7-(2-Methoxycarbonylvinyl)-3-hydroxychromones: New dyes with red shifted dual emission

Article abstract of DOI:10.1016/j.tetlet.2004.09.032

The titled compounds, synthesized by Heck coupling reaction, demonstrate strongly red shifted absorption and dual emission as well as enhanced solvatochromism. 7-(2-Methoxycarbonylvinyl)-3-hydroxychromones have been synthesized using Heck coupling reaction from the corresponding 7-bromo-3-hydroxychromones. Introduction of the electron acceptor (2-methoxycarbonylvinyl) group at 7-position of 3-hydroxychromone results in a 30-40 nm red shift in absorption and >50 nm red shifts of both bands in emission. This derivatization allowed us to develop dyes with absorption maxima reaching 480 nm and dual emission in the red region of the spectrum. In comparison to the parent dyes, 7-acryl-3-hydroxychromones demonstrate significantly stronger solvatochromism. This is due to the acceptor group at 7-position, which increases the transfer character of the excited state of the dyes. The new dyes are highly prospective for the development of new fluorescent probes in biological research.

Full text of DOI:10.1016/j.tetlet.2004.09.032

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 8391–8394
7-(2-Methoxycarbonylvinyl)-3-hydroxychromones:
new dyes with red shifted dual emission
*
Andrey S. Klymchenko and Yves Mely
´
´ ´
Lab. de Pharmacologie et Physicochimie, UMR 7034 du CNRS, Faculte de Pharmacie, Universite Louis Pasteur,
67401 Illkirch, France
Received 23 May 2004; revised 2 September 2004; accepted 6 September 2004
Available online 23 September 2004
Abstract—7-(2-Methoxycarbonylvinyl)-3-hydroxychromones have been synthesized using Heck coupling reaction from the corre-
sponding 7-bromo-3-hydroxychromones. Introduction of the electron acceptor (2-methoxycarbonylvinyl) group at 7-position of
3-hydroxychromone results in a 30–40nm red shift in absorption and >50nm red shifts of both bands in emission. This derivatiza-
tion allowed us to develop dyes with absorption maxima reaching 480nm and dual emission in the red region of the spectrum. In
comparison to the parent dyes, 7-acryl-3-hydroxychromones demonstrate significantly stronger solvatochromism. This is due to the
acceptor group at 7-position, which increases the transfer character of the excited state of the dyes. The new dyes are highly pros-
pective for the development of new fluorescent probes in biological research.
Ó 2004 Elsevier Ltd. All rights reserved.
Recently it was demonstrated that 3-hydroxychromones
(3HC) are powerful fluorescence probes of intermolecu-
lar interactions in condensed media.1 Due to an excited-
state intramolecular proton transfer (ESIPT),2 these
dyes exist in two excited state forms, normal N* and
tautomer T*, which emit quanta of different energy
resulting in two-band fluorescence.3 These two bands
are highly sensitive to a variety of properties of the
microenvironment, which allows the application of these
dyes as sensors of polarity,1,4 and electric fields,5 and
also as probes to study polymers,6 reverse micelles,7,8
lipid membranes and proteins.9–12
which shifted the absorption of the dyes up to 400nm
and increased strongly the N* band emission in medium
polarity environments.13,14 We succeeded to improve the
fluorescent quantum yield of these dyes by substitution
of 2-phenyl with 2-benzofuryl heterocycle.15,16 Introduc-
tion in the latter cycle of electron donor groups (see dye
2) allowed shifting absorption and emission of the dyes
further to the red, to increase the relative intensity of the
N* band and to increase the two-band sensitivity of the
dyes to solvent polarity.17,18 In contrast, introduction of
an electron donor group (methoxy) from the opposite
side of the chromophore resulted in exactly opposite
The unique sensing properties of 3HC dyes can be real-
ized in full for studying biological systems only if their
absorption and fluorescence properties meet the
demands of modern fluorescence microscopy. In this
respect, the main disadvantages of the parent 3HC, 3-
hydroxyflavone, is its absorption in the ultraviolet, low
fluorescent quantum yield and low relative intensity of
the N* band with respect to the T* band in most media.
These properties were significantly improved with the
introduction of a 40-dialkylmino group (see dye 1),
N
N
1
8
2
O
MeOOC
MeOOC
O
O
7
6
3
O
O
H
4
5
H
O
1
1a
N
N
O
O
O
O
O
O
O
Keywords: 3-Hydroxychromones; Fluorescent dyes; Dual emission;
Solvatochromism.
O
H
H
*
Corresponding author. Tel.: +33 390 244115; fax: +33 390 244313;
e-mail: aklymchenko@aspirine.u-strasbg.fr
2a
2
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.09.032

