A soluble and reusable colorimetric sensor based on the covalent attachment of a triarylpentenedione to poly(ethylene glycol)

Article abstract of DOI:10.1002/ejoc.200500004

3-(4-Aminophenyl)-1,5-diphenylpent-2-ene-1,5-dione (PDO) is a colorimetric sensor of Lewis acid cations that has been covalently attached to a poly(ethylene glycol) (PEG) scaffold. The resulting PDO-PEG is soluble in water and other organic solvents and a

Full text of DOI:10.1002/ejoc.200500004

FULL PAPER
A Soluble and Reusable Colorimetric Sensor Based on the Covalent
Attachment of a Triarylpentenedione to Poly(ethylene glycol)
Mercedes Alvaro,[a] Carmela Aprile,[a] Hermenegildo García,*[a] and Encarna Peris[a]
Keywords: Sensors / Polymers / Heterocycles / Cyclization / Colorimetry
3-(4-Aminophenyl)-1,5-diphenylpent-2-ene-1,5-dione (PDO)
is a colorimetric sensor of Lewis acid cations that has been
covalently attached to a poly(ethylene glycol) (PEG) scaffold.
The resulting PDO-PEG is soluble in water and other organic
solvents and acts as a colorimetric sensor of Lewis acid cat-
ions by undergoing cyclization into the triarylpyrylium form.
In water, PDO-PEG also undergoes the colorimetric change
in the presence of Brönsted acids, which may limit its useful-
ness. This modified PEG is recoverable by precipitation from
cold ethanol and reusable after reopening of the triarylpyryl-
ium heterocycle to the active pentenedione form. The test-
ing-recovery-reuse cycle has been performed five times
without noticeable differences in the response. In contrast to
PDO-PEG, PDO on its own is insoluble in water and remains
completely soluble in ethanol. This solubility behavior of
PDO makes the molecule unsuitable for use as a reusable
and recoverable sensor.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
in microporous zeolites.[13] The problem of this methodol-
ogy, however, is the long response time required to observe
the color change, which arises due to the mass-transfer phe-
nomena between the solid and the liquid phases. In order
to reduce the response time, one alternative that could cir-
cumvent this problem would be the covalent binding of the
sensor molecule to a soluble polymer, thus rendering ana-
lyte sensing a homogeneous rather than a heterogeneous
process. The polymeric backbone could serve to both in-
crease the solubility of the sensing molecule in the desired
medium and to permit the recovery of the sensor after use.
In fact, the strategy consisting of attaching an organic
molecule covalently to a polymeric scaffold is gaining im-
portance in homogeneous catalysis to recover soluble cata-
lysts.[14–19] Polymers,[20–22] sol-gels,[23–25] and hydrogels[26]
have often been used in combination with a sensor to better
adapt it to water and other media where the analyte is pres-
ent. Herein we report the preparation of a 4-(p-ami-
nophenyl)-2,6-diphenylpent-2-ene-1,5-dione covalently at-
tached to a water-soluble poly(ethylene glycol) and compare
its performance with that of a water-insoluble, low-molecu-
lar-weight polystyrene recently prepared by us.[13] The use
of soluble poly(ethylene glycol) as a scaffold for catalysts or
reagents is a topic of much current interest. Their solubility
properties allow the reactions to be performed in homogen-
eous solution in some solvents and quantitatively recover
the polymer by precipitation with diethyl ether after use.
We show here that attachment of a PDO sensor to poly-
(ethylene glycol) polymers allows the recovery and reuse of
the polymeric sensors which, in some solvents, behave like
the equivalent sensor not bound to the polymer. Unat-
tached, molecular PDO is not soluble in water, and remains
Introduction
Colorimetric sensors undergo visual changes during the
sensing phenomena and they do not require any instrumen-
tal technique to reveal the presence of analyte.[1–5] One type
of chromogenic sensor that has recently received some at-
tention has a chemical structure derived from 1,3,5-triaryl-
pent-2-ene-1,5-dione (PDO).[6–9]
Derivatives of this parent PDO having a covalently at-
tached selective complexing unit have been reported for the
sensing of metal ions as well as some inorganic anions and
cis configured dicarboxylic acids.[8–11] Although there are
many other colorimetric sensors that are soluble in water,[12]
a possible limitation of this family of sensors based on PDO
is their poor solubility in aqueous medium, which means
that either organic co-solvents or pure organic solvents
must be used to achieve sufficient solubility. Another as-
pect, related to the solubility, worthy of being explored for
sensors based on the PDO structure is the possibility to
recover and reuse the molecule.
