Synthesis, structures, photoluminescent and electroluminescent properties of boron complexes with anilido-imine ligands

Article abstract of DOI:10.1016/j.ica.2010.01.027

Syntheses of three new N-arylanilido-arylimine bidentate Schiff base type ligand precursors, ortho-C₆H₄[NH(2,6-ⁱPr₂C₆H₃)](CH{double bond, long}NAr¹) [Ar¹ = p-FC_6<

Full text of DOI:10.1016/j.ica.2010.01.027

Inorganica Chimica Acta 363 (2010) 1441–1447
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structures, photoluminescent and electroluminescent properties
of boron complexes with anilido-imine ligands
Xiaoming Liu a,b, Yi Ren a, Hong Xia b, Xiao Fan a, Ying Mu a,
*
a State Key Laboratory of Supramolecular Structure and Materials, School of Chemistry, Jilin University, Changchun 130012, PR China
b State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Technology, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 August 2009
Received in revised form 16 January 2010
Accepted 22 January 2010
Available online 29 January 2010
Syntheses of three new N-arylanilido-arylimine bidentate Schiff base type ligand precursors, ortho-
C6H4[NH(2,6-iPr2C6H3)](CH@NAr1) [Ar1 = p-FC6H4 (2a); C6H5 (2b); p-OMeC6H4 (2c)], and their four-coor-
dinated boron complexes, ortho-C6H4[N(2,6-iPr2C6H3)](CH@NAr1)BF2 [Ar1 = p-FC6H4 (3a); C6H5 (3b);
p-OMeC6H4 (3c)] are described. The boron complexes 3a–3c were synthesized from the reaction of
BF3(OEt2) with the lithium salt of their corresponding ligand. All complexes were characterized by 1H
and 13C NMR spectroscopy and molecular structures of complexes 3a and 3c were determined by X-
ray crystallography. The photophysical properties of complexes 3a–3c were briefly examined. All three
complexes display bright green fluorescence in solution and in the solid state. Electroluminescent devices
with complex 3c as the emitter were fabricated. These devices were found to give green emission with
maximum current efficiency of 2.92 cd/A and maximum luminance of 670 cd/m2.
Keywords:
Boron complexes
Electroluminescence
Photoluminescence
Schiff base ligands
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
in OLEDs [11]. We have previously reported a series of N-arylani-
lido-arylimine complexes for green light emission with high effi-
Luminescent coordination compounds have been extensively
studied in recent years due to their potential uses in organic
light-emitting devices (OLEDs) [1], nonlinear optics [2], and fluo-
rescent sensors [3]. For developing the OLED technology, design
and synthesis of coordination complexes with high emission effi-
ciency have been one of the most active research areas in the fields
of coordination and organometallic chemistry [4]. In recent years,
boron compounds, especially the four-coordinated boron com-
plexes with bidentate ligands, such as the 8-hydroxyquinoline
[5], pyridyl-7-azaindole [4b], pyridyl-phenol [6], pyridyl-pyrrolide
[7], and Schiff base ligands [8], have attracted much attention due
to their good luminescent properties, electron-transporting prop-
erties and low cost. In comparison with three-coordinated boron
compounds, which require bulky substituents (such as mesityl)
to be stabilized, the four-coordinated boron complexes are in gen-
eral stable and can be obtained readily. Nevertheless, the boron
complexes with Schiff base ligands as emission materials are still
scarce and emission efficiency is also low [8,9]. We have recently
reported a number of luminescent complexes with chelating
N-arylanilido-arylimine bidentate Schiff base type ligands [10].
Complexes of this type are currently interested for their simple
synthesis, tunable photophysical properties by changing the elec-
tronic property of substituents on the ligands, and good stability
ciency, and the emission color of complexes in solution can be
tuned by the ortho-substituents on the rotatable aryl rings of the
ligands. Generally, ways to tune the emission color of the com-
plexes are varying the electronic property of substituents on the
ligands and changing the conjugated length of chromophores
[12]. As a continuation of the previously reported work, we have
synthesized more boron complexes of this type with electron-
withdrawing or electron-donating substituents on their ligands
and examined their luminescent properties. Herein, we wish to re-
port the synthesis, characterizations, photoluminescent (PL), and
electroluminescent (EL) properties of three new complexes,
ortho-C6H4[N(2,6-iPr2C6- H3)](CH@NAr1)BF2 [Ar1 = p-FC6H4 (3a);
C6H5 (3b); p-OMeC6H4 (3c)].
2. Experimental
2.1. General procedures
Manipulations of air and moisture sensitive materials were car-
ried out under high purity nitrogen atmosphere using standard
Schlenk line or glove-box techniques. Toluene, hexane and tetrahy-
drofuran (THF) were dried by refluxing over sodium/benzophe-
none and distilled under nitrogen prior to use. BF3(OEt2), nBuLi,
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PED-
OT:PSS), poly (N-vinyl carbazole) (PVK) and 1,3,5-tris(2-N-phenyl-
benzimidazolyl)benzene (TPBI) were purchased from Aldrich and
* Corresponding author. Tel.: +86 431 85168376; fax: +86 431 85193421.
