Synthesis and spectral properties of a hexameric pyrene-fluorene chromophore based on cyclotriphosphazene

Article abstract of DOI:10.1016/j.poly.2014.06.049

A new fluorene substituted methoxybenzene-pyrene chromophore (3), [1-(9,9-dimethylfluorene)-8-(4-methoxyphenyl)]pyrene, was synthesized by the Suzuki cross-coupling reactions of 1,8-dibromopyrene with 4-methoxyphenylboronic acid and 9,9-dimethylfluorene-2-boronic acid pinacol ester, respectively. The hexameric compound (5), hexakis{1-oxyphenyl-8-(9,9-dimethylfluorene)pyrene} cyclotriphosphazene, was prepared by a nucleophilic displacement reaction of hexachlorocyclotriphosphazene, N₃P₃Cl₆, with (4), [1-(9,9-dimethylfluorene)-8-(4-hydroxyphenyl)]pyrene, which was obtained from the deprotection of compound (3). The newly synthesized compounds were fully characterized by standard spectroscopic techniques. The spectral properties of compound (5) were investigated by thermogravimetric analysis, differential scanning calorimetry, UV-Vis and fluorescence spectroscopy, and cyclic voltammetry. In addition, fluorescence quantum yields, fluorescence lifetimes and fluorescence quenching behavior by 1,4-benzoquinone were investigated in dichloromethane. It was found that the hexameric compound (5) emitted in the blue-light spectral region with a maxima of 455 nm, with a quantum yield of 0.63 in dichloromethane. Compound (5) was amorphous in nature and exhibited excellent thermal stability, with a decomposition temperature of 498 °C and a high glass-transition temperature of 58°C.

Full text of DOI:10.1016/j.poly.2014.06.049

Polyhedron 81 (2014) 436–441
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and spectral properties of a hexameric pyrene-fluorene
chromophore based on cyclotriphosphazene
⇑
Hüsnüye Ardıç Alidag˘ı, Bünyemin Çoßsut, Adem Kılıç, Serkan Yeßsilot
The Department of Chemistry, Gebze Institute of Technology, Gebze, Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A new fluorene substituted methoxybenzene-pyrene chromophore (3), [1-(9,9-dimethylfluorene)-8-(4-
methoxyphenyl)]pyrene, was synthesized by the Suzuki cross-coupling reactions of 1,8-dibromopyrene
with 4-methoxyphenylboronic acid and 9,9-dimethylfluorene-2-boronic acid pinacol ester, respectively.
The hexameric compound (5), hexakis{1-oxyphenyl-8-(9,9-dimethylfluorene)pyrene} cyclotriphospha-
Received 1 May 2014
Accepted 18 June 2014
Available online 2 July 2014
zene, was prepared by
a nucleophilic displacement reaction of hexachlorocyclotriphosphazene,
Keywords:
N3P3Cl6, with (4), [1-(9,9-dimethylfluorene)-8-(4-hydroxyphenyl)]pyrene, which was obtained from
the deprotection of compound (3). The newly synthesized compounds were fully characterized by
standard spectroscopic techniques. The spectral properties of compound (5) were investigated by
thermogravimetric analysis, differential scanning calorimetry, UV–Vis and fluorescence spectroscopy,
and cyclic voltammetry. In addition, fluorescence quantum yields, fluorescence lifetimes and fluores-
cence quenching behavior by 1,4-benzoquinone were investigated in dichloromethane. It was found that
the hexameric compound (5) emitted in the blue-light spectral region with a maxima of 455 nm, with a
quantum yield of 0.63 in dichloromethane. Compound (5) was amorphous in nature and exhibited
excellent thermal stability, with a decomposition temperature of 498 °C and a high glass-transition
temperature of 58 °C.
Cyclotriphosphazene
Pyrene
Fluorene
Thermally stable
Fluorescence
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
light emitting diodes (OLEDs), light harvesting materials for photo-
catalysis and fluorescent sensors for organic or inorganic analyzers
Cyclic phosphazenes, composed of alternating nitrogen and
phosphorus atoms, are a commonly used building block with an
inorganic (P@N)n (n = 3 or 4) core, and are extensively used as an
interior core or a pendent group to develop dendrimers or poly-
mers, respectively. Cyclic and polymeric phosphazenes are an
important class of inorganic heterocyclic ring systems [1,2]. They
readily undergo nucleophilic substitution reactions with alkoxides,
aryloxides or amines on halocyclophosphazenes or high polymers
[3–5] and their physical and chemical properties can be tailored
via appropriate substituted groups on the phosphorus atoms [6].
