Synthesis of a dendrimeric phenoxy-substituted cyclotetraphosphazene and its non-covalent interactions with multiwalled carbon nanotubes

Article abstract of DOI:10.1016/j.poly.2013.09.011

A dendrimeric phenoxy-substituted cyclotetraphosphazene has been prepared for the non-covalent functionalization of multiwalled carbon nanotubes (MWCNTs). The synthesized cyclotetraphosphazene derivatives (2-4) and dendrimeric cyclotetraphosphazene (5) have been characterized by standard spectroscopic techniques. The functionalization of MWCNTs with compound 5 has been accomplished and the resulting MWCNT-5 hybrid has been characterized by ³¹P NMR, ¹H NMR, Raman spectroscopy, X-ray diffraction (XRD), energy dispersive X-ray (EDX), fluorescence, thermal gravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM) techniques. The results showed that the dendrimeric compound 5 has been attached to the MWCNTs via non-covalent π-π interactions between the phenoxy groups on the dendrimeric compound 5 and the extended π system of the MWCNTs. The MWCNT-5 hybrid exhibited significant dispersion stability in common organic solvents.

Full text of DOI:10.1016/j.poly.2013.09.011

Polyhedron 67 (2014) 344–350
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis of a dendrimeric phenoxy-substituted cyclotetraphosphazene
and its non-covalent interactions with multiwalled carbon nanotubes
Elif Okutan a,b, Bünyemin Çosßut a, Saadet Beyaz Kayıran c, Mahmut Durmußs a, Adem Kılıç a,
Serkan Yeßsilot a,
⇑
a Department of Chemistry, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey
b Department of Chemistry and Pharmacy, Friedrich-Alexander Universität Erlangen, Nürnberg, 91054 Erlangen, Germany
c Department of Chemistry, Kocaeli University, 41380 Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 May 2013
Accepted 2 September 2013
Available online 21 September 2013
A dendrimeric phenoxy-substituted cyclotetraphosphazene has been prepared for the non-covalent func-
tionalization of multiwalled carbon nanotubes (MWCNTs). The synthesized cyclotetraphosphazene deriv-
atives (2–4) and dendrimeric cyclotetraphosphazene (5) have been characterized by standard
spectroscopic techniques. The functionalization of MWCNTs with compound 5 has been accomplished
and the resulting MWCNT-5 hybrid has been characterized by 31P NMR, 1H NMR, Raman spectroscopy,
X-ray diffraction (XRD), energy dispersive X-ray (EDX), fluorescence, thermal gravimetric analysis
(TGA) and high-resolution transmission electron microscopy (HRTEM) techniques. The results showed
Keywords:
p–p stacking
Phosphazene
Carbon nanotube
that the dendrimeric compound 5 has been attached to the MWCNTs via non-covalent ‘‘
between the phenoxy groups on the dendrimeric compound 5 and the extended
p
p
–
p
’’ interactions
system of the
MWCNTs. The MWCNT-5 hybrid exhibited significant dispersion stability in common organic solvents.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
the p–p interaction [15]. So far, polynuclear aromatic compounds
[16,17], conjugated polymers [18] and also dendrimers like phtha-
locyanine [19] and porphyrin [20] have been used to modify CNTs.
Dendrimers have drawn considerable attention for the functional-
ization of CNTs because of their size, symmetric, highly branched
and non-entangled architecture [21]. For instance, dendrimeric
cyclophosphazene frameworks can provide valuable advantageous
purposes towards useful materials for CNT-dendrimeric phospha-
zene materials.
