Synthesis and spectrophotometric PH sensing applications of poly-2-[4-(diethylaminophenyl)imino]-5-nitro-phenol and its Schiff base monomer for two different PH ranges

Article abstract of DOI:10.1007/s10895-011-0983-3

Poly-2-[4-(diethylaminophenyl)imino]-5-nitrophenol (PEAPINP) and 2-[4-(diethylaminophenyl)imino]-5-nitro-phenol (EAPINP) were prepared as new pH sensors. Spectrophotometric and spectrofluorometric response of the novel sensors in various pH values were investigated. EAPINP has the ability to respond linearly at alkaline P^H values, 8 to 9, and can be utilized in absorption and wavelength radiometric methods. PEAPINP has the ability to respond linearly at lower pH values, 6 to 7, and can be used as an alternative pH sensor in this range. The new sensors are yellow-colored in acidic and neutral media and red-colored in alkaline P^Hs. With their colorimetric responses at different pH ranges EAPINP and PEAPINP can be used to develop color-tunable P^H sensors.

Full text of DOI:10.1007/s10895-011-0983-3

J Fluoresc (2012) 22:495–504
DOI 10.1007/s10895-011-0983-3
ORIGINAL PAPER
Synthesis and Spectrophotometric PH Sensing Applications
of Poly-2-[4-(diethylaminophenyl)imino]-5-nitro-phenol
and its Schiff Base Monomer for Two Different PH Ranges
İsmet Kaya & Aysel Aydın & Mehmet Yıldırım
Received: 26 January 2011 /Accepted: 16 September 2011 /Published online: 5 October 2011
#
Springer Science+Business Media, LLC 2011
Abstract Poly-2-[4-(diethylaminophenyl)imino]-5-nitro-
phenol (PEAPINP) and 2-[4-(diethylaminophenyl)imino]-
5-nitro-phenol (EAPINP) were prepared as new pH sensors.
Spectrophotometric and spectrofluorometric response of the
novel sensors in various pH values were investigated.
EAPINP has the ability to respond linearly at alkaline PH
values, 8 to 9, and can be utilized in absorption and
wavelength radiometric methods. PEAPINP has the ability
to respond linearly at lower pH values, 6 to 7, and can be
used as an alternative pH sensor in this range. The new
sensors are yellow-colored in acidic and neutral media and
red-colored in alkaline PHs. With their colorimetric
responses at different pH ranges EAPINP and PEAPINP
can be used to develop color-tunable PH sensors.
have been applied in a variety of industrial, environmental,
and medicinal areas [2–4]. With the development of
wavelength-radiometric techniques, the design of molecular
structure with dual absorption and emission which depends
on the PH value has been an attractive challenge [3, 5, 6].
Despite these advantages, only a few examples of such
systems have been reported so far [5, 7–9]. Arshak et al. were
reviewed the current state-of-the-art methods to measure PH
levels using polymer based materials [10]. Most common
fluorescent PH sensors are suitable for PH from 4 to 9. They
are promising to be applied in biological applications,
determining neutral PH values of normal body fluids [3].
One advantage of PH sensors over the PH electrodes is their
wide PH scales. There are, however, only a few optical PH
sensors for use at high or low PHs [11–16]. There is still need
for developing of PH sensors available in different PH ranges.
Polyazomethines (PAMs), with their useful properties
have attracted much attention of researchers so far [17, 18].
PAMs and their oligophenol derivatives have been widely
investigated with their optical, electrochemical, thermal, and
conductivity measurements [19–21]. Some chelate derivatives
of PAMs were also developed as alternative gas sensing
materials [22]. Additionally, Schiff base substituted oligophe-
nols could be used as antimicrobial agents [23]. Schiff base
substituted oligophenols have been studied by Kaya and co-
workers for the last decade. This class of the polymers was
mainly found to be electroactive as well as semi-conductive
materials [24, 25]. Their conductivities were increased by
doping with iodine [26]. Coupling selectivity of the Schiff
base substituted oligophenols were also studied and two
coupling mechanisms were reported including C-O-C and C-
C couplings [19, 25]. The formation of C-O-C coupling in the
polymer structure decreases the phenolic –OH proton
numbers and consequently acidity of the polymer. This results
in lower acidity of the oligophenol than the Schiff base
H
.
.
.
Keywords Polyazomethine P sensor Fluorescence
.
Schiff base Differential scanning calorimetry
Introduction
During the last 20 years, global research and development
(R&D) on the field of sensors has expanded exponentially in
terms of financial investment, the published literature, and the
number of active researchers [1]. Glass PH electrodes are
unsuitable for certain applications such as determination of
intracellular PH, microscopy studies as well as measurement
of extreme PH values (<1 or >9). In contrast to the
electrochemical methods, optical PH sensors based on
absorbent and fluorescent probes without such drawbacks
:
:
İ. Kaya (*) A. Aydın M. Yıldırım
Faculty of Sciences and Arts, Department of Chemistry,
Çanakkale Onsekiz Mart University,
TR-17020, Çanakkale, Turkey
e-mail: kayaismet@hotmail.com

