Synthesis and spectroscopic properties of some new difluoroboron bis-β-diketonate derivatives

Article abstract of DOI:10.1016/j.saa.2014.04.078

Six new bis-β-diketones (RCOCH₂CO-C₇H ₇N-COCH₂COR) were synthesized from 3,5-diacetyl-2,6- dimethylpyridine via Claisen condensation with the corresponding esters, and then reacted with boron trifluoride etherate to afford difluoroboron bis-β-diketonate derivatives. Their spectroscopic properties were investigated by UV-vis, FTIR, ¹H NMR and fluorescence spectroscopic techniques. It was found that these boron complexes exhibited violet or blue fluorescence emission at 422-445 nm and possessed high extinction coefficients. The results indicate that the extending π-conjugation can increase the fluorescence intensity and quantum yield for these boron complexes. Especially, the compound 2b displayed the stronger fluorescence intensity and the highest fluorescence quantum yield (Φ_u = 0.94) in these boron compounds. However, compounds 2c and 2d had the lower fluorescence intensity and quantum yield as a result of the heavy atom effect of the chlorine atom in the molecules.

Full text of DOI:10.1016/j.saa.2014.04.078

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 209–213
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and spectroscopic properties of some new difluoroboron
bis-b-diketonate derivatives
⇑
Yan Pi, Dun-Jia Wang , Hua Liu, Yan-Jun Hu, Xian-Hong Wei, Jing Zheng
Hubei Collaborative Innovation Center for Rare Metal Chemistry, Hubei Key Laboratory of Pollutant Analysis and Reuse Technology, Hubei Normal University, Huangshi 435002, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Synthesis and characterization of the
bis-b-diketones and their
difluoroboron complexes.
Spectroscopic properties of
difluoroboron bis-b-diketonate
derivatives.
Fluorescence behavior and quantum
yields for difluoroboron complexes
under UV radiation.
a r t i c l e i n f o
a b s t r a c t
Article history:
Six new bis-b-diketones (RCOCH2CO-C7H7N-COCH2COR) were synthesized from 3,5-diacetyl-2,6-dimeth-
ylpyridine via Claisen condensation with the corresponding esters, and then reacted with boron trifluor-
ide etherate to afford difluoroboron bis-b-diketonate derivatives. Their spectroscopic properties were
investigated by UV–vis, FTIR, 1H NMR and fluorescence spectroscopic techniques. It was found that these
boron complexes exhibited violet or blue fluorescence emission at 422–445 nm and possessed high
Received 19 February 2014
Received in revised form 7 April 2014
Accepted 17 April 2014
Available online 26 April 2014
extinction coefficients. The results indicate that the extending
p-conjugation can increase the fluores-
Keywords:
Difluoroboron complexes
Bis-b-diketone
Fluorescence
Quantum yield
cence intensity and quantum yield for these boron complexes. Especially, the compound 2b displayed
the stronger fluorescence intensity and the highest fluorescence quantum yield (Uu = 0.94) in these boron
compounds. However, compounds 2c and 2d had the lower fluorescence intensity and quantum yield as a
result of the heavy atom effect of the chlorine atom in the molecules.
Ó 2014 Elsevier B.V. All rights reserved.
Spectroscopic properties
Introduction
difluoroboron b-diketonates were reported [10–12]. In addition,
we also have investigated photoluminescence behavior of some
In the past decade, the design and synthesis of stable tetracoor-
dinate difluoroboron complexes have been attracted attention due
to their outstanding photophysical and photochemical properties
[1,2]. Especially, the difluoroboron bis-b-diketonate complexes
are classic fluorescent molecules with large molar extinction
coefficients, two-photon absorption cross sections and high emis-
sion quantum yields [3,4]. Their features have been widely
explored as imaging cells fluorophores [5], photochemical reagents
[6], semiconductors [7], laser dyes [8], organic light-emitting
diodes [9] and many other modern technologies. Recently, the
photophysical and photochemical properties of some novel
new difluoroboron complexes with b-diketone ligands [13,14].
But until now the difluoroboron bis-b-diketonate compounds
have been little studied. As an extension of this work, herein we
present the synthesis and spectroscopic properties of six difluorob-
oron complexes with the new bis-b-diketones. Structures of the
obtained compounds were demonstrated by means of FT-IR, NMR,
MS, elemental analysis, UV–vis and fluorescence spectroscopy.
