Synthesis, characterization and fluorescence properties of boron difluoride pyridyl-β-diketonate derivatives

Article abstract of DOI:10.1016/j.saa.2012.11.102

Five pyridyl-β-diketones were synthesized by Claisen condensation of ethyl nicotinate with various aryl methyl ketones in benzene in the presence of sodium amide as the base, and then reacted with boron trifluoride diethyl etherate in dichloromethane to a

Full text of DOI:10.1016/j.saa.2012.11.102

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 419–422
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization and fluorescence properties of boron difluoride
pyridyl-b-diketonate derivatives
⇑
Dun-Jia Wang , Yan-Fang Kang, Ben-Po Xu, Jing Zheng, Xian-Hong Wei
Hubei Key Laboratory of Pollutant Analysis and Reuse Technology, College of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi 435002, PR China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Some new boron difluoride pyridyl-b-diketonate derivatives were synthesized by Claisen condensation of
aryl methyl ketones with ethyl nicotinate and followed by complexation with boron trifluoride etherate.
Their optical properties were studied by UV–vis absorption and fluorescence spectroscopy.
"
"
"
Synthesis and characterization of the
pyridyl-b-diketones and its BF2
complexes.
Fluorescence properties of boron
difluoride pyridyl-b-diketonate
derivatives.
Intense fluorescence of BF2
complexes under UV radiation and
high quantum yield.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2012
Received in revised form 23 November 2012
Accepted 27 November 2012
Available online 7 December 2012
Five pyridyl-b-diketones were synthesized by Claisen condensation of ethyl nicotinate with various aryl
methyl ketones in benzene in the presence of sodium amide as the base, and then reacted with boron
trifluoride diethyl etherate in dichloromethane to afford some new boron difluoride pyridyl-b-diketonate
derivatives. The compounds obtained were characterized using FTIR, 1H NMR, elemental analysis and
mass spectrometry. Their optical properties were studied in DMF by UV–vis absorption and fluorescence
spectroscopy. The results showed that these boron complexes exhibited intense fluorescence in the blue–
green region (420–490 nm) under UV radiation with a relatively high quantum yield. Especially, com-
pounds 4b and 5b displayed much higher quantum yield as compared to compounds 1b, 2b and 3b.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Boron difluoride pyridyl-b-diketonate
b-Diketone
Synthesis
Characterization
Fluorescence
Quantum yield
Introduction
rescent probes, laser dyes, light-emitting diodes, and nonlinear op-
tics [3–6]. By far, the synthesis and photoluminescence properties
In recent years, organic electroluminescent devices have at-
tracted considerable investigations to develop different materials
and various devices [1,2]. It is well known that complex com-
pounds of boron difluoride with b-diketonate ligands containing
aromatic cycles at the carbonyl carbon atoms possess intense lumi-
nescence with high quantum yields, two-photon absorption and
large molar extinction coefficients, which leads to their use as fluo-
of many boron difluoride b-diketonate derivatives were reported
[7,8]. In addition, some solid-state structures of boron difluoride
complexes with b-diketones were also studied [9,10]. Recently,
we have reported spectroscopic properties of some new diaroyl-
methanatoboron difluoride derivatives [11]. As a continuation of
our previous work, herein we describe the synthesis and character-
ization of some novel boron difluoride pyridyl-b-diketonate deriv-
atives. The variety of the aromatic substituent groups for the
chelate ring enable the study of varying electronic effects on their
luminescence properties and intense emission [3,12,13]. Therefore,
⇑
Corresponding author. Tel.: +86 714 6515602; fax: +86 714 6573832.
E-mail address: dunjiawang@163.com (D.-J. Wang).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.11.102

