A new polymerizable fluorescent PET chemosensor of fluoride (F-) based on naphthalimide-thiourea dye

Article abstract of DOI:10.1016/j.saa.2012.01.008

A novel N-allyl-4-amino-substituted 1,8-naphthalimide dye, containing thiourea functional group with intense yellow-green fluorescence was successfully synthesized. Copolymerization was done with styrene. The photophysical characteristics of dye and its copolymer in solution and solid film were investigated in the presence of halide ions. The results reveal that the fluorescence emissions of the monomer dye and also its polymer were 'switched off' in the presence of fluoride ions. The dye showed spectral shifts and intensity changes in the presence of more fluoride ions which lead to detect certain fluoride concentrations of 10-150 mM at visible wavelengths. By adding the fluoride ions, green-yellow to purple color changes occurs and the green fluorescence emission quenches, all of which easily observed by naked eyes. These phenomena are essential for producing a dual responsive chemosensor for fluoride ions. The polymeric sensor, in the film state exhibited a fast response to the fluoride ions.

Full text of DOI:10.1016/j.saa.2012.01.008

Spectrochimica Acta Part A 90 (2012) 85–92
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new polymerizable fluorescent PET chemosensor of fluoride (F−) based on
naphthalimide–thiourea dye
Parvaneh Alaeia, Shohre Rouhania,∗, Kamaladin Gharanjiga, Jahanbakhsh Ghasemib
a Institute for Color Science and Technology, Department of Organic Colorants, Tehran, Iran
b Chemistry Department, Faculty of Sciences, K. N. Toosi University of Technology, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel N-allyl-4-amino-substituted 1,8-naphthalimide dye, containing thiourea functional group with
intense yellow-green fluorescence was successfully synthesized. Copolymerization was done with
styrene. The photophysical characteristics of dye and its copolymer in solution and solid film were inves-
tigated in the presence of halide ions. The results reveal that the fluorescence emissions of the monomer
dye and also its polymer were ‘switched off’ in the presence of fluoride ions. The dye showed spectral
shifts and intensity changes in the presence of more fluoride ions which lead to detect certain fluoride
concentrations of 10–150 mM at visible wavelengths. By adding the fluoride ions, green-yellow to pur-
ple color changes occurs and the green fluorescence emission quenches, all of which easily observed by
naked eyes. These phenomena are essential for producing a dual responsive chemosensor for fluoride
ions. The polymeric sensor, in the film state exhibited a fast response to the fluoride ions.
© 2012 Elsevier B.V. All rights reserved.
Received 4 October 2011
Received in revised form
30 November 2011
Accepted 2 January 2012
Keywords:
Synthesis
1,8-Naphthalimide
Photophysical
Fluorescence quenching
PET sensor
Fluoride anions
1. Introduction
hydrogen-bond donors. The majority of the reported fluoride sen-
sors are based on colorimetric changes or fluorescent quenching
To date, many hundreds of naphthalimide derivatives have
been synthesized and found wide applications as fluorescent
sensors. Because of the easily modified N-head group and the
4-substituent and their well-studied emission properties, 1,8-
naphthalimides have found applications in many different areas
of physical and biomedical studies [1]. In recent years, more
attention has been paid to the design, synthesis and applications
of novel fluorescent polymer materials [2,3]. It has been shown
that photoinduced electron transfer (PET) proceeding in the 1,8-
naphthalimide also takes place in the solid polymer films of its
copolymer with some conventional commercial monomers [4,5].
Hence, these functionalized copolymers can also act as sensors
for transition metal ions or protons [6–9]. Considerable efforts
have been made to design chemosensors for anions. The reason
for this huge interest is the importance of detection and quantifi-
cation of anions in disciplines such as biology and environmental
science. Among the interests in biologically functional anions, flu-
oride is of particular importance owing to its established role in
dental care and treatment of osteoporosis. As the smallest and the
most electronegative atom, fluoride has unique chemical proper-
ties and can form the strongest hydrogen-bond interaction with
[10].
