Dual spectroscopic responses of pyridinium hemicyanine dyes to anions

Article abstract of DOI:10.1002/cjoc.201180343

The absorption and fluorescence spectroscopic responses of three pyridinium hemicyanine dyes to anions F^-, Cl^-, Br^-, I^-, H₂PO₄^-, HSO₄ ^- and OAc^- were investigated. At lower concentrations of OAc^- (less than 1 equiv.), both the absorption and the fluorescence intensities of 1-3 were more effectively changed than F^- at identical concentrations. At higher concentrations of OAc^- (more than 1 equiv.), the interaction was opposite for each compound. ¹H NMR results indicated the interaction between 1, 2 or 3 and F^- proceeded through hydrogen bonding. The results showed that these dyes are promising to develop dual fluorescence and chromogenic chemosensors toward F^- and OAc^- according to the subtle difference in the affinity of F ^- and OAc^-. Pyridinium hemicyanine dyes bearing hydroxyl groups on the benzene ring provide a simple class of anion receptors which are capable of selectively reporting the presence of F^- and OAc ^- in CH₃CN by color changes. Copyright

Full text of DOI:10.1002/cjoc.201180343

FULL PAPER
Dual Spectroscopic Responses of Pyridinium Hemicyanine
Dyes to Anions
Xie, Puhui*,a(谢普会)
Guo, Fengqi*,b(郭丰启)
Zhang, Leia(张磊)
Zhang, Dashenga(张大生)
a College of Sciences, Henan Agricultural University, Zhengzhou, Henan 450002, China
b Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450001, China
The absorption and fluorescence spectroscopic responses of three pyridinium hemicyanine dyes to anions F－ ,
Cl , Br , I , H2PO4 , HSO4 and OAc－ were investigated. At lower concentrations of OAc－ (less than 1
equiv.), both the absorption and the fluorescence intensities of 1— 3 were more effectively changed than F－ at iden-
tical concentrations. At higher concentrations of OAc－ (more than 1 equiv.), the interaction was opposite for each
compound. 1H NMR results indicated the interaction between 1, 2 or 3 and F－ proceeded through hydrogen
bonding. The results showed that these dyes are promising to develop dual fluorescence and chromogenic chemo-
sensors toward F－ and OAc－ according to the subtle difference in the affinity of F－ and OAc－ .
－
－
－
－
－
Keywords hemicyanine, absorption spectra, fluorescence spectra, chemosensors, anions
Introduction
compounds, may be modulated via hydrogen bonding or
deprotonation of the hydroxy moiety by small anions
possessing high electron density such as F－ and HO－ .10
It is significant to develop simple systems without com-
plicated synthesis. The dual action of 1— 3 toward F－
would thus yield a combined fluorescent and colorimet-
ric based sensor in a single molecule. The receptors
containing hydroxy groups displayed the F－ and OAc－
recognition due to hydrogen bonding and/or intermo-
lecular proton transfer in the sensor-anion complexes,
but most of those receptors could not distinguish these
two anions.11-13 Detailed investigation in this work
found acetate increased the absorbance of the new band
and decreased the fluorescence intensity of 1— 3 more
pronounced than F－ at lower concentration (less than 1
equiv.). At higher concentrations, the interaction was
opposite for each compound. Acetate could even de-
crease the absorbance of the newly formed absorption
band in some degree, resulting in partially faded color.
As anions play important roles in a wide area of
chemical, medical, industrial, environmental and bio-
logical processes, considerable attention has been fo-
cused on the design of chemosensors that can recognize
and detect anion species selectively through visible,
electrochemical, nuclear magnetic and optical re-
sponses.1-5 Color changes, recognized by the naked eye,
are conveniently applied because they may be used as
dip-stick sensors. Color variation can be related to either
structural or conformational changes in chemosensors
when an interaction is happened.2,5,6 Chemosensors are
designed according to the binding of a specific anion
with their receptor sites, with a chromophore on a
chemosensor responsible for translating the receptor-
anion association into an optical signal.
Among the biologically important anions, fluoride is
of particular importance due to its crucial role in dental
care7,8 and osteoporosis.9 The addition of fluoride in
drinking water and toothpastes has become widespread
due to the valuable effects of fluoride in human health.
High doses of fluoride are hazardous and can lead to
dental or skeletal fluorosis. Therefore, convenient
methods for the detection of fluoride have become a hot
topic.