In a recent paper we have reported the covalent attach-
ment of a PDO derivative to silica particles or its inclusion
[a] Instituto de Tecnología Química y Departamento de Química,
Universidad Politécnica de Valencia,
Av. De los Naranjos s/n, 46022 Valencia, Spain
Fax: +49-6221-487-672
E-mail: hgarcia@qim.upv.es
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DOI: 10.1002/ejoc.200500004
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M. Alvaro, C. Aprile, H. García, E. Peris
FULL PAPER
completely soluble in ethanol. However, our study of the of 2,6-diphenylpyrylium with N-allylaniline in DMF fol-
sensing response reveals that when attached to poly(ethyl- lowed by heterocyclic ring opening with a base. The actual
ene glycol), the PDO-based sensor is not appropriate for synthetic steps, reaction conditions, and the required rea-
use in an acidic aqueous medium due to the chemical insta- gents are summarized in Scheme 1.
bility of the sensing dione to Brönsted acids.
Alternatively, the precursor of the sensor molecule 4-[4-
(allylamino)phenyl]-2,6-diphenylpyrylium was co-polymer-
ized with an excess of styrene to form a co-polymer in
which pyrylium ions are connected to a polystyrene (PS)
backbone, as previously reported by us.[13] By controlling
the pyrylium/styrene molar ratio, as well as the polymeriza-
tion conditions, a co-polymer with adequate solubility in
ethanol, dichloromethane, and other organic solvents, but
Results and Discussion
Preparation and Characterization of the PDO Sensor
Attached to Poly(ethylene glycol)
The key step in the preparation of modified poly(ethylene insoluble in water, was obtained. The actual sensing form,
glycol) (PEG) containing terminal sensing diones is a radi- namely the open pentenedione form, was again obtained by
cal chain addition of a mercapto group to a terminal car- hydrolytic ring opening in a mixture of ethanol and water
bon-carbon double bond connected to an appropriately in the presence of sodium acetate as base to give PDO-
functionalized PDO derivative. This radical-mediated ad- PS. Scheme 2 depicts the synthetic route followed to obtain
dition of mercapto groups to vinylic functions has pre- PDO-PS. As expected, PDO-PS is insoluble in water but
viously been used by many groups for the efficient covalent soluble in ethanol or dichloromethane.
attachment of metallic complexes to modified solid surfaces
The PDO-PEG and PDO-PS polymers were purified by
under mild conditions.[27–31] To obtain the mercapto-func- dialysis in water (PDO-PEG) or methanol (PDO-PS) in or-
tionalized PEG with the desired solubility properties, com- der to ensure the absence of residual pentenedione or pyryl-
mercially available PEG (mol. wt. 5000 D) was reacted with ium dye not covalently bonded to the polymer backbone
thionyl chloride to convert the primary hydroxyl groups that could interfere later in the sensing tests. Thin-layer
into the corresponding chlorides, which were subsequently chromatography showed that the dialyzed polymers were
substituted by treating then with 2-mercaptoethylamine. free from PDO precursors The presence of PDO in the
This modified PEG containing terminal mercapto groups water-soluble PDO-PEG polymer was confirmed by the
1
was subsequently treated under an inert atmosphere with 4- presence of a singlet at δ = 4.79 ppm in its H NMR spec-
[4-(allylamino)phenyl]pent-2-ene-1,5-dione perchlorate with trum in solution, as well as signals in the aromatic region.
AIBN as radical initiator to give 4-[4-(allylamino)phenyl]- These NMR signals, particularly the one at δ = 4.79 ppm
pent-2-ene-1,5-dione by aromatic electrophilic substitution attributable to the methylene protons in position 4 of the
Scheme 1.
