Synthesis, photoluminescence properties and theoretical insights on 1,3-diphenyl-5-(9-anthryl)-2-pyrazoline and -1H-pyrazole

Article abstract of DOI:10.1002/bio.2404

1,3-Diphenyl-5-(9-anthryl)-2-pyrazoline and 1,3-diphenyl-5-(9-anthryl)-1H- pyrazole with an anthryl chromophore were synthesized and characterized using ¹H NMR, ¹³C NMR, FT-IR, mass spectrometry and elemental analysis. Their optical properties were characterized by UV-vis absorption and fluorescence spectroscopy. It was observed that the absorption and fluorescence spectra of the two compounds showed a red shift with respect to that of anthracene. Pyrazole exhibited high fluorescent quantum yields (Φ_f = 0.90 in toluene) while pyrazoline showed nearly no fluorescence in solution. The significant fluorescence divergence of the two similar compounds was investigated theoretically through density functional theory (DFT) calculations. The energetically lowest-lying state S₁ in the pyrazoline exhibited both characteristics of locally excited and electron-transfer states that resulted in the fluorescence quenching of anthryl chromophore whereas the S₁ state in the pyrazole corresponded to an optically allowed state that led to high fluorescence quantum yields in solutions. Copyright 2012 John Wiley & Sons, Ltd. Copyright

Full text of DOI:10.1002/bio.2404

Research article
Received: 13 March 2012,
Revised: 30 May 2012,
Accepted: 8 June 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2404
Synthesis, photoluminescence
properties and theoretical insights on
1,3-diphenyl-5-(9-anthryl)-2-pyrazoline
and -1H-pyrazole
Baoli Dong,a Mingliang Wanga* and Chunxiang Xub
ABSTRACT: 1,3-Diphenyl-5-(9-anthryl)-2-pyrazoline and 1,3-diphenyl-5-(9-anthryl)-1H-pyrazole with an anthryl chromophore
were synthesized and characterized using 1H NMR, 13C NMR, FT-IR, mass spectrometry and elemental analysis. Their optical
properties were characterized by UV–vis absorption and fluorescence spectroscopy. It was observed that the absorption and
fluorescence spectra of the two compounds showed a red shift with respect to that of anthracene. Pyrazole exhibited high
fluorescent quantum yields (Φf = 0.90 in toluene) while pyrazoline showed nearly no fluorescence in solution. The significant
fluorescence divergence of the two similar compounds was investigated theoretically through density functional theory (DFT)
calculations. The energetically lowest-lying state S1 in the pyrazoline exhibited both characteristics of locally excited and
electron-transfer states that resulted in the fluorescence quenching of anthryl chromophore whereas the S1 state in the
pyrazole corresponded to an optically allowed state that led to high fluorescence quantum yields in solutions. Copyright ©
2012 John Wiley & Sons, Ltd.
Keywords: pyrazoline; pyrazole; fluorescence; DFT
melting point apparatus and uncorrected. Elemental analyses were
performed using a CE instruments EA-1100 (CE Instruments, Ltd., Wigan,
Introduction
1,3,5-Triaryl-2-pyrazoline derivatives have attracted much interest
due to their favorable photophysical properties including high
hole-transport efficiency and excellent blue emission (1–4). They
have been widely used as whitening and brightening reagents
for synthetic fibers as fluorescence probes in biological research
and clinical diagnosis such as novel hole-transport materials for
increasing the efficiency of an organic light-emitting diode
(OLED) device (5–9). In their oxidized forms, pyrazoles, are also
important nitrogen-containing heterocyclic compounds and have
received much attention due to their applications as ultraviolet
stabilizers and analytical reagents in the complexation of transition
metal ions (10–12). Ma et al. reported that the introduction of
condensed rings into the 5-position of pyrazoline ring could
increase the melting point of the compounds and be beneficial
to their optoelectronic application (13). In the current study,
we prepared 1,3-diphenyl-5-(9-anthryl)-2-pyrazoline (P1) with an
anthracene ring in the 5-position. We oxidized this pyrazoline to
1,3-diphenyl-5-(9-anthryl)-1H-pyrazole (P2) using two very simple
oxidation methods. Interestingly, the optical properties of the
two compounds changed from nearly no fluorescence to bright
emission in solution. Through density functional theory (DFT)
calculations, we investigated the factors that led to a significant
difference in the optical properties of the 1,3,5-triaryl-2-pyrazoline
and its corresponding pyrazole.
