorption and
Figure 4. Photographic images of thermal bleaching of 1 in the
presence of different metal ions in THF in dark at room temperature.
slowest and the fastest, respectively. We chose 1-Zn as the
example for the subsequent studies in this work, because it had
a suitable thermal bleaching rate that is not too slow or too fast
for measurement. In contrast to metal ions, the photochromism
of 1-Zn was not affected by anions such as F−, Cl−, Br−, I−,
SCN−, Ac−, and SO42−, unless strong ligands such as EDTA2−
were used to compete with 1 in Zn(II) binding (Supporting
Information Figure S5).
In addition to metal ion, other factors for thermal bleaching
rate were also studied. Initially, 1-Zn-Enol-Close had no
absorption band above 500 nm, so the absorbance at 554 nm
should be proportional to the concentration of 1-Zn-Keto-
Open formed upon UV irradiation. Thus, the recovery kinetics
from 1-Zn-Keto-Open to 1-Zn-Enol-Close could be moni-
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tored by measuring the absorbance at 554 nm over time. As
shown in Figure 5a, elevated temperature could accelerate the
Figure 5. (a) The thermal fading kinetics of 1-Zn in THF at different
temperature. Inset: The curves fitted with one-order reaction dynamics
ln A = −kt to calculate the thermal bleaching speed constant: k = 7.39
× 10−3 s−1 (T = 10 °C), k = 9.85 × 10−3 s−1 (T = 15 °C), k = 1.39 ×
10−2 s−1 (T = 20 °C), k = 1.84 × 10−2 s−1 (T = 25 °C), k = 2.38 ×
10−2 s−1 (T = 30 °C), k = 3.03 × 10−2 s−1 (T = 35 °C), k = 3.73 ×
10−2 s−1 (T = 40 °C). [1] = 10 μmol/L, [Zn(II)] = 100 μmol/L, λ =
554 nm. (b) The data were fitted with Arrhenius expressions.
Figure 6. The thermal fading kinetics of 1-Zn at different volume
fractions of THF in DCM at 25 °C. [1] = 10 μmol/L, [Zn(II)] = 100
μmol/L, λ = 554 nm. (a) Absorbance change. (b) The curves fitted
with one-order reaction dynamics ln A = −kt to calculate the thermal
bleaching speed constant: k = 1.80 × 10−2 s−1 (100%), k = 6.00 × 10−3
s−1 (80%), k = 2.88 × 10−3 s−1 (60%), k = 7.74 × 10−4 s−1 (40%), k =
1.65 × 10−4 s−1 (20%), k = 3.31 × 10−5 s−1 (0%).
conversion of 1-Zn-Keto-Open to 1-Zn-Enol-Close. All seven
curves obtained at different temperatures were fitted well with
first-order reaction kinetics (Figure 5a, inset). Thus, the rate
constants (k) could be calculated to evaluate the activation
energy (Ea) according to the Arrhenius expression (ln k = −Ea/
RT + constant). The Ea for the recovery reaction of 1-Zn-Keto-
Open to 1-Zn-Enol-Close was calculated as 43.58 kJ/mol. This
result suggested the lifetime of 1-Zn-Keto-Open could be
increased by lowering temperature.
To study the effects of different solvents on the photo-
chromic reaction of 1-Zn, 13 types of common solvents were
chosen in which 1-Zn could easily dissolve. According to the
previous reports,56 we thought the solvent polarity might be a
determinant factor for the photochromic properties of 1-Zn.
However, as shown in Supporting Information Table S1, the
solvent polarity did not seem to be the only determinant factor.
Although the photochromism was valid in most solvents with
low polarity (less than 2.0), it was still so in some of the
solvents with dipole moment more than 2.0 (such as
acetonitrile). It seemed that the chemical structure of the
solvent molecule was also important. In protic solvents such as
alcohols, and some aprotic but high-polar solvents such as
pyridine, DMF, and DMSO, 1-Zn exhibited no photochromic
property. In other solvents investigated, 1-Zn exhibited
photochromic properties with different kinetics. Excellent
reversible photochromic property of 1-Zn was observed in
ethers including THF, 1,4-dioxane, and diethyl ether. On the
basis of these results, solvent effect was also used to control the
dynamics of the recovery reaction. For this purpose, THF−
DCM solvent mixtures were utilized because the recovery was
fast in THF but slow in DCM. As illustrated in Figure 6, the
reaction rate increased with increasing THF fraction in the
solvent mixture, and the rate constants could be tuned from
10−2 to 10−5 s−1 at 25 °C, which suggested the half-life of
thermal recovery reaction from 1-Zn-Keto-Open to 1-Zn-
Enol-Close could be as short as several minutes or as long as
several hours at 25 °C.