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A Soluble and Reusable Colorimetric Sensor
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Scheme 2.
pentenedione moiety, are also observed in all PDO deriva-
tives. Figure 1 shows an expansion of the H NMR spec- variation in the combustion chemical analyses before and
The PDO loading in the polymer was quantified by the
1
trum of PDO-PEG.
after PEG functionalization and before and after covalent
attachment of the sensor. This technique gives a percentage
of PEG functionalization of about 40% of the maximum
theoretical value. This estimation was confirmed more sim-
ply by 1H NMR spectroscopy using a commercial PEG
having terminal methyl groups (PEG-OMe). Known
amounts of PEG-OMe and PDO-PEG were dissolved in
CDCl3 and the peak integral used to quantify the amount
of PDO present relative to the methyl groups of PEG-OMe.
Figure 1. Expansions of part of the 1H NMR spectrum of PDO-
PEG.
PDO-PEG as a Soluble, Reusable Sensor
The purpose of this work was to develop a recoverable
colorimetric sensor based on PDO that could be used in
water and other solvents. It is appropriate to comment that
previous reports on the use of chemosensors based on the
pentenedione–pyrylium interconversion have not used pure
water as solvent, but mostly a mixture of water and organic
solvents or pure organic solvents.[10,11] Unfortunately, it
soon became evident that in spite of the solubility of PDO-
PEG in water and that the changes in the color are visually
observable as expected, this system does not exhibit the se-
lectivity previously shown in the literature for related deriv-
atives containing selective complexing units attached to
PDO.[10] Thus, aqueous solutions of PDO-PEG
(5 mgmL–1) change the color from yellow to red in the pres-
ence of strong Lewis acid cations such as Fe3+, Cu2+, and
Pb2+ in concentrations between 10–2 and 10–4 m. Other me-
tal cations that do not exhibit sufficiently strong Lewis acid-
ity do not produce a response in the PDO-PEG sensor. This
is the case of alkali metal ions and alkaline-earth cations.
As reported in an earlier work on PDO attached to silica
particles,[13] PDO responds also to Brönsted acids. For this
reason, most of the experiments in aqueous media were
conducted in HEPES buffered solution at pH 5. The range
of pH values at which PDO-PEG is stable (pH Ͼ 3) is,
however, not a major limitation for physiologically relevant
cations, considering the pH of physiological fluids. As an
example, to illustrate the visual changes produced upon
sensing, Figure 3 shows the color change of PDO-PEG in
water when Fe3+ was added to a solution of the sensor.
The presence of PDO in the dialyzed polymers was also
determined by transmission UV/Vis spectroscopy. PDO and
the corresponding closed pyrylium form (TP) have charac-
teristic bands in the UV/Vis spectrum at 420 nm and
520 nm, respectively. These absorption bands are responsi-
ble for the change in color during the sensing experiments.
Figure 2 shows the transmission UV/Vis spectra for aque-
ous solutions of PDO-PEG and TP-PEG; the differences
between the two spectra are clear. Due to the low amount
of organic dye attached to the polymer chain, the character-
istic absorption bands for the open PDO and heterocyclic
forms appear on top of a baseline that grows in absorbance
towards shorter wavelengths due the background of the
polymer scaffold.
Figure 2. Transmission UV/Vis spectra of dichloromethane solu-
tions of PDO-PEG (spectrum a) and TP-PEG (spectrum b). The
arrows show the λmax characteristic of PDO and TP precursors that
are responsible for the color changes.
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M. Alvaro, C. Aprile, H. García, E. Peris
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Figure 3. Photograph of two cells containing aqueous solutions of
PDO-PEG (0.5 mgmL–1) in the presence (left) or absence (right)
of Fe(NO3)3 at a final concentration of 2×10–4 m.
The use of dichloromethane as solvent instead of water,
or the presence of other Lewis acid cation analytes, pro-
duces identical color changes provided that appropriate sol-
uble metal salts are used. Importantly, PDO-PEG in water
Scheme 3.
also changes color at acidic pH, thus indicating that shorten the response time we studied the influence of add-
Brönsted acids are also able to produce the colorimetric re- ing NaCl as we expected that addition of Na+ would have
sponse. This behavior is, in fact, not completely unexpected a positive influence by replacing analyte at the spurious
considering that the color changes are related to a cycliza- complexing sites in the polymer backbone (see Scheme 3).
tion from the open PDO to the closed pyrylium form and However, addition of a 10–2 m solution of NaCl has only a
vice versa. Since it is known that formation of pyrylium marginal effect on the response time, although some short-
heterocycles from unsaturated pentenediones can be pro- ening of the time from addition of the analyte until the
moted by Brönsted or Lewis acids we anticipated that pure color change was noticed.