The most important advantage of functionalized copolymer as
a sensor is firmly bonded to the main chain which eliminates any
migrations of chromoionophors from the matrix. Copolymers of
1,8-naphthalimide with vinyl monomers have been obtained using
free radical mass polymerization [11], cation solution polymer-
ization [12] and condensation polymerization [13]. To the best of
our knowledge no report is available on polymerizable base sensor
for F− ions. The chemosensors employ the photoinduced electron
transfer process that is quenched upon cation complexation or
anion bonding and the fluorescence is switched on, i.e. this is an
‘off–on’ sensor type. If the complexation with the receptor unit
is somehow perturbed, the interaction can take place at another
chelating site and may result in fluorescence quenching, i.e. the
sensor is of an ‘on–off’ type [14].
In this paper, the design and synthesis of new polymeriz-
able 1,8-naphthalimide dye containing phenyl thiourea functional
group was reported. The ability of dye to copolymerize with vinyl
monomers of styrene was examined by free radical copolymeriza-
tion. The obtained dye and copolymer were characterized by FTIR,
1H NMR, 13C NMR, elemental analysis, DSC, GPC, UV–visible and
fluorescence spectroscopic techniques. The photophysical charac-
teristics of synthesized dye in various solvents and a transparent
copolymer in solid film were recorded. The absorption and fluo-
rescent spectra of DMF solution of the dye and the transparent
∗
Corresponding author. Tel.: +98 21 22944184.
E-mail addresses: rouhani s@yahoo.com, rouhani@icrc.ac.ir (S. Rouhani).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.01.008

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P. Alaei et al. / Spectrochimica Acta Part A 90 (2012) 85–92
Scheme 1. Synthesis of dye: (a) NBS, DMF, room temperature, 75%; (b) Na2Cr2O7, AcOH, reflux 5 h, 80%; (c) allylamine, EtOH, reflux 4 h, 92%; (d) ethylenediamine, DMF,
reflux 5 h, 60.2%; (e) isothiocyanate, CHCl3, 75%.
copolymer in solid film were investigated in the presence of halide
ions.
6 DSC under the nitrogen atmosphere at a heating rate of
10 ◦C min−1
.
2.3. Dye synthesis
2. Experimental
2.3.1. The intermediate synthesis
2.1. Materials
Synthesis of the aforementioned polymerizable dye 6 is shown
in Scheme 1. The intermediate, compound 3, was synthesized from
acenaphthene, according to the literature [15,16]. Compound 4 was
prepared at 92% yield by the reaction of 3 with allylamine and it was
subsequently converted into compound 5 by refluxing the solution
of ethylenediamine and 4 in DMF [5].
All solvents were of analytical grade. Acenaphthene, allyamine,
ethylenediamine, phenyl isothiocyanate were purchased from
Merck Chemical Co. and used as received. After purifying by distil-
lation under vacuum the resulting styrene (St) was used. Dibenzoyl
peroxide (DBP) was also used as the initiator for free radical copo-
lymerization.
2.3.2. Synthesis of 1-(2-{[1,3-dioxo-2-(prop-2-en-1-yl)-2,
2.2. Analysis
3-dihydro-1H-benzo [de]isoquinolin-6yl] amino}
ethyl)-3-phenylurea (6)
UV–visible spectrophotometric measurements of the dye and
styrene copolymer (St-co-dye) in DMF solution were performed
on a CECIL CE9200 spectrophotometer. The fluorescence spectral
of the dye, the copolymer in DMF solution and as a thin film, was
measured on an Osean optic usb2000flg spectrophotometer. Fluo-
rescence quantum yield was determined based on the absorption
and fluorescence spectra, using fluorescein as reference (˚st = 0.95)
[14]. For all the absorption and fluorescent measurements, the dye
concentration in DMF was 5 × 10−5 mol l−1. The concentration of
polymer in solution for all measurements was 5 g L−1. A thin poly-
meric film, used for the fluorescent investigations, was obtained
from depositing a 10% solution of copolymer in chloroform on
a glass surface. The thickness of the copolymer film was 35 ␮m.