E-mail address: ymu@jlu.edu.cn (Y. Mu).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.01.027

1442
X. Liu et al. / Inorganica Chimica Acta 363 (2010) 1441–1447
used as received. The ortho-C6H4F(CH@NC6H4) was synthesized
according to literature procedure [13]. 1H and 13C NMR spectra
were measured on either a Bruker AVANCE-500 NMR spectrometer
or a Varian Mercury-300 NMR spectrometer. The elemental analy-
sis was performed with a Perkin–Elmer 2400 microanalyzer. UV–
Vis absorption spectra were recorded on an UV-3100 spectropho-
tometer. Fluorescence measurements were carried out on a RF-
5301PC spectrophotometer. Room-temperature luminescence
quantum yields were measured at a single excitation wavelength
(360 nm) referenced to quinine sulfate in sulfuric acid aqueous
solution (U = 0.546).
116.1, 116.6, 122.2, 122.3, 123.8, 127.5, 132.2, 134.6, 134.8,
147.4, 149.8, 162.9 (CH@NAr) ppm.
i
2.2.4. Ortho-C6H4[NH(C6H3 Pr2-2,6)](CH@NC6H5) (2b)
A solution of nBuLi (30.32 ml, 48.5 mmol) in hexanes was added
slowly to a solution of 2,6-diisopropylaniline (6.02 g, 48.5 mmol) in
THF (40 ml) at ꢀ78 °C. The reaction mixture was allowed to warm
to room temperature and stirred overnight. The resulting solution
i
of LiNH(C6H3 Pr2-2,6) was transferred into a solution of ortho-
C6H4F(CH@NC6H5) (9.66 g, 48.5 mmol) in THF (40 ml) at 25 °C.
After stirring for 2 h, the reaction was quenched with H2O
(25 ml), extracted with n-hexane, and the organic phase was evap-
orated to dryness in vacuo to give the crude product. Pure product
(12.09 g, 33.9 mmol, 70%) was obtained as yellow crystals by
recrystallization from ethanol at –20 °C. Anal. Calc. for C25H28N2
(356.23): C, 84.23; H, 7.92; N, 7.86. Found: C, 84.16; H, 7.85; N,
7.75%. 1H NMR (300 MHz, CDCl3): d 1.15 (d, 6H, CH(CH3)2), 1.17
(d, 6H, CH(CH3)2), 3.17 (sept, 2H, CH(CH3)2), 6.27 (d, 1H, Ph-H),
6.70 (t, 1H, Ph-H), 7.05–7.19 (m, 5H, Ph-H), 7.24–7.32 (m, 4H,
Ph-H), 7.86 (d, 1H, Ph-H), 8.67 (s, 1H, CH@NAr), 10.62 (s, 1H, NH)
ppm. 13C {1H} NMR (75 MHz, CDCl3): d 22.8 (CH(CH3)2), 24.9
(CH(CH3)2), 28.5 (CH(CH3)2), 112.1, 115.5, 116.7, 120.9, 122.1,
123.1, 123.8, 125.6, 127.4, 129.3, 132.1, 134.6, 147.5, 149.8,
151.5, 163.1 (CH@NAr) ppm.
2.2. Syntheses of compounds
2.2.1. Ortho-C6H4F(CH@NC6H4F-4) (1a)
A mixture of ortho-fluorobenzaldehyde (5.85 g, 47.1 mmol), p-
fluoroaniline (5.23 g, 47.1 mmol) and MgSO4 (1.0 g) in n-hexane
(30 ml) was stirred for 2 h. The mixture was filtered and the filtrate
was concentrated to about 10 ml and kept at ꢀ20 °C overnight.
Pure product was obtained as yellow crystals (7.67 g, 35.3 mmol,
75%). Anal. Calc. for C13H9NF2 (217.21): C, 71.88; H, 4.18; N, 6.45.
Found: C, 71.76; H, 5.25; N, 6.34%. 1H NMR (300 MHz, CDCl3): d
7.06–7.13 (m, 3H, Ph-H), 7.21–7.27 (m, 3H, Ph-H), 7.47 (t, 1H,
Ph-H), 8.17 (t, 1H, Ph-H), 8.76 (s, 1H, CH@NAr) ppm. 13C {1H}
2
NMR (75 MHz, CDCl3): d 115.2 (d, JCF = 23 Hz, Ph), 122.4 (d,
i
2
3
2.2.5. Ortho-C6H4[NH(C6H3 Pr2-2,6)](CH@NC6H4OMe-4) (2c)
2JCF = 9 Hz, Ph), 123.8 (d, JCF = 9 Hz, Ph), 127.8 (d, JCF = 3 Hz, Ph),
2
3
3
A solution of nBuLi (30.32 ml, 48.5 mmol) in hexanes was added
slowly to a solution of 2,6-diisopropylaniline (6.02 g, 48.5 mmol) in
THF (40 ml) at ꢀ78 °C. The reaction mixture was allowed to warm
to room temperature and stirred overnight. The resulting solution
133.0 (d, JCF = 9 Hz, Ph), 147.9 (d, JCF = 3 Hz, Ph), 153.2 (d, JCF
5 Hz, Ph), 159.8 (d, JCF = 5 Hz, CH@NAr), 162.8 (d, JCF = 252 Hz,
CF), 163.1 ppm.
