Synthesis and spectroscopic properties of benzo- and naphthofuryl-3-hydroxychromones

Article abstract of DOI:10.1139/v01-066

With the focus of designing new fluorescent probes, four new 3-hydroxy-chromone derivatives bearing benzofuran and naphthofuran groups were synthesized. They show bathochromic absorption shifts relative to 3-hydroxyflavone with the ability of retention to display the excited-state proton transfer. Disruption of the planarity by the methyl group in the furan ring leads to a decrease of both the extinction coefficient and the contribution of long wavelength absorption band, while molecules without a methyl group showed two distinct absorption bands. Shifts to longer wavelengths are also observed in fluorescent spectra, and the absence of the methyl group results in a dramatic increase of fluorescence quantum yield and lifetime. Of the extended 3-hydroxychromone derivatives, 3-hydroxy-2-naphtho[2,1-b]furan-2-yl-chromone has shown comparable, and in some cases better, absorption and fluorescence properties than the 3-hydroxychromones synthesized so far, which make it a highly promising candidate as molecular probe for analytical chemistry, biophysics, and cellular biology.

Full text of DOI:10.1139/v01-066

358
Synthesis and spectroscopic properties of
benzo- and naphthofuryl-3-hydroxychromones
Andrey S. Klymchenko, Turan Ozturk, Vasyl G. Pivovarenko, and
Alexander P. Demchenko
Abstract: With the focus of designing new fluorescent probes, four new 3-hydroxy-chromone derivatives bearing
benzofuran and naphthofuran groups were synthesized. They show bathochromic absorption shifts relative to
3-hydroxyflavone with the ability of retention to display the excited-state proton transfer. Disruption of the planarity
by the methyl group in the furan ring leads to a decrease of both the extinction coefficient and the contribution of long
wavelength absorption band, while molecules without a methyl group showed two distinct absorption bands. Shifts to
longer wavelengths are also observed in fluorescent spectra, and the absence of the methyl group results in a dramatic
increase of fluorescence quantum yield and lifetime. Of the extended 3-hydroxychromone derivatives, 3-hydroxy-2-
naphtho[2,1-b]furan-2-yl-chromone has shown comparable, and in some cases better, absorption and fluorescence
properties than the 3-hydroxychromones synthesized so far, which make it a highly promising candidate as molecular
probe for analytical chemistry, biophysics, and cellular biology.
Key words: benzo- and naphthofuryl-3-hydroxyflavone, synthesis, electronic spectra, fluorescence, excited state proton
transfer.
Résumé : Dans le but de développer de nouvelles sondes fluorescentes, on a synthétisé quatre nouveaux dérivés de la
3-hydroxychromone portant des groupes benzofurane et naphtofurane. Par rapport à la 3-hydroxyflavone, ils présentent
des déplacements bathochromes d’absorption tout en conservant leur habilité à donner lieu à un transfert de proton
dans l’état excité. La présence du groupe méthyle dans le noyau furane conduit à un manque de planéité qui provoque
une diminution du coefficient d’extinction ainsi que de la contribution de la bande d’absorption de grande longueur
d’onde alors que les molécules sans groupe méthyle présentent deux bandes d’absorption distinctes. On observe aussi
des déplacements vers les longueurs d’onde plus élevées dans les spectres de fluorescence et l’absence du groupe mé-
thyle conduit à une augmentation dramatique du rendement quantique de fluorescence et du temps de vie. Parmi les
dérivés de la 3-hydroxychromone allongés, il a été démontré que, comparées à celles des 3-hydroxychromones synthéti-
sés jusqu’à maintenant, l’absorption et les propriétés de fluorescence de la 3-hydroxy-2-naphto[2,1-b]furan-2-
ylchromone sont au moins comparables et, quelquefois meilleures; il s’agit donc d’un candidat plein de promesses
comme sonde moléculaire pour la chimie analytique, la biophysique et la biologie cellulaire.
