Constitutional isomers of dendrimer-like pyrene substituted cyclotriphosphazenes: Synthesis, theoretical calculations, and use as fluorescence receptors for the detection of explosive nitroaromatics

Article abstract of DOI:10.1039/c9nj03695d

Two constitutionally isomeric bis-pyrenyl phenol dendrons (4 and 6) and their dendrimer-like cyclotriphosphazene derivatives (5 and 7) with different architectural motifs on the periphery of the cyclotriphosphazene ring are designed and synthesized, and their structural characterization is carried out using standard spectroscopic techniques (elemental analysis, ¹H, ³¹P and ¹³C NMR spectroscopies and MALDI-TOF mass spectrometry). The thermal and photophysical properties of these newly synthesized compounds are also investigated. The geometric optimization of the constitutional isomeric dendrons and their dendrimer-like derivatives, HOMO-LUMO energy levels and molecular interactions are performed using density functional theory calculations at the wB97XD/6-31G(d) level. The fluorescence detection behaviors of 5 and 7 are evaluated for nitro aromatic compounds (NACs). Results of measurements show that explosive nitroaromatics efficiently quenched fluorescence emission, whereas other tested competitive species (toluene, RDX, picric acid, benzene and 20 metal ions) induced no spectral changes, suggesting that these dendrimer-like cyclotriphosphazene derivatives (5 and 7) could be promising candidates to be applied as sensors for explosive nitro aromatics with high selectivity and sensitivity.

Full text of DOI:10.1039/c9nj03695d

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Constitutional isomers of dendrimer-like pyrene
substituted cyclotriphosphazenes: synthesis,
theoretical calculations, and use as fluorescence
receptors for the detection of explosive
nitroaromatics†
Cite this: New J. Chem., 2019,
43, 16738
Husniye Ardic Alidagi, Sureyya Oguz Tumay,
Ahmet S- enocak,
˘
¨
¨
O¨ mer Faruk Çiftbudak, Bunyemin Ços-ut
and Serkan Yes-ilot
*
¨
Two constitutionally isomeric bis-pyrenyl phenol dendrons (4 and 6) and their dendrimer-like
cyclotriphosphazene derivatives (5 and 7) with different architectural motifs on the periphery of the
cyclotriphosphazene ring are designed and synthesized, and their structural characterization is carried out
using standard spectroscopic techniques (elemental analysis, 1H, 31P and 13C NMR spectroscopies and
MALDI-TOF mass spectrometry). The thermal and photophysical properties of these newly synthesized
compounds are also investigated. The geometric optimization of the constitutional isomeric dendrons and
their dendrimer-like derivatives, HOMO–LUMO energy levels and molecular interactions are performed
using density functional theory calculations at the wB97XD/6-31G(d) level. The fluorescence detection
behaviors of 5 and 7 are evaluated for nitro aromatic compounds (NACs). Results of measurements
show that explosive nitroaromatics efficiently quenched fluorescence emission, whereas other tested
competitive species (toluene, RDX, picric acid, benzene and 20 metal ions) induced no spectral changes,
suggesting that these dendrimer-like cyclotriphosphazene derivatives (5 and 7) could be promising
candidates to be applied as sensors for explosive nitro aromatics with high selectivity and sensitivity.
Received 16th July 2019,
Accepted 1st October 2019
DOI: 10.1039/c9nj03695d
rsc.li/njc
because of their various functions and increased interaction sites.
Among environmental pollutants, nitroaromatic compounds
1. Introduction
The design and synthesis of dendrimers has been increasingly (NACs) are not only hazardous organic contaminants but also
studied in recent decades because they have an important role dangerous explosives. Therefore, the detection of NACs is a very
in various advanced technological applications such as catalysts,1 important research topic for applications such as security, indus-
biological drug delivery2 and chemical sensors.3 Among all trial processes and the environment.16–19 For instance, excessive
the dendrimers, phosphorus-containing dendrimers are very intake of toxic levels of NACs from drinking and ground water is a
attractive, not only due to their chemical properties, but also threat to living organisms.20 Pyrene and its derivatives have electron
because using 31P NMR allows them to be easily characterized rich chemical structures and they are suitable materials for fluores-
even those with sophisticated structures.4–9 Increasing industrial cence detection.21,22 It is well known that electron rich molecules
activity through technological development has improved such as pyrene derivatives can interact with NACs which are
living standards as well as caused environmental pollution. electron deficient. As a result of this interaction fluorescence
Therefore, the desire to obtain a safer environment increases signals of electron rich molecules are quenched via electron
the demand for effective and time saving methods to detect transfer.23 Generally, p–p interaction between pyrene moieties
traces of hazardous chemicals. The use of covalent organic and NACs caused fluorescence quenching of pyrene molecules.
polymers and dendrimers,10,11 small molecules12 and MOFs13–15 In addition, planar structures like p-conjugated pyrene improves
can improve the binding or interaction properties of analytes the p–p interaction with NACs.24,25 The fluorescence quenching of
p-conjugated pyrene occurs with non-radiative relaxation which is
based on a photo-induced electron transfer process from the LUMO
Department of Chemistry, Gebze Technical University, Gebze, Kocaeli, Turkey.
