Tuning the luminescence and electroluminescence of diphenylboron complexes of 5-substituted 2-(2′-pyridyl)indoles

Article abstract of DOI:10.1021/om020446j

To examine the effect of substituent groups on the luminescence of BPh₂(X-2-PI) complexes, three new air-stable boron complexes BPh₂(F-2-PI) (5a), BPh₂(Cl-2-PI) (5b), and BPh₂(CH₃O-2-PI) (5c) were synthesized and characterized, where F-2-PI = 5-fluoro-2-(2′-pyridyl)indole, Cl-2-PI = 5-chloro-2-(2′-pyridyl)indole, and CH₃O-2-PI = 5-methoxyl-2-(2′-pyridyl)indole. In these complexes, the 5-substituted 2-PI ligand chelates in a tretrahedral fashion to the boron center. Compounds 5a-c are luminescent, with 5a having the highest emission efficiency. Compared with the emission maximum of BPh₂(2-PI) (516 nm), the emission maximum of 5a and 5b is blue-shifted to 490 and 487 nm, respectively, while the emission of 5c is red-shifted to 532 nm, indicating the possibility of tuning the luminescence of these complexes by varying the substituent groups on the 2-PI ligand. An electroluminescent device using compound 5a as the emitter and the electron transport material has been fabricated.

Full text of DOI:10.1021/om020446j

Organometallics 2002, 21, 4743-4749
4743
Tu n in g th e Lu m in escen ce a n d Electr olu m in escen ce of
Dip h en ylbor on Com p lexes of 5-Su bstitu ted
2-(2′-P yr id yl)in d oles
Qinde Liu,† Maria S. Mudadu,‡ Hartmut Schmider,† Randolph Thummel,*,‡
Ye Tao,§ and Suning Wang*,†
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada,
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, and
Institute for Microstructural Sciences, National Research Council, 100 Sussex Drive,
Ottawa, K1A 0R6, Canada
Received J une 4, 2002
To examine the effect of substituent groups on the luminescence of BPh2(X-2-PI) complexes,
three new air-stable boron complexes BPh2(F-2-PI) (5a ), BPh2(Cl-2-PI) (5b), and BPh2(CH3O-
2-PI) (5c) were synthesized and characterized, where F-2-PI ) 5-fluoro-2-(2′-pyridyl)indole,
Cl-2-PI ) 5-chloro-2-(2′-pyridyl)indole, and CH3O-2-PI ) 5-methoxyl-2-(2′-pyridyl)indole. In
these complexes, the 5-substituted 2-PI ligand chelates in a tretrahedral fashion to the boron
center. Compounds 5a -c are luminescent, with 5a having the highest emission efficiency.
Compared with the emission maximum of BPh2(2-PI) (516 nm), the emission maximum of
5a and 5b is blue-shifted to 490 and 487 nm, respectively, while the emission of 5c is red-
shifted to 532 nm, indicating the possibility of tuning the luminescence of these complexes
by varying the substituent groups on the 2-PI ligand. An electroluminescent device using
compound 5a as the emitter and the electron transport material has been fabricated.
In tr od u ction
ticularly useful in electroluminescent (EL) displays
because of their relatively high stability and volatility.2,3
Although blue luminescent materials are one of the key
color components necessary for full-color displays, they
are still scarce. Recently, we reported two blue/green
luminescent boron compounds,5 BPh2(2-PI) (2-PI ) 2-(2′-
pyridyl)indole) and BPh2(2-PazI) (2-PazI ) 2-(2′-py-
ridyl)-7-azaindole). It was found that although the
Luminescent organic/organometallic compounds have
attracted much attention recently due to their potential
applications in electroluminescent displays.1-4 Lumi-
nescent chelate complexes have been shown to be par-
† Queen’s University.
‡ University of Houston.
§ Institute for Microstructural Sciences.