=
3
1
i
of LiNH(C6H3 Pr2-2,6) was transferred into a solution of ortho-
2.2.2. Ortho-C6H4F(CH@NC6H4OMe-4) (1c)
C6H4F(CH@NC6H4OMe-4) (11.12 g, 48.5 mmol) in THF (40 ml) at
25 °C. After stirring for 2 h, the reaction was quenched with H2O
(25 ml), extracted with n-hexane, and the organic phase was evap-
orated to dryness in vacuo to give the crude product. Pure product
(13.49 g, 34.9 mmol, 72%) was obtained as yellow crystals by
recrystallization from ethanol at ꢀ20 °C. Anal. Calc. for C26H30N2O
(386.53): C, 80.79; H, 7.82; N, 7.25. Found: C, 80.68; H, 7.75; N,
7.39%. 1H NMR (300 MHz, CDCl3): d 1.14 (d, 6H, CH(CH3)2), 1.17
(d, 6H, CH(CH3)2), 3.16 (sept, 2H, CH(CH3)2), 3.82 (s, 3H, OCH3),
6.25 (d, 1H, Ph-H), 6.68 (t, 1H, Ph-H), 6.93 (d, 2H, Ph-H), 7.11 (t,
1H, Ph-H), 7.20 (d, 2H, Ph-H), 7.24 (t, 1H, Ph-H), 7.31 (d, 2H, Ph-
H), 7.39 (d, 1H, Ph-H), 8.66 (s, 1H, CH@NAr), 10.68 (s, 1H, NH)
ppm. 13C {1H} NMR (75 MHz, CDCl3): d 22.8 (CH(CH3)2), 25.9
(CH(CH3)2), 28.5 (CH(CH3)2), 55.5 (OCH3), 111.9, 114.5, 115.3,
116.9, 121.9, 123.8, 127.3, 131.7, 134.3, 134.9, 144.6, 147.5,
149.6, 157.9, 161.4 (CH@NAr) ppm.
A mixture of ortho-fluorobenzaldehyde (5.85 g, 47.1 mmol), 4-
methoxyanilline (5.80 g, 47.1 mmol) and MgSO4 (1.0 g) in n-hex-
ane (30 ml) was stirred for 2 h. The mixture was filtered and the fil-
trate was concentrated to about 10 mL and kept at –20 °C
overnight. Pure product was obtained as yellow crystals (8.41 g,
36.7 mmol, 78%). Anal. Calc. for C14H12NFO (229.25): C, 73.35; H,
5.28; N, 6.11. Found: C, 73.44; H, 5.40; N, 6.01%. 1H NMR (300
MHz, CDCl3): d 3.83 (s, 3H, OCH3), 6.94 (d, 2H, Ph-H), 7.11 (t, 1H,
Ph-H), 7.20–7.28 (m, 4H, Ph-H), 7.43 (t, 1H, Ph-H), 8.80 (s, 1H,
CH@NAr) ppm. 13C {1H} NMR (75 MHz, CDCl3): d 55.4 (OCH3),
2
2
114.3, 115.7 (d, JCF = 21 Hz, Ph), 122.3 (Ph), 124.2 (d, JCF = 9 Hz,
3
3
Ph), 124.4 (d, JCF = 4 Hz, Ph), 127.6 (d, JCF = 4 Hz, Ph), 132.4 (d,
2JCF = 9 Hz, Ph), 144.7, 151.1 (d, JCF = 5 Hz, CH@NAr), 158.5 (Ph),
162.6 (d, JCF = 252 Hz, CF) ppm.
3
1
i
2.2.3. Ortho-C6H4[NH(C6H3 Pr2-2,6)](CH@NC6H4F-4) (2a)
i
A solution of nBuLi (30.32 mL, 48.5 mmol) in hexanes was added
to a solution of 2,6-diisopropylaniline (6.02 g, 48.5 mmol) in THF
(40 mL) at ꢀ78 °C. The reaction mixture was allowed to warm to
room temperature and stirred overnight. The resulting solution
2.2.6. Ortho-C6H4N(C6H3 Pr2-2,6)(CH@NC6H4F-4)BF2 (3a)
A solution of nBuLi in hexanes (0.33 ml, 0.52 mmol) was added
dropwise to a solution of 2a (0.19 g, 0.52 mmol) in toluene (5 ml)
at –78 °C. The reaction mixture was allowed to warm to room tem-
perature and stirred for two hours. The resulting mixture was
added dropwise to a solution of BF3(OEt2) (75 mg, 0.53 mmol) in
toluene (5 ml) at ꢀ78 °C with stirring. The reaction mixture was
then allowed to warm to room temperature and stirred for 12 h.