Mots clés : benzo- et naphtofuryl-3-hydroxyflavone, synthèse, spectres électroniques, fluorescence, transfert de proton
dans l’état excité. Klymchenko et al. 363
Introduction
isomerization of the normal excited form of the flavonol to a
phototautomer. These two excited isomers return to the
ground state with the emission of photons of different ener-
gies separated by 5000–6000 cm–1 (60–80 nm). The posi-
tions and the ratio of intensities of these two bands exhibit
strong dependence on solvent polarity (4) and hydrogen
bonding with the solvent molecules (2, 4, 5). These particu-
lar features suggest that 3-hydroxyflavones can be effec-
tively used as fluorescence probes to investigate the
structure and physical properties of biological membranes
such as polarity, hydration, transmembrane potential, and the
transformations of their structure under various conditions
(6, 7). For example, interdigitated gel phase formation of
lipid membrane bilayer in the presence of alcohols (8, 9) can
be followed by 3-hydroxyflavone probes (10).
So far, 4-amino derivatives of 3-hydroxyflavone (1a–d)
have been widely investigated as fluorescent membrane
probes and fluorescent chelators (11, 12). However, most
modifications of the structure were focused on extensions of
3-hydroxyflavone system at the chromone fragment, while
extensions of phenyl side have not been extensively explored.
3-Hydroxyflavones (1a–d) have interesting specific spec-
troscopic properties, which make them important chromo-
phores for the preparation of new fluorescence probes and
molecular sensing devices for applications in analytical
chemistry, biophysics, and cellular biology (1, 2). They ex-
hibit dual fluorescence due to excited state intramolecular
proton transfer (ESIPT) from the 3-hydroxy moiety to the
carbonyl group. The latter is characterized by a significant
Stokes shift of about 10 000 cm–1 (3). This results in the
Received November 28, 2000. Published on the NRC
Research Press Web site on April 24, 2001.
S. Klymchenko, T. Ozturk, and A.P. Demchenko.1
TUBITAK Marmara Research Center, Gebze-Kocaeli 41470,
Turkey.
V.G. Pivovarenko. Kiev Taras Shevchenko University,
Department of Chemistry, 252017, Ukraine.
1Corresponding author (telephone: (0262) 641-2300 (ext.
4003); fax: (0262) 641-2309; e-mail: dem@rigeb.gov.tr).
Can. J. Chem. 79: 358–363 (2001)
DOI: 10.1139/cjc-79-4-358
© 2001 NRC Canada

Klymchenko et al.
359
proton transferred tautomeric form. Furthermore, having
shifted the absorption and emission bands to longer wave-
lengths, such heterocyclic systems are expected to serve
better in biological systems, since most cells respond poorly
to short wavelength excitation (13), and intrinsic cellular
pigments interfere with the analysis. Therefore, it is highly
desirable to locate the emission peaks in a region where
background fluorescence (auto-fluorescence) from cellular
pigments is very low. It is, also, well known that longer
wavelength light is much less attenuated, and reduced Ra-
leigh and Raman scattering with longer wavelength would
yield much lower background signal. Obviously, having ex-
tended conjugation systems would be advantagous to reach
these goals.
Although all the chromones (2–6) have shown absorption
and fluorescence spectra at longer wavelengths compared
with the 3-hydroxyflavone (1a), naphthofurylhydroxy-
chromone 5 exhibits superior properties of both higher quan-
tum yields and relatively longer wavelength absorption and
fluorescence spectra. These properties make it one of the
best fluorescence probe candidates among the 3-
hydroxychromones synthesized so far.
Experimental
Melting points were determined on a Kofler hostage mi-
croscope melting point apparatus and are uncorrected.
Microanalyses were performed with a Carlo Erba 1106 Ele-
mental Analyzer. Proton NMR spectra were recorded at
200 MHz on JEOL PMX 270 MHz spectrometer.
Tetramethylsilane (TMS) was used as the internal standard
in all NMR spectra run in CDCl3 or DMSO-d6. Mass spectra
were recorded on a Kratos MS-25 mass spectrometer with
an ionization potential of 70 eV. All column chromatography
were performed on silica gel (Merck, Kieselgel 60H, Art 7736).