orbital of p-conjugated pyrene to the electron deficient NACs.26–28
E-mail: yesil@gtu.edu.tr
Phosphorus-containing dendrimers are an important class of
inorganic macromolecules that have a wide range of applications
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9nj03695d
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ranging from catalysis, materials chemistry, and even nano purchased from Merck Chemical Co., and freshly distilled over
medicine.29,30 Cyclophosphazenes which are phosphorus con- metallic sodium and benzophenone prior to use. Sodium
taining inorganic heterocycles provide important properties in carbonate (Na2CO3), sodium sulfate (Na2SO4) and the deuterated
the field of smart dendrimers, and they are used very often as solvents (CDCl3, D2O) for NMR spectroscopy were obtained from
cores.31–33 Recently, cyclophosphazene cores have been receiving Merck Chemical Co. Pyrene-1-boronic acid, palladium tetrakis-
more attention in the sensing field due to their extended surface triphenylphosphine (Pd(PPh3)4), 4-methoxybenzene boronic acid,
areas which result in more contact with the analyte, which could Br2, CCl4, K2CO3 were purchased from Aldrich Chemical Co.
carry directly six or eight functional groups with simple synthetic The HBr (48%, aqueous), tetrabutylammoniumbromide (TBAB),
methodology. For example, highly selective fluorescent probes 1,4-dioxane, cesium carbonate (Cs2CO3), Aliquat and 3,5-dibromo-
based on cyclophosphazene cores carrying phenanthrolines,34 phenol were purchased from Alfa Aesar Chemical Co. All the other
BODIPY,35 rhodamine,36 imidazole,37 pyrene38 and curcumin39 reagents and solvents were reagent grade quality and obtained
were prepared for the sensing Al3+, Fe2+, Fe3+, Cu2+, Hg2+ and from commercial suppliers. All reactions were carried out in an
picric acid, respectively.
argon atmosphere with anhydrous solvents.
In a recent publication, a series of pyrene functionalized
cyclotriphosphazenes, representing small molecules, and their
2.2. Equipment and calculations
sensing behaviors towards NACs were described.40 This work A Thermo Finnigan FlashEA 1112 instrument was used for the
showed that the bis-, tetra- and/or hexa-pyrene substituted elemental analysis. The mass spectra were obtained using a
cyclotriphosphazenes exhibited intramolecular excimer emission Bruker Daltronics Microflex mass spectrometer equipped with
arising from non-covalent p–p and CH–p stacking interactions due nitrogen UV-Laser and an average of 50 shots in the linear
to the geometric orientation of the pyrene units on the cyclotripho- modes used for analysis. Infrared spectra and UV-Vis spectra
sphazene core. It was also found that these materials had intra- were recorded by a PerkinElmer Spectrum 100 Optica FT-IR
molecular interactions which contributed to effective fluorescence spectrometer and a Shimadzu UV-2001 UV spectrophotometer,
quenching of pyrene moieties when they were tested with NACs and respectively. Fluorescence emission spectra and fluorescence
appeared to be suitable for sensor applications for determination of lifetime were measured with a Varian Eclipse spectrofluoro-
explosive nitro aromatics with high selectivity and sensitivity. These meter and a Horiba-Jobin-Yvon-SPEX FluoroLog 3-2iHR equipped
findings have motivated further investigations into the geometric with a FluoroHub-B single photon counting controller. The
orientations and sensing behaviors when six more pyrene units were TCSPC module was used for signal acquisition in fluorescence
attached onto a cyclotriphosphazene core, hence, the design and lifetime measurements and an excitation wavelength 390 nm was
synthesis of these types of molecular constructions was considered. chosen. Silica gel plates F254 indicator (Kieselgel 60, 0.25 mm
In designing dendrimer-like materials, constitutional isomers which thickness) and silica gel (Kieselgel 60, 230–400 mesh), purchased
are compounds of the same molecular formula but have a different from Merck, were used for analytical thin layer chromatography
chemical structure were also considered, because the effects on (TLC) and column chromatography, respectively. The 1H, 13C
chemical, physical and biological properties may also be significant. and 31P NMR spectra were recorded using a Varian 500 MHz
To prepare the dendrimer-like derivatives, including ones with more spectrometer and the samples were dissolved in CDCl3. Thermal
than six pyrene units, firstly bis-pyrenyl phenol dendrons with two gravimetric analysis (TGA) was carried out using a Mettler Toledo
constitutionally isomeric forms such as ‘‘bent’’ (4) or ‘‘V’’ shaped (6) TGA/SDTA851 instrument (heating rate of 10 1C minꢀ1), and
were synthesized separately by convergent methodology using a differential scanning calorimetry (DSC) measurements were
sequence of Suzuki cross-coupling reactions between pyrene boronic performed with a Mettler Toledo DSC 821e equipped with Stare
acid and pyrene bromide derivatives. Then, their dendrimer-like software (scanning rate of 10 1C minꢀ1) and operated under a
cyclotriphosphazenes (5 and 7) containing twelve pyrene units with nitrogen flow (50 mL minꢀ1).
different architectural motifs as constitutional isomers were easily
synthesized by nucleophilic substitution reactions. Standard spectro-
2.3. Synthesis
scopic techniques were used for characterization of newly synthe- Compounds (1) and (2) were prepared according to procedures
sized compounds (elemental analysis, 1H, 31P and 13C NMR in the literature.41,42
spectroscopies and MALDI-TOF mass spectrometry). Dendrons
2.3.1. Synthesis of 8-(4-methoxyphenyl)-1,10-bipyrene (3).
and their derivatives were analysed by density functional theory A mixture of pyrene-1-boronic acid (0.69 g, 2.83 mmol),
calculation at the wB97XD/6-31G(d) level. Their thermal and photo- Pd(PPh3)4 (0.06 g, 0.05 mmol) and K2CO3 (1.4 g, 10.32 mmol)
physical properties, and sensing of NACs were also investigated.
