Coumarin Photocaging Groups Modified with an Electron-Rich Styryl Moiety at the 3-Position: Long-Wavelength Excitation, Rapid Photolysis, and Photobleaching

Article abstract of DOI:10.1002/anie.201800713

A new class of coumarin photocaging groups modified with an electron-rich styryl moiety at the 3-position was constructed. The large π-conjugated structure and stabilization of the carbocation intermediates by electron donors endowed the new photocaging groups with excellent long-wavelength absorption, large two-photon absorption cross-sections, and high uncaging quantum yields. Moreover, the new photocaging groups displayed unique photobleaching properties after photocleavage as a result of the intramolecular cyclization rearrangement of a carbocation intermediate to form five-membered ring byproducts and block the styryl conjugation at the 3-position. These superior properties of the new photocaging groups are extremely beneficial for high-concentration samples and thick specimens, thus extending the application of photocaging groups in many fields.

Full text of DOI:10.1002/anie.201800713

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Coumarin Photocages Modified with an Electron-Rich Styryl
Moiety at the 3-Position: Long Wavelength Excitation, Rapid
Photolysis and Photobleaching
Authors: Linyong Zhu, Qiuning Lin, Lipeng Yang, Zhiqiang Wang,
Yujie Hua, Dasheng Zhang, Bingkun Bao, Chunyan Bao, and
Xueqing Gong
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800713
Angew. Chem. 10.1002/ange.201800713
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10.1002/anie.201800713
Angewandte Chemie International Edition
COMMUNICATION
Coumarin Photocages Modified with an Electron-Rich Styryl
Moiety at the 3-Position: Long Wavelength Excitation, Rapid
Photolysis and Photobleaching
Qiuning Lin,‡ Lipeng Yang,‡ Zhiqiang Wang, Yujie Hua, Dasheng Zhang, Bingkun Bao, Chunyan
Bao, Xueqing Gong and Linyong Zhu*
Abstract: A new class of coumarin photocages modified with an
electron-rich styryl moiety at the 3-position was constructed. The large
π-conjugated structure and stabilization of the carbocation
intermediates by electron-donors endowed the new photocages
excellent long wavelength absorption, large two-photon absorption
cross-sections and high uncaging quantum yields. Moreover, the new
photocages displayed unique photobleaching property after
besides the absorption wavelength and uncaging efficiency, the
competitive absorption by the photolysis byproducts would also
affect the application of PPGs, especially for high concentration
or thick samples due to the inner filtering of light rapidly attenuates
the valid light intensity,20 which significantly prolongs the uncaging
time and decreases the overall conversion. Hence, the
development of PPGs with long-wavelength absorption and a
high uncaging efficiency as well as photobleaching at the
irradiation wavelength is of great significance.
Herein, we developed a new class of coumarin photocages
modified with electron-rich (donor) styryl moieties (D-S) at the 3-
position (Figure 1a). Due to the 1) large π-conjugated structure,
2) stabilization of the carbocation intermediates by electron-
donors, and 3) destruction of the 3-position π-conjugation during
the photocleavage process, we first assembled the superior
properties of long wavelength excitation, large TP absorption
cross-section, high uncaging quantum yield and excellent
photobleaching into a class of ideal photocages. We anticipate
that these advantages will significantly promote the performance
of PPGs, thus benefiting the application of PPGs in many areas.
First, as shown in Figure 1b and Figure S1, coumarin
photocages conjugate at 3 positions with D-S moieties of p-
photocleavage as
a result of the intramolecular cyclization
rearrangement of carbocation intermediate to form five-membered
ring byproducts and block the styryl conjugation at the 3-position.
These superior properties of the new photocages are extremely
beneficial for high concentration samples and thick specimens, thus
extending the application of photocages in many fields.