Absorption spectra were recorded on a Cary 3 Bio-spectro-
photometer (Varian) in the concentration range of 10–5 M.
Fluorescence measurements were recorded on a Quanta
Master spectrofluorometer (Photon Technology Interna-
tional). Quantum yields were measured by reference to 3-
hydroxyflavone (φF = 0.046 in acetonitrile, 0.024 in ethanol,
0.29 in toluene). Fluorescence lifetimes with ≈0.5 ns resolu-
tion were recorded on Strobe Master time-resolved
fluorometer (Photon Technology International), which has
nitrogen-filled pulsed lump as a light source (excitation
To modify and improve the chromophore properties, we
have synthesized four new chromones bearing benzofuran
(3 and 4) and naphtofuran groups (5 and 6). It is expected
that such systems may display improved fluorescent proper-
ties due to the following reasons: (i) their conjugation sys-
tems are larger than 3-hydroxyflavone, therefore, absorption
and emission bands are expected to shift to longer wave-
lengths; (ii) furan oxygen might compete for the 3-hydroxy
group’s proton, which should increase the sensitivity of the
molecule to hydrogen bonding interactions with the sur-
rounding molecules; (iii) the presence of methyl group in the
furan ring should lead to a decrease of planar structure sta-
bility, which is a crucial factor for fluorescence properties;
and (iv) the furan ring is a π-electron-abundant system, and
it is known that an electron donor in position two of the
chromone ring increases the transfer of electron density to
the carbonyl group in the excited state, which usually leads
to the enhancement of the proton transfer and stabilizes the
© 2001 NRC Canada

360
Can. J. Chem. Vol. 79, 2001
380 nm) and stroboscopic principle for registration of emis-
sion decay curves.
3-Hydroxy-2-(benzo[b]furan-2-yl)chromone (2) was syn-
thesized using the procedure described elsewhere (15).
15.9 Hz, 1H), 7.02 (d, J = 15.9 Hz, 1H), 7.48–8.07 (m, 9H),
8.40 (d, J = 8.1 Hz, 1H), 13.03 (s, 1H, H-exchangeable).
Anal. calcd. for C22H16O3: C 80.47, H 4.91; found: C 80.30,
H 4.87.
3-Hydroxy-2-(3-methylbenzo[b]furan-2-yl)chromone (3)
To a solution of 3-methylbenzo[b]furan-2-carbaldehyde
(9) (0.1 g, 0.63 mmol) and 2-hydroxyacetophenone (7)
(0.085 mL, 0.71 mmol) in ethanol (5 mL) was added 70%
potassium hydroxide (0.11 g, 2 mmol) dropwise. The mix-
ture was left stirring at room temp for 24 h, and to the resul-
tant red (E)-1-(2-hydroxyphenyl)-3-(3-methylbenzo[b]furan-2-
yl)-2- propen-1-one (14) solution was added 30% hydrogen
peroxide (0.5 mL, 4.4 mmol). After stirring for an additional
2 h, the precipitate was filtered, washed with ethanol (5 mL),
and crystallized from ethanol to obtain 2 (40 mg, 22%), mp
172°C. EI-MS m/z: 292.1 (M+), 275.0, 263.1, 249.0, 235.1,
221.0, 171.0, 144.0, 121.0, 115.0. 1H NMR (200 MHz,
CDCl3) δ: 2.58 (s, 3H), 6.90 (s, 1H, H-exchangeable), 7.29–
7.78 (m, 7H), 8.28 (dd, J = 8.0, 1.3 Hz, 1H). Anal. calcd. for
C18H12O4: C 73.97, H 4.14; found: C: 73.75, H: 4.14.