Experimental
Materials and methods
⇑
3,5-Diacetyl-2,6-dimethylpyridine was prepared by our group
according to the method described in the literature [15,16]. Substi-
Corresponding author. Tel.: +86 714 6515602; fax: +86 714 6573832.
E-mail address: dunjiawang@163.com (D.-J. Wang).
http://dx.doi.org/10.1016/j.saa.2014.04.078
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

210
Y. Pi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 209–213
tuted benzoic acid esters, potassium and quinine sulfate were pur-
chased from Shanghai Chemical Reagent Company Ltd. (Shanghai,
China). Boron trifluoride diethyl etherate was purchased from
Sigma Chemical Co. Other reagents were of analytical grade purity
and used without further purification.
Elemental analysis (C, H, N) was determined with a Perkin–
Elmer 2400 elemental analyzer. Infrared spectra were performed
on a Nicolet FTIR 5700 spectrophotometer with KBr pellets. 1H
NMR and 13C NMR spectra were recorded on an Avance III™
300 MHz NB Digital NMR spectrometer in CDCl3 or DMSO–d6.
Electrospray ionization mass spectra (ESI–MS) were determined
with a Finnigan LCQ Advantage Max spectrometer. Melting points
were measured using X-4 digital melting-point apparatus and are
uncorrected.
stirred at room temperature for 24 h and then poured into 100 mL
ice-water. The aqueous layer was adjusted to pH = 6 and the prod-
ucts were precipitated. The resulting precipitates were filtered and
recrystallized from acetone to give bis-b-diketones (1a–1f).
1,10-(2,6-dimethylpyridine-3,5-diyl)bis(butane-1,3-dione)(1a):
Colorless needle crystals, yield 45%, mp 89–90 °C; 1H NMR
(300 MHz, CDCl3): d 15.88 (s, br, 2H, enol OH), 7.86 (s, 1H, PyAH),
5.88 (s, 2H, enol CH), 2.73 (s, 6H, PyACH3), 2.20 (s, 6H, CH3); 13C
NMR (300 MHz, CDCl3): d 193.2, 185.6, 158.2, 136.0, 128.8, 100.8,
25.5, 24.0; IR (KBr):
m 3441 (w), 3020 (m), 2980 (m), 1599 (s),
1513 (s), 1432 (s), 1374 (w), 1228 (s), 1148 (m), 1080 (m), 1024
(m), 933 (m), 832 (m), 768 (m) cmꢁ1; ESI–MS m/z: 276.12
[M+1]+; Anal. Calcd. for C15H17NO4: C 65.44, H 6.22, N 5.09; found
C 65.69, H 6.20, N 5.07.
The optical properties of the obtained compounds were investi-
gated with UV–vis absorbance and fluorescence spectroscopies in
DMSO solution. The emission spectra of the samples were carried
out on a Hitachi F-4500 Fluorescence spectrophotometer. The
UV–vis spectra were obtained with Hitachi U-3010 spectropho-
3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-phenylpropane-1,3-
dione) (1b): White crystals, yield 59%, mp 158–159 °C; 1HNM R
(300 MHz, CDCl3): d 16.59 (s, br, 2H, enol OH), 8.05 (s, 1H, PyAH),
7.96 (d, J = 8.8 Hz, 4H, ArAH), 7.55–7.47 (m, 6H, ArAH), 6.56 (s, 2H,
enol CH), 2.81 (s, 6H, PyACH3); 13C NMR (300 MHz, CDCl3): d 187.7,
185.4, 158.2, 136.0, 134.7, 132.9, 129.6, 128.8, 127.3, 97.2, 24.1; IR
tometer. The
method using quinine sulfate in 0.1 mol Lꢁ1 sulfuric acid
Us = 0.55, kex = 366 nm, at room temperature) as a standard [17].
U values were determined according to literature
(KBr):
m 3443 (w), 3053 (m), 2939 (w), 1597 (s), 1502 (s), 1424 (s),
(
1381 (w), 1250 (s), 1184 (m), 1091 (m), 926 (m), 819 (m), 773 (m),
The fluorescence quantum yield (Uu) was calculated by the
685 (m) cmꢁ1; ESI–MS m/z: 400.13 [M+1]+; Anal. Calcd. for
following equation:
C
25H21NO4: C 75.17, H 5.30, N 3.51; found C 75.28, H 5.28, N 3.53.