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molecular structures of these two compounds are almost
identical, compared with that of BPh2(2-PI) (λmax ) 516
nm), the emission of BPh2(2-PazI) is ∼40 nm blue-
shifted (λmax ) 476 nm). The reason for this shift is that
replacement of an indole CH by the more electronega-
tive N atom lowers the HOMO level, increasing the
HOMO-LUMO gap, leading to a blue shift of the
emission energy. Earlier molecular orbital calculations
indicated that the HOMO level of BPh2(2-PI) consists
almost entirely of atomic orbitals from the indole ring.5
Hence, a substituent on the indole ring could have a
significant impact on the luminescence of the corre-
sponding boron complex. With the appropriate choice
of a substituent group X, it may be possible to drive the
emission energy of the boron complex more to the blue
region.
(5) Liu, S.-F.; Wu, Q.; Schmider, H. L.; Aziz, H.; Hu, N.-X.; Popovic,
Z.; Wang, S. J . Am. Chem. Soc. 2000, 122, 3671.
10.1021/om020446j CCC: $22.00 © 2002 American Chemical Society
Publication on Web 10/01/2002

4744 Organometallics, Vol. 21, No. 22, 2002
Liu et al.
133.5 (indolyl), 129.5 (d, J C-F ) 10.3 Hz, indolyl), 122.5 (py),
As part of our continuing search for stable blue
emitters, we have examined the effect of three different
substituents in the 5-position of indole on the lumines-
cence of their diphenylboron complexes BPh2(X-2-PI).
Two ligands containing electron-withdrawing fluoro and
chloro groups, 5-fluoro-2-(2′-pyridyl)indole (F-2-PI) and
5-chloro-2-(2′-pyridyl)indole (Cl-2-PI), and one ligand
containing an electron-donating methoxy group, 5-meth-
oxy-2-(2′-pyridyl)indole (CH3O-2-PI), were synthesized,
and the corresponding boron compounds were prepared.
The details of the syntheses, structures, and lumines-
cent and electroluminescent properties of the boron
complexes are reported.
120.3 (py), 112.2 (d, J C-F ) 9.7 Hz, indolyl), 112.1 (d, J C-F
)
26.3 Hz, indolyl), 105.8 (d, J C-F ) 23.2 Hz, indolyl), 100.8 (d,
J C-F ) 4.9 Hz, indolyl). Anal. Calcd for C13H19FN2: C, 73.58;
H, 4.25; N, 13.21. Found: C, 73.30; H, 3.98; N, 13.03.
5-Ch lor o-2-(2′-p yr id yl)in d ole (4b). In the manner de-
scribed for 4a , a mixture of 3b (1.86 g, 7.6 mmol) and PPA
(12.00 g) provided 4b as a white solid, which was recrystallized
1
from absolute EtOH (1.14 g, 65%); mp 173-175 °C. H NMR
(DMSO-d6, δ, ppm): 11.86 (s, 1H), 8.63 (dd, 1H, J ) 4.8, 0.9
Hz), 8.00 (dd, 1H, J ) 8.1, 0.9 Hz), 7.88 (td, 1H, J ) 7.5, 1.8
Hz), 7.62 (d, 1H, J ) 2.1 Hz), 7.48 (d, 1H, J ) 8.1 Hz), 7.34
(ddd, 1H, J ) 8.7, 4.8, 1.5 Hz), 7.13 (s, 1H), 7.11 (dd, 1H, J )
8.7, 2.1 Hz). 13C NMR (CDCl3, δ, ppm): 150.1 (py), 149.3
(indolyl), 138.2 (py), 137.0 (py), 135.1 (indolyl), 130.3 (indolyl),
126.0 (indolyl), 123.7 (py), 122.6 (py), 120.6 (indolyl), 120.2
Exp er im en ta l Section
(indolyl), 112.6 (indolyl), 100.3 (indolyl). Anal. Calcd for C13H19
-
All starting materials were purchased from Aldrich Chemi-
cal Co. except triphenylboron, which was purchased from
Strem Chemical Co. 1H NMR spectra were recorded on a
ClN2: C, 68.42; H, 3.95; N, 12.28. Found: C, 68.07; H, 3.78;
N, 12.17.