General procedure for synthesis of 3-hydroxychromone
derivatives
To a solution of appropriate 1-(2-hydroxyphenyl)-2-
propen-1-one derivative (15–17) (0.22 mmol) in ethanol
(5 mL) was added subsequently 70% potassium hydroxide
(0.1 g, 1.8 mmol) and 30% hydrogen peroxide (0.4 mL,
3.5 mmol). The mixture was then heated at reflux for 1 h,
cooled to room temp, and the precipitate was filtered,
washed with water, and crystallized from appropriate sol-
vent.
2-(6-Ethoxy-3-methylbenzo[b]furan-2-yl)-3-hydroxychromone
(4)
Crystallized from acetonitrile, yield 31%, mp 186°C. EI-
MS m/z: 336.1 (M+), 307.1, 265.1, 251.1, 237.1, 223.1,
1
187.1, 165.1, 160.1, 154.1, 140.1, 131.1, 121.0, 77.0. H
NMR (200 MHz, CDCl3) δ: 1.46 (t, J = 7.1 Hz, 3H), 2.55 (s,
3H), 4.09 (q, J = 7.1 Hz, 2H), 6.85 (s, 1H, H- exchangeable),
6.94 (dd, J = 8.6, 2.1 Hz, 1H), 7.09 (d, J = 2.0 Hz, 1H),
7.39–7.59 (m, 2H), 7.72 (td, J = 6.8, 1.4 Hz, 1H), 8.26
(dd, J = 8.1, 1.4 Hz, 1H). Anal. calcd. for C20H16O5: C
71.42, H 4.79; found: C 71.38, H 4.73.
General procedure for synthesis of 1-(2-hydroxyphenyl)-
2-propen-1-one derivatives
To a solution of appropriate benzo- or naphtho[b]furan-2-
carbaldehyde (0.57 mmol) and 2-hydroxyacetophenone (7)
(0.07 mL, 0.58 mmol) in ethanol (5 mL) was added 70% po-
tassium hydroxide (0.07 g, 1.25 mmol) dropwise, and the
mixture was left stirring for 24 h. In the case of the naphtho-
derivatives, the mixture was heated in water bath at 50°C for
the same time. Then, the mixture was poured into water
(20 mL), neutralized with dilute HCl, and the resultant pre-
cipitate was filtered and washed with water to yield the
product, which is pure enough to use for the next step.
3-Hydroxy-2-naphtho[2,1-b]furan-2-yl-chromone (5)
Crystallized from acetic acid, yield 40%, mp 253°C. EI-
1
MS m/z: 328.1 (M+), 299.1, 271.1, 215.1, 164.1, 152.1. H
NMR (200 MHz, DMSO-d6) δ: 7.47–8.01 (m, 7H), 8.09 (d,
J = 7.8 Hz, 1H), 8.16 (d, J = 7.8 Hz, 1H), 8.30 (s, 1H),
8.43 (d, J = 7.8 Hz, 1H), 10.35 (s, 1H, H-exchangeable).
Anal. calcd. for C21H12O4: C 76.82, H 3.68; found: C 76.61,
H 3.69.
(E)-3-(6-Ethoxy-3-methylbenzo[b]furan-2-yl)-1-(2-
hydroxyphenyl)-2-propen-1-one (15)
3-Hydroxy-2-(1-methylnaphtho[2,1-b]furan-2-yl)-chromone
(6)
Purified over silica gel column (hexane–ethyl acetate,
5:1), yield 49%, mp 106°C. EI-MS m/z: 322.1 (M+), 305.1,
276.1, 265.1, 229.1, 202.1, 189.1, 174.1, 161.0, 147.0,
Crystallized from acetic acid, yield 31%, mp 187°C. EI-
1
MS m/z: 342.1 (M+), 299.1, 285.1, 271.1, 185.1. H NMR
1
133.0, 121.0. H NMR (200 MHz, CDCl3) δ: 1.48 (t, J =
(200 MHz, DMSO-d6) δ: 2.77 (s, 3H) 7.47–8.00 (m, 7H),
8.11 (d, J = 8.0 Hz, 1H), 8.17 (d, J = 8.0 Hz, 1H), 8.52 (d,
J = 8.2 Hz, 1H), 9.68 (s, 1H, H-exchangeable). Anal. calcd.
for C22H14O4: C 77.19, H 4.12; found: C 77.1, H 4.05.