The multi-functional nature of chlorocyclophosphazenes allows
these compounds to be utilized as dendrimer cores [22]. N3P3Cl6
and N4P4Cl8 offer six and eight branching points respectively,
which is significantly more than the ones available with conven-
tional organic cores. Additionally, depending on the nature of the
side groups, cyclic phosphazene derivatives and macromolecules
show high chemical and thermal stabilities and they can also dis-
play various properties suitable for a wide range of applications
[23–28]. Even though some poly(organophosphazenes) have been
successfully used for the coating of CNTs based on non-covalent
interactions [8,29–31], dendrimeric cyclic phosphazene-CNT hy-
brid materials have not been reported. In our previous works, we
have demonstrated that poly(organophosphazene)s are very
attractive candidates for an advanced dispersing agent of carbon
nanotubes [8,9]. Poly(organophosphazenes) are reasonably flexible
and usually possess a linear or branched structure, which may lead
to relatively unstable dispersions because they can wrap
Carbon nanotubes (CNTs) have attracted particular interest
since their discovery in the 1990s because of their extraordinary
properties [1]. CNTs have been incorporated into many application
areas, such as optoelectronics, nanotechnology and chemical sens-
ing [2–4]. The key problem for the application of CNTs is their poor
solubility, which is caused by bundling of carbon nanotube walls
owing to van der Waals interactions between the walls of CNTs
[5]. To this end, many surface modification approaches have been
proposed to address the solubility problem of CNTs, including
covalent and non-covalent functionalization [6–10]. Whilst non-
covalent functionalization can be achieved without destroying
the extended p-network of CNTs, covalent functionalization results
in permanent structural modification of CNTs, which may cause a
loss of their unique properties [11]. Among the many non-covalent
solubilization approaches, the sorption of aromatic compounds on
CNTs has been studied by several groups [12,13]. Aromatic com-
pounds can interact strongly with the graphitic walls of CNTs
through effective p–p stacking [14]. The size, shape and the envi-
ronment of the aromatic system generally affects the affinity of
⇑
Corresponding author. Address: Department of Chemistry, Gebze Institute of
Technology, P.O. Box: 141, Gebze 41400, Kocaeli, Turkey. Tel.: +90 262 6053137;
fax: +90 262 6053101.
E-mail address: yesil@gyte.edu.tr (S. Yesßilot).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.09.011

E. Okutan et al. / Polyhedron 67 (2014) 344–350
345
themselves around either nanotubes or nanotube bundles. CNT-
2.3. Synthesis
dendrimeric cyclophosphazene systems which disrupt the van
der Waals interactions that cause the insolubility of CNTs show
promise. In this respect, the development of new non-covalent
CNT-dendrimeric cyclophosphazene hybrid materials showing
high thermal stability and solubility in widely used solvents would
be important for the applications of CNT-based systems.
1,1,3,3,5,5,7,7-Octaphenoxy-cyclotetraphosphazatetraene, [NP
(OC6H5)2]4, (1) was prepared according to procedures described
previously [33].
2.3.1. Synthesis of compound (2)
In this research, procedures of non-covalently introducing a
phenoxy-substituted dendrimeric cyclotetraphosphazene (5) onto
the side walls of MWCNTs by non-covalent adsorption and the
spectroscopic, thermal and morphological properties of the soluble
MWCNT-5 nanocomposite, using 31P NMR, XRD, EDX, TGA and
HRTEM techniques, are reported. The fluorescence, Raman and 1H
NMR spectra confirmed the non-covalent functionalization of
MWCNTs with phenoxy groups on the dendrimeric compound.
The hybrid material shows both high thermal stability and
excellent fluorescence quenching efficiency. Also the hybrid
(MWCNT-5) showed significant dispersion stability in common or-
ganic solvents, such as dichloromethane, chloroform, N,N-dimeth-
ylformamide, dimethyl sulfoxide and tetrahydrofuran.
A solution of sodium phenoxide was prepared by the dropwise
addition of phenol (2.1 g, 23 mmol) to a sodium hydride (1.1 g,
28 mmol, 60%) dispersion in dry THF (60 mL) at 0 °C. Octachlorocy-
clotetraphosphazene (1.6 g, 3.3 mmol) in dry THF (30 mL) was
added dropwise to the reaction mixture of sodium phenoxide,
and the resulting mixture was stirred for 0.5 h. The reaction mix-
ture was then stirred for 1 day under an atmosphere of argon
and the reaction was followed by TLC. Sodium chloride was re-
moved by filtration, the solvent was removed under reduced pres-
sure and the resulting product was subjected to column
chromatography using benzene:petroleum ether (1:1) as the elu-
ent. Yield: 2.5 g (80%). Anal. Calc. for [C42H35ClN4O7P4] Found: C,
58.26; H, 4.10; N, 6.40%; requires: C, 58.18; H, 4.07; N, 6.46%. MS
(ESI) m/z (%) Calc.: 866; found: 867 (100) [M+H]+. 31P{1H} NMR
(toluene-d8), assigned as an AB2C spin system d: À4.064 [1P,
P(OPh)Cl, A], À11.65 [2P, P(OPh)2, B2], À11.05 [1P, P(Ph)2, C],
2. Experimental
2
2
2JAB = 55 Hz, JAC = 18 Hz, JCB = 79 Hz. 1H NMR (CDCl3) d: 7.01–
7.29 (m, 35H, ArCH). {1H}13C NMR (CDCl3) d: 151.40 (ArC),
144.30 (ArC), 129.53 (ArCH), 125.44 (ArCH), 124.90 (ArCH),
121.77 (ArCH), 121.28 (ArCH).
