Absorption and fluorescent studies of 3-hydroxychromones

Article abstract of DOI:10.1007/s10895-015-1623-0

The synthesis and spectral studies of variously substituted 3-hydroxychromones have been carried out. A key relationship between the structural motif of synthesized 3-hydroxychromones (3-HCs) and their fluorescent properties was found. The chromones substituted with electron-donating group at 4′-position expressed the red shift of the N and T band and also exhibited the increased fluorescent intensity ratio while the chromones with electron-withdrawing group showed the blue shift of the N and T band. Therefore, these 3-HCs may behave as the possible fluorescent probes.

Full text of DOI:10.1007/s10895-015-1623-0

J Fluoresc (2015) 25:1159–1163
DOI 10.1007/s10895-015-1623-0
SHORT COMMUNICATION
Absorption and Fluorescent Studies of 3-Hydroxychromones
Radhika Khanna1 & Ramesh Kumar1 & Aarti Dalal1 & Ramesh C. Kamboj1
Received: 8 May 2015 /Accepted: 5 July 2015 /Published online: 16 July 2015
#
Springer Science+Business Media New York 2015
Abstract The synthesis and spectral studies of variously
substituted 3-hydroxychromones have been carried out. A
key relationship between the structural motif of synthe-
sized 3-hydroxychromones (3-HCs) and their fluorescent
properties was found. The chromones substituted with
electron-donating group at 4′-position expressed the red
shift of the N* and T* band and also exhibited the increased
fluorescent intensity ratio while the chromones with
electron-withdrawing group showed the blue shift of the
N* and T* band. Therefore, these 3-HCs may behave as
the possible fluorescent probes.
are strongly governed by the structure of the 3-
hydroxychromone chromophore, which assign these a wide
variety of high technological utility. Many 3-HCs have been
synthesized and analysed for the development of new fluores-
cent probes for solvent polarity [2–4], ionic binding [5] and
electric fields [6, 7] with applications in the field of polymers
[8], reverse micelles [9, 10], host-guest complexes [11], lipid
vesicles [7, 12–15], cellular membranes [16] and proteins
[17–21]. These molecules have also been found to show fas-
cinating photochemistry furnishing the oxirane scaffold [22].
The 3-HCs have also been used as important substrates for the
synthesis [23–27] of many photo-labile molecules which
yielded many polycyclic exotic molecules on
photoirradiation.
.
.
Keywords 3-Hydroxychromones Fluorescence
.
Absorption ESIPT
Keeping in view the applications and significant photo-
chemistry elicited by these 3-HCs, in the present communica-
tion, we have made an attempt to synthesise, determine and
analyse their absorption and fluorescent properties. And also
the effect of different substituents located at 4′-position of 3-
HCs and intensity ratio of N* and T* emission bands was
assessed. Herein, we have demonstrated that, there is possibil-
ity of driving the spectroscopic properties in the desired direc-
tion by providing proper substitutions in the chromophoric
system.
Introduction
The 3-hydroxychromones (3-HCs) derivatives have attracted
the interest of researchers in the recent years as they exhibit
dual emission due to the excited state intramolecular proton
transfer (ESIPT) that results in two well-separated fluores-
cence bands belonging to excited state normal (N*) and tau-
tomer (T*) forms [1] (Fig. 1). The spectroscopic properties of
these fluorescence bands and the interplay of their intensities
Methods and Materials
General
* Ramesh C. Kamboj
All the chemicals used in the study were procured from the
market and were of highest quality available. The solvents
required were purified by the standard procedures.
rckamboj@hotmail.com
1
Melting points were determined in open capillaries and are
thus uncorrected. IR spectra were recorded on a Perkin-Elmer
Department of Chemistry, Kurukshetra University,
Kurukshetra 136119, Haryana, India

