A 3-hydroxychromone with dramatically improved fluorescence properties

Article abstract of DOI:10.1016/S0040-4039(01)01723-3

A new 3-hydroxychromone derivative, 2-(6-diethylaminobenzo[b]furan-2-yl)-3-hydroxychromone, has been synthesized by a concise route. Possessing dual emission common for 3-hydroxyflavones, it exhibits strong red shifts of both absorption and fluorescence spectra, which makes it the longest wavelength fluorescent dye among all known chromones. It also demonstrates a significant increase in fluorescence quantum yield in aprotic solvents and shift in solvent-polarity-dependent switch between normal and tautomer emissive forms. This derivative offers new possibilities in designing novel molecular sensors.

Full text of DOI:10.1016/S0040-4039(01)01723-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7967–7970
A 3-hydroxychromone with dramatically improved
fluorescence properties
Andrey S. Klymchenko,a,c,* Turan Ozturk,a Vasyl G. Pivovarenkob and Alexander P. Demchenkoa,c
aTUBITAK Marmara Research Center, Gebze-Kocaeli 41470, Turkey
bDepartment of Chemistry, Kyiv National Taras Shevchenko University, 01033 Kyiv, Ukraine
cA. V. Palladin Institute of Biochemistry, 9 Leontovicha str., 02030 Kyiv, Ukraine
Received 19 July 2001; revised 5 September 2001; accepted 13 September 2001
Abstract—A new 3-hydroxychromone derivative, 2-(6-diethylaminobenzo[b]furan-2-yl)-3-hydroxychromone, has been synthesized
by a concise route. Possessing dual emission common for 3-hydroxyflavones, it exhibits strong red shifts of both absorption and
fluorescence spectra, which makes it the longest wavelength fluorescent dye among all known chromones. It also demonstrates a
significant increase in fluorescence quantum yield in aprotic solvents and shift in solvent-polarity-dependent switch between
normal and tautomer emissive forms. This derivative offers new possibilities in designing novel molecular sensors. © 2001 Elsevier
Science Ltd. All rights reserved.
The presence of two well-separated fluorescence bands
belonging to excited-state normal (N*) and tautomer
(T*) forms is a characteristic feature of 3-hydroxy-
flavone (A, Fig. 1) derivatives.1,2 Introduction of an
electron donor group in the 4%-position increased the
charge-transfer nature of N* form and made the distri-
bution between these forms in emission very sensitive to
solvent polarity. This property resulted in a variety of
applications of substituted 3-hydroxyflavones as probes
in the studies of organized ensembles such as micelles,3,4
phospholipid vesicles,5,6 protein molecules7 and poly-
mers.8 However, this unique property has remained
unexplored in applications of fluorescence microscopy
to living tissues and cells, which requires new fluores-
cent sensors. In order to decrease photo-damage,
remove self-fluorescence and use common light sources
in the visible region, the probe has to be excited at 450
nm, or longer wavelengths, and its fluorescence quan-
tum yield should not be lower than that of the com-
monly used dyes. During our previous studies,9 we
replaced the 2-phenyl group of 3-hydroxyflavone with
2-benzo[b]furanyl and, in having extended conjugation,
the product, 2-(2-benzo[b]furanyl)-3-hydroxychromone
(B), exhibited better fluorescence properties than 3-
hydroxyflavone (A). In the next step, we have made an
attempt to increase the charge-transfer character of
3-hydroxychromone in the normal excited state (N*).
In analogy with 4%-diethylamino-3-hydroxyflavone (C),
a p-electron donor group, diethylamino, was introduced
to the 6-position of benzofuran ring. Our results show
that this group has dramatically improved the fluores-
cence properties of 2-(2-benzo[b]furanyl)-3-hydroxy-
chromone (B). To our best knowledge, this novel
product, 2 - (6 - diethylaminobenzo[b]furan - 2 - yl) - 3-
hydroxychromone (D), has the longest wavelength of
absorption and fluorescence among all the known
flavones and chromones to date.
O
O
O
OH
OH
O
O
A
B
N
N
O
O
O
OH
OH
O
O
C
D
Figure 1. 3-Hydroxychromones A–D.
2-(6-Diethylaminobenzo[b]furan-2-yl)-3-hydroxychro-
mone (D) was synthesized in four steps starting from
* Corresponding author. Tel.: +90-262-641-2300/3625; fax: +90-262-
641-2300; e-mail: andrey@rigeb.gov.tr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01723-3

