Syntheses and structures of new luminescent B(III) complexes: BPh 2(2-(2-pyridyl)naphtho[b]imidazole) and BF 2(2-(2-pyridyl)naphtho[b]imidazole)

Article abstract of DOI:10.1016/j.jorganchem.2005.10.022

A novel chelating ligand, 2-(2-pyridyl)naphtho[b]imidazole (PNI) has been synthesized and structurally characterized. The PNI molecule reacts readily with BPh₃ and BF₃ to form BPh ₂(2-(2-pyridyl)naphtho[b]imidazole) (1) and BF ₂(2-(2-pyridyl)naphtho[b]imidazole) (2), respectively. The structures of PNI and 1 were determined by single crystal X-ray diffraction, while that 2 by spectroscopic methods. The structure of 1 reveals that the boron center is four-coordinated. The electroluminescent device using 1 as the emitting layer shows bright yellow luminescence with a maximum emission peak at 540 nm. Complexes 1 and 2 form a new family of organometallic emitting materials which could be of interest for practical applications.

Full text of DOI:10.1016/j.jorganchem.2005.10.022

Journal of Organometallic Chemistry 691 (2006) 799–804
www.elsevier.com/locate/jorganchem
Syntheses and structures of new luminescent B(III) complexes:
BPh2(2-(2-pyridyl)naphtho[b]imidazole) and
BF2(2-(2-pyridyl)naphtho[b]imidazole)
a,
b
b
,b
,c
*
*
Tsun-Ren Chen *, Rong-Hong Chien , Ming-Shiun Jan , Anchi Yeh , Jhy-Der Chen
a
Department of Electrical Engineering, Yung-Ta Institute of Technology, Ping-Tung, Taiwan, ROC
b
Department of Chemical Engineering, Chengshiu University, Kaohsiung, Taiwan, ROC
Department of Chemistry, Chung-Yuan Christian University, Chung-Li, Taiwan, ROC
c
Received 21 September 2005; received in revised form 16 October 2005; accepted 16 October 2005
Available online 28 November 2005
Abstract
A novel chelating ligand, 2-(2-pyridyl)naphtho[b]imidazole (PNI) has been synthesized and structurally characterized. The PNI mol-
ecule reacts readily with BPh3 and BF3 to form BPh2(2-(2-pyridyl)naphtho[b]imidazole) (1) and BF2(2-(2-pyridyl)naphtho[b]imidazole)
(2), respectively. The structures of PNI and 1 were determined by single crystal X-ray diffraction, while that 2 by spectroscopic methods.
The structure of 1 reveals that the boron center is four-coordinated. The electroluminescent device using 1 as the emitting layer shows
bright yellow luminescence with a maximum emission peak at 540 nm. Complexes 1 and 2 form a new family of organometallic emitting
materials which could be of interest for practical applications.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Electroluminescence; Imidazole; Boron; Intramolecular charge transfer (ICT)
1. Introduction
of imidazoles are good chelating ligands and can force the
resultant metal complexes into special geometries [11], and
the attachment of the pyridyl group at 2-position of imi-
dazoles would allow the ligands to form the stable com-
plexes with metals. Therefore, the imidazole ligands
containing pyridyl group (PNI) were prepared in this work,
and the complexes, BPh2(2-(2-pyridyl)naphtho[b]imidaz-
ole) (1) and BF2(2-(2-pyridyl)naphtho[b]imidazole) (2),
have been synthesized for the first time. The details of syn-
theses, structures, and luminescent properties of those
boron complexes are reported.
