Organoboron compounds with an 8-hydroxyquinolato chelate and its derivatives: Substituent on structures and luminescence

Article abstract of DOI:10.1021/ic0489746

Four new luminescent organoboron complexes have been synthesized and fully characterized. These compounds are four-coordinate boron chelated by either 8-hydroxyquinolato (q) or functionalized 8-hydroxylquinolato ligands, including BPh₂(5-(1-naphthyl)-q) (1), BPh₂(5-(2-benzothienyl)-q) (2), B(2-benzothienyl)₂q (3), and B(2-benzothienyl) ₂-(2-Me-q) (4). All four compounds have a tetrahedral geometry as established by X-ray diffraction analyses. In solution, compounds 1-4 have an emission maximum at 534, 565, 501, and 496 nm, respectively, at room temperature. They emit similar colors in the solid states without red shifts of the emission band due to the lack of significant intermolecular interactions in the crystal lattices. The substituent group at C5 or C2 position of the 8-hydroxyquinolato ligand has been observed to have a significant impact on the emission energy and the emission quantum efficiency of the boron complexes. Molecular orbital calculations (Gaussian 98) showed that the electronic transition of 1 and 2 is a π-πz.ast; transition centered on the functionalized 8-hydroxyquinolato group and the electronic transition of 3 and 4 is an interligand charge transfer from the 2-benzothienyl ligand to the hydroxyquinolato ring. A double-layer electroluminescent device using 3 as the emitter has been fabricated, which produced a broad emission band with a significant contribution of exciplex emission.

Full text of DOI:10.1021/ic0489746

Inorg. Chem. 2005, 44, 601−609
Organoboron Compounds with an 8-Hydroxyquinolato Chelate and Its
Derivatives: Substituent Effects on Structures and Luminescence
Yi Cui,† Qin-De Liu,† Dong-Ren Bai,† Wen-Li Jia,† Ye Tao,‡ and Suning Wang*,†
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, K7L 3N6 Canada, and Institute
for Microstructural Science, National Research Council of Canada, Ottawa, K1A 0R6 Canada
Received July 28, 2004
Four new luminescent organoboron complexes have been synthesized and fully characterized. These compounds
are four-coordinate boron chelated by either 8-hydroxyquinolato (q) or functionalized 8-hydroxylquinolato ligands,
including BPh2(5-(1-naphthyl)-q) (1), BPh2(5-(2-benzothienyl)-q) (2), B(2-benzothienyl)2q (3), and B(2-benzothienyl)2-
(2-Me-q) (4). All four compounds have a tetrahedral geometry as established by X-ray diffraction analyses. In
solution, compounds 1−4 have an emission maximum at 534, 565, 501, and 496 nm, respectively, at room
temperature. They emit similar colors in the solid states without red shifts of the emission band due to the lack of
significant intermolecular interactions in the crystal lattices. The substituent group at C5 or C2 position of the
8-hydroxyquinolato ligand has been observed to have a significant impact on the emission energy and the emission
quantum efficiency of the boron complexes. Molecular orbital calculations (Gaussian 98) showed that the electronic
transition of 1 and 2 is a π−π* transition centered on the functionalized 8-hydroxyquinolato group and the electronic
transition of 3 and 4 is an interligand charge transfer from the 2-benzothienyl ligand to the hydroxyquinolato ring.
A double-layer electroluminescent device using 3 as the emitter has been fabricated, which produced a broad
emission band with a significant contribution of exciplex emission.
Introduction
boron complexes protected by bulky substituents have also
been reported recently to be useful emitters for OLEDs.1a-d,4
Alq3 (q ) 8-hydroxyquinolato) and its derivatives have been
investigated extensively for their uses in OLEDs.5a-h Gal-
Luminescent organoboron compounds have recently re-
ceived considerable attention due to their potential applica-
tions in organic light emitting devices (OLEDs).1,2 In the
study on blue luminescent materials based on 7-azaindolyl
ligands, we revealed that boron compounds in general are
more stable than the corresponding aluminum compounds
due to the increased covalency of boron-ligand bonds,
compared to aluminum-ligand bonds.3 Three-coordinate
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G.; Breeze, S. R.; Wang, S. Angew. Chem., Int. Ed. 1999, 38, 985.
(b) Hassan, A.; Wang, S. J. Chem. Soc., Chem. Commun. 1998, 211.
(c) Liu, W.; Hassan, A.; Wang, S. Organometallics 1997, 16, 4257.
(d) Ashenhurst, J.; Brancaleon, L.; Hassan, A.; Liu, W.; Schmider,
H.; Wang, S.; Wu, Q. Organometallics 1998, 17, 3186.
(4) Jia, W. L.; Bai, D. R.; McCormick, T.; Liu, Q. D.; Motala, M.; Wang,
R. Y.; Seward, C.; Tao, Y.; Wang, S. Chem. Eur. J. 2004, 10, 994.
(5) (a) Yu, J. S.; Chen, Z. J.; Sakuratani, Y.; Suzuki, H.; Tokita, M.;
Miyata, S. Jpn. J. Appl. Phys. 1999, 38, 6762. (b) Sapochak, L. S.;
Padmaperuma, A.; Washton, N.; Endrino, F.; Schmett, G. T.; Marshall,
J.; Fogarty, D.; Burrows, P. E.; Forrest, S. R. J. Am. Chem. Soc. 2001,
123, 6300. (c) Shoji, E.; Miyatake, K.; Hill, A. R.; Hay, A. S.;
Maindron, T.; Jousseaume, V.; Dodelet, J. P.; Tao, Y.; D’Iorio, M. J.
Polym. Sci., Part A: Polym. Chem. 2003, 41, 3006. (d) Chen, C. H.;
Shi, J. M. Coord. Chem. ReV. 1998, 171, 161 and references therein.
(e) Co¨lle, M.; Gmeiner, J.; Milius, W.; Hillebrecht, H.; Bru¨tting, W.
AdV. Funct. Mater. 2003, 13, 108. (f) Pohl, R.; Anzenbacher, P., Jr.
Org. Lett. 2003, 5, 2769. (g) Pohl, R.; Montes, V. A.; Shinar, J.;
Anzenbacher, P., Jr. J. Org. Chem. 2004, 69, 1723. (h) Halls, M. D.;
Schlegel, H. B. Chem. Mater. 2001, 13, 2632. (i) Chen, B. J.; Sun, X.
W.; Li, Y. K. Appl. Phys. Lett. 2003, 82, 3017. (j) Wang, L.; Jiang,
X.; Zhang, Z.; Xu, S. Displays 2000, 21, 47. (k) Sapochak, L. S.;
Burrows, P. E.; Garbuzov, D.; Ho, D. M.; Forrest, S. R.; Thompson,
M. E. J. Phys. Chem. 1996, 100, 17766.
* To whom correspondence should be addressed. E-mail:
wangs@chem.queensu.ca.
† Queen’s University.
‡ National Research Council of Canada.