-
HO
I
-
I +
+
N
N
OH
1
2
-
OCH3
OH
I
+
N
We set out to choose pyridinium hemicyanine dyes
as chromophores (Figure 1). Their large dipole moments
originate from an intramolecular charge transfer (ICT)
states. The ground state, and therefore the color of the
3
Figure 1 Pyridinium hemicyanine dyes 1, 2 and 3.
* E-mail: pxie2007@yahoo.com.cn (P. Xie), fqguo@zzu.edu.cn (F. Guo); Tel.: 0086-0371-63554844 (P. Xie), 0086-0371-68019032 (F. Guo)
Received March 25, 2011; revised May 4, 2011; accepted May 30, 2011.
Project supported by Ministry of Education, the People’s Republic of China (No. [2009]8) and the Education Department of Henan Province (No.
2010B150011).
Chin. J. Chem. 2011, 29, 1975— 1981
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Xie et al.
FULL PAPER
Experimental
1
iodide (2): H NMR (DMSO-d6, 400 MHz) δ: 10.45 (s,
1H), 8.89 (d, J＝6.0 Hz, 1H), 8.48—8.42 (m, 2H), 7.97
(d, J＝16 Hz, 1H), 7.88—7.84 (m, 1H), 7.77 (dd, J＝
1.2, 8.0 Hz, 1H), 7.61 (d, J＝16 Hz, 1H), 7.32—7.28 (m,
1H), 6.97—6.90 (m, 2H), 4.32 (s, 3H); MS (EI) m/z (%):
212.1 ([M－I]＋ , 100). Anal. calcd for C14H14NOI•
0.4H2O: C 48.55, H 4.31, N 4.04; found C 48.58, H 4.30,
N 4.05.
General experiments
The anion salts n-Bu4NF, n-Bu4NCl, n-Bu4NBr,
n-Bu4NI, n-Bu4NHSO4, n-Bu4NOAc and n-Bu4NH2PO4
were purchased from Aladdin-reagent Inc. All the sol-
vents for spectroscopic measurement were AR grade
and were dried and distilled before use.
NMR spectra were recorded with a 400 MHz Varian
spectrometer. Electrospray ionization mass spectra
(ESI-MS) were measured on an LC-MSD-Trap-SL sys-
tem. Elemental analyses were carried on an elemental
analyzer (Flash EA 1112, Thermo Electron SPA, Italy).
Absorption spectra were measured on a TU1901 UV-vis
spectrometer. Fluorescence spectra were collected on
the SPEX Fluorolog-3 spectrometer. Stock solutions of
pyridinium hemicyanine dyes 1—3 and anions were
prepared in CH3CN solutions. Then they were diluted to
(E)-1-Methyl-2-[4-(hydroxyl-3-methoxybenzyl)-
vinyl]-pyridinium iodide (3): H NMR (DMSO-d6, 400
1
MHz) δ: 9.82 (s, 1H), 8.82 (d, J＝6.4 Hz, 1H), 8.47—
8.40 (m, 2H), 7.88 (d, J＝16.0 Hz, 1H), 7.83—7.79 (m,
1H), 7.46 (d, J＝1.6 Hz, 1H), 7.38 (d, J＝16.0 Hz, 1H),
7.29 (dd, J＝1.6, 8.4 Hz, 1H), 6.88 (d, J＝8.0 Hz, 1H),
4,34 (s,＋3H), 3.87 (s, 3H); MS (EI) m/z (%): 242.1
([M－I] , 100). Anal. calcd for C15H16NO2I•0.8H2O: C
46.96, H 4.62, N 3.65; found C 46.99, H 4.61, N 3.66.
－5
－1
3×10 mol•L with CH3CN. Titration experiments
were performed by placing 3 mL of the diluted solution
in a quartz cuvette of 1 cm optical path length, then
adding the stock solutions of anions incrementally by
means of a micro-pipette. Spectra were recorded 5 s
later after each addition.
Results and discussion
Absorption responses of 1— 3 to F－ and OAc－
The absorption spectra displayed maximum peaks at
371, 353 and 383 nm for 1, 2 and 3 in CH3CN solutions
respectively. The 12 nm red shift for 3 compared to 1 is
due to electron donating effect of the methoxy group on
the benzene ring. The electron donating effect on the
benzene ring would decrease the energy gap from π to
π* transition, leading to red-shift in the absorption spec-
tra of 3. The 18 nm blue shift for 2 compared to 1 is due
to steric effect of the hydroxyl group in 2-position on
the benzene ring, which can be proved by the density
functional theory (DFT) calculations performed with the
Gaussian 03 software (Table S1). The results showed
that the dihedral angle of 1 (1.6°) is less than that of 2
(4.7°). And the energy difference between the HOMO
orbital and LUMO orbital for 1 is slightly lower than
that for 2.