7968
A. S. Klymchenko et al. / Tetrahedron Letters 42 (2001) 7967–7970
3-diethylaminophenol 1 (Scheme 1). It was trans-
formed to a sodium salt with sodium hydride, and
then reacted with bromoacetaldehyde diethylacetal in
DMSO at 50°C, using KI as a catalyst. The ether 2
was converted to the aldehyde 310 in POCl3 and
DMF at 50°C, so that the synthesis of this highly
functionalized aldehyde, 6-diethylaminobenzo[b]furan-
2-carbaldehyde 3, has been achieved in two steps. To
our best knowledge, this is a new and concise route
leading to such an aldehyde. Condensation of the
aldehyde 3 with 2-hydroxyacetophenone in the pres-
ence of excess KOH in aqueous ethanol yielded 2-
demonstrates record long-wavelength fluorescence of
both N* and T* forms. Second, introduction of
diethylamino group in 6-position of the benzofuran
ring leads to dramatic long-wavelength shifts—up to
180 nm in ethanol for N* form and up to 66 nm in
toluene for T* form (Table 1). This is significantly
higher than the effect of the same group in 4%-posi-
tion of 3-hydroxyflavone (A)—up to 123 nm in etha-
nol for N* form and up to 40 nm for T* form in
toluene. Such a strong effect of the donor group on
the fluorescence profile of B can provide an opportu-
nity to design new sensitive fluorescent sensors of
ions.14,15
propen-1-one
4,
which
was
then,
applying
Algar–Flynn–Oyamada reaction,11 treated with 30%
hydrogen peroxide to form chromone D.12
As a result of increase in charge transfer character of
normal excited state, an extremely high sensitivity of
D to solvent polarity is observed. The emission band
of N* form shifts from 460 nm in hexane to 600 nm
in ethanol, displaying strong positive solvato-
fluorochromy (140 nm, 5072 cm−1), which is signifi-
cantly higher than that of the analog C, with shorter
conjugation (73 nm, 3102 cm−1) (Table 1). It is
important to note that non-substituted A and B show
very small positive solvatofluorochromy. Another
excellent feature of chromone D is that the ratio of
intensities of two forms, N*/T*, changes from 0.1 to
ꢀ5 with the growth of solvent polarity from hexane
(ET[30]=31.0)16 to ethyl acetate (ET[30]=38.1) (Fig.
4). On the other hand, chromones A–C show pre-
dominantly emission of the T* form in this range of
solvents. In the more polar solvent, acetonitrile
(ET[30]=45.6), the T* form emission of D is already
not observed. These dramatic changes of partition
between emission of T* and N* forms with such a
small variation in the polarity of non-polar solvents is
a unique feature of D among 3-hydroxychromones,
which normally display significant spectral changes in
a range of more polar solvents. Considering the
sequence of chromones A, C and D, it could be con-
cluded that both the introduction of donor group and
increase of the dipole length favor the charge transfer
character of the excited state of 3-hydroxychromones
and shift dramatically the sensitivity of charge trans-
Absorption and fluorescence properties of 3-hydroxy-
chromone D were studied with respect to parent com-
pounds A–C in six solvents of different polarity:
hexane, toluene, ethyl acetate, chloroform, acetonitrile
and ethanol.13 In all the solvents, chromone D shows
significant red shift of absorption spectra, compared
with A–C (Fig. 2), approaching ca. 450 nm. Compar-
ing the absorption maxima of A with C, and B with
D (Table 1), it can clearly be noted that the red shift,
produced by introduction of p-electron donor, diethyl-
amino, to benzofuran ring of B (84 nm, 5177 cm−1 in
toluene) is considerably higher than in the case of the
introduction of the same donor to the phenyl ring of
A (65 nm, 4645 cm−1 in toluene). The 40 nm differ-
ence between absorption maxima of C and D is con-
sidered to be an effect of the chromophore extension.
Low solvent sensitivity of absorption spectra is
observed for all four compounds. The increase of
molar extinction coefficient of D up to 1.3 times
(Table 1), compared with B and C, is the result of
gathering their extended conjugation and p-electron
donor group, respectively, in one molecule.
The most remarkable effects are observed in fluores-
cence (Fig. 3). First, in all the solvents, chromone D
1)NaH, THF
N
N
2)DMSO, KI
OEt
OEt
OEt
Br
OH
O
OEt
2
1
50 oC
POCl3
DMF
N
N
O
30% H2O2
KOH, EtOH, rt
D
OH
KOH, EtOH, rt
HO
O
O
O
O
3
4
Figure 2. Normalized absorption spectra of chromones A–D
in toluene.
Scheme 1.