´
A. S. Klymchenko, Y. Mely / Tetrahedron Letters 45 (2004) 8391–8394
8392
effects.18 Thus, we have concluded that electron donor
groups at the 2-aryl group increases the charge transfer
from the 2-aryl to the chromone cycle in the N* excited
state of the dyes, while 7-donor group provides the
opposite effect, probably by decreasing the electron
acceptor properties of the chromone cycle.18 Therefore,
introduction of an electron acceptor group at 7-position
could be extremely interesting to provide further
enhancement in the charge transfer character of the
3HC dyes.
poured into water and neutralized with 1M hydrochlo-
ric acid. The obtained precipitate of the corresponding
product 1a or 2a was filtered, dried and crystallized from
butanol.23
Absorption and emission spectra of the dyes 1a and 2a
were studied in five hydrophobic solvents of different
polarity and the data were compared with those for
the parent compounds 1 and 2.24 The results show that
in all the studied solvents, the one band absorption spec-
trum of the new dyes is significantly shifted to the red
with respect to the parent dyes (Table 1, Fig. 1). Similar
strong red shifts are observed for both emission bands in
the fluorescence spectra (Table 1, Fig. 1). Furthermore,
these shifts are accompanied by a dramatic increase in
the relative intensity of the N* band with respect to
the T* band.
In the present work we report on the synthesis of 3HC
dyes bearing at 7-position an electron acceptor (2-meth-
oxycarbonylvinyl) group (see dyes 1a and 2a). Our
results show that this group provides dramatic red shifts
in the absorption and emission spectra of the dyes,
increases the N* band relative intensity and enhances
their solvatochromism.
Previously we showed that a 7-donor (methoxy) group
provides blue shifts in absorption and emission maxima
and decrease in the I /I ratio.18 Thus, the 7-acceptor
group (2-methoxycarbonylvinyl) provides the opposite
spectroscopic effects. In this respect, the effects of the
7-acceptor group are very similar to those of a donor
aryl group at 2-position18 and, therefore, can be related
to the increase in the charge transfer character of the
excited state of 3HC dye as a result of the increase in
the acceptor properties of its chromone cycle.
The target 3HC dyes were synthesized in five steps from
3-bromophenol (Scheme 1). The latter was acylated with
acetic anhydride in pyridine and the product was con-
verted into 40-bromo-20-hydroxyacetophenone 3 in the
presence of aluminium chloride at 110°C. Applying a
recently developed procedure19 based on Algar–Flynn–
Oyamada method,20,21 acetophenone 3 was converted
into the corresponding 3HC 4 or 5 in two steps.22 At
*
N
*
T
the first step,
3 was condensed with 4-(diethyl-
amino)benzaldehyde or 6-(diethylamino)benzo[b]furan-
2-carbaldehyde17 in dry DMF in the presence of 3mol
excess of sodium methoxide. On the next step the reac-
tion mixture was diluted with two volumes of ethanol
followed by addition of 12mol excess of sodium meth-
oxide and 10mol excess of 35% hydrogen peroxide.
After reflux for 3–5min, the mixture was poured into
ice, neutralized and the precipitate of the corresponding
product 4 or 5 was filtered. The purified 4 or 522 was
converted into corresponding 1a or 2a by palladium-cat-
alyzed Heck coupling reaction with methyl acrylate
(Scheme 1). Solution of 100lmol of 4 or 5, 150lmol
of methyl acrylate, 1lmol of palladium (II) acetate
and 2lmol tri-o-tolylphosphine in 0.4mL of N,N-diiso-
propylethylamine and 1mL of DMF was heated for 4h
at 120°C. After cooling, the reaction mixture was
Similar to the parent compounds the new dyes 1a and 2a
demonstrate a dual emission that is strongly sensitive to
solvent polarity. Indeed, an increase in solvent polarity
from heptane (ET(30) = 31.1)25 to trichloroethylene
(ET(30) = 35.9) results in an increase of the relative
intensity of the N* band accompanied by its red shift
(Fig. 2). Noticeably, the increase in the I /I * ratio with
*
N
T
solvent polarity is larger in the case of dyes 1a and 2a as
compared to their parent compounds (Table 1). Indeed,
1a and 2a show a 12-fold growth of the IN*/I ratio in
*
T
toluene compared to heptane, while the IN*/I ratio of
*
T
1 and 2 increases only 4.4 and 7 times, respectively. This
suggests that the 7-acceptor group enhances the sensitiv-
ity of the new dyes to solvent polarity. Previously, we
have shown that the sensitivity of the dual emission of
3-hydroxychromones can be improved by increasing
the electron donor properties of the 2-aryl, which was
connected with the increase in the charge transfer from
the 2-aryl to the chromone ring in the N* excited state.18
However, the T* state was less dependent on the donor
properties of 2-aryl, probably because in this state the
1-oxygen heteroatom is involved more in the electron
donation than the 2-aryl group.18 In dyes 1a and 2a,
7-acceptor group produces a very similar effect. Thus,
Br
OH
O
Br
OH
AcCl
Py
AlCl
3
0
110 C
3
ArCHO
NaOMe
DMF
both the higher values of the I /I * ratio and the stron-
*
EtOH
reflux
N
T
H O
NaOMe
2
2
ger solvatochromism of 1a and 2a, compared to their
parent compounds 1 and 2, is probably connected with
their more pronounced charge transfer in the excited
state, due to an increase in the acceptor properties of
the chromone heterocycle. The increased charge transfer
character for the new compounds is also evidenced from
the strong fluorescence quenching in more polar solvents
like ethyl acetate (u = 0.10 for 1a) and acetone
(u = 0.008 for 1a). A very similar quenching with
MeOOC
Pd(OAc)
2
O
O
Ar
OH
Br
O
Ar
(o-Tol) P
3
i-Pr EtN
2
OH
DMF
COOMe
O
1a (49 %) and 2a (41 %)
4 (45 %) and 5 (21 %)
Scheme 1. Synthesis of 3-hydroxychromones 1a and 2a.

