Phosphazene based multicentered naked-eye fluorescent sensor with high selectivity for Fe3+ ions

Article abstract of DOI:10.1021/ic402783x

A novel on/off fluorescent rhodamine-based hexapodal Fe³⁺ probe (L) containing a cyclotriphosphazene core was synthesized by an azide-alkyne "click-reaction". The synthesized compounds (1-5 and L) were characterized by FT-IR; ¹H, ¹³C, and ³¹P NMR; and MALDI MS spectrometry. The optical sensor features for the Fe³⁺ complex of L were investigated by UV-vis and fluorescence spectroscopy. The stoichiometry of L-Fe³⁺ complex was found to be 1:3 (ligand/metal ion), and the detection limit of L was determined as 4.8 μM (0.27 mg L ^-1) for Fe³⁺ ions. The reusability of the sensor was tested by the addition of ethylenediamine to L-Fe³⁺ complex solutions followed by the addition of Fe³⁺ solution.

Full text of DOI:10.1021/ic402783x

Article
pubs.acs.org/IC
Phosphazene Based Multicentered Naked-Eye Fluorescent Sensor
with High Selectivity for Fe3+ Ions
Reyhan Kagit,† Mehmet Yildirim,‡,§ Ozgur Ozay,∥ Serkan Yesilot,⊥ and Hava Ozay*,†
†Department of Chemistry, Inorganic Chemistry Laboratory, Faculty of Science and Arts, Canakkale Onsekiz Mart University,
Canakkale 17020, Turkey
‡Department of Materials Science and Engineering, Faculty of Engineering, Canakkale Onsekiz Mart University, Canakkale 17020,
Turkey
§Department of Chemistry, Polymer Synthesis and Analysis Laboratory, Faculty of Science and Arts, Canakkale Onsekiz Mart
University, Canakkale 17020, Turkey
∥Lapseki Vocational School, Department of Chemistry and Chemical Processing Technologies, Canakkale Onsekiz Mart University,
Lapseki/Canakkale 17800, Turkey
⊥Department of Chemistry, Gebze Institute of Technology, Gebze/Kocaeli 41400, Turkey
S
* Supporting Information
ABSTRACT: A novel on/off fluorescent rhodamine-based hexapodal Fe3+
probe (L) containing a cyclotriphosphazene core was synthesized by an azide−
alkyne “click-reaction”. The synthesized compounds (1−5 and L) were
characterized by FT-IR; 1H, 13C, and 31P NMR; and MALDI MS spectrometry.
The optical sensor features for the Fe3+ complex of L were investigated by
UV−vis and fluorescence spectroscopy. The stoichiometry of L−Fe3+ complex
was found to be 1:3 (ligand/metal ion), and the detection limit of L was
determined as 4.8 μM (0.27 mg L−1) for Fe3+ ions. The reusability of the
sensor was tested by the addition of ethylenediamine to L−Fe3+ complex
solutions followed by the addition of Fe3+ solution.
instrumental methods such as inductively coupled plasma
atomic emission or mass spectroscopy (ICP-AES, ICP-MS),
atomic absorption spectroscopy (AAS), etc. For this reason,
several studies have focused on construction of selective,
sensitive and reversible ion sensors using spectral methods.
Both spectrophotometric and spectrofluorometric methods are
available for optical chemosensors.19,20 These sensing applica-
tions generally depend on quenching/growing of emission or
absorption peaks. Fluorescence signaling supplies high
sensitivity and easy handling as well as being cheaper to use.
Many studies have been carried out to decrease the detection
limits while generating high selectivity. Development of a new
generation of optical sensors is still needed for environmental
and biological requirements. The Fe3+ ion in the structure of
many enzymes is involved as a catalyst for oxygen metabolism
and electron transfer mechanisms in the human body.21,22
However, a high concentration of Fe3+ ions has a toxic effect on
living organisms and causes diseases such as Parkinson’s disease
and Alzheimer’s disease.23,24 Therefore, a few colorimetric
sensors have been developed for this iron ion species.
Rhodamine derivatives are the most common among these
INTRODUCTION
■
Phosphazene compounds, which are characterized by the
double bond in between phosphorus and nitrogen atoms,
comprise a large class of inorganic compounds.1−5 These
compounds have been the center of interest for many
researchers, as halogen atoms bound by phosphorus atoms in
their structure may easily undergo a substitution reaction with
nucleophilic compounds. The vast majority of studies relevant
to phosphazenes are based on substitution reactions of these
compounds with nucleophilic compounds such as amines,
alcohols, and phenols.6−8 Some phosphazene derivatives and
polymers have applications in various industrial and medical
areas such as the production of inflammable textile fibers and
elastomers,9 liquid crystals,10 anticancer11 and antibacterial
reagents,12 synthetic bone and inert biomaterials for tissue
engineering,13,14 and multicenter coordination compounds.15 In
addition to this, the cyclic phosphazenes are interesting
compounds as a core for the synthesis of dendrimeric structures
due to substituted groups on phosphorus atoms positioned
above and below the plane of the phosphazene ring.8,16−18
Therefore, phosphazene compounds are a very interesting class
of inorganic−organic hybrid compounds.
