Syntheses, structures, and electroluminescence of new blue/green luminescent chelate compounds: Zn(2-py-in)2(THF), BPh2(2-py-in), Be(2-py- in)2, and BPh2(2-py-aza) [2-py-in = 2-(2-pyridyl)indole; 2-py-aza = 2-(2

Article abstract of DOI:10.1021/ja9944249

Four novel blue/green luminescent compounds, Zn(2-py-in)₂(THF) (1), BPh₂(2-py-in) (2), Be(2-py-in)₂ (3), and BPh₂(2-py-aza) (4), where 2-py-in = 2-(2-pyridyl)indole and 2-py-aza = 2-(2-pyridyl)-7-azaindole, have

Full text of DOI:10.1021/ja9944249

J. Am. Chem. Soc. 2000, 122, 3671-3678
3671
Syntheses, Structures, and Electroluminescence of New Blue/Green
Luminescent Chelate Compounds: Zn(2-py-in)2(THF), BPh2(2-py-in),
Be(2-py-in)2, and BPh2(2-py-aza) [2-py-in ) 2-(2-pyridyl)indole;
2-py-aza ) 2-(2-pyridyl)-7-azaindole]
Shi-Feng Liu,† Qingguo Wu,† Hartmut L. Schmider,† Hany Aziz,‡ Nan-Xing Hu,‡
Zoran Popovic´,‡ and Suning Wang*,†
Contribution from the Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada,
and Xerox Research Center of Canada, 2660 Speakman DriVe, Mississauga, Ontario L5K 2L1, Canada
ReceiVed December 20, 1999. ReVised Manuscript ReceiVed February 15, 2000
Abstract: Four novel blue/green luminescent compounds, Zn(2-py-in)2(THF) (1), BPh2(2-py-in) (2), Be(2-
py-in)2 (3), and BPh2(2-py-aza) (4), where 2-py-in ) 2-(2-pyridyl)indole and 2-py-aza ) 2-(2-pyridyl)-7-
azaindole, have been synthesized and fully characterized. The 2-py-in ligand and 2-py-aza ligand in the new
compounds are chelated to the central atom. Compounds 2-4 are air stable and readily sublimable, with a
melting point above 250 °C. In the solid state, compounds 1-4 have an emission maximum at λ 488, 516,
490, and 476 nm, respectively. The structures of compounds 2 and 4 are similar. The blue shift of emission
energy displayed by compound 4, in comparison to that of 2, is attributed to the presence of an extra nitrogen
atom in the 2-py-aza ligand as confirmed by ab initio calculations on compounds 2 and 4. Electroluminescent
devices of compounds 3 and 4 were fabricated by using N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPB) as
the hole transporting layer, Alq3 (q ) 8-hydroxyquinolato) as the electron transporting layer, and compound
3 or 4 as the light emitting layer. At 20 mA/cm2 the EL device of 3 has an external efficiency of 1.06 cd/A
while the EL device of 4 has an external efficiency of 2.34 cd/A, demonstrating that compounds 3 and 4 are
efficient and promising emitters in electroluminescent devices.
Introduction
components required for full-color EL displays and are still rare.
We reported recently that boron and aluminum complexes
containing 7-azaindole ligands produce a bright blue lumines-
Luminescent organic/organometallic compounds have at-
tracted much attention recently because of their potential
applications in electroluminescent displays.1-3 Luminescent
chelate complexes have been shown to be particularly useful
in electroluminescent (EL) displays because of their relatively
high stability and volatility.3 The most well-known example of
such chelate compounds is Alq3, q ) deprotonated 8-hydroxy-
quinoline, a green emitter and an electron transporter in organic
EL devices.1,4 We have been interested in blue luminescent
chelate compounds because they are one of the key color
cence.5 Blue electroluminescent devices using B2R2(7-azaindole)2-
(O) (R ) Ph, ethyl) have been fabricated successfully.5a Due
to the geometry of the 7-azaindole ligand, it can only bind to
the central atom as either a terminal ligand or a bridging ligand.5
We anticipated that if we can modify the 7-azaindole ligand so
that it can chelate to a central atom, we could improve the
stability and the performance of the EL devices where 7-aza-
indole based compounds are used as the emitter. The new
ligands we chose to investigate are 2-(2-pyridyl)-7-azaindole
and 2-(2-pyridyl)indole. The 2-(2-pyridyl)indole ligand was
chosen because it has a structure similar to that of 2-(2-pyridyl)-
7-azaindole but can only function as a chelating ligand, thus
providing us an opportunity to do a comparative study on the
effect of bonding modes and electronic factors on luminescence
† Queen’s University.
‡ Xerox Research Center of Canada.
(1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(b) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65,
3611. (c) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.;
Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807. (d)
Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K. Jpn. J.
Appl. Phys. 1993, 32, L514. (e) Bulovic, V.; Gu, G.; Burrows, P. E.; Forrest,
S. R. Nature 1996, 380, 29.
(2) (a) Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys.
1988, 27, L713. (b) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990,
56, 799. (c) Tao, X. T.; Suzuki, H.; Wada, T.; Sasabe, H.; Miyata, S. Appl.
Phys. Lett. 1999, 75, 1655. (d) Shen, Z.; Burrows, P. E.; Bulovic, V.; Borrest,
S. R.; Thompson, M. E. Science 1997, 276, 2009. (e) Aziz, H.; Popovic, Z.
D.; Hu, N.-X.; Hor, A.-M.; Xu, G. Science 1999, 283, 1900.
