Synthesis and properties of fullerene (C60) substituted cyclophosphazene derivatives

Article abstract of DOI:10.1016/j.inoche.2014.09.006

A series of fullerene (C₆₀) substituted cyclophosphazene derivatives (2a-4a) were synthesized for the first time by 1,3-dipolar cycloaddition of the 1-formylphenoxy (aryloxy)cyclotriphosphazenes (2-4) (aryloxy = phenoxy, 1-naphthoxy and 1-pyren

Full text of DOI:10.1016/j.inoche.2014.09.006

Inorganic Chemistry Communications 49 (2014) 1–4
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and properties of fullerene (C60) substituted
cyclophosphazene derivatives
⁎
Elif Okutan , Bünyemin Çoşut, Serkan Yeşilot
Department of Chemistry, Gebze Institute of Technology, 41400 Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2014
Received in revised form 1 September 2014
Accepted 7 September 2014
Available online 8 September 2014
A series of fullerene (C60) substituted cyclophosphazene derivatives (2a–4a) were synthesized for the first time
by 1,3-dipolar cycloaddition of the 1-formylphenoxy (aryloxy)cyclotriphosphazenes (2–4) (aryloxy = phenoxy,
1-naphthoxy and 1-pyrenoxy) and C60. All the obtained compounds were characterized by 1H NMR, 13C NMR, 31
P
NMR, UV–Vis, fluorescence spectroscopy, MALDI-TOF and elemental analysis. The thermal behaviors of the syn-
thesized compounds were investigated by Thermal Gravimetric Analysis (TGA).
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Cyclotriphosphazene
Fullerene
NMR
Fluorescence
Since the discovery of the bulk production of fullerenes [1], synthetic
chemists have been fascinated by the idea of developing chemical
modifications of these carbon cages [2–6]. Years of research experience
have shown that the curved external surfaces of the fullerenes can be
considered as a powerful building block to the design of larger
assemblies introducing new chemical, geometric, thermal and
photophyisical properties [7,8]. In addition, these carbon nanomaterials
have pioneered to alternate fluorescent materials that do not have the
toxic effects [9]. For example soluble fullerenes have been studied as po-
tential materials in biomedical field [10–13]. However, these materials
have showed very low fluorescence efficiencies which make them inap-
propriate in this field. Therefore, the synthesis of photoluminescent and
biocompatible and biodegradable fullerene derivatives for applications
is challenging. For instance, cyclophosphazene frameworks can
provide valuable advantageous purposes towards useful materials for
fullerene-phosphazene materials. The multi-functional nature of
chlorocyclophosphazenes allows these compounds to be utilized as
branching points, which is significantly more than the ones available
with conventional organic cores [14]. Also cyclic phosphazene deriva-
tives and macromolecules show high chemical and thermal stabilities
and display various properties suitable for a wide range of applications
depending on the nature of their side groups [15,16]. Even though
to the cyclophosphazene. These systems can open a pathway to con-
struct more efficient donor–acceptor systems in which phosphazene
core potentially acts as thermally stable, biocompatible/biodegradable
branching unit. Furthermore, phosphazene units will provide a new
toolset for the control of physical and chemical properties of targeted
molecules. The structures of these compounds were characterized by
MALDI-TOF, 1H NMR, 13C NMR, 31P NMR and elemental analysis.
Photophysical and thermal behaviors of these compounds were investi-
gated by UV–Vis, fluorescence and TGA analysis.
In this work, the synthetic procedures for preparation of the com-
pounds are shown in Scheme 1. Hexachlorocyclotriphosphazene was
reacted with 4-hydroxybenzaldehyde in the presence of potassium car-
bonate in THF to yield 1,1,3,3,5-pentachloro-5-(4-formylphenoxy)
cyclotriphosphazatriene (1). Compounds 2–4 were synthesized by
treating compound 1 with excess of phenol, 1-naphtol and 1-
hydroxypyrene in the presence of cesium carbonate in THF at 60 °C.
C60–phosphazene derivatives 2a–4a were obtained reaction between
the predesigned phosphazene derivatives having aldehyde group (2–
4) and C60 in the presence of an amino acid (sarcosine) in a manner sim-
ilar to that reported previously [18]. In this approach azomethine ylides
which exhibit 1,3 dipole character react smoothly and in good yields
with C60 to give fulleropyrrolidines. The entire procedure for synthesis
is relatively easy and it is expected that this reaction pathway could
be used to prepare many other C60–phosphazene derivatives because
cyclophosphazene frameworks can provide numerous advantageous
purposes towards useful materials for C60-–phosphazene materials.
Cyclic phosphazene derivatives (1–4) and C60–fullerene derivatives
(2a–4a) were characterized by 1H, 13C and 31P NMR, MALDI-TOF and
elemental analysis. All of the results were consistent with the predicted
structures and assigned formulations. The unequivocal attribution of all
one poly(organophosphazene) was used for the synthesis of C60
–
poly(organophosphazene) material [17], C60–cyclophosphazene frame-
works had not been reported. Thus, we synthesized cyclophosphazene–
fulleropyrrolidine derivative by 1,3-dipolar cycloaddition in which five
aromatic units (phenoxy, 1-naphtoxy and 1-pyreneoxy) were bonded
⁎
Corresponding author.
E-mail address: eokutan@gyte.edu.tr (E. Okutan).
http://dx.doi.org/10.1016/j.inoche.2014.09.006
1387-7003/© 2014 Elsevier B.V. All rights reserved.