7.1 Hz, 3H), 2.41 (s, 3H), 4.11 (q, J = 7.1 Hz, 2H) 6.87–7.05
(m, 4H), 7.41–7.55 (m, 4H), 7.65 (d, J = 14.8 Hz, 1H), 7.89
(d, J = 14.8 Hz, 1H), 8.00 (dd, J = 8.0, 1.1 Hz, 1H), 13.05
(s, 1H, H-exchangeable). Anal. calcd. for C20H18O4: C
74.52, H 5.63; found: C 73.8, H 5.56.
Results and discussion
(E)-1-(2-Hydroxyphenyl)-3-naphtho[2,1-b]furan-2-yl-2-
propen-1-one (16)
The syntheses of the target molecules 2–6 were carried
out by employing the well-established Algar–Flynn–
Oyamada reaction (14). Condensation of 2-hydroxy-
acetophenone (7) with the aldehydes 8–12, synthesized as
described elsewhere (15–19), in basic solution gave the in-
termediates 13–17, respectively (Scheme 1). Treatment of
these intermediates with hydrogen peroxide in basic ethanol
solution led to the formation of 3-hydroxychromones 2–6,
respectively.
Crystallized from acetonitrile, yield 95%, mp 198°C. EI-
MS m/z: 314.1 (M+), 297.1, 285.1, 257.1, 239.1, 221.0,
1
194.1, 181.1, 165.1. H NMR (200 MHz, CDCl3) δ: 6.99 (d,
J = 15.9 Hz, 1H), 7.03 (d, J = 15.9 Hz, 1H), 7.30–8.18 (m,
10H,), 8.15 (d, J = 8.0 Hz, 1H), 12.91 (s, 1H, H-
exchangeable). Anal. calcd. for C21H14O3: C 80.24, H 4.49;
found: C 80.03, H 4.44.
(E)-1-(2-Hydroxyphenyl)-3-(1-methylnaphto[2,1-b]furan-2-
yl)-2-propen-1-one (17)
Crystallized from butanol, yield 64%, mp 174°C. EI-MS
m/z: 328.1 (M+), 310.1, 235.1, 208.1, 195.1, 178.1, 121.0.
1H NMR (200 MHz, CDCl3) δ : 2.83 (s, 3H) 6.98 (d, J =
All the 3-hydroxychromones (2–6) possess absorption
spectra at longer wavelengths compared with 3-hydroxy-
flavone (1a) in various solvents (Fig. 1: acetonitrile (A), eth-
anol (E), toluene (T)), and of all the 3-hydroxychromones
(2–6), 3-hydroxy-2-naphthofurylchromone (5) is found to
© 2001 NRC Canada

Klymchenko et al.
361
Scheme 1.
Fig. 1. Absorption spectra of chromones 1a, 2–6 in different sol-
vents: acetonitrile (A); ethanol (E); and toluene (T).
have the longest wavelength position of absorption spec-
trum. This result is related to the extension of the π-conjuga-
tion system. The effect of addition of methyl group at C-3
position of the furan ring can be observed as decrease of ab-
sorption at long-wavelength part of the spectra. Thus, the
absorption spectra of chromones 3, 4, and 6 bearing methyl
group at C-3 position of the furan ring, show only one ab-
sorption band at shorter wavelengths, with a small shoulder
at longer wavelengths, while compounds without a methyl
group (2 and 5) show two distinct absorption bands. One of
the possible suggestions for these observed differences could
be that the methyl group dramatically effects the hydrogen
bonding capabilities of 3-OH group with solvent, but it is
unlikely in view of the following reasons. First, this cannot
explain the presence of long-wavelength band in absorption
spectra of compounds 2 and 5 for all the solvents, including
nonpolar toluene. Second, it is in contradiction with ob-
served small differences between absorption spectra of
chromones 2 and 5 in toluene and in acetonitrile, where the
blue shift is correlated with the increase of polarity (Figs. 1A
and T). The latter means that hydrogen bonding of 3-OH
group with acetonitrile is either very weak, even for the case
of non-methylated chalcones, or unimportant for energetic
profile of the molecules. The critical role of methyl group in
absorption of compounds 3, 4, and 6 can be explained by a
steric factor that may lead to statistical non-planarity be-
tween benzo- and naphthofuran and chromone rings, which
has been already studied on 2′-, 3′-, and 4′-methyl-3-
hydroxyflavones (20). Therefore, it can be suggested that
planarity of the chromone molecule is the necessary condi-
tion for existence of the long-wavelength band.