were added to the solution of 4-(8-bromopyren-1-yl)phenol (2)
(1 g, 2.58 mmol) in 30 mL of dioxane. The reaction mixture was
stirred vigorously with heating at 100 1C for four days, with TLC
monitoring. After completion of the reaction, the mixture was
poured into 10 mL of HCl that included ice followed by
extraction with dichloromethane via a separating funnel. The
2. Experimental
2.1. Materials
Recrystallization with n-hexane was used for purification of combined organic layer was dried over Na2SO4 and filtered. The
hexachlorocyclotriphosphazene (trimer, N3P3Cl6) (Sigma-Aldrich solvent was then removed under reduced pressure. The crude
Chemical Co). Tetrahydrofuran (THF) and toluene were compound was further purified by column chromatography
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using dichloromethane : n-hexane (1 : 3, v/v) as the mobile
2.3.4. Synthesis of 3,5-di(pyrene-1-yl)phenol (6). Pyrene-1-
phase on silica gel. Compound 3 was the eluate (0.75 g, 57%, boronic acid (2.93 g, 12 mmol), 3,5-dibromophenol (1 g,
m.p.: 236 1C). FT-IR (umax/cmꢀ1): 3035 (ArC–H), 2957, 2931, 4 mmol), TBAB and Na2CO3 (2 M, 20 mL) were dissolved in a
2908, 2835 (C–H, CH3), 1607 (ArCQC), 1494 (ArC–C), 1240 toluene/THF (60 : 5 mL) solvent mixture. The mixture was
(ArC–O), 1179 (CH3–O). 1H NMR: d (ppm) 8.35 (dd, 2H, J = purged with argon for 20 min, followed by the addition of
12.1, 8.2 Hz), 8.27 (d, 1H, J = 7.9 Hz), 8.24 (d, 1H, J = 8.1 Hz), (Pd(PPh3)4) (0.14 g, 0.12 mmol). The two-phase mixture was
8.21–8.13 (m, 6H), 8.01 (dd, 2H, J = 15.2, 6.2 Hz), 7.87 (d, 1H, J = then stirred vigorously under heating at 100 1C for three days
9.0 Hz), 7.68 (d, 1H, J = 8.5 Hz), 7.62 (d, 1H, J = 9.6 Hz), 7.51 with TLC monitoring. After cooling to room temperature, the
(d, 2H, J = 7.2 Hz), 7.0–6.98 (m, 4H), 3.85 (s, 3H). {1H} 13C NMR: mixture was diluted with water and extracted with CH2Cl2. The
d (ppm) 158.93, 135.77, 131.56, 128.91, 128.82, 127.88, 127.65, combined organic layer was dried over Na2SO4 and filtered.
127.55, 127.47, 127.33, 126.06, 125.79, 125.70, 125.61, 125.51, The solvent was then removed under reduced pressure, the
125.22, 125.12, 125.03, 124.97, 124.80, 124.56, 124.49, 113.78, crude compound was subjected to column chromatography
55.34. [M]+, (m/z, %): 508.70 (100). Elemental analysis: (found: using THF : n-hexane (1 : 3, v/v) as the mobile phase on silica
C 92.61%, H 4.25%, C39H24O (508), required: C 92.10%, H 4.76%). gel. Compound 6 was the eluate (1.33 g, 68%, m.p.: 246 1C).
2.3.2. Synthesis of 4-([1,10-bipyren]-8-yl)phenol (4). 8-(4-Methoxy- FT-IR (umax/cmꢀ1): 3306 (O–H), 3043 (ArC–H), 1589 (ArCQC),
phenyl)-1,10-bipyrene (3) (0.82 g, 1.62 mmol) was stirred in 50 1485, 1424 (ArC–C), 1171 (ArC–O). 1H NMR: d (ppm) 8.41
mL of HBr at room temperature for 3 h. Aliquat (1.35 g, (s, 2H), 8.25–8.02 (m, 18H), 7.50 (s, 1H), 5.15 (s, 1H). {1H}
4.2 mmol) was added into the mixture and stirred at 130 1C 13C NMR: d (ppm) 155.46, 142.89, 137.03, 131.48, 130.97,
for 14 h. After completion of the reaction, the mixture was 130.78, 128.53, 127.68, 127.55, 127.54, 127.40, 126.06, 125.67,
poured into 100 mL of water followed by extraction with 125.24, 125.20, 124.99, 124.94, 124.90, 124.68, 116.50. [M + H]+,
dichloromethane via a separating funnel. The combined (m/z, %): 495.345 (100). Elemental analysis: (found: C 92.36%,
organic layer was dried over Na2SO4 and filtered. The solvent H 4.25%, C38H22O (494), required: C 92.28%, H 4.48%).