The chemistry of organoboron complexes have attracted
much more attention recently because they are of interest
for practical applications [1–3]. Since an organic light emit-
ting diode (OLED) was reported by Tang and Vanslyke [4],
LEDs based on organic materials have generated consider-
able interest and enabled the development of low-cost, full-
color, flat-panel displays [5–8]. The best-known EL metal
complex used in OLED is Alq3 which is not only a good
emitter but also a highly efficient electron-transporting
material, where q is the 8-hydroxyquinolinato ligand
[9,10]. Via the modification of the ligand of metal complex,
the emission spectra of devices and other properties, such
as thermostability and carrier mobility, can be tuned. Most
2. Results and discussion
2.1. Syntheses and characterization
2.1.1. The PNI ligand
The synthesis of PNI was carried out using a modified lit-
erature method reported previously for related compounds
*
Corresponding author. Tel./fax: +886 8 7333632.
E-mail addresses: trchen@mail.ytit.edu.tw, lspccj@ms14.hinet.net
(T.-R. Chen).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.10.022

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T.-R. Chen et al. / Journal of Organometallic Chemistry 691 (2006) 799–804
H2N
N
PPA
+
HN
COOH
H2N
N
N
BX3
N
1: X = - C6H5
2: X = - F
N
B
X
X
Scheme 1.
[12,13] in high yield (Scheme 1). The structure of PNI was
determined by single-crystal X-ray diffraction analysis.
Fig. 1 shows an ORTEP diagram of PNI. The dihedral
angle between imidazol moiety and quinoline ring, 8ꢁ,
reveals that this molecule approximates to a plane structure
stabilized by an intramolecular hydrogen bonding, N(1)–
both of 1 and 2 are quite stable in the atmosphere of nitro-
gen. The DSC showed the melting point of complexes 1
and 2 were 276 and 250 ꢁC, respectively, which may serve
as an advantage for OLED device fabrication because the
materials having high melting point could provide the
device with greater longevity [14,15].
˚
Hꢀ ꢀ ꢀN(3) (2.578 A). The C(5)–C(6) bond length of
˚
1.464(10) A is shorter than a single bond length, indicative
2.1.3. Structure of 1
of the conjugation of the imidazol moiety and quinoline
ring, which also show that the plane structure is stabilized
by the delocalization of p-electrons of the heterocyclic
system.
Single crystals of 1 were grown by slow diffusion of hex-
ane into a concentrated solution of complex 1 in dichloro-
methane. The molecular structure of 1 is shown in Fig. 2
and the selected bond lengths and angles are listed in Table
1. In this structure, the boron atom in 1 has a typical tet-
rahedral geometry. The PNI ligand is bidentately chelated
to the boron center through its pyridyl and imidazole
nitrogen atoms and two phenyl groups occupy the other
boron sites. The bond length of B–N(2) imidazole bond
2.1.2. The complexes of 1 and 2
Complexes 1 and 2 were prepared by reacting PNI with
appropriate boron compounds in dry THF (Scheme 1).
Both the complexes are air-stable in the solid state and in
solution. The thermogravimetric analysis (TGA) scans
under nitrogen for 1 and 2 powders showed weight loss
of 10% at 337 and 310 ꢁC, respectively, which reveal that
˚
(1.558(4) A) is shorter than that of the B–N(1) pyridyl
˚
bond (1.624(4) A), indicating that the imidazole nitrogen
is anionic and hence a better donor than the neutral
Fig. 1. An ORTEP drawing of the molecular structure of PNI with 20%
thermal and labeling schemes.
Fig. 2. An ORTEP drawing of the molecular structure of complex 1 with
20% thermal and labeling schemes.