excitation of 3-hydroxychromone ring, and the long wave-
length absorption, as in the case of planar compounds 2 and
5, is the result of charge transfer from the benzo- and
naphthofuran part to the 3-hydroxychromone system. Small
long-wavelength absorption shoulders in the spectra of 3, 4,
and 6 can be explained by less efficient π-electron
delocalization between the furan and chromone parts as a re-
sult of distortion of the planarity. Non-planarity results not
only in a decrease of intensity of the long-wavelength ab-
sorption band, but also in a decrease of the extinction coeffi-
cient (Table 1), while the introduction of a strong electron
donating group, such as ethoxy, to the benzofuryl group (4),
shifts the absorption band to longer wavelengths in compari-
son with its analogues (2 and 3) (Figs. 1A, E, and T).
Fluorescence spectra of 2–6 showed dual fluorescence in
all solvents which were used for the absorption measurements
(Figs. 2A, E, and T). Dual fluorescent behavior is well ex-
plained to be due to the excited state intramolecular proton
In the light of the previous studies (20) we can assume
that the absorption at short-wavelength is due to the normal
© 2001 NRC Canada

362
Can. J. Chem. Vol. 79, 2001
Table 1. Spectral properties of chromones 1a, 2–6.
Absorption
wavelength
(nm)
Position of
long-wavelength
peak (nm)
Extinction
Fluorescence
quantum
yield
Lifetime of
long-wavelength
band (ns)
coefficient (× 10–5)
(mol–1 cm–1)
Solvent
1a
2
Acetonitrile
Ethanol
Toluene
339
343
343
525
531.5
528
0.143
0.133
0.144
0.046
0.024
0.29
0.94
0.5
2.66
Acetonitrile
Ethanol
Toluene
338
367
365
542
544
547
0.165
0.198
0.253
0.177
0.122
0.396
2.09
1.41
3.87
3
Acetonitrile
Ethanol
Toluene
357
361
364
565
560
572.5
0.15
0.131
0.171
0.033
0.061
0.134
0.68
0.71
2.15
4
Acetonitrile
Ethanol
Toluene
373
378
383
580
—
586
0.190
0.177
0.212
0.0272
0.118
0.058
≤0.5
≤0.5
0.71
5
Acetonitrile
Ethanol
Toluene
376
398
403
559
555
562.5
0.348
0.243
0.331
0.14
0.22
0.371
1.93
1.77
3.08
6
Acetonitrile
Ethanol
Toluene
370
375
379
576
—
582
0.229
0.176
0.234
0.052
0.129
0.09
0.83
≤0.5
0.87
transfer (ESIPT), which is the result of the partial
isomerization of the excited normal isomer to
phototautomer. These two forms, then, return to the ground
state with the emission of different energies exhibiting two
fluorescent bands, at short and long wavelengths for the nor-
mal and phototautomer excited forms, respectively.
more, the effect of methyl group was found to cause an
increase in the IN* /IT* ratios for compounds 3, 4, and 6.