(1) (a) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714. (b) Noda,
T.; Ogawa, H.; Shirota, Y. AdV. Mater. 1999, 11, 283. (c) Shirota,
Y.; Kinoshita, M.; Noda, T.; Okumoto, K.; Ohara, T. J. Am. Chem.
Soc. 2000, 122, 11021. (d) Noda, T.; Shirota, Y. J. Lumin. 2000, 87,
1168. (e) Kinoshita, M.; Kita, H.; Shirota, Y. AdV. Funct. Mater. 2002,
12, 780. (f) Doi, H.; Kinoshita, M.; Okumoto, K.; Shirota, Y. Chem.
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(2) (a) Liu, S.-F.; Wu, Q.; Schmider, H. L.; Aziz, H.; Hu, N.-X.; Popovic,
Z.; Wang, S. J. Am. Chem. Soc. 2000, 122, 3671. (b) Liu, Q. D.;
Mudadu, M. S.; Schmider, H.; Thummel, R.; Tao, Y.; Wang, S.
Organometallics 2002, 21, 4743.
10.1021/ic0489746 CCC: $30.25
Published on Web 01/05/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 3, 2005 601

Cui et al.
Synthesis of BPh2(5-(1-naphthyl)-q) (1). The synthesis of 1
involves three steps as described below.
lium(III) analogues of 8-hydroxyquinoline derivatives have
also been investigated.5i-k In contrast, the investigation on
boron compounds that contain the ligand 8-hydroxyquinoline
or its derivatives has been rather limited. A few years ago
we reported our investigation on BR2q (R ) ethyl, phenyl,
and 2-naphthyl) and their electroluminescent properties,6
which demonstrated that boron compounds are promising
emitters for OLEDs. In order to understand the electronic
effects of substituents and the ligands on the luminescent
properties of the BR2q or BR2q′ family, we have extended
our investigation to boron compounds with the general
formula of BAr2q or BAr2q′ where Ar ) phenyl or 2-ben-
zothienyl, q′ ) 2-methyl-q, 5-(1-naphthyl)-q, or 5-(2-
benzothienyl)-q. The synthetic aspects and luminescent
properties of the new boron compounds are reported herein.
A preliminary study on the electroluminescent properties of
B(2-benzothienyl)2q is also described herein.
5-(1-Naphthyl)-8-methoxyquinoline (L1a). An oven-dried re-
sealable Schlenk flask with magnetic stirring bar was cooled to
room temperature under a nitrogen purge. A condenser was
connected for reflux. The flask was charged with 5-bromo-8-
methoxyquinoline (0.476 g, 2 mmol), (1-naphthyl)boronic acid
(0.413 g, 2.4 mmol), NaHCO3 (0.400 g, 4.8 mmol), and Pd(PPh3)4
(69 mg, 0.06 mmol, 3% mmol), and then it was evacuated and
backfilled three times with nitrogen again. A separate flask
containing a mixture of toluene (45 mL), water (10 mL), and ethanol
(10 mL) was degassed for 1 h before being transferred to the above
flask via cannula. The resulting mixture was refluxed with vigorous
stirring for 18 h. The reaction mixture was cooled to room
temperature and separated. The aqueous phase was extracted with
dichloromethane (3 × 40 mL) combined with the organic phase,
dried over MgSO4, and decanted for evaporation of the solvents
under reduced pressure. The residue was purified by a column
chromatography using ethyl acetate/hexane (4/1) as the eluent to
1
Experimental Section
afford L1a in 92% yield. H NMR (CDCl3, δ, ppm): 8.97 (1H,
dd, J ) 1.5 Hz, 4.0 Hz), 7.98 (1H, d, J ) 8.2 Hz), 7.96 (1H, d, J
) 8.2 Hz), 7.72 (1H, dd, J ) 1.5 Hz, 8.5 Hz), 7.60 (1H, dd, J )
7.0, 8.2 Hz), 7.48-7.54 (3H, m), 7.38 (1H, d, J ) 8.3 Hz), 7.33
(1H, dd, J ) 1.1 Hz, 6.5 Hz), 7.29 (1H, dd, J ) 4.3 Hz, 8.6 Hz),
7.20 (1H, d, J ) 7.9 Hz), 4.21 (3H, s)
Solvents for reactions were freshly distilled over appropriate
drying agents prior to use. Reactions that required oxygen-free
conditions were carried out under inert atmosphere of nitrogen in
1
oven-dried glassware using standard Schlenk techniques. H and
13C NMR spectra were recorded on Bruker Avance 300 (300 MHz
1
for H, 75.3 MHz for 13C) or Bruker Avance 400 (400 MHz for
5-(1-Naphthyl)-8-hydroxyquinoline (L1b). A solution of BBr3
(1.0 M in heptane, 3.7 mL, 3.7 mmol) was added to a solution of
5-(naphth-1-yl)-8-methoxyquinoline (L1a) (0.500 g, 1.75 mmol)
in 25 mL of dichloromethane at 0 °C under nitrogen. The reaction
mixture was warmed to room temperature and underwent further
reflux with stirring for 12 h. H2O (20 mL) was then added, and the
resulting mixture was stirred for 30 min. The organic phase was
separated. The aqueous layer was neutralized with saturated
NaHCO3 to pH 7-8 and was extracted with CH2Cl2 (3 × 40 mL).
The combined organic phases were washed with brine. The
combined dichloromethane solution was dried over MgSO4 and
decanted. After the solvent was removed under reduced pressure,
the residue was purified by column chromatography using THF/
hexane (1/1) as the eluent, giving a green solid of 5-(1-naphthyl)-
8-hydroxyquinoline, L1b, in 45% yield. 1H NMR (CDCl3, δ,
ppm): 8.82 (1H, d, J ) 3.3 Hz), 7.98 (1H, d, J ) 8.1 Hz), 7.96
(1H, d, J ) 8.0 Hz), 7.79 (1H, d, J ) 8.5 Hz), 7.60 (1H, t, J ) 8.1
Hz), 7.54-7.47 (3H, m), 7.41 (1H, d, J ) 8.4 Hz), 7.37 (1H, d, J
) 7.8 Hz), 7.33 (1H, t, J ) 7.9 Hz), 7.31 (1H, d, J ) 8.5 Hz), 6.99
(1H, s).
1H, 100.6 MHz for 13C) spectrometers at room temperature. 11B
NMR spectra were also recorded on a Bruker Advance 400 (128.4
MHz for 11B) spectrometer using BF3 (Et2O) as the external
reference. Elemental analyses were performed by Canadian Mi-
croanalytical Service, Delta, British Columbia. UV-Vis spectra
were obtained on a Hewlett-Packard 8562A diode array spectro-
photometer. Excitation and emission spectra were recorded on a
Photon Technologies International QuantaMaster model C-60
spectrometer at room temperature. Redox potentials were measured
in CH2Cl2 solution using 0.1 M NBu4PF6 as the electrolyte on a
CV-50W BAS voltammetric analyzer by using a Ag/AgCl electrode
as the reference electrode and a platinum electrode as the working
electrode at room temperature. TLC was carried out on SiO2 (silica
gel F254, Whatman). Flash chromatography was carried out on
silica (silica gel 60, 70-230 mesh). Melting points were determined
with a Fisher-Johns melting point apparatus without correction.
