Monomolecular Layers of Pyrene as a Sensor to Dicarboxylic Acids

Article abstract of DOI:10.1021/jp0372385

A new photoinduced luminescence film was prepared by immobilizing pyrene on a quartz plate surface via a flexible long spacer. The film displayed combined monomer and excimer emission of pyrene both in wet and dry states, and the excimer might be formed mainly by direct excitation of ground-state dimers and/or monomers in aggregates. Both the monomer emission and the eximer emission were sensitized by introduction of some dicarboxylic acids. This phenomenon was ascribed to the insertion of dicarboxylic acids into the space between the neighboring spacers. In contrast, introduction of monocarboxylic acids, e.g., formic acid and acetic acid, had little influence on the emission of the functional film. Response of the film to dicarboxylic acids was reversible, and the film was stable within a number of months if it was properly preserved. These characteristics guaranteed the film as a potential sensing material for common dicarboxylic acids.

Full text of DOI:10.1021/jp0372385

J. Phys. Chem. B 2004, 108, 1207-1213
1207
Monomolecular Layers of Pyrene as a Sensor to Dicarboxylic Acids
Lining Gao, Yu Fang,* Xiangpeng Wen, Yuangang Li, and Daodao Hu
School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an 710062, China
ReceiVed: October 26, 2003
A novel photoinduced luminescence film has been prepared by immobilizing pyrene on a quartz plate surface
via a flexible long spacer. The film shows combined monomer and excimer emission of pyrene both in wet
and dry states, and the excimer may be formed mainly by direct excitation of ground-state dimers and/or
monomers in aggregates. Classical Birks’ scheme plays a little role in the formation of the excimers. The
emission of the film is sensitive to the presence of dicarboxylic acids, including ethanedioic acid, malonic
acid, succinic acid, etc. The emission in the monomer and excimer region increases with addition of the
acids. The time needed for the emission to reach equilibrium depends on the nature of the acids. It has been
shown that the longer the chains of the acids, the more time is needed. This observation is explained by
considering the conformational reorganization of the immobilized pyrene due to insertion of the dicarboxylic
acids into the space between neighboring spacers. Experimental results from similar studies using formic
acid and acetic acid are in support of this explanation. Furthermore, the response of the film to dicarboxylic
acids is reversible.
Introduction
For selective sensing of dicarboxylic acids, imino units were
intentionally introduced into a spacer, via which Py was
In recent years, monomolecular assemblies on solid surfaces
have attracted growing attention due to their potential application
in creation of novel electronic, photoelectronic, and sensing
immobilized on a substrate surface. Imino units were chosen
due to their character to form hydrogen bonds with the
1
2
1
-4
carboxyhydroxy group. Dicarboxylic acids are commonly
found in liquid fertilizers and water-soluble organic compounds
films. Chemical immobilization of small molecules onto the
substrate surface is one of the important approaches to prepare
1
3,14
5
(WSOC) in atmospheric aerosols.
Their identification and
monomolecular assemblies and provides a convenient way to
determination are commonly conducted by an esterification-
produce surfaces with specific chemical functionalities that allow
the precise turning of surface properties.5,6 The properties of
gas chromatography technique, which is quite complicated and
1
3,15
time consuming.
It was expected that the specific inter-
the modified surfaces depend on a number of factors, including,
at least, the nature of the small molecules, the properties of the
substrate surface, the structure of the spacer, which links the
small molecule and the substrate, and the density of the small
molecules on the substrate surface. Clearly, a variety of
functional films with ideal properties may be prepared by careful
selection and combination of the small molecules, the substrate,
and the spacer.
action of the imino group with the carboxyhydroxy group would
result in insertion of a dicarboxylic acid into the spacer layer,
altering the spatial distribution of the sensing element, Py. This
variation must be accompanied by a profile change in the
fluorescence emission spectrum of the sensing element since
the emission is characterized by both monomer emission and
excimer emission, and the relative intensities of the two
emissions are sensing element distribution dependent. On the
basis of this consideration, the quartz plate was selected as a
substrate due to its resistance to common solvents and ease to
be functionalized. Py was chosen again due to similar reasons
On the basis of this methodology, a number of functional
films with pyrene (Py) as a fluorescence-active molecule have
been prepared and studied as sensing materials in our
7
-11
laboratory.