(3) (a) Hu, N.-X.; Esteghamatian, M.; Xie, S.; Popovic, P.; Ong, B.;
Hor, A.-M.; Wang, S. AdV. Mater 1999, in press. (b) Hamada, Y.; Sano,
T.; Fujii, H.; Nishio, Y.; Takahashi, H.; Shibata, K. Appl. Phys. Lett. 1997,
71, 3338. (c) Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata,
K. Chem. Lett. 1993, 905. (d) Baldo, M. A.; Lamansky, S.; Burrows, P.;
Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 5. (e) Wu, Q.;
Esteghamatian, M.; Hu, N.-X.; Popovic, Z.; Enright, G.; Tao, Y.; D’Iorio,
M.; Wang, S. Chem. Mater. 2000, 12, 79.
(4) Schmidbaur, H.; Lettenbauer, J.; Wilkinson, D. L.; Mu¨ller, G.;
Kumberger, O. Z. Naturforsch. 1991, 46b, 901.
(5) (a) Wu, Q.; Esteghamatian, M.; Hu, N.-X.; Popovic, Z. D.; Enright,
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, 3186-3195.
10.1021/ja9944249 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/29/2000

3672 J. Am. Chem. Soc., Vol. 122, No. 15, 2000
Liu et al.
Synthesis of 1-(Benzenesulfonyl)-2-(2-pyridyl)indole. The same
procedure used for 1-(benzenesulfonyl)-2-(2-pyridyl)-7-azaindole was
employed. From 1-benzenesulfonylindole (3.88 g, 15.1 mmol) and
2-bromopyridine (1.84 g, 11.6 mmol) was obtained 1-(benzenesulfonyl)-
2-(2-pyridyl)indole (3.78 g, 97%). 1H NMR in CDCl3 (δ, ppm, 25
°C): 8.71 (1H, ddd, J ) 4.9, 1.6, 0.9 Hz), 8.22 (1H, dd, J ) 8.3, 0.6
Hz), 7.82 (1H, td, J ) 7.5, 1.7 Hz), 7.74 (1H, d, J ) 7.8 Hz), 7.67
(2H, m), 7.25-7.53 (7H, m), 6.90 (1H, s).
Synthesis of 2-(2-Pyridyl)-7-azaindole. A solution of 1-(benzene-
sulfonyl)-2-(2-pyridyl)-7-azaindole (2.0 g, 5.97 mmol), ethanol (340
mL), and 10% aqueous NaOH (34 mL) was heated at reflux overnight.
The resulting mixture was concentrated, and the residue was dissolved
in CH2Cl2. The organic solution was washed with water and aqueous
Na2CO3, dried, and concentrated. After being run through a chromato-
graphic column using eluant CHCl3/CH3OH, (20:1), the residue afforded
pure 2-(2-pyridyl)-7-azaindole (0.90 g, 77%). Mp 215-216 °C. 1H
NMR in CDCl3 (δ, ppm, 25 °C): 10.90 (1H, br s), 8.69 (1H, ddd, J )
4.8, 1.6, 1.0 Hz), 8.46 (1H, dd, J ) 4.8, 1.6 Hz), 7.98 (1H, dd, J )
7.8, 1.3 Hz), 7.85 (1H, m), 7.76 (1H, td, J ) 7.8, 1.7 Hz), 7.24 (1H,
ddd, J ) 7.8, 4.8, 1.2 Hz), 7.12 (1H, dd, J ) 7.8, 4.8 Hz), 6.99 (1H,
d, J ) 1.8 Hz). Anal. Calcd for C12H9N3: C, 73.83; H, 4.65; N, 21.52.
Found: C, 73.26; H, 4.48; N, 21.58.
of the complex. Three novel complexes with 2-(2-pyridyl)indole
and one new boron compound with 2-(2-pyridyl)-7-azaindole
have been synthesized and characterized structurally. The details
of syntheses, structures, and luminescent and electroluminescent
properties of these new compounds are reported herein.
Experimental Section
All starting materials were purchased from Aldrich Chemical Co.
and used without further purification, unless otherwise stated. Tetrahy-
drofuran, hexane, and toluene solvents were distilled from sodium and
benzophenone under a nitrogen atmosphere. Dichloromethane was
distilled from P2O5 under a nitrogen atmosphere. 1H NMR spectra were
recorded on Bruker Avance 300 MHz spectrometers. Excitation and
emission spectra were recorded on Fluoromax-2, Instruments S.A., Inc.
Elemental analyses were performed by Guelph Chemical Laboratories
Ltd, Guelph, Ontario and Canadian Microanalytical Service, Ltd, Delta,
British Columbia. 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 on a Fisher-Johns
melting point apparatus. The syntheses of 2-(2-pyridyl)-7-azaindole and
2-(2-pyridyl)indole were based on a modified literature method6 for
related compounds.
Synthesis of 1-Benzenesulfonyl-7-azaindole. To a vigorously stirred
mixture of 7-azaindole (5.91 g, 50 mmol), 50% aqueous NaOH (50
mL), water (75 mL) and tetrabutylammonium bromide (1.61 g, 5 mmol)
was added dropwise PhSO2Cl (9.71 g, 55 mmol) in benzene (50 mL)
over an hour at ambient temperature. After the mixture had been stirred
at ambient temperature for 2 h, the aqueous phase was removed. The
organic phase was washed twice with 1 M aqueous NaHCO3 (30 mL),
water (30 mL), and saturated brine (30 mL), and then dried over Na2-
SO4. The solvent was evaporated in a vacuum. The residue was washed
with anhydrous ether to give white powder (11.2 g, yield 87%).