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J Fluoresc (2012) 22:495–504
monomer model. Resultantly, the Schiff bases containing
phenol units that can response towards different PHs could
work in different PH range than their oligophenol derivatives.
For this reason, in this study, we declare development of new
spectrophotometric PH sensors for both alkaline (PH 8.0–9.0)
and acidic (PH 6.0–7.0) conditions based on 2-[4-(diethyla-
minophenyl)imino]-5-nitro-phenol (EAPINP) and its oligo-
phenol derivative. In the range of PH 6–10, EAPINP and
PEAPINP cause color changes and allow the production of
color-tunable PH sensors. A linearly response in the PH range
of 8–9 was obtained for EAPINP and in the range of PH 6–7
for PEAPINP using spectrophotometric measurements.
Herein, we report the design and syntheses of novel PH
sensors those are sensitive in acidic-basic environments.
The sensing mechanism of the novel sensors is also
suggested that clarifying the different available PH ranges
for the monomer and polymer.
NaOCl as the oxidant [24–26]. EAPINP (0.313 g, 0.001 mol)
was dissolved in an aqueous solution of KOH (10%,
0.001 mol) and placed into a 50 ml three-necked round-
bottom flask which was fitted with a condenser, thermometer,
stirrer, and an addition funnel containing 2 ml of NaOCl
aqueous solution (30% by weight). After heating up to 60 °C,
NaOCl was added drop by drop over about 20 min. Reaction
was maintained for 5 h. Then, the reaction mixture was
cooled at the room temperature, 0.001 mol HCl (37%) added
for neutralization, and kept for 24 h at the room temperature
for precipitation of the polymer. The mixture was filtered,
washed with hot water (3×25 ml) and methanol (2×20 ml) to
separate the mineral salts and unreacted monomers, respec-
tively. The obtained polymer was dried in a vacuum oven at
60 °C for 24 h. The reaction design is given in Scheme 2.
Characterization Techniques
The solubility tests were done using 1 mg sample and 1 ml
solvent at 25 °C. The infrared spectra were measured by
Perkin Elmer Spectrum One FT-IR system. The FT-IR
spectra were recorded using universal ATR sampling
accessory within the wave numbers of 4,000–550 cm−1.
The synthesized compounds were also characterized using
1H- and 13C-NMR spectra (Bruker AC FT-NMR spectrom-
eter operating at 400 and 100.6 MHz, respectively) and
recorded by using deuterated DMSO-d6 as a solvent at 25 °C.
Tetramethylsilane was used as internal standard. The number
average molecular weight (Mn), weight average molecular
weight (Mw) and polydispersity index (PDI) were deter-
mined by size exclusion chromatography (SEC) techniques
of Shimadzu Co. For SEC investigations, an SGX (100Å
and 7 nm diameter loading material) 3.3 mm i.d.×300 mm
columns was used; eluent: DMF (0.4 ml/min), polystyrene
standards were used. A refractive index detector (RID) was
used to analyze the products at 25 °C.
Experimental
Materials
4-diethylaminobenzaldehyde, 2-amino-5-nitro-phenol,
methanol, ethanol, acetone, dioxane, ethyl acetate, toluene,
n-hexane, dichloromethane, CHCl3, THF, DMF, and KOH
were supplied by Merck Chemical Co. and used as
received. 30% aqueous solution of sodium hypo chlorite,
NaOCl was supplied from Paksoy Chemical Co. (Turkey).
Preparation of 2-[4-(diethylaminophenyl)imino]-5-nitro-
phenol (EAPINP)
EAPINP was prepared by the condensation of 4-
diethylaminobenzaldehyde (0,177 g, 0.001 mol) with 2-
amino-5-nitro phenol (0,154 g, 0.001 mol) in methanol
(30 ml) achieved by boiling the mixture under reflux for 5 h
at 70 °C (Scheme 1). The precipitated 2-[4-(diethylamino-
phenyl)imino]-5-nitro-phenol (EAPINP) was filtered,
recrystallized from methanol and dried in a vacuum
desiccator (yield, 70%) [27].
Optical Measurements
Fluorescence and absorption spectra were recorded by
Shimadzu RF-5301PC spectrofluorophotometer and Perkin
Elmer Lambda 25, respectively. Measurements were carried
out in ethanol-water (2/1, v/v) solutions in the PH range of 6–
10. PHs of the solutions were adjusted by 0.1 mol L−1
concentrated buffer solutions of Na2HPO4-NaH2PO4. Excita-
tion wavelength for emission measurements and emission
wavelength for excitation measurements were 510 and
Synthesis of Poly-2-[4-(diethylaminophenyl)imino]-5-nitro-
phenol (PEAPINP)
PEAPINP was synthesized through oxidative polycondensa-
tion of 2-[4-(diethylaminophenyl)imino]-5-nitro-phenol using
Scheme 1 Synthesis of 2-[4-
(diethylaminophenyl)imino]-5-
nitro-phenol (EAPINP)

