Styryl conjugated coumarin caged alcohol: Efficient photorelease by either one-photon long wavelength or two-photon NIR excitation

Article abstract of DOI:10.1021/ol203188h

The synthesis and photorelease properties of a new phototrigger for alcohols are described. Compared to ester 4 caged by the reported [7-(diethylamino)coumarin-4-yl]methoxycarbonyl (DEACM) phototrigger, the caged ester 3 shows an efficient single-photon photolysis efficiency upon irradiation of long wavelength light (λ = 475 nm) and a stronger two-photon photolysis sensitivity with 800 nm laser light. Its promising properties and the efficient photorelease of adenosine make it very useful as a caging group for biological applications.

Full text of DOI:10.1021/ol203188h

ORGANIC
LETTERS
2012
Vol. 14, No. 2
572–575
Styryl Conjugated Coumarin Caged
Alcohol: Efficient Photorelease by
Either One-Photon Long Wavelength
or Two-Photon NIR Excitation
Chunyan Bao,† Guanshui Fan,† Qiuning Lin,† Bo Li,‡ Shuiyu Cheng,† Qi Huang,† and
Linyong Zhu*,†
Shanghai Key Laboratory of Functional Materials Chemistry, Institute of Fine
Chemicals, East China University of Science and Technology, Shanghai, 200237,
P. R. China, and Key Laboratory for Polar Materials and Devices, Ministry of
Education, East China Normal University, Shanghai, 200241, P. R. China
Linyongzhu@ecust.edu.cn
Received November 29, 2011
ABSTRACT
The synthesis and photorelease properties of a new phototrigger for alcohols are described. Compared to ester 4 caged by the reported
[7-(diethylamino)coumarin-4-yl]methoxycarbonyl (DEACM) phototrigger, the caged ester 3 shows an efficient single-photon photolysis efficiency
upon irradiation of long wavelength light (λ = 475 nm) and a stronger two-photon photolysis sensitivity with 800 nm laser light. Its promising
properties and the efficient photorelease of adenosine make it very useful as a caging group for biological applications.
Phototriggers (or caging groups) for a variety of func-
tional groups have numerous applications in biotechnology
and cell biology.1À3 In contrast to most other protecting
strategies, the removal of this category only requires light
and allows a very mild activation of sensitive molecules,
which permits functional molecules to evolve to intracel-
lular distribution with high special and temporal precision.
To date, most phototriggers need ultraviolet (UV) light to
achieve cleavage, which is a major drawback because the
presence of UV light is damaging to cells and it also pro-
vides poor penetration due to light scattering and absor-
bance by intrinsic biological chromophores.4 To promote
the application in biological areas, two approaches are ef-
fective in designing biologically suitable phototriggers: one
method is to create new phototriggers that can absorb at
longer wavelengths,5 or modify the existing phototriggers
by different substitutions.6 However this approach always
encounters difficulties in synthesizing and elongating the
conjugation of the phototriggers without adversely affect-
ing the bond cleavage rates and selectivity. Another method
is to use two-photon excitation with near-infrared (NIR)
light that is more suitable for biomedical application be-
cause of the minimization of cell damage while maintain-
ing higher spatial resolution. However, to the best of our
† East China University of Science and Technology.
‡ East China Normal University.
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switches and Caged Biomolecules: Goeldner, M., Givens, R. S., Eds.;
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€
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r
10.1021/ol203188h
Published on Web 12/27/2011
2011 American Chemical Society