This allows for the preparation of various types of dendrimers
and polymers for varied applications, including thermal stability,
catalytic properties, electrical conductivity, liquid crystals,
biomedical activity and luminescent materials [5–7].
[8]. In the area of active OLED materials, both small molecules and
polymers are the current preferred candidates [9,10]. Cyclic phos-
phazene based materials are a suitable platform as luminescent
materials because they provide high thermal stability and the func-
tional groups are projecting in three dimensions, thus producing a rigid
spherical core for attaching the dendrons of interest. Moreover, when
attaching organic chromophores onto cyclic phosphazene cores, their
optical and electronic properties are not noticeably affected compared
to the molecular analogue [11–13]. These rigid spheres have been
shown to have amorphous properties that are known to be important
for electroluminescent devices. Hence, there has been considerable
interest in fluorescent compounds based on cyclic phosphazene cores
for use in OLEDs [14–18].
Pyrene and its electron-rich aromatic derivatives have been
widely used in many applications, such as materials science and
organic electronic devices [19]. For example, fluorene-pyrene
functional materials have been extensively well-documented as
active ingredients for electroluminescence devices [20–26].
Pyrenyl-functionalized fluorene chromophores have demonstrated
great potential and advantages for OLEDs [20–23], and organic
semi-conductors [24–26]. Recently cyclophosphazene derivatives
The field of luminescent materials is attracting interest because
of their many applications, including emitting materials for organic
⇑
Corresponding author. Address: Department of Chemistry, Gebze Institute of
Technology, P.O. Box: 141, Gebze 41400, Kocaeli, Turkey. Tel.: +90 262 6053014;
fax: +90 262 6053101.
E-mail address: yesil@gyte.edu.tr (S. Yesßilot).
http://dx.doi.org/10.1016/j.poly.2014.06.049
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

H.A. Alidag˘ı et al. / Polyhedron 81 (2014) 436–441
437
decorated with fluorene and pyrene [11,14,15,27] groups were
reported for the development of electroluminescent devices. To
the best of our knowledge, there is no report on the synthesis of
both chromophores being linked to cyclotriphosphazene together.
Hence, we thought that it would be interesting to prepare a
recorded on a Perkin Elmer Spectrum 100 Optica FT-IR spectrome-
ter. Fluorescence excitation and emission spectra were recorded
on a Varian Eclipse spectrofluoremeter using 1 cm pathlength cuv-
ettes at room temperature. Mass spectra were acquired in the linear
mode with an average of 50 shots on a Bruker Daltonics Microflex
mass spectrometer equipped with a nitrogen UV-Laser operating
at 337 nm. Analytical thin layer chromatography (TLC) was per-
formed on silica gel plates (Merck, Kieselgel 60, 0.25 mm thickness)
with F254 indicator. Column chromatography was performed on sil-
ica gel (Merck, Kieselgel 60, 230–400 mesh; for 3 g crude mixture,
100 g silica gel was used in a column of 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 solution on a Varian 500 MHz spectrometer. The
thermal properties of the compounds were investigated on a Mettler
Toledo TGA/SDTA 851 thermogravimetric analysis (TGA) and
differential scanning calorimeter (DSC) DSC 821e equipped with
Mettler Toledo Stare software at a heating rate of 10 °C minꢀ1 under
a nitrogen flow (50 mL minꢀ1). Electrochemical behaviors were
investigated using a CHIModel840B electrochemical analyzer.
hexameric fluorene-pyrene chromophore based on
a cyclic
phosphazene core, which may allow favorable thermal and optical
properties as desired for electroluminescent devices.
Herein, we reported the design and synthesis of a new fluorene
substituted methoxybenzene-pyrene chromophore using Suzuki
cross-coupling reactions. After that, a hexameric structure was pre-
pared by a nucleophilic displacement reaction of hexachlorocyclo-
triphosphazene, N3P3Cl6, with the newly synthesized pyrene-
fluorene chromophore (Fig. 1). The chemical structures of the
newly synthesized compounds were characterized by standard
spectroscopic techniques. Selected properties of the hexameric
structure were investigated by thermogravimetric analysis (TGA),
differential scanning calorimetry (DSC), UV–Vis, fluorescence
spectroscopy and cyclic voltammetry (CV).
