Modulation of the solvent-dependent dual emission in 3-hydroxychromones by substituents

Article abstract of DOI:10.1039/b302965d

3-Hydroxychromones (3HCs) are fluorescent dyes, which respond to solvent perturbations by shifts and changes in the relative intensity of the two well-separated bands in the emission spectra. These bands originate from an excited state intramolecular prot

Full text of DOI:10.1039/b302965d

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Modulation of the solvent-dependent dual emission in
3
-hydroxychromones by substituents
abc
Andrey S. Klymchenko,* Vasyl G. Pivovarenko, Turan Ozturk and
b
d
ce
Alexander P. Demchenko
a
Facult e´ de Pharmacie, Laboratoire de Pharmacologie et Physicochimie, Universit e´ Louis
Pasteur, BP 24, 67401, Illkirch, France. E-mail: aklymchenko@aspirine.u-strasbg.fr;
Fax: +33 3 90 24 43 13; Tel: +33 3 90 24 42 62
b
c
d
e
Department of Chemistry, Kyiv National Taras Shevchenko University, 01033, Kiev, Ukraine.
E-mail: pvg@univ.kiev.ua; Tel: +38 044 2296365
TUBITAK Research Institute for Genetic Engineering & Biotechnology, Gebze-Kocaeli 41470,
Turkey. E-mail: dem@rigeb.gov.tr; Fax: +90 262 6412309; Tel: +90 262 6412300
Chemistry Department, Middle East Technical University, Ankara, Turkey.
E-mail: trozturk@yahoo.com
A. V. Palladin Institute of Biochemistry, Kiev 01030, Ukraine. E-mail: adem@biochem.kiev.ua;
Fax: +38 044 229-63-65; Tel: +38 044 2241157
Received (in Montpellier, France) 13th March 2003, Accepted 12th May 2003
First published as an Advance Article on the web 4th August 2003
3
-Hydroxychromones (3HCs) are fluorescent dyes, which respond to solvent perturbations by shifts and
changes in the relative intensity of the two well-separated bands in the emission spectra. These bands originate
from an excited state intramolecular proton transfer (ESIPT) reaction, which can be modulated by different
factors, including modifications in the 3HC chromophore. In view of the great importance of 3HCs as
prospective basic elements of molecular sensors, we have performed the first systematic study on the correlation
between 3HC structure and spectroscopic properties. Two series of known and newly synthesized 2-phenyl-
3
-hydroxychromones and 2-(2-benzo[b]furanyl)-3-hydroxychromones with varied electron-donor substituents,
introduced on opposite sides of the chromophore, were compared in solvents of different polarities. The
substitution of 2-phenyl for 2-(2-benzo[b]furanyl) and introduction of electron donors on the 2-aryl group not
only shift the absorption and fluorescence spectra to the red, but also strongly modulate the ESIPT behavior,
resulting in a dramatic increase of the intensity ratio of the two emission bands, IN*/IT* . In contrast,
introduction of a 7-methoxy group results in exactly the opposite spectroscopic effects. All the studied 3HC
dyes demonstrate a linear increase in ln(IN*/IT*) with the solvent polarity parameter E
T
(30). Substitution of
2
-phenyl for 2-(2-benzo[b]furanyl) or introduction of electron donors on the 2-aryl group in 3HC increases the
sensitivity of their IN*/IT* ratio to solvent polarity and shifts the optimal range of ratiometric polarity sensing
to less polar solvents. The opposite effects are observed for 7-methoxy derivatives. These results allow a new
generation of two-band fluorescent sensors based on 3HC that operate by the ESIPT mechanism to be
proposed. By proper substituents their photophysical and sensing properties can be tuned over broad ranges.
3
4
0
micelles and polymers. 4 -Dialkylamino-3-hydroxyflavones
a typical example is flavone 3, Chart 1), due to their more
Introduction
(
5
3
-Hydroxychromone (3HC) derivatives are very interesting
advanced fluorescence properties, recently aroused the inter-
est of researches as two-band fluorescent dyes with a ratio-
metric response to different perturbations. They were proposed
compounds because they exhibit an excited state intramolecu-
lar proton transfer (ESIPT) reaction in such a manner that the
emission of two excited state species, reactant and reaction
6
7
as sensors of ions and electric fields. As microenvironment-
sensitive probes they were applied in micelles, model and cell
1
8
product, can be observed simultaneously. This results in two
9
membranes, and protein molecules.
10
emission bands, well-separated on the wavelength scale. The
band in the blue region originates from the normal excited
state (N*) and the other, shifted dramatically to the red, arises
from an excited tautomer state (T*). The spectroscopic beha-
vior of these bands as well as the interplay of their intensities
depends strongly on the structure of the 3HC chromophore
and the parameters of its microenvironment, which suggests
a variety of high technology applications. The most extensively
studied representative of this family is 2-phenyl-3-hydroxy-
chromone (1; 3-hydroxyflavone, 3HF). It was applied as a laser
Meantime, a wider application of 3HC derivatives as probes
in molecular and cellular biology and in nanoscale sensor
technologies has been limited so far due to several important
problems. One of them is the need to improve and optimize
for particular applications their spectroscopic properties. The
absorption maximum, in the near UV region for 3HF, should
be shifted to longer wavelengths and the fluorescence quantum
yield has to be increased. To resolve this problem the 2-phenyl
group of 3HF was substituted with benzo and naphthofuryl
1b
11
dye with a large Stokes’ shift and as a fluorescence probe
sensitive to hydrogen bonding perturbations in proteins,
groups, and, finally, a dialkylamino group was introduced at
0
1
e
2
the 6 position of 2-(2-benzo[b]furanyl)-3-hydroxychromone
1
336
New J. Chem., 2003, 27, 1336–1343
DOI: 10.1039/b302965d
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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in aprotic solvents as a result of dielectric stabilization of the
N* state, which due to its charge transfer character may
5
,13
possess a larger dipole moment than the T* state.