It has been demonstrated that the intensity and/
8
as described before, and a long, flexible spacer containing an
or the profile of the fluorescence emission of the films are
sensitive to the purity of water and the presence of nitrite in
8
imino structure was designed via combination of ethylenedi-
amine and 3-glycidoxypropyltrimethoxysilane. It is expected that
with this designation a novel film of selective responses to
dicarboxylic acids may be obtained. This article reports the
results.
9
aqueous phase and nitromethane in methanol, ethanol, or
water.10 Simple embedding of Py into a chitosan (CS) film
7
results in a medium polarity sensitive film. It is to be noted
that all the sensing films reported from our laboratory have been
designed with a short or even no spacer, and the sensing
principles of the films are based on the hydrophobic aggregation
of the sensing element, Py, and its dependence on the presence
of analytes. Therefore, it should be more practical to introduce
recognition sites or structures on the substrate surface or into
the monomolecular layer in terms of actual device implementa-
tion.
Experimental Section
Reagents. Py (Acros, 96%) was recrystallized from an
ethanolic solution and then extracted with Soxhlet extractor by
ethanol. 3-Glycidoxypropyltrimethoxysilane (GPTS, Acros,
97%) was used directly without further purification. Ethylene-
diamine (EDA) was washed with KOH solution and then
distilled. N,N-Dimethylformamide was freshly distilled from
calcium hydride before use. Dichloromethane, trichloromethane,
and toluene were washed with concentrated sulfuric acid and
*
Author to whom correspondence may be addressed. Tel: +86-29-
307534, +86-29-5308531. Fax: +86-29-5307774. E-mail: yfang@
snnu.edu.cn.
5
1
0.1021/jp0372385 CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/06/2004

1208 J. Phys. Chem. B, Vol. 108, No. 4, 2004
Gao et al.
then distilled. Diethyl ether was washed with ferrous sulfate
solution to remove peroxide and then distilled. Thionyl chloride
was rectified three times in the presence of pure sulfur (sublimed
grade). Other reagents were of analytical grade at least. Water
used in this work was deionized and then double distilled.
TABLE 1: Advancing Contact Angles (θ) of Various Quartz
Plate Surfaces and Water
quartz platesa
1
2
3
4
θ (deg)
41.4 ( 1.2 21.1 ( 1.4 55.3 ( 1.6 56.8 ( 1.1
a
(1) stands for the original clean quartz plate, (2) the activated quartz
Instrumentation. Fluorescence measurements were per-
formed at room temperature on a Perkin-Elmer LS-50B
fluorescence/phosphorescence/luminescence spectrometer and
a time-correlated single photon counting Edinburgh FLS 920
fluorescence spectrometer with a front-face method. Analyses
of C, H, and N were conducted on a Perkin-Elmer 2400 CHN
elemental analyzer. Pressed KBr disks for all the powder
samples were used for the transmission infrared spectroscopy
measurements, and their FTIR spectra were obtained with a Bio-
Rad FTIR spectrometer. The reflection-absorption infrared
spectroscopy measurements of the films were conducted on a
Nicolet Nexus 670 FTIR spectrometer, and their Raman spectra
were recorded by using a Nicolet Almega laser scattering Raman
spectrometer. A Bruker AM400 NMR spectrometer was used
plate with hydroxyl surface, (3) the quartz plate with expoxide surface,
and (4) the plate with pyrene surface.
Results and Discussion
Functionalization of the Quartz Plate Surface. In the early
1
980s, Sagiv and co-workers successfully assembled a highly
ordered and tightly organized monomolecular layer on glass
surface by employing long-chain alkyl trichlorosilane.18 Since
then, various organic silicon derivatives have been developed
and widely used to build monomolecular layers on glass surfaces
in order to alter its surface properties and introduce some
1
9-22
reactive groups.