420
D.-J. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 419–422
we introduce the electron-donating 4-methyl, 4-methoxyl, elec-
tron-withdrawing 4-fluoro and 4-phenyl groups into the aromatic
substituents of the chelate ring (Fig. 1) for investigating their spec-
troscopic and fluorescent properties.
reaction mixture was stirred at 80 °C for about 7 h until the dark
yellow product precipitated. The precipitate was filtered off and
washed with 5% acetic acid until pH = 6. The crude products were
recrystallized from ethanol (95%) to obtain the pyridyl-
b-diketones. Compounds 1a–5a were confirmed by elemental
analysis, FTIR, 1H NMR, and mass spectroscopy (the data are in
the Supplementary Information).
Experimental
Materials
General procedure for the synthesis of the boron complexes (1bꢀ5b)
Ethyl nicotinate was prepared by our group according to the lit-
erature [14]. Acetophenone, 4-methylacetophenone, 4-fluoroace-
tophenone, 4-methoxy-acetophenone, 4-phenylacetophenone,
quinine sulfate and sodium amide were purchased from Shanghai
Chemical Reagent Company Ltd. (Shanghai, China). Other reagents
used were of analytical grade, and were used without further
purification.
To a solution of the pyridyl-b-diketones (2.2 mmol) in dry
dichloromethane (25 mL), boron trifluoride diethyl etherate
(0.42 mL, 3.3 mmol) was added dropwise. The reaction mixture
was stirred at 40 °C for 5 h. After removal of the solvent, the resi-
due was filtered and washed with dichloromethane. The solid
was sublimated at 200 °C under 10ꢀ3 mmHg to obtain the boron
complexes. The title compounds 1b–5b were characterized by ele-
mental analysis, FTIR, 1H NMR, and mass spectroscopy (the data
are in the Supplementary Information).
Methods and instruments
Infrared spectra were recorded on a Nicolet FTIR 5700 spectro-
photometer with KBr pellets. 1H NMR spectra were measured on
an Avance III™ 300 MHz NB Digital NMR spectrometer in CDCl3
or DMSO-d6 solution with TMS as internal standard. Electrospray
ionization mass spectra (ESI-MS) were performed with a Finnigan
LCQ Advantage Max spectrometer. Elemental analysis (C, H, N)
was performed with a Perkin–Elmer 2400 elemental analyzer.
Melting points were determined using X-4 digital melting-point
apparatus and are uncorrected.
Results and discussion
Synthesis of boron difluoride pyridyl-b-diketonate derivatives
The synthetic procedure was accomplished in two steps
(Scheme S1). Firstly, the pyridyl-b-diketones 1a–5a were prepared
via Claisen condensation of the aryl methyl ketones with ethyl nic-
otinate using sodium amide as the condensing agent. Secondly, the
boron difluoride complexes 1b–5b were obtained by reacting the
pyridyl-b-diketones with boron trifluoride diethyl ether (BF3ꢂEt2O)
in dichloromethane. The crude products were isolated by simple
filtration. The pure boron complexes were provided by sublimation
at 200 °C under 10ꢀ3 mmHg. The advantages of this synthetic pro-
cedure were the easily available starting materials, mild reaction
and high purity products.
The UV–vis spectra were obtained with Hitachi U-3010 spectro-
photometer. The emission spectra of the sample were carried out
on a Hitachi F-4500 Fluorescence spectrophotometer. The
U values
were determined according to literature method using quinine sul-
fate in 0.1 mol Lꢀ1 sulfuric acid (Us = 0.55, kex = 366 nm, at room
temperature) as a standard [15]. The fluorescence quantum yields
U
were calculated from the relation shown in the following
equation:
Fu As n2u
Spectroscopic characterization
Uu
¼
ꢁ
ꢁ
ꢁ
Us
ð1Þ
Fs Au n2s
The IR spectra of the title compounds showed several weak
vibrations in the range of 3100–3050 cmꢀ1 assigned to the stretch-
ing vibrations for unsaturated CAH. Strong absorption bands in the
regions of 1603–1592 cmꢀ1 and 1549–1524 cmꢀ1 were due to the
C@O and enolic C@C stretching vibrations [16,17]. The strong
absorptions in the region of 1368–1364 cmꢀ1 belonged to the
BAO stretching vibrations and those in the region of
1179–1032 cmꢀ1 were attributed to the BAF and BAO stretching
vibrations [16,18]. Obviously, these results indicated that the bor-
on complexes of the pyridyl-b-diketones with boron trifluoride
were formed.
where the subscripts u and s denote test and standard respectively,
U
is the fluorescence quantum yield, A is the absorbance at the exci-
tation wavelength, F is the integrated emission spectrum, and n is
the refractive index for the solvent.
General procedure for the synthesis of the pyridyl-b-diketones
(1aꢀ5a)
The mixture of sodium amide (1.4 g, 36 mmol) and ethyl nico-
tinate (6.65 g, 44 mmol) in benzene (50 mL) was placed in a
three-necked, round-bottom flask fitted with a reflux condenser
and heated to 50 °C. Then a solution of aryl methyl ketones
(22 mmol) in benzene was added dropwise to the mixture. The
The 1H NMR spectra of boron difluoride pyridyl-b-diketonate
derivatives show pronounced changes in comparison with those
of the pyridyl-b-diketones. In the pyridyl-b-diketones, the keto-
CH2 protons displayed a single peak at d = 4.61–4.69 ppm, the
vinylic protons exhibited a single peak at d = 6.81–6.91 ppm and
the enolic protons revealed a single peak at d = 16.72–16.88 ppm.
But in their boron complexes, the 1H NMR spectra did not show
the presence of not only the enolic proton but also the keto-CH2
proton signals. Only their vinylic protons exhibited a single peak
at d = 7.99–8.14 ppm. However, these proton signals were shifted
1.18–1.26 ppm to lower field with respect to the corresponding
vinylic signal in the parent enol [16,17], which due to the with-
draw of electronic density from the chelate ring by the fluorine
atoms.
F
F
B
O
O
N
R
R = H, CH3, F, OCH3, Phenyl
1
2
3
4
5
The ESI mass spectra of the title compounds 1b–5b were mea-
sured and compared to confirm elemental compositions. The
Fig. 1. Structures of boron difluoride pyridyl-b-diketonate derivatives 1b–5b.