These features suggest that 3HCs may act as unique two-
band ratiometric polarity sensors with extremely high sensi-
tivity. In order to design efficient molecular sensors based on
3
HC it is primarily needed to resolve the question of how
the structure of 3HC derivatives determines their spectroscopic
and sensing properties. In this work, we provide comparative
solvatochromic studies of known and newly synthesized 3HC
derivatives (Chart 1). We demonstrate that, by providing
proper substitutions in the chromophore system, there is the
possibility of driving the spectroscopic properties in the desired
direction (longer wavelengths of light absorption and emission,
higher quantum yield) and of optimizing the two-color
ratiometric response to the selected range of solvent polarity.
Experimental
Instrumentation
Melting points of synthesized compounds were determined on
a B u¨ chi 512 melting point apparatus and are presented as
uncorrected values. Microanalyses were performed with a
Carlo Erba 1106 Elemental Analyzer. Proton NMR spectra
were recorded at 200 MHz on a JEOL PMX 270 MHz spectro-
meter. Tetramethylsilane (TMS) was used as the internal stan-
dard in all NMR spectra run in CDCl or [D ]DMSO. Mass
Chart 1 Chemical structures of the studied chromones.
3
6
(2), resulting in the development of chromone 4. The latter
shows superior absorption and fluorescence properties to those
reported to date for 3HC derivatives. In order to obtain a
further improvement of these properties, a more detailed study
of an extended range of 3HC derivatives has to be undertaken
and some general relationships need to be derived.
The other problem is that the ESIPT behavior, which con-
trols the redistribution of intensities between the two emission
bands, IN*/IT* , still cannot be predicted and modulated in the
desired range. This is the problem of major interest, because its
solution can provide a clue for designing new molecular sen-
sors and switches with a dramatic two-band fluorescence ratio-
metric response allowing fluorescent imaging in real color. The
literature data show that the presence of the electron-donating
spectra were recorded on a Kratos MS-25 mass spectrometer
using EI or FAB methods. All column chromatography was
performed on silica gel (Merck, Kieselgel 60H, Art 7736).
Absorption spectra were recorded on a Cary 3 Bio spectro-
photometer (Varian). Fluorescence spectra were recorded on
a Quanta Master spectrofluorometer (Photon Technology
International).
1
2
Reagents and solvents
All the reagents were purchased from Aldrich-Sigma Chemical
Company. Solvents for synthesis were of reagent quality and
were appropriately dried if necessary. For absorption and
fluorescence studies the solvents were of spectroscopic grade.
Flavone 1 was purchased from Aldrich Chemical Company.
0
4
-dialkylamino group in 3HF increases the IN*/IT* ratio dra-
5
c
5b,16
matically. Even stronger augmentation of the IN*/IT* ratio
was recently reported for dialkylamino-substituted chromone
4
Chromones 2 and 3 were prepared as described elsewhere.
1
with respect to parent compound 2. However, no systema-
2
6-Diethylaminobenzo[b]furan-2-carbaldehyde (5). To 11.5
mmol of 3-diethylaminophenol in 10 ml of dry THF, 13.8
mmol of NaH (55% in paraffin) was slowly added while stir-
ring. The solvent was evaporated and to the resultant mixture
dissolved in 8 ml of DMSO, 14.0 mmol of bromoacetaldehyde
diethylacetal and 11.5 mmol of KI were added. After heating
tic data on the effect of electron-donor substitutions at differ-
ent positions of the 3HC chromophore on the IN*/IT* ratio
are presently available.
Regarding solvent effects, a very strong perturbation of the
spectroscopic behavior has been reported for parent 3HF 1
by intermolecular H-bonding with protic solvents and for
1
e
ꢀ
at 50 C for 3 h, water was added. The product, N,N-diethyl-
dialkylamino-substituted 3HF 3 by the increase of solvent
polarity. In both cases, the IN*/IT* ratio can be increased dra-
3-(2,2-diethoxyethoxy)aniline, was extracted with benzene,
purified on silica gel column (hexane–ethylacetate, 4 : 1), and
used directly for the next step. Yield was 90%.
5
matically, up to complete disappearance of the T* emission
band. As it was recently shown the IN*/IT* ratio of 3 on the
logarithmic scale increases linearly with solvent polarity as a
To 26.9 mmol of POCl , cooled in an ice bath, 8.5 mmol
3
of DMF was added, while stirring. Then, 7.1 mmol of N,N-
diethyl-3-(2,2-diethoxyethoxy)aniline was added and stirring
1
3
function of dielectric constant. A similar effect was reported
1
4a
ꢀ
for chromone 4
with respect to the solvent polarity para-
In this respect, several other examples can
be found in the literature on dyes exhibiting the ESIPT reac-
was continued for 14 h at 50 C. The mixture was poured into
ice, neutralized and extracted with ethylacetate. The product
1
4b
meter E
T
(30).
5 was purified on silica gel column (hexane–ethylacetate,
4 : 1). Yield 21%. IR n 1655 cm (HC=O); H NMR (200
1
5
ꢁ1
1
tion and showing the possibility of solvent dual emission.
Meanwhile, the presence of the N* band in their emission is
associated with their conformational isomerization and disrup-
tion of their intramolecular hydrogen bonds. Therefore, the
connection between the IN*/IT* ratio of these dyes and solvent
polarity is not straightforward. In 3HCs 3-OH and 4-carbonyl
groups are members of the same chromone heterocycle and
therefore conformational changes are not involved in their
ESIPT reaction coordinate. The N* band in 3HCs can appear
MHz, CDCl ) d 1.21 (6H, t, J 7.1 Hz), 3.33 (3H, q, J 7.1
3
Hz), 6.70 (1H, d, J 2.3 Hz), 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-MS m/z
+
217.1 (M ), 202.1, 193.1, 174.0, 159.0, 145.0, 118.0, 89.0.