Synthesis of 1— 3
Synthesis of 2-methyl-N-methyl pyridinium io-
dide 9.8532 g (105.88 mmol) of 2-methylpyridine and
14.9862 g (105.54 mmol) of iodomethane were dis-
solved in 20 mL of acetonitrile. The mixture was re-
fluxed for 10 h. After cooling to room temperature, the
mixture was concentrated under reduced pressure and
then ethyl ether was added and stirred. The precipitate
was filtered and washed twice with ethyl ether. 10.686 g
of salts were resulted. The crude product was used di-
rectly without further purification.
Synthesis of (E)-1-methyl-2-[4-(hydroxybenzyl)-
vinyl]-pyridinium iodide (1) 0.4237 g (1.79 mmol)
2-methyl-N-methyl pyridinium iodide and 0.2836 g
(2.32 mmol) 4-hydroxybenzaldehyde were dissolved in
10 mL ethanol with three drops of triethyl amine. The
mixture was refluxed for 12 h and then concentrated
under reduced pressure. The precipitate was filtered and
washed twice with ethyl ether. The crude product was
chromatographed with dichloromethane/ethanol1 (V∶
V ＝2 ∶1) and finally get the product. 1H NMR
(DMSO-d6, 400 MHz) δ: 10.21 (s, 1H), 8.82 (d, J＝6.8
Hz, 1H), 8.48—8.40 (m, 2H), 7.88 (d, J＝16.0 Hz, 1H),
7.82 (d, J＝6.0 Hz, 1H), 7.35 (d, J＝16.0 Hz, 1H), 7.26
(d, J＝8.4 Hz, 2H), 6.88 (d, J＝8.4 Hz＋, 2H), 4.32 (s,
3H); MS (EI) m/z (%): 212.1 ([M－I] , 100). Anal.
calcd for C14H14NOI•0.5H2O: C 48.29, H 4.34, N 4.02;
found C 48.26, H 4.35, N 4.03.
The binding ability of 1, 2 and 3 for anions was in-
vestigated using the UV-vis absorption method.14 Figure
2 shows the absorption spectra of 1, 2 or 3, with the
－5
concentration of 3.0×10 mol/L in CH3CN upon titra-
tion with F－ or OAc－ respectively. For 1, the absorb-
ance at 371 nm decreased with the increase of F－
concentration, at the same time a new absorption band
at about 543 nm appeared and enhanced (Figure 2a).
The changes of absorption spectra indicated obvious
interaction between F－ and 1. The relationship between
the absorbance at 543 nm and the equivalents of F－ was
almost linear for 1 within 2 equiv. of F－ (Figure 3a).
Non-linear fitting could not get a good fitting in the
whole range. The modulation in the electron-donating
capabilities of the hydroxyl group in the presence and
absence of F－ directly influences the ICT from the ben-
zene ring to the pyridinium ring. In the presence of F－,
the ICT effect was enhanced. The color of the solution
changed from light yellow to strong pink upon addition
of 6 equiv. of F－ (Figures 4 and 5). The absorption at
371 nm was almost switched off and that at 543
Similar procedures were carried out for the synthesis
of 2 and 3, using 2-methyl-N-methyl pyridinium iodide
to react with 2-hydroxybenzaldehyde and 4-hydroxy-3-
methoxybenzaldehyde respectively.
(E)-1-Methyl-2-[2-(hydroxybenzyl)vinyl]-pyridinium-
1976
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Chin. J. Chem. 2011, 29, 1975— 1981

Dual Spectroscopic Responses of Pyridinium Hemicyanine Dyes to Anions
nm was switched on. However, the response of 1 to
OAc－－ is quite different. Upon titration of 1 equiv.