5-Meth oxyl-2-(2′-p yr id yl)in d ole (4c). A solution of 3c
(0.70 g, 2.9 mmol) in glacial HOAc (10 mL) was refluxed under
Ar for 28 h. After cooling to room temperature, 10% NaOH
was added until pH 14 was obtained. The mixture was
extracted with CH2Cl2 (3 × 50 mL), and the combined organic
phase was washed with brine (20 mL), dried (MgSO4), and
concentrated. Chromatography on silica gel eluting with
CH3Cl/EtOAc (4:1) provided 4c, which was recrystallized from
benzene/hexane (0.17 g, 11%); mp 128-129 °C. 1H NMR
(DMSO-d6, δ, ppm): 11.53 (s, 1H), 8.60 (d, 1H, J ) 4.8 Hz),
7.95 (d, 1H, J ) 8.1 Hz), 7.84 (td, 1H, J ) 7.5, 1.8 Hz), 7.34 (d,
1H, J ) 8.7 Hz), 7.27 (dd, 1H, J ) 7.2, 5.1 Hz), 7.05 (s, 2H),
6.77 (dd, 1H, J ) 8.7, 2.1 Hz), 3.76 (s, 1H). 13C NMR (CDCl3,
δ, ppm): 154.5 (indolyl), 150.7 (py), 149.1 (indolyl), 137.5 (py),
136.8 (py), 132.4 (indolyl), 129.6 (indolyl), 122.0 (py), 120.0 (py),
114.0 (indolyl), 112.3 (indolyl), 102.6 (indolyl), 100.7 (indolyl),
55.9 (methyl). Anal. Calcd for C13H19N2O: C, 75.00; H, 5.36;
N, 12.50. Found: C, 74.87; H, 5.27; N, 12.34.
1
Bruker Advance spectrometer at 300 MHz for H and 75 MHz
for 13C. Elemental analyses of C, H, and N were performed by
Canadian Microanalytical Service, Ltd, Delta, British Colum-
bia, and QTI, Rt 22 East, Whitehouse, NJ . Excitation and
emission spectra were obtained with a Photon Technologies
International QuantaMaster Model 2 spectrometer. UV-vis
spectra were recorded on a Hewlett-Packard 8562A diode array
spectrophotometer. The syntheses of boron compounds were
carried out under a nitrogen atmosphere.
4′-F lu or op h en ylh yd r a zon e of 2-Acetylp yr id in e (3a ). A
mixture of 2-acetylpyridine (0.61 g, 5.0 mmol), 4-fluorophen-
ylhydrazine hydrochloride (0.97 g, 6.0 mmol), and glacial AcOH
(2 drops) in absolute EtOH (10 mL) was refluxed for 3 h. After
cooling to room temperature, the precipitate was filtered to
provide 3a as an orange solid (1.12 g, 97%); mp 252-254 °C.
1H NMR (DMSO-d6, δ, ppm): 10.43 (s, 1H), 8.70 (d, 1H, J )
5.7 Hz), 8.40 (t, 1H, J ) 7.5 Hz), 8.23 (d, 1H, J ) 8.1 Hz), 7.74
(t, 1H, J ) 6.0 Hz), 7.59-7.63 (m, 2H), 7.14 (td, 2H, J ) 9.0,
0.6 Hz), 2.39 (s, 3H).
4′-Ch lor op h en ylh yd r a zon e of 2-Acetylp yr id in e (3b). In
the manner described for 3a , a mixture of 2-acetylpyridine
(0.61 g, 5.0 mol), 4-chlorophenylhydrazine hydrochloride (1.18
g, 6.0 mmol), and glacial AcOH (2 drops) in absolute EtOH (8
mL) provided 3b as an orange solid (1.16 g, 95%); mp 243-
246 °C. 1H NMR (DMSO-d6, δ, ppm): 10.39 (s, 1H), 8.72 (d,
1H, J ) 5.4 Hz), 8.37 (t, 1H, J ) 7.5 Hz), 8.25 (d, 1H, J ) 8.4
Hz), 7.74 (t, 1H, J ) 6.9 Hz), 7.57 (d, 2H, J ) 9.0 Hz), 7.36 (d,
2H, J ) 8.7 Hz), 2.40 (s, 3H).