2,3,6,7-Tetrahydro-1H,5H-furo[2,3-f]pyrido[3,2,1-ij]quinoline-
10-carbaldehyde (6). To 11.5 mmol of 9-formyl-8-hydroxyjulo-
lidine in 10 ml of dry THF, 13.8 mmol of NaH (55% in
New J. Chem., 2003, 27, 1336–1343
1337

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paraffin) was slowly added while stirring. The solvent was eva-
porated and to the resultant mixture dissolved in 8 ml of
DMSO, 14.0 mmol of bromoacetaldehyde diethylacetal and
(1H, m), 7.48 (1H, d, J 8.8 Hz), 7.64 (1H, s), 7.64–7.75 (2H,
m), 8.25 (1H, dd, J 8.2); EI-MS m/z 349.2 (M ), 334.1,
305.1, 276.1, 248.1, 167.0.
+
ꢀ
1
1.5 mmol of KI were added. After heating at 60 C for 24 h,
water was added. The product, 8-(2,2-diethoxyethoxy)-2,3,6,7-
tetrahydro-1H,5H-pyrido[3,2,1-ij]quinoline-9-carbaldehyde, was
extracted with ethyl acetate, purified on silica gel column
2
-(6-Diethylaminobenzo[b]furan-2-yl)-3-hydroxy-7-methoxy-
chromone (4m). Prepared from 2-hydroxy-4-methoxyacetophe-
none and aldehyde 5. Crystallized from toluene; yield 15%, mp
14 C. UV (ethanol) e(438 nm) 41 000 l mol cm ; H NMR
200 MHz, CDCl ) d 1.23 (6H, t, J 7.0 Hz), 3.45 (4H, q, J 7.0
Hz), 3.93 Hz (3H, s), 6.75 (1H, dd, J 8.7, 2.2 Hz), 6.93 (1H, d,
J 2.2 Hz), 6.99 (1H, dd, J 8.8, 2.3 Hz), 7.05 (1H, d, J 2.3 Hz),
ꢀ
ꢁ1
ꢁ1
1
(dichloromethane–ethylacetate, 10 : 1) and used directly for
the next step. Yield was 60–70%.
2
(
3
The latter product was kept in an excess of 50% sulfuric acid
ꢀ
at 100 C for 4 h affording 2,3,6,7-tetrahydro-1H,5H-furo[2,3-
f]pyrido[3,2,1-ij]quinoline-10-carbaldehyde (6), pure enough
7.47 (1H, d, J 8.7 Hz), 7.57 (1H, s), 8.12 (1H, d, J 8.8 Hz);
EI-MS m/z 379.1 (M ), 364.1, 355.1, 324.1.
1
for the next step. Yield 20%. H NMR (200 MHz, CDCl
+
3
) d
.00 (4H, m), 2.83 (2H, t, J 6.3 Hz), 2.98 (2H, t, J 6.5 Hz),
2
3
.26 (4H, m), 7.08 (1H, s), 7.33 (1H, s), 9.57 (1H,s); EI-MS
3-Hydroxy-2-(2,3,6,7-tetrahydro-1H,5H-furo[2,3-f]pyrido[3,2,1-
+
m/z 241.1 (M ), 212.0, 184.1
ij]quinolin-10-yl)chromone (4j). Prepared from 2-hydroxyaceto-
phenone and aldehyde 6. Crystallized from butanol; yield 26%;
mp 295 C (dec.). UV (ethanol) e(458 nm) 44 000 l mol cm
ꢀ
ꢁ1
ꢁ1
;
General strategy and modified general procedure for
the synthesis of new 3-hydroxychromones
1
H NMR (200 MHz, [D ]DMSO) d 1.93 (4H, m), 2.79 (2H, t, J
6
7
7.49 (1H, m), 7.52 (1H, s), 7.60–7.82 (2H, m), 8.10 (1H, dd, J
8
.3 Hz), 2.95 (2H, t, J 6.4 Hz), 3.20 (4H, m), 7.11 (1H, s), 4.41–
The common two-step procedure for the preparation of 3-
1
7
hydroxychromones was found to be inefficient for the synth-
+
.0, 1.1 Hz); EI-MS m/z 373.1 (M ), 344.1, 316.1
esis of new 3HC derivatives having electron-donor groups, 3m,
3
j, 4m and 4j. Its first step, condensation of the corresponding
aldehydes with 2-hydroxyacetophenones, which requires
sodium hydroxide in aqueous ethanol, takes place for several
weeks without complete transformation. Then, oxidative
Absorption and fluorescence studies
The solutions of 3HC derivatives for absorption and fluores-
cence spectroscopy were used in concentrations corresponding
to an absorbance close to 0.1. Quantum yields j for 3, 3m, 3j,
0
heterocyclization of the obtained 1-(2 -hydroxyphenyl)-2-
propene-1-ones (Algar–Flynn–Oyamada reaction
1
7a
) with
4, 4m and 4j were determined with respect to a solution of 3
hydrogen peroxide and sodium hydroxide in aqueous ethanol
provides only traces of the target chromones. Therefore, for
both of the steps we applied a recently developed modified
5
b
in ethanol as the reference (j ¼ 0.52). Excitation wavelength
for the fluorescence studies was 350 nm for 1 and 2 and 420
nm with all other dyes. All the spectroscopic data for com-
1
8
procedure.