Detection of heavy metals by optical methods has been
favored due to their relatively lower cost compared to
Received: November 6, 2013
Published: February 5, 2014
© 2014 American Chemical Society
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a
Scheme 1. Synthesis of L
a
Conditions: (i) Cs2CO3, THF, rt, 48 h; (ii) NaBH4, THF/MeOH, rt, 24 h; (iii) PBr3, THF, rt, 24 h; (iv) NaN3, DMF, 90 °C, 24 h; (v) EtOH,
reflux, 24 h; (vi) CuI, PMDETA, THF, 60 °C, 24 h.
chemosensors.23−27 Rhodamine derivatives are colorless with a
tricyclic lactam structure, and their color changes as a result of
the opening of this lactam ring in the presence of an ion.28−31
Yang et al. synthesized three rhodamine-based Fe3+ selective
sensors, and they used these sensors for Fe3+ imaging in living
cells.23 In addition, Zheng et al. developed a europium-based
metal−organic framework containing a terpyridine group for
Fe3+.24 Recently, a few tripodal rhodamine derivatives have
been reported as fluorescent sensors for Al3+, Hg2+, and Cu2+
ions.32−34 We synthesized L, a new 1,2,3-triazole ring
functionalized hexapodal rhodamine derivative on a cyclo-
triphosphazene core. We also demonstrated the sensor
behavior of L for selective colorimetric and fluorometric
detection of Fe3+ ion and determined the detection limit of L as
4.8 μM (0.27 mg L−1) for Fe3+.
azide 4 and alkyne 5 in the presence of copper(I) catalyst as
shown Scheme 1 with 65% yield.
For this purpose first, compounds 4 and 5 were synthesized.
Compound 1 was obtained from the reaction of hexachlor-
ocyclotriphosphazene and 4-hydroxybenzaldehyde in the
presence of Cs2CO3 in dry THF with 92% yield. Then, this
compound was reduced to compound 2. Compound 3 was
obtained from the reaction of 2 with PBr3 with 60.92% yield.
The synthesis of azide 4 was achieved by the reaction of
compound 3 and NaN3 with 88% yield. Compound 5 was
synthesized according to the literature procedure.35 The
reaction of rhodamine 6G and propargyl amine produced 5
as a pale pink solid with 68% yield.
1
The structures of all compounds were characterized by H,
13C, 31P NMR; HR-MS; and MALDI-TOF MS analysis (see
Experimental Section and Supporting Information Figures S1−
S22). Only a singlet peak was observed in the 31P NMR spectra
of compounds 1−4 and L. This shows that all phosphazene
compounds were fully substituted. As an example, the 31P NMR
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Compounds. The
naked-eye sensor L was synthesized by the “click-reaction” of
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of L showed that all phosphorus atoms are identical because
there is a single peak at 8.25 ppm (Supporting Information
Figure S20). In the 1H NMR spectrum of L, the signals related
to protons in the triazole ring and methylene group directly
attached to the nitrogen atom of the triazole ring were observed
as singlets at 6.97 and 5.24 ppm, respectively (Supporting
Information Figure S19).
the solution color of L converts from colorless to pink and
yellow under sunlight and UV light, respectively. However,
there is no visible change in the presence of other cation types.
This colorimetric difference allows the use of L as a color-
tunable Fe3+ sensor for qualitative determination.
The UV−vis spectrum recorded for L in THF/H2O (99:1,
0.01 M HEPES buffer, pH = 7.4) indicated an absorption
maximum at λ = 532 nm relating to the delocalized xanthane
form of rhodamine group formed by opening of the spirolactam
ring.32 According to the literature, certain transition-metal ions
can selectively bind with rhodamine-based molecules which
results in opening of the spirolactam ring and generation of the
xanthene form.36 This conversion is determined by UV−vis and
fluorescence analyses as a new band formation in the visible or
NIR region.37 A similar structural change is expected for Fe3+
induced L as shown in Figure 2. This structure for the L−Fe3+
complex was confirmed by Job’s plot (see Supporting
Information Figure S26a,c). The binding mechanism was also
confirmed by FT-IR spectrum of L−Fe3+ complex (Supporting
Information Figure S27). The characteristic CO stretching
frequency related to the amide group of the rhodamine unit was
observed at 1684 cm−1. This stretching frequency shifted to
1648 cm−1 as a shoulder in the presence of 10 equiv of Fe3+ ion.