To introduce an expoxide group on the
plate, GPTS was employed to react with the substrate, a quartz
plate. It is expected that the expoxide group will react with the
amino group of PSEDA, and form an imino structure within
the spacer.
1
to measure the H NMR spectra of the samples. The advancing
contact angles of the films were measured by using a JY-82
contact angle goniometer.
Table 1 shows the advancing contact angle data of the
surface and water at 18 °C. Reference to the Table, it is revealed
that the contact angle decreased significantly from 41 to 21°
after treatment with the “piranha solution”. Further treatment
of the surface with GPTS resulted in a sharp increase in the
data (from 21 to 55), indicating the surface becoming less
hydrophilic. The angle slightly increased further after treatment
with PSEDA. These results are consistent with the expectation
from the chemical composition of the surface, of which the
composition changed from a less hydroxyl group, more hydroxyl
group, expoxide group, and Py along with the treatment (cf.
Scheme 1).
The stability of Py on the plate surface was examined by
monitoring the fluorescence emissions of the systems and the
medium, within which the plate had been immersed, as a
function of time at 380 nm with 340 nm as the excitation
wavelength. It was demonstrated that the emission intensities
and emission profile of the film, the control, and the medium
hardly changed with time, suggesting clearly that leaking of
Py from the plate surface was negligible and should not affect
the follow-up measurements. Actually, the emission of the film
was stable within several days when it was kept in aqueous
phase. Moreover, 3 months, at least, preservation of the film in
a dry and clean place has little effect upon its behavior in
aqueous medium.
Synthesis of N-1-Pyrenesulfonyl-ethylenediamine (PSE-
DA). A literature method16 was employed for the preparation
of pyrenesulfonyl chloride (PSC). The coarse PSC was purified
by chromatography on silica gel with dichloromethane as eluent,
Rf ) 0.85. Anal. Calcd for PSC C16H9SO2Cl: C, 63.89; H, 3.00.
Found: C, 63.60; H, 2.99. Mp of the purified PSC is 172-174
16
1
°
C (not corrected, literature value 172 °C ). H NMR (CHCl3-
1
d): δ (ppm) 9.1 (1H), 8.8 (1H), 8.1-8.5 (7H) (detailed H NMR
-
1
spectra are given in the Supporting Information). IR (cm ):
364 (s), 1170 (s), 845 (s), 819 (m), 753 (m). PSEDA was
1
1
6
prepared by adopting a modified literature method. Here,
CHCl3 was employed as a solvent to replace CH2Cl2 in order
to increase the successive reaction temperature. The resulting
solution was kept as a PSEDA solution and used directly in the
following surface reactions.
For characterization, PSEDA was isolated from the above
solution in its salt form by acidification of the solution with
0.1 M HCl in diethyl ether. The fine white precipitate was
filtered and dried at room temperature. Anal. Calcd for the salt
of PSEDA C18H17N2SO2Cl: C, 59.92; H, 4.72; N, 7.77.
1
Found: C, 59.22; H, 4.74; N, 7.42. H NMR (DMSO-d): δ
-
1
(
3
ppm) 9.0 (1H), 8.2-8.6 (8H), 3.0 (2H), 2.7 (2H). IR (cm ):
043 (m), 1590 (m), 1497 (m), 1326 (s), 1160 (s), 851 (s), 758
(m), 716 (m).
Activation and Silanization of the Quartz Plate Surface.
Successful chemical coupling of Py on the plate surface is
further confirmed by FTIR and Raman spectroscopy studies (cf.
parts a and b of Figure 1). The ultrastrong absorption of the
A clean quartz plate (∼0.9 cm × 2.5 cm) was treated in a
piranha solution”17 (7/3, V/V, 30% H2O2/98% H2SO4) at 98
C for 1 h, then rinsed thoroughly with plenty of water, and
“
°
-1
Si-O bond near 1000 cm makes the absorptions of other
finally dried at 100 °C in a dust-free oven for 1 h. The activated
quartz plate was immersed in a warm (50 °C) toluene solution
of GPTS (0.6%, V/V), containing a trace of water, for 12 h.