-Hydroxyacetophenones were condensed with the appro-
1
2
pounds 3 and 4 reported previously were reproduced and
corrected. Deconvolution of fluorescence spectra in the cases
when the two bands were overlapped was made using the
program Siano, kindly provided by the author (Dr. A.O.
Doroshenko from Karazin University, Kharkov, Ukraine). The
program uses an iterational non-linear least-squares method
based on the Fletcher–Powell algorithm. The shapes of indivi-
dual emission bands were approximated by a log-normal func-
2
priate aldehydes in the presence of sodium methoxide in
DMF for 1–6 h. The mixture was diluted with ethanol, then,
subsequently, 20 mol excess of sodium methoxide and 15
mol excess of 30% hydrogen peroxide were added. The mixture
was refluxed for less than 1 min, cooled to room temperature
and poured into water. After neutralization with diluted
HCl, the resultant precipitate was filtered. The target chro-
mones 3m, 3j, 4m, 4 and 4j were purified by recrystallization
or column chromatography. The obtained 3HCs were pure
1
9
tion, which accounts for the asymmetry of the spectral
bands.
1
according to thin-layer chromatography and H NMR spectra.
Results
0
4
-Diethylamino-3-hydroxy-7-methoxyflavone (3m). Prepared
from 2-hydroxy-4-methoxyacetophenone and 4-diethylamino-
benzaldehyde. Crystallized from ethanol; yield 36%; mp
Absorption and fluorescence properties of the new 3-hydroxy-
chromones were studied in six solvents of different polarity:
hexane, toluene, ethyl acetate, chloroform, acetonitrile and
ethanol. We found that all the new compounds exhibit two
solvent-dependent bands in the fluorescence spectra, which is a
characteristic feature of 3-hydroxychromones. Comparative
studies of the 3HC derivatives reveal the key relationships
between the chromophore structure and its spectral properties.
ꢀ
ꢁ1 ꢁ1 1
cm ; H
1
72 C. UV (ethanol) e(406 nm) 42 000 l mol
NMR (200 MHz, CDCl ) d 1.22 (6H, t, J 7.0 Hz), 3.44 (4H,
3
q, J 7.0 Hz), 3.92 (3H, s), 6.76 (2H, d, J 9.1 Hz), 6.87 (1H,
s), 6.94 (1H, s), 6.96 (1H, d, J 6.6 Hz), 8.10 (1H, d, J 6.6
1
,5
+
Hz), 8.12 (2H, d, J 9.1 Hz); EI-MS m/z 339.1 (M ), 324, 295.
3-Hydroxy-2-(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]qui-
nolin-9-yl)chromone (3j). Prepared from 2-hydroxyacetophe-
Effects of chromophore extension
none and 9-formyljulolidine. Yield 35%; purified on silica gel
ꢀ
column chromatography (dichloromethane); mp 207 C. UV
cm
2-(Benzo[d]furan-2-yl)-3-hydroxychromones (benzofurylchro-
mones, BF-3HCs) 2, 4, 4j and 4m exhibit absorption spectra
shifted to longer wavelengths in comparison with their 2-
phenyl analogs (flavones, 3HFs) 1, 3, 3j and 3m, respectively
(Fig. 1).
ꢁ
1
ꢁ1
1
(
MHz, CDCl
ethanol) e(429 nm) 41 000 l mol
;
H NMR (200
3
) d 2.00 (4H, m), 2.84 (4H, t, J 6.3 Hz), 3.28
(
(
4H, t, J 5.7 Hz), 6.88 (1H, s), 7.33–7.41 (1H, m), 7.54–7.69
2H, m), 7.75 (2H, S), 8.21 (1H, dd, J 8.1, 1.2 Hz); EI-MS
m/z 333 (M ), 304.
+
In the fluorescence spectra the extension of conjugation by
one furan heterocycle also results in red shifts of the N* and
T* bands. These shifts are much stronger for dialkylamino-
substituted chromones in comparison to their non-substituted
analogs (Table 1). Thus, in chloroform, compound 4 exhibits
N* and T* bands shifted to the red, with respect to 3, by
2-(6-Diethylaminobenzo[b]furan-2-yl)-3-hydroxychromone (4).
Prepared from 2-hydroxyacetophenone and aldehyde 5.
ꢀ
Crystallized from acetonitrile; yield 25% mp 226 C. UV (etha-
ꢁ1
ꢁ1
1
; H NMR (200 MHz,
nol) e(444 nm) 36 000 l mol cm
ꢁ
1
CDCl ) d 1.23 (6H, t, J 7.1 Hz), 3.45 (4H, q, J 7.1 Hz), 6.76
2440 and 1750 cm , respectively, while for 2, with respect to
ꢁ
3
1
1, the corresponding shifts are only 660 and 710 cm .
(
1H, dd, J 8.8, 2.2 Hz), 6.87 (1H, d, J 2.2 Hz), 7.38–7.46
1
338
New J. Chem., 2003, 27, 1336–1343

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opposite sides of the 3HC molecule. Chromones 3 and 4 with
an introduced diethylamino group, show absorption spectra
strongly shifted to longer wavelengths with respect to their
non-substituted analogs 1 and 2 (Fig. 1). Introduction of a
0
diethylamino group at the 6 position of chromone 2 results
in a red shift of the absorption maximum by 78–88 nm
ꢁ
1
0
(
position of 3-hydroxyflavone 1, this shift is somewhat smaller,
6
4800–5320 cm ). When this group is introduced at the 4
ꢁ1
1–69 nm (4470–4860 cm ). Moreover, chromones 3j and 4j
with stronger p-electron donation from the nitrogen fixed in
the julolidine cycle show an additional red shift of the absorp-
tion maxima with respect to the corresponding diethylamino
derivatives 3 and 4 (Table 1). In contrast, chromones 3m
and4m with the introduced electron-donor methoxy group
at the 7 position show absorption spectra shifted to the blue
(
Fig. 1).