J Fluoresc (2012) 22:495–504
497
Scheme 2 Synthesis of poly-2-
[4-(diethylaminophenyl)imino]-
5-nitro-phenol (PEAPINP)
574 nm; and 246 and 306 nm for EAPINP and PEAPINP,
respectively.
partially soluble in acetone, THF, dichloromethane, CHCl3,
and ethyl acetate. Also it was insoluble in apolar solvents
like n-hexane and toluene.
According to the FT-IR spectra the structure of EAPINP
is confirmed by growing imine (CH = N) peak at
1,605 cm−1 with disappearing of the C = O and –NH2
peaks of 4-diethylaminobenzaldehyde and 2-amino-5-
nitro-phenol, respectively. O-H stretch of phenolic
hydroxy is also observed at 3,281 cm−1. Additionally,
at the FT-IR spectrum of PEAPINP imine (HC = N) and
O-H stretch peaks shift up to 1,625 and 3,355 cm−1,
respectively. This is because of the polyconjugated structure
of the polymer. Polymer structure of PEAPINP is also
confirmed by broad peaks as the result of the polymerization.
Other characteristic FT-IR values are also shown below.
Results and Discussion
Solubilities and Structures of the Compounds
EAPINP have red colored-powder forms whereas its
polyphenol derivative, PEAPINP, is black colored. EAPINP
is completely soluble in many organic solvents such as
acetone, THF, DMF, dichloromethane, and CHCl3. How-
ever it’s partially soluble in methanol, ethanol, n-hexane,
toluene, ethyl acetate, and dioxane. PEAPINP was com-
pletely soluble in DMF, DMSO, and H2SO4 while it is
Fig. 1 a 1H-NMR and b 13C-
NMR spectra of EAPINP

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Fig. 2 a 1H-NMR and b 13C-
NMR spectra of PEAPINP
1H and 13C-NMR analyses results of the synthesized
respectively. According to the 1H-NMR results of the
compounds the structures of EAPINP and PEAPINP are
confirmed by specific –OH and CH = N peaks observed at
1
compounds are given below. The H and 13C-NMR spectra
of EAPINP and PEAPINP are also shown in Figs. 1 and 2,
Scheme 3 Protonation–depro-
tonation equilibria of EAPINP