2. Experimental
2.3. Fluorescence quantum yields and lifetimes
2.1. Materials
The fluorescence quantum yield (UF) was determined by the
comparative method (Eq. (1)) [28].
Hexachlorocyclotriphosphazene (trimer) (Otsuka Chemical Co.
Ltd) was purified by fractional crystallization from n-hexane. The
deuterated solvent (CDCl3) for NMR spectroscopy and the follow-
ing chemicals were obtained from Merck: tetrahydrofuran (THF),
toluene, 1,4-dioxane, potassium carbonate (K2CO3), sodium sulfate
(Na2SO4), sodium carbonate (Na2CO3), hydrochloric acid (37%);
Aldrich: pyrene, 9,9-dimethylfluorene-2-boronic acid pinacol
ester, palladium tetrakis(triphenylphosphine) (Pd(PPh3)4), bro-
mine, Aliquat; Alfa Aesar: 4-methoxybenzeneboronic acid, cesium
carbonate (Cs2CO3), tetrabutylammonium bromide (TBAB); Fluka:
hydromic acid (HBr). All other reagents and solvents were reagent
grade quality and obtained from commercial suppliers.
F:AStd:n2
FStd:A:n2Std
UF ¼
UFðStdÞ
ð1Þ
where F and FStd are the areas under the fluorescence emission
curves of the sample (5) and the standard, respectively. A and AStd
are the respective absorbances of the samples and standard at the
excitation wavelengths, respectively. The refractive indices (n) of
the solvents were employed in calculating the fluorescence quan-
tum yields in different solvents. Quinine sulfate (in 0.1 M H2SO4)
(UF = 0.54) [29] was employed as the standard. Both the sample
and standard were excited at the same wavelength. The concentra-
tion of the solutions at the excitation wavelength was fixed at
2 ꢁ 10ꢀ7 mol dmꢀ3
.
Natural radiative (T0) lifetimes were
2.2. Equipment
determined using the Photochem CAD program, which uses the
Elemental analyses were carried out using a Thermo Finnigan
Flash 1112 Instrument. UV–Vis spectra were recorded with a
Shimadzu 2001 UV spectrophotometer. Infrared spectra were
Strickler-Berg equation [30]. The fluorescence lifetimes (TF) were
evaluated using Eq. (2).
sF
s0
UF
¼
ð2Þ
The rate constant for the florescence (kF) values were calculated
using Eq. (3).
kF ¼ UF
=
sF
ð3Þ
2.4. Fluorescence quenching by 1,4-benzoquinone (BQ)
Fluorescence quenching experiments on the hexameric deriva-
tive (5) were carried out by the addition of different concentrations
of BQ to a fixed concentration of the compound, and the concentra-
tions of BQ in the resulting mixtures were 0, 0.008, 0.016, 0.024
and 0.032 M. The fluorescence spectrum of compound (5) at each
BQ concentration was recorded, and the changes in fluorescence
intensity related to BQ concentration by the Stern-Volmer (S-V)
equation [31] (Eq. (4)):
O
O
P
N
N
P
N
O
P
O
O
O
I0
¼ 1 þ KSV½BQꢂ
ð4Þ
I
where I0 and I are the fluorescence intensities of the fluorophore in
the absence and presence of quencher, respectively. [BQ] is the
concentration of the quencher and KSV is the Stern-Volmer constant,
which is the product of the bimolecular quenching constant (kq) and
TF, as is expressed in Eq. (5).
Fig. 1. Molecular structure of the hexameric compound (5).

438
H.A. Alidag˘ı et al. / Polyhedron 81 (2014) 436–441
0.12 g (24%), m.p.: 220 °C. FT-IR (m
max/cmꢀ1): 3291 (O–H), 3036
KSV ¼ kq:sF
ð5Þ
(ArC–H), 2960, 2890 (C–H, CH3), 1609, 1599 (ArC@C), 1518
(ArC–C), 1063 (ArC–O). 1H NMR (CDCl3, ppm) d: 7.19–8.35 (m,
19H, ArCH), 5.07 (s, 1H, –OH), 1.63 (s, 6H, CH3); {1H}13C NMR
(CDCl3, ppm) d: 155.78, 155.01, 153.34, 144.88, 138.34, 138.21,
138.01, 137.86, 135.57, 133.48, 132.25, 130.63, 129.88, 128.69,
127,55, 127,37, 127.06, 127.01, 126.34, 125.42, 124.74, 124.58,
124.26, 124.11, 121.14, 120.87, 115.87, 47.25, 27.87. Elemental
analysis, Found: C 91.42, H 5.38; C37H26O (486) requires: C 91.33,
H 5.39%. MS (MALDI) m/z (%): 487 (100) [M+H]+.