8-Hydroxyquinoline, 8-hydroxyquinaldine, BPh3, BBr3, 1-bro-
monaphthalene, and 1-benzothiophene were purchased from Aldrich
Chemical Co. 5-Bromo-8-methoxyquinoline was synthesized ac-
cording to literature methods,7 and (1-naphthyl)boronic acid and
(2-benzothienyl)boronic acids were prepared according to the
general procedures for the preparation of aryl boronic acids.8 BPh2q
and BPh2(2-Me-q) were prepared using previously reported
procedures.6a
Synthesis of Compound 1. Triphenylborane (80 mg, 0.33 mmol)
dissolved in THF (10 mL) was added to a solution of 5-(naphth-
1-yl)-8-hydroxyquinoline (L1b) (90 mg, 0.33 mmol) stirring in THF
(15 mL). The reaction mixture became yellow with green lumi-
nescence after being stirred for a few minutes. After stirring for 12
h under reflux with TLC monitoring the completion of the reaction,
the mixed solution was concentrated by vacuum. The residue was
dissolved in CH2Cl2 and transferred to a small vial, on which hexane
was slowly laid. The solution was allowed to stand at room
temperature for 2 days for slow evaporation and diffusion, which
gave greenish crystals of 1 in 72% yield with mp 255-256 °C. 1H
NMR (CDCl3, δ, ppm): 8.64 (1H, d, J ) 5.05 Hz,), 8.02-7.98
(3H, m), 7.74 (1H, d, J ) 7.83 Hz,), 7.64-7.62 (3H, m), 7.56-
7.49 (6H, m), 7.42-7.29 (8H, m). 13C NMR (CDCl3, δ, ppm):
158.44, 139.37, 138.19, 137.56, 135.44, 134.30, 133.82, 132.74,
132.04, 132.03, 128.51, 128.48, 128.04, 127.66, 127.65, 127.06,
126.44, 126.13, 125.90, 125.53, 124.19, 122.71, 109.48. 11B NMR
(6) (a) Wu, Q.; Esteghamatian, M.; Hu, N. H.; Popovic, Z.; Enright, G.;
Tao, Y.; D’Iorio, M.; Wang, S. Chem. Mater. 2000, 12, 79. (b) Liu,
S.-F.; Seward, C.; Aziz, H.; Hu, N.-X.; Popovic´, Z.; Wang, S.
Organometallics, 2000, 19, 5709.
(7) (a) Trecourt, F.; Mallet, M.; Mongin, F.; Queguiner, G. Synthesis 1995,
9, 1159. (b) Hopkins, T. A.; Meerholz, K.; Shaheen, S.; Anderson,
M. L.; Schmidt, A.; Kippelen, B.; Padias, A. B.; Hall, H. K.;
Peyghambarian, J. N.; Armstrong, N. R. Chem. Mater. 1996, 8, 344.
(8) (a) Kumar, S.; Kim, T. Y. J. Org. Chem. 2000, 65, 3883. (b) Nadin,
A.; Sa´nchez Lo´pez, J. M.; Owens, A. P.; Howells, D. M.; Talbot, A.
C.; Harrison, T. J. Org. Chem. 2003, 68, 2844. (c) Jotterand, N.;
Pearce, D. A.; Imperiali, B. J. Org. Chem. 2001, 66, 3224. (d) Trecourt,
F.; Mongin, F.; Mallet, M.; Queguiner, G. J. Heterocycl. Chem. 1995,
32, 1261. (e) Trecourt, F.; Mallet, M.; Mongin, F.; Queguiner, G.
Tetrahedron 1995, 51, 11743.
602 Inorganic Chemistry, Vol. 44, No. 3, 2005

Organoboron Compounds with 8-Hydroxyquinolato Chelate
(CDCl3, δ, ppm): 11.86 ppm. Anal. Calcd (%) for C31H22NBO:
C, 85.53; H, 5.09; N, 3.22; Found: C, 85.13; H, 5.03; N, 3.13.
Synthesis of BPh2(5-(2-benzothienyl)-q) (2). As described for
the formation of 1, the synthesis of 2 also involves three steps as
described below.
dd, J ) 1.01 Hz, 5.05 Hz,), 8.48 (1H, dd, J ) 1.01 Hz, 8.34 Hz),
7.84-7.72 (5H, m), 7.68 (1H, dd, J ) 8.08 Hz, 5.05 Hz), 7.54 (s.
2H), 7.36-7.24 (6H, m). 13C NMR (CDCl3, δ, ppm): 157.95,
142.35, 141.36, 140.19, 139.60, 136.94, 133.11, 128.40, 127.20,
123.72, 123.49, 123.25, 122.97, 122.27, 118.24, 113.27, 110.46.
11B NMR (CDCl3, δ, ppm): 8.97 ppm. Anal. Calcd for C25H16-
NOBS2: C, 71.26; H, 3.83; N, 3.32. O, 3.8; B: 2.57; S, 15.22.
Found: C, 70.86; H, 3.93; N, 3.21.
5-(1-Benzothien-2-yl)-8-methoxyquinoline (L2a). As in the
manner described for L1a, to a mixture of 5-bromo-8-methoxy-
quinoline (0.476 g, 2 mmol), (2-benzothienyl)boronic acid (0.400
g, 2.25 mmol), NaHCO3 (0.400 g, 4.8 mmol), and Pd(PPh3)4 (60
mg, 5.1 mmol, 2.5%) were added a degassed solution of toluene
(40 mL), water (10 mL), and ethanol (10 mL) via cannula. The
resulting mixture was refluxed with vigorous stirring for 18 h. The
crude product was purified by a column chromatography using ethyl
Synthesis of B(2-benzothienyl)2(2-methyl-q) (4). To the solu-
tion of benzothiophene (0.730 g, 5.4 mmol) in 20 mL of THF was
added 3.9 mL (1.6 M, 6.2 mmol) of n-BuLi slowly at 0 °C. After
being stirred for 1 h, the solution changed from light green to a
clear brown color. BBr3 (1.0 M in heptane, 1.8 mL, 1.8 mmol,)
was then added, and the color of the mixture turned colorless. The
solution was stirred for 1.5 h, and then 0.290 g (1.8 mmol) of
2-methyl-8-hydroxyquinoline dissolved in 10 mL THF in a separate
flask was added via double needle to the above solution. The
reaction mixture became light yellow immediately with green
luminescence. After being stirred for 3 h, the solution was
evaporated to dryness under vacuum, and 5 mL of CH2Cl2 was
added. The product was transferred to a small vial for crystallization
after filtration by laying the hexane on the top of the CH2Cl2 layer.