Similarly to the shifts in absorption spectra, the shifts of
Fig. 1 Normalized absorption spectra of the studied 3-hydroxychro-
mones in toluene.
both the N* and T* bands in emission depend strongly on
the nature and position of the electron-donor substituents in
3
2
-hydroxychromones. For both chromone series, 1-3-3j and
-4-4j, an increase of electron donation on the 2-aryl side
The intensity ratio of these bands, IN*/IT* , which is an
important characteristics of the ESIPT reaction, is also affected
dramatically by the nature of the 2-aryl substituent. In all the
studied solvents, BF-3HCs 2, 4, 4j and 4m exhibit much higher
results in sequential red shifts of the emission bands (Table
). It is stronger in the case of the N* band compared to the
1
T* band. Introduction of the electron-donor methoxy group
on the opposite side of the 3-hydroxychromone moiety at the
7
IN*/IT* ratios than the corresponding flavones 1, 3, 3j and 3m
Table 1), so that the ESIPT reaction is depressed for the for-
(
position results for flavone 3m in a very uncommon observa-
mer. This effect is especially strong for dialkylamino-substi-
tuted chromones. Whereas for 2, IN*/IT* is less than 2 times
higher than that of 1, it becomes 8–15 times higher for 4, 4j
and 4m in comparison with corresponding dyes 3, 3j and 3m.
For BF-3HCs we observe also a significantly increased fluor-
escence quantum yield in most of the studied solvents. Usually
it is 2–5 times higher than that of the corresponding flavones
tion: shifts of the N* and T* bands in opposite directions
(Fig. 2). This increases the separation between these bands
by 12–15 nm (450–630 cm ). In the case of 4m, the introduced
ꢁ1
methoxy group affects mainly the N* band, shifting it to the
blue by 8–16 nm (300–490 cm ), while the T* band does
ꢁ1
not undergo any considerable and systematic shifts in the
different studied solvents (Table 1).
(
Table 1).
A particularly strong effect of the electron-donor groups is
observed on the fluorescence intensity ratio, IN*/IT* . Chro-
mones 3 and 4, compared to the corresponding parent com-
pounds 1 and 2, show dramatically increased IN*/IT* ratios,
by 84 and 210-fold, respectively (Table 1). It can be noted that
in the case of BF-3HC 4, the effect of introduction of the
Effects of different electron donors
The newly synthesized dyes allow comparison of spectroscopic
effects produced by introduction of electron-donor groups on
a
Table 1 Spectroscopic properties of the studied 3-hydroxychromones
ꢂ
ꢂ
ꢂ
ꢂ
/nm
abs
max
N
max
T
max
abs
max
N
max
T
Solvent
Hexane
E
T
(30) 3HF
l
/nm
l
/nm
l
/nm
I
N*/IT* % j BF-3HC
l
/nm
l
/nm
l
max
IN*/IT* % j
31.0
33.1
38.1
39.1
45.6
53.7
3m
3
394
403
414
343
400
408
422
340
394
401
415
344
407
413
430
339
397
404
420
343
406
412
429
—
562
553
564
527
572
566
582
529
573
570
586
521
566
560
579
525
577
571
598
532
568
—
—
28
14
13
29
30
14
17
4m
4
431
439
451
364
439
446
457
358
429
436
446
365
445
453
468
358
431
437
450
366
438
444
458
450
457
480
406
498
508
532
410
524
537
567
415
535
545
578
418
566
582
618
425
590
600
626
588
583
602
547
610
613
634
545
618
623
655
541
616
621
—
0.053
0.078
0.247
0.008
0.242
0.550
1.75
29
20
23
41
30
26
44
17
30
26
35
36
46
46
38
17
34
25
425
440
—
0.010
0.021
—
3
1
j
4j
2
b
Toluene
EtOAc
3m
3
447
456
480
—
0.018
0.044
0.174
—
4m
4
3
1
j
4j
2
3.6
11
5.0
11
0.029
0.914
3m
3
464
475
507
404
475
481
511
394
498
509
541
402
515
523
552
0.062
0.253
1.01
0.008
0.252
0.669
1.71
0.028
0.350
1.30
—
4m
4
c
c
c
2.67
c
3
1
j
4j
2
10.8
0.020
1.58
19
24
19
50
CHCl
3
3m
3
4m
4
c
c
c
4.2
—
3
1
j
4j
2
5.1
542
643
—
0.040
c
8.7
—
CH
3
CN
3m
3
9.0 4m
9.0
32
2.9
50
4
c
c
3
j
4j
2
—
—
5.0
1
0.270
c
4.56
543
—
0.220
—
—
15
22
EtOH
3m
3
4m
4
d
—
—
52
12
—
—
9.0
2.0
3
j
—
4j
—
a
abs
max
Nꢂ
Tꢂ
l
is the position of the absorption maximum, l
and l
are the positions of the fluorescence maxima of the N* and T* forms, respectively,
b
max
max
c
j is the fluorescence quantum yield. Data on j for 1 and 2 are from Ref. 20. Ref. 5c. The value was evaluated from the results of the deconvolu-
tion. Ref. 5b.
d
New J. Chem., 2003, 27, 1336–1343
1339

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Fig. 3 Positions of N* band maxima of the studied chromones as a
function of the solvent polarity index E (30). The solvents are toluene,
ethyl acetate, chloroform, acetonitrile, ethanol and methanol.