2.1. Materials
Octachlorocyclotetraphosphazene (Otsuka Chemical Co. Ltd.)
was purified by fractional crystallization from n-hexane. The deu-
terated solvents (CDCl3 and toluene-d8) for NMR spectroscopy and
the following chemicals were obtained from Merck; cyclohexene,
ethanol, phenol, Pd(OH)2, Cs2CO3, NaH, acetone, triethylamine, sil-
ica gel 60, tetrahydrofuran Ni(NO3)2Á6H2O, MgO and HCl (37%).
1,8,9-Anthracenetriol for the MALDI matrix was obtained from Flu-
ka. Acetylene (99.8%), helium (99.98%) and hydrogen (99.998%)
gases were purchased from Yalız Industrial Medical Gases Inc.
2.3.2. Synthesis of compound (3)
4-(Benzyloxy) phenol (0.54 g, 2.7 mmol), dry and finely pow-
dered cesium carbonate (0.91 g, 2.8 mmol) were dissolved in dry
THF (10 mL) under an argon atmosphere. The solution was trans-
ferred into a dropping funnel (50 mL) and slowly dropped into a
solution of 1,1,3,3,5,5,7-heptaphenoxy-7-chlorocyclotetraphosph-
azatetraene (2 g, 2.3 mmol) (2) in dry THF (10 mL) under an argon
atmosphere in a three necked round bottomed flask (50 mL). The
reaction mixture was refluxed under argon for 24 h and followed
by TLC, until it indicated that no starting material remained. The
precipitated salt (CsCl) was filtered off and the solvent was re-
moved under reduced pressure. The crude product was purified
by column chromatography using silica gel 60 (70–230 mesh) as
the adsorbent and dichloromethane:n-hexane (1:2) as the eluent.
1,1,3,3,5,5,7-Heptaphenoxy-7-[(4-benzyloxy)phenoxy]-cyclo-
tetraphosphazatetraene (3) was obtained as a viscous oil; Yield:
1.65 g (68%). Anal. Calc for [C55H46N4O9P4] Found: C, 64.16; H,
4.55; N, 5.36%; requires: C, 64.08; H, 4.50; N, 5.43%. MS (ESI) m/z
(%) Calc.: 1030; found: 1163 (100) [M+Cs]+. 31P{1H} NMR (tolu-
ene-d8) d: À12.45 (br, s, 4P). 1H NMR (CDCl3) d: 6.61–7.33 (m,
44H, ArCH), 4.89 (br s, 2H, CH2). {1H}13C NMR (CDCl3) d: 155.69
(ArC), 151.60 (ArCH), 145.37 (ArC), 137.28 (ArC), 129.39 (ArCH),
128.87 (ArCH), 127.68 (ArCH), 124.56 (ArCH), 121.26 (ArCH),
115.38 (ArCH), 70.64 (CH2).
2.2. Equipment
Elemental analyses were carried out using a Thermo Finnigan
Flash 1112 Instrument. UV–Vis spectra were recorded with a
Shimadzu 2101 UV spectrophotometer. Fluorescence excitation
and emission spectra were recorded on a Varian Eclipse spectroflu-
orometer using 1 cm pathlength cuvettes at room temperature.
Mass spectra were acquired in linear modes with an average of
50 shots on a Bruker Daltonics Microflex mass spectrometer
(Bremen, Germany) equipped with a nitrogen UV-Laser operating
at 337 nm. Analytical thin layer chromatography (TLC) was
performed on silica gel plates (Merck, Kieselgel 60, 0.25 mm thick-
ness) with F254 indicator. Column chromatography was performed
on silica gel (Merck, Kieselgel 60, 230–400 mesh; for 3 g. crude
mixture, 100 g. silica gel was used in a column 3 cm in diameter
and 60 cm in length) and preparative thin layer chromatography
was performed on silica gel 60 P F254.