T.-R. Chen et al. / Journal of Organometallic Chemistry 691 (2006) 799–804
801
Table 1
2.1.4. Electronic absorption and photoluminescence spectra
˚
Selected bond distance (A) and angles (ꢁ)
At room temperature and low concentration
(1 · 10ꢁ5 M), the absorption spectrum of PNI in dichloro-
methane consist of two discrete bands (Fig. 4). A strong
absorption which can be assigned to the p–p* transition
centered at 282 nm. The other intense band centered at
344 nm shows a vibrational separation of 1300 cmꢁ1 with
the m0,0 transition at 3.03 · 104 cmꢁ1. This lower energy
PNI
Bond lengths
N(1)–C(5)
N(1)–C(1)
N(2)–C(6)
N(2)–C(7)
N(3)–C(6)
N(3)–C(16)
C(4)–C(5)
C(5)–C(6)
C(7)–C(8)
Bond angles
1.325(8)
1.353(8)
1.349(8)
1.384(8)
1.350(8)
1.407(8)
1.410(9)
1.464(10)
1.406(8)
C(5)–N(1)–C(1)
C(6)–N(2)–C(7)
C(6)–N(3)–C(16)
C(2)–C(1)–N(1)
N(1)–C(5)–C(4)
N(1)–C(5)–C(6)
C(4)–C(5)–C(6)
N(2)–C(6)–C(5)
N(3)–C(6)–C(5)
116.2(6)
103.3(5)
107.7(5)
124.1(6)
123.7(6)
115.4(7)
120.9(7)
123.2(7)
123.0(7)
band possesses
a
reasonably high absorptivity
(e ꢂ 104 dm3 molꢁ1 cmꢁ1) and a bathochromic shift with
increasing polarity of solvent, which are consistent with
the intramolecular charge transfer (ICT) character for the
concerned p–p* transition involving the filled p orbital
(HOMO) located on the imidazol ring and the unoccupied
p* orbital (LUMO) located on the quinoly ring [17–23].
For the absorption spectrum of 1 (Fig. 4), based on the
comparisons with the spectrum of PNI, we attribute the
bands at 372, and 278 nm to metal-perturbed ligand-local-
ized (LC) transition. Interestingly, the BPh2 group lowers
the p ! p* energy gap of PNI, which could be attributed
to the fact that the p system of molecule is more extensive
when the BPh2 group bonds to PNI, resulting in the
decrease of the HOMO–LUMO energy gap. For the
absorption spectrum of 2, the bands at 267, and 345 nm
are also attributed to LC transition, which is similar to that
of PNI, indicating that the BF2 group can hardly affect the
p system of PNI.
1
Bond angles
N(1)–C(1)
N(1)–C(5)
N(1)–B
N(2)–C(6)
N(2)–C(7)
N(2)–B
N(3)–C(6)
N(3)–C(8)
C(17)–B
C(23)–B
Bond angles
1.339(4)
1.359(4)
1.624(4)
1.361(4)
1.385(4)
1.558(4)
1.316(4)
1.398(4)
1.607(5)
1.611(5)
C(1)–N(1)–C(5)
C(5)–N(1)–B
C(6)–N(2)–B
C(6)–N(3)–C(8)
N(2)–B–C(23)
N(2)–B–N(1)
C(23)–B–N(1)
C(1)–N(1)–B
C(6)–N(2)–C(7)
C(7)–N(2)–B
N(2)–B–C(17)
C(17)–B–C(23)
C(17)–B–N(1)
119.5(3)
113.1(2)
113.8(2)
101.9(2)
111.1(3)
94.8(2)
111.6(3)
127.5(3)
104.3(2)
141.7(3)
114.2(3)
114.5(3)
109.0(2)
pyridyl nitrogen atom [2]. The two five-membered rings are
nearly coplanar with a dihedral angle of 2.6ꢁ between these
two rings. Fig. 3 shows a packing diagram for 1. The inter-
planar distances between imidazol moiety and pyridyl ring
Fig. 5 shows the photoluminescence (PL) spectra of 1 in
various concentrations, excited with 355 nm laser line. At
low concentration, 1 · 10ꢁ8 M in DMF, only one emission
band was observed with a maximum at 413 nm, which cor-
responds to the relaxation of 1 from the excited state of a
single molecule into ground state. Interestingly, when the
concentration was increased to 1 · 10ꢁ7 M, a new emission
band having a maximum at 550 nm was observed, which
can be attributed to the formation of excimers by exciting
monomers that happened to be in ground-state [24–27].
When the solution became more concentrated, reaching
1 · 10ꢁ4 M, the monomer emission disappeared and only
the excimer emission existed.