J Fluoresc (2012) 22:495–504
499
1
10.02 and 8.44; and 10.67 and 9.62 ppm, respectively.
However, it’s clearly observed that the –OH peak integra-
tion of PEAPINP is lower than that of EAPINP. This is
probably because of the C-O-C coupling of the monomer
units. Kaya et al. have previously studied the radical
polymerization mechanism of polyphenols [19]. This
mechanism suggests that phenol-based monomers can be
polymerized by C-C and C-O-C coupling of monomer
units. The C-C coupling mechanism proceeds by intermo-
lecular combination of the monomer units at the ortho and
para positions of phenolic –OH. However, C-O-C coupling
occurs by combination of a phenoxy radical with an ortho
or para position of another monomer unit. Also, Kaya and
co-workers calculated the C-C/C-O-C coupling rates by
comparison of phenolic –OH integrations of oligophenols
and corresponding monomers using H-NMR spectra [25].
As a result of Figs. 1 and 2, PEAPINP is obtained with
forming of both coupling mechanisms. Other NMR results
are also given below.
EAPINP: FT-IR (cm−1): v(O-H) 3,281 s, v(C-H Phenyl)
3,105 m, v(C-H Aliphatic) 2981, 2940, 2,900 m, v(C = N)
1,605 s, v(C = C phenyl) 1574, 1544, 1,505 s, v(C-O)
1267 s. 1H-NMR (DMSO): δ ppm, 10.02 (s, 1H, -OH), 8.44
(s, 1H, -CH = N-), 7.81(s, 1H, Ar-Ha), 7.64 (d, 2H, Ar-Hd),
7.52 (d, 1H, Ar-Hb), 7.23 (d, 1H, Ar-Hc), 6.74 (d, 2H, Ar-
He), 3.42 (s, 4H, -CH2-), 1.11 (s, 6H, -CH3). 13C-NMR
(DMSO): δ ppm, 162.31 (C5-H), 152.36 (C11-ipso), 151.20
(C1-ipso), 146.04 (C6-ipso), 142.95 (C9-ipso), 132.14 (C3-
H), 124.48 (C4-ipso), 123.06 (C7-H), 118.78 (C8-H), 115.84
(C10-H), 111.01 (C2-H), 44.44 (−CH2-), 12.74 (−CH3).
Fig. 3 Spectrophotometric re-
sponse of EAPINP (a) and
PEAPINP (b) related to different
PH values

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PEAPINP: FT-IR (cm−1): v(O-H) 3,355 s, v(C-H Phenyl)
3,099 m, v(C-H Aliphatic) 2,975 m, v(C = N) 1,625 s, v(C =
1
C phenyl) 1587, 1552, 1,523 s, v(C-O) 1,272 s. H-NMR
(DMSO): δ ppm, 10.67 (s, -OH), 9.62 (s, -CH = N-), 8.01(d,
Ar-Ha), 7.64 (d, Ar-Hc), 7.44 (d, Ar-Hb), 3.42 (s, -CH2-),
1.12 (s, -CH3). 13C-NMR (DMSO): δ ppm, 166.68 (C5-H),
154.81 (C9-ipso), 152.41 (C11-ipso), 147.99 (C1-ipso),
147.21 (C6-ipso), 136.54 (C3-H), 132.43 (C4-ipso), 125.20
(C8-ipso), 121.21 (C7-H), 116.75 (C10-ipso), 111.05
(C2-H), 44.46 (−CH2-), 12.78 (−CH3).
According to the SEC chromatogram PEAPINP has two
fractions. The number-average molecular weight (Mn),
weight average molecular weight (Mw), polydispersity
index (PDI), and fraction percentage values are calculated
as 6,400, 8,400 gmol−1, 1.310, and 58%; and 3760, 3,790 g
mol−1, 1.008, and 42%, respectively. Totally; Mn, Mw, and
PDI values of PEAPINP is obtained as 5,290, 6,460 g
mol−1, and 1.221, respectively. According to these results
PEAPINP contains approximately 17–21 repeated units.
Optical Characteristics
Diethylamino group of EAPINP is the corresponding site
from stable spectral change depending on the PH values
[28]. Diethylamino is an electro-donating group in neutral
and basic conditions. However, in the acidic conditions this
group becomes an electron withdrawing site, which causes
decreasing in conjugation of the molecule and inhibition of
the photoinduced electron transfer (PET) between the
electro-donor phenolic –OH and electro-acceptor –NO2
groups [28, 29]. At the high PH values, diethylamino
group donates electrons into the molecule which increases
the conjugation of the molecule. Consequently, in the
basic conditions the molecule is in the red-colored form
due to high conjugation whereas in the yellow-colored
form in the acidic media. Schematic illustration of
protonation/deprotonation mechanism of EAPINP is
shown in Scheme 3.
Fig. 5 Color changes of EAPINP (a) and PEAPINP (b) at various PH
values: (a) 6.0, (b) 7.0, (c) 8.0, (d) 9.0, and (e) 10.0
The normalized absorption spectra of EAPINP in
different PH values are given in Fig. 3a. At the absorption
spectra specific spectral changes are obtained as shifting of
Table 1 Spectral response of EAPINP at various PH values
PH
UV (1max/nm)
Fluorescence
EAPINP
EAPINP
PEAPINP
PEAPINP
aIEx
bIEm
cIEx
dIEm
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
410
410
410
412
417
428
440
447
452
407
411
414
416
417
418
418
418
418
300
1072
344
279
136
132
120
110
110
292
1128
344
268
112
102
95
885
664
842
918
902
905
924
937
902
867
624
836
887
868
873
891
891
886
86
86
a Maximum excitation intensity of EAPINP at 510 nm, emission
wavelength for excitation: 574 nm
b Maximum emission intensity of EAPINP at 574 nm, excitation
wavelength for emission: 510 nm
c Maximum excitation intensity of PEAPINP at 248 nm, emission
wavelength for excitation: 306 nm
Fig. 4 Relative spectrophotometric response (Rr) of EAPINP and
PEAPINP in the PH range of 6–10
d Maximum emission intensity of PEAPINP at 305 nm, excitation
wavelength for emission: 247 nm