knowledge, most phototriggers’ sensitivity to two-pho-
ton excitation is low due to inefficient two-photon un-
caging cross-section δu.6,7 Thus, development of new
phototriggers that are sensitive to long wavelength or
NIR light appears to be an essential step toward bio-
logical applications.
2-nitrobenzyl phototriggers with absorbance nor-
mally located at ca. 350 nm and δu reaching only ca.
0.01 GM within the 700À750 nm range,7b,15 coumarin-
based phototriggers have longer wavelength absorp-
tion and larger δu.12c,13c,14b The mechanism for the
photorelease has been extensively investigated; they
are members of arylalkyl-type photoremovable pro-
tecting groups, and the photolysis upon cleavage of
a CÀO bond produces a leaving group and a solvent-
trapped coumarin as a photo-byproduct (as shown
in Scheme 1).13c Recently, we reported a substituted
7-amino coumarin as a phototrigger for a controlled
drug release system and anticancer therapy.12a This
paper illustrates development of a coumarin platform
to design new phototriggers with efficient photorelease
by either one-photon long wavelength or two-photon
NIR excitation.
Scheme 1. Structures of Esters 1À5 and the Photolysis
Mechanism for Esters
During the process, para-substituted styryl was intro-
duced to the 7-position of coumarin-based phototriggers,
with the expectation that the electron-donating substitu-
tion and elongated π-conjugation could make the absor-
bance of the phototriggers red shift and enlarge their molar
absorption coefficient. Beyond the anticipated red shift,
the introduction of electron-donating conjugated chromo-
phores (D-π-A backbone formation) is expected to exhibit
an improved cross section for two-photon absorption
(TPA) δa (Scheme 1).16 Thus, the caged model compounds
(esters 1À3) were synthesized by the protecting of 4-methoxy
-benzyl alcohol. The corresponding photorelease properties
were examined and compared to those of ester 4 from the
known [7-(diethylamino)coumarin-4-yl]methoxycarbonyl
Among the developed phototriggers (such as
2-nitrobenzyl,6,8 benzoin,9 7-nitroindoline,10 phenacyls11),
coumarin-based phototriggers arouse more interest
and have been applied to cage carboxylates, phos-
phates, amines, and also alcohols compounds.12À14
Compared with the most popular 4,5-dimethoxy
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J.; Vriz, S.; Bensimon, D.; Jullien, L. Angew. Chem., Int. Ed. 2008, 47,
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Scheme 2. Synthesis of Photosensitive Protecting Groups 1À3
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Am. Chem. Soc. 2010, 132, 10645–10647. (b) Babinm, J.; Pelletier, M.;
Lepage, M.; Allard, J.; Morris, D.; Zhao, Y. Angew. Chem., Int. Ed.
2009, 48, 3329–3332. (c) Lu, M.; Fedoryak, O. D.; Moister, B. R.; Dore,
T. M. Org. Lett. 2003, 5, 2119–2122.
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Kotzur, N. J. Org. Chem. 2010, 75, 2790–2797. (b) Hagen, V.; Dekowski,
B.; Kotzur, N.; Lechler, R.; Wiesner, B.; Briand, B.; Beyermann, M.
Chem.;Eur. J. 2008, 14, 1621–1627. (c) Hagen, V.; Dekowski, B.;
Nache, V.; Schmidt, R.; Geißler, D.; Lorenz, D.; Eichhorst, J.; Keller,
S.; Kaneko, H.; Benndorf, K.; Wiesner, B. Angew. Chem., Int. Ed. 2005,
44, 7887–7891.
(DEACM) phototrigger.12b The photophysical examina-
tion shows that 3 provides prospective long wavelength
absorption with a maximum at 407 nm (ca. 30 nm red
shift) and large TPA cross section with 309 GM (increased
(14) (a) Mizukami, S.; Hosoda, M.; Satake, T.; Okada, S.; Hori, Y.;
Furuta, T.; Kikuchi, K. J. Am. Chem. Soc. 2010, 132, 9524–9525. (b)
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2007, 9, 4717–4720. (c) Nomura, W.; Narumi, T.; Ohashi, N.; Serizawa,
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BioChem 2011, 12, 535–539.
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W. W. Biophys. J. 1999, 76, 489–499.
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Org. Lett., Vol. 14, No. 2, 2012
573