This shift in the frequency is the most important evidence of
the binding of the rhodamine unit to Fe3+ ion. Similar shifts in
the frequency of the CO bond stretching related to the
binding rhodamine unit to metal ions have been reported
previously.37
Optical Responses of L in the Presence of Fe3+. To
investigate the selectivity of L, various cations were added to a
solution of L in THF/H2O (99:1, 0.01 M HEPES buffer, pH =
7.4). Rhodamine derivatives are colorless with a tricyclic lactam
structure, and their color and luminescence changes as a result
of the opening of this lactam ring in acidic pH. Therefore, the
pH titration control experiments were carried out for free L and
L−Fe3+ in the pH range from 1 to 12 (Supporting Information
Figure S23). These experiments revealed that the solution of L
did not show any obvious fluorescence in the pH range from 5
to 12 while the solution of L emits strong fluorescence in acidic
media (pH = 1−4). After the addition of Fe3+ ions, strong
fluorescence intensity was observed in the pH range from 5 to
8. Therefore, the emission measurements were carried out
using HEPES as pH-controlled at pH = 7.4. In order to
determine the response time of L, after the addition of Fe3+ ion
the UV−vis spectra were recorded at certain time intervals and
the absorption change at 532 nm was investigated (Supporting
Information Figure S24). All measurements were carried out 1
h after the addition of Fe3+ ion because the fluorescence
intensity of L−Fe3+ mixture reached equilibrium in approx-
imately 45 min. The photostability of L in the absence and
presence of Fe3+ ion was investigated for 60 min in THF/H2O
(99:1, 0.01 M HEPES buffer, pH = 7.4) (Supporting
Information Figure S25). Slit width and excitation wavelength
were 3 and 500 nm. The fluorescence intensity of L and L−
Fe3+ remain the same with starting value. Therefore, it can be
said that L and L−Fe3+ complex ion have excellent photo-
stability.
Selectivity of L was investigated against solutions of Na+, K+,
Ca2+, Ba2+, Mg2+, Ag+, Mn2+, Hg2+, Cu2+, Ni2+, Co2+, Pb2+, Cd2+,
Zn2+, Fe2+, Fe3+, and Cr3+ in THF/H2O (99:1, 0.01 M HEPES
buffer, pH = 7.4). A significant absorption band formation at
532 nm is recorded for only the Fe3+ induced L solution among
the tested cations (Figure 3a). Spectral changes of L solutions
containing Fe3+ with various concentrations (0−50 μM) are
shown in Figure 3b.
Figure 1 represents color differences of L in THF/H2O
(99:1, 0.01 M HEPES buffer, pH = 7.4) upon exposure to
various cation types with the same concentration. Selective
binding of L only with Fe3+ is visible. In the presence of Fe3+
The peak values at 532 nm are plotted versus Fe3+
concentration (Supporting Information Figure S28a), and a
linear relationship is observed with a good regression coefficient
(R2 = 0.9822) in the concentration range from 4 to 50 μM. The
regression equation was A = 0.0117 [Fe3+] (μM) to 0.0472. As
a result, the following equation derived from the linear
absorption change could be used to determine the Fe3+ content
of any tested sample: [Fe3+] (μM) = (A + 0.047)/0.012, where
A is the absorbance of the sample solution. The formation of a
new peak at 532 nm is attributed to the breakage of the
spirolactam ring structure of L and subsequent formation of the
xanthene form.37
The spectral change in emission bands could also be used in
optical sensing applications. To evaluate L as a possible
spectrofluorimetric Fe3+ sensor, the photoluminescence (PL)
spectra were obtained. Figure 4a shows the PL spectra of L in
THF/H2O (99:1, 0.01 M HEPES buffer, pH = 7.4) in the
absence and presence of various cations. The selective binding
of L with Fe3+ among all other metal ions is visible in the PL
spectra. When the L solution containing 50 μM Fe3+ is excited
at 500 nm an emission band with high intensity centered at 550
nm appears. There is nearly no change in the presence of other
cations. This allows the use of L as a selective spectrofluoro-
metric sensor for Fe3+. The quantum yield of the Fe3+ and L
solution was calculated according to the comparative method
and found to be 5.4%. The quantum yield of L in the absence of
Figure 1. Solution color of L exposed to various types of cations under
sunlight (top) and UV light (bottom).
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Figure 2. Proposed sensing mechanism for L−Fe3+ complex.