Recrystallization from ethyl acetate and hexane produces a colorless
crystalline product. Mp 127-128 °C. 1H NMR in CDCl3: δ 8.44 (1H,
d, J ) 4.8 Hz), 8.21 (2H, d, J ) 7.4 Hz), 7.86 (1H, d, J ) 7.8 Hz),
7.75 (1H, d, J ) 4.0 Hz), 7.42-7.64 (3H, m), 7.19 (1H, dd, J ) 7.8,
4.8 Hz), 6.61 (1H, d, J ) 4.0 Hz).
Synthesis of 2-(2-Pyridyl)indole. This compound was obtained by
the same procedure as above. From 1-(benzenesulfonyl)-2-(2-pyridyl)-
indole (2.15 g, 6.44 mmol) was obtained 2-(2-pyridyl)indole (1.0 g,
1
80%). Mp 154-155 °C. H NMR in CDCl3 (δ, ppm, 25 °C): 9.70
(1H, br), 8.60 (1H, dd, J ) 4.9, 1.0 Hz), 7.83 (1H, dd, J ) 8.0, 1.0
Hz), 7.44 (1H, td, J ) 7.6, 1.7 Hz), 7.69 (1H, d, J ) 7.9 Hz), 7.42
(1H, dd, J ) 8.0, 0.9 Hz), 7.10-7.31 (3H, m), 7.04 (1H, d, J ) 1.2
Hz).
Zn(2-py-in)2(THF) (1). Butyllithium (1.6 M in hexane, 0.75 mL,
1.2 mmol) was added slowly to 15 mL of THF solution containing
2-(2-pyridyl)indole (0.233 g, 1.2 mmol) at -78 °C under N2. After the
mixture was stirred for half of an hour, zinc chloride (0.50 M in THF,
1.2 mL, 0.6 mmol) was added. The mixture was warmed to room
temperature, stirred for 3 h, concentrated, and recrystallized from CH2-
Cl2. Yellow crystals of 1 (0.220 g, yield 82%) were obtained. Mp 271-
272 °C. 1H NMR in CDCl3 (δ, ppm, 25 °C): 7.98-8.06 (4H, m), 7.80-
7.91 (2H, m), 7.71-7.74 (2H, m), 7.25 (2H, s), 7.00-7.12 (8H, m),
3.75 (m, 4H), 1.75 (m, 4H). Anal. Calcd for C26H18N4Zn: C, 69.11;
H, 4.02; N, 12.40. Found: C, 68.32; H, 4.05; N, 12.05.
Synthesis of 1-Benzenesulfonylindole. The same procedure used
for 1-Benzenesulfonyl-7-azaindole was employed. From indole (5.85
g, 50 mmol) and PhSO2Cl (9.71 g, 55 mmol) 1-benzenesulfonylindole
1
was obtained (9.8 g, yield 76%). Mp 76-78 °C. H NMR in CDCl3
B(2-py-in)Ph2 (2). 2-(2-Pyridyl)indole (0.194 g, 1.0 mmol) was
dissolved in 10 mL of toluene. After the mixture was stirred for 10
min, BPh3 (1.0 mmol, 0.240 g) was added to the solution under N2,
and the mixture was heated for 4 h at 80 °C. After the solution was
cooled to room temperature, the mixture was evaporated to dryness in
a vacuum and moved to a drybox. The mixture was dissolved in THF.
Yellow crystals of compound 2 were obtained in 65% yield. Mp 255
(δ, ppm, 25 °C): 8.03 (1H, d, J ) 8.2 Hz), 7.88-7.94 (2H, m), 7.59
(1H, d, J ) 3.7 Hz), 7.41-7.58 (3H, m), 7.20-7.38 (3H, m), 6.68
(1H, d, J ) 3.7 Hz).
Synthesis of 1-(Benzenesulfonyl)-2-(2-pyridyl)-7-azaindole. To a
solution of 1-benzenesulfonyl-7-azaindole (3.89 g, 15.1 mmol) in 25
mL of anhydrous THF at 0 °C was slowly added a solution of LDA
(1.5 M in cyclohexane, 11.2 mL, 16.6 mmol). The resulting mixture
was stirred for 30 min at this temperature, then a solution of anhydrous
ZnCl2 (0.5 M in THF, 33.2 mL) was added. The mixture was stirred at
room temperature for another 30 min. In a separated flask a solution
of 2-bromopyridine (1.84 g, 11.6 mmol) in 10 mL of anhydrous THF
was added to a solution containing a catalyst prepared by reaction of
a Pd(PPh3)2Cl2 (0.326 g, 0.464 mmol) in 5 mL of anhydrous THF with
diisobutylaluminum hydride (1.0 M in hexane, 0.94 mL, 0.928 mmol),
and the mixture was stirred at ambient temperature for 10 min. The
resulting mixture was transferred via cannula to the solution of
1-(benzenesulfonyl)-2-(7-azaindolyl) zinc chloride prepared above. The
mixture was refluxed for 6 h under N2, cooled to room temperature,
and poured into saturated aqueous Na2CO3. The aqueous phase was
extracted with diethyl ether, and the organic extracts were concentrated
to give a brown residue that was purified by a chromatographic column
using hexane/ethyl acetate (6:1) as the eluant to obtain 1-(benzene-
sulfonyl)-2-(2-pyridyl)-7-azaindole (3.6 g, yield 92%). 1H NMR in
CDCl3 (δ, ppm, 25 °C): 8.75 (1H, ddd, J ) 4.8, 1.6, 0.9 Hz), 8.50
(1H, dd, J ) 4.8, 1.6 Hz), 8.19 (2H, m), 7.81-7.88 (2H, m), 7.72 (1H,
d, J ) 7.8 Hz), 7.36-7.60 (4H, m), 7.22 (1H, dd, J ) 7.8, 4.8 Hz),
6.77 (1H, s).
1
3
°C. H NMR (CDCl3, δ, ppm, 25 °C): 8.39 (1H, d, J ) 5.7 Hz),
3
7.99-7.88 (2H, m), 7.69 (1H, d, J ) 7.8 Hz), 7.28-7.18 (12H, m),
7.10 (1H, s), 7.08-6.69 (2H, m). Anal. Calcd for C25H19N2B: C, 83.82;
H, 5.35; N, 7.82. Found: C, 84.20; H, 5.06; N, 7.87.