The ratios of Io/I were calculated and plotted against [BQ],
according to Eq. (4), and KSV was determined from the slope.
2.5. Synthesis
1,8-Dibromopyrene (1) was synthesized according to the litera-
ture procedure [32].
2.5.1. Synthesis of [1-bromo-8-(4-methoxyphenyl)]pyrene (2)
A mixture of 4-methoxybenzene boronic acid (0.4 g, 2.6 mmol),
(1) (1 g, 2,8 mmol), Pd(PPh3)4 (0.06 g, 0.05 mmol) and dry potas-
sium carbonate (1.6 g, 11.2 mmol) in dry 1,4-dioxane (40 mL)
was stirred, under argon for 72 h at 60 °C. The reaction mixture
was poured into a solution of ice with concentrated hydrochloric
acid (3:1). The organic phase was extracted with dichloromethane,
dried over anhydrous sodium sulfate and evaporated. The residue
was purified by column chromatography, with the eluent
2.5.4. Synthesis of hexakis{1-oxyphenyl-8-(9,9-dimethylfluorene)
pyrene} cyclotriphosphazene (5)
The trimer N3P3Cl6 (0.05 g, 0.14 mmol) and compound (4)
(0.56 g, 1.2 mmol) were dissolved in dry THF (10 mL) under an
argon atmosphere. After stirring for 15 min at 40 °C, dry and finely
powdered cessium carbonate (0.56 g, 1.73 mmol) was added
portionwise over 15 min with efficient stirring. The reaction
mixture was stirred under an argon atmosphere at 80 °C for 24 h.
The reaction mixture filtered off, the volatile materials were
evaporated under vacuum and the product was purified by
preparative TLC on silica gel using n-hexane:THF (2:1) as the
eluent. Compound (5) was obtained as a solid; Yield: 0.16 g
n-hexan:CH2Cl2 (3:1). Yield 0.43 g (40%), m.p.: 141 °C. FT-IR (mmax
/
cmꢀ1): 3041 (ArC–H), 2961, 2928, 2905, 2834 (C–H, CH3), 1606,
1569 (ArC@C), 1517 (ArC–C), 1261, 1240 (ArC–O), 1100 (CH3–O),
661 (ArC–Br). 1H NMR (CDCl3, ppm) d: 7.11–8.44 (m, 12H, ArCH),
3.94 (s, 3H, OCH3); {1H}13C NMR (CDCl3, ppm) d: 130.13, 130.06,
128.99, 128.30, 128.24, 127.72, 127.02, 126.96, 126.69, 125.80,
125.73, 125.70, 125.47, 125.37, 125.18, 125.17, 113.97, 109.98,
55.42. Elemental analysis, Found: C 71.05, H 3.78; C23H15BrO
(387) requires: C 71.33, H 3.90%. MS (MALDI) m/z (%): 388 (100)
[M+H]+.
(36%). Tg: 58 °C. FT-IR (m
max/cmꢀ1): 3038 (ArC–H), 2950, 2875
(C–H, CH3), 1611, 1586 (ArC@C), 1520 (ArC–C), 1270 (P@N), 1065
(ArC–O), 960 (P–O). 1H NMR (CDCl3, ppm) d: 7.42–8.24 (m, 90H,
pyrene and fluorene CH), 6.98–7.36 (m, 24H, phenyl H), 1.61
(s, 36H, CH3); {1H}13C NMR (CDCl3, ppm) d: 168.93–120.03.
31P NMR (CDCl3) d: 9.32. Elemental analysis, Found: C 87.21,
H 4.85, N 1.32; C222H150N3O6P3 (3045) requires: C 87.46, H 4.96,
N 1.38%. MS (MALDI) m/z (%): 3046.141 (100) [M+H]+.