Figure 4. (a) Fluorescence spectra of L (5 μM) in the presence of
different metal ions (10 equiv). Inset: The digital camera image of L (5
μM) and Fe3+ (50 μM) mixture at 365 nm. (b) Fluorescence intensity
changes of L (5 μM) upon addition of different amounts of Fe3+ in
THF/H2O (99:1, 0.01 M HEPES buffer, pH = 7.4).
Figure 3. (a) Absorption spectra of L (5 μM) in the presence of
different metal ions (10 equiv). Inset: The digital camera image of L (5
μM) and Fe3+ (50 μM) mixture in sunlight. (b) Absorption spectra of
L (5 μM) upon addition of different amounts of Fe3+ in THF/H2O
(99:1, 0.01 M HEPES buffer, pH = 7.4).
Fe3+ was <0.01%. This indicates a huge increase in the
fluorescence band with exposure to Fe3+.
under the same experimental conditions. The relative standard
deviation (RSD %) was calculated as 2.2%. Thus, it may be said
that the proposed sensor has high accuracy and precision.
Possible interference from other cations in the spectro-
fluorometric response to Fe3+ was tested. A series of L
solutions, each of which simultaneously contains the same
concentration of Fe3+ and another tested cation type, were used
for testing. Figure 5 shows the changes in the intensity of the
emission peak (550 nm). The peak intensity of the solution is
not significantly affected by the presence of other cations
indicating the presence of stable complexation between L and
Fe3+. As a result, the sensing measurement is appropriate even
in the presence of high concentrations of guest metals.
A significant increase in the peak intensity at 550 nm is
obtained with increasing concentration of Fe3+ (Figure 4b). A
linear relationship is observed in the concentration range from
4 to 30 μM. The regression equation and regression coefficient
were I = 20.7183 [Fe3+] (μM) to 89.062 and R2 = 0.9801,
respectively. The correlation is suitable for quantitative
determination of Fe3+. The linear equation with a formula of
[Fe3+ ]= (I + 89.06)/20.72 is derived from the intensity change
at 550 nm versus Fe3+ concentration (Supporting Information
Figure S28b), where I is the peak intensity at 550 nm and
[Fe3+] is the concentration of Fe3+ in micromolar. Additionally,
in order to determine the reproducibility or precision of the
sensor, five measurements were carried out for 10 μM of Fe3+
To determine the stoichiometry of the L−Fe3+ complex the
method of continuous variations (Job’s plot) was used. The
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by another suitable ligand is favorable for this aim. Recovery
studies of L after complexation with Fe3+ were carried out using
ethylenediamine (EDA).43 The fluorescence intensity of the
EDA-exposed complex solution is quenched significantly,
indicating an ion exchange from L−Fe3+ complex to EDA
(Figure 6). Repeated use of L was tested by exposure to Fe3+,
Figure 5. Fluorescence changes of the solution containing L and 10
equiv of Fe3+ upon addition of 10 equiv of competing metal ions: 1,
blank (L + Fe3+); 2, Na+; 3, K+; 4, Ca2+; 5, Ba2+; 6, Fe2+; 7, Mg2+; 8,
Ni2+; 9, Co2+; 10, Zn2+; 11, Cu2+; 12, Hg2+; 13, Mn2+; 14, Cr3+; 15,
Cd2+; 16, Ag+; 17, Pb2+.
Figure 6. Reversible fluorescence response of L to Fe3+ ions.
stoichiometry for the L−Fe3+ complex was determined as 1:3
from Job’s plot (see Supporting Information Figure S26a,c).
This is also confirmed by the following equation used to
calculate the stability constant for the L−Fe3+ complex
(Supporting Information Figure S29).34,38
and nearly complete recovery was obtained in the PL intensity.
As a result, the present sensor is appropriate for repeated use to
construct an on−off Fe3+ sensor.
Anions which are found in environmental samples, in
addition to metal ions, can affect the detection limit of the
metal ion sensor. Figure 7 shows the fluorescence intensity
⎡
⎤
Fmax − F
F − F
log
= n log C + log Kas
⎢
⎥
⎦
M
⎣
min
Here Fmax and Fmin limit the fluorescence intensity at the
respective wavelength, n denotes n:1 M/L stoichiometry, and
Kas is the apparent stability constant. According to fluorescence
data, stability constant and stoichiometry were calculated as
4.37 × 1014 and as approximately 3 (3.0691).34 Additionally,
the stability constant of the L−Fe3+ complex was calculated as
3.40 × 1012 from UV−vis data. The regression equation and
regression coefficient were log[Amax − A/A − Amin] = −2.7045
× log[Fe3+] + 12.531 and R2 = 0.9816, respectively.