Be(2-py-in)2 (3). Butyllithium (1.6 M in hexane, 0.75 mL, 1.2 mmol)
was added slowly to 15 mL of THF solution containing 2-(2-pyridyl)-
indole (0.233 g, 1.2 mmol) at -78 °C under N2. After the mixture was
stirred for half an hour, berylium chloride (0.048 g, 0.6 mmol) was
added. The mixture was warmed to room temperature, stirred for 3 h,
concentrated, and recrystallized from CH2Cl2. Light yellow crystals of
3 (0.160 g, yield 67%) were obtained. Mp >300 °C. 1H NMR in CDCl3
(δ, ppm, 25 °C): 7.88-8.00 (4H, m), 7.70 (2H, dt, J ) 5.4, 1.2 Hz),
7.65-7.70 (2H, m), 7.16 (2H, s), 6.90-7.04 (8H, m). Anal. Calcd for
C26H18N4Be: C, 78.98; H, 4.56; N, 14.17. Found: C, 77.82; H, 4.49;
N, 13.95. This compound was analyzed three times using crystals of
compound 3 at two analytical companies.
BPh2(2-py-aza) (4). Triphenyl boron (0.291 g, 1.2 mmol) was added
to 20 mL of THF solution containing 2-(2-pyridyl)-7-azaindole (0.234
g, 1.2 mmol) under N2. The mixture was stirred at reflux for 4 h. After
the mixture was cooled to room temperature, it was concentrated by
vacuum and transferred to an inert atmosphere drybox. The residue
was recrystallized from CH2Cl2/hexane, yielding colorless crystals of
(6) (a) Sakamoto, T.; Kondo, Y.; Takazawa, N.; Yamanaka, H. J. Chem.
Soc., Perkin Trans. 1996, 1927-1934. (b) Amat, M.; Hadida, S.; Pshen-
ichnyi, G.; Bosch, J. J. Org. Chem. 1997, 62, 3158-3175.
1
compound 4 (0.307 mg, yield 71%). Mp 291-292 °C. H NMR in

New Blue/Green Luminescent Chelate Compounds
J. Am. Chem. Soc., Vol. 122, No. 15, 2000 3673
Table 1. Crystallographic Data
1
2
3
4
formula
fw
space group
a/Å
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
C30H26N4OZn
523.91
P41212
13.872(2)
13.872(2)
15.043(2)
90
90
90
2894.6(8)
4
1.202
C25H19N2B
358.23
P21/n
9.191(5)
15.607(14)
13.510(7)
90
102.347(15)
90
1893(2)
4
1.257
0.73
45.6
C26H18N4Be
395.45
Pnna
23.417(5)
13.460(3)
13.346(3)
90
C24H18N3B
359.22
P212121
8.916(6)
11.965(12)
17.726(9)
90
90
90
1891(2)
4
1.262
90
90
4206.6(16)
8
1.249
0.75
45
Z
Dc, g cm-3
µ, cm-1
8.75
50
2637
0.75
50
3726
o
2θ max
,
reflns measd
2649
2759
reflns used (Rint)
no. of variables
final R [I > 2 σ(I)]
2544 (0.032)
191
2472 (0.10)
253
2759 (0.00)
281
3338(0.025)
253
R1a ) 0.0556,
wR2b ) 0.1425
R1 ) 0.0669,
wR2 ) 0.1550
1.037
R1a ) 0.0864,
wR2b ) 0.1306
R1 ) 0.2354,
wR2 ) 0.1781
1.069
R1a ) 0.0650,
wR2b ) 0.1440
R1 ) 0.1489,
wR2 ) 0.1808
1.069
R1a ) 0.0603,
wR2b ) 0.1173
R1 ) 0.1211,
wR2 ) 0.1449
1.018
R (all data)
goodness-of-fit on F2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w[(Fo - Fc2)2]/∑[w(Fo )2]]1/2. w ) 1/[σ2(Fo ) + (0.075P)2], where P ) [max(Fo , 0) + 2Fc2]/3.
2
2
2
2
CDCl3 (δ, ppm, 25 °C): 8.51 (1H, m), 8.47 (1H, dd, J ) 4.5, 1.6 Hz),
7.98-8.13 (3H, m), 7.20-7.42 (11H, m), 7.10 (1H, s), 7.03, dd, J )
8.0, 4.5 Hz). Anal. Calcd for C24H18N3B: C, 80.24; H, 5.05; N, 11.70.
Found: C, 78.84; H, 5.10; N, 11.45. Again this sample was also
analyzed three times at two different analytical companies using crystals.
The carbon content is however consistently low in all analyses.
Incomplete combustion of carbon in samples 3 and 4 could be the cause
of the low carbon.
1-naphthyl-N,N′-diphenylbenzidine (NPB) was employed as the hole
transport layer in all devices. The device structures and the thickness
of each layer are listed in Table 3. To obtain the photoluminescence
(PL) spectra, a thin film (100 nm) deposited on a quartz substrate was
measured with a fluorescence spectrophotometer. The current-voltage
characteristics were measured using a Keithley 238 current/voltage unit.
The light intensity of the EL device was measured by a Minolta Chroma
Meter CS100. The EL spectrum was obtained by an in-house setup
made up of a series of electronic components including a monochro-
mator (Instruments SA Inc), a photomultiplier tube, and a photon
counter.