Photocages, also termed as photoremovable protecting groups
(PPGs), are molecules capable of releasing active groups with
light activation at a specific wavelength.1,2 Due to the non-invasive,
non-contamination and high spatiotemporal precision of light
stimulus control, PPGs are widely used in various fields of
chemistry, biology and materials science.3-5 The traditional PPGs
such as o-nitrobenzyl,6 benzoinyl(desyl),7 4-hydroxyphenacyl8
and coumarinyl9 moieties are all absorptive in the UV-light region
that possesses low tissue penetration and causes unwanted
photochemical side reactions.10
The new development of PPGs is expected to have long-
wavelength absorption that leads to deeper tissue penetration
and matches the common laser wavelengths.3, 11 In these years,
many efforts have been made to develop visible and near-IR light-
induced uncaging groups based on o-nitrobenzyl,12 coumarin,13
BODIPY,14 cyanine15 and quinone16 derivates either by one- or
two-photon excitation. Among them, the coumarin-based
photocages attract much attentions due to the rapid photolysis
and large two-photon (TP) absorption cross-section. Recently,
Ellis-Davies and Jullien successfully extended the maximal
absorption wavelength of coumarin-based photocages to the blue
ethoxystyryl
(1a),
4’-N,N-dimethylaminostyryl
(1b),
or
conformationally cycloalkyl-locked julolidinestyryl (1c), and its
upgraded analogues with bis-julolidine (1d) or sulfur atom
substitution (2b and 2d) were synthesized. In contrast, the
coumarin photocages linked by bald styryl (1e) or electron
withdrawing p-methylbenzotestyryl (1f) groups at the same
position and the traditional 7-diethylaminocoumarin (DEAC) and
o-nitrobenzyl (o-NB) PPGs were also prepared. The p-
methoxybenzonic acid (p-mBA) was chosen as model cargo.
The photophysical and photochemical properties of all of the
light region (450 nm) by 3-position π-conjugating with
a
acrylamide moiety17 or 2-position substituting by sulfur atom.18
However, further redshift absorption is accompanied by a drop in
the uncaging efficiency.19 In addition, it is worth mentioning that
[a]
Dr. Q. Lin, L. Yang, Z. Wang, Y. Hua, D. Zhang, B. Bao, Prof. C.
Bao, Prof. X. Xue, Prof. L. Zhu
Key Laboratory for Advanced Materials
East China University of Science and Technology
130# Meilong Road, Shanghai, 200237 (China)
E-mail: linyongzhu@ecust.edu.cn
Figure 1. a) Scheme illustrating the photolysis reaction of coumarin-based
PPGs with D-S-conjugation at the 3-position: long wavelength excitation, rapid
photolysis and photobleaching properties. b) Structures of PPGs 1a-1f, 2b, 2d,
DEAC and o-NB caged model compounds discussed in this work.
‡
These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201800713
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COMMUNICATION
Table 1. Photophysical and photochemical properties of PPGs 1a-1f, 2b,
2d, DEAC and o-NB. a
PPGs
λmax
Φf
εmax
Φu
εmaxΦ
σmax
σmaxΦu
u
1a
1b
1c
1d
2b
430
446
456
467
490
0.47
0.02
N
3.0
4.1
4.4
3.5
3.0
0.45
0.2
13.5
8.2
58.6
198
N
26.4
39.6
N
0.19
0.41
0.4
8.36
14.35
12.0
1.16
17.5
1.19
1.6
0.03
N
43.2
N
17.7
N
0.29 (570 nm)
2d
515
N
2.5
0.7
N
N
0.17 (600 nm)
Figure 2. a) Normalized absorbance spectra showed the long-wavelength
absorption of D-S-conjugated PPGs 1a-1d, 2b and 2d. b) Temporal evolution
of the uncaging process indicated the faster photolysis of D-S-conjugated PPGs
2b and 2d than those of DEAC and o-NB. c) Large π-conjugation enhanced the
two-photon absorption cross-sections of 1a, 1b, 1d, 1e and 1f compared to
common DEAC. d) The conversion rates of 1b under TP excitation at 730 nm
and 800 nm were much faster than those of DEAC. All of the tested samples
(10-5 M) were carried out in a 9:1 (v:v) mixture of methanol and DI water.
1e
428
443
380
338
0.53
0.61
0.14
N
3.2
4.4
2.0
0.6
0.05
67.8
80.1
30.8
N
3.39
3.20
2.16
N
1f
0.04
0.07
0.16
1.76
1.4
DEAC
o-NB
0.96
aSymbols and abbreviations: λmax
, absorption maximum (nm); Φf,
fluorescence quantum yield; εmax, molar extinction coefficient (104 M-1cm-1);
Φu, uncaging quantum yield; εmaxΦu, uncaging efficiency upon one-photon
excitation (103 M-1cm-1); σmax, maximum two-photon cross-section within
730-880 nm (GM, 1GM = 10-50 cm4s/photon); σmaxΦu, uncaging efficiency
upon two-photon excitation (GM); and N, not detected.
styryl group had a Φu value of 0.05, which was closest to that of
the DEAC coumarin parent. The above results demonstrated that
the electron-rich effect in the 3-position substituent group of styryl
is the key factor for the rapid photocleavage. In addition, the sulfur
atom substitution also substantially enhanced the photolysis rate,
as the Φu values of the photocages 2b and 2d enlarged to 0.4 and
0.7, respectively. Combined with the enhanced molar extinction
coefficients (ε), the uncaging efficiency εΦu values for most of the
D-S-conjugated coumarin-based PPGs have approached or even
exceeded 10000 M-1cm-1. Importantly, if the uncaging efficiency
εΦu of 1400 M-1cm-1 for DEAC or 960 M-1cm-1 for o-NB was
regarded as an usable standard of PPGs, the effective uncaging
wavelengths of 2b and 2d can extend to 570 nm and 600 nm
respectively (shown in Table 1 and Figure 2b), which easily
matches the wavelength of commonly used lasers. Hence, the
superiority of rapid photolysis and long wavelength excitation
make the coumarin-based PPGs conjugated with electron-rich
stryl moieties at the 3-position rank among the top families of
photocages.