ca. 150 times). In the end, the controlled photochemical
release of bioactive adenosine caged by the phototrigger
3 was successfully performed, which validated the po-
tential application in biological systems.
application. Second, the photolysis of ester 3wasselected to
be performed to compare with ester 4 under the same
photolysis conditions (as shown in Table 1). Although the
photolysis quantum yield of ester 3 is smaller than that of
ester 4, the overall photolysis efficiency Φchemε, the photo-
lysis quantum yield and molar absorptivity, exhibits the
benefit along with the red shift of the irradiation wave-
length for ester 3 (more than three times larger than that of
ester 4 at 430 nm). Ester 3 also shows efficient photolysis
even upon 475 nm irradiation. The concrete photolysis
process was also performed and analyzed by HPLC. As
shown in Figure 2, ester 3 photodecomposes gradually with
Esters 1À4 were prepared from compounds 1À3 and
DEACM alcohol respectively via two steps: first, the
alcohols were activated by coupling to 4-nitrophenyl chlo-
roformate in the presence of DMAP (4-(dimethylamino)-
pyridine),7b and then the intermediate products were al-
lowed to react with 4-methoxybenzyl alcohol to yield final
esters. Compounds 1À3, the photosensitive protect-
ive groups, were prepared from commercially available
7-amino-4-methyl coumarin (10) in five steps by (1)
diazotation with NaNO2 followed by nucleophilic
substitution with iodide to obtain 9;17 (2) oxidation
by SeO2 and then reduction by NaBH4 to yield 8; (3)
protection of benzyl hydroxyl by 3,4-dihydro-2H-
pyran;18 (4) Heck coupling reaction with correspond-
ing styrenes; and (5) deprotection of benzyl hydroxyl to
obtain compounds 1À3 (Scheme 2).18
Table 1. Selected Photophysical and Chemical Properties of
Esters 1À5a
λmax
(nm)
εmax
Φchem
δ800nm
(GM)
ester
( Â 10À4 b
)
( Â 103)c
;
Φchem
ε
1
2
3
349
366
407
5.77
2.75
2.88
;
;
45.4f
85.5f
309.5f
0.26g
2.3f
0.12g
303.5f
0.24g
The absorption spectra of the esters are showed in
Figure 1. Ester 3 shows the biggest red shift of maximum
absorption, and its absorption band extends up to 500 nm,
which suggests that the donating power of the substituted
;
0.83
8.28d
2.46e
2.29d
4
5
377
407
1.24
2.97
45.0
0.79
e
;
8.18d
2.36e
a All one-photon experiments were done in CH3CN:H2O (9:1)
solution. b Molar absorptivity at maximum absorption wavelength
(cmÀ1
M
À1). c Photolysis quantum yields for consumption of esters
upon irradiation of 420 nm light. d Photolysis quantum yield and molar
absorption at 430 nm. e Photolysis quantum yield and molar absorption
at 475 nm. f TPA (δa) cross section at 800 nm (0.60 mJ cmÀ2 per pulse).
3
g Two-photon uncaging (δu) cross section at 800 nm (0.60 mJ cmÀ2
per pulse).
3
Figure 1. Comparison of UVÀvis spectra for esters 1À5.
groups has a great effect on the absorption of the photo-
triggers. Compared to that of ester 4, the above results con-
firm our design intention that the introduction of electron-
donating substitution and elongated conjugation will
make the absorbance of the phototriggers red-shift as well
as enlarge their molar absorption coefficient.
To demonstrate these new designed phototriggers with
improved photochemical properties, the photolysis experi-
ments were examined. First, the hydrolytic stability of these
estersunder simulated physiological conditions were check-
ed, which shows that all of the remaining rates of the esters
under dark conditions are higher than 93.5%. It indicates
that these phototriggers are stable enough for biological
Figure 2. Time course of single photon photolysis of esters 3, 4,
and 5 at 475 nm (28 mW/cm2). Samples (10À4 M) were irradiated
in CH3CN/H2O (9:1) solution.
the elongation of irradiation time to release 4-methoxyben-
zyl alcohol. The photolysis curve for ester 3 shows approx-
imate single-exponential decay, suggesting no undesired
secondary effect that interferes with photolysis throughout
the reaction.14b However, for ester 4, there is almost no
(17) Mao, W.; Wang, T.; Zeng, H.; Wang, Z.; Chen, J.; Shen, J.
Bioorg. Med. Chem. Lett. 2009, 19, 4570–4573.
ꢀ
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3058–3061.
574
Org. Lett., Vol. 14, No. 2, 2012