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J Fluoresc (2015) 25:1159–1163
Energy
Table 1 Melting point data of 3-HCs
O
O
R
+
O
R
+
3-HC No.
R
R’
R^
R^’
M. pt.
T*
ESIPT
N*
O-
O
H
O-
1
H
H
H
H
170 °C
192 °C
206 °C
228 °C
182 °C
175 °C
221 °C
184 °C
140 °C
182 °C
169 °C
232 °C
192 °C
219 °C
210 °C
H
2
H
Cl
H
H
h
h
abs
abs
3
H
CH3
OCH3
OCH3
H
H
H
4
H
H
H
5
H
OCH3
H
OCH3
H
O
R
O
O
R
6
CH3
CH3
CH3
CH3
CH3
Cl
T
N
O
H
O-
7
Cl
H
H
O
H
+
8
CH3
OCH3
OCH3
H
H
H
9
H
H
Tautomer
Normal
10
11
12
13
14
15
OCH3
H
OCH3
H
Fig. 1 The dual fluorescent behavior of 3-hydroxychromones
is represented in an energy diagram. Upon excitation, the normal excited
species (N*) undergoes an excited intramolecular proton transfer (ESIPT)
to give the excited tautomer (T*)
Cl
Cl
H
H
Cl
CH3
OCH3
OCH3
H
H
Cl
H
H
Cl
OCH3
OCH3
IR spectrophotometer. H and 13C spectra were recorded in
1
CDCl3/DMSO on a 300 MHz/400 MHz and 75.4 MHz/
100.6 MHz Bruker spectrometer using TMS as an internal
standard.
yield 3-HCs as light yellow solid. The melting points
(Table 1), IR and 1H-NMR spectral data of these 3-HCs over-
lapped with the data reported in the literature [29].
Synthesis of 3-HCs (Scheme 1)
Absorption and Fluorescence Spectra of 3-HCs
The 3-HCs synthesized above were purified to the highest
level of purity and then crystallized. Absorption and fluo-
rescence spectra (Figs. 2, 3 and 4) of these purified and
crystallized chromones were recorded on T90+ UV/VIS
Spectrophotometer and Shimadzu RF-5301Pc
Spectrofluorophotometer. The solutions of 3-HCs were
made in methanol of spectroscopic grade. In all cases
the concentrations of the solutions were made such that
absorption was close to 0.1 and these concentrations were
between 3 and 5 μM. The solutions of these 3-HCs were
also prepared in acetonitrile, the polar aprotic solvent and
ethanol separately to observe solvatochromism.
A General Procedure for the Synthesis of Chalcone A
A solution of substitued 2-hydroxyacetophenone (0.05 mol)
and substituted benzaldehyde (0.055 mol) in absolute ethanol
and sodium hydroxide (0.1 mol) was stirred for 8-9 h. The
dark red mixture was poured on ice-HCl to obtain A as yellow
solid which was crystallized from EtOH.
A General Procedure for the Synthesis of 3-HCs
from Chalcone A
To a well stirred suspension of A (0.01 mol) in MeOH was
added aq. KOH (20 %). This mixture was cooled to 0 °C. To
this dark red solution was added H2O2 (30 %) drop-wise under
Algar-Flynn-Oyamada(AFO) reaction conditions [28] till the
colour changed to yellow and the stirring was continued for
3 h. The reaction mixture was neutralized with ice-HCl to give
light yellow precipitates, crystallized (chloroform-ethanol) to
Results and Discussion
The fluorescent spectra of these 3-HCs in methanol, a polar
protic solvent exhibited two bands: one situated between 400
R"
R"
Scheme 1 Synthesis of
3-hydroxychromones. Reagents
and conditions: (a) NaOH/EtOH,
stirring overnight; (b) KOH,
R'
R'
R"
R'
CHO
OH
O
O
OH
b
R"'
a
+
R"'
CH3
R
R
OH
R
H2O2 (30 %), 0 °C, Stirring (2 h)
R"'
O
O
3-HCs (1-15)
A

J Fluoresc (2015) 25:1159–1163
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1200
1000
800
600
400
200
0
1200
1000
800
600
400
200
0
1
2
3
4
5
11
12
13
14
15
300
400
500
600
300 350 400 450 500 550 600
Fig. 2 Fluorescence spectra of 3-hydroxychromones 1, 2, 3, 4 and 5 (in
methanol)
Fig. 4 Fluorescence spectra of 3-hydroxychromones 11, 12, 13, 14 and
15 (in methanol)
and 473 nm and another between 476 and 576 nm. A look of
these fluorescent spectra revealed that there is definitely a
close relationship between the 3-hydroxychromone structural
motifs and their spectral behaviors (Table 2). These results of
the fluorescence spectra indicate that the tethering of electron-
donating group at 4′-position of 3-hydroxychromone results in
the red-shifts of the N* and T* bands. Simultaneously, this also
increases the intensity ratio of N* and T* bands (Fig. 5). While
the electron withdrawing group reflected the blue-shifts of the
N* and T* bands simultaneously decreasing their intensity
ratio. Such a strong effect of the donor group on the fluores-
cence profile of 3-HCs may provide a prospect to design new
sensitive fluorescent sensors of ions. No appreciable change in
these absorption and fluorescent properties were observed by
replacing methanol with ethanol as another polar protic sol-
vent. The absorption and fluorescent spectra of all these
chromones were also determined in acetonitrile (polar aprotic
solvent). It was found that the fluorescent intensity ratio of
these 3-HCs was significantly larger in acetonitrile, polar
aprotic solvent than that in methanol (polar protic solvent).
This is because of the H-bonding interaction of the protic
solvents with the carbonyl and –OH functionalities of the
chromones which inhibit the ESIPT reactions of 3-HCs. This
kind of interaction is absent in aprotic solvents.
These results on fluorescent behavior of the
chromones can find an application as highly promising
candidates in environment-sensitive probes vis-a-vis
possible labels for biological applications in body fluids,
cells and tissues.
Table 2 Spectrophotometric data of the studied 3-hydroxychromones
3HCs
λ
abs max/ nm
λN*max/ nm
λT*max/ nm
IN*/IT*
1
340
326
346
330
354
346
316
348
354
358
348
348
352
360
360
403
395
426
463
–
528
517
570
573
576
548
476
566
572
580
538
527
540
544
548
0.442
0.389
0.945
0.962
–
2
3
1200
1000
4
5
6
413
415
417
473
–
0.522
0.474
0.913
0.953
–
7
800
6
8
7
9
600
8
10
11
12
13
14
15
423
420
427
444
–
0.369
0.341
0.417
0.982
–
9
400
10
200
0
where Bλ abs max^ is the position of the absorption maximum; BλN*
300 350 400 450 500 550 600
max
and λT* max^ are the positions of the fluorescence maxima of the N* and
Fig. 3 Fluorescence spectra of 3-hydroxychromones 6, 7, 8, 9 and 10
(in methanol)
T* forms respectively

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Acknowledgments One of the authors (Radhika Khanna) wishes to
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