Fluorescent properties of coumarins with dual functions constructed by two sequential reactions

Article abstract of DOI:10.1016/j.tetlet.2012.08.085

A series of coumarins with two aryl moieties was constructed from 3-azide-6-bromocoumarin scaffold by means of two sequential reactions, Huisgen 1,3-cycloaddition and Suzuki-Miyaura coupling reactions. Evaluation of their fluorescence properties indicated that the mode of fluorescent change by substituted group on each aryl group would be different. So this structure could be promising candidates as multi-analyte type fluorescent sensors, whose mode of spectroscopic changes, such as fluorescence intensity, and absorption or fluorescence maximum wavelength, are different for each analyte.

Full text of DOI:10.1016/j.tetlet.2012.08.085

Tetrahedron Letters 53 (2012) 5916–5919
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Tetrahedron Letters
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Fluorescent properties of coumarins with dual functions constructed by
two sequential reactions
⇑
Tomoya Hirano, Haruko Kubo, Takuya Shiraishi, Kenichi Hiromoto, Takashi Fujiwara, Hiroyuki Kagechika
Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of coumarins with two aryl moieties was constructed from 3-azide-6-bromocoumarin scaffold by
means of two sequential reactions, Huisgen 1,3-cycloaddition and Suzuki–Miyaura coupling reactions.
Evaluation of their fluorescence properties indicated that the mode of fluorescent change by substituted
group on each aryl group would be different. So this structure could be promising candidates as multi-
analyte type fluorescent sensors, whose mode of spectroscopic changes, such as fluorescence intensity,
and absorption or fluorescence maximum wavelength, are different for each analyte.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 18 July 2012
Revised 16 August 2012
Accepted 22 August 2012
Available online 31 August 2012
Keywords:
Fluorescence
Coumarin
Click chemistry
Sensor
Small molecule-based fluorophores are useful and versatile
molecules because of their low cost and minimal technical
limitation, which could make them utilized in various applications
such as fluorescent labeling to biomolecules and development of
fluorescent sensors.1 Diversity-oriented synthesis and the
construction of chemical compound libraries are thought to be
an effective approach for finding small molecules with functional
activities.2 Recently, diversity-oriented fluorescence library ap-
proach (DOLFA)3 has been applied to various dyes, including styryl
dyes,4 benzimidazole dyes,5 xanthene dyes,6 and coumarin dye,7
and has yielded several practically useful small molecule-based
fluorophores and excellent fluorescent sensors for ions, proteins,
or specific types of cells.3–8 For this approach, conjugating
reactions, which have orthogonal character in relation to various
functional groups, of core dye structure would be desirable. Huis-
gen 1,3-cycloaddition reaction has been applied, and yielded novel
fluorophores with valuable fluorescent properties.9 In some
sensors, triazole ring formed by Huisgen reaction could become a
part of recognition unit for the analyte, like metal cations and
reactive nitrogen species.10
and this property could be utilized as sensor candidates. In this Let-
ter, we tried to combine these two reactions, Huisgen 1,3-cycload-
dition reaction and Suzuki–Miyaura coupling reaction, and applied
them to one coumarin scaffold to expand the variety of the fluores-
cent property, and explore potential availability as a fluorescent
sensor, especially as a multi-analyte type fluorescent sensor.
We set 3-azido-6-bromocoumarin derivative (3) as a starting
material, and its 3-azide group and 6-bromo group, which can re-
act with various arylacetylenes via Huisgen cycloaddition reaction,
or with various boronic acid derivatives via Suzuki–Miyaura cou-
pling reaction, respectively, to form a series of coumarin deriva-
tives represented by general formula 4 (Fig. 1b). The synthesis of
scaffold molecule 3 and construction of compounds 4 are illus-
trated in Scheme 1. 3-Azidocoumarin derivatives have been pre-
pared from the corresponding 3-nitrocoumarins,7b so initially we
prepared 6-bromo-3-nitrocoumarin, which was reduced to the
amino compound, then transformed into azide 3. However, the
yield of each step was relatively low (data not shown). Since vari-
ous 3-substituted coumarin moieties have been synthesized from
the acetate ester of salicylaldehyde,12 we employed the reported
method to transform azidoacetate ester 6, prepared from bromo-
salicylaldehyde 5 and azidoacetyl chloride, into 3-azidocoumarin
3 in moderate yield by utilizing proazaphosphatrane (7). Com-
pound 3 was reacted with arylacetylenes (8a–8e) via Huisgen
cycloaddition reaction to yield 4a–4e, and these products were fur-
ther reacted with arylboronic acids (9a–9e) to construct a series of
coumarin derivatives 4aa–4ee (Scheme 1).
Previously, our group reported 6-arylcoumarin derivatives (2),
whose structures were constructed via Suzuki–Miyaura coupling
reaction between 6-bromocoumarin (1) and various arylboronic
acid moieties (Fig. 1a).11 The fluorescence properties of 6-aryl-
coumarins (2) are influenced by the substituent group (R); that
is, the fluorescence intensity was largely changed, while the shift
of the fluorescence maximum wavelength was less than 20 nm,
The fluorescence properties of coumarin derivatives bearing
hydrogen, dimethylamino or cyano group(s) as R1 and/or R2 groups
are summarized in Figure 2 and Table 1. The fluorescence intensity
⇑
Corresponding author. Tel.: +81 3 5280 8032; fax: +81 3 5280 8127.
E-mail address: kage.chem@tmd.ac.jp (H. Kagechika).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.085