The detection limit is an important parameter in sensor
applications. The detection limits of L were calculated
according to the fluorescence titration curve. A good linear
relationship was obtained in the range 4−30 μM, and the
detection limit of L was obtained as 4.8 μM (0.27 ppm) for
Fe3+.38 This value is lower than the suggested water quality
standards for iron ions (0.3 ppm) in drinking water by the
WHO and EPA.39,40 Several rhodamine-based sensors with 1:1
and 1:2 stoichiometry for Fe3+ ions are reported in the
literature. For example, while the detection limit for Fe3+ has
been reported as 100 μM by Weerasinghe et al.,41 Chai et al.,27
Chen et al.,42 and Yang et al.23 reported it to be 20, 10, and 5
μM, respectively. It may be said that L is a better sensor from
these reported systems for Fe3+ due to its detection limit and
1:3 stoichiometry. Furthermore, L−Fe3+ has excellent photo-
stability. However, a system with lower detection limit than
reported here was reported by Liu and Wu (2.2 μM).19 In the
reported study, the reversibility of the sensor, the effect of
competing anions on the sensing process, and the photo-
stability of sensor and sensor−Fe3+ complex ion have not been
investigated. Thus, there are many deficiencies of the reported
system by Liu and Wu for a practical application. Our proposed
rhodamine based sensor L is an interesting compound with
hexapodal structure containing six rhodamine moieties on a
cyclotriphosphazene core. Additionally, L has attractive features
such as good photostability, high selectivity, and sensitivity.
Reversible usage is an important feature for optical
chemosensors. This feature allows improvement of an on−off
sensor and decreases the analysis cost. An ion exchange process
Figure 7. Fluorescence changes of the solution containing L and 10
equiv of Fe3+ upon addition of 20 equiv competing anions: 1, blank (L
−
−
+ Fe3+); 2, AcO−; 3, F−; 4, NO2 ; 5, HSO4 ; 6, SCN−; 7, CN−; 8, Br−;
−
−
2‑
9, Cl−; 10, I−; 11, H2PO4 ; 12, NO3 ; 13; PO43‑; 14, CO32‑; 15, SO4
.
changes of L (5 μM) and Fe3+ (50 μM) mixture with the
addition of AcO−, F−, NO2 , HSO4 , SCN−, CN−, Br−, Cl−, I−,
−
−
−
−
3‑
2‑
H2PO4 , NO3 , PO4 , CO3 , and SO42‑ anions (100 μM). As is
clearly shown in the figure, AcO−, CO3 , SCN−, and CN−
2‑
anions cause a decrease in sensitivity of the sensor because they
lead to a decrease in fluorescence intensity of the L−Fe3+
complex. However, SCN− and CN− anions are less common in
aqueous media compared to other ions.
CONCLUSION
■
In this study, we report the synthesis of hexapodal L and its use
as a naked eye fluorescent on−off sensor in the selective
detection of Fe3+. L was synthesized from the “click-reaction” of
a hexaazide-substituted phosphazene compound 4 with a
rhodamine 6G derivative alkyne compound 5. It was found
that L is an excellent selective naked eye sensor for Fe3+ ions in
the presence of other metal ions and anions. Moreover, L
provided remarkable fluorescence intensity and absorbance
change as a result of the opening of the spirolactam ring. The
complex stoichiometry of L−Fe3+ was found as 1:3 (L/M).
Also, the detection limit of L was 4.8 μM (0.27 ppm) for Fe3+.
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(10.00 g, 18.47 mmol) via a syringe through the septum at −10 °C.
The reaction mixture was magnetically stirred at room temperature for
24 h. The solvent was removed by rotary evaporator and to the viscose
oily residue was added distilled water (250 mL). The resulting white
precipitate was collected by filtration and washed with distilled water
until neutral. The crude product was purified by column
chromatography on silica gel with CHCl3/hexane (4:1) solvent
mixture to give 3 as a white crystal solid (3.49 g, 60.92%): mp 149−
150 °C. FTIR-ATR (νmax, cm−1): 1602 (aromatic CC), 1264 and
1159 (PN), 952 (POC). 1H NMR (500 MHz, CDCl3, 25 °C):
δ 7.25 (d, J = 8.54 Hz, 12H, ArH), 6.90 (d, J = 8.54 Hz 12H, ArH),
4.48 (s, 6H, CH2Br). 13C NMR (125 MHz, CDCl3, 25 °C): δ 150.18
(ArC), 134.51 (ArC), 130.27 (ArCH), 121.23 (ArCH), 32.91 (CH2).
