The complexation of flavone derivatives with alkali and alkaline earth metal cations studied by spectroscopic methods

Article abstract of DOI:10.1039/a904825a

A new kind of fluoroionophore, typified by flavone quasi-crown ether [4'-(dimethylamino)-3,6-(tetraethylene glycol)flavone] and flavone lariat- crown ether, were synthesized and characterized. The photophysical changes upon complexation with alkali and al

Full text of DOI:10.1039/a904825a

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The complexation of Ñavone derivatives with alkali and alkaline earth
metal cations studied by spectroscopic methods
Huaping Li, Hongzhi Xie, Penfei Wang and Shikang Wu*
Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing, 100101, China
Received (in Montpellier, France) 16th June 1999, Accepted 14th December 1999
A new kind of Ñuoroionophore, typiÐed by Ñavone quasi-crown ether [4@-(dimethylamino)-3,6-(tetraethylene
glycol)Ñavone] and Ñavone lariat-crown ether, were synthesized and characterized. The photophysical changes
upon complexation with alkali and alkaline earth metal cations are explained in terms of the enhancement of a
cation-induced intramolecular charge transfer process, which results from the interaction between the bound cation
and the carbonyl group of the Ñavone. The size and shape of the quasi-crown or quasi-cryptand and the charge
density of the bound cation control the magnitude of the photophysical changes. Based on the determination of
stability constants between di†erent cations and the macrocyclic ligand, our results indicate that the selectivity of
Ñavone quasi-crown ether and Ñavone lariat-crown ether is better for the Ca2` cation.
Fluorescence chemosensors is a new research Ðeld that has
grown very rapidly.1h4 Obviously, it is closely related to the
development of supralmolecular chemistry and extensive
studies on the mechanism of Ñuorescence emission.5,6 Struc-
turally, Ñuorescence chemosensors consist of three moieties:7
a receptor, which is responsible for selectively binding a
foreign species; a linker, which is a ““bridgeÏÏ used as a trigger
between receptor and Ñuorophore; and a Ñuorophore, whose
photoproperties are sensitive to the binding between receptor
and metal cations. Crown ethers, spherands and cryptands
have served as template structures for cationic receptors, espe-
cially for the recognition of alkali and alkaline earth
cations.8h10 The elegance of this work includes the designed
architecture of the receptors, with consideration of the size
and shape of the receptor site in comparison with the guest,
which are used to rationalize observed binding data and to
design new systems with unique properties.11 The linker is a
very important moiety in Ñuorescence chemosensor design,
keeping in mind that the strongest perturbation of the photo-
physical properties of the Ñuorophore by the bound cation is
sought. In many cases, the receptor is linked to the Ñuoro-
phore via a linker that belongs to both the Ñuorophore and
receptor. Recently, there has emerged a few Ñuorescence che-
mosensors in which the linker is not connected directly to the
receptor; instead, it is part of the Ñuorophore and, by partici-
pating in the complexation, it acts as a trigger. For example,
Valeur et al.6 reported the direct interaction between the
bound cation and the carbonyl group of coumarin derivatives
with lariat-crown ether. In this case the e†ect of bound cation
on the carbonyl group is similar to the observation of a
solvato-kinetic e†ect of coumarin derivatives in solvents with
di†erent polarity. In this paper, we report the synthesis and
the photophysical properties of a new class of Ñuorescence
chemosensorÈÑavone quasi-crown ether (FQC) and Ñavone
lariat-crown ether (FLC). The designed structure contains
both an electron-donating amino group and an electron-
withdrawing carbonyl group in the Ñavone moiety, so that it
can undergo intramolecular charge transfer upon excitation
by light. We also studied the photophysical changes upon
complexation with alkali or alkaline earth metal cations.
Experimental
Synthesis of FQC and FLC
4º-Dimethylamino-2,5-dihydroxychalcone. A mixture of 2,5-
dihydroacetophenone (3.04 g, 20 mmol, synthesized according
to ref. 12), p-dimethylaminobenzaldehyde (4.47 g, 30 mmol)
and KOH (30 g) in methanol (50 mL) and water (25 mL) solu-
tion was stirred at room temperature for 24 h. After mixing
with 32 g acetic acid and 300 mL water, the obtained clear
solution turned a fresh red and a brown solid precipitated.