BP h 2(F -2-P I) (5a ). A mixture of 4a (50 mg, 0.24 mmol) and
triphenylboron (57 mg, 0.24 mmol) was dissolved in dry
toluene (pretreated with P2O5 and freshly distilled). The
solution was refluxed for 6 h under nitrogen. After removing
the solvent under vacuum, the residue was dissolved in THF/
toluene (1:3) and the solution was allowed to stand at room
temperature under nitrogen. Slow evaporation of solvent gave
yellowish green crystals of 5a after 2 weeks (52 mg, 58%), mp
290-292 °C. 1H NMR (CD2Cl2, δ, ppm): 8.46 (dt, 1H, J ) 5.8,
1.1 Hz), 8.10 (ddd, 1H, J ) 8.1, 7.3, 1.4 Hz), 8.02 (dt, 1H, J )
8.1, 1.1 Hz), 7.38 (m, 3H), 7.27 (m, 9H), 7.19 (tt, 1H, J ) 4.7,
0.5 Hz), 7.16 (s, 1H), 6.87 (td, 1H, J ) 9.0, 2.6 Hz). 13C NMR
(CD2Cl2, δ, ppm): 158.6 (J C-F ) 234.2 Hz, indolyl), 149.8
(indolyl), 143.2 (py), 142.2 (py), 138.9 (ph), 136.4 (indolyl), 133.8
(ph), 133.3 (J C-F ) 10.5 Hz, indolyl), 128.3 (ph), 127.6 (ph),
122.7 (py), 119.7 (py), 115.1 (J C-F ) 9.8 Hz, indolyl), 112.7 (J C-F
4′-Meth oxyp h en ylh yd r a zon e of 2-Acetylp yr id in e (3c).
In the manner described for 3a , a mixture of 2-acetylpyridine
(0.97 g, 8.0 mol), 4-methoxyphenylhydrazine hydrochloride
(1.68 g, 9.6 mmol), and glacial AcOH (2 drops) in absolute
EtOH (11 mL) provided 3c as an orange solid (1.76 g, 91%);
1
mp 218-220 °C. H NMR (DMSO-d6, δ, ppm): 10.34 (s, 1H),
) 26.8 Hz, indolyl), 106.7 (J C-F ) 23.2, indolyl), 99.0 (J C-F
)
8.67 (d, 1H, J ) 5.4 Hz), 8.41 (t, 1H, J ) 7.8 Hz), 8.21 (d, 1H,
J ) 8.4 Hz), 7.73 (t, 1H, J ) 6.3 Hz), 7.57 (d, 2H, J ) 7.5 Hz),
6.89 (d, 2H, J ) 8.7 Hz), 3.73 (s, 3H), 2.37 (s, 3H).
5.8 Hz, indolyl) (one quaternary carbon hidden). Anal. Calcd
for C25H18BFN2: C, 79.83; H, 4.79; N, 7.45. Found: C, 79.72;
H, 4.79; N, 7.44.
5-F lu or o-2-(2′-p yr id yl)in d ole (4a ). A mixture of 3a (0.61
g, 2.6 mmol) and polyphophoric acid (PPA, 4.13 g) was
mechanically stirred at 120 °C for 3 h. After cooling to room
temperature, 10% NaOH was added until pH 14 was obtained.