T
The common feature of all studied 3-hydroxychromones is
the high sensitivity of the intensity ratio of the emission
bands, IN*/IT* , to solvent polarity (Table 1). The logarithm
of this ratio shows an almost linear increase with solvent
polarity for all studied 3HCs with relatively good correlation
coefficients of the linear fits (Table 2). For dialkylamino-sub-
stituted 3HCs this is demonstrated in Fig. 4. A considerable
and systematic deviation from this linearity is observed only
for the dialkylamino-substituted 3HF dyes in chloroform,
the solvent with a tendency for specific H-bond interactions,
resulting in an increase of its apparent polarity. Interestingly,
this deviation is the strongest for 3m and negligible for 3j
Fig. 2 Fluorescence spectra of the studied flavones 3, 3j and 3m in
chloroform (A) and benzofurylchromones 4, 4j and 4m in toluene
(B). Excitation wavelength of 420 nm.
diethylamino group is stronger than that for flavone 3. Fixing
the amino group in chromones 3j and 4j results in additional
growth of IN*/IT* , up to 4-fold (Table 1), which can be due
to a further increase of the electron donation. The opposite
effect is observed on introduction of a methoxy group on the
opposite side of the chromophore, at the 7 position in chro-
mones 3m and 4m; the IN*/IT* ratio decreases by 4 and 3 times,
respectively (Fig. 2).
(
Fig. 4).
It is remarkable that the level of sensitivity to the polarity
of the IN*/IT* ratio is also determined by the chromone
structure. The slopes of the linear fits for BF-3HCs 4, 4j
and 4m are steeper than those for the corresponding flavones
3, 3j and 3m, representing a higher sensitivity to solvent
polarity (Table 2). Furthermore, the electron-donor substitu-
ents can modulate this sensitivity. For both chromone series,
The presence of an electron-donor group exerts a signifi-
cant influence on fluorescence quantum yield. However, the
magnitude and direction of the effect depend strongly on the
nature of the solvent. The general trend observed is higher
quantum yields of 3m and 4m in most of the studied solvents
1-3-3j and 2-4-4j, an increase of electron donation at the 2-
aryl side increases the sensitivity of IN*/IT* to solvent polar-
ity (Table 2). Meanwhile, for methoxy-substituted 3m and
4
3
m, it is lower in comparison to the corresponding parents
and 4.
In both series of compounds the fluorescence quantum yield
(
Table 1).
j exhibits broad variations. For the 3HF derivatives 3 and 3m,
the j values follow a similar trend, being low in acetonitrile
and high in ethanol (Table 1). BF-3HCs 2 and 4 exhibit rela-
tively high j values (18–44%) in all studied solvents. Mean-
time, 4j, with the strongest electron-donor substituent,
demonstrates in highly polar solvents a dramatic j decrease,
from 44% in toluene down to 2% in ethanol (Table 1). A simi-
lar, though less dramatic j drop is observed for compound 4.
It is interesting to note that the introduction of the methoxy
group always increases the j values and makes them substan-
tially less solvent-dependent for both flavone 3m and BF-3HC
4m (Table 1).
Solvent effects
The correlation of spectroscopic effects with the solvent polar-
ity was established based on the most popular empirical polar-
T
ity scale, E (30), which is based on the solvatochromic shifts of
1
4b
betaine dye. According to our data, the absorption spectra
of all studied chromones are almost independent of solvent
polarity, and only in the case of the BF-3HCs 4, 4j and
4m was a small positive solvatochromy detected (Table 1).
This is in line with the recent detailed solvatochromic data
on 3, showing that its absorption maximum is almost polarity
insensitive.
1
3
In contrast, fluorescence spectra show a clear dependence on
solvent polarity and this property is strongly dependent on the
nature of the 2-aryl group. For the parent 3-hydroxyflavone 1,
both emission bands do not reveal any solvent-dependent
shifts (Fig. 3, Table 1). For non-substituted BF-3HC 2, the
N* band is already solvent polarity dependent, while its T*
band is not. The introduction of a diethylamino group results
in a dramatic increase of the solvent dependence of the N*
band in the compounds of both the flavone and BF-3HC
series. This effect is much stronger for the N* band, which
demonstrates a gradual shift to the red with an increase in
polarity (Fig. 3). The T* band positive solvatofluorochromy
is considerable only for the BF-3HCs (Table 1).
a
T
Table 2 Data obtained from linear fits of ln(IN*/IT*) vs. E (30)
3
HF Slope
0.260
O
E(30)
r
BF-3HC Slope
O
E(30)
r
1
3
3
59.0
48.4
43.0
37.9
0.996
2
0.170
0.400
0.475
0.525
61.4
38.0
35.9
33.0
0.972
0.994
0.991
0.992
m
0.250
0.341
0.524
0.885 4m
0.949
0.992 4j
4
3j
a
T
The slope is that of the corresponding linear fit. OE(30) is the E (30)
index of a virtual solvent, in which the two emission bands are equal in
intensity. r is the correlation factor of the linear fit.
1
340
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be explained by the decrease in p-electron-acceptor ability of
the chromone part. Thus, the effect of electron donation
from the 7 position on the absorption and emission spectra
in 3HC chromophore is the opposite of that at the 2 position.
Importantly, the electron-donor substituents at both sides of
the chromophore affect more strongly the position of the N*
emission band as compared to that of the T* band. This means
that increase or decrease of the charge transfer character of the
excited state due to substituents stabilizes or destabilizes,
respectively, the N* state with respect to the T* state, which
should substantially affect the ESIPT reaction.
We can analyze the factors that make 3HCs strongly solva-
tochromic. The small solvent dependence of the emission max-
ima observed for non-substituted 3HCs 1 and 2 shows that
their dipole moments do not increase significantly on electro-
2
1
nic excitation. The N* band becomes strongly solvent depen-
dent with introduction of dialkylamino substituents, which
increases the charge-transfer character and, correspondingly,
the dipole moment of the N* state. The latter is supported
by the recent data on direct measurements of dipole moments
0
22
of 4 -dialkylamino-substituted 3HF. Our results show also
that the charge separation of the T* state is small and it is
not influenced significantly by the provided substitutions.