was then removed under reduced pressure. The crude com-
2.3.5. Synthesis of hexakis[3,5-di(pyrene-1-yl)phenoxy] cyclo-
pound was further purified by column chromatography using triphosphazene (7). Cesium carbonate (0.5 g, 1.5 mmol) was
dichloromethane : n-hexane (3 : 1, v/v) as the mobile phase on added to the THF solution (20 mL) of compound 2 (0.5 g,
silica gel. Compound 4 was the eluate (0.5 g, 50%, m.p.: 179 1C). 1.0 mmol) at 40 1C. After the mixture was stirred for 15 min,
FT-IR (umax/cmꢀ1): 3394 (OH), 3043 (ArC–H), 1606 (ArCQC), trimer (0.045 g, 0.13 mmol) in THF (15 mL) solution was added
1
1490, 1445 (ArC–C), 1260 (ArC–O). H NMR: d (ppm) 8.34 (dd, dropwise and refluxed at 80 1C for five days under an argon
2H, J = 13.5, 8.0 Hz), 8.27 (d, 1H, J = 7.7 Hz), 8.24 (d, 1H, J = atmosphere. After the reaction was completed, filtration was
7.7 Hz), 8.20–8.13 (m, 6H), 8.01 (d, 2H, J = 8.9 Hz), 7.87 (d, 1H, performed and the solvent was then removed under reduced
J = 9.3 Hz), 7.67 (d, 1H, J = 9.0 Hz), 7.62 (d, 1H, J = 9.6 Hz), 7.45 pressure. The crude product was subjected to column chroma-
(d, 2H, J = 7.5 Hz), 7.0 (s, 2H), 6.92 (d, 2H, J = 7.0 Hz), 5.02 tography using dichloromethane:n-hexane (2: 1) as the mobile
(s, 1H). {1H} 13C NMR: d (ppm) 155.05, 135.78, 131.83, 128.91, phase. Compound 7 was the eluate (0.2 g, 50%). 1H NMR:
128.81, 128.25, 127.85, 127.64, 127.55, 127.46, 127.34, 126.06, d (ppm) 8.05 (d, 12H, J = 9.2 Hz), 7.91 (d, 12H, J = 7.6 Hz), 7.80
125.78, 125.71, 125.58, 125.51, 125.22, 125.02, 124.96, 124.79, (s, 12H), 7.68 (d, 12H, J = 8.9 Hz), 7.62 (t, 12H, J = 7.6 Hz), 7.57
124.57, 124.49, 115.16. [M]+, (m/z, %): 494.40 (100). Elemental (d, 12H, J = 7.9 Hz), 7.44–7.34 (m, 36H), 7.22 (s, 6H), 7.19
analysis: (found: C 92.55%, H 4.21%, C38H22O (494), required: (d, 12H, J = 9.1 Hz). {1H} 13C NMR: 151.00, 142.81, 135.63,
C 92.28%, H 4.48%).
130.99, 130.37, 129.92, 127.96, 127.20, 126.84, 124.78, 124.58,
2.3.3. Synthesis of hexakis[4-([1,10-bipyren]-8-yl)phenoxy] 124.47, 124.38, 124.34, 122.37. 31P NMR: d (ppm) 10.33. Elemental
cyclotriphosphazene (5). Cesium carbonate (0.291 g, 0.9 mmol) analysis: (found: C 88.53%, H 4.07%, N 1.30%, C228H126N3O6P3
was added to the THF solution (150 mL) of compound 4 (0.3 g, (3096), required: C 88.44%, H 4.10%, N 1.36%). [M + H]+, (m/z, %):
0.6 mmol) at 40 1C. After the mixture was stirred for 15 min, 3097.57 (100).
trimer (0.025 g, 0.07 mmol) in THF (100 mL) solution was
added dropwise and refluxed at 80 1C for 24 h under an argon
2.4. Fluorescence quantum yields and lifetimes
atmosphere. After the reaction was completed, filtration
was performed and the solvent was then removed under
reduced pressure. The crude product was subjected to column
chromatography using dichloromethane : cyclohexane (2 : 1) as
the mobile phase. Compound 5 was the eluate (0.1 g, 45%).
The fluorescence quantum yield (FF) of the target compounds
(5 and 7) were determined by the comparative method (eqn (1)):43
F ꢁ AStd ꢁ n2
Ff ¼ FFStd
(1)
2
FStd ꢁ A ꢁ nStd
1H NMR: d (ppm) 8.40–7.38 (m, 102H, pyrene protons), 7.21– Quinine sulfate (in 0.1 M H2SO4) (FF = 0.54)44 was used as a
7.04 (m, 24H, phenyl protons). {1H} 13C NMR: d (ppm) 155.96, standard and in this equation, F, FStd, A, AStd are the areas
144.36, 134.70, 132.69, 132.02, 131.69, 129.30, 127.38, 125.56, under the fluorescence emission and absorbance curves of the
125.43, 125.11, 125.06, 124.63, 124.56, 118.90, 116.64. 31P NMR: target compounds (5 and 7) and the standard at the excitation
d (ppm) 9.13. Elemental analysis: (found: C 88.65%, H 4.15%, wavelengths, respectively. Due to the dissolution of target
N 1.10%, C228H126N3O6P3 (3096) required C 88.44%, H 4.10%, compounds (5 and 7) and the standard in different solvents,
N 1.36%). [M + H]+, (m/z, %): 3097.46 (100).
refractive indices (n) were used for calculating the fluorescence
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quantum yields. To determine the fluorescence quantum yield via characterization using standard spectroscopic techniques.
of the target compounds (5 and 7), the concentration of each Compound 3 was synthesized using the same method as that
solution was adjusted to 2 ꢂ 10ꢀ7 M at the excitation wave- for compound 2, by means of palladium catalyzed Suzuki cross-
length. Also, directly measured fluorescence lifetimes of target coupling reactions of pyrene-1-boronic acid with a bromide
compounds (5 and 7) were calculated according to mono or derivative of compound 2 in the presence of K2CO3 as a base
di-exponential calculation.
in 1,4-dioxane. Dendron 4 was obtained after a deprotection
reaction of compound 3. For that purpose, compound 3 was
refluxed with Aliquat and aqueous hydrobromic acid for 14 h.