The brown solid was recrystallized from 95% ethanol, giving
5.6 g of a brown powder, mp: 206È208 ¡C. 1HNMR (DMSO-
d ) d: 3.0 (s, 6H), 6.7È7.8 (m, 9H), 9.2 (s, 1H), 12.4 (s, 1H). MS
6
(m/z): 283, 147, 134, 121, 103, 77, 53.
4º-Dimethylamino-2,7-dihydroxyÑavone. Hydrogen peroxide
(30%, 20 mL) was added dropwise to a DMSO (50 mL) solu-
tion of 4@-dimethylamino-2,5-dihydroxychalcone (5 g, 17.6
mmol) and KOH (20%, 25 mL) at 0 ¡C. After stirring for 4 h
at room temperature, a blackish green solution was formed.
After mixing the solution with glacial acetic acid and 300 mL
water a solid product precipitated. After Ðltration, washing
and recrystallization from 95% ethanol, a yellow powder was
obtained, mp: 280È282 ¡C. 1HNMR (CD COCD ) d: 3.05 (s,
3
3
6H), 3.4 (m, 2H), 6.8 (d, 2H), 7.2È7.6 (m, 3H), 8.2 (d, 2H). MS
(m/z): 297, 268, 240, 149, 105, 89, 77, 52, 43.
1,11-Dibromo-3,6,9-trioxaundecane.
A solution of tetra-
ethylene glycol (17.3 mL) dissolved in dry ether (30 mL) was
stirred under a dry atmosphere and cooled down to 0 ¡C. Into
the solution, 8.5 mL of phosphorous tribromide was added
dropwise, while keeping the temperature of the solution below
5 ¡C. After completion of the addition, the solution was
allowed to warm up to room temperature and stirred for 4 h.
The reaction mixture was then poured into a crushed ice bath,
the separated organic layer was washed with a 10% sodium
carbonate in water solution. The product was dried over
New J. Chem., 2000, 24, 105È108
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105

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anhydrous potassium carbonate and the solvents evaporated
ligand, while I and A are the absorption and Ñuorescence
intensity of the complex in solution at a given wavelength.
F
to yield a yellow liquid. 1HNMR(CDCl ) d: 3.4 (t, 4H), 3.63 (s,
3
8H), 3.7È3.8 (t, 4H). IR: 2868 (m ), 1180 (m
) cm~1.
The values of A /(A [ A) or I0/(I0 [ I) are plotted against
CH2
CH2vOvCH2
0
0
F
F
[M]~1 and the stability constants K are then given by the
S
ratio of intercept to slope.
4º-(Dimethylamino)-2,7-(3,6,9-trioxaundecane-1,11-dioxy)-
Ñavone (FQC). Anhydrous sodium carbonate (2 g) and 200
mL of DMF were placed in a dry three-neck round-bottom
Ñask with two capillary dropping funnels and a reÑux con-
denser. 1,11-Dibromo-3,6,9-trioxaundecane (2.8 g, 8.8 mmol)
was dissolved in 80 mL of DMF in one funnel. 4@-
Dimethylamino-2,7-dihydroxyÑavone (1.3 g, 8.8 mmol), dis-
solved in 80 mL of DMF, was stored in the other one. The
two solutions were added dropwise into the Ñask at the same
rate at 100 ¡C, then the mixture was reÑuxed overnight. After
completion of the reaction, the color of the mixture was a
blackish green with Ñuorescence. The remaining DMF was
removed by distillation. The remaining mixture was diluted
with water and extracted from chloroform (3 ] 50 mL). A
strongly Ñuorescent component could be separated from the
concentrated extracted solution. Yellow crystals were
obtained after recrystallization, mp: 280È282 ¡C, 1HNMR
It should be remembered that the stability constants
obtained in the ground state are di†erent from those of the
excited state. Comparison of the two constants may provide
us with some additional information.
Instrumentation and materials
The proton NMR spectra were recorded at 300 MHz on a
Varian Germana-300. IR spectra were obtained on a Perkin-
Elmer 938G spectrophotometer using KBr discs. Mass spectra
were recorded on a Finnigan 4021C spectrometer. Absorption
and Ñuorescence spectra were recorded on a Hitachi 330
UV-Vis spectrophotometer and MPF-4 Ñuorescence spectro-
photometer, respectively.
Acetonitrile, purchased from Aldrich Chemical Inc.
