kept in the dark for 7 days was irradiated (690 nm diode laser
at 200 mW cmꢀ2) and the fluorescence was determined. It
showed similar kinetic data with those of the fresh sample
(Fig. 4b), indicating that the aminoacrylate was stable in
medium at least up to 7 days in the dark.
In conclusion, we proposed and proved the concept of
‘‘click and photo-unclick chemistry’’ using nucleophile-yne
type reaction and photo-oxidative cleavage of electron-rich
olefins using singlet oxygen. Among aminoacrylate, amino-
acrylamide, and amioacrylthiolate, aminoacrylate seemed to
be best suited for applications for the release of active com-
pounds due to its fast photo-oxidation without unnecessary
oxidation products. In addition, we proved that the amino-
acrylate linker was cleaved rapidly by the irradiation of long
wavelength visible light (690 nm) and was stable under dark
conditions in the biological medium. This combination of click
and photo-unclick chemistry would find important applica-
tions in the spatio-temporal release of not only drugs but also
other bioactive molecules. Since the release can be triggered by
tissue penetrable low energy light, this simple but unique
chemistry will be applicable in the visible light-controlled
release of biologically important molecules at the tissue level.
This research was supported by the Department of Defense
[Breast Cancer Research Program] under award number
(W81XWH-09-1-0071). Views and opinions of and endorse-
ments by the author(s) do not reflect those of the US Army or
the Department of Defense. We thank Dr. AbuGafar M. L.
Hossion for his discussion and suggestion.
Fig. 3 Model compound, PS-L-Rh, for monitoring the cleavage of
the linker using FRET.
Fig. 4 Photocleavage of 17 in media: (a) fluorescence intensity
(excitation at 525 nm) after the irradiation, (b) photocleavage of 17
with or without irradiation in media: *17 kept for 7 days in the media
in the dark before the experiment.
5% fetal bovine serum) using FRET (fluorescence resonance
energy transfer), compound 17 (PS-L-Rh) was designed and
prepared by conjugating two dyes [hydroxyl-dithiaporphyrin
(PS, Chart 2) and rhodamine B (Rh B)] with the aminoacrylate
linker (Fig. 3).11 In PS-L-Rh, Rh B is a donor and PS
(dithiaporphyrin) is an acceptor of the FRET. Fluorescence
(lem: 575 nm, excitation at 525 nm) of the Rh group is
quenched by the PS group when they are close via the linker.
However, once the two dyes are apart after the cleavage of the
linker, the fluorescence intensity of the Rh group increases
dramatically since the FRET is not possible. Time-dependent
increase of Rh emission upon irradiation (690 nm diode laser
at 200 mW cmꢀ2) was first confirmed with PS-L-Rh in CHCl3.
Complete (B100%) cleavage was achieved in 10 min, giving
about 8-fold increase in Rh fluorescence intensity. This rate is
consistent with the cleavage data of 16 (in CDCl3, 90%
cleavage in 10 min) monitored by NMR (Fig. S1 and
Table S1, ESIw). Compound 17 successfully released hydroxyl
dithiaporphyrin, after irradiation (ESIw). The conversion yield
from 17 to hydroxyl-dithiaporphyrin by the photo-unclick
reaction seemed to be high (estimated by TLC, >80%). Then,
PS-L-Rh in media was irradiated using the same irradiation
conditions. It showed B100% cleavage in 30 min (Fig. 4). The
slower cleavage in medium may be, in part, due to the lower
concentration of oxygen (0.27 mM in media vs. 2.4 mM in
CHCl3 at atmospheric pressure)12 and the shorter lifetime of
singlet oxygen (2 ms in media vs. 60 ms in CHCl3).13
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The stability of the aminoacrylate linker of PS-L-Rh in
media and CHCl3 was investigated by monitoring the fluores-
cence emission (575 nm) of the Rh group excitation at 525 nm.
Up to 7 days, no increase of the Rh emission was observed
(o ꢁ 12%). To ensure the intactness of the linker, the PS-L-Rh
13 Modern Molecular Photochemistry, ed. N. J. Turro, University
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6517–6519 6519