FTIR spectra were recorded on a SPECTRUM ONE spectrometer
using KBr pellets. 1H NMR and 13C NMR spectra were recorded
with Brucker DRX AVANCE NMR spectrophotometer at 500 MHz
and 125 MHz in DMSO, respectively. Molecular weights and poly-
dispersity index of synthesized polymer were determined by Gel
Permeation Chromatography (GPC) with a Atilent 1100 RI (reflec-
tometer index) PLGell 10 ␮m, 300 mm (500 A, l03 A, 10 A in a series)
relative to polystyrene standards in chloroform solution. Differen-
tial scanning calorimetric (DSC) and glass transition temperatures
(Tg) of the copolymer was performed using Perkin-Elmer-pyris
This dye was synthesized by treating 5 (0.27 mmol) with phenyl
isothiocyanate (0.27 mmol) in chloroform and stirring the reaction
mixture at room temperature for 5 days under nitrogen. The pre-
cipitate was collected by vacuum filtration and washed off with
chloroform to obtain the desired product as a yellow-orange solid
(65%) after recrystalization in ethanol [17,18]. mp. 224.22 ◦C. 1H
NMR (500 MHz, DMSO-d6): 9.69 (1H, s, PhNH), 8.69 (1H, d, J = 8.3 Hz,
naphthalimide, H-5), 8.43 (1H, d, J = 7.1 Hz, naphthalimide, H-7),
8.25 (1H, d, J = 8.5 Hz, naphthalimide, H-2), 7.94 (1H, b s, NHCS),
7.90 (1H, b s, NH.), 7.70 (1H, t, J = 7.5 Hz, naphthalimide, H-6),
7.12–7.30 (5H, C6H5), 6.96 (1H, d, J = 8.5 Hz, naphthalimide, H-3),
5.9 (1H, m, CH ), 5.07 (2H, CH2), 4.61 (2H, d, J = 3.9 Hz, N CH2),
3.85 (2H, CH2), 3.61 (2H, CH2). 13C NMR (125 MHz, DMSO-d6): 181.4
(C S), 163.4 (C O), 163.0 (C O), 152.0 (naphthalimide, C-4), 135.1
(benzene, C-1), 134.1 (naphthalimide, C-2,7), 131.6 (CH ), 129.6
(benzene, C-3ꢀ,5ꢀ), 129.5 (benzene, C-4ꢀ), 125.4 (benzene, C-2ꢀ),
125.2 (benzene, C-6ꢀ), 124.4 (naphthalimide, C-9), 122.6 (naph-
thalimide, C-8), 121.1 (naphthalimide, C-10), 116.8 ( CH2), 104.9
(naphthalimide, C-3), 43.0, 42.1 (CH2 aliphatic); FTIR (KBr): ␯ 3357,
3234.2 cm−1 (N H str. three secondary amine), 3179 cm−1 (C
H
str. aromatic), 2928 cm−1 (C H str. aliphatic), 1684 cm−1 (C C str.
allyl), 1669, 1643 cm−1 (C O str. carbonyl), 1586 cm−1 (C C str.
aromatic), 1242 cm−1 (C S str.). Anal. Calcd (%) for C24H22N4O2S

P. Alaei et al. / Spectrochimica Acta Part A 90 (2012) 85–92
87
(MW 430.52): C, 66.95; H, 5.15; N, 13.01; found: C, 67.02; H, 5.5; N,
12.5.
2.4. Copolymerization
Free radical copolymerization of styrene with the produced dye
was carried out in bulk [19–23]. The polymerization in styrene was
carried out in an ampoule flushed with pure dry nitrogen, in the
presence of 0.1 wt.% of dye in styrene and 0.5 wt.% of DBP within the
monomer mixture. The ampoule was sealed and heated at 80 ◦C for
24 h. The obtained transparent colored polymers were dissolved in
chloroform and precipitated in ethanol four times until the filtrate
became colorless. In the last step the polymer was purified and
desiccated under vacuum at 40 ◦C to reach a constant weight.
Poly(St-co-dye); yield: 55%, Tg: 83.29 ◦C, FTIR (KBr): ␯ 3451 cm−1
(N H str. secondary amine), 3067, 3034 cm−1 (C H str. aromatic),
2921 cm−1 (C H str. aliphatic), 1614, 1634 cm−1 (C O str. car-
bonyl), 1604, 1483 cm−1 (C C str. aromatic), 1018 cm−1 (C N str.
amine).
Scheme 2. Schematic presentation of poly(St-co-dye).
Table 1
Results of polymerization and characterization.
Polymer
Mn × 10−5 (g/mol)
Mw × 10−5 (g/mol)
Mw/Mn
3.42
3. Results and discussion
poly(St-co-dye)
0.97684
3.3461
3.1. Synthesis and characterization of dye
Mn, number average molecular weight; Mw, weight average molecular weight.