UK). ESI-mass spectra were recorded on a Micromass Quatrro II triple
quadrupole mass spectrometer (Waters Corporation, Milford, MA, USA).
Infrared spectra were obtained with a Bruker Tensor 27 FT-IR spectrometer
(Bruker Optik Asia Pacific Ltd. Honk Kong, China) 1H and 13C NMR spectra
were recorded at 297 K on a Bruker Avance 300 MHz NMR spectrometer
(Bruker, China) using CDCl3 as solvent and TMS as internal standard.
UV–vis spectra were recorded on a Shimadzu UV-3600 spectrometer
(Shimadzu Scientific Instruments, Colombia, MD, USA). Fluorescence
spectra were obtained on a Horiba FluoroMax 4 spectrofluorometer (Horiba
Instruments Co. Ltd., Shanghai, China). Fluorescence quantum yields were
determined using a solution of quinine sulfate in 0.05 N H2SO4 as a
standard (Φf = 0.55). Excitation wavelengths were set at 350 nm for
anthracene and 365 nm for pyrazoline and pyrazole. 1,3-diphenyl-5-
(9-anthryl)-2-pyrazoline and 1,3-diphenyl-5-(9-anthryl)-1H-pyrazole were
synthesized according to the method shown in Fig. 1.
Synthesis of 1, 3-diphenyl-5-(9-anthryl)-2-pyrazoline
A mixture of acetophenone (10 mmol), 9-anthrylaldehyde (10 mmol) and
3mol/L aqueous sodium hydroxide (6 mL) in ethanol (30 mL) was stirred
at room temperature for 3 h. The resulting solid was filtered, dried and
* Correspondence to: Mingliang Wang, School of Chemistry and Chemical
Engineering, Southeast University, Nanjing 211189, PR China.
E-mail: wangmlchem@seu.edu.cn
a
School of Chemistry and Chemical Engineering, Southeast University,
Nanjing 211189, PR China
Experimental
All materials were commercially available and used without further
b
State Key Laboratory of Bioelectronics, Southeast University, Nanjing
purification. Melting points were recorded on electrothermal digital
210096, PR China
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B. Dong et al.
Figure 1. Synthesis of pyrazoline and pyrazole (P1 and P2).
crystallized from ethyl acetate/acetic acid (1:1 v/v) mixed solution
resulting in yellow crystals of 3-(9-anthryl)-1-phenylprop-2-en-1-one.
Next, 3-(9-anthryl)-1-phenylprop-2-en-1-one (3 mmol), phenylhydrazine
(6 mmol) and 37 % HCl (4 mL) were dissolved in ethanol (40 mL). The
mixture was then stirred for 6 h at 78ꢀC resulting in a yellow solid. The
crude product was isolated and recrystallized from ethyl acetate as follows:
Yield: 80 %; light yellow; mp: 238–239 ꢀC. FT-IR (KBr, cm-1): 3054, 1592, 1492,
1391, 1326, 1119, 882, 752, 681, 669. 1H NMR (CDCl3): d / ppm, 8.55
(d, J = 15.6 Hz, 2H), 8.31 (d, J = 9.3 Hz, 1H), 8.04-8.14 (m, 3H), 7.70-7.82
(m, 2H), 7.44-7.76 (m,7H), 7.27 (s, 1H), 6.99 (s, 3H), 6.63-6.67 (m, 1H),
4.01-4.06 (m, 1H), 3.54-3.64 (m, 1H). 13C NMR (CDCl3): d / ppm
147.18, 132.80, 129.92, 128.81, 126.93, 125.96, 124.88, 123.25, 119.51,
113.77, 61.12, 42.47. ESI-MS: 399.17 (M + H)+. Anal. calcd. for C29H22N2: C
87.41; H 5.56; N 7.03. Found: C 87.50; H 5.62; N 6.88.