X-ray Crystallographic Analysis. All crystals were obtained either
from CH2Cl2/hexane solutions or from THF/hexane solutions. All
crystals were mounted on glass fibers except compound 1, which was
sealed in a glass capillary under nitrogen. All data were collected on a
Siemens P4 single-crystal diffractometer with graphite-monochromated
Mo KR radiation, operating at 50 kV and 35 mA at 23 °C. The data
for 1 and 4 were collected over 2θ 3-50° while the data for 2 and 3
were collected over 2θ 3-45°. Three standard reflections were
measured every 197 reflections. No significant decay was observed
for all samples. Data were processed on a Pentium PC using Siemens
SHELXTL software package7 (version 5.0) and corrected for Lorentz
and polarization effects. Neutral atom scattering factors were taken from
Cromer and Waber.8 The crystals of 3 and 4 belong to the orthorhombic
space groups Pnna and P212121, respectively, uniquely determined by
systematic absences. The crystals of 1 belong to the tetragonal space
group P41212 while the crystals of 2 belong to the monoclinic space
group P21/n, uniquely determined by systematic absences. All structures
were solved by direct methods. The coordinated THF molecule in
compound 1 is disordered. Partial modeling on this THF ligand was
achieved. All non-hydrogen atoms were refined anisotropically, except
the disordered atoms. The positions for all hydrogen atoms except those
attached to the disordered THF molecule were calculated and their
contributions in structural factor calculation were included. The
crystallographic data for compounds 1-4 are given in Table 1.
Results and Discussion
Ligands. Our previous work has established that the depro-
tonated 7-azaindole ligand is an efficient blue emitter when
bound to two central atoms in a bridging mode.5 Although the
bridging mode of the 7-azaindole ligand contributes significantly
to the overall stability of the complex, it often results in the
formation of dinuclear and polynuclear complexes that are
difficult to sublime due to the high molecular weight. We
therefore decided to modify the 7-azaindole ligand so that it
can bind to a central atom by a chelate mode. We attached a
2-pyridyl group at the 2 position of indole and 7-azaindole using
the procedure outlined in Scheme 1, resulting in the formation
of new ligands 2-(2-pyridyl)indole (2-py-in) and 2-(2-pyridyl)-
7-azaindole (2-py-aza). In the synthesis of 2-(2-pyridyl)indole,
the required 2-indolylzinc chloride was easily obtained by
metalation of 1-(benzenesulfonyl)indole with LDA followed by
the treatment of the resulting 2-lithioindole with anhydrous
ZnCl2. Cross-coupling reactions of 2-indolylzinc chloride with
2-bromopyridine in the presence of 2 mol % of a catalyst
prepared from PdCl2(PPh3)2 and DIBAH (DIBAH ) diisobu-
tylaluminum hydride) in refluxing THF afforded the 2-(2-
pyridyl)indole. The 2-(2-pyridyl)-7-azaindole was synthesized
by the same strategy. Unlike the 2-py-aza ligand which has two
potential bonding modes, bridging and chelate, the 2-py-in
ligand has only the chelate bonding mode, making the reaction
simpler. Despite the fact that both compounds have similar
structures, the melting point of 2-py-aza is about 50 °C higher
than that of 2-py-in, the possible cause of which is likely the
difference of hydrogen bonds of these two molecules in the solid
Fabrication of Electroluminescent Devices. The EL devices using
3 and 4 as the emitting layer were fabricated on an indium-tin oxide
(ITO) substrate, which was cleaned by an ultraviolet ozone cleaner
immediately before use. Organic layers and a metal cathode composed
of magnesium silver alloy (Mg0.9Ag0.1) were deposited on the substrate
by conventional vapor vacuum deposition. Prior to the deposition, all
the materials were purified via a train sublimation method.9 N,N′-Di-
(7) SHELXTL crystal structure analysis package, Bruker Axs, Analytical
X-ray System, Madison, Wisconsin, 1995; Version 5.
(8) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch Press: Birmingham, AL, 1974; Vol. 4, Table 2.2A.
(9) Wagner, H. J.; Loutfy, R. O.; Hsiao, C. K. J. Mater. Sci. 1982, 17,
2781.

3674 J. Am. Chem. Soc., Vol. 122, No. 15, 2000
Liu et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg)
Compound 1
Zn(1)-N(2)
Zn(1)-N(1)
Zn(1)-O(1)
N(1)-C(5)
N(1)-C(1)
N(2)-C(6)
N(2)-C(13)
1.968(3)
2.189(4)
2.204(6)
1.330(6)
1.347(6)
1.376(5)
1.376(5)
N(2)-Zn(1)-N(2A)
N(2)-Zn(1)-N(1)
N(2A)-Zn(1)-N(1)
N(1)-Zn(1)-N(1A)
N(2)-Zn(1)-O(1)
N(1)-Zn(1)-O(1)
138.8(2)
79.75(14)
106.96(13)
161.4(2)
110.60(11)
80.69(11)
Compound 2
N(1)-C(1)
N(1)-C(5)
N(1)-B(1)
B(1)-N(2)
B(1)-C(20)
B(1)-C(14)
N(2)-C(6)
N(2)-C(13)
1.329(8)
1.358(8)
1.633(9)
1.561(9)
1.587(10)
1.602(10)
1.369(7)
1.370(8)
N(2)-B(1)-C(20)
N(2)-B(1)-C(14)
C(20)-B(1)-C(14)
N(2)-B(1)-N(1)
C(20)-B(1)-N(1)
C(14)-B(1)-N(1)
115.1(6)
111.2(6)
115.4(7)
93.5(6)
109.1(6)
110.3(6)
Figure 1. A diagram showing the molecular structure of compound 1
with 50% thermal ellipsoids and labeling schemes.
Compound 3
was synthesized first. However, the complex is not stable in
the air, which led us to investigate the synthesis of more stable
beryllium and boron compounds. The three systems investigated
involve using ZnCl2, BeCl2, and BPh3 as the starting materials,
respectively. The reactions of Li[2-py-aza] with ZnCl2 or BeCl2
produced a dark colored solution and no clean products could
be isolated while the reaction of Li[2-py-in] with ZnCl2 or BeCl2
in a 2:1 ratio yielded a bright yellow solution, from which Zn-
(2-py-in)2(THF) (1) or Be(2-py-in)2 (3) was isolated. The reason
for this drastic reaction difference of 2-py-in and 2-py-aza
toward halide compounds has not been well understood. It is
however likely that the 2-py-aza ligand forms several products
with the halides (NMR data indeed suggested this) because of
its versatile bonding modes while the only chelate bonding mode
available to 2-py-in made it possible to produce a clean product.