The ESIPT modulation
1
,23
0
and its 4 -diethyl-
Fig. 4 Dependence of the spectral parameter ln(IN*/IT*) of the
studied 3-hydroxychromones on the solvent polarity index E (30).
A) flavones 3 (S), 3j (˘) and 3m (G); (B) benzofurylchromones 4
The ESIPT rates in both parent 3HF
5
T
amino-substituted analog 3 are much higher than the rates of
emission. This allows us to assume that in 3HC dyes the major
part of the emission proceeds from both N* and T* states
(
(
S), 4j (˘) and 4m (G). The solvents are hexane, toluene, ethyl acetate,
chloroform and acetonitrile. The straight lines correspond to linear fits
of the data. Their crossing with the horizontal dashed line that corre-
sponds to ln(IN*/IT*) ¼ 0 determines OE(30) as the point of equality in
the intensity of the N* and T* bands on the polarity scale.
5a
when they reach dynamic equilibrium. Based on this consi-
deration, the intensity ratio of the two emission bands can be
correlated with the relative populations of the states at equili-
brium, which is a function of the relative energies of the corre-
sponding states. This is in accordance with the recent
spectroscopic data on dyes 3 and 4.
Discussion
13,24
As it follows from
the data on spectral shifts, the increase in the size and elec-
tron-donor ability of the 2-aryl p-electron system in the 3HC
chromophore results in selective stabilization of the N* state.
This increases the population of this state with respect to the
T* state and, therefore, increases the relative intensity of the
N* band in emission. Indeed, the red shifts of the N* band
observed as a result of these modifications are always accom-
The spectral shifts
We observe that the positions of the absorption bands depend
strongly on the length of the chromophore and on the excited
state electron-donor and -acceptor ability of the groups. In
3HC derivatives the electron-acceptor group is 4-carbonyl, while
the role of the electron donor can be played by the 1-oxygen
heteroatom or 2-aryl group. Participation of the 2-aryl
group in electronic excitation can be observed by a comparison
of the two studied series of compounds, 3-hydroxyflavones
panied by an increase in the fluorescence intensity ratio, IN*/
T* (Table 1). Meanwhile, the electron-donor methoxy group
I
at the 7 position, which destabilizes the N* state, shifting its
band to the blue for both 3m and 4m, produces the opposite
effect—a decrease of the IN*/IT* ratio (Fig. 3). Thus, the
ESIPT reaction in 3HC system can be modulated in opposite
directions by p-electron donation from the two opposite sides
of the chromophore.
With the increase of solvent polarity, the same correlation is
observed: the increase in the IN*/IT* ratio is always accompa-
nied by a red shift of the N* band (Table 1). Thus, the increase
of solvent polarity results in selective dielectric stabilization
of the N* state with respect to the T* state, which increases
its relative population and, therefore, the relative intensity of
the corresponding band.
(
3HFs) and 2-benzofuryl-3-hydroxychromones (BF-3HCs).
Since benzofuryl in comparison to phenyl has a higher p-elec-
tron density and a stronger electron-donor ability, the absorp-
tion and emission spectra of BF-3HCs are shifted to the red.
The other factor that may contribute to the observed red shift
is the smaller size of the five-membered furan ring in compar-
ison to the six-membered benzene ring, which can allow a more
planar orientation with respect to the 3HC system resulting in
a better conjugation. The introduction of an electron-donor
diethylamino group in the 2-aryl group of 3HC should increase
the charge transfer character of the excited state, which
explains the strong red shift in absorption and emission spectra
of 3 and 4 with respect to 1 and 2. Fixing the amino group into
the julolidine cycle results in an additional red shift in absorp-
tion and emission spectra. This effect is opposite to that
expected from twisted intramolecular charge transfer (TICT),
which involves rotation of the dialkylamino group. The latter
2
0
The observed effects of chemical substituents and solvent
polarity reveal the essential common features, which suggests
that ESIPT in both of these cases is controlled by the same
mechanism—a change in the relative energies of the N* and
T* states.
0
was proposed previously for 4 -dimethylamino-3-hydroxyfla-
5
c
vone, an analog of 3. Our results show that TICT probably
does not take place in chromones 3 and 4 and the most impor-
tant factor determining the spectroscopic properties of 3j and
The quantum yields
The fluorescence quantum yield j of 3-hydroxychromones
is known to vary significantly from solvent to solvent.
1
,5c,20
4
j is the increase of electron donation.
The blue shift of absorption and N* emission bands
observed with the introduction of the 7-methoxy group can
This is especially characteristic for 3-hydroxyflavones, the j
values of which do not follow a simple dependence on solvent
New J. Chem., 2003, 27, 1336–1343
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range is dramatically shifted to lower polarities. Introduction
of the diethylamino group and its fixation results in a strong
sequential decrease of OE(30) . In contrast, the introduction
of the 7-methoxy group provides a change in the opposite
direction—the point of band equality on the polarity scale is
shifted in the direction of higher polarities.
polarity but are mostly influenced by specific solute-solvent
interactions.
1
3,20
In contrast, BF-3HCs show relatively higher
j values in most of the tested solvents and their solvent-depen-
1
1,14a
dent variations are smaller.