1H, 13C and 31P NMR spectra
were recorded in CDCl3 and toluene-d8 solutions on a Varian
500 MHz spectrometer. The thermal properties of the compounds
were investigated on a Mettler Toledo TGA/SDTA 851 Thermo-
gravimetric Analysis (TGA) instrument. The X-ray diffraction
(XRD) patterns of the samples were taken at room temperature
with 2h = 5–40° on a BRUKER D8 Advance X-ray diffractometer
2.3.3. Synthesis of compound (4)
1,1,3,3,5,5,7-Heptaphenoxy-7-[(4-benzyloxy)phenoxy]-cyclo-
tetraphosphazatetraene (3) (1.05 g, 1 mmol) was dissolved in dry
THF (10 mL) under an argon atmosphere and cyclohexene
(10 mL), palladium (II) hydroxide (0.4 g, 20 wt% on carbon) and
ethanol (10 mL) were added to this solution. The mixture was re-
fluxed for 24 h under an argon atmosphere and filtered. All of the
solvents were removed under reduced pressure. 1,1,3,3,5,5,7-Hep-
taphenoxy-7-[(4-hydroxy)phenoxy]cyclotetraphosphazatetraene
(4) was obtained by crystallization from dichloromethane/n-hex-
ane (1:1); Yield: 0.75 g (78%). Anal. Calc for [C48H40N4O9P4] Found:
C, 61.33; H, 4.35; N, 5.89%; requires: C, 61.28; H, 4.29; N, 5.96%. MS
(ESI) m/z (%) Calc.: 940; found: 963 (100) [M+Na]+. 31P{1H} NMR
equipped with a sealed tube copper target (k Cu Ka = 1.54056 Å).
The structural analysis was performed on an automated Renishaw
InVia Reflex Raman microscopy system (Renishaw Plc., New Mills,
Wotton-under-Edge, UK) equipped with a 514 nm laser, which was
used for all of the experiments. The morphology of the composite
(MWCNT-5) was characterized using a Philips-FEI G2 F20 S-Twin
HRTEM. The NMR simulation program, available free of charge,
used was gNMR [32].

346
E. Okutan et al. / Polyhedron 67 (2014) 344–350
(toluene-d8) d: À12.41 (br, s, 4P). 1H NMR (CDCl3) d: 6.58–7.20 (m,
39H, ArCH), 5.60 (s, 1H, OH). {1H}13C NMR(CDCl3) d: 152.77 (ArC–
OH), 151.60 (ArC), 145.04 (ArC), 129.41 (ArCH), 124.57 (ArCH),
122.25 (ArCH), 121.26 (ArCH), 115.82 (ArCH).
3. Results and discussion
It is well known [16] that aromatic organic molecules, such as
benzene, anthracene or pyrene, can interact with the surfaces of car-
bon nanotubes surfaces by p–p stacking, hence we chose the phen-
oxy substituent as a prototypical electron rich system for the
synthesis of dendrimeric cyclophosphazene frameworks, which
were expected to interact with MWCNTs and increase the stability
of MWCNT dispersions in common solvents. Firstly, octa-
2.3.4. Synthesis of compound (5)
N4P4Cl8 (0.03 g, 0.064 mmol) and compound
4 (0.49 g,
0.51 mmol) were dissolved in dry THF (10 mL) under an argon
atmosphere. After stirring for 15 min. at 40 °C, dry and finely pow-
dered cesium carbonate (0.195 g, 0.6 mmol) was added portion-
wise over 15 min. with efficient stirring. The reaction mixture
was stirred under an argon atmosphere at 80 °C for 24 h. The vol-
atile materials were evaporated under vacuum and the product
was purified by preparative TLC on silica gel using n-hexane:THF
(1:1) as the eluent. Compound 5 was obtained as a viscous oil;
Yield: 0.18 g (37%). Anal. Calc for [C384H312N36O72P36] Found: C,
59.96; H, 4.15; N, 6.47%; requires C, 59.91; H, 4.09; N, 6.55%. MS
(MALDI-TOF) m/z (%) Calc.: 7697; found: 7698(100) [M+H]+. 31P
{1H} NMR (CDCl3) d: À12, 46 (br, s, 36P). 1H NMR (CDCl3) d:
7.19–6.51 (m, 312H, ArCH). {1H} 13C NMR (CDCl3) d: 142.35
(ArC), 134.39 (ArCH), 133.74 (ArC), 132.83 (ArCH), 129.99 (ArCH).
phenoxycyclotetraphosphazatetraene (1) was synthesized as
a
small-molecule model [33], (Scheme 1), and the MWCNT-1 nano-
composite was prepared according to the experimental section. Sol-
ubility tests showed that the MWCNT-1 composite is insoluble in
organic solvents. This result can be explained by the p–p interactions
between compound 1 and the MWCNTs being so weak that these
interactions are not enough to provide solubility of the MWCNTs. Be-
cause studies had indicated that the p–p interactions between aro-
matic molecules and CNTs increased with the increasing number
of aromatic rings in the adsorbate molecule [14], the dendrimeric
compound 5, containing more phenoxy units, was designed. The syn-
thetic route to the dendrimeric compound 5 is shown in Scheme 1.