These results correspond to the increased IN* /IT* ratio for 3-
hydroxyflavone with a methyl group at position 2 of the
phenyl ring (20).
a
It is interesting to note that the molecules with methyl
group in furan ring (3, 4, and 6) demonstrate stronger red
shifted (by 22–28 nm, which is 670–880 cm–1) fluorescent
bands compared with the corresponding non-methylated
derivatives (2 and 5) (Table 1). Taking into account that the
position of the long-wavelength maximum of 3-hydroxy-
flavones is not very sensitive to 2- or 3- substituents of the
phenyl ring, the fact of the red shift, caused by such a weak
electron donor as methyl group, is important and requires
further investigation. Since a red shift is observed in both
polar and nonpolar solvents arguments regarding the differ-
ences in hydrogen bonding between solvent and molecules 2
and 3, and 5 and 6 do not explain this effect. It could be sug-
gested that the non-planar molecules, with the steric hin-
drance of the methyl group, relax to a more stable planar
excited conformation characterized by lower energies with
an emission at longer wavelengths. Red shifts in fluores-
cence due to flattening of a molecule in an excited state is
known for some heterocyclic compounds (21). Since, fluo-
rescence of the phototautomer of 3-hydroxychromones re-
quires a planar conformation between the 2-aryl and
chromone rings (20), it is understandable that the red shift,
caused by flattening of the non-planar chromones with the
methyl group in furan ring 3, 4, and 6, is much stronger for
the long-wavelength fluorescence band. For instance, in
acetonitrile the long-wavelength band of 3 (compared with
2) is red shifted by 24 nm (782 cm–1), whereas the short-
wavelength band is red shifted by just 6 nm (339 cm–1).
Fluorescent measurements of compounds 2–6 indicate
that replacement of the phenyl group of 3-hydroxyflavone
(1a) for longer aromatic systems, such as benzo- or
naphthofuryl, leads to a significant red shift (ca. 50 nm) (Ta-
ble 1). In polar solvents, such as acetonitrile and ethanol,
two distinctive fluorescent bands are observed, while only
the long-wavelength band is recorded in nonpolar solvent
(toluene). The exception is naphthofurylchromone 5, which
showed considerable short-wavelength fluorescence band in
both polar and nonpolar solvents. It appears that the stron-
gest among the solvent effects is the change in ratio of inten-
sities of normal excited form emitting at shorter wavelength
to phototautomer form emitting at longer wavelength
(IN* /IT*). This can be explained as the preference of hydro-
gen bonding of the 3-OH group with the solvent as com-
pared to intramolecular hydrogen bonding of 3-OH with the
carbonyl group. However, in acetonitrile this ratio is still
very small, especially for the case of non-methylated
chromones (2 and 5), which supports the above statement
based on absorption data that hydrogen bonding of 3-OH
group of the chromones with acetonitrile is very weak and
does not significantly change the energetic profile of the
molecules.
The fluorescence quantum yield of compounds 2 and 5,
was found to be higher than that of 3-hydroxyflavone, and
the addition of methyl group in C-3 position decreases the
fluorescence quantum yield considerably (Table 1). Further-
© 2001 NRC Canada

Klymchenko et al.
363
Fig. 2. Fluorescence spectra of chromones 1a, 2–6 in different
solvents: acetonitrile (A); ethanol (E); and toluene (T) (excitation
for: 1a (340 nm); 2 and 3 (360 nm); 4–6 (390 nm)).
make benzo- and naphthofuryl-3-hydroxychromones highly
promising candidates for fluorescence probes design. Cur-
rently, the preparation of membrane probes and fluorescent
chalators using the chromophoric systems 2 and 5 are under-
way, and the results will be published in due course.
Acknowledgements
We are grateful to A. D. Roshal (Institute of Chemistry of
Kharkov State University, Kharkov 310077, Ukraine) for
lifetime measurements, to V. P. Khilya (Kiev Taras
Shevchenko University, Department of Chemistry, 252017,
Ukraine) for providing some chemicals, and to TUBITAK
Marmara Research Center, Turkey for the financial support.
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intensity, which also indicates its dramatic influence on re-
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In conclusion, we have synthesized a series of 3-
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spects better, absorption and fluorescence properties than the
3-hydroxychromones synthesized so far. They have compa-
rably higher quantum yield, display absorption and fluores-
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