The solvent was removed in vacuo and the residue was extracted
with n-hexane (30 ml). The extraction was concentrated to about
20 ml and kept at –20 °C to give the product as yellowish-green
crystals (170 mg, 0.41 mmol, 78%). Anal. Calc. for C25H26BF3N2
(422.29): C, 71.10; H, 6.21; N, 6.63. Found: C, 71.02; H, 6.32; N,
6.70%. 1H NMR (500 MHz, CDCl3): d 1.02 (d, 6H, CH(CH3)2), 1.23
(d, 6H, CH(CH3)2), 3.05 (sept, 2H, CH(CH3)2), 6.32 (d, 1H, Ph-H),
6.73 (t, 1H, Ph-H), 7.17–7.62 (m, 9H, Ph-H), 8.38 (s, 1H, CH@NAr).
i
of LiNH(C6H3 Pr2-2,6) was transferred into a solution of ortho-
C6H4F(CH@NC6H4F-4) (10.53 g, 48.5 mmol) in THF (40 mL) at
25 °C. After stirring for 2 h, the reaction was quenched with H2O
(25 ml), extracted with n-hexane, and the organic phase was evap-
orated to dryness in vacuo to give the crude product. Pure product
(12.88 g, 34.4 mmol, 71%) was obtained as yellowish-green crystals
by recrystallization from ethanol at ꢀ20 °C. Anal. Calc. for
C25H27N2F (374.49): C, 80.18; H, 7.27; N, 7.48. Found: C, 80.26;
H, 7.15; N, 7.41%. 1H NMR (300 MHz, CDCl3): d 1.15 (d, 6H,
CH(CH3)2), 1.17 (d, 6H, CH(CH3)2), 3.15 (sept, 2H, CH(CH3)2), 6.27
(d, 1H, Ph-H), 6.69 (t, 1H, Ph-H), 7.05–7.19 (m, 5H, Ph-H), 7.24–
7.32 (m, 3H, Ph-H), 7.39 (d, 1H, Ph-H), 8.63 (s, 1H, CH@NAr),
10.54 (s, 1H, NH) ppm. 13C {1H} NMR (75 MHz, CDCl3): d 22.8
(CH3), 24.9 (CH(CH3)2), 28.5 (CH(CH3)2), 112.1, 115.4, 115.8,
13C {1H} NMR (125 MHz, CDCl3):
(CH(CH3)2), 28.7 (CH(CH3)2), 112.8, 116.6, 116.8, 116.9, 122.6,
d 24.5 (CH(CH3)2), 25.3

X. Liu et al. / Inorganica Chimica Acta 363 (2010) 1441–1447
1443
Table 1
124.8, 126.4, 128.3, 133.5, 137.9, 147.9, 152.7, 160.5 (CH@NAr)
ppm.
Crystal data and structural refinements details for 3a and 3c.
3a
3c
i
2.2.7. Ortho-C6H4N(C6H3 Pr2-2,6)(CH@NC6H5)BF2 (3b)
Formula
C25H26BF3N2
422.29
293(2)
0.71073
monoclinic
P2(l)/e
C26H29BF2N2O
434.32
293(2)
A solution of nBuLi in hexanes (0.33 ml, 0.52 mmol) was added
dropwise to a solution of 2b (0.19 g, 0.52 mmol) in toluene (5 ml)
at ꢀ78 °C. The reaction mixture was allowed to warm to room
temperature and stirred for two hours. The resulting mixture
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
0.71073
triclinic
ꢀ
Pl
was added dropwise to
a solution of BF3(OEt2) (75 mg,
a (Å)
b (Å)
12.81(3)
12.69(3)
14.60(3)
90
109.64(7)
90
2236(9))
2
1.254
888
9.316(4)
10.610(4)
12.901(6)
89.941(15)
70.934114)
79.117(14)
1181.0(9)
2
0.53 mmol) in toluene (5 ml) at –78 °C with stirring. The reaction
mixture was allowed to warm to room temperature and stirred
for 12 h. The solvent was removed in vacuo and the residue was
extracted with n-hexane (30 ml). The extraction was concentrated
to about 20 ml and kept at –20 °C to give the product as yellow-
ish-green crystals (140 mg, 0.35 mmol, 66%). Anal. Calc. for
C25H27BF2N2 (404.30): C, 74.27; H, 6.73; N, 6.93; Found: C,
74.31; H, 6.65; N, 6.77%. 1H NMR (300 MHz, CDCl3): d 1.04 (d,
6H, CH(CH3)2), 1.15 (d, 6H, CH(CH3)2), 3.06 (sept, 2H, CH(CH3)2),
6.26 (d, 1H, Ph-H), 6.69 (t, 1H, Ph-H), 7.03–7.17 (m, 5H, Ph-H),
7.23–7.34 (m, 4H, Ph-H), 7.86 (d, 1H, Ph-H), 8.38 (s, 1H, CH@NAr)
ppm. 13C {1H} NMR (75 MHz, CDCl3): d 24.5 (CH(CH3)2), 25.3
(CH(CH3)2), 28.7 (CH(CH3)2), 112.8, 116.1, 116.3, 124.1, 124.3,
128.2, 130.1, 133.4, 135.7, 137.5, 138.6, 147.8, 152.2, 161.1
(CH@NAr) ppm.