J Fluoresc (2012) 22:495–504
501
absorption edges. Figure 3a shows the red shifts in the edge of
the absorption spectrum of EAPINP when the PH increases.
Maximum absorbance wavelengths (1max) are also plotted on
the right side of Fig. 3a. This region is also corresponding
from the color change between yellow-red colors. However,
the spectral changes of that region are limited in the range of
PH 7–10. Also, linearly response of this area is limited in the
range of PH 8 to 9. This indicates that the use of EAPINP as a
spectrophotometric PH sensor is useful in the PH range of 8–
9. The regression coefficient of EAPINP obtained in the
range of PH 8 to 9 is 0.9997. As a result the following
equation (Eq. 1) derived from spectral response of EAPINP
in the range of PH=8–9 could be used in PH measurements:
when PH of the media is increased the absorption edge of
the spectrum clearly shifts up to the higher wavelengths.
1max values of PEAPINP are also plotted on the right side
of Fig. 3b. However, on the contrary of spectral response of
EAPINP, PEAPINP has linearly response limited in the
range of PH=6–7. As seen in Fig. 3, EAPINP and
PEAPINP have nearly opposite responses in different PHs.
Scheme 3 presents the protonation/deprotonation of
EAPINP which mainly occurred at phenolic –OH during
conversion in neutral and basic media. However, as
mentioned above the polycondensation reaction of EAPINP
proceeds by C-O-C and C-C couplings. As a result of C-O-
C coupling of the monomer units phenolic hydroxy group
numbers of PEAPINP decrease which also causes decreas-
ing of acidity of the oligomer. Due to lower number of –OH
protons the polymer doesn’t present clear spectral change in
the basic conditions. Resultantly, PEAPINP could be used
as a spectrophotometric sensor at higher acidic conditions
lmax ꢀ 232:83
pH ¼
ð1Þ
23
The normalized absorbance spectra of PEAPINP in
different PH values are also given in Fig. 3b. Similarly,
Fig. 6 PL spectra of EAPINP at
various PH values: a Emission
spectra, 1Ex 510 nm, inset; the
changes of PL intensities at
532 nm, b excitation spectra,
1Em 574 nm, inset; the changes
of PL intensities at 522 nm. Slit:
1Ex 5 nm, 1Em 5 nm