´
A. S. Klymchenko, Y. Mely / Tetrahedron Letters 45 (2004) 8391–8394
8393
Table 1. Spectroscopic properties of the studied chromonesa
Solvent ET(30)
3HC
kmax abs (nm)
kmax fl N* (nm)
kmax fl T* (nm)
IN /IT
*
u
*
Heptane 31.1
1
403
425
439
457
425
482
457
511
553
600
583
629
0.010
0.054
0.078
0.158
0.14
0.36
0.20
0.26
1a
2
2a
CCl4 32.4
1
411
434
450
471
442
499
481
536
560
605
596
649
0.026
0.180
0.277
0.511
0.21
0.41
0.31
0.26
1a
2
2a
CS2 32.8
1
423
448
466
487
459
518
502
556
578
625
618
666
0.049
0.386
0.487
0.958
0.16
0.39
0.26
0.26
1a
2
2a
Tolueneb 33.9
Trichloroethylene 35.9
1
408
434
446
470
456
531
508
571
566
612
613
657
0.044
0.652
0.55
0.14
0.45
0.26
0.24
1a
2
2a
1.30/1.9c
1
412
439
452
478
462
534
515
577
563
610
609
655
0.119
1.55c
0.943
2.9c
0.19
0.50
0.46
0.15
1a
2
2a
a ET(30)—empirical polarity index of solvents from Ref. 25. kmax abs—positions of absorption maxima, kmax fl N* and kmax fl T*—positions of the
fluorescence maxima of the N* and T* bands. u—the fluorescence quantum yield determined with 1 as a reference (u = 0.52 in ethanol, see Ref. 13).
b Data from Ref. 18.
c Values evaluated from deconvolution of the spectra to two bands approximated by log-normal function using the Siano program kindly provided
by the author (A.O. Doroshenko from the Karazin University, Kharkov, Ukraine).
1.2
1.0
TCE
Toluene
CS2
(A)
(A)
(B)
0.9
0.6
0.3
0.8
0.6
0.4
0.2
CCl4
Heptane
0.0
1.6
0.0
1.0
(B)
0.8
0.6
0.4
0.2
0.0
1.2
0.8
0.4
0.0
500
600
700
400
500
600
700
Wavelength, nm
Wavelength, nm
Figure 2. Fluorescence spectra of dyes 1a (A) and 2a (B) in solvents of
different polarity. TCE refers to trichloroethylene.
Figure 1. Absorption and fluorescence spectra of dyes 1a (A) and 2a
(B) (dashed lines) in toluene with respect to their parent compounds 1
and 2 (solid lines).
chromone), bearing the strongest 2-aryl electron donor
group studied so far on 3-hydroxychromones.18
increase in solvent polarity was observed for the
julolidine analog of dye 2 (3-hydroxy-2-(2,3,6,7-tetra-
hydro-1H,5H-furo[2,3-f]pyrido[3,2,1-ij]quinolin-10-yl)-
In conclusion, we have synthesized new 3-hydroxychro-
mone dyes characterized by significantly red shifted

´
A. S. Klymchenko, Y. Mely / Tetrahedron Letters 45 (2004) 8391–8394
8394
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7-acceptor group in the 3-hydroxychromone ring. The
latter methodology provides a facile route to the devel-
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shifted absorption and emission, which are highly
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Acknowledgements
22. 7-Bromo-40-(diethylamino)-3-hydroxyflavone (4): Crystal-
lized from butanol; yield 45%; mp 156°C; 1H NMR
(300MHz, CDCl3) 1.23 (6H, t, J 7.0Hz), 3.45 (4H, q, J
7.0Hz), 6.77 (2H, J 9.4Hz), 6.85 (1H, s), 7.49 (1H, dd, J
8.5, 1.9Hz), 7.76 (1H, d, J 1.9Hz), 8.08 (1H, d, J 8.5), 8.13
(2H, d, J 9.4Hz); ESI m/z (M+ + 1) 388.1. 7-Bromo-2-
(6-(diethylamino)benzo[b]furan-2-yl)-3-hydroxychromone (5):
This work was supported with CNRS research grants.
A.S.K. is presently a fellow from the European Commu-
nity (TRIoH integrated project). Laboratories of Mau-
´
rice Goeldner, Andre Mann are acknowledged for
providing support for organic synthesis.
1
Crystallized from DMF; yield 21%; mp 257°C; H NMR
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