The synthesis of the desired dye is shown in Scheme 1. The
dye made from readily available acenaphthene as a starting mate-
rial. The precursor was 6, made in four steps at 60% yield form
4-bromo-1,8-naphthalic anhydride 3; then reacted with allylamine
in ethanol and nucleophilic displacement was done using an
excess of ethylenediamine (neat), followed by recrystallization in
ethanol. Consequently phenyl isothiocyanate reacted with 4-(2-
aminoethylamino)-1,8-naphthalimide 5 at room temperature in
dry CHCl3 to give the dye at 65% yield.
weights (Mw and Mn) and the molecular weight distribution,
Mw/Mn, confirm the formation of a high molecular weight polymer.
3.2.2. Characterization of the polymer produced
The difference between dye and its copolymer was determined
by FTIR spectroscopy. FT-IR spectra of poly(St-co-dye), monomer
dye and pure styrene are shown in Fig. 1. Because of the low
percentage of dye in the polymer (0.1 wt.%), complete overlapping
of the spectral bands was observed. Comparison between the
spectra (a) and (b) shows a considerable difference in the spectral
The dye was characterized by FTIR, 1H NMR, 13C NMR,
elemental analysis and UV–visible spectroscopy. The results
recorded by FTIR spectra of the dye shows the strong imid-
groups vibration about 1600 cm−1; the vibration of allyl-group
and amino-groups appeared at around 1680–1690 cm−1 and
3100–3300 cm−1,respectively. The 1H NMR spectra in DMSO-d6 of
the dye display the existence of hydrogen around naphthalimide
and phenyl rings, allyl group and amino-hydrogen, as a result of
nucleophilic displacement. The dye showed the presence of two
thiourea protons at 9.69 ppm, whereas the amino proton appeared
as a broad singlet peak at 7.94 ppm.
range of 720–790 cm−1, 1600–1700 cm−1 and 3300–3500 cm−1
.
The C H out-of-plane bending vibrations of the dye are observed
in spectrum (c) at 772 and 730 cm−1. In spectrum (a) the rocking
vibrations (CH2)n of pure styrene appear as doublet peaks in the
solid state at 697 and 766 cm−1. Spectrum (b) indicates the over-
lapping of these two types of vibration in the range studied. The
stretching vibration of C C of the dye is seen at 1684 cm−1. Stretch-
ing vibration of C C bond of pure styrene is observed at 1683 cm−1
.
3.2. Polymerization investigations
Polystyrene is a widely used polymer that attributes to many
applications. Experiments for the copolymerization of styrene with
dye was carried out in bulk with 0.1 wt.% of dye and 0.5 wt.%
DBP, according to a procedure described elsewhere [19–23]. After
the copolymerization of styrene with dye, a solid and transparent
yellow polymer with an intense yellow-green fluorescence was
obtained. The polymer was purified after successive and selec-
tive dissolving and precipitating with properly chosen solvents of
chloroform and ethanol. The colored copolymer was isolated from
ethanol .It is a good solvent for the monomer dye but not for its
polymer. The copolymer’s retained color and fluorescence, indi-
cates that the compound was chemically bonded to the polymer
chain (Scheme 2).
3.2.1. Molecular weight estimation
The molecular weight characteristics of the poly(St-co-dye)
have been determined using GPC, listed in Table 1. The molecular
Fig. 1. FT-IR spectra of (a) pure styrene, (b) poly(St-co-dye) and (c) dye.

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P. Alaei et al. / Spectrochimica Acta Part A 90 (2012) 85–92
solvents with different dielectric constants were used to investigate
this effect on the photophysical properties of the dye.
The absorption spectra of the dye demonstrated a structured
naphthalimide band with two peaks at 281 nm and a broad one
at 440 nm. Table 2 presents the spectral characteristics of the
monomeric 1,8-naphthalimide dye and its polymer in DMF solu-
tion and as a thin film; the absorption (ꢀA) and fluorescence (ꢀF)
maxima, the extinction coefficient (log ε), Stokes shift (ꢁA–ꢁB),
oscillator strength (f), quantum yield of fluorescence (˚F) and
energy yield of fluorescence (EF). Fig. 3 shows the excitation and
fluorescence spectra of the dye in DMF solutions at 10−5 M and
solid poly(St-co-dye) in DMF and as a thin film. The dye exhibits
a yellow-green color with absorption maxima at ꢀA = 440 nm.