A Suzuki cross-coupling reaction was also used for dendron 6, but
unlike the previous reaction, the biphasic system was used in this
reaction and 6 was obtained with better yield in a single step. The
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of the designed compounds (1–7) is outlined in reaction of pyrene-1-boronic acid with 3,5-dibromophenol in the
Scheme 1 and Fig. 1. The 1,8-dibromopyrene (1) and 1-bromo-8- presence of 2 M Na2CO3 (aq) as a base and n-Bu4NBr (TBAB) as a
(4-methoxyphenyl)pyrene (2) were synthesized according to phase transfer catalyst in THF/toluene (1 : 4, v/v) was realized to
methods found in the literature41,42 and confirmed prior to use obtain dendron 6. Nucleophilic substitution reactions of 4 and 6
Scheme 1 Synthetic routes for the preparation of compounds (1–7). Reagents and conditions: (i) Br2, CCl4; (ii) 4-methoxybenzeneboronic acid, K2CO3,
dioxane, Pd(PPh3)4; (iii) pyrene-1-boronic acid, K2CO3, dioxane, Pd(PPh3)4; (iv) Aliquat, HBr, 130 1C; (v) N3P3Cl6 (trimer), Cs2CO3, THF; (vi) pyrene-1-
boronic acid, 2 M N2CO3, THF, toluene, Pd(PPh3)4, TBAB.
Fig. 1 Molecular structures of constitutional isomers (5) and (7).
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than that of 5, which was probably due to the higher bulky
pyrene chromophores on 7 causing the pyrene groups to be too
close to each other and so intra/intermolecular interactions
occur and therefore, 7 was more resistant to decomposition. As
shown in Fig. S14 (ESI†), the glass transition temperatures (Tg)
of 5 and 7, which were measured by DSC, were observed at
around 59 1C and 40 1C, respectively. It can be said that the
compounds are amorphous because only Tg peaks are observed
in the DSC thermograms.
3.3. Photophysical properties
Fig. 2 1H NMR spectrum of compound 3 in CDCl3.
The optical properties of the constitutional isomers (5 and 7)
and their dendrons (4 and 6) were evaluated with UV-Vis and
fluorescence spectroscopies. Electronic absorption and fluores-
cence emission spectral data of 5 and 7 and their dendrons
(4 and 6) were measured in CH2Cl2 which are shown in Fig. 3
and the molar absorption coefficient of all the compounds in
different solvents were calculated (Table 1). All the compounds
demonstrated absorption maxima at 280 nm and 349 nm which
were attributed p–p* transitions of the pyrene moiety45 (Fig. 3)
and maximum emission bands were shown at 449, 457, 392 and
479 nm, when excited 325 nm, for 4, 5, 6 and 7, respectively.
The UV-Vis absorptions of 5 and 7 were nearly same as those of
their dendrons (4 and 6), because of the optical inactivity of
cyclophosphazene core46,47 which means that the pyrenyl
groups had no effective ground state interaction (Fig. 3a). On
the other hand, although the fluorescence emission spectra of 4
and 5 were very similar to each other, the fluorescence emission
spectra of 6 and 7 were completely different (Fig. 3b). As can be
seen from Fig. 3b and 4 which is the dendron of 5, demonstrated
excimer emission at B460 nm in different solvents like its
cyclotriphosphazene-based dendrimer derivative (5). However,
6 which is the dendron of 7, showed classic pyrene-like monomer
emission between 380–410 nm, whereas 7 showed red-shifted
with trimer with cesium carbonate as a base gave dendrimer-like
compounds (5 and 7), respectively, which were constitutional
isomer compounds as produced under an inert argon atmosphere.
The chemical structures and purities of compounds were con-
firmed by 1H, 13C and 31P NMR, MALDI-TOF MS, and elemental
analyses and were found to be in good agreement with the structures
shown in the Experimental section and ESI† (Fig. S1–S12). The
1H NMR spectrum of compound 3 is shown in Fig. 2. Aromatic
proton signals on the pyrene ring were observed between
8.35–7.50 ppm and the benzene ring proton signals were
around 7.00 ppm. The proton signal of the methoxy group
was observed at 3.85 ppm. The integration of the aromatic
proton signals to the methoxy group signal in the 1H NMR
spectrum of compound 3 gave approximately a 7 : 1 proton
ratio, which confirmed the suggested structure. As shown in
Fig. S5 (ESI†), the proton signal of the –OH on the benzene
group of 4 was observed at 5.02 ppm and it disappeared after
the D2O exchange. The carbon signal of the methoxy group on
compound 3 was seen at 55.34 ppm which disappeared after
the deprotection reaction as shown in Fig. S6 (ESI†). Two
compounds (5 and 7) have the same chemical formula, there-
fore the peak group representing the protonated molecular ion
peaks were obtained at almost the same value according to the
MALDI-MS results (Fig. S7, ESI†). As can be seen from Fig. S8
(ESI†), in the proton-decoupled 31P NMR spectra of 5 and 7, only the
singlet peaks (at 9.13 and 10.32 ppm, respectively), were observed
which were evidence of full substitution of the three phosphorus
atoms with identical units. It was noted that the resonance
values for the phosphorus atoms of the compounds were at
slightly different places due to the different architectural pyrene
motifs on the periphery of the cyclotriphosphazene ring.