The mixture was extracted with CH2Cl2 (3 × 30 mL), and the
combined organic phase was washed with brine (20 mL), dried
(MgSO4), and concentrated. Chromatography on silica gel,
eluting with EtOAc, followed by sublimation (115-120 °C, 0.5
mmHg) provided 4a (0.22 g, 39%) as a white solid; mp 147-
148 °C. 1H NMR (DMSO-d6, δ, ppm): 9.76 (s, 1H), 7.78 (dd,
1H, J ) 4.8, 0.9 Hz), 7.78 (d, 1H, J ) 7.5 Hz), 7.73 (t, 1H, J )
7.2 Hz), 7.25-7.31 (m, 2H), 7.19 (t, 1H, J ) 6.0 Hz), 6.92-
6.99 (m, 2H). 13C NMR (CDCl3, δ, ppm): 158.3 (d, J C-F ) 233.2
Hz, indolyl), 150.4 (py), 149.3 (indolyl), 138.6 (py), 137.0 (py),
BP h 2(Cl-2-P I) (5b). In the manner described for 5a , a
mixture of 4b (50 mg, 0.21 mmol) and triphenylboron (47 mg,
0.21 mmol) provided 5b as yellowish green crystals (41 mg,
50%), mp 297-298 °C. 1H NMR (CD2Cl2, δ, ppm): 8.47 (dt,
1H, J ) 5.9, 1.2 Hz), 8.10 (ddd, 1H, J ) 8.2, 7.3, 1.4 Hz), 8.02
(dt, 1H, J ) 8.1, 1.2 Hz), 7.72 (dd, 1H, J ) 2.0, 0.4 Hz), 7.39
(ddd, 1H, J ) 7.3, 5.9, 1.4 Hz), 7.27 (m, 10H), 7.19(dt, 1H, J )
8.8, 0.7 Hz), 7.14 (d, 1H, J ) 0.8 Hz), 7.05 (dd, 1H, J ) 8.8,
2.1 Hz). 13C NMR (CD2Cl2, δ, ppm): 149.4 (indolyl), 143.0 (py),
142.0 (py), 138.4 (ph), 137.6 (indolyl), 134.0 (indolyl), 133.5
(ph), 128.1 (ph), 127.4 (ph), 125.6 (indolyl), 123.8 (py), 122.6
(py), 121.6 (indolyl), 119.5 (indolyl), 115.1 (indolyl), 98.3
(indolyl) (one quaternary carbon hidden). Anal. Calcd for

5-Substituted 2-(2′-Pyridyl)indoles
Organometallics, Vol. 21, No. 22, 2002 4745
Ta ble 1. Da ta Collection a n d P r ocessin g
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
P a r a m eter s for 5a -c
(d eg) for 5a -c a n d BP h 2(2-p y-in )2
5a
5b
5c
Complex 5a
B(1)-N(2)
B(1)-C(1)
B(1)-C(7)
B(1)-N(1)
C(24)-F(1)
1.5538(15)
1.6072(16)
1.6165(17)
1.6326(16)
1.3686(14)
N(2)-B(1)-C(1)
N(2)-B(1)-C(7)
C(1)-B(1)-C(7)
N(2)-B(1)-N(1)
C(1)-B(1)-N(1)
C(7)-B(1)-N(1)
112.91(9)
111.51(9)
117.76(10)
94.34(8)
109.09(9)
108.58(9)
formula
fw
space group
a/Å
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
C
25H18BFN2
C
25H18BClN2
C
26H21BN2O
376.22
C2/c
18.486(3)
11.5197(18)
18.403(3)
90
95.083(3)
90
3903.8(11)
8
392.67
C2/c
18.383(5)
11.632(3)
18.799(5)
90
98.814(5)
90
3972.3(18)
8
