k
S-SO/kI-SO, was 1.78 in the MB solution, compared to 3.93
for light-induced bleaching (Table 1). This is likely because
S-SO produces singlet oxygen more efficiently than I-SO.
Heavy atoms, like the additional sulfur in S-SO, increase
the probability of singlet-triplet intersystem crossing and
1
4
promote singlet oxygen formation.
Introduction of a cyano group blocked destruction of the
dyes by singlet oxygen, yet the cyano-substituted dyes still
underwent light-induced bleaching (albeit much more slowly).
This likely involves a reactive oxygen species other than
singlet oxygen, e.g., superoxide. Superoxide has been
detected in solutions of cyanine dyes upon illumination with
light, and is generated by electron transfer from the singlet
excited state of the dye to an oxygen molecule.1
5,16
This
would be facilitated by higher energy excited states, con-
sistent with our observation that the dye with shorter
excitation wavelengths bleaches more rapidly (Table 1:
k
S-SO-CN/kI-SO-CN ) 0.56).
Figure 2. Absorbance decay of I-SO, S-SO, I-SO-CN, and
S-SO-CN dyes in MeOH upon illumination with light.
Our results are consistent with a report in the literature
that the photostability of cyanine dyes is improved by
1
7
polyfluorination. Electron-accepting fluorine groups were
attached to the aromatic rings rather than to the polymethine
chain of a benzthiazole-based cyanine dye. The observed
effect was moderate: incorporation of eight fluorines in the
dye molecule resulted in about a 3.5-fold decrease in the
dye photobleaching rate.
These results point toward cyano substitution as a general
strategy for improving dye performance in imaging applica-
tions. We are currently preparing other merocyanine dyes
with cyano substituents in the polymethine chain to further
address the mechanism of the photobleaching reaction. Work
is in progress to prepare water-soluble analogues of cyano-
substituted merocyanine dyes with reactive groups for
attachment to proteins. These dyes will be used in the
construction of new photostable biosensors for live cell
imaging studies. Results of these studies will be reported in
due course.
by using the photosensitizer Methylene Blue (MB), which
has a high quantum yield for singlet oxygen production and
allows singlet oxygen to be introduced into the system
without excitation of the merocyanine dye.13 Light from a
tungsten lamp was filtered to permit selective excitation of
MB. Figure 3 shows that irradiation of the MB resulted in
Acknowledgment. Financial support from the NIH
(
grants GM57464 and GM64346) and Panomics, Inc. is
Figure 3. Methylene Blue-sensitized photobleaching of S-SO (A)
and S-SO-CN (B). Samples were irradiated with filtered tungsten
lamp light (λ > 630 nm) for 40 min [dye] ) [Methylene Blue] )
gratefully acknowledged.
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
3.33 µM.
OL070780H
43% bleaching of S-SO but no bleaching of S-SO-CN.
Under the same conditions, I-SO and I-SO-CN dyes
showed 27% and 0% bleaching. Thus introduction of -CN
completely blocked detectable bleaching by singlet oxygen.
The greater reactivity of S-SO vs I-SO toward singlet
oxygen can be attributed to the greater electron donating
capacity of the benzothiazole relative to the indolenine ring
system.
(
13) Murov, S. L.; Carmichael, I.; Hag, G. L., Handbook of Photochem-
istry, 2nd ed.; Dekker: New York, 1993.
14) Koziar, J. C.; Cowan, D. O. Photochemical Heavy-Atom Effects.
Acc. Chem. Res. 1978, 11, 334-341.
15) Clapp, P. J.; Armitage, B. A.; O’Brien, D. F. Two-Dimensional
(
(
Polymerization of Lipid Bilayers: Visible-Light-Sensitized Photoinitiation.
Macromolecules 1997, 30, 32-41.
(16) Chen, P.; Li, J.; Qian, Z.; Zheng, D.; Okakasaki, T.; Hayami, M.
Study on the Photooxidation of a Near-infrared-absorbing Benzothiazolone
Cyanine Dye. Dyes Pigm. 1998, 37, 213-222.
The relative rates of S-SO and I-SO bleaching are
different when bleaching is solely due to singlet oxygen
(17) Renikuntla, B. R.; Rose, H. C.; Eldo, J.; Waggoner, A. S.; Armitage,
B. A. Improved photostability and fluorescence properties through poly-
(Figure 3) or to light (Table 1). The relative bleaching rate,
fluorination of a cyanine dye. Org. Lett. 2004, 6, 909-12.
Org. Lett., Vol. 9, No. 15, 2007
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