(Å)
C
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc, (Mg mꢀ3
F (0 0 0)
)
1.221
460
Crystal size (mm)
h Range for data collection
Limiting indices
0.30 ꢁ 0.22 ꢁ 0.18 0.31 ꢁ 0.22 ꢁ 0.15
3.05–27.48°
ꢀ16 6 h 6 15
ꢀ16 6 k 6 16
ꢀ17 6 l 6 18
4652/0/284
1.036
3.32–27.48°
ꢀ12 6 h 6 10
ꢀ13 6 k 6 13
ꢀ16 6 l 6 16
5350/0/294
1.030
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
r
(I)]
R1a = 0.0840
wR2b = 0.1179
R1a = 0.1891
wR2b = 0.1463
R1a = 0.0724
wR2b = 0.1363
R1a = 0.1780
wR2b = 0.1704
0.173, ꢀ0.192
R indices (all data)
i
2.2.8. Ortho-C6H4N(C6H3 Pr2-2,6)(CH@NC6H4OMe-4)BF2 (3c)
Largest difference in peak and hole 0.142, ꢀ0.147
A solution of nBuLi in hexanes (0.33 ml, 0.52 mmol) was added
dropwise to a solution of 2c (200 mg, 0.52 mmol) in toluene (5 ml)
at –78 °C. The reaction mixture was allowed to warm to room tem-
perature and stirred for two hours. The resulting mixture was
added dropwise to a solution of BF3(OEt2) (75 mg, 0.53 mmol) in
toluene (5 ml) at –78 °C with stirring. The reaction mixture was al-
lowed to warm to room temperature and stirred for 12 h. The sol-
vent was removed in vacuo and the residue was extracted with n-
hexane (30 ml). The extraction was concentrated to about 20 ml
and kept at ꢀ20 °C to give the product as yellowish–green crystals
(180 mg, 0.41 mmol, 81%). Anal. Calc. for C26H29BF2N2O (434.33): C,
71.90; H, 6.73; N, 6.45. Found: C, 71.81; H, 6.65; N, 6.57%. 1H NMR
(500 MHz, CDCl3): d 0.95 (d, 6H, CH(CH3)2), 1.16 (d, 6H, CH(CH3)2),
3.00 (sept, 2H, CH(CH3)2), 3.80 (s, 3H, OCH3), 6.23 (d, 1H, Ph-H),
6.65 (t, 1H, Ph-H), 6.91–7.51 (m, 9H, Ph-H), 8.31 (s, 1H, CH@NAr).
(e Åꢀ3
)
P
P
a
R1
=
||Fo| – |Fc||/ |Fo|.
P
P
wR2 = [ [w(F2o ꢀ F2c )2]/ [w(F2o)2]]1/2
.
b
using detergent and alcohol in sequence, followed by oxygen plas-
ma cleaning. PEDOT was spin coated onto the substrate under
clean-room conditions yielding a layer of 100 nm, which was
baked at 110 °C for 10 min to remove the residual water. Spin-
coating with solutions containing 3c (2–8 wt%) in polymer host
(PVK) (10 mg/ml) in chloroform gives the active layer with a thick-
ness of 80 nm. The electron-transporting layer TPBI and the cath-
ode LiF layer were deposited by thermo-evaporation, and
followed by a thick Al capping layer. The luminance of devices
was recorded on a PR650 spectrometer. Current–voltage and light
intensity measurements were carried out at room temperature and
ambient conditions.
13C {1H} NMR (125 MHz, CDCl3):
(CH(CH3)2), 28.7 (CH(CH3)2), 56.0 (OCH3), 113.0, 115.0, 116.4,
116.6, 124.7, 125.7, 128.2, 130.7, 133.4, 135.9, 137.1, 137.4,
148.0, 152.4, 159.9 (CH@NAr) ppm.
d 24.6 (CH(CH3)2), 25.3
3. Results and discussion
2.3. X-ray measurements
3.1. Synthesis of the ligands and their boron complexes
The single crystals of 3a and 3c suitable for X-ray structural
analysis were obtained from n-hexane. Diffraction data were col-
lected at 293 K with a Rigaku R-AXIS RAPID IP diffractometer
The synthetic routes for the ligands and their boron complexes
are shown in Scheme 1. The new pre-ligand compounds, ortho-
C6H4F(CH@NAr) [Ar = 4-FC6H4 (1a); 4-(MeO)C6H4 (1c)] were pre-
pared according to a literature procedure [10a] in high yields by
condensation reaction of ortho-fluorobenzaldehyde with one
equivalent of relevant amine in hexane. The reactions were also
tested in ethanol, which gives the products in slightly lower yields.