3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(2-chlorophenyl)pro-
Fu As n2u
pane-1,3-dione) (1c): White powder, yield 65%, mp 160–162 °C; 1H
NMR (300 MHz, CDCl3): d 16.10 (s, br, 2H, enol OH), 8.07 (s, 1H,
PyAH), 7.70–7.67 (m, 2H, ArAH), 7.49–7.38 (m, 6H, ArAH), 6.48
(s, 2H, enol CH), 2.82 (s, 6H, PyACH3); 13C NMR (300 MHz, CDCl3):
d 186.4, 186.2, 158.8, 136.5, 135.4, 132.1, 131.9, 130.8, 130.2,
Uu
¼
ꢂ
ꢂ
ꢂ
Us
ð1Þ
Fs Au n2s
where the subscripts u and s denote test and standard respectively,
is the fluorescence quantum yield, A is the absorbance at the
U
excitation wavelength, F is the integrated emission spectrum, and
n is the refractive index for the solvent.
128.9, 127.1, 102.4, 24.3; IR (KBr):
m 3431 (w), 3079 (m), 2980
(w), 1594 (s), 1498 (s), 1448 (m), 1388 (w), 1248 (s), 1173 (m),
1035 (m), 950 (m), 804 (m), 765 (s) cmꢁ1; ESI–MS m/z: 468.12
[M]+; Anal. Calcd. for C25H19Cl2NO4: C 64.12, H 4.09, N 2.99; found
C 63.95, H 4.11, N 2.98.
Synthetic methods
The difluoroboron complexes were synthesized in two steps
(Scheme 1). Firstly, the bis-b-diketone ligands 1a–1f were pre-
pared via Claisen condensation of 3,5-diacetyl-2,6-dimethylpyri-
dine with the appropriate esters using potassium tert-butanolate
as the condensing agent in benzene. Secondly, the difluoroboron
complexes 2a–2f were obtained by reacting the bis-b-diketones
with an excess of boron trifluoride diethyl ether (BF3ꢃEt2O) in
dichloromethane. The crude products were isolated by filtration
and recrystallized from DMF–CHCl3 (1:1) solution to give the title
compounds 2a–2f.
3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(4-chlorophenyl)pro-
pane-1,3-dione) (1d): White powder, yield 51%, mp 187–188 °C; 1H
NMR (300 MHz, CDCl3): d 16.53 (s, br, 2H, enol OH), 8.03 (s, 1H,
PyAH), 7.90 (d, J = 8.7 Hz, 4H, ArAH), 7.47 (d, J = 8.7 Hz, 4H, ArAH),
6.51 (s, 2H, enol CH), 2.80 (s, 6H, PyACH3); 13C NMR (300 MHz,
CDCl3): d 187.4, 184.2, 158.4, 139.3, 136.0, 133.1, 129.4, 129.1,
128.6, 97.1, 24.1; IR (KBr):
m 3442 (w), 3066 (m), 2939 (w), 1592
(s), 1500 (s), 1477 (m), 1393 (w), 1253 (m), 1177 (m), 1095 (s),
806 (m), 725 (m) cmꢁ1; ESI–MS m/z: 468.25 [M]+; Anal. Calcd. for
C25H19Cl2NO4: C 64.12, H 4.09, N 2.99; found C 64.29, H 4.07, N 2.98.