˚
are in the range of 3.4–3.8 A, which implies that the inter-
layer p–p interaction may play an important role in pro-
ducing bathochromic shift of absorption maximum in
solid state and red shift of PL spectra in higher concentra-
tion in solution [16].
UV
1.0
PNI in CH2Cl2
1 in in CH2Cl2
2 in in CH2Cl2
PNI in DMF
0.8
1 in DMF
2 in DMF
0.6
0.4
0.2
0.0
300
400
Wavelength (nm)
Fig. 4. UV–vis spectra of PNI, 1 and 2 in N,N0-dimethylformamide and
Fig. 3. An illustration of the stacking present in crystal 1.
dichloromethane.

802
T.-R. Chen et al. / Journal of Organometallic Chemistry 691 (2006) 799–804
ing the vacuum under 3 · 10ꢁ6 Torr. Before the deposition,
all of the organic materials were purified by a train subli-
mation method. The thickness of the organic layers and
cathode were measured by quartz-crystal monitor.
PL spectra
of 1 in DMF
thin film
1.0
0.8
0.6
0.4
0.2
0.0
1X10-4
1X10-5
1X10-6
1X10-7
1X10-8
M
M
M
M
M
This device possesses a yellow emission (CIE coordi-
nates x = 0.45 and y = 0.53) and the EL peak at 543 nm,
Fig. 7. Though the pattern of EL spectrum of this device
is similar to that of PL spectrum of 1 in higher concentra-
tion, the EL peak is shifted about 20 nm, which could be
attributed to the interaction occurring at the interface
between emitting layer and ETM. The voltage–current den-
sity and luminance–voltage characteristics of this device,
Fig. 8, shows that the turn-on voltage is 3.5 V with a max-
imum luminance of 332 cd/m2 at 10 V and a maximum
luminance efficiency of 0.02 lm/W at 7 V.
300
400
500
600
700
800
Wavelength (nm)
Fig. 5. Photoluminescence spectra of complex 1 in various concentrations,
excited with 355 nm laser line.
In conclusion, we have demonstrated that the novel PNI
ligand is capable of chelating to B(III) center and the
resulting complexes possess appreciable photoluminescent
efficiency and have a very high thermal stability, which
are important characters for practical application in the
electronic fields, especially for OLED.
The photoluminescence (PL) spectra of the solutions of
2, excited with 355 nm laser line, are illustrated in Fig. 6.
Both at low concentration and at high concentration, only
one emission band was observed with a maximum at
435 nm, which corresponds to the relaxation of 2 from
the excited state of a single molecule into ground state.
The excimer emission was not observed for 2.
The complexes 1 and 2 possess the appreciable PL quan-
tum yields, Uf = 0.8 and 0.2, respectively, in DMF relative
to 3-(2-benzothiazolyl)-7-diethyl-aminocoumarin (C540).
1.0
EL of 1
0.8
2.1.5. Electroluminescence study
0.6
0.4
0.2
In order to evaluate the electroluminescence characteris-
tic of PNI–boron complex, a three-layer OLED having the
configuration of ITO/NPB (50 nm)/1 (50 nm)/Alq3
(50 nm)/MgAg (100 nm) was fabricated, in which N,N0-
bis-(1-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine
(NPB) was used as a hole-transport material (HTM), the
complex 1 was used as an emitter, and tris(8-quinnolino-
lato) aluminum (Alq3) was employed as an electron-trans-
porting material (ETM). The organic layers and the
cathode were sequentially deposited by conventional vac-
uum-vapor deposition in the same chamber without break-
300
400
500
Wavelength (nm)
Fig. 7. Electroluminescence spectrum of the EL device.
600
700
800
400
PL spectra
of 2 in DMF
1.0
0.8
350
300
250
200
150
100
50
1X10-3M
1X10-4M
1X10-6M
0.10
0.05
0.00
IV
LV
0.6
0.4
0.2
0.0
0
300
400
500
600
700
800
2
4
6
8
10
Wavelength (nm)
Voltage (V)
Fig. 6. Photoluminescence spectra of complex 2 in various concentrations,
excited with 355 nm laser line.