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E. Okutan et al. / Inorganic Chemistry Communications 49 (2014) 1–4
Scheme 1. Synthetic pathway to synthesize phosphazene–fullerene derivatives (2a, 3a, 4a).
1H and resonances of the compounds 2a–4a dissolved in CDCl3/CS2
were performed using standard 1D NMR experiments. The diastereoiso-
meric signatures can be easily confirmed. Indeed, comparison of the 1H
NMR spectra of compounds 2a–4a shows different chemical shifts for
the protons of the pyrrolidine ring and aromatic protons on the cyclic
phosphazene. As an example 1H NMR spectrum of compound 4a was
given (Fig. 1). The doublets (one for each H of the CH2 group of the pyr-
rolidine) resonate at δ 4.85 and 4.10 ppm for endo and exo protons. As
expected the proton of the asymmetric center on the pyrrolidine and
the N-CH3 protons were situated as two singlets at 4.60 ppm and
2.49 ppm respectively. The resonances of aromatic protons on the
phosphazene ring are situated between 6.94 and 8.26 ppm as multiplet.
The integration of aromatic part (49H) to the each aliphatic proton sup-
ports the molecular structure.
The proton-decoupled 31P NMR spectrum of compounds 2–4
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, compounds 2–4 have been assigned as A2B spin
systems and the corresponding parameters (see Supplementary data).
The integration of the A parts of the A2B pattern to the remaining B
parts of the proton-decoupled 31P NMR spectra of compounds 2–4
shows nearly 2:1 phosphorus ratio, which supports the suggested struc-
tures. Similar A2B patterns were expected in the 31P NMR spectra of
compounds 2a–4a. However, due to the stereogenic carbon centers in
pyrolidine groups, the two diastereotopic phosphorus atoms in com-
pounds 2a–4a result in AB patterns in 31P NMR. Since the two phospho-
rus in compounds 2a–4a have very similar chemical shifts and a large
2J(P,P) coupling constants, second order 31P NMR spectra with ABC spin
Fig. 1. 1H NMR spectrum of compound 4a in CDCl3–CS2.