Brönsted acids such as HNO3 and HCl could also promote
the response of the sensor, as turned out to be the case.
In contrast to water or dichloromethane, an appropriate
solvent in which the response time was only a few minutes
One fact that was, however, unexpected was the ex- is ethanol. In this solvent at room temperature the solubility
tremely slow response of the PDO-PEG sensor in aqueous of the PDO-PEG is low but still sufficient to test the pres-
solution and other organic solvents. At the concentration ence of analytes in a few minutes at a PDO-PEG concentra-
used, the response requires tens of minutes at room tem- tion of 0.05 mgmL–1. In contrast to PDO-PEG, and in
perature, and is considerably slower than simple derivatives agreement with what is known for polymer solubility, PDO-
in organic solvents. Under the typical conditions used (Fig- PS is insoluble in water but is extremely soluble in ethanol,
ure 3) Fe3+exhibits a clear response in 20 min, which is a dichloromethane, acetonitrile, and many other organic sol-
somewhat faster response than those of Cu2+ and Pb2+
.
vents. In support of the influence of the PEG chain slowing
This response time also depends on the solvent, and is no- down the response time of PDO-PEG, PDO-PS responds
tably slower in dichloromethane and water. We believe that to the presence of analytes in organic solvents in a few min-
this is due to the complexing ability of the PEG backbone, utes and in all cases considerably faster than PDO-PEG.
which could interact with the cations via the aminophenyl This reduced response time for PDO-PS as compared to
group (Scheme 3). This type of complexation between PEG PDO-PEG lends support to the influence of the nature of
and metal ions is well documented in the literature[32,33] and the polymer chain on the sensing process.
is the basis of the high catalytic activity of PEGs in the
The main purpose of our work was to show the reusabil-
presence of cesium carbonate in organic solvents due to the ity of soluble polymers containing PDO sensing units. With
sequestration of cesium and enhancement of the basicity of the aim of showing this reusability, we devised a procedure
free carbonate.[34,35]
consisting of the recovery of the sensing polymer by precipi-
The sensor response evidently requires an interaction be- tating it from cold ethanol. Solutions of 1 mg of TP-PEG
tween the Lewis or Brönsted acid and the para amino group in 1 mL of ethanol were almost quantitatively recovered by
of the aryl ring at the 3-position of the pentenedione sys- concentrating the solution to 0.2 mL and filtering the pre-
tem, which provokes the cyclization to the pyrylium ring. cipitated solid. In contrast, unbound TP+ remains in solu-
A slow response was also found with dichloromethane as tion under the same conditions. The red TP-PEG solid was
solvent, and it was not shortened by increasing the concen- then dissolved in a 10–2 m aqueous solution of sodium ace-
tration of PDO-PEG. In fact, a high PDO-PEG concentra- tate and the solution stirred at room temperature for 1 h.
tion can be unfavorable from the sensitivity point of view, This treatment under basic conditions produced the ring
since the visual color change from red to yellow is less opening of the pyrylium back to the open PDO sensing
clearly observed for highly concentrated solutions. To form (Scheme 4). This aqueous solution of yellow PDO-
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A Soluble and Reusable Colorimetric Sensor
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Scheme 4.
PEG was concentrated under reduced pressure before re- selectivity can be improved by adding appropriate complex-
precipitating in cold ethanol. The recovered PDO-PEG was ing units to the structure, although the complexing ability
reused for a consecutive run. This cycle (sensing/recovery/ of PEG also plays a role in the sensing. The presence of
heterocycle reopening/reuse) was repeated five times with- complexing units on the N atom of the PDO moiety can
out significant variation in the visual response of the ana- also influence positively the response time in some solvents.
lyte. Scheme 4 summarizes the procedure for the reuse of On the other hand, the influence of Brönsted acids limits
the recoverable PDO-PEG sensor.
the use of the PDO-TP sensing system in aqueous solution
Figure 4 shows the UV/Vis spectra recorded for solutions to neutral and basic pHs.
during some of the cycles to illustrate the interconversion
between the open and closed forms of the PDO-PEG sensor
upon consecutive reuse.