(spectroscopic grade), was used as solvent for the determi-
nation of absorption and Ñuorescence spectra without further
puriÐcation, but it was checked by UV-Vis and Ñuorescence
spectrophotometry prior to use. There was no absorption
before 210 nm and no Ñuorescence of impurities was observed.
Alkali metal and alkaline earth metal perchlorates, purchased
from Beijing Chemical Reagent Inc., were of the highest
quality available and vacuum dried over P O prior to use.
(CDCl ) d: 3.1 (s, 6H), 3.1È3.2 (t, 2H), 3.35 (t, 2H), 3.45 (t, 2H),
3
3.7 (t, 2H), 3.8 (t, 2H), 4.2 (t, 2H), 4.5 (t, 2H), 6.8 (d, 2H), 7.2 (d,
1H), 7.4 (d, 1H), 7.9 (d, 1H), 8.1 (d, 2H). MS (m/z): 455, 412,
323, 280, 207, 162, 148, 132, 91, 77, 45.
3-Hydroxy-6-methyl-4º-N,N-dimethylaminoÑavone
(A).
2
5
Hydrogen peroxide (30%, 10 mL) was added dropwise to a
methanol (40 mL) solution of 4@-N,N-dimethylamino-2-
hydroxy-5-methylchalcone (2 g, synthesized according to ref.
13) and 20% NaOH solution (10 mL) at 0È5 ¡C over 30 min.
After being stirred for 24 h at room temperature, the bright
yellow solution was neutralized with acetic acid. The yellow
precipitate was Ðltered and washed with water. After drying
the product was recrystallized from xylene to give 1.5 g (yield
B71%) of A as orange crystals, mp: 226È228 ¡C. MS (m/z):
295.
Results and discussion
We previously reported the spectroscopic and photophysical
properties of 4,4-dimethylaminoÑavone.14 Its emission proper-
ties are strongly inÑuenced by environmental stimuli such as
the polarity and viscosity of the medium and the tem-
perature.15,16 Flavone is a Ñuorophore in which the 4@-
dimethylamino is an electron-donating group and the
carbonyl group is an electron-acceptor, hence it is a donorÈ
acceptor conjugated p-electron system, which can undergo
intramolecular charge transfer upon excitation. It can thus be
anticipated that the introduced cation in close proximity with
the donor or acceptor moiety will change the photophysical
properties of the Ñuorophore because the bound cation would
a†ect the efficiency of intramolecular charge transfer.17 Roshal
et al.18 have studied some Ñavone derivatives in which the
amino electron-donating group was replaced by a monoaza-
crown ether. Complexation reduced the conjugated system
and resulted in a blue shift of the absorption spectrum. Con-
versely, a cation interacting with the carbonyl group enhances
its electron-withdrawing character, thus a red shift of the
absorption and Ñuorescence spectra of this compound is
expected.
As exempliÐed by Fig. 1, the addition of alkali and alkaline
earth metal perchlorates to solutions of FQC or FLC induces
large red shifts in the absorption spectra of these crowns. An
increase in the molar extinction coefficient also occurs. Fur-
thermore, the alkaline earth cations lead to a stronger shift
than the alkali metal cations. The emission spectra of free and
bound FQC are very similar in shape and show a large red
shift in the presence of cation perchlorates. A concomitant
quenching of Ñuorescence intensity resulting from the
enhancement of intramolecular charge transfer is also
observed. For instance, the absorption spectrum of FQC is
red-shifted about 36 nm upon complete complexation by
Ca2`. The spectral characteristics of free and fully complexed
FQC and FLC are listed in Table 1. These data show that the
order of the wavelength shift in the absorption and Ñuores-
cence spectra is Mg2` [ Ca2` [ Li2` [ Na` with both com-
pounds. Moreover, the Stoke shift (*l) seems also to exhibit a
similar ordering. Obviously, the results are related to the
charge density of the cations. The interactions between FQC
Flavone lariat-crown ether (FLC). To a vigorously stirred
dry DMF (15 mL) solution of A (0.59 g), tert-butoxide pot-
assium (0.23 g) and chloromethyl-12-crown-4 (0.49 g) were
added at room temperature. The mixture was heated to 80È
90 ¡C and maintained for 70 h. At the end of this time the
mixture was cooled down, Ðltered and concentrated. The con-
centrated solution was poured into water (50 mL) and
extracted with ethyl acetate (3 ] 50 mL). The organic layer
was dried over sodium sulfate and the solvent then evaporated
under vacuum. The crude product was puriÐed by column
chromatography and recrystallized from ethyl acetateÈ
petroleum. Pure FLC (0.12 g) was obtained as yellow crystals,
mp: 67È72 ¡C. 1HNMR (CDCl ) d: 2.41 (s, 3H), 3.02 (s, 6H),
3
3.54È3.85 (m, 14H), 3.91È4.10 (m, 3H), 6.78È7.02 (d, 2H), 7.35È
7.39 (m, 2H), 7.95 (s, 1H), 8.05È8.10 (d, 2H). MS (m/z): 483
(M`). Elem. anal. calcd for C
N, 2.90; found: C, 67.21; H, 7.11; N, 2.67.