The plate was washed with toluene and trichloromethane for
tens of times, respectively, to ensure that it was free of unbound
GPTS on the plate surface.
bonds much weaker, and thereby it is difficult to recognize the
modification by IR absorptions of the relative bonds, except
Si-O, of which the absorption shifted from 1011.1 to 1026.9
after surface modification with GPTS, a strong evidence of
chemical binding of the reagent to the plate surface. Further
treatment of the GPTS-modified quartz plate surface with
PSEDA, however, did not alter the peak position significantly
(less than 3 cm ). This observation is not difficult to understand
considering that the reaction occurs a number of bonds away
from the Si-O bond. Chemical coupling of Py on the plate
surface was directly confirmed by Raman spectroscopy mea-
surements. Reference Figure 1b reveals that modification of the
activated quartz plate surface with GPTS does not cause any
significant change in the profile of the Raman spectrum.
Chemical Coupling of Py on the Plate Surface. The
trialkoxysilane-treated quartz plate was macerated into a PSEDA
solution in CHCl3 (cf. synthesis of N-1-pyrenesulfonyl-ethyl-
enediamine) at 61 °C for 12 h. To remove unreacted PSEDA,
the plate was rinsed with plenty of CHCl3 and then extracted
with CH2Cl2 in a Soxhlex extractor for 5 h. The plate was further
rinsed with CH2Cl2, acetone, and water, respectively, after the
extraction.
-1

Pyrene Sensor to Dicarboxylic Acids
J. Phys. Chem. B, Vol. 108, No. 4, 2004 1209
SCHEME 1. Synthesis and Immobilization of 2-Pyrenesulfonylethylenediamine onto the Quartz Plate Surface
Reaction of PSEDA, however, with the GPTS modified quartz
plate surface produces two significant bands at 1617.9 and
430 and 490 nm (cf. the time-gate 0-2 spectrum), respectively,
are also important contributors of the TRES results, indicating
that some excimer may originate from direct excitation of the
ground-state dimers (static excimer), and some static excimers
may be distorted due to difficulties in reorientation of the
fluorophore.
-
1
-1
1
939.3 cm , respectively. The band at 1617.9 cm is just
25
within the double-bond stretching region between 1500 and 1700
-
1
cm and may be assigned to the vibrational motion of the
23
carbon-carbon stretching of the fused rings of Py. The origin
-
1
of the band at 1939.3 cm is not clear. It is probably an
overtone or a combination band.
Distorted excimer emission originating from partially over-
lapped Pys has been reported by a number of groups.2
6,28-30
It
Photophysical Behavior of Py Immobilized on the Plate
is to be noted that the position of the excimer emission of the
film in aqueous medium is around 490 nm (cf. Figure 2), a
wavelength corresponding to a perfect “sandwichlike” structure,
an indication of quite even distribution of Py on the plate
surface. This argument is also supported by steady-state and
TRES studies of the emission of the film in the dry state (Figures
4 and 3b). Compared with that of the film in the wet state, the
excimer emission in the dry state is much broader and
blueshifted. In addition, by careful examination of the emission
in the dry state, it can be noted that there is a small defect around
450 nm, an indication of coappearance of the distorted and
perfect Py excimer emissions. Focusing our attention to the
monomer emissions of the film at both wet and dry states
(Figures 2 and 4), it is observed that the ratio of the relative
intensities of the two sharp peaks (I400/I380) of the monomer
emission is greater than 1 for the film at the dry state. For the
film at the wet state, however, the ratio is less than 1. These
observations may be rationalized by considering the appear-
ance of the partially overlapped excimer, which is ironically
confirmed by TRES measurement (cf. Figure 3b). By inter-
rogation of the TRES spectra, it should not be difficult to
conclude that both excimers are main contributors for the
middle time-gate spectra (2-4 and 4-10 ns) and that the
distorted excimer is short-lived and the main contributor for
the early time-gate spectrum (0-2 ns). The dramatic appearance
of the partially overlapped Py excimer in the dry state may
be a result of the collapse of the long flexible spacer due to
lack of extension condition, such as miscible solvent. It should
not be difficult to imagine that collapse of the spacer makes
the motion and reorientation of the immobilized Py more
difficult, and thereby opportunity for formation of perfect
excimer decreases. On the basis of these discussions and
inspiration by literature studies, a model for the photophysical
processes occurring at the plate surface is proposed, and shown
in Scheme 2.