Synthesis of 1,3-diphenyl-5-(9-anthryl)-1H-pyrazole
A mixture of P1 (1 g) and activated carbon (0.1 g) in 30 mL HoAc was
placed in a 100-mL three-necked flask under an oxygen atmosphere
and stirred at 118ꢀC for 4 h. The reaction mixture was then filtered and
the obtained clear solution was poured into ice water slowly. The crude
product was collected by filtration and column chromatographed (silica
gel) to produce P2 as follows. Yield: 71%; yellow solid; mp: 154–156 ꢀC.
FT-IR (KBr, cm-1): 3048, 1592, 1497, 1403, 1356, 1326, 1119, 1018, 764,
693. 1H NMR (CDCl3): d / ppm 8.52 (s, 1H), 8.03-8.05 (t, J = 7.5 Hz, 2H),
7.99-8.01 (d, J = 7.5 Hz, 2H), 7.78-7.79 (t, J = 8.0 Hz, 2H), 7.42-7.48
(m, 4H), 7.36-7.40 (m, 3H), 7.17-7.19 (m, 2H), 6.95-6.98 (m, 4H). 13C NMR
(CDCl3): d / ppm 152.02, 140.54, 133.10, 131.24, 128.77, 126.80, 125.88,
124.84, 123.13, 108.60. ESI-MS: 397.25 (M + H)+. Anal. calcd. for
C29H20N2: C 87.85; H 5.08; N 7.07. Found: C 87.97; H 5.18; N 6.85.
Results and discussion
Compound P1 had two chromophores - an anthryl group in
the 5-position and 1,3-diphenyl-2-pyrazoline moiety. In P2, the
anthryl group was directly linked to the p-system of the pyrazole
ring by a single bond. The two compounds had similar structures
but interestingly, P1 had nearly no fluorescence while P2 had
strong blue fluorescence in solution. Under irradiation with
254 nm ultraviolet in chloroform solution, P1 oxidized to P2
gradually and the solution produced blue fluorescence. Figure 2
shows the variation trend of absorption and fluorescence
spectra of P1 in chloroform solution under UV irradiation. It
Figure 2. The variation trend of absorption and fluorescence spectra of P1 in
chloroform solution under 254 nm ultraviolet irradiation.
was observed that fluorescence intensity increased gradually
over time and the absorption spectra changed after irradiation.
The absorption and fluorescence spectra of P1 and P2 in
solutions are shown in Figs. 3 and 4. The spectroscopic
parameters of P1, P2 and anthracene are summarized in Table 1.
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Photoluminescence properties of pyrazoline and pyrazole
Figure 4. Fluorescence spectra of P1 and P2 excited at 365 nm in various solvents
Figure 3. UV-Vis absorption spectra of P1 and P2 in various solvents (10À4M).
(10À4M).
P1 and P2 exhibited broad absorption bands arising from the
anthryl group located in the range of 325 to 410 nm. Compared
with the absorption of anthracene, the absorption bands of P1
and P2 were red shifted of about 10 nm and possessed
higher molar absorption coefficients at the maximum of the
absorption band.