Compound 2 was synthesized by the reaction of 7 equivalents of
phenol with 1 equivalent of N4P4Cl8 in the presence of NaH in THF.
Then compound 2 was reacted with 4-benzyloxyphenol in the pres-
ence of Cs2CO3 in THF to obtain compound 3. The 4-benzyloxyphen-
oxy unit of compound 3 was converted to a 4-hydroxyphenoxy
group by hydrogenolysis with cyclohexene in a mixture of THF/eth-
anol, in the presence of Pd(OH)2 as a catalyst, to give compound 4 in
good yield (90%). Compound 5 was obtained from a nucleophilic dis-
placement reaction of 4 with N4P4Cl8 under an argon atmosphere
with cesium carbonate as a base. All the products were purified by
column chromatography and/or preparative TLC techniques.
2.3.5. Synthesis and purification of MWCNTs
Pristine MWCNTs (p-MWCNTs) were synthesized by the cata-
lytic decomposition of acetylene (C2H2) over a nickel/magnesium
oxide catalyst (Ni/MgO, 5 wt% Ni). The catalyst was prepared via
the impregnation method from Ni(NO3)2Á6H2O solution in ethanol.
The supported catalyst was placed in a horizontal fixed-bed flow
reactor in a quartz boat, which was inserted in a quartz tube and
was calcined in air at 600 °C. After the calcinations, the catalyst
was reduced in a hydrogen flow (20 mL/min) for 2 h at 600 °C. To
grow the p-MWCNTs, the decomposition of the acetylene flow
(60 mL/min) was carried out at 650 °C for 30 min, after that the reac-
tor was cooled to room temperature under a helium atmosphere and
the p-MWCNTs were collected from the quartz boat. The p-MWCNTs
were oxidated thermally at 350 °C for 3 h and sonicated in hydro-
chloric acid (37% by volume) for 2 h at 40 °C. The solution was cen-
trifuged at 4000 rpm for 1 h and the excess of acid was removed. The
acid treated material was redispersed in ethanol to remove the acid,
at least 5 times and with the help of an ultrasonic bath. The purified
MWCNTs were collected by centrifugation at 4500 rpm for 30 min
and dried under vacuum for 2 days at 60 °C.
The new compounds were characterized by 1H, 13C and 31P NMR
spectroscopy, mass spectrometry and elemental analysis. All of the
results were consistent with the predicted structures, as shown in
the experimental section. The elemental analysis results and the
mass spectral data for the newly synthesized cyclic compounds 2–
5 were consistent with the assigned formulations. While the mass
spectra of compounds 2–4 were obtained by ESI mass spectrometry,
the mass spectrum of compound 5 was measured by MALDI-TOF-
MS using 1,8,9-anthracenetriol as the MALDI matrix. As an example,
the mass spectrum of 5 shows the peak group representing the pro-
tonated molecular ion at 7698 Da (Fig. 1). The proton-decoupled 31
P
NMR spectrum of compound 2 showed a non-first order 31P NMR
spectrum, so it was necessary to carry out simulation analysis to
get the appropriate spectral parameters. Following this analysis,
compound 2 has been assigned as an AB2C spin system (Fig. 2),
and the corresponding parameters have been given in the experi-
mental section. The integration of the A part of the AB2C pattern
to the remaining B2C part of the proton-decoupled 31P NMR spec-
trum of compound 2 shows a nearly 1:3 phosphorus ratio, which
supports the suggested structures. Although similar AB2C patterns
were expected in the 31P NMR spectra of compounds 3, 4 and 5,
the resonances were observed as broad singlets at À12.45, À12.41
and À12.46 ppm, respectively, rather than a clear AB2C pattern.
The MWCNT-5 composite was prepared by the treatment of
compound 5 with MWCNTs, obtained by the Chemical Vapor
Deposition (CVD) method (1:1; w:w) using stirring and sonicating
in N-methyl-2-pyrrolidone (NMP) (Scheme 2). Solutions of the
MWCNT-5 hybrid in different solvents, including dichloromethane,
tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide and
chloroform, with same concentration (3 mg/mL) were prepared.
The MWCNT-5 composite was dispersed in each solvent by sonica-
tion in an ultrasonic bath for 10 min. Homogeneous solutions were
all present after 3 days from the preparation.