A. S. Klymchenko et al. / Tetrahedron Letters 42 (2001) 7967–7970
Table 1. Spectroscopic properties of compounds A–Da
7969
Solvent
umax
umax
umax
€
m×10−3
abs (nm)
fl N* (nm)
fl T* (nm)
Hexane
Toluene
C
D
A
B
C
D
C
D
C
D
A
B
C
D
A
B
C
D
403
439
343
364
408
446
401
437
413
453
339
358
404
437
343
366
412
444
450
458
–
553
586
528
547
568
613
571
618
562
611b
325
542
571
–
0.12
33.2
44.8
14.4c
29.9
30.7
37.6
31.1
37.3
31.5
37.5
14.3c
29.9
31.1
36.6
13.3c
29.4
31.5
35.9
0.22
0.29c
0.39d
0.12
414
452
508
474
537
482
545
390
418
509
582
400
423
523
600
0.24
0.039
0.18
AcOEt
CHCl3
CH3CN
0.11
0.36
0.046c
0.18d
0.10
0.23
EtOH
532
544
0.024c
0.12d
0.52e
0.10
–
–
a umax abs, positions of absorption maxima, umax fl N* and umax fl T*, positions of fluorescence maxima of N* and T* forms (dashes signify that
corresponding maxima are not resolved). € is the fluorescence quantum yield determined with C as the reference (€=0.52 in ethanol), and
m-molar coefficient of extinction (ml mol−1 cm−1).
b Band appears as a shoulder.
c Ormson, M. S.; Brown, R. G.; Vollmer, F.; Rettig, W. J. Photochem. Photobiol. 1994, 81, 65.
d Ref. 5.
e Chou, P.-T.; Martinez, M. L.; Clements, J. H. J. Phys. Chem. 1993, 97, 2618.
fer–proton transfer switch to less polar solvents. Thus,
A displays significant solvent sensitivity of partition
between N* and T* forms in the range of polar sol-
vents, acetonitrile–ethanol–trifluoroethanol,17 so are C
in the solvents of medium polarity, ethyl acetate–aceto-
nitrile–ethanol, and D in low polar solvents, hexane–
toluene–ethyl acetate (Fig. 4).
In polar aprotic solvents, chromone D shows much
higher fluorescence quantum yield than A–C (Table 1).
The higher quantum yield of benzo/naphthofuryl-3-
hydroxychromones, non-substituted at 3-position of
furan ring, was previously reported with respect to
Figure 3. Normalized fluorescence spectra of chromones A–D
3-hydroxyflavone.9 Presumably, this is due to their
in toluene.
more planar structure in steady state compared with
3-hydroxyflavones, which are known to be non-
planar18,19 with an angle of ca. 28° between chromone
and phenyl units.
In conclusion, this work is not only confined to show-
ing the unique fluorescence sensitivity of D to solvents.
In a more general sense, we have demonstrated the fine
tuning in switching between two well-resolved emissive
states with almost ideal ‘all-or-none’ switchability by
environmental stimuli. By designing substituents at the
site of electron-donor, diethylamino group, in line with
similar research based on substituted 3-hydroxy-
flavones,14,15 new sensing properties can be induced
with better prospects. In this work we also report, for
the first time, a concise route to 6-diethylaminoben-
Figure 4. Normalized fluorescence spectra of D in ethyl acet-
ate (E), hexane (H), and toluene (T).
zo[b]furan-2-carbaldehyde, which can provide an easy
access to the synthesis of various new compounds.

7970
A. S. Klymchenko et al. / Tetrahedron Letters 42 (2001) 7967–7970
J 7.1 Hz), 3.33 (3H, q, J 7.1 Hz), 6.70 (1H, d, J 2.3 Hz),
Acknowledgements
6.76 (1H, dd, J 8.8, 2.3 Hz), 7.40 (1H, s), 7.49 (1H, d, J
8.8 Hz), 9.61 (1H, s); EI m/z 217.1 (M+), 202.1, 193.1,
174.0, 159.0, 145.0, 118.0, 89.0.
We thank TUBITAK Marmara Research Center
(Turkey) for providing financial support.
11. Algar, J.; Flynn, J. Proc. R. Irish Acad. 1934, B42, 1.
12. 2-(6-Diethylaminobenzo[b]furan-2-yl)-3-hydroxychro-
1
mone, mp 226°C; H NMR (200 MHz, CDCl3) 1.23 (6H,
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