Next, the two-photon absorption properties of the new PPGs
were determined by a TP-induced fluorescence method22 within
730-880 nm. As shown in Table 1 and Figure 2c, the TP
absorption cross-sections of the examined molecules within the
measured wavelength range were almost higher than that of
DEAC. Then, the TP photolysis of photocages 1b and DEAC
were further carried out at 730 nm and 800 nm. Figure 2d showed
that the TP-uncaging rate of 1b was much faster than that of
DEAC at both 730 nm and 800 nm. This result was consistent
with the TP uncaging efficiency σΦu of 1b (39.6 GM at 730 nm
and 7.3 GM at 800 nm) that was estimated to be 10-25 times
higher than that of DEAC (1.7 GM at 730 nm and 0.73 GM at 800
PPGs were characterized and are summarized in Table 1.
Compared to the traditional DEAC, the absorption maximum of
PPGs 1a-1f, 2b and 2d are significantly red-shifted to the blue to
green light region of 430-515 nm (Figure 2a). For PPGs 1c and
1d, each julolidine ring contributes a ~10 nm red shift because of
the greater conjugation of the amino group with the aromatic
core.21 In addition, the enhanced intramolecular charge-transfer
(ICT) through the 3d empty atomic orbitals of the sulfur atom18
made the thione PPGs of 2b and 2d red-shift 40-50 nm, thus
further broadening the absorption band even to the red light
region. Besides the absorption wavelength redshift, the molar
extinction coefficients of the new PPGs (ranging from 25000 to
44000 M-1cm-1) were all larger than that of the DEAC coumarin
parent.
The photolysis of all the PPGs were then conducted under
their corresponding excitation wavelength with a light intensity of
10 mW/cm2. The photolysis evolution recorded by HPLC analysis
indicated the smooth photolysis process of all of the PPGs with a
high yield of p-mBA (Figure 2b and Figure S2). The PPGs 1a-1d,
2b and 2d with D-S-conjugation displayed the behavior
associated with a “fast” (t1/2 < 110 s) photorelease of p-mBA with
a uncaging quantum yield Φu of 0.19 to 0.7, which are
approximately 3 to 10 times higher than that of the common
DEAC (0.07) and can rival or even exceed the DEAC450 (Φu =
0.39) developed by Ellis-Davies recently.17 However, the electron-
withdrawing-styryl conjugation analogue 1f displayed the
behavior associated with a “slow” photorelease with a timescale
of 16 min (Φu = 0.04). The photocage 1e conjugated with a bald
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Figure 4. a) Speculation of the photolysis mechanism for the 3-position styryl-
conjugated coumarin-based PPGs. b) Calculated energy profiles of the
photolysis process of 1b and 1f supported the photolysis promotion by the
electron-rich effect and the assumption of intramolecular cyclization
rearrangement to form five-membered ring byproduct. c) 1H NMR spectra of the
cyclized byproduct of 1b.
Figure 3. a) Time course UV-Vis spectra during photolysis display the
photobleaching behavior of 1a (left) and 2b (right). b) The absorbance ratio of
after to before photolysis (Aafter/Abefore) revealed the photobleaching behavior of
the new PPGs 1a-1f, 2b and 2d compared to DEAC and o-NB. c) Partial 1H
NMR spectra of 1b show that the peaks of the 3-position double bond
disappeared after photolysis.
inducing the photobleaching behavior.
nm). Thus, the unique structure of styryl-conjugation also
endowed our new PPGs with excellent two-photon photolysis
properties.