31P NMR (202 MHz, CDCl3, 25 °C): δ 8.51 (s). MALDI MS m/z
Calcd for C42H36Br6N3O6P3: 1251.105. Found: 1251.415. Anal. Calcd
for C42H36Br6N3O6P3: C, 40.32; H, 2.90; N, 3.36. Found: C, 40.85; H,
2.85; N, 3.64.
Synthesis and Characterization of 4. The solution of 3 (3.00 g,
2.40 mmol) and NaN3 (1.56 g, 24.00 mmol) in DMF (10 mL) was
heated at 90 °C for 24 h under argon atmosphere. At the end of this
time, the reaction mixture was cooled to room temperature and
poured into rapidly magnetically stirred distilled water (200 mL). The
mixture was stirred for 15 min, and the resulting solid was collected by
filtration. Afterward, the dried solid was isolated as white solid by
column chromatography on silica gel with EtOAc/hexane (3:7)
EXPERIMENTAL SECTION
■
General Experimental Description. The reagents and all
solvents except THF were purchased from chemical companies and
were used without further purification. THF was dried by using Na
and benzophenone. Distilled water was used in all studies. Perchlorate
salts of Na+, K+, Ca2+, Ba2+, Mg2+, Ag+, Mn2+, Hg2+, Cu2+, Ni2+, Co2+,
Pb2+, Cd2+, Zn2+, Fe2+, Fe3+, and Cr3+ ions; tetrabutylammonium salts
of AcO−, F−, NO2 , HSO4 , SCN−, CN−, Br−, Cl−, I−, HPO42‑, and
−
−
NO3 anions; and potassium salts of PO43‑, CO32‑, and SO4 anions
were used in sensor studies. 4-(2-Hydroxyethyl)piperazine-1-ethane-
sulfonic acid (HEPES) was used as buffer media. All UV−vis
absorption and fluorescence spectra were recorded using PG
Instruments T80+ and Shimadzu RF-5301PC spectrofluorophotom-
−
2‑
1
eter, respectively. H, 13C, and 31P NMR spectra were recorded on
Bruker Biospin (400 MHz) or Varian Unity INOVA (500 MHz)
spectrophotometer by using tetramethylsilane and 85% H3PO4 as
1
interval reference for H NMR and 31P NMR, respectively. CDCl3 or
DMSO-d6 was used in all NMR measurements. Mass spectra of the
compounds were recorded on Water SYNAT (HRMS ES+) and
Bruker Microflex LT MALDI-TOF MS spectrometers. FT-IR was
recorded with a Perkin-Elmer FT-IR instrument by using an ATR
apparatus with 4 cm−1 resolution between 4000 and 650 cm−1. The
analytical data of compounds were obtained using a LECO CHNS-932
elemental analyzer. Compounds 1−5 were similarly synthesized
according to literature procedures with minor modifications.1,2,35,44
All UV−vis absorption and fluorescence intensity measurements were
repeated three times. The averaged values were given in the figures.
Caution! Metal perchlorate salts and azides are potentially explosive
compounds under certain conditions such as heat, pressure, and light.
Therefore, even small amounts of these compounds should be used with
caution.
solvent mixture (2.16 g, 88%): mp 125−126 °C. FTIR-ATR (νmax
,
cm−1): 1605 (aromatic CC), 1264 and 1177 (PN), 959 (PO
1
C). H NMR (500 MHz, CDCl3, 25 °C): 7.16 (d, J = 8.46 Hz, 12H,
ArH), 6.97 (d, J = 8.46 Hz, 12H, ArH), 4.32 (s, 12H; CH2N3). 13C
NMR (125 MHz, CDCl3, 25 °C): δ 150.36 (ArC), 132.11 (ArCH),
129.35 (ArCH), 121.23 (ArC), 54.12 (CH2N3). 31P NMR (202 MHz,
CDCl3, 25 °C): δ 8.41 (s). MALDI MS m/z Calcd for C42H36N21O6P3
+ (H+): 1024.244. Found: 1024.431. Anal. Calcd for C42H36N21O6P3:
C, 49.27; H, 3.54; N, 28.73. Found: C, 49.43; H, 3.48; N, 28.81.