After standing for a few days, light green crystals of compound 4
were obtained in 53% yield with mp 208-209 °C. 1H NMR (CDCl3,
δ, ppm): 8.38 (1H, d, J ) 8.34 Hz,), 8.84-7.78 (4H, m), 7.67
(1H, t, J ) 8.08 Hz), 7.61 (2H, s), 7.42 (1H, d, J ) 8.59 Hz),
7.44-7.23 (6H, m), 2.78 (3H, s). 13C NMR (CDCl3, δ, ppm):
158.05, 154.85, 143.12, 142.09, 140.53, 137.66, 132.36, 129.32,
127.26, 126.13, 125.00, 124.31, 124.19, 123.96, 122.86, 113.88,
111.07, 21.91. 11B NMR (CDCl3, δ, ppm): 8.75 ppm. Anal. Calcd
(%) for C26H18NOBS2: C, 71.73; H, 4.17; N, 3.22. Found: C,
71.29; H, 4.13; N, 3.25.
1
acetate/hexane (4/1) as the eluent to afford L2a in 52% yield. H
NMR (CDCl3, δ, ppm): 8.98 (1H, dd, J ) 1.6 Hz, 4.1 Hz), 8.60
(1H, dd, J ) 1.7 Hz, 8.6 Hz), 7.90 (1H, d, J ) 8.2 Hz), 7.85 (1H,
d, J ) 7.3 Hz), 7.67 (1H, d, J ) 8.0 Hz), 7.45-7.37 (2H, m), 7.45
(1H, dd, J ) 1.2 Hz, 7.2 Hz), 7.38 (1H, s), 7.12 (1H, d, J ) 8.1
Hz), 4.16 (3H, s).
5-(Benzothien-2-yl)-8-hydroxyquinoline (L2b). To a solution
of 5-(benzothien-2-yl)-8-methoxyquinoline (L2a) (0.294 g, 1 mmol)
in 20 mL of dichloromethane was added a solution of BBr3 (1.0 M
in heptane, 2 mL, 2 mmol) at 0 °C with stirring under nitrogen.
The reaction mixture was warmed to room temperature and
underwent further reflux overnight. Following the same method of
workup as described for L1b afforded L2b in 50% yield. 1H NMR
(DMSO, δ, ppm): 9.11 (1H, d, J ) 4.2 Hz), 8.06 (1H, d, J ) 7.7
Hz), 7.98 (1H, d, J ) 7.0 Hz), 7.95 (1H, d, J ) 7.6 Hz), 7.88 (1H,
d, J ) 8.0 Hz), 7.67 (1H, s), 7.49-7.43 (3H, m), 7.38 (1H, s),
4.80 (1H, br).
Synthesis of 2. A THF solution (10 mL) of triphenylborane (58
mg, 0.22 mmol) was transferred to a flask containing 5-(2-
benzothienyl)-8-hydroxyquinoline (L2b) (61 mg, 0.22 mmol)
dissolved in 20 mL of THF. The solution became luminescent
yellow after being stirred for a few minutes. The solution was under
reflux with stirring for overnight. Orange-yellow single crystals of
2 were obtained via slow evaporation and diffusion of solvents in
56% yield in the same manner as for 1. Mp 210-211 °C. 1H NMR
(CDCl3, δ, ppm): 8.92 (1H, d, J ) 7.8 Hz), 8.66 (1H, d, J ) 4.5
Hz), 7.92-7.86 (3H, m), 7.72 (1H, dd, J ) 5.01 Hz, 8.51 Hz),
7.51-7.48 (4H, m), 7.45-7.39 (4H, m) 7.34-7.27 (6H, m). 13C
NMR (CDCl3, δ, ppm): 159.56, 140.67, 140.37, 140.10, 138.22,
138.17, 135.10, 134.99, 132.42, 128.34, 128.07, 127.53, 127.42,
125.17, 124.94, 123.99, 123.69, 123.42, 122.58, 118.89, 110.15.
11B NMR (CDCl3, δ, ppm): 12.10 ppm. Anal. Calcd (%) for C29H20-
NOBS: C, 78.92; H, 4.57; N, 3.17. Found: C, 78.40; H, 4.52; N,
3.48.
X-ray Crystallography Analyses. Single-crystals of 1-4 ob-
tained from CH2Cl2/hexane solution were mounted on glass fibers
in a brass pin, and the data were collected on a Siemens P4 single-
crystal X-ray diffractometer with a SMART CCD-1000 detector
and graphite-monochromated Mo KR radiation operating at 50 kV
and 30 mA at 25 °C. No significant decay was observed during
the data collection. Data were processed on a Pentium PC using
Siemens SHELXTL software package (version 5.10).9 Neutral atom
scattering factors were taken from Cromer and Waber.10 Empirical
absorption correction was applied to all crystals. The structures were
solved by direct methods. All non-hydrogen atoms were refined
anisotropically. The positions of hydrogen atoms were calculated,
and their contributions in structural factor calculations were
included. The crystal data are summarized in Table 1. Selected bond
lengths and angles are given in Table 2.
Synthesis of B(2-benzothienyl)2q (3). To the solution of
benzothiophene (0.730 g, 5.4 mmol) in 20 mL of THF was added
3.9 mL (1.6 M, 6.2 mmol) of n-BuLi slowly. After being stirred
for 1 h, the solution changed from light green to a clear brown
color. BBr3 (1.0 M in heptane, 1.8 mL, 1.8 mmol) was then added,
and the color of the mixture turned to colorless. The solution was
stirred for 1.5 h, and 0.260 g (2.0 mmol) of 8-hydroxyquinoline
dissolved in 10 mL of THF in a separate flask was added via a
double needle to the above solution. The reaction mixture became
light yellow immediately with green luminescence. After being
stirred for 3 h, the solution was evaporated to dryness under
vacuum, and 5 mL of CH2Cl2 was added. The product was
transferred to a small vial for crystallization after filtration by laying
the hexane on the top of the CH2Cl2 layer. After standing for a
few days, light green crystals of compound 3 were obtained in 51%
yield with mp 204-205 °C. 1H NMR (CDCl3, δ, ppm): 8.73 (1H,
Quantum Yield Measurement. Quantum yields of compounds
1-4 were determined relative to 9,10-diphenylanthracene in CH2-
Cl2 at 298 K (Φr ) 0.95).11 The absorbance of all the samples and
the standard at the excitation wavelength were approximately
0.098-0.109. The quantum yields were calculated using previously
known procedures.12
Fabrication of Electroluminescent Devices. The EL device of
3 was fabricated on an indium-tin oxide (ITO) substrate. Organic
layers were deposited on the substrate by conventional vapor
(9) SHELXTL NT Crystal Structure Analysis Package, Version 5.10;
Bruker AXS, Analytical X-ray System: Madison, WI, 1999.
(10) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch Press: Birmingham, AL, 1974; Vol. 4, Table 2.2A
(11) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993.
(12) Demas, N. J.; Crosby, G. A. J. Am. Chem. Soc. 1970, 29, 7262.
Inorganic Chemistry, Vol. 44, No. 3, 2005 603

Cui et al.