3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(4-fluorophenyl)pro-
Synthesis of bis-b-diketone ligands 1a–1f
pane-1,3-dione) (1e): Bright yellow crystals, yield 46%, mp 177–
179 °C; 1H NMR (300 MHz, CDCl3): d 16.58 (s, br, 2H, enol OH),
8.02 (s, 1H, PyAH), 8.01–7.96 (m, 4H, ArAH), 7.18 (m, 4H, ArAH),
6.50 (s, 2H, enol CH), 2.80 (s, 6H, PyACH3); 13C NMR (300 MHz,
CDCl3): d 187.0, 184.8, 167.4, 164.0, 158.3, 136.0, 131.2, 131.1,
A suspension of freshly cut potassium (1.56 g, 40 mmol) in tert-
butanol (30 mL) was placed in a 100 mL round bottom flask with
stirring and the mixture was heated up to 80 °C. After the potassium
was completely exhausted, the excess tert-butanol was evaporated
under reduced pressure and the white residue was dissolved in ben-
zene (20 mL). To this solution a mixture of 3,5-diacetyl-2,6-
dimethyl-pyridine (1.19 g, 10 mmol) and the required ester
(25 mmol) in benzene (30 mL) was slowly added. The mixture was
129.9, 129.7, 129.4, 116.2, 115.9, 97.0, 24.1; IR (KBr):
m 3445 (w),
3057 (m), 2961 (w), 1602 (s), 1515 (s), 1350 (w), 1237 (s), 1157
(m), 1096 (m), 831 (m), 769 (m) cmꢁ1; ESI–MS m/z: 436.24
Scheme 1. Synthetic routes of compounds 1a–1f and 2a–2f.

Y. Pi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 209–213
211
[M+1]+; Anal. Calcd. for C25H19F2NO4: C 68.96, H 4.40, N 3.22;
found C 69.11, H 4.38, N 3.24.
Calcd. for C25H17B2F6NO4: C 56.55, H 3.23, N 2.64; found C 56.68,
H 3.24, N 2.65.
3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(4-(tert-butyl)phenyl)
propane-1,3-dione) (1f): White crystals, yield 48%, mp 154–156 °C;
1H NMR (300 MHz, CDCl3): d 16.62 (s, br, 2H, enol OH), 8.03 (s, 1H,
PyAH), 7.90 (d, J = 8.7 Hz, 4H, ArAH), 7.51 (d, J = 8.7 Hz, 4H, ArAH),
6.53 (s, 2H, enol CH), 2.80 (s, 6H, PyACH3), 1.39 (s, 18H, C(CH3)3);
13C NMR (300 MHz, CDCl3): d 187.3, 185.5, 158.1, 156.9, 136.0,
Difluoroboron 3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(4-(tert-
butyl)phenyl) propane-1,3-dionate) (2f): Yellow powder, yield 79%,
mp 182–184 °C; 1H NMR (300 MHz, DMSO–d6): 8.76 (s, 1H, PyAH),
8.18 (d, J = 8.7 Hz, 4H, ArAH), 7.81 (d, J = 8.7 Hz, 4H, ArAH), 7.37 (s,
2H, C@CH), 2.80 (s, 6H, PyACH3), 1.40 (s, 18H, C(CH3)3); IR (KBr):
m
3089 (m), 2964 (m), 1577 (s), 1492 (s), 1494 (m), 1384 (s), 1264
(m), 1190 (s), 1052 (s), 811 (m), 691 (m) cmꢁ1; ESI–MS m/z:
607.98 [M+1]+; Anal. Calcd. for C33H35B2F4NO4: C 65.27, H 5.81, N
2.31; found C 65.43, H 5.80, N 2.32.
132.0, 129.7, 127.2, 125.8, 97.1, 35.1, 31.1, 24.0; IR (KBr):
m 3441
(w), 3057 (m), 2961 (m), 2863 (m), 1600 (s), 1500 (s), 1480 (m),
1388 (m), 1259 (m), 1186 (m), 1130 (m), 842 (m), 792 (m) cmꢁ1
;
ESI–MS m/z: 512.39 [M+1]+; Anal. Calcd. for C33H37NO4: C 77.47, H
7.29, N 2.74; found C 77.64, H 7.26, N 2.75.
Results and discussion
1H NMR spectra
Synthesis of difluoroboron complexes 2a–2f
To a solution of bis-b-diketone ligands 1a–1f (2.0 mmol) in
dichloromethane (40 mL), the excess boron trifluoride diethyl
etherate (1.0 mL, 7.9 mmol) was added dropwise. The reaction
mixture was stirred for 6 h under reflux. The solvent was removed
in vacuo and the precipitates were filtered off, washed with dichlo-
romethane. The crude products were recrystallized from DMF–
CHCl3 (1:1) solution to obtain difluoroboron complexes 2a–2f in
good yields.