2
4
Surface. As revealed by Karpovich and Blanchard, the
translational and rotational motions of a fluorophore im-
mobilized on a substrate surface should be very different from
those of the fluorophore in free state due to restrictions from
the surrounding structures and the substrate. This difference may
result in different photophysical behavior of the fluorophore in
immobilized state to that of it in free state. Figure 2 depicts the
steady-state fluorescence excitation and emission spectra of the
functional plate in aqueous medium as functions of excitation
and emission wavelengths. Examination of the figure reveals
that the emission is composed of two sharp peaks and one broad
band. The peaks and the band may be attributed to Py monomer
emission and excimer emission, respectively. Compared with
the results reported in the literature,2
2,25,26
a perfect “sandwich-
like” structure may be adopted by the excimer due to its
emission centering around 490 nm. Furthermore, the right-hand
side of the excitation spectrum is broadened and redshifted along
with the analysis wavelength shifting to longer wavelength,
suggesting that some of the immobilized Py may exist in
associated states.2
7-28
This tentative conclusion is further
supported by time-resolved (fluorescence) emission spectroscopy
TRES) studies.
Figure 3a shows the TRES measurement results of the film
in aqueous medium with time gates being set at 0-2, 2-4, 4-6,
-8,10-30, and 120-200 ns, respectively. Time 0 corresponds
to the time at which the excitation pulse reached its maximum
intensity. Interrogation of the Py excimer formation via TRES
reveals that for the film, contribution from monomer emission
to the spectrum decreases dramatically and the maximum
emission shifts gradually to longer wavelength along with the
time-gate moves away from the excitation pulse. Domination
of monomer emission in the early time-gate spectrum and that
of excimer emission in the middle and late time-gate spectra
indicates that some immobilized Pys are in their lonely monomer
state and that the excimer is mainly dynamic in nature (dynamic
excimer).25 Careful examination of the early time-gate (0-2
ns) spectrum reveals that excimer emissions centering around
(
6
The model, even though a little bit oversimplified, is also
supported by the results from fluorescence lifetime measure-
ments. Table 2 lists the decay data of the film, at various analysis

1210 J. Phys. Chem. B, Vol. 108, No. 4, 2004
Gao et al.
Figure 2. Excitation and emission spectra of the functionalized quartz
plate in aqueous medium.
SCHEME 2. A Proposed Model Describing the
Photophysical Processes Occurring on the Functionalized
Quartz Plate Surface
TABLE 2: Four Exponential Fits to the Decay of the
Functionalized Quartz Plate in Aqueous Medium Monitored
at Various Emission Wavelengths with an Excitation
Wavelength of 353 nm
λ
em (nm)
A
1
(%)
A
2
(%)
A
3
(%)
A
4
(%)
ø2
3
4
4
5
5
80
00
50
00
50
49.2
45.6
17.4
6.1
42.0
43.4
39.8
18.5
15.2
5.8
6.7
28.1
45.4
48.1
3.0
4.3
14.8
30.0
32.1
1.49
1.62
1.49
2.49
2.98
4.6
to the decay of the film measured at 450 nm at the wet state.
Clearly, a substantial improvement in the “goodness of fitting”
2
is apparent (as evidenced by the reduction in ø ). The lifetimes
from the fit are 0.9, 3.4, 12.3, and 42.1 ns, respectively, which
have been tentatively attributed, according to the model and
the TRES and steady-state emission studies, to a quenched
excited monomer (M*), an excited monomer (M2*), a distorted
excimer (E1*), and a perfect excimer (E2*), respectively.