H
NO : C, 67.06; H, 6.88;
27 33
7
Determination of the stability constants9
The stability constants can be obtained from the variation of
either the absorption or Ñuorescence intensity at the appropri-
ate wavelengths according to the following equations:
A /(A [ A) \ e /(e ] e ) ] (1/K [M] ] 1)
0
0
L
L
ML
S
or
I0/(I0 [ 1) \ e U /(e U [ e
U
) ] (1/K [M] ] 1)
F
F
L
L
L
L
ML ML
S
where e and e are the molar extinction coefficients of ligand
L
ML
and complex, respectively, and U and U are the Ñuores-
L
ML
cence quantum yields of ligand and complex, respectively. I0
F
and A are the absorption and Ñuorescence intensity of free
0
106
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Na` is difficult to complex with these crown compounds
owing to its low charge density and big ionic diameter (0.19
nm). In Table 1, the data given for Na` refer to partial com-
plexation only and should be used with care when comparing
the data.
The e†ect of gradual addition of di†erent perchlorate salts
on the absorption and Ñuorescence spectra of FQC and FLC
in acetonitrile is shown in Fig. 2 for FLC. It is seen that for
alkaline earth metal perchlorates, the absorption spectra show
a well-deÐned isosbestic point in the whole range of concen-
tration, whereas for lithium perchlorate the isosbestic point is
only observed at high concentration due to its poor binding
capability. From these spectra we are able to plot the absorp-
tion changes of the Ñavone derivatives against the concentra-
tion of alkali and alkaline earth metal perchlorates; an
example is given in Fig. 3. The ratio of A /(A [ A) can then
0
0
be plotted vs. [M]~1, as in Fig. 4, to give a set of quite good
straight lines. As it is well known that the BenesiÈHildebrand
equation is derived on the basis of a 1 : 1 stoichiometric ratio,
the linear relationship obtained indicates that the stoichiomet-
ric ratio of the hostÈguest complexes studied in this work is
1 : 1. In addition, it is also worth noting that Na` and K` can
cause a red shift of the electronic spectra and Ñuorescence
quenching of FQC or FLC at low concentrations. But their
binding with the ligands is difficult to observe at low concen-
trations. From Fig. 3 it can also be seen clearly that the
binding ability of Na` (and even of Li` to a certain extent) to
Fig. 1 Absorption and Ñuorescence spectra of FQC in acetonitrile
and its complexes with di†erent perchlorate salts. (A) Absorption and
(B) Ñuorescence spectra: (curve 1) FQC, (curve 2) FQC-Li`, (curve 3)
FQC-Na`, (curve 4) FQC-Mg2`, (curve 5) FQC-Ca2`.
or FLC and the cations were modelled and analyzed by PC
Model 6.0 (Serena Software) software. However, in the calcu-
lation the solvent was not taken into account; thus the con-
Ðgurations obtained only provide an indication of what can
happen. The conÐgurations obtained from the calculations for
FQC and FLC complexed with the four di†erent cations are
based on a 15-C-5 crown ether and a 15-C-5 crytand, respec-
tively (as shown in Scheme 1).