Fig. 3 The normalized (a) UV-vis and (b) fluorescence spectra of 4–7 in
CH2Cl2 (lex = 325 nm).
3.2. Thermal properties
Table 1 Epsilon values of 4–7 in different solvents
e (L molꢀ1 cmꢀ1
)
4
5
6
7
The thermal stabilities of the dendrimer-like compounds
(5 and 7) were measured by thermogravimetric analysis (TGA).
The measurements were obtained at a heating rate of 10 1C per
min at +25 1C to +700 1C in a nitrogen (N2) atmosphere. The
compounds were highly thermally stable with decomposition
temperatures of 477 1C and 494 1C, respectively (Fig. S13, ESI†).
The char yield that remained was 45% for 5 and 70% for 7
at 700 1C. It is seen from Fig. S13 (ESI†), that both the
decomposition temperature and the char yield of 7 were higher
CH
Dioxane
THF
Toluene
Dichloromethane
MeOH
18 300
77 320
36 140
50 340
37 400
33 700
28 420
LSa
LS
4020
LS
121 800
95 840
—
75 060
—
—
LS
79 080
78 900
70 960
70 560
13 220
29 620
LS
310 620
322 060
395 460
333 640
LS
LS
LS
ACN
Water
a
Low-solubility.
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Table 2 Lifetimes (tF) of 5, 7 and their NAC complexes
Compound + NAC
tF0 (ns)
tF (ns)
tF0/tF
5
7
Nitrobenzene
Nitrophenol
2,4-DNT
2.359 ꢃ 0.026
2.122 ꢃ 0.035
2.208 ꢃ 0.032
2.197 ꢃ 0.034
2.187 ꢃ 0.027
14.996 ꢃ 0.025
14.002 ꢃ 0.016
12.957 ꢃ 0.017
13.959 ꢃ 0.022
1.10
1.06
1.07
1.08
1.06
1.14
1.23
1.15
TNT
Nitrobenzene
Nitrophenol
2,4-DNT
16.003 ꢃ 0.013
Fig. 4 The normalized solid-state fluorescence emission spectra of (a) 5
and (b) 7 at 25 1C and after annealing at 200 1C (lex = 325 nm).
TNT
emission at B479 nm in CH2Cl2. The red-shifts of the consti-
tutional isomers (5 and 7) from their dendrons were calculated
as 8 nm and 87 nm, respectively, (Fig. 3b). To identify these red-
shift differences, theoretical calculations were carried out by
using wB97XD with the 6-31G(d) method. As explained in more
detail in the theoretical part, p-(1,40-bipyren-8-yl)phenol (4), has
an effective intramolecular non-covalent p–p interaction when
its structural isomer 3,5-di(pyren-1-yl)phenol (6) has no inter- or
intramolecular non-covalent p–p interactions. For these rea-
sons, it was decided that, the intramolecular non-covalent p–p
interactions of 4 caused pyrene-based excimer emission which
was responsible for the red shifting when the classic pyrene like
monomer emission of 6 occurred because of the lack of inter- or
intramolecular interactions. The strong red-shifted emission of
5 and 7 were very similar to the intermolecular pyrene excimer
emission which was a concentration-dependent phenomenon
and did not occur at 10ꢀ5 M and below.48 Therefore, it was
decided that this red shift was attributed to an intramolecular
These results suggested that, the molecular conformation of 5
and 7 shows insignificant changes from solution to solid state.
To determine the effect of temperature on the optical proper-
ties of 5 and 7, their fluorescence spectra which were recorded
after annealing at 200 1C for 24 h under an argon atmosphere
were compared to their fluorescence spectra at room temperature.
As can be seen from Fig. 4, the molecular structures of 5 and 7
remained stable at high temperatures. The fluorescence quantum
yields (Ff) of all the compounds were determined in CH2Cl2
according to eqn (1) with quinine sulfate in 0.1 M H2SO4 solution
as a standard and were calculated to be 0.69 and 0.77. In addition,
fluorescence lifetime measurements (tF) were carried out with
mono or di-exponential calculations for all compounds (Fig. S24,
ESI†). The fluorescence lifetimes (tF) of 5 and 7 with NACs are
summarized in Table 2.