Table 1. Crystallographic Data of 1-4
1
2
3
4
formula
fw
space group
a/Å
b/Å
c/Å
C31H22BNO
435.324
P21/c
16.850(5)
7.776(2)
18.372(6)
90
107.837
90(5)
2291.6(12)
4
C29H20BNOS
441.352
P1h
C25H16BNOS2
421.344
P21/n
12.242(4)
17.188(6)
19.354(7)
90
91.748(6)
90
4071(2)
8
C26H18BNOS2
435.37
P21/c
18.36(3)
17.41(2)
13.799(19)
90
100.97(2)
90
4329(10)
8
1.336
0.265
56.68
7.9708(16)
10.50(2)
14.353(3)
74.011(4)
87.716(4)
89.802(4)
1158.2(4)
2
1.266
0.162
56.44
8263
R, deg
â, deg
γ, deg
V/Å3
Z
Dc/(g cm-3
)
1.262
0.075
56.72
14684
1.375
0.279
56.64
2847
µ/mm-1
2θmax/deg
reflns measured
30173
reflns used (Rint
no. variables
final R [I > 2σ(I)]
)
5222 (0.1483)
308
5189 (0.0154)
311
9567 (0.0345)
541
10130 (0.0319)
579
R1a ) 0.0711
wR2b ) 0.1244
R1 ) 0.3101
wR2 ) 0.1630
0.707
R1a ) 0.0418
wR2b ) 0.0885
R1 ) 0.0893
wR2 ) 0.1012
0.855
R1a ) 0.059
wR2b ) 0.1511
R1 ) 0.1256
wR2 ) 0.1737
0.898
R1a ) 0.0490
wR2b ) 0.1216
R1 ) 0.1013,
wR2 ) 0.1409
0.905
R (all data)
GOF on F2
2
2
2
2
2
2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w[(Fo - Fc )2]/∑[w(Fo )2]]1/2. w ) 1/[σ2(Fo ) + (0.075P)2], where P ) [max (Fo , 0) + 2Fc ]/3.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1-4
1. The parent molecules L1a and L2a were obtained by using
Pd catalyzed Suzuki coupling13 between the appropriate
boronic acid and 5-bromo-8-methoxyquinoline. Demethyla-
tion14 of L1a and L2a in the presence of BBr3 led to the
isolation of L1b and L2b in 45% and 50% yields, respec-
tively. The boron chelate compounds 1 and 2 were isolated
in 52% and 50% yields, from the reaction of BPh3 with the
ligands L1b and L2b, respectively, in THF under reflux.
For the synthesis of 3 and 4, the precursor compound B(2-
benzothienyl)3 was first prepared in situ without a full
characterization by the reaction of n-BuLi with ben-
zothiophene to yield (2-benzothienyl)lithium, followed by
1
B(1)-O(1)
B(1)-N(1)
B(1)-C(1)
B(1)-C(7)
1.535(5)
1.646(5)
1.578(6)
1.606(6)
O(1)-B(1)-C(1)
O(1)-B(1)-C(7)
C(1)-B(1)-C(7)
O(1)-B(1)-N(1)
C(1)-B(1)-N(1)
C(1)-B(1)-N(1)
111.9(4)
109.3(4)
119.0(4)
97.6(3)
108.2(4)
108.5(4)
2
B(1)-O(1)
B(1)-N(1)
B(1)-C(1)
B(1)-C(7)
1.5323(19)
1.6383(19)
1.594(2)
O(1)-B(1)-C(1)
O(1)-B(1)-C(7)
C(1)-B(1)-C(7)
O(1)-B(1)-N(1)
C(1)-B(1)-N(1)
C(7)-B(1)-N(1)
109.73(13)
111.07(12)
116.29(13)
97.73(11)
109.54(11)
110.90(12)
1.596 (2)
1
the addition of /3 equiv of BBr3.15 The reaction of 8-hy-
3
B(1)-O(1)
B(1)-N(1)
B(1)-C(1)
B(1)-C(9)
1.503(4)
1.618(4)
1.603(5)
1.593(6)
O(1)-B(1)-C(1)
O(1)-B(1)-C(9)
C(1)-B(1)-C(9)
O(1)-B(1)-N(1)
C(1)-B(1)-N(1)
C(9)-B(1)-N(1)
111.3(2)
110.7(2)
114.1(2)
99.6(2)
107.8(2)
112.5(2)
droxyquinoline or 8-hydroxyquinaldine with the B(2-ben-
zothienyl)3 solution resulted in the formation of compounds
3 and 4, respectively (isolated yield 51%, 52%, respectively).
Schemes 1 and 2 summarize all the synthetic steps involved
for the syntheses of the boron compounds. The four boron
4
1
compounds were characterized by H, 13C, and 11B NMR,
B(1)-O(1)
B(1)-N(1)
B(1)-C(11)
B(1)-C(19)
1.528(3)
1.630(4)
1.603(4)
1.593 (4)
O(1)-B(1)-C(19)
O(1)-B(1)-C(11)
C(19)-B(1)-C(11)
O(1)-B(1)-N(1)
C(19)-B(1)-N(1)
C(11)-B(1)-N(1)
109.00(19)
110.51(18)
114.2(2)
98.48(15)
114.61(18)
108.95(4)
elemental analyses, and single-crystal X-ray diffractions. 11
B
NMR chemical shifts of the four compounds are in the same
range as those observed for diphenyl(2-aminoethoxy)boranes
derivatives’ spectra,16 consistent with a tetrahedral geometry.
(13) (a) Miyaura, N. AdV. Met.-Org. Chem. 1998, 6, 187 and references
therein. (b) Suzuki, A. J. Organomet. Chem. 1999, 576, 147 and
references therein.
vacuum deposition. N,N′-Bis(1-naphthyl)-N,N′-diphenylbenzidine
(NPB) was employed as the hole transport layer. The cathode
composed of LiF and Al was deposited on the substrate by
conventional thermal vacuum deposition. The active device area
is 1.0 × 5.0 mm2. The current/voltage characteristics were measured
using a Keithley 238 Source Measure Unit. The EL spectra and
the luminance for the devices were measured by using a Photo
Research-650 Spectra Colorimeter.
(14) (a) Wolfe, J. P.; Singer, R. A.; Yang, B. H.; Buchwald, S. L. J. Am.
Chem. Soc. 1999, 121, 9550. (b) Silva, N. O.; Abreu, A. S.; Ferreira,
P. M. T.; Monteiro, L. S.; Queiroz, M.-J. R. P.; Eur. J. Org. Chem.
2002, 2524. (c) Gust, R.; Keilitz, R.; Schmidt, K.; von Rauch, M. J.
Med. Chem. 2002, 45, 3356. (d) Queiroz, M.-J. R. P.; Dubest, R.;
Aubard, J.; Faure, R.; Guglielmetti, R. Dyes Pigm. 2000, 47, 219. (e)
Lopez-Alvarado, P.; Avendano, C.; Menendez, J. C. J. Chem. Soc.,
Perkin Trans. 1 1997, 229. (f) Batt, D. G.; Maynard, G. D.; Petraitis,
J. J.; Shaw, J. E.; Galbraith, W.; Harris, R. R. J. Med. Chem. 1990,
33, 360. (g) Amin, S.; Huie, K.; Hussain, N.; Balanikas, G.; Carmella,
S. G.; Hecht, S. S. J. Org. Chem. 1986, 51, 1206.
Results and Discussion
(15) (a) Wrackmeyer, B.; Milius, W.; Molla, E. Z. Naturforsch., B: Chem.