E. Okutan et al. / Inorganic Chemistry Communications 49 (2014) 1–4
3
systems have been obtained (see Supporting information). The reso-
nances were observed as broad overlapped peaks rather than clear
ABC patterns. To obtain appropriate parameters simulation program
has been also used for the 31P NMR spectra of C60–fullerene derivatives
(2a–4a) (Fig. 2).
Fullerene nanomaterials exhibit extremely low fluorescence effi-
ciencies. Therefore, the synthesis of photoluminescent fullerene
nanomaterials for practical use in different applications is challenging
[9]. To investigate the photophysical properties of C60–phosphazene de-
photon counting (TCSPC) technique in CDCl3, excited at 390 nm. Our
values of the lifetimes were found to be 0.0051, 0.041 and 0.098 ns re-
spectively (see Supplementary data). These results indicate a strong
electronic interaction between phosphazene moiety and the fullerene.
Comparative TGA results of fullerene, synthesized phosphazene de-
rivatives with aromatic side groups (2–4) and all C60–phosphazene con-
jugates (2a–4a). As expected fullerene has no decomposition between
25 and 600 °C. Compound 2 has single step decomposition between
325 and 395 °C with the percent weight loss of 97.5% between 260
and 425 °C. Compound 2a has two step decompositions between 175
and 250 °C (10%) and 355–440 °C (37%) with total weight loss of 42%
at 600 °C. This weight loss is equal to the decomposition of the
phosphazene derivative bonded to the fullerene through pyrrolidino
group. The thermal stability of naphtoxy (3) and pyreneoxy (4)
substituted cyclophosphazenes is higher than the phenoxy substituted
phosphazene (2). Compound 3 starts to lose its functionalities at
370 °C up to 600 °C with a total weight loss of 72%. Compound 3a
shows one-step decomposition of one naphtoxy group and methyl
group between 130 and 180 °C (24%) with a total weight loss of 40%
at 600 °C. This decomposition can be attributed to the loss of naphtoxy
groups on phosphazene ring. Compound 4 has a first step 7% weight loss
170–205 °C and a second step decomposition from 390 to 450 °C with
weight loss of 51%. The pyreneoxy-substituted phosphazene–C60 deriv-
ative (4a) has a two-step decomposition between 130 and 235 °C (15%)
and 390–470 °C (18%). The char yield of the compound at 600 °C is 65%.
The C60 phosphazene derivatives (2a–4a) show better thermal stability
than the phosphazene compounds (2–4) due to the excellent thermal
stability of fullerene. The char yields of the derivatives increases with
the increasing size of the aromatic compounds on phosphazene because
of the higher π–π interaction between aromatic sides and fullerene
which makes derivative more stable.
rivatives, the absorbance and fluorescence emission of C60
–
phosphazene derivatives in chloroform were analyzed. The absorption
spectrum of phosphazene derivatives shows a broad shoulder with
peak maxima for 2a and 3a at 270 nm and 330 nm, and for 4a at 268,
278, 330 and 345 nm in chloroform. The UV–Vis absorption spectra
for pyrene phosphazene (4) and derivatives (C60–pyrene) 4a at almost
same concentration in chloroform are depicted. In fact, the observed ab-
sorption spectrum is close to a superposition of the spectra of typical 4a
and 4 compounds and is consistent with the UV–visible spectra of other
pyrene–fullerene derivatives [19]. The absorption at longer wave-
lengths beyond 400 nm is due entirely to the pyrene–C60 cage [20]. In
fluorescence emission spectra of the C60–phosphazene derivatives 2a–
4a in chloroform (excited at same wavelength, 390 nm), peaks were ob-
served at 448 nm. Pyrene excimer formation is a well-known phenom-
enon in organic solutions. In our previous work hexakis(pyrenyloxy)
cyclotriphosphazene exhibited an intramolecular excimer emission
arising from the non-covalent π–π and CH–π stacking interactions
among the pyrenyloxy moieties which were investigated by fluores-
cence spectroscopy [21]. Compound 4 exhibits a fluorescence maximum
near 448 nm, where most of the incident radiation is absorbed by the
pyrene phosphazene parts and not the fullerene which is coherent
with the previous results. Compound 4a shows a typical pyrene–
phosphazene emission spectrum but the intensity is reduced relative
to that of pyrene–phosphazene by a factor of ~10. Thus, the pyrene
phosphazene fluorescence is almost entirely quenched by the attached
fullerene moiety. The characteristic C60 fluorescence at ~700 nm is too
weak to be observed on our instrument. Fluorescence quantum yields
of compounds 2a–4a were determined by a standard procedure using
2-aminopyridine as standard (ΦF = 0.60). We observed that the quan-
tum yields of compounds 2a–4a are 0.12%, 0.75% and 0.82% respectively.
The lifetimes were also measured with the time correlated single
In this contribution we presented the synthesis of C60–phosphazene
derivatives with covalent linkage between both moieties by an 1,3-
dipolarcycloaddition of phosphazene derivatives having aldehyde
group and fullerene C60 in the presence of sarcosine. For this purpose
we have synthesized phosphazene derivatives which could serve as a
versatile template for designing more efficient donor–acceptor systems.
All compounds were fully characterized by standard spectroscopic tech-
niques. The photophysical behaviors of compounds were studied by
UV–Vis absorption and fluorescence spectroscopies. The fluorescence
Fig. 2. (A) Normal and (B) simulated 31P NMR spectra of compound 4a in CDCl3–CS2.

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Acknowledgment
The authors wish to thank Dr. H. Maid and Dr. W. Bauer for helpful
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Appendix A. Supplementary material
Spectral and thermal data of the compounds are provided in SI file.
This material is available free of charge via the internet. Supplementary
data to this article can be found online at http://dx.doi.org/10.1016/j.
inoche.2014.09.006.
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