3.4. Theoretical calculations
excimer emission arising from the non-covalent p–p stacking The Gaussian 16 package was used for all theoretical computa-
interactions.49 To determine the effect of the solvent on the tions which were carried out using DFT to obtain more infor-
optical properties of the target compounds (5 and 7) and their mation about the optimized structures of the new compounds
dendrons (4 and 6), various solvents such as acetonitrile (ACN), and their molecular interaction among pyrene groups.50 Geo-
dichloromethane, 1,4-dioxane, methanol (MeOH), cyclohexane metry optimization studies were carried out by wB97XD51 with
(CH), tetrahydrofuran (THF) and toluene (TOL) were used. As a basis set of 6-31G(d) for describing noncovalent dispersion
can be seen in Fig. S15–S18 (ESI†), polar solvents (ACN, MeOH interactions and the nearest values to the crystal structure
and water) and CH for 5 and 7 and water for 4 and 6 could not could be obtained with this method.40 A conductor-like polar-
be used in measurements because of the low-solubility of izable continuum model (CPCM) was adopted to consider the
the compounds. The absorption and emission maxima of the solvent effects using the dielectric constant of THF (e = 7.6)
compounds were not affected by the change of solvents. The in the same level: wB97XD/6-31G(d). The HOMO and LUMO
concentration effect on the pyrene monomer/excimer for- energy values were estimated by using a single point time-
mation, and fluorescence spectra of all the compounds were dependent DFT (TD-DFT) method at similar levels and perform-
evaluated at different concentrations (5 ꢂ 10ꢀ6 M, 1 ꢂ 10ꢀ6
M
ing the wB97XD/6-31G(d). The DFT-optimized geometries and
and 2 ꢂ 10ꢀ7 M) in the above solvents. According to Fig. S19–S22 HOMO–LUMO orbitals of compounds 4, 5, 6, and 7 are shown
(ESI†), it can be seen that pyrene monomer/excimer formation in Fig. 5 and interactions via intramolecular noncovalent dis-
of precursors and target compounds was independent from persion of 4, 6, and 7 and also the distances of the pyrene group
concentration. In addition, it was decided that, the excimer for 5 are shown in Fig. 6 among the pyrene units. The selected
emission of 4, 5 and 7 arose from intramolecular non-covalent intramolecular pyrene groups were in the range of 3.095 to
p–p stacking interactions because as mentioned before, inter- 3.931 Å for 4, 5 and 7, which were accepted values for p–p
molecular pyrene excimer formation is
a
concentration- interactions less than 4.0 Å.49 Although the bent shaped dendron
dependent phenomenon and does not occur at 10ꢀ5 M and (4) had intramolecular p–p interactions between 3.095 and
below.48 To evaluate the solid-state fluorescence properties of 3.313 Å, the V-shaped dendron (6) did not have any interaction
the target compounds (5 and 7), fluorescence emission spectra due to distances of 4.097 and 4.794 Å as shown in Fig. 6. Both
were measured at 25 1C as a solid, which is shown in Fig. S23 dendrimer-like compounds (5 and 7) have p–p and CH–p intra-
(ESI†). Their solid-state fluorescence emission spectra were molecular interactions due to the distance of the pyrene units.
nearly the same as those spectra obtained with dilute solutions, Compound 5 had only face-to-face interactions but compound 7
except for small red shifts at maximum wavelength. not only had face-to-face but also face-to-tail interactions which
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Fig. 5 Optimized geometries and HOMO–LUMO orbitals of compounds 4, 5, 6 and 7 using wB97XD/6-31G(d) levels.
Fig. 6 Intramolecular p–p and CH–p interactions in the structures of compounds 4, 5, 6 and 7.
were the main factor of excimer emission. The p–p intramolecular (ESI†). Geometric optimization studies were also carried out by
interactions for 5 among the pyrene groups were between 3.432 wB97XD with a basis set of 6-31G(d) to understand the solvent
and 3.931 Å. The separation of p–p intramolecular interactions of effect on noncovalent dispersion interactions among the
compound 7 were 3.293–3.737 Å (Table 3). Moreover, the separa- pyrene units for compound 7. As seen in Fig. S25a (ESI†) and
tion of CH–p intramolecular interactions between the pyrene Table S2 (ESI†) there was no remarkable change in the CH–p
groups for 4, 5 and 7 were in the range of 2.654 to 3.567 Å, and p–p interactions inside THF in comparison with the gas
2.870 to 3.638 Å and 3.126 to 3.840 Å, respectively, however 6 had phase calculation for compound 7 (Fig. 6 and Table S1, ESI†).