The long wavelength band for the dye has an extinction coef-
ficient of 18,458 l mol−1 cm−1. The dyes’ molar absorption (ε) in
the longest wavelength band of the absorption spectra is higher
than 10,000 l mol−1 cm−1 (Table 2), indicating that this is a charge
transfer (CT) band, due to (␲ → ␲*) character of the S0 → S1 tran-
sition [26]. In DMF solution, the compound under study displayed
yellow-green fluorescence due to the charge transfer in the 1,8-
naphthalimide moieties from the electron donating group at C-4
position to the carbonyl groups. The dye emission was observed in
the visible region with well-pronounced maxima (ꢀF) at 525 nm.
Viewing the application of the poly(St-co-dye) as a heteroge-
neous sensor, its spectral characteristics have been studied in DMF
solution and the solid state in the form of a thin polymeric film
(35 ␮m) (Fig. 3). The poly(St-co-dye) has an absorption maximum
at 438 and 376 nm and emits fluorescence at 523 and 505 nm
(Table 2). The absorption spectra have broad bands in the visible
region. The copolymer is yellow and shows intense green fluores-
cence in DMF and solid state. The maxima in thin polymeric film are
hypsochromically shifted in comparison to that of the dye taken in
DMF solution. The shift is probably caused by the firmly fixed chro-
mophore naphthalimide system in the solid state. Thus, there is a
minimal possibility for conformation changes [27].
Fig. 2. UV–vis spectra of dye and colored poly(St-co-dye) in chloroform solution.
Spectrum (b) indicates that these two types of vibrations disappear
in the studied range. Also the bands at 3357 cm−1 and 3451 cm−1
are related to the stretching vibration of N H in the dye and the
poly(St-co-dye). Compared with pure styrene, characteristic peaks
at 3357 cm−1 and 3451 cm−1 of N H in pure styrene disappeared.
3.2.3. Providing an evidence for a chemical bond between
copolymer and dye
The presence of a covalent bond between the monomer dye
units and the main polymer chain has been proved by UV–visible,
fluorescence spectroscopy and by Gel Permeation Chromatogra-
phy (GPC) [24]. The amount of dye incorporated into the polymer
molecules determined using UV–visible spectroscopy. UV–Visible
absorption spectra of the precipitated polymer in chloroform solu-
tion were compared to its monomer dye. Likewise, dye (1 g) and
colored polymer (1 g) were dissolved in chloroform (25 mL). UV–vis
spectra were taken and depending on the relation of the absorp-
tion of the colored polymer to that of its dye, the percent content
of the covalently bound was calculated. Fig. 2 demonstrates the
UV–visible spectra of dye and colored poly(St-co-dye). In these
spectra, no change in ꢀmax was registered. This was an indica-
tion that there were no changes in the basic chromophore dye
either during the polymerization or as a result of its bonding to
the polymer chain. For this reason the free chromophore and the
copolymer with styrene possesses identical ꢀmax. This makes it
possible to determine the percent content of the covalently bound
chromophore [24]. The experimental data concerning quantity of
the dye, chemically bound to the polymer, is 75%. It can be clearly
seen over 75% of the initial dye that to the polymer chain are
attached which means that they are fully consumed and incorpo-
rated into the styrene chain during the polymerization process.
As seen in Fig. 3, fluorescence curve is approximately a specular
image of the absorption one in DMF solution. This indicates preser-
vation of the fluorophore molecular structure in the exited state.
The overlap between the absorption and the fluorescence bands is
small and an aggregation effect at the concentration of 10−5 mol l−1
has not been observed [24].
An important characteristic of dyes is the oscillator strength (f)
which can be calculated using Eq. (1), where ꢂꢁ1/2 is the width of
the absorption band (in cm−1) at 1/2 εmax [28].
f = 4.32 × 10−9 ꢂꢃ1/2εmax
(1)
The values obtained for the dye and its polymer 0.304 and 0.341
in DMF and the poly(St-co-dye), in thin film, is 0.166.