T. Hirano et al. / Tetrahedron Letters 53 (2012) 5916–5919
5917
in dichloromethane was greater than that in more polar solvents
such as acetonitrile for every compound, (Table 1). In acetone
and methanol, those fluorescent properties were similar as those
in acetonitrile (data not shown). The fluorescence of the dimethyl-
amino-substituted series (designated as ‘b’) was weaker than that
of the hydrogen (designated as ‘a’) and cyano (designated as ‘c’)
series. The absorption maximum wavelength varied by about
20 nm, depending on the functional group at the 6-aryl ring (R2),
as in the case of the compounds with R1 = H, 4aa, 4ab, and 4ac,
whose absorption maximum wavelengths were 412, 425, and
406 nm, respectively. A similar trend was also observed in the
compounds with R1 = NMe2 (4ba, 4bb, and 4bc) or CN (4ca, 4cb,
and 4cc). On the other hand, the compounds with R2 = H, 4aa,
4ba, and 4ca, showed a smaller variation of the absorption maxi-
mum wavelength according to R1 group, and the maxima were
412 nm for 4aa, 415 nm for 4ba, and 416 nm for 4ca. This trend
was similar to that of compounds with R2 = NMe2 (4ab, 4bb, and
4cb) and R2 = CN (4ac, 4bc, and 4cc). Similar characteristics were
also observed for the fluorescence maximum wavelengths,
although 4bb showed only residual fluorescence, and its maximum
wavelength was not determined. These data suggested that chang-
ing the R2 group, hydrogen (shown with black line in Fig. 2), elec-
tron-donating dimethylamino group (shown with blue line in
Fig. 2) and electron-withdrawing cyano group (shown with green
line in Fig. 2), could induce the change of fluorescence intensity
with a moderate shift of the absorption or fluorescence maximum
wavelength. Thus, considering the scaffold (4) as sensor candi-
dates, the transformation of the R2 group by reacting or binding
with each analyte species is expected to induce the shift of the
absorption or fluorescence maximum wavelength as well as the
change of fluorescence intensity. On the other hand, changing the
R1 group, hydrogen (shown with solid line in Fig. 2), electron-
donating dimethylamino group (shown with dashed line in
Fig. 2), and electron-withdrawing cyano group (shown with dotted
line in Fig. 2), could induce a fluorescence intensity change with lit-
tle wavelength shift. So, as sensor candidates, the transformation of
R1 group by reacting or binding with each analyte species is
Figure 1. Synthesis of a series of coumarins (2 and 4) from scaffold molecules 1 and
3.
Scheme 1. Synthesis of coumarin compounds 4.