In the 1H NMR spectra, the characteristic chemical shifts for the
bis-b-diketone ligands and boron complexes are summarized in
Table 1. From Table 1, the free bis-b-diketone ligands showed a sin-
glet for the enolic protons at d = 16.62–15.88 ppm, the pyridine
protons at d = 8.07–7.86 ppm and the methine protons at
d = 6.56–5.88 ppm. However, the enolic proton signals were not
observed in difluoroboron complexes 2a–2f. Their methine proton
and pyridine proton signals exhibited a single peak at d = 5.88–7.59
and d = 8.49–8.87 ppm, which shifted 0.46–1.11 ppm and 0.63–
0.85 ppm, respectively, to lower field with respect to the corre-
sponding proton signal in the free bis-b-diketone ligands. This is
because of difluoroboron complexes formation arising from the
electron-withdrawing effect of the chelate ring by the fluorine
atoms [18]. In addition, the chemical shifts of other protons have
few changes.
Difluoroboron 1,10-(2,6-dimethylpyridine-3,5-diyl)bis(butane-1,3-
dionate) (2a): White powder, yield 71%, mp 209–210 °C; 1H NMR
(300 MHz, DMSO–d6): d 8.49 (s, 1H, PyAH), 6.34 (s, 2H, C@CH),
2.79 (s, 6H, PyACH3), 2.23 (s, 6H, CH3); IR (KBr):
m 3030 (w),
2983 (m), 1582 (s), 1501 (s), 1395 (s), 1248 (m), 1200 (s), 1078
(s), 869 (w), 808 (w) cmꢁ1; ESI–MS m/z: 371.53 [M+1]+; Anal.
Calcd. for C15H15B2F4NO4: C 48.57, H 4.08, N 3.78; found C 48.76,
H 4.07, N 3.79.
Difluoroboron 3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-phenyl-
propane-1,3-dionate) (2b): Yellow powder, yield 76%, mp 272–
273 °C; 1H NMR (300 MHz, DMSO–d6): d 8.77 (s, 1H, PyAH), 8.25
(d, J = 8.0 Hz, 4H, ArAH), 7.83–7.72 (m, 6H, ArAH), 7.37 (s, 2H,
IR spectra
The IR spectra of difluoroboron complexes exhibited noticeable
changes as compared with those of bis-b-diketone ligands. The
characteristic, strong absorption bands of the free ligands and
boron complexes are listed in Table 2. The free ligands 1a–1f
showed the strong absorption bands at 1602–1592 cmꢁ1 and
1515–1498 cmꢁ1 assigned to the C@O and enolic C@C stretching
vibrations. However, the C@O and C@C absorption peaks of boron
C@CH), 2.80 (s, 6H, PyACH3); IR (KBr):
m 3078 (m), 2971 (w),
1572 (s), 1490 (s), 1458 (m), 1397 (s), 1250 (m), 1187 (s), 1057
(s), 832 (m), 780 (m), 693 (m) cmꢁ1; ESI–MS m/z: 496.01 [M+1]+;
Anal. Calcd. for C25H19B2F4NO4: C 60.66, H 3.87, N 2.83; found C
60.87, H 3.86, N 2.84.
Difluoroboron 3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(2-chloro-
phenyl) propane-1,3-dionate) (2c): White powder, yield 86%, mp
99–101 °C; 1H NMR (300 MHz, DMSO–d6): d 8.80 (s, 1H, PyAH),
7.88 (d, J = 7.5 Hz, 2H, ArAH), 7.75–7.63 (m, 6H, ArAH), 7.59 (s,
complexes 2a–2f were at 1572–1582 cmꢁ1 and 1500–1487 cmꢁ1
,
which red-shifted 8–30 cmꢁ1 with respect to those of the corre-
sponding bis-b-diketone ligands [19]. This is because that the
C@O bonds were converted into the vibrating structure of the
C@O ? BF2 bond and CAOABF2 bond. Further information on the
IR spectroscopy for boron complexes is given by the disappearance
of corresponding OAH vibration band which exists in the free
ligand due to the keto–enol tautomerism of the bis-b-diketones.
2H, C@CH), 2.84 (s, 6H, PyACH3); IR (KBr):
m 3051 (m), 2926 (w),
1578 (s), 1489 (s), 1442 (m), 1388 (s), 1282 (m), 1233 (m), 1177
(s), 1075 (s), 813 (m), 772 (m), 711 (m) cmꢁ1; ESI–MS m/z:
564.39 [M+1]+; Anal. Calcd. for C25H17B2Cl2F4NO4: C 53.25, H
3.04, N 2.48; found C 53.46, H 3.02, N 2.48.