Compounds 1a and 1c were characterized by 1H NMR spectroscopy
and elemental analyses. The free ligands 2a–2c were synthesized
in good yields (>70% isolated yields) via a nucleophilic aromatic
displacement of fluoride in 1a–1c by LiNH(2,6-iPr2C6H3) in THF.
Compounds 2a–2c were characterized by 1H and 13C NMR spec-
troscopy along with elemental analyses. The 1H NMR spectra of
these compounds exhibit resonances in the region of d 8.63–8.67
for the imine CH protons, and the NH resonances appear at the
characteristically low field (d 10.5–10.7). The boron difluoride
complexes 3a–3c were synthesized in reasonable yields by reac-
equipped with graphite-monochromated Mo
Ka radiation
(k = 0.71073 Å). Details of the crystal data, data collections, and
structure refinements are summarized in Table 1. The structures
were solved by direct methods [14] and refined by full-matrix
least-squares on F2. All non-hydrogen atoms were refined aniso-
tropically and the hydrogen atoms were included in idealized posi-
tion. All calculations were performed using the SHELXTL [15]
crystallographic software packages.
2.4. Device fabrication
Indium-tin-oxide (ITO)-coated glass with a sheet resistance of
<50
X h
–1 was used as substrate. The substrate was pre-patterned
by photolithography to form the pattern of devices in size of
4 mm2. Pre-treatment of ITO includes a routine chemical cleaning

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X. Liu et al. / Inorganica Chimica Acta 363 (2010) 1441–1447
Scheme 1. Synthetic procedure of ligands 2a–2c and complexes 3a–3c.
tion of BF3(OEt2) with the lithium salt of a corresponding free li-
gand that was generated in situ by treating the free ligand with
nBuLi in toluene at ꢀ78 °C. These complexes are well soluble in
dichloromethane, diethyl ether, toluene and THF, but sparingly sol-
uble in saturated hydrocarbon solvents. Complexes 3a–3c were all
characterized by elemental analyses, 1H and 13C NMR spectros-
copy, and satisfactory analytic results were obtained. In the 1H
NMR spectra of these complexes, the resonances for the imine
CH protons appear at d 8.38 (3a, 3b) and 8.31 (3c), which are obvi-
ously shifted toward high field in comparison with the correspond-
ing signals of the free ligands. In the same way, the 13C NMR
resonances (d 159.9–161.1) for the imine CH carbons in these com-
plexes also shift to high field. The NH signals of the free ligands dis-
appear in the 1H NMR spectra of complexes 3a–3c, which is
indicative of the formation of B–N bond in these complexes. The
1H NMR resonances for the two methyls of the isopropyl units in
complexes 3a–3c are inequivalent due to that the rotation of the
2,6-iPr2C6H3 group about the N-aryl bond is blocked or becomes
slow on the NMR time scale [10a,10b] in these chelating com-
plexes. In addition, both 1H and 13C NMR spectra reveal that these
complexes show Cs-symmetry in solution.
Fig. 1. Molecular structure of complex 3a (thermal ellipsoids are drawn at the 30%
probability level).
3.2. Description of structures
complexes of this type [10d]. In both complexes, the six-membered
chelating ring is nearly planar with the boron atom lying 0.3499 Å
(for 3a) and 0.3045 Å (for 3c) out of the plane, respectively. The
imine C@N bonds in both complexes retain their double bond char-
acter, being 1.296(4) Å (for 3a) and 1.304(3) Å (for 3c), respec-
tively. The B–N (amido) distances [1.506(6) Å for 3a, 1.531(4) Å
for 3c] are shorter than the B–N (imine) distances [1.568(5) Å for
3a, 1.578(4) Å for 3c], which is consistent with the fact that the
B–N (amido) bond is a covalent bond while the B–N (imine) bond
is a coordination bond. The B–N (amido) distances in 3a and 3c are
shorter than the value (1.578(5) Å) of B–N (amido) bond length in a
related boron complex 5,7-difluoro-2-(20-pyridyl)indoleBPh2 [4b].
The dihedral angles between the six-membered chelating ring
Molecular structures of complexes 3a and 3c were determined
by X-ray crystallographic analysis. The crystals of both complexes
suitable for crystal structure analysis were obtained from n-hexane
at room temperature. The ORTEP drawings of molecular structures
of complexes 3a and 3c are shown in Figs. 1 and 2, respectively.