2.5.2. Synthesis of [1-(9,9-dimethylfluorene)-8-(4-methoxyphenyl)]
pyrene (3)
A mixture of compound (2) (0.5 g, 1.3 mmol), 9,9-dimethylfluo-
rene-2-boronic acid pinacol ester (0.83 g, 2.58 mmol), tetrabutyl-
ammonium bromide (0.06 g, 0.18 mmol) and Na2CO3 (2 M, 6 mL)
were dissolved in 20 mL of toluene. The solution was purged with
argon for 20 min, and then tetrakis(triphenylphosphine) palladium
(0.05 g, 0.04 mmol) was added and the reaction mixture was
heated with stirring at 110 °C. The reaction progress was followed
by TLC and after 24 h was worked up. The cooled mixture was
extracted with CH2Cl2, and the extract was washed with saturated
brine and dried over Na2SO4. The crude product so obtained was
purified by silica gel chromatography and eluted with CH2Cl2:
n-hexane (1:3) as the eluent to obtain (3). Yield 0.27 g (42%),
3. Results and discussion
3.1. Synthesis and structural characterization
The palladium-catalysed Suzuki cross-coupling reaction of aryl-
boronic acids or arylboronic acid pinacol esters with arylhalides is
an extremely useful tool in organic synthesis. This reaction for
carbon-carbon bond formation proceeds under mild conditions, is
largely unaffected by the presence of water and yields non-toxic
byproducts [33]. Because of this, the synthesis of a fluorene
substituted methoxybenzene-pyrene chromophore was accom-
plished by Suzuki cross-coupling reactions. The synthetic routes
for the preparation of compounds (1)–(5) are shown in Scheme 1.
1,8-Dibromopyrene (1) was prepared according to a literature pro-
cedure. 1,8-Dibromopyrene (1) was reacted with the commercially
available 4-methoxybenzeneboronic acid to get the mono-bromi-
nated compound (2) by employing the Pd(PPh3)4 catalytic system.
Compound (2) was further reacted with 9,9-dimethylfluorene-2-
boronic acid pinacol ester to achieve compound (3). Compound
(4) was obtained by the deprotection reaction of compound (3)
on refluxing the compound in a solution of concentrated hydrobro-
mic acid with aliquat for 24 h. Compound (5) was obtained from a
nucleophilic displacement reaction of (4) with N3P3Cl6 under an
argon atmosphere, with cesium carbonate as a base. The products
were purified by column chromatography and/or preparative TLC
techniques. The newly synthesized compounds were characterized
by 1H, 13C and 31P NMR, mass spectrometry (MALDI-TOF
techniques) and elemental analysis. All the results were consistent
with the predicted structures, as shown in the experimental
section. As an example, the 1H NMR spectra of compounds (3)
and (4) are shown in Fig. S1. The aromatic protons are shown in
the region d = 7–8.5 ppm. The proton signals of the two methyl
groups on the fluorene ring and aromatic methyl group are
m.p.: 283 °C. FT-IR (m
max/cmꢀ1): 3033 (ArC–H), 2955, 2833 (C–H,
CH3), 1607, 1570 (ArC@C), 1516 (ArC–C), 1244 (ArC–O), 1104
(CH3–O). 1H NMR (CDCl3, ppm) d: 7.13–8.30 (m, 19H, ArCH), 3.96
(s, 3H, OCH3), 1.62 (s, 6H, CH3); {1H}13C NMR (CDCl3, ppm) d:
159.01, 153.87, 140.34, 138.97, 138.34, 138.19, 137.56, 133.68,
131.67, 130.41, 130.23, 129.61, 128.98, 128.96, 127,86, 127,49,
127.35, 127.09, 125.34, 125.22, 124.94, 124.68, 124.46, 124.41,
120.14, 119.87, 113.87, 55.43, 47.05, 27.27. Elemental analysis,
Found: C 90.76, H 5.59; C38H28O (500) requires: C 91.17, H 5.64%.
MS (MALDI) m/z (%): 501 (100) [M+H]+.
2.5.3. Synthesis of [1-(9,9-dimethylfluorene)-8-(4-hydroxyphenyl)]
pyrene (4)
0.5 g (1 mmol) of compound (3) in 30 mL of concentrated HBr
was stirred at room temperature for 3 h. 1.5 g (4.65 mmol) of
aliquat was added to the reaction mixture and after 24 h at
130 °C the mixture was poured into water (100 mL). The product
was extracted with CH2Cl2. The organic phase was filtrated and
dried on Na2SO4. The solvent was removed under reduced pres-
sure. The crude product was purified by column chromatography
silica gel 60 (70–230 mesh) as the adsorbent and CH2Cl2:n-hexane
(3:1) as the eluent. Compound (4) was obtained as a solid; Yield:

H.A. Alidag˘ı et al. / Polyhedron 81 (2014) 436–441
OMe
439
of compound (5) is shown in Fig.S2. The spectrum provided defin-
itive characterization, and the peak representing the protonated
molecular ion of (5) was observed at 3046.1. Only a singlet peak
(at 9.3 ppm) was observed in the proton-decoupled 31P NMR
spectrum of compound (5), indicating that the three phosphorus
atoms are fully substituted with identical units (Fig. S3).