For the BF-3HCs presented
in this work, the j values are also high in most of the studied
solvents (Table 1). The dramatic decrease in quantum yield of
the 3HF 1 in hydrogen-bond-acceptor solvents has recently
been proposed to be a result of hydrogen bonding of the
Thus, we demonstrate that the ratiometric sensing effect of
chromones presented in this work, due to the variation of sub-
stituents, may already cover the whole polarity range of known
solvents (Fig. 5). For non-substituted 1 and 2 the range of
optimal sensitivity is between ethanol and water. For flavone
3 it is in the range of medium polarity, between ethyl acetate
and acetonitrile. For chromone 4, it is already in the range
of low polar solvents, between toluene and ethyl acetate. An
additional shift towards lower polarity can be achieved by fixa-
tion of the amino group, while the opposite can be achieved
by introduction of the 7-methoxy group (Table 2). Since the
polarity scale reflects a number of universal and specific
3
-hydroxy group with solvent molecules, which can produce
additional distortion of the planarity of 2-phenyl with respect
2
0
to the 3HC moiety. Similar j variations are observed for
flavones 3 and 3m, with the exception of 3j, which possesses the
strongest electron-donor group in the 4 position. In the case
0
of BF-3HCs, these variations are much less pronounced, which
is probably due to the smaller size of the furan heterocycle,
which does not allow a significant distortion of the planarity
by the solvent.
The pronounced j decrease for 3j, 4 and 4j in the highly
polar solvent ethanol may be connected with the strong elec-
tronic asymmetry in their excited states and their strong solva-
1
4b
solute-solvent interactions, these results point to the possibi-
lity of adapting the ratiometric response to a particular specific
range of interactions that may be required for the design of
highly specified sensors.
2
5
tion with the formation of electron traps. This effect is most
clearly observed in the case of 4j, which possesses the strongest
electron donor and the most extended conjugation. Therefore,
the substituent that restores the p-electronic symmetry can
provide an increase in the j value, which is exactly what is
observed with the introduction of the 7-methoxy group.
Conclusions
On the basis of 3-hydroxychromone we suggest a new genera-
tion of two-band fluorescence dyes that operate by the ESIPT
mechanism, providing high two-band sensitivity to microenvir-
onment polarity. The photophysical and sensing properties of
the dyes can be finely tuned by introduction of the appropriate
groups at the 2 and 7 positions of 3-hydroxychromone. (a) The
absorption and fluorescence spectra can be shifted to longer
wavelengths and the IN*/IT* ratio can be increased by substitu-
tion of 2-phenyl for 2-(2-benzo[b]furanyl) and by introduction
of an electron donor in the 2-aryl group. The change of spec-
troscopic properties in the opposite direction can be provided
by introduction of an electron-donor methoxy group at the
Solvent polarity sensing
Polarity is a simple integrating characteristic of weak non-
covalent interactions in the condensed phase, which may be
an important characteristic of unknown media on the molecu-
lar dimension scale. Before addressing the problem of polarity
sensing in these media,
often tested for their sensitivity to solvent polarity. The linear
1
4b
prospective fluorescent sensors are
dependence of ln(IN*/IT*) on the solvent polarity index E (30),
T
previously reported for BF-3HC 4,
1
4a
and demonstrated pre-
sently for all the studied 3HCs, is a very important property
of these dyes, making them prospective polarity sensors with
the possibility to perform quantitative measurements. In the
analysis presented below we use the solvent polarity scaling
as the tool for evaluating the sensitivity of our new dyes to
external perturbations and for deriving the general rules of
their response.
T
The slope of the linear fit of ln(IN*/IT*) vs. E (30) may serve
as an important parameter to characterize the sensitivity of the
two-band ratiometric response to solvent polarity. The results
presented in Fig. 4 show that both the substitution of 2-phenyl
with 2-benzofuryl and the introduction of electron donors at
2
-aryl makes the 3HC chromophore more sensitive to polarity.
This probably reflects a stronger increase in dipole moment of
the N* state compared to the T* state. In this respect, it is
remarkable that the 7-methoxy group produces the opposite
effect. Thus, the variation of the 2-aryl group and the introduc-
tion of electron-donor substituents on different sides of the
3HC chromophore can provide a fine modulation of dye
sensitivity to solvent polarity.
In order to measure the intensity ratio of the two bands most
easily and with high precision, these bands have to be of com-
parable intensity. It is important therefore to specify experi-
mental conditions in which the two N* and T* bands are
observed in equal intensities. Regarding polarity sensing, we
introduce a new parameter, OE(30) , the E (30) polarity index
T
of such a virtual solvent, in which the two emission bands
are equal in intensity. Its values can be obtained by a linear
interpolation of the functions presented in Fig. 4 at ln(IN*
/
Fig. 5 The two-band fluorescence ratiometric responses of chro-
mones 2, 3 and 4 adapted for three different ranges of solvent polarity:
polar, medium-polar and low-polar, respectively. The spectrum that
refers to water was recorded in the presence of 10% methanol.
Excitation wavelength of 350 nm for 2 and 420 nm for 3 and 4.
IT*) ¼ 0. The data of Table 2 show that this parameter
depends strongly on the chromone structure. The BF-3HC
dyes 4, 4j and 4m exhibit much lower values of OE(30) than
the corresponding flavones 3, 3j and 3m. So, their sensing
1
342
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Biophys. Acta, 1998, 1369, 119–130; (b) S. M. Dennison,
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7
position. The fluorescence quantum yield in 3-hydroxy-
chromones can be increased by substitution of 2-phenyl for
-(2-benzo[b]furanyl) and by introduction of a 7-methoxy
9
2
5
5, 1127–32; (c) G. Duportail, A. S. Klymchenko, Y. Mely and
group. (b) The two-band sensitivity of 3-hydroxychromones to
solvent polarity can be increased and its optimal polarity range
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donor in the 2-aryl group. The change of sensing properties in
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derivative can be chosen with optimal color-change sensitivity.
These new design possibilities make the studied family of
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