Selected bond lengths and angles for both complexes are given in
Table 2. The X-ray analysis reveals that both complexes 3a and
3c are four-coordinate and adopt a distorted tetrahedral geometry
for the boron center, which is chelated by the bidentate ligand via
the imine and amido nitrogen atoms. The structural parameters of
both complexes are in line with those reported for known boron

X. Liu et al. / Inorganica Chimica Acta 363 (2010) 1441–1447
1445
Table 3
UV–Vis and fluorescent properties of compounds 2a–2c and 3a–3c.
Compound
e
Ab k
(nm)
Em k
(nm)
Quantum
Conditions
(Mꢀ1 cmꢀ1
)
yields (/)a
2a
2b
2c
3a
3b
3c
9851
389
387
392
438
437
438
420
430
424
433
424
435
516
514
518
513
515
514
0.021
0.019
0.025
0.50
Hexane, 298 K
Solid, 298 K
Hexane, 298 K
Solid, 298 K
Hexane, 298 K
Solid, 298 K
Hexane, 298 K
Solid, 298 K
Hexane, 298 K
Solid, 298 K
Hexane, 298 K
Solid, 298 K
10 205
10 643
10 145
11 391
12 812
0.49
0.56
a
Determined using quinine sulfate in 0.1 M sulfuric acid as a standard.
Fig. 2. Molecular structure of complex 3c (thermal ellipsoids are drawn at the 30%
probability level).
the formation of the B–N bonds changes the emission energy of
*
the ligand, owing to the lowering of the energy gap between
p
and
p. Second, after coordination with the B atom, the ligand be-
Table 2
comes more rigid, which can reduce the loss of energy via vibra-
tional motions and increase the emission efficiency. As a result,
the quantum yields of these boron complexes are relatively high
in solution. All complexes 3a–3c emit bright green fluorescence
in the solid state when irradiated with an exciting light. The emis-
sion maxima of complexes 3a–3c in both solution and the solid
state are close to each other, indicating that the substituents F, H
and OMe on the ligands of complexes 3a–3c impose limited effect
on the energy gap of these complexes. The emission maxima of
these complexes in the solid state are close to their corresponding
emission maxima in solution.
Selected bond lengths (Å) and angles (°) for 3a and 3c.
Complex 3a
B–F(1)
B–F(2)
B–N(l)
B–N(2)
C(l)–N(l)
C(7)–N(2)
F(2)–B–F(l)
F(2)–B–N(l)
1.370(4)
1.388(4)
1.531(4)
1.578(4)
1.359(3)
1.304(3)
F(1)–B–N(l)
F(2)–B–N(2)
F(l)–B–N(2)
N(l)–B–N(2)
C(l)–N(l)–B
C(15)–N(l)–B
C(7)–N(2)–B
C(8)–N(2)–B
111.8(3)
107.4(3)
108.6(2)
107.6(3)
123.8(3)
116.9(2)
121.7(2)
119.3(2)
109.4(3)
111.8(2)
Complex 3c
B–N(l)
B–N(2)
B–F(l)
B–F(2)
N(l)–C(l)
N(2)–C(7)
N(l)–B–N(2)
F(l)–B–N(2)
1.506(6)
C(l)–N(l)–B
F(2)–B–N(2)
C(14)–N(l)–B
C(7)–N(2)–B
C(8)–N(2)–B
F(l)–B–F(2)
Fll)–B–N(l)
F(2)–B–N(l)
123.1(3)
109.5(3)
116.9(3)
120.0(3)
120.8(3)
108.2(3)
111.6(3)
111.5(3)
1.568(5)
1.390(5)
1.365(5)
1.358(4)
1.296(4)
3.4. Electroluminescent properties
Complex 3c was chosen for the study of EL properties of this
type of complexes due to its relatively high quantum yield in solu-
tion. Polymer OLEDs with 3c as the dopant emitter and the poly-
mer PVK as the host were fabricated and examined. The polymer
PVK was chosen as the host because PVK is a good hole-transport-
ing material and its emission band overlaps the absorption band of
3c as shown in Fig. 3. It is well known that a dopant-host doped
emitter system can significantly improve device performance in
terms of EL efficiency, emission color and operational lifetime
108.6(3)
107.2(3)
and the aromatic ring at the amido nitrogen are 97.6° for 3a and
94.5° for 3c, while the dihedral angles between the six-membered
chelating ring and the aromatic ring at the imine nitrogen (38.9°
for 3a and 59.9° for 3c) are relatively small. The F–B–F bond angles
(108.2(3)° for 3a, 109.4(3)° for 3c) of both complexes are close to
the ideal tetrahedral angles (109.49°).