The reactions of BPh3 with 2-py-in and 2-py-aza produced a
bright yellow compound, BPh2(2-py-in) (2), and a pale yellow
compound, BPh2(2-py-aza) (4), respectively. Compound 1 is
air-sensitive while compounds 2-4 are stable in the air. The
stability difference between compound 1 and 2-4 is attributed
to the fact that the Zn-N(indole) bonds are highly polarized or
ionic in comparison to the corresponding B-N and Be-N bonds
which are mostly covalent, hence more stable. A similar trend
has been observed in aluminum complexes versus boron
complexes when 7-azaindole ligands are involved.5
N(4)-C(26)
N(4)-C(19)
N(4)-Be(2)
N(3)-C(14)
N(3)-C(18)
N(3)-Be(2)
N(2)-C(13)
N(2)-C(6)
N(2)-Be(1)
N(1)-C(5)
N(1)-C(1)
N(1)-Be(1)
1.364(6)
1.373(5)
1.665(5)
1.336(6)
1.359(6)
1.747(6)
1.373(5)
1.387(5)
1.668(5)
1.343(6)
1.347(6)
1.753(6)
N(4A)-Be(2)-N(4)
N(4A)-Be(2)-N(3)
N(4)-Be(2)-N(3)
129.3(6)
111.75(19)
93.7(2)
N(4A)-Be(2)-N(3A) 93.7(2)
N(3)-Be(2)-N(3A)
N(2A)-Be(1)-N(2)
N(2)-Be(1)-N(1A)
N(2)-Be(1)-N(1)
N(1A)-Be(1)-N(1)
119.0(6)
126.1(6)
111.56(17)
93.29(19)
124.1(6)
Compound 4
N(1)-C(13)
N(1)-C(17)
N(1)-B(1)
N(3)-C(23)
N(3)-C(24)
N(2)-C(24)
N(2)-C(18)
N(2)-B(1)
C(7)-B(1)
C(1)-B(1)
1.343(5)
1.354(5)
1.637(5)
1.337(5)
1.344(5)
1.353(5)
1.392(5)
1.550(5)
1.604(5)
1.616(5)
N(2)-B(1)-C(7)
N(2)-B(1)-C(1)
C(7)-B(1)-C(1)
N(2)-B(1)-N(1)
C(7)-B(1)-N(1)
C(1)-B(1)-N(1)
113.4(3)
109.6(3)
118.9(3)
94.8(3)
108.7(3)
108.6(3)
Scheme 1. The Synthesis of Ligands
Another important and common feature among compounds
1-4 is the substantial increase of melting points in comparison
to those of the free ligand. For example, the free ligand 2-(2-
pyridyl)indole has a melting point of 155 °C but the complexes
1-3 have melting points above 250 °C, especially compound
3, which has a melting point greater than 300 °C, a highly
desired property for the practical application in electrolumines-
cent devices. Compounds 2-4 can be sublimed readily under
vacuum. No attempts were made to sublime compound 1 due
to its poor stability. Compounds 1-4 have been fully character-
ized by NMR, elemental, and single-crystal X-ray diffraction
analyses. Selected bond lengths and angles for all compounds
are listed in Table 2.
state. In the solid state, 2-py-aza has a week emission at
λmax ) 420 nm while 2-py-in does not show any visible emission
at all.
Complexes. The complexes of 2-(2-py)indole and 2-(2-py)-
7-azaindole were synthesized by two approaches: (a) substitu-
tion of halide ligands by 2-py-in or 2-py-aza anions produced
by the reactions of butyllithium with the neutral ligand or (b)
the direct reaction of BPh3 with the neutral 2-py-in or 2-py-aza
ligands. A zinc(II) complex with the new ligand, 2-py-indole,
Structure of Compound 1. The structure of compound 1 is
shown in Figure 1. The molecule of 1 has a C2 symmetry while
Table 3. EL Device Data
hole transport
(thickness, nm)
emitting layer
(thickness, nm)
electron transport
(thickness, nm)
eff. at 20 mA/cm2,
cd/A
device
C.I.E. (x, y)
A
B
C
NPB (60)
NPB (60)
NPB (60)/Bicarb (20)
3 (75)
4 (20)
4 (20)
0.306, 0.500
0.279, 0.463
0.200, 0.325
1.06
0.57
2.34
Alq3 (40)
Alq3 (40)

New Blue/Green Luminescent Chelate Compounds
J. Am. Chem. Soc., Vol. 122, No. 15, 2000 3675
Figure 2. A diagram showing the molecular structure of compound 2
with 50% thermal ellipsoids and labeling schemes.
Figure 3. A diagram showing the molecular structure of compound 3
with 50% thermal ellipsoids and labeling schemes.
the crystal is chiral because of the chiral space group P41212.
The structure of 1 can be best described as a distorted trigonal
bipyramid with O(1), N(2), and N(2) on the basal plane (the
sum of bond angles on the basal plane is 358°.). Two 2-(2-py)-
indole ligands are chelated to the Zn(II) center. The shortest
bond is between Zn(1) and the negatively charged indole atom
N(2), 1.968(3) Å, while the longest bond is between Zn(1) and
the disordered THF oxygen atom O(1), 2.204(6) Å. Five-
coordinate Zn(II) complexes are quite common and many
examples have been reported previously.10 The structure of 1
resembles that10c of [Zn(8-aminoquinoline)2(H2O)]2+. Com-
pound 1 loses the coordinate THF molecule readily when
subjected to vacuum. The poor stability of 1 in air is believed
to be caused by the reaction of the negatively charged 2-(2-
py)indole ligand with moisture, resulting in the rupture of Zn-
N(indole) bonds.