The Stokes shift values are 3663 and 4283 cm−1 for the dye and
its polymer in DMF, and is 6793 cm−1 for the poly(St-co-dye) in
solid thin film. The value of Stokes shift for poly(St-co-dye) is higher
than its monomer. This means that the conformation change in
the polymer macromolecules is more pronounced, owing to the
presence of styrene units and their influence on the chromophoric
1,8-naphthalimide structure [25,29].
3.3. Spectroscopic properties
The basic spectral characteristics of 1,8-naphthalimides depend
on the polarization of naphthalimide molecules, due to the electron
donor–acceptor interaction occurring between the substituent at
position C-4 and the carbonyl groups from the imide moiety of the
chromophoric system [25]. This interaction strongly depends on
the medium, especially, on the polarity of organic solvents. Organic
The fluorescence quantum yield (˚F) was determined on the
basis of the absorption and fluorescence spectra of the dye using
Table 2
Photophysical charactristic of dye in DMF solution at 10−5 M and poly(St-co-dye) in DMF and as a thin film.
a
Compounds
ꢀA (nm)
ꢀF (nm)
Log ε (l mol−1 cm−1
)
ꢁA–ꢁB (cm−1
)
ꢂꢁ1/2 (cm−1
)
f
˚
F
EF
Dye
440
438
376
525
523
505
4.26
–
–
3663
3711
6793
3813
4283
2090
0.304
0.341
0.166
0.365
–
–
0.306
–
–
Poly(St-co-dye) in DMF
Poly(St-co-dye) in thin film
a
EF, energy yield of fluorescence [32].

P. Alaei et al. / Spectrochimica Acta Part A 90 (2012) 85–92
89
Fig. 3. Excitation (absorbance) and fluorescence emission spectra of (a): the dye in DMF solution; (b) poly(St-co-dye) as a thin film and (c) poly(St-co-dye) in DMF solution.
fluorescein as a reference ˚F = 0.95 [14,29–31]. The fluorescence
quantum yield is determined on the basis of the absorption and
fluorescence spectra of the dyes taken in DMF. As it can be seen
from the data in Table 2, the 1,8-naphthalimide dye has quantum
yield values of ˚st = 0.365 in DMF.
solution and ˚st = 0.156 in methanol solution. The results obtained
could be explained by the formation of hydrogen bonding effect,
photo-induced electron transfer (PET) processes in more polar sol-
vents [33].
Also in all organic solvents under study, the monomeric
1,8-naphthalimide displayed yellow-green color and intense fluo-
rescence. The dye has yellow-green color with absorption maxima
at 423–443 nm and fluorescence emission maxima at 509–541 nm
in all the organic solvents at room temperature.
As seen from the data collected in Table 3 and the plot in Fig. 4
the solvent dielectric constant (ε) does not significantly influence
the position of the absorption maxima, while in more polar sol-
vents the fluorescence maxima are bathochromically shifted. As
the dipole moment of the molecule is enhanced upon excitation
due to electron density redistribution, the excited molecule is bet-
ter stabilized in polar solvents because of the stronger interactions
with the solvent dipoles. This effect causes the red shift in the flu-
orescence maxima [27]. For the calculated Stokes shift values one
observes that in the less polar medium, the value of Stokes shift
was lower than that obtained in the more polar medium (Table 3).
As seen from the data in Table 3, the monomer dye has
quantum yield values between ˚st = 0.532 in carbon tetrachloride
3.4. Photophysical characteristics of the dye in the presence of
different halide ions
The photophysical properties of the dye as a receptor in the pres-
ence of different halide ions (F−, Cl−, Br− and I−) were investigated
in DMF solution in respect of its potential sensor application. Prior
to the assessment, the sensor ability of the monomer bonded to a
polystyrene chain was determined.
3.4.1. Colorimetric investigations
The interactions of the dye with various halide ions were
first investigated by UV–visible titration spectra. Observable color
changes took place in DMF solution. Upon addition of 15 equiv. of
fluoride ions, the yellow-green solutions of the dye became yellow-
orange in DMF. No color changes of the receptors in DMF were
observed in the presence of chloride, bromide and iodide anions.
First the interactions of the dye with various halide ions were
investigated by UV–vis titration spectra. The halide ions (F−, Cl−,
Br− and I−) were added as tetrabutylammonium salts to 5 × 10−5 M
solutions of the dye. Fig. 5 demonstrated the absorption spectral
changes of the dye after addition of increasing amount of F−.