Fig. 8. Current–voltage and luminance–voltage characteristics of the EL
device.

T.-R. Chen et al. / Journal of Organometallic Chemistry 691 (2006) 799–804
803
3. Experimental
7.33 (m, 6H, ph and imidazole), 7.27 (m, 7H, ph and imid-
azole). MS data: 409.29 (26.643%), 332.2 (100%), 295.2
(11.59%), 245.2 (19.26%), 239.2 (19.37%), 166.1 (9.77%),
91.1 (6.69%). Anal. Calc. for C28H20N3B (MW = 409.2):
C, 82.11; H, 4.88; N, 10.26. Found: C, 82.09; H, 4.93; N,
10.24%.
3.1. General procedure
All starting materials were purchased from Aldrich
Chemical Co. 1H and 13C NMR spectra were recorded
on the Bruker 300 MHz NMR spectrometers in CDCl3.
Thermogravimetric analyses (TGA) were performed on a
Perkin–Elmer thermogravimeter (Pyris 1) under a dry
nitrogen gas flow at the heating rate of 20 ꢁC/min. Glass
transition temperature (Tg) and melting point (Tm) of
materials were determined by differential scanning calorim-
eter (DSC) of the Perkin–Elmer differential scanning calo-
rimeter (DSC-7). The absorption spectra were recorded
with the HP-8453A UV–vis photodiode array spectropho-
tometer. The EL spectrum and the Commission Internatio-
nale de IÕEclairage (CIE) co-ordinates were measured by
Pro-650 Spectroscanner, the current–voltage (I–V) charac-
teristics were measured by Keithley 2400 Source meter.
Photoluminescence (PL) was measured by Model LS 55
luminescence Spectrometer.
3.5. BF2(2-(2-pyridyl)naphtho[b]imidazole) (2)
1
Pale yellow solid. 30% yield. M.p. > 250 ꢁC. H NMR
(500 MHz, DMF-d7, 293 K),
d ppm: 8.69 (d, 1H,
J = 6.3 Hz, pyridyl), 8.56 (d, 1H, J = 7.8 Hz, pyridyl),
8.11 (s, 2H, imidazole), 7.95 (m, 3H, pyridyl and imidaz-
ole), 7.43 (m, 3H, imidazole and imidazole). Anal. Calc.
for C16H10N3BF2 (MW = 293.09): C, 65.57; H, 3.44; N,
14.34. Found: C, 65.53; H, 3.50; N, 14.30%.
3.6. Determination of the crystal structure
The diffraction data of PNI and complex 1 were col-
lected on a Bruker AXS P4 diffractometer, which is
equipped with graphite-monochromated Mo Ka radiation
˚
3.2. Synthesis of 2-(2-pyridyl)naphtho[b]imidazole (PNI)
(k = 0.71073 A). Structure refinements were carried out
using the SHELXTL software package [28]. The structure fac-
tors were obtained after Lorentz and polarization correc-
tions. The positions of some of the heavier atoms were
located by the direct method. The remaining atoms were
found in a series of alternating difference Fourier maps
Pyridine-2-carboxylic acid (20.0 mmol, 2.46 g), 2,3-dia-
minonaphthalene (20.0 mmol, 3.16 g), and polyphosphoric
acid (PPA, 20 ml) were added to a flask. The mixture was
heated at 200 ꢁC for 4 h. After cooling to room T, the res-
idue was slowly added to deionized water (500 ml) with
stirring. The solid was collected by suction filtration and
purified by column chromatography. Colorless crystals of
Table 2
Crystal data and structure refinement parameters
Compound
PNI
1
1
PNI were obtained in 80% yield. M.p. >250 ꢁC. H NMR
Empirical formula
Formula weight
Crystal system
Space group
C
245.28
Orthorhombic
P2(1)2(1)2(1)
16H11N3
C28H20B N3
409.2
Triclinic
(500 MHz, DMF-d7, 293 K), d ppm: 13.3 (s, 1H, NH),
8.83 (d, 1H, J = 4 Hz, pyridyl), 8.58 (d, 1H, J = 7.8 Hz,
pyridyl), 8.40 (s, 1H, imidazole), 8.21 (s, 1H, imidazole),
8.13 (dd, 2H, J = 7.7, 1.7 Hz, imidazole), 8.07 (t, 1H,
J = 10, 1.2 Hz, pyridyl), 7.62 (m, 1H, imidazole), 7.45 (m,
2H, pyridyl). Anal. Calc. for C16H11N3 (MW = 245.28):
C, 78.36; H, 4.48; N, 17.14. Found: C, 78.31; H, 4.50; N,
17.11%.