It can be clearly shown that the oxygen of the carbonyl
group is also bound to the cation in FQC and FLC, while the
oxygen at the 2 position in FQC does not bind with the
cations. The involvement of the carbonyl group enhances the
efficiency of intramolecular charge transfer, resulting in photo-
physical changes in the Ñavone Ñuorophore upon complex-
ation with cations. These results are also supported by the
change in the C2O absorption in the IR spectrum upon com-
plexation with cations (see Table 2 below). In addition, from
the experimental results it is seen that the monovalent ion
Fig. 2 E†ect of the concentration of di†erent metal perchlorates on
the absorption and Ñuorescence spectra of FLC. (A) E†ect of the addi-
tion of Mg2` salt on the absorption spectrum of FLC (1 ] 10~2 mM)
in acetonitrile. Concentration of Mg2`: (curve 1) 0, (curve 2) 0.15,
(curve 3) 0.5, (curve 4) 1.0, (curve 5) 2.0, (curve 6) 3.0, (curve 7) 5.0,
(curve 8) 20.0, (curve 9) 30.0 mM. (B) E†ect of the addition of Ca2`
salt on the Ñuorescence spectrum of FLC (1 ] 10~2 mM) in acetoni-
trile. Concentration of Ca2`: (curve 1) 0, (curve 2) 0.15, (curve 3) 0.2,
(curve 4) 0.35, (curve 5) 0.5, (curve 6) 0.74, (curve 7) 0.99, (curve 8) 1.23,
(curve 9) 1.48, (curve 10) 1.73 mM.
Scheme 1 Probable molecular conÐgurations of FQC and FLC
when complexed with di†erent cations.
Table 1 The spectrum characteristics of free and complexed FQC and FLC
j
/nm
max
e/L mol~1 cm~1
Absorption
Emission
*l
Stokes shift
/103 cm~1
FQC
29 200
32 300
32 500
32 800
37 100
21 900
21 400
21 900
23 500
22 900
374
394
380
423
410
375
386
380
404
402
489
510
504
528
525
492
506
498
520
520
8.70
8.62
8.06
9.52
8.70
8.55
8.33
8.47
8.62
8.47
FQC-Li`
FQC-Na` a
FQC-Mg2`
FQC-Ca2`
FLC
FLC-Li`
FLC-Na`
FLC-Mg2`
FLC-Ca2`
a The Na` complexation is discussed in more detail in the text.
New J. Chem., 2000, 24, 105È108
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Table 2 The stability constants for complexation between FQC or FLC and di†erent metal cations
Ligand
Li`
Mg2`
Ca2`
Diameter/nm
0.17È0.22a
0.136
0.132
0.198
Log K (FQC)
È
1.51 ^ 0.01
1.68 ^ 0.02
È
2.98 ^ 0.02
3.19 ^ 0.02
3.38 ^ 0.02
3.27 ^ 0.04
1612
3.19 ^ 0.01
3.34 ^ 0.01
5.52 ^ 0.03
5.48 ^ 0.02
1616
abs
Log K (FQC)
em
È
È
È
1634
Log K (FLC)
abs
Log K (FLC)
3.02 ^ 0.03
em
l(C2O)/cm~1
1625
a Cavity size of the crown ether (15-C-5).
character and can bind with alkali and alkaline earth metal
cations. Obviously, this is due to the chemical structure of
FQC. Flavone quasi-crown ether and Ñavone lariat-crown
ether are selective in cation recognition due to the photo-
chemical properties of their Ñavone Ñuorophore, e.g., the
appropriate intramolecular charge transfer character, the high
Ñuorescence quantum yield, etc. Although their complex sta-
bility constants with Ca2` are prominent, their suitability for
the recognition of other cations is under further investigation.
Acknowledgements
We are grateful to Professor J. W. Canary (New York
University) for helpful discussions. This work was supported
by a grant (29733100) from the National Natural Science
Foundation of China.
Fig. 3 The change in absorption of FQC (1 ] 10~2 mM) in acetoni-
trile vs. the concentration of alkali and alkaline earth metal perchlor-
ates (curve 1) Na`, (curve 2) Li`, (curve 3) Mg2`, (curve 4) Ca2`. The
measured wavelength is set at the absorptive peak wavelength of each
compound.
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obtained with certainty. These are listed in Table 2.
It is apparent that the stability constants of the divalent
alkaline earth metal cations are far larger than those of the
monovalent alkali metal cations. This demonstrates that the
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and the electrochemical properties of the receptor. For diva-
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Mg2` is smaller than Ca2`, consistent with the complexation
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In summary, we Ðnd that there is some analogy in structure
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di†erences between them have also been observed. For
instance, acridan-9-one crown ether cannot complex with
alkali metal and alkaline earth metal cations but with Cu`,
Ag` and Fe2`. Conversely, FQC still has some hard base
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Fig. 4 Plots of A /(A [ A) against the inverse of the metal ion con-
0
0
centration [M]~1.
Paper a904825a
108
New J. Chem., 2000, 24, 105È108