The absorption spectrum of the dye shows four transitions
in DMF. It has a strong absorption band centered at 438 nm
(log ε = 4.26 l mol−1 cm−1) due to its ICT character and a second
band at 280 nm (n → ␲*). Upon the addition of halide ions, only F−
gave rise to changes in these absorption bands but no detectable
spectral changes were observed even in the presence of larger
excess of hundred equivalents of the corresponding anions such as
Fig. 4. Absorption and fluorescence maxima of the monomeric 1,8-naphthalimide
dye as a function of the solvent polarity parameter ET (30): (1) carbon tetrachloride,
(2) chloroform, (3) ethyl acetate, (4) dichloromethane, (5) acetone, (6) ethanol, (7)
methanol, (8) DMF, (9) acetonitrile and (10) DMSO.

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P. Alaei et al. / Spectrochimica Acta Part A 90 (2012) 85–92
Table 3
Photophysical properties of dye in organic solvents of different polarity.
Solvents
Dielectric constant (ε)
ꢀA (nm)
ꢀF (nm)
ꢁA–ꢁF (cm−1
)
˚
F
Carbon tetrachloride
Chloroform
Ethyl acetate
Dichloromethane
Acetone
Ethanol
Methanol
DMF
Acetonitrile
DMSO
2.24
4.81
6.02
8.93
20.7
24.5
32.7
36.7
37.5
46.7
423.8
431.6
428.9
430.7
431.3
441.5
440.3
438.2
432.2
443.3
512.3
509.4
513.0
511.9
516.6
533.5
541.5
525.3
524.9
533.9
4076.2
3538.6
3833.9
3682.9
3828.4
3905.9
4244.5
3783.9
4086.2
3828.0
0.532
0.420
0.272
0.479
0.391
0.235
0.156
0.363
0.433
0.331
Fig. 5. The UV–vis titration spectra of dye (5 × 10−5 M in DMF) after addition of increasing amount of F−. The right is the nonlinear curve fitting of absorbance at 438 nm as a
function of equivalent of F−
.
Cl−, Br− and I−, which made it clear that the dye could sense F− over
other halide ions. This observation could be attributed to the small
size and higher electronegativity of the F− compared to the other
halide ions. Upon titration of the dye with F−, the absorptions at 438
and 280 nm were substantially reduced with the formation of new
bands at 537 and 342 nm and concomitant formation of three isos-
bestic points at ca. 501, 380 and 288 nm. Although these isosbestic
points were very clear at lower concentrations, at higher concentra-
tions some broadenings was observed (possibly due to aggregation)
[34]. The changes in the absorption spectra of the dye upon titrat-
ing with F−, shown in Fig. 5, clearly demonstrate that by formation
of a clear isobestic point at 501 nm, the ground state is somewhat
affected upon recognition of the anion where the absorption was
shifted to longer wavelengths. Furthermore, some changes were
observed at shorter wavelengths. The concurrent changes of these
absorption peaks implied that 4-amino NH of naphthalimide and
thiourea N H were synchronously involved in F− binding and the
deprotonation of the amino moiety by F− rather than hydrogen
bonds [35]. This causes a significant increase in the charge den-
sity on the amino nitrogen with associated enhancement in the
push–pull character of the ICT transition, bathochromically shifting
the absorption. As demonstrated above, the absorption at 438 nm
was almost ‘switched off’ upon gradual addition of F− and the
537 nm absorption was ‘switched on’ [36].
seen along with the appearance of new absorption peaks at 342 nm
and 537 nm, as shown in Figs. 5 and 6.
The recognition behavior of the dye toward halide (F−, Cl−, Br−
and I−) ions was also investigated by fluorescence spectrophoto-
metric methods in DMF solution. In contrast to the changes in the
ground state (UV–visible spectra), the fluorescence of the dye was
dramatically affected upon titration of F−. The halide ions (F−, Cl−,
Br− and I−) were added as tetrabutylammonium salts to 5 × 10−5 M
solution of the dye. A dramatic quenching of fluorescence intensity
in the presence of the guest halide ions was only observed in the
case of F−. The influence of the halide ions on fluorescence quench-
ing is presented in Fig. 7. Upon addition of F−, the emission of the
Addition of some protic solvent (such as methanol) to the
titrated solution could restore the color of the solution to greenish
yellow, indicating that the protic solvent destroyed the complexa-
tion between dye and F−. Confirming that the interaction between
dye and F− was hydrogen bond in essence. Excess of F− often caused
the deprotonation of 4-amino NH of naphthalimide fragment, lead-
ing to drastic color alteration and great spectral changes. When
80 equiv. of F− was added to the dye, an orange red solution was
Fig. 6. The color change of dye 1 (5.0 × 10−5 M in DMF) after addition of 80 equiv.