OAc to 1, a pink color was generated. The absorbance
of 1 at 543 nm reached its maximum (Figure 2b, Figure
3a). Further titration of OAc－ gradually decreased the
absorbance. The absorption band at 371 nm was
red-shifted to 400 nm. And new absorption bands be-
tween 400 to 500 nm became stronger and stronger
(Figure 2b). After addition 6 equiv. of OAc－ to 1, the
color of the solution became weaker. This is possibly
due to aggregation induced by tetrabutylammonium
acetate. Upon addition of less than －1 equiv. of OAc－,
the interaction between 1 and OAc became stronger
than the interaction between 1 and F－－. However, upon
addition of more than 1 equiv. of OAc , the results were
opposite. Whereas addition of other anions such as Cl－,
－
－
－
－
Br , I , H2PO4 and HSO4 almost did not induce
obvious color change and a－bsorption change of 1.
Similar responses to F or OAc－ for 2 or 3 were
observed (Figures 2c—2g). Job’s plot exhibited a 1∶1
complex formation for each compound toward F－ or
OAc－. Upon titration with F－, the color of 2 turned
from colorless to purple (Figure 5). The absorption band
at 353 nm decreased and a new band at 546 nm evolved
and －reached its limiting value upon addition of 4 equiv.
of F (Figure 2c). The largest red-shift (193 nm) in the
Figure 2 Absorption spectra of 1, 2 and 3 (3.0× 10－ mol/L) in CH3CN upon titration of F－ and OAc－ . (a) addition of F－ to 1, (b)
addition of OAc－ to 1, (c) addition of F－ to 2; (d) addition of OAc－ to 2, (e) addition of F－ to 3, (f) addition of OAc－ to 3.
5
Chin. J. Chem. 2011, 29, 1975— 1981
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1977

Xie et al.
FULL PAPER
Figure 3 Titration curves of different anions to 1 (a), 2 (b) and 3 (c) in CH3CN respectively.
Figure 4 Color changes of 1 (2× 10－ 5 mol•L－ 1) in CH3CN upon addition of 6 equiv. of different anions.
Figure 5 Color changes of 1, 2 and 3 (3× 10－ 5 mol•L－ 1) to different equivalents of F－ and OAc－ in CH3CN.
1978
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Chin. J. Chem. 2011, 29, 1975— 1981

Dual Spectroscopic Responses of Pyridinium Hemicyanine Dyes to Anions
absorption maxima was observed after F－ was added to
the solution of 2. This was attributed to the steric effect of
the hydroxyl group on the benzene before interaction
with F－ . The deprotonated form of 2 is more delocalized
than the protonated form. With the increase of F－ con-
centration, the color of 3 turned purple (Figure 5). The
absorption band at 383 nm decreased gradually and a new
band at 560 nm evolved and reached its maximum upon
addition of 4 equiv. of F－ (Figure 2e). One isosbestic
point at 430 nm was observed. The larger red-shift and
deeper color of 3 than 1 upon titration of F－ is due to
electron-donating substituent of the methoxy group ortho
to the hydroxyl group on the benzene ring of 3. Com-
pounds 1, 2 and 3 have a much better selectivity and sen-
sitivity to F－ than OAc－ upon titration of more than one
equivalent, while they have a slightly better selectivity
and sensitivity to OAc－ than F－ upon titration of less
than one equivalent (Figures 3 and 5).
The absorption effect of newly-formed species to free
dye 3 was attributed to the reduced intensity of fluores-
cence and the blue-shifted peaks with the increase of F－
concentration. The fluorescence of 3 was more effi-
ciently quenched within less than 1 equiv. of OAc－
compared to F－ . Then it was slightly quenched until 4
equiv. of OAc－ were added. Further addition of OAc－
did not change the fluorescence spectra at all (Figure
6b).
Similar effects were observed in the fluorescence
spectra of 1 or 2 upon addition of F－ or OAc－ (Figure
7). The fluorescence of 1 or 2 was also more effectively
quenched by OAc－ within less than 1 equiv. compared
to F－ , then slightly quenched with more than 1 equiv. of
OAc－ . Further addition of OAc－ did not change the
fluorescence spectra at all. However, further addition of
F－ quenched the fluorescence of 1 or 2 more efficiently
than OAc－ (with more than 1 equiv.). The relationship
between fluorescence intensities of 1, 2 or 3 and the
equivalents of F－ was almost linear, while the relation-
ship between fluorescence intensities and equivalents of
OAc－ was nonlinear. The variation trends of the fluo-
rescence intensities coincided with the absorbance upon
addition of F－ or OAc－ . Other anions investigated in
this paper did not induce any obvious fluorescence
quenching.