388.26
P1h
7.2396(14)
11.480(2)
12.598(3)
84.200(4)
76.205(4)
78.795(4)
995.9(3)
2
Complex 5b
B(1)-N(2)
B(1)-C(1)
B(1)-C(7)
B(1)-N(1)
C(24)-Cl(1)
1.551(3)
1.596(4)
1.598(4)
1.636(3)
1.744(2)
N(2)-B(1)-C(1)
N(2)-B(1)-C(7)
C(1)-B(1)-C(7)
N(2)-B(1)-N(1)
C(1)-B(1)-N(1)
C(7)-B(1)-N(1)
112.69(19)
112.2(2)
117.7(2)
93.93(17)
108.50(19)
109.06(19)
Z
Dc/g cm-3
µ/cm-1
2θmax/deg
1.280
0.81
56.6
1.313
2.06
56.6
14066
1.295
0.78
56.6
7226
BPh2(2-py-in)2
no. of reflns measd 13878
no. of reflns used
(Rint
B(1)-N(2)
B(1)-C(1)
B(1)-C(7)
B(1)-N(1)
1.561(9)
1.602(10)
1.587(10)
1.633(9)
N(2)-B(1)-C(1)
N(2)-B(1)-C(7)
C(1)-B(1)-C(7)
N(2)-B(1)-N(1)
C(1)-B(1)-N(1)
C(7)-B(1)-N(1)
111.2(6)
115.1(6)
115.4(7)
93.5(6)
110.3(6)
109.1(6)
4663 (0.0160) 4776 (0.0542) 4554 (0.0257)
)
no. of variables
final R [I > 2σ(I)]
R1a
334
334
343
0.0376
0.1002
0.0523
0.1112
0.0469
0.0658
wR2b
R (all data)
R1
wR2
Complex 5c
0.0609
0.1100
1.010
0.1370
0.1336
0.813
0.1439
0.0792
0.744
B(1)-N(2)
B(1)-C(1)
B(1)-C(7)
B(1)-N(1)
C(24)-O(1)
1.568(2)
1.602(3)
1.615(3)
1.639(3)
1.408(2)
N(2)-B(1)-C(1)
N(2)-B(1)-C(7)
C(1)-B(1)-C(7)
N(2)-B(1)-N(1)
C(1)-B(1)-N(1)
C(7)-B(1)-N(1)
113.23(16)
113.17(16)
114.99(17)
94.22(14)
110.28(16)
108.92(15)
goodness of fit
on F2
C
25H18BClN2: C, 76.48; H, 4.59; N, 7.14. Found: C, 76.32; H,
4.64; N, 7.03.
BP h 2(CH3O-2-P I) (5c). In the same manner as described
for 5a , a mixture of 4c (50 mg, 0.22 mmol) and triphenylboron
(54 mg, 0.22 mmol) provided 5c as yellow crystals (42 mg,
49%), mp 251-253 °C. 1H NMR (CD2Cl2, δ, ppm): 8.42 (dt,
1H, J ) 5.9, 1.0 Hz), 8.08 (ddd, 1H, J ) 8.2, 7.3, 1.4 Hz), 7.97
(dt, 1H, J ) 8.1, 1.1 Hz), 7.32 (ddd, 1H, J ) 7.2, 5.9, 1.3 Hz),
7.26 (m, 10H), 7.16 (d, 1H, J ) 2.4 Hz), 7.13 (dd, 1H, J ) 9.0,
0.5 Hz), 7.10 (d, 1H, J ) 0.7 Hz), 6.77 (dd, 1H, J ) 8.9, 2.5
Hz), 3.85 (s, 3H). 13C NMR (CD2Cl2, δ, ppm): 154.9 (indolyl),
150.1 (indolyl), 143.1 (py), 142.0 (py), 137.7 (ph), 135.4 (indolyl),
133.8 (ph), 133.7 (indolyl), 128.3 (ph), 127.5 (ph), 122.0 (py),
119.3 (py), 115.4 (indolyl), 115.1 (indolyl), 103.0 (indolyl), 98.6
(indolyl), 56.2 (methyl) (one quaternary carbon hidden). Anal.
Calcd for C26H21BN2O: C, 80.45; H, 5.42; N, 7.22. Found: C,
80.28; H, 5.47; N, 7.22.