Difluoroboron 3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(4-chloro-
phenyl) propane-1,3-dionate) (2d): Light yellow powder, yield
71%, mp > 300 °C; 1H NMR (300 MHz, DMSO–d6): d 8.85 (s, 1H,
PyAH), 8.37–7.94 (m, 4H, ArAH), 7.39–7.33 (m, 4H, ArAH), 7.13
Table 1
Comparison of 1H NMR parameters for compounds 1a–5a and 1b–5b.
(s, 2H, C@CH), 2.85 (s, 6H, PyACH3); IR (KBr):
m 3088 (m), 2947
Compound
d(OAH), ppm
d(PyAH), ppm
d(C@CAH), ppm
(w), 1573 (s), 1487 (s), 1421 (m), 1374 (m), 1240 (m), 1188 (s),
1071 (s), 1050 (s), 821 (m), 705 (m) cmꢁ1; ESI–MS m/z: 564.45
[M+1]+; ESI–MS m/z: 564.19 [M+1]+; Anal. Calcd. for C25H17B2Cl2F4-
NO4: C 53.25, H 3.04, N 2.48; found C 53.43, H 3.05, N 2.49.
Difluoroboron 3,30-(2,6-dimethylpyridine-3,5-diyl)bis(1-(4-fluoro-
phenyl) propane-1,3-dionate) (2e): Yellow powder, yield 88%,
mp > 300 °C; 1H NMR (300 MHz, DMSO–d6): d 8.87 (s, 1H, PyAH),
8.18 (m, 4H, ArAH), 7.57 (m, Hz, 4H, ArAH), 7.18 (s, 2H, C@CH),
1a
2a
1b
2b
1c
2c
1d
2d
1e
2e
1f
15.88
–
16.59
–
16.10
–
16.53
–
16.58
–
16.62
–
7.86
8.49
8.05
8.77
8.07
8.80
8.03
8.85
8.02
8.87
8.03
8.76
5.88
6.34
6.56
7.37
6.48
7.59
6.51
7.13
6.50
7.18
6.53
7.37
2.81 (s, 6H, PyACH3); IR (KBr):
m 3054 (m), 2947 (w), 1572 (s),
1500 (s), 1390 (s), 1250 (m), 1163 (s), 1118 (m), 1080 (s), 852
2f
(m), 810 (m), 746 (m) cmꢁ1; ESI–MS m/z: 531.91 [M+1]+; Anal.

212
Y. Pi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 209–213
Table 2
Comparison of the characteristic IR data for compounds 1a–5a and 1b–5b.
Compound
m
(C@O), cmꢁ1
m
(C@C), cmꢁ1
m
(BAO), cmꢁ1
m
(BAF) and m
(BAO), cmꢁ1
1a
1599 (s)
1513 (s)
1374
–
(w, dCH3
)
2a
1b
2b
1c
2c
1d
2d
1e
2e
1f
1582 (s)
1597 (s)
1572 (s)
1594 (s)
1578 (s)
1592 (s)
1573 (s)
1602 (s)
1572 (s)
1600 (s)
1501 (s)
1502 (s)
1490 (s)
1498 (s)
1489 (s)
1500 (s)
1487 (s)
1515 (s)
1500 (s)
1500 (s)
1395 (s)
–
1397 (s)
–
1388 (s)
–
1374 (s)
–
1200 (s), 1078 (s)
–
1187 (s), 1057 (s)
–
1177 (s), 1075 (s)
–
1188 (s), 1071 (s)
–
1390 (s)
1388
1163 (s), 1080 (s)
–
(m, dCH3
)
2f
1577 (s)
1492 (s)
1384 (s)
1190 (s), 1052 (s)
In addition, the strong absorptions of boron complexes in the
region of 1397–1374 cmꢁ1 were due to the BAO stretching vibra-
tions and those in the range of 1200–1052 cmꢁ1 were attributed
to the BAF and BAO stretching vibrations [18,20]. Apparently,
there were no strong absorption peaks in these regions for the
bis-b-diketone ligands.