D.-J. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 419–422
421
Table 1
molecular ion peaks for these compounds were observed in agree-
UV–vis spectra data for compounds 1a–5a and 1b–5b.
ment with the Nitrogen Rule.
Compound
kmax (nm)
log emax
Compound
kmax (nm)
log emax
Photo-physical properties
1a
2a
3a
4a
5a
347
348
346
355
355
4.35
4.36
4.33
4.48
4.54
1b
2b
3b
4b
5b
366
372
368
400
402
4.40
4.48
4.38
4.53
4.42
UV–vis absorption spectra
For UV–vis absorption measurements, the concentration of
compounds 1a–5a and 1b–5b was 1.0 ꢁ 10ꢀ5 mol Lꢀ1. Their spec-
tra in DMF are shown in Fig. 2 and their spectra data are summa-
rized in Table 1. The results showed that there were a strong broad
absorption bands at 346–355 nm and 366–402 nm in compounds
1a–5a and 1b–5b, respectively. The longest absorption maxima
of compounds 1b–5b are red shifted about 20–50 nm as compared
4b
1.0
0.8
0.6
0.4
0.2
to those of compounds 1a–5a, which assigned to p–
pꢃ transitions
in the conjugated ring system by the BF2-chelating moiety.
5b
Fluorescence spectra
2b
The fluorescence spectra of compounds 1b–5b were also re-
corded in DMF at a concentration of 1.0 ꢁ 10ꢀ5 mol Lꢀ1. Their
emission spectra are shown in Fig. 3 and their spectra data are
summarized in Table 2. The maximal emission peaks of the boron
complexes are mainly located at about 420–490 nm. These com-
pounds yield blue or green emission in DMF solution at room tem-
perature. Among these boron difluoride pyridyl-b-diketonate
complexes, it can be noted that the emission spectra of substituted
phenyl compounds were shifted to the red. Especially, compounds
4b and 5b, where the benzene is linked by methoxyl and phenyl
groups, respectively, emit at higher wavelengths than other
compounds.
3b
1b
400
450
500
550
Wavelength (nm)
Fig. 3. Normalized emission spectra for compounds 1b–5b.
The highest fluorescence intensity in these boron difluoride
complexes was found for Boron difluoride 1-(4-methoxyphenyl)-
3-(3-pyridyl)-1,3-propandionate (4b). The intensity of fluorescence
decreased in the sequence, 4b > 5b > 2b > 3b > 1b. The introduc-
tion of the electron-donating 4-methyl and 4-methoxyl groups into
the benzene ring in the boron complexes resulted in a higher fluo-
rescence intensity. Meanwhile, the introduction of the electron-
withdrawing 4-fluoro group did not caused an obvious change of
fluorescence intensity. However, the introduction of 4-phenyl
group sharply increased the fluorescence intensity, which due to
Table 2
Fluorescence spectra data and quantum yields for compounds 1b–5b.
Compound
kex (nm)
kem (nm)
Uu
1b
2b
3b
4b
5b
366
372
368
400
402
417
419
446
471
487
0.17
0.43
0.36
0.72
0.65
dard because of the similarity between its absorption and fluores-
cence spectra with those of the boron complexes. The quantum
yield data are tabulated in Table 2. The results revealed that the
4-position substituted group of benzene ring had a significant ef-
fect on their fluorescence quantum yield in boron difluoride pyri-
dyl-b-diketonate derivatives. Especially, compounds 4b and 5b
exhibited much higher quantum yield (Uu = 0.72 and 0.65, respec-
tively) than that of other boron complexes, resulting from the
much larger p-conjugated systems.
Fluorescence quantum yields
The fluorescence quantum yields (Uu) of compounds 1b–5b
were referenced to that of quinine sulfate as a fluorescence stan-
0.40
larger donor character of the methoxyl moiety and larger p-conju-
gated system of the phenyl moiety. This result is in accordance
with the literature [3].
1a
2a
0.35
3a
4a
0.30
5a
1b
0.25
2b
Conclusions
3b
0.20
4b
5b
In conclusion, several new boron difluoride pyridyl-b-diketo-
nate derivatives were synthesized and their structures were con-
firmed by FTIR, 1H NMR, elemental analysis and MS
spectrometry. From the emissive properties, it was concluded that
these boron complexes exhibited intense fluorescence in the blue–
green region (420–490 nm) with a relatively high quantum yield.
Especially, compounds 4b and 5b displayed much higher quantum
yield (Uu = 0.72 and 0.65, respectively) as compared to compounds
0.15
0.10
0.05
0.00
-0.05
300
350
400
450
500
Wavelength (nm)
1b, 2b and 3b. The introduction of the electron-donating and
p-
conjugated groups into the aromatic substituents of the chelate
ring can remarkably increase the fluorescence intensity of the bor-
on complexes.
Fig. 2. UV–vis spectra for compounds 1a–5a (dotted lines) and 1b–5b (solid lines).
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)

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Appendix A. Supplementary material
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