5918
T. Hirano et al. / Tetrahedron Letters 53 (2012) 5916–5919
indicated by weak fluorescence of dimethylamino- (designated as
‘b’) or amino-substituted (designated as ‘d’) derivatives, which is
similar to that seen with fluorescein derivatives or Tokyo Green
derivatives.13 The detailed mechanistic investigations are in
progress.
Then, we focused on the fluorescence properties of compounds
(4dd, 4de, 4ed, and 4ee) bearing amine (designated as ‘d’) and
acetamide groups (designated as ‘e’) to examine the usefulness of
this approach for the development of fluorescent sensors (Table 1
and Fig. 3). A change of the R2 group from acetamide to amine
shifted the fluorescence maximum wavelength from 494 nm
(4ee) to 514 nm (4ed), and from 494 nm (4de) to 518 nm (4dd)
with a change of fluorescence intensity. The absorption maximum
wavelength was also slightly shifted, that is, 414 nm (4ee)–420 nm
(4ed) and 413 nm (4de)–420 nm (4dd) (Table 1). On the other
hand, a change of the R1 group (4ee to 4de and 4ed to 4dd) altered
the fluorescence intensity with little shift of the fluorescence max-
imum wavelength. These data suggested that compound 4ee could
be utilized as a core structure of fluorescent sensors for multiple
amide-amine transforming reactions, such as enzymatic reactions
catalyzed by peptidases, and the mode of fluorescence change
would be different, depending on the position of the substituent,
that is, one might produce a change of only the fluorescence inten-
sity, while another might produce a change of the fluorescence
intensity with a shift of the fluorescence maximum wavelength.
Therefore, it might be possible to obtain multi-input and multi-
output type sensor candidates.
Recently, some sensors of this type, whose mode of spectro-
scopic change, such as fluorescence intensity, color (absorption
or fluorescence maximum wavelength) and chemiluminescence,
is different for each input (analyte), have been reported.14 Their
structures are based on Bodipy or ruthenium complex, and they
are expected to be useful in biological analysis and in the con-
struction of molecular logic gates.15 Optimization of our 7-
diethylaminocoumarin-based scaffold might yield fluorescent
sensors of this type. In our strategy, the introduction of the rec-
ognition unit was performed in the final two sequential steps of
the synthesis, using Suzuki–Miyaura coupling and Huisgen
cycloaddition, which have orthogonal character in relation to
various functional groups. So, this might be a facile method
for the development of various multi-analyte type fluorescent
sensors.
Figure 2. Absorption and fluorescence spectra of coumarin derivatives 4. (a)
Absorption spectra and (b) fluorescence spectra excited at 410 nm of 4aa–4cc were
measured for
3 lM solutions in dichloromethane. Compounds with the same
functional group at R2 are shown in the same color (black: H; blue: NMe2; green:
CN). The same line type indicates compounds with the same functional group at R1
(solid line: H; dashed line: NMe2; dotted line: CN).
expected to induce only the change of fluorescence intensity with
little shift of wavelength. These characteristics of R1 and R2 groups
of our diethylaminocoumarin scaffold (4) are thought to be utilized
for the development of multi-analyte type fluorescent sensor with
multi mode of fluorescent change. Some mechanisms, such as
intramolecular charge transfer (ICT) and/or twisted intramolecular
charge transfer (TICT), where the interaction between 6- and 7-
substituents is thought to be important, are involved in these
photophysical properties.1 Photoinduced electron transfer (PeT)
would also participate in the control of fluorescence intensity as
In conclusion, we have synthesized a series of coumarins with
two aryl moieties from a single synthetic intermediate 3 with var-
ious arylboronic acids and arylacetylenes. The unique fluorescence
properties of these compounds suggested that the method has
Table 1
Absorption and fluorescence properties of coumarins 4 in dichloromethane and acetonitrile
Compound
In dichloromethane
In acetonitrile
Absmax (nm)
e
(Â104 MÀ1cmÀ1
)
Emmax (nm)
Q.Y.b
Absmax (nm)
e
(Â104 MÀ1cmÀ1
)
Emmax (nm)
Q.Y.b
4aa
4ab
4ac
4ba
4bb
4bc
4ca
4cb
4cc
4dd
4de
4ed
4ee
412
425
406
415
426
407
416
426
409
420
413
420
414
3.3
2.8
3.1
3.5
3.0
3.1
3.7
2.8
3.8
3.1
2.9
3.1
2.9
490
484
481
491
n.d.a
483
493
486
484
518
494
514
494
0.606
0.016
0.652
0.040
n.d.a
0.045
0.501
0.030
0.632
0.031
0.043
0.091
0.512
402
414
397
405
415
398
405
416
399
411
406
411
406
2.8
2.3
2.6
2.9
2.5
2.7
3.0
2.4
3.2
2.7
2.4
2.6
2.5
507
n.d.a
496
n.d.a
n.d.a
n.d.a
509
n.d.a
501
n.d.a
497
483
493
0.013
n.d.a
0.018
n.d.a
n.d.a
n.d.a
0.009
n.d.a
0.011
n.d.a
0.024
0.022
0.052
a
Emission maximum wavelength or quantum yield was not determined, because only residual fluorescence was observed.
Quantum yields of fluorescence were determined using that of quinine sulfate in 0.1 M H2SO4 (0.577) as a standard.
b

T. Hirano et al. / Tetrahedron Letters 53 (2012) 5916–5919
5919
Acknowledgements
The work described in this Letter was partially supported by
Grants-in-Aid for Scientific Research from The Ministry of Educa-
tion, Science, Sports and Culture, Japan (Grant Nos. 22790106
and 24950005 to T.H., 19201044 to H.K.), and by a Grant-in-Aid
for Scientific Research on Innovative Areas (Grant No. 22136013
to H.K.). T.H. also gratefully acknowledges funding from the Astel-
las Foundation and Takeda Science Foundation.
Supplementary data
Supplementary data (experimental procedures including details
of synthesis and identification) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2012.08.085.
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Figure 3. Absorption and fluorescence spectra of amino- or acetamido-substituted
compounds (4dd, 4de, 4ed, and 4ee).
potential for the development of a variety of fluorescent sensors,
including multi-analyte type sensor molecules.