Experimental Section
Synthesis of Chloride-Functionalized Poly(Ethylene Glycol): Com-
mercial poly(ethylene glycol) (PEG, mol. wt. 5000 D; 1.00 g,
0.17 mmol) was dissolved in a mixture of SOCl2 (6 mL) and di-
methyl formamide (1 mL). The reaction mixture was stirred for
24 h at 60 °C, then cooled to room temperature and the solvent
partially evaporated under reduced pressure. Residual thionyl chlo-
ride was decomposed by cautious addition of a 10–2 m aqueous
solution of K2CO3. The reaction mixture was then concentrated
and extracted with dichloromethane. The organic layer was dried
with anhydrous Na2SO4 and the solvent was removed under vac-
uum to give chloride-functionalized PEG.
Conclusions
In summary, we have reported here that a recoverable
and reusable soluble colorimetric sensor can be devised by
the covalent attachment of a pentenedione colorimetric sen-
sor to a polymeric poly(ethylene glycol) scaffold. The sys-
tem can be recovered and reused after basic treatment and
can be used for five cycles without loss of activity. Among
the major drawbacks of the system are the limited selectiv-
ity and the slow response under some circumstances. The
Synthesis of Mercapto-Functionalized Poly(Ethylene Glycol): Chlo-
ride-functionalized PEG (300 mg) was dissolved in toluene (10 mL)
and 2-aminoethanethiol hydrochloride (300 mg) was added. The
mixture was stirred at reflux temperature for 24 h then cooled to
room temperature and concentrated under reduced pressure.
Dichloromethane was added to the resulting crude and the 2-amin-
oethanethiol hydrochloride precipitate was removed by filtration.
The organic layer was evaporated to give mercapto-functionalized
PEG. Chemical analysis indicated that the modified polymer con-
tains 1.05% sulfur.
Synthesis
of
N-Allyl-N-methylaniline:[13]
N-Methylaniline
Figure 4. UV/Vis spectra of an aqueous solution of PDO-PEG
(plot a) that was used as Fe3+ sensor and, therefore, transformed
into TP-PEG (spectrum b, which was reused five times according
to the cycle shown in Scheme 4 either in the PDO-PEG form (plot
c) or after sensing with Fe3+ and transformation into TP-PEG (plot
d).
(535.5 mg, 5 mmol) was dissolved in dry THF (10 mL) and KtOBu
(561.4 mg, 5 mmol) was added. The mixture was stirred at 0 °C
under N2 for 1 h and then allyl bromide (604.9 mg, 5 mmol) was
added. The suspension was left to warm to room temperature and
magnetically stirred for 24 h. Then, the reaction was washed with
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a 1 m solution of HCl and extracted with CH2Cl2. The organic
layer was washed with NaOH, dried with anhydrous Na2SO4, and
concentrated under vacuum. N-Allyl-N-methylaniline (35%) was
obtained as a yellow oil after preparative column chromatography
with a mixture of hexane and diethyl ether (10:1) as eluent.1H
NMR (300 MHz, CDCl3, 25 °C): δ = 2.98 (s, 3 H, CH3), 3.94 (d,
J = 4.8 Hz, 2 H, CH2N), 5.14–5.22 (m, 2 H, CHCH2), 5.77–5.90
(m, 1 H, CHCH2), 7.22–7.28 (m, 5 H, ArH) ppm.13C NMR
(300 MHz, CDCl3, 25 °C): δ = 36.6, 55.9, 113.0, 116.7, 117.0, 129.7,
134.4, 150.1 ppm.
(s, PhCO-CH2), 7.20 (s, PhCOCH=C), 7.40–7.60 (ArH), 8.20–8.30
(ArH ortho to CO) ppm.
For use as a sensor, a few milligrams of PDO-PEG (typically 5 mg)
was dissolved in the appropriate solvent (3–5 mL) and then a vol-
ume of a solution containing a known concentration of the analyte
in the same solvent was added with a syringe, thus allowing the
determination of the final concentration. For re-use, the solution
was concentrated to 0.3 mL and the TP-PEG precipitated with cold
ethanol. The solid was collected and dissolved in 2 mL of a 0.5 m
aqueous solution of Na2CO3 at room temperature until the color
changed to yellow. The resulting PDO-PEG was recovered after
concentrating the solvent to 0.3 mL and precipitating the sensor
with cold ethanol.