2.3.6. Synthesis of MWCNT-1 nanocomposite
10 mg of MWCNTs were dispersed in 3 mL N-methyl-2-pyrroli-
done (NMP)bysonication for15 minatfullpower.Following addition
ofcompound 1 (10 mg, 0.0024 mmol, 1 eq.), the reactionmixture was
sonicated for 1 h and stirred for 2 days at room temperature under an
argon atmosphere. The mixture was then diluted with dichlorometh-
ane (50 mL) and washed with water (5 Â 100 mL). The solvent was
removed under reduced pressure. Finally the solid was dried under
vacuum to afford 17 mg of a black solid.
2.3.7. Synthesis of MWCNT-5 nanocomposite
20 mg of MWCNTs were dispersed in 5 mL N-methyl-2-pyrrol-
idone (NMP) by sonication for 15 min at full power. Following
addition of compound 5 (20 mg, 0.0024 mmol, 1 eq.), the reaction
mixture was sonicated for 1 h and stirred for 2 days at room tem-
perature under an argon atmosphere. The mixture was then di-
luted with dichloromethane (50 mL) and washed with water
(5 Â 100 mL). The solvent was removed on a rotary evaporator un-
der reduced pressure. Finally the solid was dried under vacuum to
afford 35 mg of a black solid. 31P{1H} NMR (CDCl3) d: À12.50 (br, s,
36P). 1H NMR (CDCl3) d: 7.20–6.59 (m, 312H, ArCH).

E. Okutan et al. / Polyhedron 67 (2014) 344–350
347
Scheme 1. Chemical structures and synthetic pathways for compounds 1–5.
Fig. 1. The MALDI-TOF MS spectrum of compound 5.
The fluorescence emission spectra of compound 5 and the
MWCNT-5 composite were measured in tetrahydrofuran with di-
lute solutions of 1 Â 10À5 mol dmÀ3 upon excitation at 260 nm
(Fig. 3). Upon excitation of the phenoxy groups, the dilute tetrahy-
drofuran solution of compound 5 shows an emission band at
302 nm due to the phenoxy groups. In contrast, after non-covalent
adsorption of compound 5 onto the side walls of MWCNTs, a dras-
tic decrease in the intensity of the fluorescence spectrum was ob-
served. The dilute tetrahydrofuran solution of MWCNT-5 did not
show an emission band, which can be rationalized by the intramo-
lecular interaction of electrons on the phenoxy groups of
the dendrimeric phosphazene with the extended system of the
MWCNTs. This fluorescence quenching was not attributed to the
decreasing solubility of compound when adsorbed on
the MWCNTs, since an excellent dispersion of MWCNT-5 nano-
composite in tetrahydrofuran was achieved (Fig. 3). The result indi-
cates that compound 5 undergoes efficient fluorescence quenching
when adsorbed on the MWCNTs. The high fluorescence quenching
p
p
5

348
E. Okutan et al. / Polyhedron 67 (2014) 344–350
Fig. 2. (a) Normal and (b) simulated 31P NMR spectra of compound 2 in toluene-d8.
Scheme 2. Preparation of the MWCNT-5 nanocomposite.
Fig. 3. Fluorescence emission spectra of compound 5 and MWCNT-5 nanocompos-
ite in THF (excitation wavelength = 260 nm).
Fig. 4. Comparative Raman spectra of MWCNTs and the MWCNT-5 nanocomposite.

E. Okutan et al. / Polyhedron 67 (2014) 344–350
349
Raman spectroscopy was used to examine the interaction be-
tween MWCNTs and compound 5. Fig. 4 shows a comparison of
the Raman spectra of MWCNTs and the MWCNT-5 nanocomposite.
In both cases, two important features of the spectra can be ob-
served, the tangential G band (1573 cmÀ1), assigned to the active
E2g vibration of graphite, and the disorder induced D band
(1342 cmÀ1). The intensity ratio of the G band versus the D band,
IG/ID, can provide important clues regarding the defectiveness of
CNTs, as the intensity of the D band is generally considered to be
directly proportional to the defect density in the hexagonal carbon
network of CNTs. The Raman spectrum of the MWCNT-5 nanocom-
posite shows that the intensity ratio of the D and G bands is almost
the same after the formation of the composite material. This result
indicates the present non-covalent modification of the MWCNTs
has little impact on the CNTs, since the D-band intensity of the
MWCNTs is comparable to the D-band intensity in the MWCNT-5
sample [37].