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J Fluoresc (2012) 22:495–504
than that of EAPINP. According to the obtained results
PEAPINP could be used as a spectrophotometric PH sensor
in the pH range of 6–7. The regression coefficient of
PEAPINP obtained in this range is 0.9966. The following
equation (Eq. 2) derived from spectral response of
PEAPINP in the range of PH=6–7 could be used in PH
measurements:
where 1max is the maximum absorption wavelength at PH of
10, 1min is the maximum absorption wavelength at PH of 6,
and 1x is the maximum absorption wavelength at the test
PH. Figure 4 clearly indicates that EAPINP and its
polyphenol derivative have opposite response towards
different PHs. In lower PHs the response of PEAPINP
sharply increases while EAPINP has nearly no response.
An opposite behaviour is observed at the higher PHs; the
response of EAPINP increases while the response of
PEAPINP is stable.
lmax ꢀ 365:17
pH ¼
ð2Þ
7
According to the obtained results the relative spectro-
photometric responses (Rr) of the synthesized compounds
are determined using the following equation (Eq. 3) and
shown in Fig. 4.
The color changes of EAPINP and PEAPINP at various
PHs are also shown in Fig. 5. As seen in Fig. 5a at PH of 7
EAPINP is in the yellow-colored form. When the PH is
increased up to 10 the color changes from yellow to red.
That agrees with the spectrophotometric measurements; in
the range of PH 7–10 the absorption edge shifts from
410 nm until 460 nm, which causes color changes and
lx ꢀ lmin
Rr ¼
ꢁ 100
ð3Þ
lmax ꢀ lmin
Fig. 7 PL spectra of PEAPINP
at various PH values: a Excita-
tion spectra, 1Em 306 nm, inset;
the changes of PL intensities at
305 nm, b emission spectra, 1Ex
247 nm, inset; the changes of PL
intensities at 248 nm. Slit:
1Ex 5 nm, 1Em 5 nm

J Fluoresc (2012) 22:495–504
503
allows the production of a color-tunable PH sensor.
However, the novel sensor can be used as a color-tunable
PH sensor for only limited PH values which is suitable in the
range of PH=7–10. Some other Schiff base derivatives have
been also synthesized as colorimetric sensors in presence of
different ions like fluoride [30]. PEAPINP has also similar
colors with darker forms in both acidic and basic conditions
as seen in Fig. 5b. However, at the lower PHs, when PH
increased from 6.0 to 7.0 a clearly color change could be
observed for PEAPINP while no change is observed for
EAPINP. Similarly, in the range of PH=8–10 the color of
EAPINP clearly changes while the color of PEAPINP
doesn’t change, agreed with the spectrophotometric
measurements as mentioned above.
while PEAPINP could be used in the range of PH=6–7
depending on their linearly spectral responses. PEAPINP
was found to have lower acidity than its monomer
compound because of the C-O-C coupling which decreases
the phenolic –OH numbers. Resultantly, we demonstrate that
due to their available active centers for proton attacks the
novel Schiff base and its polyphenol derivative can be used
as alternative spectrophotometric PH sensors for different PH
ranges including 6–7 and 8–9. EAPINP and PEAPINP can
be also used as color-tunable PH sensors in practice. As a
result, EAPINP and PEAPINP with their easily and
inexpensive productions could be alternative PH probes.
References
Different Schiff base compounds have been previously
studied as emission/excitation based PH probes [31].
Emission and excitation based spectral characteristics of
EAPINP are also determined in the PH range of 6.0–10.0.
The results are given in Table 1. The signal changes of
emission-excitation spectra of EAPINP are also monitored
at different PH ranges in Fig. 6. EAPINP exhibited the best
response between PH 6.5 and 10.0 in the direction of a
decrease in signal intensity at 574 nm and 510 nm in
emission and excitation spectra, respectively. Upon expo-
sure to the solutions between PH 6.5 and 10.0, EAPINP
exhibited 92% and 90% relative signal changes in direction
of decrease in emission and excitation intensities, respec-
tively. However, fluorescence based spectral changes of
EAPINP could not be useful in PH sensor applications due
to its irregular intensity changes.
Emission and excitation based spectral characteristics of
PEAPINP are also given in Fig. 7. According to the
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EAPINP has the maximum. Upon exposure to the solutions
between PH 6.5 and 10.0, PEAPINP exhibited 43% and
41% relative signal changes in direction of decrease in
emission and excitation intensities, respectively. However,
the absence of the linear spectrofluorometric response upon
different PHs makes the polymer un-useful to use as a
spectrofluorometric PH sensor.
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