Synthesis of 4-[4-(Allylmethylamino)phenyl]-2,6-diphenylpyrylium
Perchlorate: N-allyl-N-methylaniline (160.3 mg, 1.1 mmol) was dis-
solved in DMF (3 mL) and then 2,6-diphenylpyrylium perchlorate
(724.4 mg, 2.2 mmol), synthesized as described previously,[36,37] was
added. The mixture was stirred at reflux temperature for 3 h, then
cooled to room temperature and stirred for an additional 20 h. The
solvent was removed under reduced pressure and the resulting
brownish-red triarylpyrylium oil used for the next step without pu-
rification.
Acknowledgments
Financial support by the Spanish Ministerio de Ciencia y Tecnol-
ogía (MAT2003-1226) is gratefully acknowledged. EP thanks the
Universidad Politécnica de Valencia for a post-graduate scholar-
ship. C.A. is the recipient of a fellowship from the Ana y Jose Royo
Foundation.
Synthesis of 4-[4-(Allylmethylamino)phenyl]-1,5-diphenylpent-2-ene-
1,5-dione: Sodium acetate (183.3 mg, 2.23 mmol) was dissolved in
a mixture of water (0.6 mL), methanol (1.9 mL), and acetone
(3.8 mL), triarylpyrylium perchlorate was added, and the mixture
stirred for 15 h at room temperature and then left for 10 h without
stirring. After this time the solvent was evaporated under reduced
pressure and the residue was purified by flash chromatography with
diethyl ether as eluent to give pentenedione (PDO; 55%) as a yel-
low oil. 1H NMR (300 MHz, CDCl3, 25 °C): δ = 3.01 (s, 3 H, CH3),
3.92 (d, J = 4.5 Hz, 2 H, CH2N), 4.91 (s, 2 H, CH2COPh), 5.08–
5.24 (m, 2 H, CHCH2), 5.63–5.96 (m, 1 H, CHCH2), 7.14–7.98 (m,
14 H, ArH) ppm. 13C NMR (300 MHz, CDCl3, 25 °C): δ = 36.6,
55.9, 112.4, 116.8, 126.7, 127.0, 128.1, 128.6, 128.9, 129.2, 129.4,
129.6, 133.5, 135.7 ppm. HPLC-MS (electrospray): m/z = 396.2 [M
+ H]+. C27H25NO2: calcd. C 82.00, H 6.37, N 3.54; found C 81.26,
H 6.46, N 3.14.
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Covalent Binding of PDO to Mercapto-Functionalized PEG: Mer-
capto-functionalized PEG (300 mg) was dissolved in toluene
(5 mL) then a solution of PDO (40 mg) in toluene (5 mL) and a
catalytic amount of AIBN were added sequentially. The mixture
was stirred at reflux temperature for 20 h under an inert atmo-
sphere. Then, the solution was cooled and the solvent evaporated
under vacuum. The resulting crude was purified by dialysis in
methanol to give the PDO-PEG polymer as a yellow solid. The 1H
NMR spectrum of dialyzed PDO-PEG free from PDO (as evi-
denced by thin-layer chromatography) shows weak signals between
δ = 7.9–7.1 ppm corresponding to the aromatic protons of PDO
and a signal at δ = 4.79 ppm attributable to -CH2-CO-Ph. Due to
the relatively low PDO loading, the 13C NMR spectrum of PDO-
PEG does not show any characteristic signal of the covalently at-
tached colorimetric sensor. Evidence for the binding between the
PEG backbone and PDO was also obtained by combustion chemi-
cal analysis, IR spectroscopy, and UV/Vis spectroscopy. Elemental
analysis of PDO-PEG shows the presence of nitrogen (0.78%) and
sulfur (0.89%), values which, when compared with the initial value
of the PEG scaffold, indicate that there are 26 mmol of PDO per
100 g of PEG. This corresponds to a PEG functionalization of
about 80% of the maximum capacity. An expansion of the carbonyl
region of the IR spectrum of PDO-PEG shows two bands at 1680
and 1648 cm–1, indicating the presence of PDO. In the UV/Vis
spectrum the characteristic PDO band at 430 nm was found in the
solution spectra of PDO-PEG polymers. 1H NMR (300 MHz,
CDCl3, 25 °C): δ = 1.90, 3.60–3.80, 3.70–3.80 (t, ArN-CH2), 4.97
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