Fluorescence responses of 1, 2 and 3 to F－ and OAc－
The fluorescence intensity of 3 at 517 nm was sub-
stantially quenched upon titration of 3.2 equiv. of F－ ,
with the maximum band slightly blue-shifted (Figure 6a).
Figure 7 Relationship between fluorescence intensity of 1, 2
and 3 and the equivalents of F－ or OAc－ . 1-F－ (○); 1-OAc－ (●);
2-F－ (◊); 2-OAc－ (♦); 3-F－ (⊗); 3-OAc－ (⊕). The original
fluorescence was normalized for each compound.
Plotting the fluorescence changes at fluorescence
maximum as a function of －lg[anion] gave sigmoidal
curves in each case that switched off within one log
unit.15 These changes were fitted using a non-linear
least squares regression algorithm, giving the binding
constants lg K＝(4.75 0.02), (4.84 0.02) and (4.83
0.03) toward OAc－ for 1, 2 and 3 respectively and lg K
＝(4.12 0.01), (4.31 0.01) and (4.54 0.01) toward F－
for 1, 2 and 3 respectively. The results indicated the
affinity ability of 1, 2 or 3 is indistinguishable toward
OAc－ .
Figure 6 (a) Fluorescence spectra of 3 (3.0× 10－ mol/L) in
5
CH3CN solutions upon addition of F－ (from up to down the
equivalents of F－ are 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.4,
2.8, 3.2; (b) fluorescence spectra of 3 titrated with OAc－ (from up
to down the equivalents of OAc－ are 0, 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4,
5, 6, 7, 8. λex＝ 383 nm, ex slit＝ 5 nm, em slit＝ 10 nm).
Chin. J. Chem. 2011, 29, 1975— 1981
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Xie et al.
FULL PAPER
1H NMR responses to F－ of 2
Scheme 1 Possible interaction mechanism of 2 and F－
As an example, the interaction of 2 with excess F－
was proved by 1H NMR experiments in DMSO-d6 (Fig-
ure 8). It was found that the aromatic proton signals and
the methyl group signals underwent upfield shifts upon
addition of excess F－ . Such an effect was resulted from
the through-bond propagation onto the aromatic frame-
work of the electronic charge generated on O—H de-
protonation,16 thus leading to a shielding effect and in-
ducing upfield shift. The signal of the hydroxyl group
disappeared. An obvious triplet peak at about δ 16 ap-
-
H
F
-
O
-
+
N
I
HO
I
+
N
-
F
-
-
+
N
O
I
-
F
-
+ HF2
basicity. The interaction between OAc－ and 1, 2 or 3
was found slightly stronger than F－ . At higher concen-
trations of OAc－ , aggregation effects became more and
more effective.
17
－
peared, indicating the formation of HF
as the driv-
2
ing force, implying that the phenolic proton of 2 is com-
pletely transferred to F－ .
Conclusions
Pyridinium hemicyanine dyes bearing hydroxyl
groups on the benzene ring provide a simple class of
anion receptors which are capable of selectively report-
ing the presence of F－ and OAc－ in CH3CN by color
changes. The interactions between OAc－ and 1, 2 or 3
were found slightly stronger than F－ at lower concen-
trations (less than 1 equiv.). But the interactions be-
tween F－ and 1, 2 or 3 were found much more effi-
ciently than OAc－ at higher concentrations (more than
1 equiv.). Unlike most fluorescent sensors for F－ that
cannot tell F－ from OAc－ , compounds 1, 2 and 3
showed good selectivity for F－ over OAc－ at higher
concentrations (more than 1 equiv.). The interaction
through hydrogen bonding and further deprotonation at
the hydroxyl moiety of hemicyanine chromophores re-
sulted in dual colorimetric and fluorescent response to-
ward F－ .
Figure 8 Partial 1H NMR spectra of 2 before and after addition
of F－ in DMSO-d6.
The F－ induced deprotonation process was fully re-
versible. The addition of polar protic solvents such as
H2O and CH3OH resulted in a reverse color change
from purple to yellow. This is presumably because
protic solvent competed for F－ with the hydroxy moi-
ety, moreover, the presence of a relatively high amount
of protic solvent disfavors the formation of the depro-
tonated form of 2. However, in water-containing me-
dium, the excess addition of strong base [Bu4N]OH
could still deprotonate the hydroxyl group of 2 and in-
duce a color change.
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1981