The fluorescence spectra of P1 and P2 in the region 400–450
nm were similar to the fluorescence of anthracene and mainly
arose from anthracene chromophore. Except for P1 in toluene
solution, P1 and P2 presented similar fluorescence maxima
and exhibited nearly no variation in different solutions. The
fluorescence spectra of P1 and P2 exhibited a red shift of about
20 nm with respect to that of anthracene. Interestingly, the
difference located at the central ring in the structures of P1
and P2 resulted in a significant change in their fluorescent quan-
tum yields. The quantum yields of P1 were too low to observe
fluorescence in solutions whereas P2 possessed significantly
high quantum yields, particularly in toluene solution (Φf = 0.90),
and showed even stronger fluorescence than anthracene
(Φf = 0.34 in toluene) under the same conditions.
anthracene ring with a dihedral angle of 81.36º. For P2, the central
pyrazole ring produced dihedral angles of 6.98, 39.38 and 75.59º
with the two neighboring benzene and anthracene rings, respec-
tively. In P1, the two chromophores of 1,3-diphenyl-2-pyrazoline
and anthryl bulky p-systems were linked by an sp3 hybridized
carbon atom and should have interacted with each other as
reported by Simmons and Fukunaga (14,15). Compared with the
13C NMR and 1H NMR of 1,3-diphenyl-2-pyrazoline (14), the
1
chemical shifts of C4 and C5 in 13C NMR and pyrazoline-H in H
NMR of P1 moved to a low field, indicating an interaction
between the two chromophores. In the structure of P2, the anthryl
ring was directly linked to the p-system of the pyrazole ring by
a single bond. The two bulky p-systems of anthryl group and
1,3-diphenyl-1H-pyrazoleresulted in a large dihedral angle such
that the interaction should exist between them. The chemical
shift of C5 of P2 made a slight move to a low field relative to that
of 1,3-diphenyl-1H-pyrazole in the 13C NMR spectra (16). That is to
say, an interaction between the two chromophores occurred in P1
and P2, leading to the above-mentioned red-shift in absorption
and fluorescence spectra and enhanced molar absorption
coefficients relative to anthracene.
To further examine the significant divergence in the
fluorescence properties of P1 and P2, time-dependent density-
functional theory (TD-DFT) calculations were performed. The
small calculated difference of the dipole moments between
ground and excited states for P1 (0.16 D) and P2 (0.042 D)
To investigate the mechanism of the above-mentioned optical
properties, the ground state geometries of P1 and P2 were
optimized using density functional theory (DFT) with B3LYP hybrid
functional. For compound P1, the central pyrazoline ring had
dihedral angles of 3.01 and 13.09º with the two adjacent benzene
rings whereas the pyrazoline ring was nearly perpendicular to the
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B. Dong et al.
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Photoluminescence properties of pyrazoline and pyrazole
Figure 5. Frontier orbitals of P1 and P2 in excited state.
correlated well with the small variations in their absorption and
fluorescence spectra in solutions. The main transition orbitals from
the energetically lowest-lying excited state (S1 state) to ground
state (S0 state) were HOMO-LUMO transitions in P1 and P2
(Table 2). As observed from Fig. 5, the HOMO orbital of P1 was
distributed on the whole molecular while the LUMO orbital was
located on the anthryl moiety and the two orbitals only
overlapped on the anthryl moiety. As a result, the S1 state of P1
corresponding to an emissive locally excited singlet state and
electron-transfer state simultaneously (17) can be termed an
incomplete electron-transfer state leading to relative low oscillator
strength for the S1-S0 transition. To the best of our knowledge,
there have been no studies on this type of S1 state that exhibits
both characters of locally excited and electron-transfer states. In
addition, these observations explain the phenomenon that P1
had an extremely and theoretically low fluorescent quantum yield.
In P2, the HOMO and LUMO orbitals were both located on the
anthryl moiety, resulting in a larger overlap degree relative to P1.
The HOMO-LUMO transition was an optically allowed p-p*
transition and led to the locally excited state S1. Hence, the S1-S0
transition of P2 had high oscillator strength and was responsible
for the strong fluorescence of P2. Combined with the higher molar
absorption coefficient, it can be concluded that P2 had stronger
fluorescence than anthracene.
Acknowledgements
This project was supported by the National Basic Research
Program of China (2011CB302004) and Open Project of Southeast
University Key Laboratory of Environmental Medicine Engineering
of Ministry of Education (2010EME009).
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