The X-ray diffraction patterns of the MWCNTs and MWCNT-5
nanocomposite are presented in Fig. 5. The typical peak of the
MWCNT structure, centered at 2h = 26°, is clearly observed and
indicates that the interplanar distance is about 0.342 nm. The
influence of the dendrimeric phosphazene attachment on the
MWCNT structure is shown in the diffraction pattern of the
MWCNT-5 nanocomposite. The MWCNT-5 nanocomposite showed
the characteristics of the parent compounds (compound 5 and
MWCNT). Indeed, it exhibited three broad peaks situated 2h = 9°,
14° and 21°, and one peak centered at 2h = 26°, corresponding to
compound 5 and MWCNT respectively.
Fig. 5. Comparative X-ray diffraction patterns of MWCNTs and MWCNT-5
nanocomposite.
The thermal stability of MWCNTs, compound
5 and the
MWCNT-5 composite were investigated by TGA (10 °C/min, 25–
700 °C, N2) (Fig. 6). The TGA curve of MWCNTs shows no significant
decomposition between 25 and 700 °C. Hence this result suggests
that the purification procedure efficiently removed metallic resi-
dues and amorphous carbon nanoparticles. The TGA curve of com-
pound 5 shows the main decomposition is between 340 and 600 °C
with a 46% weight loss and this is in the expected decomposition
temperature range of phenoxy-substituted dendrimeric cyclic
phosphazenes [38]. In the thermal decomposition curve of
MWCNT-5, an essential weight reduction was observed in the re-
gion 350–700 °C, due to the presence of compound 5. The last
decomposition of MWCNT-5 is at about 491 °C. The char yield of
the nanocomposite at 700 °C is 89%.
Fig. 6. TGA results of MWCNTs, compound 5 and the MWCNT-5 nanocomposite.
Fig. 7A shows the HRTEM image of the MWCNT-5 assemblies.
The measured MWCNT diameter is about 28.3 nm and the internal
diameter is about 5.8 nm. Fig. 7A also illustrates the HRTEM image
of compound 5 strands coiled around the MWCNT. An ultra-thin
efficiency of this CNT-aromatic conjugate is consistent with data in
the literature [34–36] and supports the notion that the phenoxy
parts of the dendrimeric compound 5 directly interact with the
surface of the nanotubes surface.
Fig. 7. HRTEM images and EDX analysis of MWCNT-5 nanocomposite.

350
E. Okutan et al. / Polyhedron 67 (2014) 344–350
[9] E. Okutan, G.O. Aydın, F. Hacıveliog˘lu, S.K. Beyaz, S. Yesßilot, A. Kılıç, J. Mater.
Sci. 48 (2013) 201.
film amorphous layer can be clearly seen covering the outer sur-
faces of the MWCNT with a thickness of 1.9–3.9 nm. The EDX im-
age shows the elemental analysis of the individual composition
of the MWCNT-5 composite (Fig. 7B).
[10] Y.L. Zhao, J.F. Stoddart, Acc. Chem. Res. 42 (2009) 1161.
[11] D.A. Britz, A.N. Khlobystov, Chem. Soc. Rev. 35 (2006) 637.
[12] D. Lin, B. Xing, Environ. Sci. Technol. 42 (2008) 7254.
[13] S. Gotovac, H. Honda, Y. Hattori, K. Takahashi, H. Kanoh, Nano Lett. 7 (2007)
583.
4. Conclusion
[14] A. Star, T.R. Han, J.C.P. Gabriel, K. Bradley, G. Gruner, Nano Lett. 3 (2003) 1421.
[15] C.A. Hunter, J.K.M. Sanders, J. Am. Chem. Soc. 112 (1990) 5525.
[16] J. Zhang, J.K. Lee, Y. Wu, R.W. Murray, Nano Lett. 3 (2003) 403.
[17] R.J. Chen, Y. Zhang, D. Wang, H. Dai, J. Am. Chem. Soc. 123 (2001) 3838.
[18] D.R. Kauffman, A. Star, Angew. Chem., Int. Ed. 47 (2008) 6550.
[19] X. Wang, Y. Liu, W. Qiu, D. Zhu, J. Mater. Chem. 12 (2002) 1636.
[20] L. Valentini, M. Trentini, F. Mengoni, J. Alongi, I. Armentano, L. Ricco, A.
Mariani, J.M. Kenny, Diamond Relat. Mater. 16 (2007) 658.
[21] J.T. Sun, C.Y. Hong, C.Y. Pan, Polym. Chem. 2 (2011) 998.
[22] J.P. Majoral, A.M. Caminade, Chem. Rev. 99 (1999) 845.