Reference to the Table, it can be found that in most of the cases,
the four-exponential function is inadequate for fitting the decays.
This is not surprising because only a limited number of possible
fluorophore situations have been taken into account in the
evaluation of the time-resolved kinetics. The real situation might
be much more complicated. For example, for distorted excimers,
a distribution of overlap conformations might be present.
Similarly, for isolated monomers, they could either be sur-
rounded by a hydrophobic microenvironment or exposed to a
hydrophilic microenvironment. By further examination of the
data shown in the Table, it was also found that all the pre-
exponential coefficients are positive for the decays analyzed at
wavelengths beyond 400 nm, indicating that either all but a very
small fraction of the excimers are formed by direct excitation
of the ground-state dimers or excitation of one of the monomers,
which may be surrounded by a number of Py as its neighbors,
in the aggregates. Therefore, diffusion of the fluorophore within
the lifetime of the excited state of a fluorophore and/or energy
migration to an excimer-forming site (cf. Birks’ scheme)31 may
be negligible and the rising stage in the excimer decays, which
should originate from the diffusion or migration, may be absent.
Figure 1. (a) IR and (b) Raman spectra of the quartz plates of various
surface structures.
wavelengths, resultant upon four exponential analysis of the
general form
i(t) ) A + A exp(-t/τ ) + A exp(-t/τ ) +
1
1
2
2
A exp(-t/τ ) + A exp(-t/τ ) (1)
3
3
4
4
where t is the time after a light pulse, i(t) the intensity of the
emission at time t, and τ the lifetime of the corresponding
species. As expected, the decay behavior of Py immobilized
on the plate is complex, and employment of a four-exponential
decay model, at least, in describing the decay behavior of the
film is necessary (even so, in some cases, the “goodness of
2
fitting”, ø , is still larger than 1.3, cf. Table 2). As an example,
parts a and b of Figure 5 show triple and four-exponential fits

Pyrene Sensor to Dicarboxylic Acids
J. Phys. Chem. B, Vol. 108, No. 4, 2004 1211
Figure 4. Excitation and emission spectra of the functionalized quartz
plate at dry state.
SCHEME 3. Schematic Representation of the Physical
State of Pyrene Immobilized on the Quartz Plate Surface
film. The effect of malonic acid to the excimer emission is
similar to that of ethanedioic acid before its concentration
exceeds 14 mM. The effect of succinic acid, however, is much
greater than those of either ethanedioic acid or malonic acid,
and the addition starts to affect the emission at a significant
smaller concentration (cf. Figure 6b). These observations may
be rationalized by considering that (1) the introduced dicar-
boxylic acid might insert into the space between the neighboring
spacers on the substrate surface (cf. Scheme 3),and thereby alter
the distribution of the sensing element and (2) the effect should
be chain-length dependence because the longer the chain length,
the greater the disturbing effect. Clearly, change in spatial
distribution of the sensing element must result in change in the
profile and intensity of the fluorescence emission of the film.
It might be interesting to note that both the monomer emission
and the excimer emission increase along with introduction of
dicarboxylic acids. This observation might be accounted for by
considering that insertion of a dicarboxylic acid between a
ground-state dimer would increase the number of isolated
monomers and thereby increase the monomer emission (cf.
Scheme 3). However, for the sensing molecules in the ag-
gregated state, insertion of the acid would decrease the number
of them, and deduct the “self-quenching” or “inner-filtering”
Figure 3. Time-resolved emission spectra of the functionalized quartz
plate in (a) aqueous medium and (b) at dry state. The time gates are
shown in the figure.
Similar spectra data were also observed for pyrene carboxylic
acids in Langmuir-Blodgett (LB) monolayer films consisting
of stearic acid and small amounts of 16-(1-pyrenyl) hexade-
canoic acid3 and for pyrenyl carboxylic acids and anthracene
carboxylic acids in mixed LB films of ω-arylalkanoic acids and
fatty acids.3
2,33
4,35
3
6-38
Sensing Properties to Dicarboxylic Acids. As expected, the
fluorescence emission of the film is sensitive to the presence
of dicarboxylic acids, such as ethanedioic acid, malonic acid,
and succinic acid. The intensity of the monomer emission (380
nm) increases along with increasing the dicarboxylic acid
concentration (cf. Figure 6a). A similar phenomenon was also
observed for the excimer emission (480 nm) (cf. Figure 6b).