of halide ions. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)

P. Alaei et al. / Spectrochimica Acta Part A 90 (2012) 85–92
91
However, after adding the F− the formation of the anion-receptor
hydrogen bonding complex occurred and the reduction potential
of the receptor is increased, making the electron transfer more
feasible. This subsequently gives rise to enhanced fluorescence
quenching. This is clearly demonstrated in Fig. 8, showing the addi-
tions of F− to the dye in DMF. Here the fluorescence emission of
the dye is effectively quenched or completely ‘switched off’ after
the addition of 40 equivalents of F−. The addition of methanol or
water to this solution ‘re-switched on’ the emission, demonstrat-
ing that the process was fully reversible, i.e. the hydrogen bonding
interactions were broken [35].
3.5. Photophysical characteristics of poly(St-co-dye) in the
presence of different halide ions
Fig. 7. Changes in the fluorescence emission of dye in the presence of different
halide ions.
To obtain a heterogeneous fluorescent sensor, the effects of dif-
ferent halide ions on the fluorescent intensity of a poly(St-co-dye),
its thin film (35 ␮m thick), was investigated. The solid polymer
was placed in an aqueous solution comprising tetrabutylammo-
nium salts of halides at different concentrations. The effect of
the halide ions on the fluorescence of poly(St-co-dye) is signaled
by the fluorescence intensity. The rigid polymer structure of the
poly(St-co-dye) film perturbs the diffusion of halide ions and their
contact with the fluorophore receptors. Therefore poly(St-co-dye)
film needs a longer time to exhibit its sensor abilities than homo-
geneous sensors do. Its fluorescence emission in the presence of
halide ions has been investigated for a period of 7 min. As seen from
Fig. 9, the poly(St-co-dye) film reacts only to the presence of F−. The
dye was almost fully quenched, with no noticeable changes in ꢀmax
for the additions of 0–40 equiv. of the F−, as shown in Fig. 8.
The mechanism for this quenching is via PET, which takes place
from the thiourea receptor to the excited state of the naphthalim-
ide fluorophore upon anion recognition. Unlike many PET sensors
for cations, the fluorescence of the dye is ‘switched off’ rather than
‘switched on’ upon ion recognition. We propose that this quenching
process is due to the following; prior to the recognition process, the
excited state of the fluorophore is not, or is only to a minor extent,
quenched by electron transfer from the receptor to the fluorophore.
Fig. 8. Changes in the fluorescence emission spectrum of dye upon titration with F−. The right panel shows the change in the fluorescence intensity as a function of the
equivalents of F− ions added.
Fig. 9. Fluorescence spectra of poly(St-co-dye) taken in aqueous solution at different concentration of F−. The right panel is the nonlinear curve fitting of fluorescence at
505 nm as a function of equivalent of F−
.

92
P. Alaei et al. / Spectrochimica Acta Part A 90 (2012) 85–92
fluorescent spectra of the DMF solution of the dye and the trans-
parent copolymer in solid film were investigated in the presence
of halide ions. The measurements of sensing behavior to various
halide ions, that is F−, Cl−, Br−, and I−, reveal that the monomer
and the polymer were fluorescent chemosensors for fluoride ion.
The fluorescence emission of these sensors was ‘switched off’, with
significant changes in the UV–visible spectra. According to the
present investigations it can be assumed that the monomer 1,8-
naphthalimide dye is suitable for obtaining fluorescent polymer
sensors for F−. Investigations on their sensor characteristics are the
object of our future work.
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4. Conclusion
In this work, we described the synthesis and characterization
of a new polymerizable fluorescent monomer based on N-allyl-
1,8-naphthalimide containing thiourea functional group at a good
yield. The photophysical characteristics of a polymerizable 1,8-
naphthalimide dye in DMF solution and transparent copolymer
in DMF and solid film were investigated. The absorption and