To better illuminate the above hypothesis, computational
density functional theory (DFT) calculations of electron-donating
(1b) and electron-withdrawing (1f) styryl-conjugated PPGs were
first conducted. The calculated structures of the key states are
illustrated in Figure S5. The calculated energy profiles of
photolysis processes in Figure 4b show that photolysis of 1b
Further tracking the UV-Vis absorption evolution of the
photolysis process of the new PPGs, a typical photobleaching
phenomenon was observed (Figure 3a and Figure S3). The
maximum absorption band of the styryl-conjugated coumarin
gradually disappeared. Simultaneously, absorption bands at 380
nm for ketone photocages 1a-1f and at 470 nm for thione
photocages 2b and 2d were generated, which were
corresponding to the absorption of the nuclear of ketones23 or
thiones18 coumarin with non-conjugation at the 3-position. The
absorbance ratio of after to before photolysis (Aafter/Abefore) of the
new PPGs 1a-1f, 2b and 2d decreased as high as 53-fold at the
corresponding irradiation wavelength (Figure 3b). While for the
traditional PPGs DEAC or o-NB, the absorbance ratio only slightly
decreased (1.4-fold) or even oppositely increased (0.73-fold). The
1H NMR spectrum of 1b and 2b (Figure 3c and Figure S4) showed
that the peaks of the 3-position double bond of styryl moiety
disappeared after photolysis, which indicated that the π-
conjugation at the 3-position was broken during the
photocleavage process that led to the photobleaching behavior.
Based on the above results, we can speculate the photolysis
mechanism of our new PPGs as shown in Figure 4a. According
to the recognized photo-SN1-dependent photocleavage of
coumarin-based PPGs,24 first, the heterolysis of the 4-position C-
O bond formed a free ion-pair of the p-mBA anion and
carbocation intermediate (IM1) upon light excitation. Then,
intramolecular cyclization rearrangement of the carbocation, due
to the conformational flexibility of stilbene structure, was
speculated to achieve a more stable intermediate (IM1→IM2),
followed by trapping of the carbocation by the solvent (hydroxy)
to give a probable five-membered ring byproduct F rather than the
traditional 4-hydroxymethyl-coumarin byproduct.23, 24 Upon that,
the large π-conjugation at the 3-position was blocked, thus finally
gives
a smaller stabilization energy of the carbocation
intermediate IM1 (1.06 eV) compared with that of 1f (1.97 eV),
which indicates that the electron-rich effect significantly stabilizes
the carbocation intermediate and enables the D-S-conjugated
coumarin undergoing photolysis more readily and much faster.
Moreover, the energy level of the carbocation intermediate IM2 is
much lower than that of IM1 for both PPGs 1b and 1f, which
supports the assumption that IM1 would like to undergo
intramolecular cyclization rearrangement to form
a five-
membered ring intermediate. The Mulliken charges were further
calculated to testify the carbocation rearrangement (Figure S6).
Next, the photolysis byproduct of 1b was isolated and all of the
HR-MS (ESI), H1 NMR and C13 NMR spectra (Figure 4c, Figure
S7 and Figure S8) testified the cyclized structure of the byproduct
1b-F. All of the above results effectively certify that the electron-
rich effect on 3-position promoted the photolysis rate and the large
π-conjugation was finally blocked by cyclization rearrangement,
thus changing the absorption performance of our new PPGs.
According to the photolysis rate equation and Beer-Lambert
law, either the photolysis rate or light intensity with depth shows
an inverse exponential relationship with the competitive
absorption of increasing byproducts.20,
25
That signifies the
competitive absorption by byproducts severely bothers the
photolysis process, especially for high concentration or thick
samples. However, once the photocages possess the
photobleaching property, the negative disturbance of competitive
absorption can be eliminated. Thus, we demonstrated the
superiority of our new PPGs for uncaging in high concentration
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10.1002/anie.201800713
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properties of long wavelength-excitation, rapid photolysis and
photobleaching, thus indicating that the new coumarin-based
PPGs are extremely beneficial for diverse applications in
chemical synthesis, biological manipulation and material
manufacturing.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21425311 and 21774030).
Keywords: photoremovable protecting groups • coumarin
photocages • photocleavage • photochemistry • photolysis
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Entry for the Table of Contents
COMMUNICATION
Qiuning Lin,‡ Lipeng Yang,‡ Zhiqiang
Wang, Yujie Hua, Dasheng Zhang,
Bingkun Bao, Chunyan Bao, Xueqing
Gong and Linyong Zhu*
Coumarin photocages: Based on the
1) large π-conjugation, 2) carbocation
stabilization and 3) intramolecular
cyclization rearrangement, we
constructed a new class of coumarin-
based photocages with electron-rich
styryl moiety at the 3-position. The
long wavelength excitation, rapid
photolysis and photobleaching make
the new photocages benefit for high
concentration and thick specimens,
thus extending the application of
photocages to more areas.
Page No. – Page No.
Coumarin Photocages Modified with
an Electron-Rich Styryl Moiety at the
3-Position: Long Wavelength
Excitation, Rapid Photolysis and
Photobleaching
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