Figure 4. A diagram showing the molecular structure of compound 4
with 50% thermal ellipsoids and labeling schemes.
Structure of Compound 2. The structure of compound 2 is
shown in Figure 2. The boron atom in 2 has a typical tetrahedral
geometry. The 2-(2-py)indole ligand is chelated to the boron
center in a similar fashion as in compound 1. The B(1)-N(2)
(indole) bond (1.561(9) Å) is shorter than the B(1)-N(1)
(pyridyl) bond (1.633(9) Å) but similar to those of previously
reported 7-azaindole boron complexes.5 The bond angles around
the boron atom range from 93.5(6)o to 115.4(7)o. The B-C bond
lengths in 2 are also in the normal range.
Structure of Compound 3. There are two independent
molecules of 3 in the asymmetric unit of the crystal lattice, each
of which has a 2-fold rotation symmetry. The bond lengths and
angles of the two independent molecules are similar. Therefore,
only the structure of one of the molecules, Be(1), is shown in
Figure 3. The Be(II) center has a distorted tetrahedral geometry,
as indicated by the bond angles around the Be(II) center, ranging
from 93.7(2)o to 129.3(6)o. Similar tetrahedral structures of
beryllium compounds have been observed previously.3a,11 The
Be-N bond lengths (1.665(5)-1.753(6) Å) are substantially
longer than the corresponding ones in compound 2. Both steric
and electronic factors are believed to be responsible for this
discrepancy. A few four-coordinate electroluminescent Be(II)
complexes have been reported previously.3a-c Compound 3 is
however the first example of electroluminescent Be(II) com-
pounds where the Be(II) center is chelated by nitrogen donor
atoms only. Efforts to make the analogue of 3 using 2-(2-py)-
7-azaindole were unsuccessful.
Figure 5. PL (excitation and emission) spectra of compounds 1 (dashed
line) and 2 (solid line).
Structure of Compound 4. The structure of compound 4 is
similar to that of compound 2 as shown in Figure 4. The B-N
and B-C bond lengths and angles are also similar to those of
2. Despite the fact that the 2-(2-pyridyl)-7-azaindole ligand has
three nitrogen binding sites, only two of the nitrogen atoms N(1)
and N(2) are bound to the boron center in a similar fashion as
in compound 2. The bond lengths and angles around the boron
center in compound 4 are similar to those of 2. Although the
molecular structure of compound 4 resembles that of 2, these
two compounds crystallize in two different space groups:
compound 2 being in the centric, monoclinic space group
P21/n, while compound 4 is in the chiral, orthorhombic space
group P212121. This difference is caused undoubtedly by the
replacement of a CH (C(12)) in 2-(2-pyridyl)indole by a nitrogen
atom (N(3)) in 2-(2-pyridyl)-7-azaindole. It is however surpris-
ing that a subtle difference in the ligands could cause such a
dramatic difference in the packing of molecules 2 and 4 in the
crystal lattice. Even more surprising is the fact that the melting
(10) (a) Prince, R. H. In ComprehensiVe Coordination Chemistry;
Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon Press:
New York, 1987; Vol. 5, Chapter 56.1. (b) Auf der Heyde, T. P. E. Acta
Crystallogr. 1984, B40, 582. (c) Kerr, M. C.; Preston, H. S.; Ammon, H.
L.; Huheey, J. E.; Stewart, J. M. J. Coord. Chem. 1981, 11, 111. (d) Smith,
H. W. Acta Crystallogr. 1975, B31, 2701.
(11) Tamoto, N.; Adachi, C.; Nagai, K. Chem. Mater. 1997, 9, 1077.

3676 J. Am. Chem. Soc., Vol. 122, No. 15, 2000
Liu et al.
Figure 6. Ab inito calculation results on HOMO and LUMO levels of 2 and 4.
point of compound 4 is about 40 °C higher than that of 2.
Although the crystal lattice difference must contribute somewhat
to the melting point difference between 2 and 4, electronic
factors must also contribute to some extent because the nitrogen
atom, more electronegative than the carbon atom, can cause
some difference in crystal lattice energy, thus resulting in
melting point difference. Compound 4 is much more stable
toward air than dinuclear compounds B2R2(O)(7-aza)2 where
the 7-azaindole ligand acts as a bridging ligand.5a,b The enhanced
stability of 4 can be attributed to the chelate bonding mode of
the 2-py-7-azaindole ligand. Similarly, the chelate effect is
believed to contribute to the overall stability of compounds 2
and 3.
Photoluminescent and Electroluminescent Properties of
Compounds 1-4. Compounds 1-4 produce a bright blue/green
color in solution and the solid state upon irradiation by UV
light. The emission maximum of compounds 1-4 is at 488,
516, 490, and 476 nm, respectively. The PL spectra of powder
samples of 1-4 match those of films. The fact that compounds
1-4 have a similar emission band in solution, solid, or films
indicates that the luminescence observed in these compounds
is a molecular property, attributable to a π*-π transition of
the 2-(2-py)indole ligand or the 2-(2-py)-7-azaindole ligand. The
luminescence of compounds 1-4 is in sharp contrast to those
of the free ligands which have either no emission at all in the
visible region (2-(2-py)indole) or a weak emission at 420 nm
(2-(2-py)-7-azaindole). The dramatic red shift of emission energy
of 2-(2-py)-7-azaindole complex 4 in comparison to that of free
2-(2-py)-7-azaindole can be attributed to the removal of the
proton from 2-(2-py)-7-azaindole, which is consistent with the
results of our earlier studies on luminescent di(2-pyridyl)amine
and 7-azaindole complexes that revealed that the deprotonation
of the ligand decreases the π*-π gap, thus resulting in a red
shift in emission energy.5 The role of the central ion in
compounds 1-4 is therefore simply to stabilize the negatively
charged ligand via the formation of coordination bonds.5
Compounds 2 and 4 have a similar structure but the emission
energy of 4 has a ∼40 nm blue shift relative to that of 2. The
replacement of a CH in the indole ligand by a N atom in the
7-azaindole ligand apparently increases the π*-π energy gap,
which is not surprising since the nitrogen atom is more
electronegative than the carbon atom, thus capable of lowering
the HOMO level and increasing the HOMO and LUMO gap.