[23] A.S. Sawhney, C.P. Pathak, J.A. Hubbell, Macromolecules 26 (1993) 581.
[24] J.L. Sassus, M. Graffeuil, P. Castera, J.F. Labarre, Inorg. Chim. Acta 108 (1985) 23.
[25] X.Y. Zhou, S.N. Lvov, H.R. Allcock, Macromolecules 35 (2002) 6490.
[26] N. Chattopadhyay, B. Haldar, A. Mallick, S. Sengupta, Tetrahedron Lett. 46
(2005) 3089.
In summary, a new hybrid system consisting of MWCNTs and a
dendrimeric cyclotetraphosphazene with aromatic phenoxy side
groups (5) was successfully prepared via non-covalent adsorption.
The dendrimeric cyclotetraphosphazene (5) interacted strongly
with the surface of the MWCNTs via ‘‘p–p’’ stacking interactions,
as evidenced by drastic quenching of the phenoxy side groups on
the dendrimeric phosphazene fluorescence, Raman and 1H NMR
spectra. The resulting MWCNT-5 hybrid showed high thermal sta-
bility and solubility in common organic solvents. We anticipate
that the presented method will open a new route for designing var-
ious CNT-dendrimer composites without provoking damage to the
[27] A.W. Freeman, S.C. Koene, P.R.L. Malenfant, M.E. Thompson, J.M.J. Frechet, J.
Am. Chem. Soc. 122 (2000) 12385.
conjugated
p
system of the hybrid nanomaterial. Besides, these hy-
[28] H.R. Allcock, J.D. Bender, R.V. Morford, E.B. Berda, Macromolecules 36 (2003)
3563.
[29] J. Fu, X. Huang, Y. Huang, J. Zhang, X. Tang, Chem. Commun. 9 (2009) 1049.
[30] P. Zhang, X. Huang, J. Fu, Y. Huang, X. Tang, Macromol. Mater. Eng. 295 (2010)
437.
[31] H.J. Park, H.Y. Heo, S.C. Lee, M. Park, S.S. Lee, J. Kim, J.Y. Chang, J. Inorg
Organomet. Pol. Mater. 16 (2006) 359.
[32] P.H.M. Budzelaar, <http://home.cc.umanitoba.ca/~budzelaa/gNMR/gNMR-
license.html>.
[33] H.R. Allcock, A.A. Dembek, M.N. Mang, G.H. Riding, M. Parvez, K.B. Visscher,
Inorg. Chem. 31 (1992) 2734.
[34] A. Bianco, K. Kostarelos, C.D. Partidos, M. Prato, Chem. Commun. 5 (2005) 571.
[35] H. Durmaz, A. Dag˘, Ü. Tunca, G. Hizal, J. Polym. Sci., Part A: Polym. Chem. 50
(2012) 2406.
[36] S. Park, J.O. Huh, N.G. Kim, S.M. Kang, K.B. Lee, S.P. Hong, Y. Do, I.S. Choi, Carbon
46 (2008) 706.
[37] P. Li, H. Liu, Y. Ding, Y. Wang, Y. Chen, Y. Zhou, Y. Tang, H. Wei, C. Cai, T. Lu, J.
Mater. Chem. 22 (2012) 15370.
brid materials are very interesting for further biological applica-
tions, considering the biocompatibility and biodegradability of
phosphazene compounds.
References
[1] S. Ijima, Nature 354 (1991) 56.
[2] M. Bansal, R. Srivastava, C. Lal, M.N. Kamalasananb, L.S. Tanwara, Nanoscale 1
(2009) 317.
[3] M. in het Panhuis, J. Mater. Chem. 16 (2006) 3598.
[4] J.L. Delgado, P.A. Bouit, S.M. Filippone, A. Herranza, N. Martin, Chem. Commun.
46 (2010) 4853.
[5] B. Baykal, V. Ibrahimova, G. Er, E. Bengü, D. Tuncel, Chem. Commun. 46 (2010)
6762.
[6] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chem. Rev. 106 (2006) 1105.
[7] P. Sign, S. Campidelli, S. Giordani, D. Bomifazi, A. Bianco, M. Prato, Chem. Soc.
Rev. 38 (2009) 2214.
[38] B. Çoßsut, M. Durmusß, A. Kılıç, S. Yesßilot, Inorg. Chim. Acta 366 (2011) 161.
[8] E. Okutan, G.O. Aydın, F. Hacıveliog˘lu, A. Kılıç, Polymer 52 (2011) 1241.