distances among 3.260–7.335 Å which means that there was not Moreover, geometric optimization was studied for compound 7
any interaction and all the data are summarized in Table S1 with the addition of TNT with same basis set for describing the
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Table 3 Comparison of selected intramolecular p–p interaction distances
in compounds 4, 5, 6 and 7 (Å)
metal ions were taken from their stock solutions and then the
final volume was adjusted to 2 mL with THF. The stock
solutions were prepared in THF, water and acetone, for 5 and
7, NACs and the nitrate salts of the interfering metal ions. To
evaluate the fluorescence sensor properties of 5 and 7, 200 ppm
of 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT),
nitrobenzene (NB), nitrophenol (NP), toluene, benzene, phenol
and 1 ꢂ 10ꢀ5 M of mono-, di-, tri-valent metal ions (K+, Li+, Na+,
Ag+, Cs+, Zn2+, Ba2+, Ca2+, Cd2+, Cu2+, Co2+, Fe2+, Mn2+, Pb2+,
Hg2+, Ni2+, Mg2+, Fe3+, Cr3+ and Al3+) were individually added to
2 ꢂ 10ꢀ7 M THF solutions of 5 and 7. According to the results
shown in Fig. 8, the fluorescence signals of 5 and 7 did not
change with addition of mono-, di-, tri-valent metal ions and
toluene, benzene, phenol when the NACs (TNT, 2,4-DNT, NB,
NP) caused almost complete fluorescence quenching of 5 and 7
whose signal was based on a pyrene moiety with excitation at
325 nm. According to the calculated HOMO and LUMO ener-
gies of 4, 5, 6 and 7 and NACs using theoretical calculations
(wB97XD method) for investigation of the fluorescence quench-
ing mechanism, which are given in Fig. 7, the HOMO and
LUMO energy levels of 5 and 7 were congruent with the LUMO
energy levels of NACs. This situation provided important evi-
dence for the effective electron transfer mechanism between
the HOMO of 5, 7 and the LUMO of the NACs.25,40 To gain
greater information about the fluorescence quenching mecha-
nism, the fluorescence lifetime can be used, because the
fluorescence lifetime of the fluorophore was not affected by
the interaction with the analyte at static quench whereas in
dynamic quenching, the fluorescence lifetime of the fluoro-
phore must change.52,53 So, the fluorescence lifetimes of NP,
NB, 2,4-DNT and TNT complexes of compound 5 and 7 were
measured (Fig. S24, ESI†). As can be seen in Table 2, the
fluorescence lifetimes of 5 and 7 were clearly unchanged with
their NAC complexes (compound 5 + NAC, compound 7 + NAC)
and their to/t were nearly equal to 1. These results demon-
strated that CT complex formation had occurred between
compounds 5, 7 and the NACs. In addition, static quenching
was effective at fluorescence quenching.26,54,55 The fluores-
cence receptor properties of 5 and 7 towards NACs showed
that, 0.2 mM of 5 and 7 have a more intense fluorescence signal
than the same experimental conditions with pyrene substituted
4
5
6
7
1
2
3
4
5
6
7
3.095
3.195
3.212
3.277
3.198
3.226
3.313
1
2
3
4
5
6
7
8
3.931
3.896
3.738
3.670
3.693
3.432
3.527
3.805
1
2
3
4
5
5.712
4.097
4.794
7.191
7.928
1
2
3
4
5
6
7
8
3.293
3.737
3.296
3.391
3.546
3.560
3.557
3.699
change of intramolecular interaction in compound 7 and the
intermolecular interaction between compound 7 and TNT according
to Job’s plot analysis (Fig. S25 and S26, ESI†). Fig. S26b (ESI†) shows
the CH–p and p–p intermolecular and intramolecular interactions
and the results are summarized in Table 2. Interaction among the
pyrene units did not change significantly when the TNT molecule
was close to compound 7. In addition, CH–p and p–p intermolecular
interactions between TNT and compound 7 were in the range of
2.938 to 3.731 Å, which was an acceptable value for ac p–p interaction
of less than 4.0 Å and the results were agreement with the
florescence results. According to the results from the DFT calcula-
tions, whereas 4, 5 and 7 have excimer emission taking place
within a molecule, 6 only has monomer emission which did not
exist within a molecule and the theoretical results are in perfect
agreement with the fluorescence data. In addition, the LUMO energy
of cyclophosphazene-based dendrimer 5, which was obtained from
dendron 4, decreased from ꢀ0.08 to ꢀ0.24 whereas the LUMO
energy of 7, which was obtained from 6, decreased from 0.03 to
ꢀ0.26. Thus, the decreased LUMO values of 5 and 7 caused efficient
electron transfer from LUMOs of 5 and 7 to the LUMOs of NACs
because of the closed values of LUMOs (Fig. 7).
3.5. Fluorescence receptor properties
The fluorescence sensing response of 5 and 7 towards NACs
with aromatic forms without nitro groups and interfering metal
ions were investigated. All the fluorescence measurements
were performed in quartz spectroscopic cuvettes at room tem-
perature using a micropipette. An appropriate quantity of
compounds 5 and 7, NACs and nitrate salts of the interfering
cyclophosphazene-based small molecules which were reported
40
˘
by Alidagı et al. More importantly, when the quenching effi-
ciency of pyrene functionalized cyclotriphosphazenes (5 and 7)
towards NACs (B92%) was compared with pyrene substituted
cyclophosphazene-based small molecules (B63%) under the
same analytical conditions, it was found that dendrimer-like
compounds showed far better fluorescence detection sensitivity
towards NACs which indicated that they were more applicable
for the determination of NACs than pyrene substituted
cyclophosphazene-based small molecules which was previously
reported.40 In addition, it is important to mention that 5 and 7
have a competitive detection limit and selectivity, when com-
pared with other dendrimer and porous materials-based
sensors,56–58 which highlight that they are important candidates
for a new type of fluorescence sensor material. As a result of all the
Fig. 7 HOMO and LUMO levels of 4–7 and NACs.
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at the wB97XD/6-31G(d) level. The DFT results supported the
experimental findings about both the photophysical behaviours
and the fluorescence receptor properties against nitroaromatic
compounds. The dendrimer-like compounds demonstrated
highly sensitive and selective receptor properties for the detec-
tion of explosive nitroaromatics (NP, NB, 2,4-DNT and TNT).
It was also found that pyrene-enriched molecules, such as the
dendrimer-like compounds (5 and 7), presented, increased
sensitivity against NACs compared to the small molecule ana-
logs that were used previously.31 It is also noted that although
the synthesized constitutional isomers have similar properties
in terms of the thermal stability, photophysical and receptor
properties, isomer (7) is superior to (5) because it can be more
easily synthesized, as it requires fewer reaction steps, with a
higher yield compared to the other isomer (5).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Thanks is extended to the TUBITAK under the grant number
110T142 for the funding opportunities.
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