Figure 7. (a) Structure of 4. (b) Absorption and (c) fluorescence
spectra of 4-Zn in THF before and after UV irradiation. (d) The
thermal fading kinetics of 4-Zn in THF at 25 °C, λ = 554 nm. Inset:
The curves fitted with one-order reaction dynamics. [4] = 10 μmol/L,
[Zn(II)] = 100 μmol/L.
of recovery in the dark from 4-Zn-Keto-Open to 4-Zn-Enol-
Close was much slower than that of 1-Zn under the same
condition (Figure 7d). The recovery rate constant was
calculated as 3.33 × 10−4 s−1 for 4-Zn, which is only 2% of
that of 1-Zn at 25 °C.
Besides the tunable thermal bleaching rate, the fatigue
resistance of 1-Zn was investigated to evaluate its performance
as a reversible photochromic complex. As shown in Figure 8a,
1-Zn was toggled repeatedly between the two states of 1-Zn-
Keto-Open and 1-Zn-Enol-Close 10 times, and the absorbance
at 554 nm stayed almost constant without any apparent
degradation.
Chemical substitutions on 1 could also influence the kinetics
of the recovery reaction in the dark. Control compound 4 was
synthesized in lack of a diethylamino group as compared to 1
(Figure 7a). The complex 4-Zn exhibited reversible photo-
chromism similar to that of 1-Zn (Figure 7b and c), but its rate
To gain further insight into the photochromic properties, the
calculations were performed on 1-Zn-Keto-Open and 1-Zn-
Enol-Close with the Gaussian 09 program. The geometries
were optimized with B3LYP hybrid density function and the 6-
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Figure 8. Fatigue resistance of 1-Zn upon irradiation with 365 nm and
standing in dark alternatively. (a) In THF, [1] = 10 μmol/L, [Zn(II)]
= 100 μmol/L, λ = 554 nm. (b) In poloxamer 407, [1] = 1 μmol/g,
[Zn(II)] = 10 μmol/g, λ = 560 nm.
31G (d,p) basis set. For both 1-Zn-Keto-Open and 1-Zn-Enol-
Close, HOMO and LUMO are localized at the rhodamine B
part and the salicylaldehyde hydrazone part, respectively
(Figure 9), suggesting their roles in the photochromic property.
Figure 10. (a) Photographic images of 1-Zn in poloxamer 407 upon
irradiation at 365 nm, and the thermal bleaching in dark at 25 °C. (b)
Absorption and (c) fluorescence spectra of 1-Zn in poloxamer 407
before and after UV irradiation. [1] = 1 μmol/g, [Zn(II)] = 10 μmol/
g.
Figure 11. Generating letters on 1-Zn in poloxamer 407 upon UV
irradiation. [1] = 1 μmol/g, [Zn(II)] = 10 μmol/g. Size: 4.2 cm × 2
cm.
the next round of irradiation. As shown in Supporting
Information Figure S8, the thickness of the polymer layer
that UV light could penetrate is less than 1 mm, because the
polymer was not transparent. The fatigue resistance of 1-Zn in
poloxamer 407 was found as good as that in THF (Figure 8b).
As the ozone layer becomes thinner, UV radiation pollution
is increasingly serious. Excessive UV radiation will cause an
additional risk of skin cancers and cataracts, so the detection of
UV radiation becomes significant.57−59 According to the
definition of UV index by the world health organization
(WHO), the UV radiation reaching the earth’s surface is largely
composed of UVA (315−400 nm), with only a small fraction of
UVB (280−315 nm) component.60 We found that both of 325
and 405 nm UV light could lead to the photochromism of 1-Zn
in poloxamer 407, and the wavelength range fitted well with the
UVA range. Therefore, a test bar for UV radiation pollution was
developed on the basis of 1-Zn in poloxamer 407 sealed in a
colorimetric cell. Under different UV radiant intensities as
monitored by a commercial UV meter, the cell showed different
intensities of color changes (Figure 12), which could be used as
a UV strength sensor suitable for naked eye detection.