X-r a y Cr ysta llogr a p h ic An a lysis. Crystals were obtained
from THF/toluene solutions and were mounted on glass fibers.
Data were collected on a Siemens P4 single-crystal X-ray
diffractometer with a CCD-1000 detector and graphite-mono-
chromated Mo KR radiation, operating at 50 kV and 30 mA
at 25 °C. The data collection ranges over the 2θ range are
4.18-56.6° for 5a , 4.38-56.6° for 5b, and 3.34-56.7° for 5c.
No significant decay was observed for all samples. Data were
processed on a PC using the Bruker SHELXTL software
package6 (version 5.10) and are corrected for Lorentz and
polarization effects. The crystal structures of 5a ,b are isomor-
phous and belong to the monoclinic space group C2/c. The
crystals of 5c belong to triclinic space group P1h. All structures
were solved by direct methods. All non-hydrogen atoms were
refined anisotropically. All aromatic hydrogen atoms were
located directly from difference Fourier maps. The hydrogen
atoms of the methyl group in 5c were calculated, and their
contribution were included. The crystallographic data for
compounds 5a -c are given in Table 1. Selected bond lengths
and angles for 5a -c are listed in Table 2.
substrate. Organic layers and the metal cathode layer were
deposited on the substrate by conventional vapor vacuum
deposition. Prior to the deposition, all organic materials were
purified via a train sublimation method.7 N,N′-Di-1-naphthyl-
N,N′-diphenylbenzidine (NPB) was employed as the hole
transport layer in the device. The device stucture can be
expressed as ITO/NPB(50 nm)/BPh2(F-2-PI)(50 nm)/Al(150
nm)/Ag(80 nm).
Qu a n tu m Yield Mea su r em en ts. Quantum yields of the
complexes were determined relative to 9,10-diphenylan-
thracene in CH2Cl2 at 298 K (Φr ) 0.95).8 The absorbances of
all the samples and the standard at the excitation wavelength
were approximately 0.088-0.108. The quantum yields were
calculated using previously known procedures.9 The excitation
wavelength is 376 nm for compounds 5a ,b and BPh2(2-PI) and
396 nm for compound 5c. The emission of the standard was
measured with both 376 and 396 nm excitation, showing
aborbances of 0.106 and 0.0933, respectively.
Resu lts a n d Discu ssion
Syn th eses a n d Str u ctu r es. The 2-PI derivatives
4a -c were prepared by the two-step Fischer synthe-
sis according to a modified procedure reported previ-
ously.10 Treatment of 2-acetylpyridine (1) and the ap-
propriate phenylhydrazines 2a -c afforded the corre-
sponding phenylhydrazones 3a -c in nearly quantitative
yields. Polyphosphoric acid (PPA) is often an excellent
acidic medium for the subsequent Fischer cyclization.
The reaction of 3a ,b in PPA at 100-120 °C furnishes
the corresponding PI derivatives 4a ,b in 39 and 65%
yield, respectively. However, the cyclization of 3c in PPA
(7) Wagner, H. J .; Loutfy, R. O.; Hsiao, C. K. J . Mater. Sci. 1982,
17, 2781.
F a br ica tion of Electr olu m in escen t Devices. The EL
device using 5a as the emitting layer and the electron
transport layer was fabricated on an indium-tin oxide (ITO)
(8) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993.
(9) Demas, N. J .; Crosby, G. A. J . Am. Chem. Soc. 1970, 92, 7262.
(10) (a) Wu, F.; Chamchoumis, C. M.; Thummel, R. P. Inorg. Chem.
2000, 39, 584. (b) Wu, F.; Hardesty, J .; Thummel, R. P. J . Org. Chem.
1998, 63, 4055. (c) Thummel, R. P.; Hegde, V. J . Org. Chem. 1989, 54,
1720.
(6) SHELXTL NT, Crystal Structure Analysis Package, Version
5.10; Bruker Axs, Analytical X-ray System: Madison, WI, 1999.