To rationalize the blue-shift displayed by compound 4 relative
to that of 2, we performed ab initio calculations on the
Restricted-Hartree-Fock (RHF) level using a standard split-
valence polarized (6-31G*) basis set, employing the Gaussian98
suite of programs.12 The geometric parameters employed in the
calculations were from crystal structure data. In Figure 6, we
plotted the energies of the Highest Occupied Molecular Orbitals
(HOMO’s) and Lowest Unoccupied Molecular Orbitals (LU-
MO’s) for compounds 2 and 4, together with surfaces on which
those orbitals attain values of (0.05 atomic units. As one may
see, the HOMO’s of both compounds are π orbitals localized
on the fused ring systems, i.e., the indole portion of 2, and the
7-azaindole part of 4, respectively. Therefore, the energy of the
HOMO stands to be lowered by the replacement of a C-H
(12) 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. D.; Fox, J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6;
Gaussian, Inc.: Pittsburgh, PA, 1998.

New Blue/Green Luminescent Chelate Compounds
J. Am. Chem. Soc., Vol. 122, No. 15, 2000 3677
Figure 7. Top: J-V curve of device A. Bottom: L-J curve of device
A.
Figure 9. Top: J-V curves (empty triangle, device B; dashed line,
device C). Bottom: L-J curve (empty triangle, device B; dashed line,
device C).
Figure 8. PL (empty square) and EL (solid diamond) spectra of
compound 3 and device A.
group in 2 by a more electronegative N in 4. The fact that this
lowering is still rather small might be assigned to the nodal
surfaces near the nitrogen atoms in the HOMO of 4, while the
HOMO in 2 has appreciable values near the nitrogen. On the
other hand, the lowering effect from the nitrogen is considerably
smaller for the LUMO’s that are virtually identical for the two
compounds, and largely localized on the pyridine portion of
the ligand, i.e., away from the indole or 7-azaindole group. The
overall effect is a widening of the gap by a relatively small
amount from 2 to 4. This is consistent with the observation of
a shift in the fluorescence from green in 2 to blue in 4. Although
the energy value of the HOMO-LUMO gap from Hatree-Fock
calculations does not match the experimentally measured
emission energy because only the ground state was taken into
consideration in the calculation, the trend predicted by the
calculation is qualitatively correct.
Figure 10. PL (solid line) and EL (empty triangle, device B; dashed
line, device C).
point (291 °C) and chemical stability. The electroluminescent
properties of 4 were therefore also investigated. The electrolu-
minescent device (A) for compound 3 was fabricated by using
NPB (N,N′-di-1-naphthyl-N,N′-diphenylbenzidine) as the hole
transport layer, 3 as the light emitting layer, and the electron
transport layer. The takeoff voltage of device A is ∼7 V as
shown in Figure 7. The external device efficiency is 1.06 cd/A.
The EL maximum of device A is at 515 nm, approximately
matching that of PL (Figure 8), confirming that the light in
device A is from compound 3. When Alq3 was used as the
electron transport layer, a similar electroluminescence as that
of device A was obtained.
Among compounds 1-3 which contain the same chro-
mophore, i.e., 2-(2-py)indole, 3 has the highest thermal stability
(highest melting point) and chemical stability, thus most
promising for practical applications. The electroluminescent
properties of 3 were therefore investigated. As the only 2-(2-
py)-7-azaindole complex, compound 4 has a fairly high melting
For compound 4 two EL devices (B and C) were fabricated
by using NPB as the hole transport layer, 4 as the light emitting
layer, and Alq3 as the electron transport layer. The takeoff
voltage for both devices is at ∼8 and 9 V, respectively (Figure
9). The EL maximum for device B is at 515 nm, a substantial
shift from that of PL (476 nm) of 4 as shown in Figure 10. The

3678 J. Am. Chem. Soc., Vol. 122, No. 15, 2000
Liu et al.
possible cause for this red shift is the formation of an exciplex
between the NPB layer and the emitting layer. Similar phe-
nomena have been observed previously in EL devices using
boron complexes as the light emitting layer and NPB as the
hole transport layer.3e To eliminate the exciplex, a bicarbazole
layer is added between the NPB layer and the compound 4 layer
in device C. The EL spectrum of device C indeed matches well
the PL of compound 4 with the emission maximum at 490 nm.
Furthermore, as shown in Figure 9, device C is much more
efficient than that of device B (at 20 mA/cm2 the external
efficiency for device B is 0.57 cd/A while device C has an
efficiency of 2.34 cd/A.). The relatively low efficiency of device
B is consistent with the formation of an exciplex, which is
known to reduce the efficiency of the EL device.11
produce a bright blue/green light in electroluminescent devices.
The new boron and beryllium compounds are promising
materials in electroluminescent applications. Efforts to further
improve electroluminescent performances of the new com-
pounds are underway.
Acknowledgment. We thank the Natural Science and
Engineering Research Council of Canada and Xerox Research
Foundation for financial support.
Supporting Information Available: Tables of atomic
coordinates and isotropic thermal parameters, bond lengths and
angles, anisotropic thermal parameters, and hydrogen parameters
for 1-4 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
In summary, four new blue/green luminescent compounds
have been synthesized by using modified indole and 7-azaindole
ligands. Two of these compounds have been demonstrated to
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