Synthesis and Characterization of 5. To a solution of
rhodamine 6G (2.40 g, 5.00 mmol) in methanol (25 mL) was
added propargyl bromide (1.60 mL, 25.00 mmol), and the reaction
mixture was refluxed for 24 h. After the evaporation of the solvent, the
dark red residue was purified by column chromatography on silica gel
with CH2Cl2/hexane solvent mixture to give a pale pink solid (1.54 g,
68%): mp 253−254 °C. FTIR-ATR (νmax, cm−1): 3442 and 3278
(NH), 1700 (CO). 1H NMR (300 MHz, DMSO-d6, 25 °C): 7.81
(m, 1H), 7.49 (m, 1H), 6.98 (m, 1H), 6.27 (s, 2H), 6.13 (s, 2H), 5.06
(s, 2H), 3.77 (s, 2H), 3.13 (m, 4H), 2.69 (s, 1H), 1.87 (s, 6H), 1.21
(m, 6H). 13C NMR (75 MHz, DMSO-d6, 25 °C): δ 166.91 (CO),
154.37, 151.57, 148.17, 133.43, 129.93, 128.73, 128.26, 124.05, 122.96,
118.51, 104.32, 96.32, 79.33 (ArC), 73.05 (CCH), 64.79 (C
CH), 37.96 and 28.61 (NCH2), 17.52 and 14.63 (CH3). HRMS
(ES+) m/z Calcd for C29H29N3O2 + (Na+): 474.196. Found: 474.197.
Anal. Calcd for C29H29N3O2: C, 77.13; H, 6.47; N, 9.31. Found: C,
76.90; H, 3.51; N, 9.36.
Synthesis and Characterization of 1. The solution of
hexachlorocyclotriphosphazene (3.48 g, 10.00 mmol) in dry THF
(50 mL) was dropwise added to a magnetically stirred solution of 4-
hydroxybenzaldehyde (7.46 g, 61.00 mmol) and Cs2CO3 (39.75 g, 121
mmol) in dry THF (200 mL) under argon atmosphere. Afterward the
reaction mixture was stirred at room temperature for 48 h. At the end
of this time, the insoluble salts were filtered, and the solvent was
removed under reduced pressure. The white solid residue was
extracted with 3 × 50 mL CH2Cl2. Then, the combined CH2Cl2
phase was extracted with 3 × 25 mL water and 3 × 25 mL brine,
respectively. The organic phase was dried with Na2SO4. Solvent was
removed under reduced pressure, and crude product was recrystallized
from ethylacetate to give a white crystal solid (7.92, 92%): mp 159−
160 °C. FTIR-ATR (νmax, cm−1): 1703 (CO), 1584 (aromatic C
1
C), 1206 and 1155 (PN), 960 (POC). H NMR (500 MHz,
CDCl3, 25 °C): δ 9.90 (6H, COH), 7.71 (d, Ar−H), 7.12 (d, Ar−H).
31P NMR (202 MHz, CDCl3, 25 °C): δ 7.04 (s). MALDI MS m/z
Calcd for C42H30N3O12P3 + (H+): 862.115. Found: 862.071. Anal.
Calcd for C42H30N3O12P3: C, 58.55; H, 3.51; N, 4.88. Found: C,
58.82; H, 3.44; N, 4.91.
Synthesis and Characterization of L. To a solution of
compound 4 (0.1024 g, 0.10 mmol) and compound 5 (0.32 g, 0.72
mmol) in dry THF (25 mL) was added N,N,N′,N″,N″-pentam-
ethyldiethylenetriamine (PMDETA, 0.25 mL, 1.20 mmol), and the
solution was purged with argon for 15 min. Afterward, copper(I)iodide
(0.0114 g, 0.06 mmol) was added to the reaction mixture. The mixture
was degassed with argon, and it was stirred at 60 °C for 24 h under
argon atmosphere. At the end of this time, the reaction mixture cooled
to room temperature, and the solvent was evaporated under reduced
pressure. The residue was purified by column chromatography on
silica gel with CHCl3/methanol (9:1) solvent mixture to give a pale
orange solid (0.24 g, 65%): mp 235−237 °C. FTIR-ATR (νmax, cm−1):
3377 (NH), 1683 (CO), 1198 and 1159 (PN), 952 (PO
Synthesis and Characterization of 2. NaBH4 (1.45 g, 38.28
mmol) was added to a solution of compound 1 (5.00 g, 5.80 mmol) in
THF/methanol (1:1, 300 mL) mixture at room temperature. The
reaction mixture was stirred for 24 h. Afterward, the solvent was
evaporated, and the obtained crude solids were recrystallized to form a
mixture of ethanol and water (9:1) to give compound 2 as a white
solid (4.87 g, 96%): mp 218−219 °C. FTIR-ATR (νmax, cm−1): 3310
(OH), 1604 (aromatic CC), 1157 (PN), 954 (POC). 1H
NMR (500 MHz, DMSO-d6, 25 °C): δ 7.18 (d, J = 7.81 Hz, 12H,
ArH), 6.79 (d, J = 7.81 Hz, 12H, ArH), 5.25 (br s, 6H, CH2OH), 4.45
(s, 12H, CH2OH). 13C NMR (125 MHz, DMSO-d6, 25 °C): δ 149.02
(ArC), 139.83 (ArCH), 128.11 (ArCH), 120.51 (ArC), 62.69 (CH2).