Br
Br
Br
ii
i
(2)
(1)
iii
3.2. Thermal properties
The thermal properties of compound (3) and the hexameric com-
pound (5) were evaluated using thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC). The thermogravimetric
analysis (TGA) of the compounds was used to measure their thermal
decomposition temperatures (Td). The Td of compounds (3) and (5)
were determined in the temperature range from +25 °C to +700 °C
at a heating rate of 10 °C/min, at the level of 5% weight loss
(Fig. 2). The TGA showed that the two materials were highly stable
in nitrogen, with the thermal decomposition temperatures (Td) of
346 and 498 °C, for (3) and (5), respectively. Differential scanning
calorimetry (DSC) studies were carried out to determine the glass
transition and melting temperatures (Tg) of the compounds. Com-
pound (3) has a sharp melting point of 283 °C, but has no glass tran-
sition temperature observed during subsequent heating/cooling
scans between ꢀ20 and 300 °C (Fig. 3a). The hexameric compound
(5) exhibited glass transition behavior at around 58 °C, as is shown
in Fig. 3b. In general, for application in light emitting electrolumi-
nescence devices the materials should have a high thermal stability
and good film forming ability from solution [34]. The thermal
properties for compound (3) show crystalline behavior in the DSC
measurements. Thus it is not suitable for the preparation of an a
morphologically stable film because a glass transition temperatures
was not observed. On the other hand, hexameric compound (5) dis-
played good thermal properties, and in addition, it has excellent sol-
ubility in common organic solvents and maintains a completely
amorphous state. It should be suggested that the hexameric
material may have potential applications in small molecules for
organic light-emitting diodes (OLEDs) [34].
OH
OMe
V
iV
(5)
(4)
(3)
Scheme 1. Synthetic routes for the preparation of compounds (1)–(5). Reagents
and conditions: (i) Br2, CCl4; (ii) 4-methoxyphenylboronic acid, Pd(PPh3)4, K2CO3,
1,4-dioxane; (iii) 9,9-dimethylfluorene-2-boronic acid pinacol ester, Pd(PPh3)4, 2 M
Na2CO3, toluene; (iv) HBr, Aliquat, 130 °C,14 h; (v) N3P3Cl6 (trimer), Cs2CO3, THF,
80 °C and 24 h.
Fig. 2. TGA thermograms of (3) and (5) from 25 to 700 °C at a heating rate of 10 °C/
min under an N2 flow of 50 mL/min.
3.3. Photophysical properties
observed at d = 4 and 1.6 ppm for compound (3), respectively. The
integration of the aromatic proton signals to the methyl group sig-
nals in the 1H NMR spectrum of compound (3) gave approximately
a 19:3:6 proton ratio, that confirmed the suggested structure. After
the deprotection reaction of (3), only the proton signals of the two
methyl groups on the fluorene ring were observed for compound
(4), as expected (Fig. S1b). The observed the MALDI-MS spectrum
The absorption and fluorescence spectra of compounds (3) and
(5) were measured in CH2Cl2 (2 ꢁ 10ꢀ7 mol dmꢀ3). It is well known
that chlorophosphazenes (N3P3Cl6 or N4P4Cl8) are optically trans-
parent outside the vacuum UV range and do not interfere with
the photophysical properties of the attached chromophores. Hence,
it would be expected that the absorption and emission spectra of
the hexameric compound (5) could be similar to the attached
Fig. 3. DSC thermograms in an N2 atmosphere of (a) compound (3) at a heating/cooling rate of 10 °C/min between ꢀ20 and 300 °C, (b) compound (5) from 25 to 180 °C at a
heating rate of 10 °C/min.

440
H.A. Alidag˘ı et al. / Polyhedron 81 (2014) 436–441
chromophores. The UV–Vis absorption spectrum of the hexameric
compound (5) was similar to that of compound (3) and showed
characteristic absorption bands of the pyrene molecules [19],
except for a small difference in the range 280–320 nm (Fig. 4).