In summary, we have developed a new photochromic system
based on rhodamine B salicylaldehyde hydrazone metal
complex. The molecules showed absorption “turn-on” and
fluorescence “turn-off” response upon UV irradiation both in
solution and in solid matrix. UV light promoted the
isomerization of the salicylaldehyde hydrazone moiety from
the enol-form to the keto-form, and subsequently induced the
spirolactam ring-opening in the rhodamine B part and caused
the photochromic reaction. The system exhibited good fatigue
resistance, and the lifetime of the ring-open state could be
tuned by metal ions, temperatures, solvents, and chemical
Figure 9. Frontier canonical Kohn−Sham valence MO envelopes of 1-
Zn-Enol-Close and 1-Zn-Keto-Open, with dominant AO contribu-
tions. Value of contour envelopes is 0.025 au.
The molecular conformational change was small before and
after UV irradiation (Supporting Information Figure S6),
supporting the good fatigue resistance of 1-Zn.
For applications as photochromic materials, the compound is
more preferred to be embedded in solid state than in solution.
Therefore, the photochromic properties of 1-Zn in solid matrix
were also studied. Complex 1-Zn in poloxamer 407 (poly-
(ethyleneglycol)-block-poly(propyleneglycol)-block-poly-
(ethyleneglycol) with average molecular weight of 14 600,
which is an ether polymer) solid was prepared by slow
evaporation of solvent from 1-Zn and poloxamer 407 in THF
(see the Supporting Information). Similar to that of 1-Zn in
THF, the absorbance of 1-Zn in poloxamer 407 solid at 554 nm
enhanced dramatically, while the fluorescence at 595 nm
decreased upon UV irradiation, displaying as the color change
of the powder from light yellow to purple (Figure 10,
Supporting Information video 2). Surprisingly, the fluorescence
quenching of 1-Zn upon UV irradiation was much more
efficient in poloxamer 407 than that in THF, and the quantum
yield of the solid fluorescence changed from 23% to 1.3%
(Supporting Information Figure S7).
Interestingly, 1-Zn in poloxamer 407 was capable of serving
as material for photoprinting. Letters were preorganized and
recorded on 1-Zn in poloxamer 407 (Figure 11, Supporting
Information video 3). Upon UV irradiation, letters were
successfully visualized, and they gradually disappeared until
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ASSOCIATED CONTENT
* Supporting Information
Details on experimental procedures. Additional experimental
data, including photos, videos, absorption spectra, fluorescence
spectra, etc. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
448.
AUTHOR INFORMATION
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■
Corresponding Authors
tongaj@tsinghua.edu.cn
tangbenz@ust.hk
́
(33) Zhu, W.; Yang, Y.; Metivier, R.; Zhang, Q.; Guillot, R.; Xie, Y.;
Tian, H.; Nakatani, K. Angew. Chem., Int. Ed. 2011, 50, 10986.
(34) Perrier, A. l.; Maurel, F. O.; Jacquemin, D. Acc. Chem. Res. 2012,
45, 1173.
Notes
The authors declare no competing financial interest.
(35) Fukaminato, T.; Doi, T.; Tamaoki, N.; Okuno, K.; Ishibashi, Y.;
Miyasaka, H.; Irie, M. J. Am. Chem. Soc. 2011, 133, 4984.
(36) Berberich, M.; Natali, M.; Spenst, P.; Chiorboli, C.; Scandola, F.;
ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Natural
Science Foundation of China (nos. 21175079, 21375074, and
21390410) and the Research Grants Council of Hong Kong
(HKUST2/CRF/10).
Wurthner, F. Chem.Eur. J. 2012, 18, 13651.
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(37) Cheng, H.-B.; Zhang, H.-Y.; Liu, Y. J. Am. Chem. Soc. 2013, 135,
10190.
(38) Yang, Y.; Hughes, R. P.; Aprahamian, I. J. Am. Chem. Soc. 2012,
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