photolysis due to nonirradiation wavelength excitation at
475 nm. The remarkable photolysis upon irradiation of
long wavelength light for ester 3 exhibits an advantage
especially for some biological systems that are hardly
sensitive to UV light. The incomplete photolysis for the
ester 3 may be attributed to an internal filtering effect from
the photo-byproduct (compound 3).
Table 2. Two-Photon Photolytic Consumption Rates (K) at
Different Excitation Density for Esters 3 and 4a
esters
K1
K2
K3
K4
3
4
0.01572
0.00761
0.00814
0.00785
0.00396
0.00387
0.00189
À
Two-photon uncaging cross sections (δu) of the esters
were measured using a fs-pulsed Ti:sapphire laser at 800
nm with fluorescein as an external standard.19 As summar-
ized in Table 1, ester 3 shows notably large δa at 800 nm
and is ∼150 times higher than that of ester 4 (∼309 GM,
thelargest forphototriggersasknown), which confirmsthe
introduction of a D-π-A backbone exhibits improved a
cross section for TPA. The progress of photolysis was
a The two-photon excitation density for K1ÀK4 was 0.60, 0.5, 0.4,
and 0.3 mJ cmÀ2 per pulse, respectively. The unit for K is nmol/min.
The values were taken from the HPLC analysis by taking a small aliquot
3
(100 μL) of the photolysis solution after certain time intervals.
quadratic dependence of the uncaging rate K as a function
of the incident laser intensity confirms that the photo-
release is truly induced by the two-photon excitation.20
These results indicate that the two-photon photolysis
efficiency is improved substantially for our designed
phototrigger that will enhance the feasibility for biological
application.
Last, a caged purine nucleoside (ester 5) using
{7-[(4-(dimethylamino)styryl]coumarin-4-yl}methoxycar-
bonyl as a phototrigger was synthesized, and the synthetic
route was similar to that for ester 3 (see Supporting
Information). The photorelease properties are also sum-
marized in Table 1, which reveals similar results to those
for ester 3. Upon irradiation, either one-photon (λ = 475
nm, 28 mJ/s) or two-photon excitation (λ = 800 nm, 0.6
mJ/cmÀ2), free adenosine is released gradually by control-
ling the irradiation time (see Figures 2À3).
In conclusion, we successfully prepared a new photo-
trigger based on {7-[(4-(dimethylamino)styryl]coumarin-
4-yl}methoxycarbonyl for alcohols, which are confirmed
to regulate the release of the caged alcohols under irradia-
tion of long wavelength light (475 nm) or two-photon NIR
light (800 nm). Its salient advantages of long wavelength
excitation and efficient two-photon sensitivity increase its
usefulness for caging biologically active molecules contain-
ing a hydroxyl functionality.
Figure 3. Time course of two-photon photolysis of esters 3, 4,
and 5 at 800 nm (0.60 mJ cmÀ2 per pulse). Samples (10À4 M)
were irradiated in CH3CN/H2O (9:1) solution.
3
measured by HPLC and graphed as a function of time
(Figure 3). It shows that the photolysis of ester 3 is much
faster than that of ester 4, and the value of δu arrives at 0.26
GM, which is more than twice that of ester 4. It suggests
that ester 3 provides more efficient two-photon sensitivity
at 800 nm and is sufficiently high for physiological use.
Compared with δa, the relative smaller increase of δu for
ester 3 is attributed to the decreased photolysis quantum
yield accompanying the elongation of the conjugation.6
Photolysis measurements with varying two-photon excita-
tion density were also performed for esters 3 and 4 under
the same conditions. Their photolytic consumption rates
(K) (Table 2) indicate that the two-photon photolysis rate
of ester 3 is about two times faster than that of ester 4,
which is coincident with their δu value. The approximate
Acknowledgment. We thank the NSFC(20903039,
21073062), “Chen Guang’’ project (09CG25), Shanghai
Rising-star program “10QA1401600”, and Fundamental re-
search funds for the central university for financial support.
Supporting Information Available. Synthesis and ex-
perimental procedures, and characterization data for all
new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
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Org. Lett., Vol. 14, No. 2, 2012
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