Sci. 1996, 51, 1811. (b) Brown, H. C.; Racherla, U. S. J. Org. Chem.
1986, 51, 427. (c) Brown, H. C.; Racherla, U. S. Tetrahedron Lett.
1985, 26, 4311.
Syntheses. Two new ligands, 5-(1-naphthyl)-8-hydroxy-
quinoline (L1b) and 5-(2-benzothienyl)-8-hydroxyquinoline
(L2b), have been obtained by procedures shown in Scheme
604 Inorganic Chemistry, Vol. 44, No. 3, 2005

Organoboron Compounds with 8-Hydroxyquinolato Chelate
Scheme 1
Scheme 2
sponding quinoline ring. No significant π-π stacking was
observed in the crystal lattices of the four compounds. The
dihedral angle between the naphthyl ring and quinoline ring
in 1 is 87.7°; they are almost perpendicular to each other
due to the steric hindrance of ortho hydrogen atoms. The
benzothienyl ring in 2 displays a rotational disorder with 50%
occupancy for each disordered sit and a dihedral angle of
46.1° (48.1°) with the quinoline ring. The much smaller
dihedral angle in 2 is clearly caused by the much reduced
steric interactions between the benzothienyl group and the
quinoline ring, compared to the interactions between the
1-naphthyl group and the quinoline ring in 1. As a conse-
All the four compounds are air-stable both in the solid state
and in solution. They are also thermally stable with the
melting points above 200 °C.
Crystal Structures of 1-4. The crystal structures of the
four boron compounds have been determined by single-
crystal X-ray diffraction analyses. The boron center in all
four compounds displays a typical tetrahedral geometry, as
shown in Figures 1-4. The hydroxyquinoline groups in all
four molecules are chelated to the boron in the same manner
to form a five-membered chelate ring. The bond angle
N-B-O of the four complexes is similar, ranging from
97.6(3)° to 99.6(2)°. Each boron center in the four com-
pounds is further bound by two carbon atoms of the two
phenyl groups (for 1 and 2) or the two benzothienyl groups
(for 3 and 4). The B-N, B-O, and B-C bond lengths are
similar to those reported previously.6,17,18 The five-membered
chelate ring in each compound is coplanar with the corre-
Figure 1. Molecular structure of 1 with labeling scheme and 50% thermal
ellipsoids. All hydrogen atoms are omitted for clarity.
(16) Ho¨pfl, H.; Farfa´n, N.; Castillo, D.; Santillan, R.; Contreras, R.;
Mart´ınez-Mart´ınez, F. J.; Galva´n, M.; Alvarez, R.; Ferna´ndez, L.;
Halut, S.; Daran, J.-C. J. Organomet. Chem. 1997, 544, 175.
Figure 2. Molecular structure of 2 with labeling scheme and 50% thermal
ellipsoids. All hydrogen atoms are omitted for clarity.
Inorganic Chemistry, Vol. 44, No. 3, 2005 605

Cui et al.
Figure 5. Emission spectra of 1-4 in CH2Cl2 at 298 K.
Table 3. Spectroscopic Dataa for BPh2q, BPh2(2-Me-q), 1-4
UV-vis excitation emission quantum
compd
(λ max, nm) (λmax, nm) (λmax, nm) yieldb (%)
conditions
Figure 3. Molecular structure of 3 with labeling scheme and 50% thermal
ellipsoids. All hydrogen atoms are omitted for clarity.
BPh2q
BPh2-
(2-Me-q)
242, 264, 396
246, 266, 396
395
395
504
497
30
34
CH2Cl2, 298 K
CH2Cl2, 298 K
1
298, 406
409
397
413
418
534
504
523
565
11
CH2Cl2, 298 K
CH2Cl2, 77 K
film, 298 K
2
232, 266,
304, 422
1.0
CH2Cl2, 298 K
448
428
394
396
394
392
533
560
501
483
496
496
CH2Cl2, 77 K
film, 298 K
CH2Cl2, 298 K
CH2Cl2, 77 K
film, 298 K
3
4
238, 264, 388
18
37
294, 304, 320
336, 354, 380
CH2Cl2, 298 K
395
393
478
490
CH2Cl2, 77 K
film, 298 K
a Concentration: [M] ) 5 × 10-6
.
b Relative to 9,10-diphenylanthracene
in CH2Cl2 at room temperature.
Figure 4. Molecular structure of 4 with labeling scheme and 50% thermal
ellipsoids. All hydrogen atoms are omitted for clarity.
molecule and a benzothienyl group from another molecule
(the shortest atomic separation distance is 3.50 Å). These
π-π stacking interactions are limited between two mol-
ecules, and no extended π-π stacking is observed for 3 and
4. It is likely that the methyl group on the quinoline ligand
in 4 prevents the π-π stacking from occurring between two
quinoline groups. Both thienyl rings in 4 are disordered in a
similar manner as the 2-benzothienyl substituent in 2. Only
one set of the disordered benzothienyl ligands is shown in
Figure 4. In addition to the π-π stacking difference, the
2-methyl group in 4 has a subtle impact on the structure.
For example, the O-B-N angle in 4 is about 1° smaller
than that in 3. The B-N and B-O bond lengths in 4 are
also somewhat longer than those in 3. Previously, Al(2-Me-
q)3 was reported5d to be unstable due to the 2-methyl group,
compared to that of Alq3. However, we did not observe any
difference in terms of thermal and chemical stability between
3 and 4.
quence, the benzothienyl and the quinoline ring form partial
conjugation as reflected by the slightly shortened bond length
between C17-C22 (1.476 Å) in 2 compared with that
between C16-C22 (1.536 Å) in 1. There is no significant
intermolecular π-π stacking interaction in the crystal lattice
of 1. In 2, there are intermolecular π-π interactions
involving a few atoms of the benzo portions of the ben-
zothienyl ring with the shortest atomic separation distance
being 3.73 Å. Compounds 3 and 4 have similar structures.
In the asymmetric unit of 3 and 4 are two independent
molecules, which form a π-stacked pair. As shown in Figures
3 and 4, the π-π stacking in 3 is between two quinoline
groups from the two independent molecules (the shortest
atomic separation distance is 3.44 Å), while in contrast, the
π-π stacking in 4 is between a quinoline group from one
Luminescent and Electronic Properties. Upon irradiation
by UV light, in solution and in the solid state at ambient
temperature compounds 3 and 4 yield a bright bluish-green
emission, while compounds 1 and 2 exhibit green and
orange-yellow luminescence, respectively. The emission
spectra in CH2Cl2 at ambient temperature for all four
compounds are shown in Figure 5. Table 3 summarizes the
luminescent data of compounds 1-4. For comparison, the
(17) (a) Niedenzu, K.; Deng, H.; Knoeppel, D.; Krause, J.; Shore, S. G.