ꢀ
P1
Unit cell dimensions
˚
a (A)
10.866(2)
14.3173(17)
23.011(2)
90
90
90
3579.7(9)
12
1.365
0.084
1536
9.2216(8)
9.6733(16)
15.2655(17)
97.220(10)
90.921(8)
115.787(8)
1212.6(3)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
3.3. Syntheses of 1 and 2
Z
Calculated density (Mg mꢁ3
Absorption coefficient (mmꢁ1
F(000)
)
1.237
0.179
470
)
Complexes 1 and 2 were prepared in dry THF by react-
ing, in a 1:1.05 molar ratio, PNI with triphenylboron or
boron trifluoride. The solutions were refluxed for 12 h
under nitrogen. Selected analytical data concerning 1 and
2 are:
Crystal size (mm)
h Range for data collection (ꢁ)
Reflections collected
0.2 · 0.5 · 0.3
1.77–24.99
4394
0.2 · 0.6 · 0.4
2.36–25.00
5039
Data/restraints/parameters
Goodness-of-fit on Fꢁ2
4177/0/515
1.054
4200/0/317
1.134
Final R indices [I > 2sigma(I)]
R1 = 0.0647
wR2 = 0.1471
R1 = 0.1263
wR2 = 0.1838
0.320 and ꢁ0.353
R1 = 0.0783
wR2 = 0.2017
R1 = 0.1125
wR2 = 0.2266
0.725 and ꢁ0.362
3.4. BPh2(2-(2-pyridyl)naphtho[b]imidazole) (1)
R indices (all data)
1
Pale yellow solid. 35% yield. M.p. > 250 ꢁC. H NMR
(500 MHz, DMF-d7, 293 K),
d ppm: 8.67 (d, 1H,
Largest difference peak
ꢁ3
˚
and hole (e A
)
J = 13 Hz, pyridyl), 8.63 (d, 1H, J = 13.5 Hz, pyridyl),
8.40 (s, 1H, imidazole), 8.29 (t, 1H, J = 15, 2 Hz, pyridyl),
8.0 (dd, 1H, J = 13, 1.2 Hz, pyridyl), 7.84 (s, 1H, imidaz-
ole), 7.81 (d, 1H, 7.5 Hz, pyridyl), 7.65 (m, 1H, imidazole),
P
P
P
P
R1 = iFo| ꢁ |Fci/ Fo|; wR2 ¼ ½ wðF 2o ꢁ F c2Þ = wðF 2oÞ2ꢃ1=2; quality-
2
P
2
1=2
of-fit ¼ ½ wðjF 2oj ꢁ jF 2c jÞ =Nobserved ꢁ Nparameters
ꢃ
;
w ¼ 1=½r2ðF 2oÞþ
2
ðapÞ þ ðbpÞꢃ; p ¼ ½maxðF 2o or 0Þ þ 2ðF 2cÞꢃ=3; a = 0.0185, b = 25.8531.

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T.-R. Chen et al. / Journal of Organometallic Chemistry 691 (2006) 799–804
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4. Supplementary material
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been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication numbers CCDC
283102. Copies of the data can be obtained, free of charge,
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Acknowledgement
We thank the National Science Council of ROC for
financial support.
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