The fluorescence spectra in CH2Cl2 are depicted in Fig. 5a. These
compounds emit light in the violet–blue and blue spectral regions
with peaks at around 415–455 nm. The hexameric compound (5)
displayed red-shifted emission wavelengths of up to 40 nm when
compared to compound (3). This red shifted emission was
attributed to an intramolecular excimer formation amongst the
pyrene-fluorene units due to the hexameric compounds
three-dimensional scaffold structure, and was consistent with
previous research papers [13,15–17,27]. The fluorescence emission
spectra of compound (5) was recorded in solvents of different
polarity, such as cyclohexane (CH), tetrahydrofuran (THF), toluene
(TOL), dichloromethane (DCM), acetonitrile (ACN) and methanol
(MeOH) (Fig. 5b). All the emission spectra in various solvents of
compound (5) show a broad and structure less excimer-like band
which was red-shifted upon increasing the solvent polarity, as
expected [35]. The fluorescence quantum yield (UF) of compound
(5) was measured in CH2Cl2 and quinine sulfate in 0.1 M H2SO4
solution was used as the standard. The fluorescence quantum yield
was found to be 0.63 for compound (5) in CH2Cl2. The quantum
yield and photo physical properties demonstrate compatibility
with previous studies of cyclophosphazenes that were prepared
for OLEDs [11,14]. The fluorescence lifetimes (TF) of (5) were calcu-
lated using the Strickler-Berg equation. Using this equation, a good
correlation has been found between the experimentally and theo-
retically determined lifetimes for the unaggregated molecules, as is
the case in this work. Thus, we suggest that the values obtained
using this equation are an appropriate measure of fluorescence
lifetimes. In addition, the rate constants for fluorescence (kF) and
radiative lifetime (T0) were also measured for (5) in this study, as
given in Table 1(i). The fluorescence quenching behaviour by 1,
4-benzoquinone (BQ) of (5) was investigated in CH2Cl2. Fig. 6
shows the quenching of (5) by BQ in CH2Cl2. It can be said that in
the presence of BQ quencher, an energy transfer occurs between
the excited hexameric compound (5) and BQ. The fluorescence
quenching of (5) by BQ in CH2Cl2 was found to obey Stern-Volmer
kinetics. The slope of the plots shown in the inset of Fig. 6 gave a
KSV value for (5). The KSV and the bimolecular quenching constant
(kq) values are summarized in Table 1(i).
3.4. Electrochemical properties
Fig. 4. The absorption spectra of (3) and (5) in CH2Cl2. Concentration = 2.00 ꢁ 10ꢀ7
The electrochemical behavior of the hexameric compound (5)
was investigated by cyclic voltammetry with a standard three
electrode electrochemical cell in 0.1 M TBAPF6 in CH2Cl2 at room
mol dmꢀ3
.
Fig. 5. The fluorescence emission spectra (normalized) of (a) compounds (3) and (5) in CH2Cl2, (b) various solvents for compound (5). Concentration: 2 ꢁ 10ꢀ7 mol dmꢀ3
.
Excitation wavelength: 345 nm.
Table 1
Spectral properties of the hexameric compound (5) in CH2Cl2.
(i) Photophysical properties of the hexameric compound (5)
kabs
kEm
UF
TF (ns)
T0 (ns)
akF(sꢀ1) (ꢁ1011
)
KSV (Mꢀ1
)
kq/1013 (Mꢀ1 sꢀ1
)
288,365
455
0.63
0.007
0.01
0.90
27.0
0.38
(ii)Thermal parameters and electrochemical properties of the hexameric compound (5)
HOMO (eV)
LUMOa (eV)
Eg (opt)a (eV)
Tg (°C)
58
Td (°C)
ꢀ5.99
ꢀ2.95
3.04
498
a
Determined from the onset of the UV–Vis absorption spectrum.

H.A. Alidag˘ı et al. / Polyhedron 81 (2014) 436–441
441
Acknowledgements
We thank the Gebze Institute of Technology (GIT) Research
Fund (2013-A-015) for partial financial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2014.06.049.
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In summary, the synthesis of the fluorene substituted methoxy-
benzene-pyrene (3) and its hexameric derivative (5), based on a
cyclic phosphazene ring, has been reported for the first time. All
the compounds were fully characterized by elemental analysis,
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