Inorg. Chem. 1992, 31, 3162. (b) Hsu, L. Y.; Mariategui, J. F.;
Niedenzu, K.; Shore, S. G. Inorg. Chem. 1987, 26, 143. (c) Kiegel,
W.; Lubkowitz, G.; Rettig, S. J.; Trotter, J. Can. J. Chem. 1991, 69,
1217.
(18) (a) Heller, G. Top. Curr. Chem. 1986, 131, 39. (b) Dal Negro, A.;
Ungaretti, L.; Perotti, A. J. Chem. Soc., Dalton Trans. 1972, 15, 1639.
(c) Binder, H.; Matheis, W.; Deiseroth, H.-J.; Han, F.-S. Z. Natur-
forsch. 1984, 39b, 1717. (d) Clegg, W.; Noltemeyer, N.; Shelderick,
G. M.; Maringgele, W.; Meller, A. Z. Naturforsch. 1980, 35b, 1499.
606 Inorganic Chemistry, Vol. 44, No. 3, 2005

Organoboron Compounds with 8-Hydroxyquinolato Chelate
Table 4. HOMO and LUMO Energy Levels for Compounds 1-4
experimentally measured
molecular orbital calculationsa
compd
HOMO (eV)
LUMO (eV)
∆Eoptical (eV)
HOMO
LUMO
∆(HOMO - LUMO)
1
2
3
4
-5.69
-5.53
-5.71
-5.70
-3.10
-3.10
-2.98
-2.94
2.59
2.43
2.73
2.76
-0.216673
-0.214950
-0.201876
-0.189335
-0.088219
-0.093304
-0.097097
-0.086039
0.1285/3.50 eV
0.1216/3.31 eV
0.1048/2.85 eV
0.1033/2.81 eV
a Unit: hartree.
previously reported data for BPh2q are also included. BPh2-
(2-Me-q) was prepared, and its spectroscopic data are also
included in Table 3. In solution, compounds 1-4 have
emission maxima at 534, 565, 501, and 496 nm, respectively.
The emission spectra of 3 and 4 are very similar to those of
BR2q (R ) ethyl, phenyl, and 1-naphthyl) which have
emission maxima at 495-504 nm reported earlier from our
group,6a indicating that changing the R group has little impact
on the emission maximum. There is, however, a small
difference between the emission maxima of 3 and 4, with
the latter having a somewhat shorter emission wavelength.
The same difference is also evident between BPh2q and BPh2-
(2-Me-q) as shown in Table 3. Previously, it has been
demonstrated for Alq′3 compounds that the HOMO level is
dominated by the phenoxy ring while the LUMO level is
dominated by the pyridyl ring of the 8-hydroxyquinoline
ligand.5d Therefore, an electron donating substituent on the
pyridyl ring will push the LUMO level up, hence increasing
the HOMO-LUMO energy gap, which has been confirmed
by a number of 2-methyl, 3-methyl, and 4-methyl substituted
8-hydroxyquinline complexes5 Al(Me-q)3. We therefore
believe that the small blue shift of emission energy from 3
to 4 can also be attributed to the electron donating 2-methyl
group in 4. The major difference between 3 and 4 is the
emission quantum efficiency: 4 is more than two times more
efficient than 3. A similar methyl substitution effect on
quantum efficiency has been observed in Al(3-Me-q)3 and
Al(4-Me-q)3 compounds.5b Consistently, the BPh2(2-Me-q)
also displays a higher quantum efficiency than that of BPh2q,
albeit a much smaller difference than that between 3 and 4.
We can therefore conclude that the 2-methyl group in the
8-hydroxyquinoline ligand can enhance the emission ef-
ficiency of the complex, compared to the nonsubstituted
ligand.
electron density to the quinoline ring more effectively than
the 1-naphthyl moiety in 1. The emission efficiency of both
1 and 2 is lower than those of 3 and 4. One notable difference
between 1-2 and 3-4, in addition to the emission energy,
is the low emission efficiency displayed by 1 and 2. A similar
drastic decrease of emission efficiency by an electron
donating group at the C5 position, compared to Alq3, has
been observed previously in the Al(5-Me-q)3 compound.5b
The trend we observed for the BPh2q′ compounds is
consistent with the trend observed for the C5 substituted Alq′3
compounds. The unusually small quantum yield of 2 may
be further attributed to the direct attachment of the thienyl
group on the 8-hydroxyquinoline chromophore, which ef-
fectively quenches the emission by perhaps the “heavy atom”
effect of the sulfur atom.19
The emission band for compounds 1-4 is somewhat blue-
shifted in CH2Cl2 at 77 K, which can be attributed to the
increased environmental rigidity19 at 77 K. The emission
band of the four compounds in the solid state is almost
identical to that in solution, an indication that there is little
intermolecular interaction present in the solid state, as
confirmed by the crystal structures. The investigation of the
effects of various solvents on emission was also carried out,
which showed that the emission wavelengths are independent
of the polarity of solvents (e.g., toluene, THF, CH2Cl2, and
CH3CN).
The LUMO levels for the four compounds were deter-
mined from the electrochemical reduction potential of the
complexes in CH2Cl2 solutions. Oxidation potentials for these
complexes could not be obtained. The energy gap between
HOMO and LUMO was estimated by using the UV-vis
absorption edge. The HOMO energy level was calculated
by using the corresponding LUMO and energy gap values.
As shown in Table 4, compound 2 has the highest HOMO
energy level (-5.53 eV versus -5.69 to 5.71 eV of 1, 3,
and 4) and the smallest band gap (2.43 eV) among the four
compounds. This trend is consistent with the corresponding
wavelength data shown in Table 3. Several BPh2L com-
pounds where L is a O, O′-chelate ligand (diolate) have been
reported recently.20 The HOMO and LUMO energy levels
of these compounds are comparable to those of 1-4.
Molecular Orbital Calculations. To further understand
the nature of luminescence, we carried out molecular orbital
calculations for the four compounds on the restricted
Hartree-Fock (RHF) level using a standard split-valence
On the basis of previous theoretical and experimental work
on 8-hydroxyquinoline derivative complexes of aluminum,5
it is anticipated that, by attaching an aryl group at the C5
position of the quinoline ring, the emission maximum should
shift toward a longer wavelength. Indeed, as shown by the
data in Table 4, the emission maximum of 1 and 2 was
shifted by ∼35 and 66 nm, respectively, compared to that6a
of BPh2q (504 nm). The red shift of 2 is more pronounced
than that of 1, which can be attributed to the relatively higher
degree of conjugation between the benzothienyl ring and the
quinoline ring in 2, compared to that between naphthyl and
the quinoline in 1, as confirmed by the crystal structural data.
The large emission red shift of 2 can also be attributed at
least partially to electronic effects since the 2-benzothienyl
moiety in 2 is a more electron rich group, and hence donate
(19) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.,;
Kluwer Academic/Plenum: New York, 1999.
(20) Lim, H. J.; Kim, S. M.; Lee, S.-J.; Jung, S.; Kim, Y. K.; Ha, Y. Opt.