31P NMR (202 MHz, DMSO-d6, 25 °C): δ 8.84 (s). MALDI MS m/z
Calcd for C42H42N3O12P3 + (H+): 874.205. Found: 874.265. Anal.
Calcd for C42H42N3O12P3: C, 57.74; H, 4.85; N, 4.81. Found: C,
57.44; H, 4.91; N, 4.72.
1
C). H NMR (300 MHz, CDCl3, 25 °C): 7.88 (m, 6H), 7.41 (m,
12H), 7.06 (m, 6H), 6.97 (m, 6H), 6.85 (d, J = 8.60 Hz, 12H), 6.75
(d, J = 8.60 Hz, 12H), 6.30 (s, 12H), 6.07 (s, 12H), 5.24 (s, 12H), 4.42
(s, 12H), 3.68 (b, 12 H), 3.18 (q, J = 6.95 Hz, 24H), 1.78 (s, 36H),
1.28 (t, J = 6.95 Hz, 36H). 13C NMR (75 MHz, CDCl3, 25 °C): δ
167.94 (CO), 153.67, 151.76, 150.04, 147.39, 144.47, 132.61,
Synthesis and Characterization of 3. To a solution of 2 (4.00 g,
4.58 mmol) in dry THF was added dropwise phosphorus tribromide
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132.11, 130.71, 128.85, 128.47, 128.06, 123.84, 122.88, 121.02, 117.76,
105.39, 96.41, 65.16, 52.65, 64.79, 38.31, 35.14, 16.64, 14.71. 31P NMR
(202 MHz, DMSO-d6, 25 °C): 8.25. MALDI MS m/z Calcd for
C216H210N39O18P3: 3729.397 Found: 3729.400. Anal. Calcd for
C216H210N39O18P3: C, 69.49; H, 5.67; N, 14.63. Found: C, 69.27; H,
5.61; N, 14.69.
UV−Vis Absorption Experiments. The stock solutions of L and
metal ions were prepared in THF/H2O (99:1, 0.01 M HEPES buffer,
pH = 7.4). All spectra were measured at 25 °C in this solvent mixture.
During the measurements, stock solutions of L and metal ions were
added into 3 mL of THF/H2O (99:1, 0.01 M HEPES buffer, pH =
7.4) mixture by using a micropipet. In order to keep a constant
concentration of the test solutions, the total volume of the added stock
solutions was maintained at less than 100 μL.
Fluorescence Measurements. Fluorescence characteristics were
determined by a Shimadzu RF-5301PC spectrofluorophotometer.
Measurements were carried out using THF/H2O (99:1, 0.01 M
HEPES buffer, pH = 7.4) solutions of L. The effects of transition metal
ions on selectively quenching−growing of the emission spectra were
investigated in solutions each of which contained 5 μM of L and 50
μM metal ions. The change of fluorescence intensities depending on
the concentration of Fe3+ was determined using a series of different
concentration Fe3+ solutions. Possible interference from other cation
types was investigated using separate solutions of L, each of which
simultaneously contained Fe3+ and another cation with excess
concentration. Reversibility of the sensor was tested using ethylene
diamine (EDA) as a guest ligand for ion exchange. Slit width and
excitation wavelength were 3 and 500 nm in all experiments.
Fluorescence quantum yield (QY) is calculated by comparative
methods using rhodamine 6G solution in ethanol as the standard.
The calculation was carried out as described in the literature by the
following equation45,46
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QY = QY ×[AX/AS]×[FS/FX]× [nX/nS]2
X
S
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ASSOCIATED CONTENT
■
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S
* Supporting Information
1H, 13C, and 31P NMR spectra, and HRMS and MALDI-MS
spectra of all intermediates and L. Fluorescence spectra, FT-IR
spectra of L and L−Fe3+ complex, Job plots, detection limits,
and apparent stability constant determination. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
(27) Chai, M.; Zhang, D.; Wang, M.; Hong, H.; Ye, Y.; Zhao, Y. Sens.
Actuators, B 2012, 174, 231−236.
■
Corresponding Author
(28) Yan, F.; Wang, M.; Cao, D.; Yang, N.; Fu, Y.; Chen, L. Dyes
Pigm. 2013, 98, 42−50.
*E-mail: havaozay@comu.edu.tr.
(29) Zhang, D.; Li, M.; Wang, M.; Wang, J.; Yang, X.; Ye, Y.; Zhao, Y.
Sens. Actuators, B 2013, 177, 997−1002.
Notes
The authors declare no competing financial interest.
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This work was financially supported by the Scientific and
Technological Research Council of Turkey (TUBITAK, Project
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