}
[17]. To test the efficiency of the Forster energy transfer, PVK films
containing different concentrations of 3c were prepared and their
PL spectra were determined with an excitation radiation of
3.3. Photoluminescent properties
Table 3 summarizes the UV–Vis and fluorescent properties of
the free ligands 2a–2c and their boron complexes 3a–3c measured
in both solution and the solid state. The absorption and emission
spectra of these compounds in n-hexane solution were determined
at room temperature. In solution, the free ligands 2a–2c show a
weak blue fluorescence emission band with the kmax around
420 nm. In contrast to the free ligands, their boron complexes
3a–3c all exhibit a bright green emission in solution with the emis-
sion maxima at 516, 518 and 515 nm, respectively, upon irradia-
tion with an exciting UV light (kext = 365 nm). The emission
energies of these complexes are significantly red shifted in com-
parison with the free ligands. The observed luminescence of these
*
complexes could be attributed to a
p –p transition of their che-
lated anilido-imine conjugated ligands. The role of the boron atom
in the luminescence of complexes 3a–3c should be twofold as
pointed out in literatures for coordination complexes [16]. First,
Fig. 3. The absorption, emission spectra of 3c in both solution and the solid state,
and photoluminescence spectrum of PVK.

1446
X. Liu et al. / Inorganica Chimica Acta 363 (2010) 1441–1447
Table 4
ent doping concentration 3c in PVK have stable green emission
from 3c with a peak at ꢂ518 nm with the applied voltage. The
8 wt% doping concentration is most suitable for obtaining high EL
efficiency. The current density (J) and luminance (L) versus applied
voltage (V) characteristics of the 8 wt% 3c doped device is shown in
Fig. 4. This device has a luminance of 141.6 cd/m2 at voltage of 13 V
and current density of 12.93 mA/cm2, and the luminance reaches
near 457 cd/m2 at voltage of 18 V and current density of 153.07
mA/cm2. The turn-on voltage of this device is 7.5 V (at brightness
of 1 cd/m2). Fig. 5 shows the Luminous Efficiency (LE) and Power
Efficiency (PE) as a function of the current density (J) of the same
device. The LEmax increases from 2.24 cd/A to 2.92 cd/A for devices
with 2–8 wt% doping concentration of 3c in PVK, and begins to de-
crease significantly for devices with the doping concentration of 3c
being larger than 8 wt%. The present study shows that the synthe-
sized new boron complex 3c can be used as emitter material in or-
ganic EL devices.
Summary of the performance of complex 3c based devices. The structure of devices is
ITO/PEDOT:PSS/PVK: 3c (80 nm)/TPBI (40 nm)/LiF (1.2 nm)/Al.
Concentration
(wt%)
LEmax
(cd/A)
PEmax
(lm/W)
Bmax
Turn-on
(V)
(cd/m2)
2
4
6
8
2.24
2.43
2.74
2.92
1.00
1.09
1.23
1.55
1460
754
666
667
7
8
9
8
345 nm. The Förster energy transfer in these films is remarkably
efficient, and the complete energy transfer from host PVK to guest
3c can be observed in the PL spectra of films with doping concen-
trations of 3c higher than 2 wt%. In the fabricated EL devices, ITO
glass and aluminum are used as the anode and cathode, respec-
tively. PEDOT and LiF are the materials for facilitating the hole
and electron injection in the two electrodes, and TPBI is used as
an electron-transporting material. The structure of the devices is
as follows: ITO/PEDOT:PSS/PVK:x wt% 3c/TPBI/LiF/Al. The emitting
layer was spin-cast onto the surface of ITO from the chloroform
solution. The thickness of the doped PVK films was in approxi-
mately 80 nm. The performance parameters of the multilayer de-
vices with doping concentrations of 2, 4, 6 and 8 wt% 3c in the
PVK host are listed in Table 4. The EL spectra of devices with differ-
4. Conclusion
In summary, three new boron complexes with N-arylanilido-
arylimine bidentate Schiff base type ligands were synthesized in
high yields, and their photophysical and EL properties were stud-
ied. All complexes 3a–3c emit bright green fluorescence with an
emission maximum of 515–518 nm in n-hexane solution and in
the solid state when irradiated with an exciting light. The emission
maxima of these complexes in the solid state are close to their cor-
responding emission maxima in solution. With complex 3c as a
dopant and PVK as the host, devices with different doping concen-
trations of 3c were fabricated and EL performances of these devices
were examined. A maximum luminescence efficiency of 2.92 cd/A
was achieved using a multilayer polymer light-emitting device
with 8 wt% doping concentration of 3c in PVK.
Supplementary data
CCDC 650832 and 650833 contain the supplementary crystallo-
graphic data for 3a and 3c. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
Fig. 4. The current density (h) and brightness (d) characteristics of 8 wt% doped
ITO/PEDOT:PSS/PVK: 3c/LiF/Al device as function of applied voltages.
This work was supported by the National Natural Science Foun-
dation of China (Nos. 20674024 and 20772044) and Ministry of
Science and Technology of China (No. 2002CB6134003).
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