Mater. 2003, 21, 211.
Inorganic Chemistry, Vol. 44, No. 3, 2005 607

Cui et al.
Figure 6. HOMO and LUMO levels of 2 and 3.
polarized (3LYP/6-311G**) basis set, employing the Gauss-
ian 98 suit of programs.21 The orbital diagrams were
generated by using the Molekel program.22 The geometric
parameters from X-ray diffraction analysis were used for the
calculation. The calculated HOMO and LUMO energy levels
and the difference for the four complexes are provided in
Table 4. One unexpected result is that the calculated
HOMO-LUMO gaps of compounds 1 and 2 are greater than
those of 3 and 4, which is obviously contradictory with the
experimentally observed optical energy gap. Clearly the
HOMO-LUMO gaps obtained from Gaussian 98 calcula-
tions should not be taken too seriously. Nevertheless, the
MO calculated gap for 1 is bigger than that of 2, which is
consistent with the observed trend. Compounds 3 and 4
possess almost the same band gaps based on MO results,
which is also consistent with experimental data. The surfaces
of HOMO and LUMO for 2 and 3 along with the energy for
each level are shown in Figure 6, as representative examples.
All contour values are (0.03 au. As can be seen from the
diagrams, the HOMO level of compound 2 is a π orbital
with contributions from both the benzothienyl substituent and
the quinoline ring. In contrast, the HOMO level of 3 consists
of two almost degenerate π orbitals with almost 100%
contributions from the benzothienyl ligand bonded to the
boron atom; one of the π orbitals is shown in Figure 6. The
π* orbital of the LUMO level for both compounds 2 and 3
involves contributions from the hydroxyquinoline ring. On
the basis of the orbital diagrams in Figure 6, the electronic
transition of 2 can be attributed to a π to π* transition
centered on the 5-benzothienyl-8-hydroxyquinoline ligand.
In contrast, the electronic transition of 3 can be attributed to
an interligand charge transfer from the benzothienyl ligand
to the hydroxyquinoline ligand. Although the experimental
data indicated that there is little difference between BPh2q
and B(2-benzothienyl)2q (3) in term of emission energy, the
molecular orbital calculation results showed that the elec-
tronic transition in 3 has a very different origin from that of
BPh2q: the former involves interligand charge transfer, and
the latter involves a hydroxyquinoline centered π f π*
transition. The similar emission energy of 3 and BPh2q seems
to be coincidental.
Electroluminescence. Electroluminescent properties for
1 and 2 were not examined due to their poor photolumines-
cent efficiency and the fact that a number of BPh2q′
compounds have been demonstrated previously for use in
EL devices, which indicated that compounds such as 1 and
2 should be able to act as an emitter in EL, if they were
sufficiently bright. Because 8-hydroxyquinoline boron com-
pounds that contain two 2-benzothienyl ligands are a class
of previously unknown molecules, we decided to carry out
a preliminary evaluation for the EL properties of compound
3. On the basis of our previous work demonstrating that
BPh2q can function as both an emitter and an electron
transport layer, a simple double-layer device with the
structure of ITO/NPB(40 nm)/3 (40 nm)/LiF (1 nm)/Al,
where NPB ) N,N′-bis(1-naphthyl)-N,N′-diphenylbenzidine,
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian, Inc.:
Pittsburgh, PA, 1998.
(22) Flkiger, P.; Lthi, H. P.; Portmann, S.; Weber, J. MOLEKEL, 4.1; Swiss
Center for Scientific Computing: Manno, Switzerland, 2000-2001.
608 Inorganic Chemistry, Vol. 44, No. 3, 2005

Organoboron Compounds with 8-Hydroxyquinolato Chelate
Figure 8. The J-V and L-V characteristics of the EL device.
The general effect of substitution at the C5 position and the
C2 position of the 8-hydroxyquinolato ligand on luminescent
properties observed for the boron compounds is consistent
with the trend observed for related Alq′3 compounds. Most
notable is the drastic decrease of emission quantum efficiency
caused by the electron donating substituent at the C5 position
as demonstrated by compounds 1 and 2, and the relatively
high emission quantum efficiency displayed by 4 that has a
methyl group at the C2 position of the hydroxyquinolato
ligand. Unlike the Al(2-Me-q)3 compound, the 2-Me-q ligand
does not appear to cause notable instability for the boron
compound, which probably can be explained by the presence
of only one 2-Me-q chelate ligand in the boron compound
that does not have as many interligand steric interactions as
the three 2-Me-q ligands do in the aluminum compound. The
replacement of the phenyl ligand in BPh2q by the 2-ben-
zothienyl ligand does not change the emission energy
significantly. However, the composition of the HOMO level
appears to have changed from the 8-hydroxyquinoline
dominated π orbital in BPh2q to an almost pure benzothienyl
π orbital in 3 and 4. The boron compounds such as BPh2q
in general display an emission band that is at a shorter
wavelength than that of Alq3. Compounds 1 and 2 are not
suitable for use as emitters in OLEDs due to their low PL
efficiency. Preliminary investigation on 3 indicates that this
type of compound has the tendency to form an exciplex with
the NPB hole transport layer and their performance in EL is
not as satisfactory as that of the Alq3.
Figure 7. The EL spectrum of the device and the PL spectra of a film of
3 and a film of a 1:1 mixture of 3 and NPB.
is fabricated. The EL device produced a broad bluish-green
emission band as shown in Figure 7, which is much broader
than and partially matches the PL spectrum of 3. The low
energy emission zone of EL may be caused by the presence
of exciplex emission between the NPB layer and 3. Indeed,
as shown in Figure 7, the PL spectrum of the film of a 1:1
mixture of 3 and NPB approximately matches the lower
energy portion of the EL spectrum. Therefore, we concluded
that the observed EL spectrum consists of emission from
the compound 3 layer and the exciplex emission between
NPB and 3. Similar exciplex emission has been observed
previously in the double-layer BPh2q EL device.6a The
tendency of the hydroxyquinoline boron chelates to form an
exciplex with the NPB layer is in sharp contrast to that of
Alq3 and derivatives, which are known not to form exciplexes
with NPB. This difference may be attributed to the structural
difference between the boron and the aluminum com-
pounds: the former has a relatively open tetrahedral geom-
etry while the latter has a less open octahedral geometry.
The turn-on voltage of the EL device is ∼10 V, and the
maximum brightness is ∼1050 Cd/m2 as shown in Figure 8.
To compare with the performance of Alq3, compound 3 is
much less superior, in addition to its tendency to form an
exciplex with the hole transport layer. As a result, no further
investigation on electroluminescent properties of our boron
compounds were conducted.
Acknowledgment. We thank the Natural Science and
Engineering Research Council of Canada for financial
support.
Conclusions
Supporting Information Available: Crystallographic data in
CIF and PDF formats. This material is available free of charge via
the Internet at http://pubs.acs.org.
By varying the substituent group on the 8-hydroxyquino-
lato ligand, four new boron compounds with emission color
ranging from blue-green to yellow-orange have been achieved.
IC0489746
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