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Absorption spectra
The absorption spectra data for compounds 1a–1f and 2a–2f in
DMSO (2.0 ꢂ 10ꢁ5 mol Lꢁ1) in the UV region are summarized in
Table 3. The results showed that boron complexes 2a–2f exhibited
high extinction coefficients (2.74 ꢂ 104–7.48 ꢂ 104 Mꢁ1 cmꢁ1),
which was typical of
p–
p⁄ transitions. Compounds 1a–1f and
2a–2f have a strong broad absorption bands at 302–344 nm and
330–364 nm (Fig. 1), respectively. Obviously, the absorption max-
ima of the boron compounds are red-shifted about 4–28 nm com-
pared to those of their corresponding parent bis-b-diketones,
250
300
350
400
450
Wavelength (nm)
Fig. 1. UV–vis spectra for compounds 1a–1f (dash lines) and 2a–2f (solid lines) in
DMSO (2.0 ꢂ 10ꢁ5 mol Lꢁ1) solution.
which derived from the extended
the BF2-chelating moiety.
p-conjugation by formation of
Fluorescence spectra and quantum yields
1.0
0.8
0.6
0.4
0.2
0.0
2a
2b
2c
2d
2e
2f
The emission spectra of complexes 2a–2f in DMSO (2.0 ꢂ 10ꢁ5
-
mol Lꢁ1) are shown in Fig. 2 and their spectra data are summarized
in Table 4. The results showed that these boron complexes yielded
violet or blue emission at 422–445 nm in DMSO solution. Gener-
ally, an increase in
p-conjugation length typically brings about a
red shift of emission, change in fluorescence intensity and quan-
tum yield [21]. In this experiment, the maximal emission peaks
of complexes 2b–2f with the extending conjugated benzene ring
are red-shifted 15–23 nm compared to that of the complex 2a con-
taining a methyl unit in the molecules. And complexes 2b–2f also
displayed the significant increased fluorescence intensity and
quantum yields, which was well consistent with the previous rule.
But in complexes 2b–2f, the substituted group of benzene ring
exhibited a significant effect on their fluorescence intensity and
quantum yields. The intensity of fluorescence decreased in the
400
450
500
550
Wavelength (nm)
Fig. 2. Normalized emission spectra for complexes 2a–2f in DMSO (2.0 ꢂ 10ꢁ5
-
mol Lꢁ1) solution.
Table 3
UV–visible spectra data for compounds 1a–1f and 2a–2f.
sequence, 2f > 2b > 2e > 2d > 2c. Evidently, the introduction of the
electron-donating tert-butyl substituent into the benzene ring in
boron complexes resulted in a stronger fluorescence intensity
and the introduction of the electron-withdrawing 4-fluoro, 2-
chloro, 4-chloro substituents gave rise to an obvious decline of
the fluorescence intensity. However, the complex 2e with high
electronegative 4-fluoro substituent in benzene ring exhibited
Compound
kmax (nm)
logemax
Compound
kmax (nm)
logemax
1a
1b
1c
1d
1e
1f
302
340
326
344
340
344
4.79
4.83
4.67
4.65
4.57
4.60
2a
2b
2c
2d
2e
2f
330
356
330
364
356
364
4.85
4.44
4.63
4.87
4.62
4.71

Y. Pi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 209–213
213
Table 4
practical method on the design and synthesis of high intense
fluorescent difluoroboron complex applicable to luminescence
materials of chemical sensors and OEL devices.
Fluorescence spectra data and quantum yields for boron complexes 2a–2f.
Compound
kex (nm)
kem (nm)
Uu
2a
2b
2c
2d
2e
2f
330
356
330
364
356
364
422
442
445
438
440
437
0.03
0.94
0.19
0.16
0.55
0.46
Acknowledgement
The authors would like to acknowledge the support from
National Natural Science Foundation of China (No. 21273065)
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Conclusions
In conclusion, we have synthesized some novel bis-b-diketone
ligands (RCOCH2CO-C7H7N-COCH2COR, R = methyl, phenyl, 2-chlo-
rophenyl, 4-chlorophenyl, 4-fluorophenyl and 4-tert-butylphenyl
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properties of these difluoroboron bis-b-diketonates were investi-
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techniques. It was found that these boron complexes possessed
high extinction coefficients and yielded violet or blue emission at
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