Upon further interrogation of the figures, it is revealed that,
unlike the effect to monomer emission, introduction of ethane-
dioic acid had little effect upon the excimer emission of the
effect, thereby enhancing excimer emission.
The explana-
tion is further supported by the results from studies of the effect
of monocarboxylic acids on the emission behavior of the film.
Reference to parts a and b of Figure 6, it can be observed
that introduction of formic acid or acetic acid had little effect
upon the emission behavior (both the profile and intensity of
the emission) of the film, indicating that alteration to the
emission behavior of the film caused by addition of dicarboxylic
acids may not be results of pH variation but results of alteration
in surface distribution of the sensing element.

1212 J. Phys. Chem. B, Vol. 108, No. 4, 2004
Gao et al.
Figure 5. (a) Triple exponential and (b) four exponential fits to the fluorescence decay of the functionalized quartz plate in aqueous medium
monitored at an emission wavelength of 450 nm with an excitation wavelength of 353 nm.
Figure 6. Plots of the (a) monomer (380 nm) and (b) excimer (480 nm) emission intensities of the functionalized quartz plate in aqueous medium
as functions of acid concentrations (excitation wavelength 353 nm).
It was also found that the time needed for establishment of
a new equilibrium after each addition of the acids is the chain
length of an acid dependence. In fact, the longer the chain length
of the acid, the more time is needed. For formic acid, acetic
acid, ethanedioic acid, malonic acid, and succinic acid, the times
are <1, <1, 80, 210, and 540 min, respectively, in support of
the statement that dicarboxylic acids function via insertion into
the space of the neighboring spacers.
It might be worth well to mention that response of the film
to dicarboxylic acid is reversible because both the monomer
emission and the excimer emission of the film could be fully
restored by washing the film with pure water after each
measurement. The sensitive response, the stability, and the
reversibility of the film may guarantee it as potential sensing
materials for common dicarboxylic acids.
the excimer emission are sensitized by introduction of some
dicarboxylic acids. This phenomenon has been attributed to the
insertion of dicarboxylic acids into the space between the
neighboring spacers. In contrast, introduction of monocarboxylic
acids, such as formic acid and acetic acid, has little effect upon
the emission of the functional film. In addition, response of the
film to dicarboxylic acids is reversible, and the film is stable
within a number of months if it is properly preserved. These
characteristics guarantee the film as a potential sensing material
for common dicarboxylic acids.
Acknowledgment. We thank the National Nature Science
Foundation of China (NNSFC, 20173035 and 20373039), the
Key Research Program of the Ministry of Education of China
(03148), and the Natural Science Foundation of Shaanxi
Province (2002B11) for financial support of this work.
Conclusions
Supporting Information Available: 1H NMR spectra of
PSC and PSEDA. This material is available free of charge via
the Internet at http://pubs.acs.org.
Steady-state and time-resolved fluorescence studies demon-
strated that the emission of the Py functionalized film in both
dry and wet state are characterized by the monomer emission
and the excimer emission of the fluorophore. However, the
excimer emission is complicated due to presence of both
References and Notes
(1) Wei, T. X.; Huang, C. H.; Xie, P. H.; Hou, Y. J.; Zhang, B. W.;
“
sandwichlike” structures and partially overlapped structures.
Liu, F. Q.; Ibrahim, K.; Qian, H. J. Phys. Chem. Chem. Phys. 2000, 2,
1333.
Fluorescence lifetime measurement and TRES studies reveal
that the excimer may be formed mainly by direct excitation of
the ground-state dimers or excitation of one of the monomers
in the aggregates. Moreover, both the monomer emission and
(
2) Taton, T. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am.
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