4746 Organometallics, Vol. 21, No. 22, 2002
Liu et al.
F igu r e 1. Molecular structure of 5a with 50% thermal
ellipsoids and labeling schemes.
F igu r e 2. Molecular structure of 5c with 50% thermal
ellipsoids and labeling schemes.
Sch em e 1
are occupied by the two phenyl groups. The B(1)-N(2)
indole bond (1.5538(15) Å) is shorter than the
B(1)-N(1) pyridyl bond (1.6326(16) Å), which is consis-
tent with the fact that the indole nitrogen is anionic and
hence a better donor than the neutral pyridyl nitrogen.
The B-C and C-F bond lengths in 5a are also in the
normal range. The structure of compound 5b is almost
the same as that of compound 5a except that the unit
cell volume of 5b (3972.3(18) Å3) is slightly larger than
that of 5a (3903.8(11) Å3), which could be due to the
relatively large chlorine atom and the longer C-Cl bond
length (1.774(2) Å) in 5b, as compared to the C-F bond
length (1.3686(14) Å) in 5a . Except for this difference,
the B-N and B-C bond lengths and bond angles in
compound 5b are quite similar to the corresponding
bond lengths and angles in compound 5a . The structural
parameters of 5a and 5b are also similar to those
reported5 previously for BPh2(2-PI).
at 90-100 °C affords a complex mixture of products.
Alternatively 3c may be cyclized under milder condi-
tions (refluxing HOAc for 18 h) to provide 4c in 11%
yield along with unreacted 3c and its hydrolysis prod-
ucts.
The structure of compound 5c is similar to those of
5a and 5b, as shown in Figure 2. Although the B-N(2)
indole bond length (1.568(2) Å) is still shorter than the
B-N(1) pyridyl bond length (1.639(3) Å) in compound
5c, it is somewhat longer than the corresponding B-N
indole bond length in compounds 5a and 5b. In contrast,
the B-N pyridyl bond length in 5c is similar to the
corresponding bond length in 5a and 5b, indicating that
the electron-withdrawing or -donating character of the
substituent at the 5-position of the indolyl group can
affect the bond length of the B-N(2) indole bond but
does not make a significant difference for the bond
length of the B-N(1) pyridyl bond. Electron-withdraw-
ing groups tend to make the bond shorter, and an
electron-donating group tends to make the bond some-
what longer. Although the molecular structure of com-
pound 5c resembles those of 5a and 5b, they crystallize
in two different space groups: 5a and 5b belong to the
monoclinic space group C2/c, while 5c belongs to the
triclinic space group P1h, and both are different from
that5 of BPh2(2-PI). This difference is clearly caused by
the size of the substituent at the 5-position of the indole
ligand. It was observed previously that the subtle
difference between BPh2(2-PI) and BPh2(2-PazI) causes
a dramatic difference in thermal properties and crystal
packing of the two compounds.5 The different space
groups and different thermal behavior of 5a -c and
BPh2(2-PI) demonstrate this fact again.
The different cyclization behavior of the phenylhy-
drazones 3a -c can be explained by considering the
mechanism of the Fischer reaction. The key step is a
[3,3]-sigmatropic rearrangement which is polarized in
such a way that the intermediate diene is stabilized by
an electron-withdrawing group in the phenyl ring.
Therefore, the intermediate involved in the reaction of
the methoxy-substituted 4c is comparatively destabi-
lized and thus more reactive.
The three boron complexes 5a -c were all obtained
in good yield by the reaction in a 1:1 ratio of triphenyl-
boron with the corresponding ligand in toluene. Single
crystals were obtained by recrystallization of the com-
plexes from THF/toluene (1:3). Complexes 5a -c were
1
characterized by H and 13C NMR, elemental analysis,
and single-crystal X-ray diffraction. All the complexes
are air-stable both in the solid state and in solution. An
important and common feature of 5a -c is their high
melting points. The melting points for 5a and 5b are
similar, in the range 290-300 °C, about 40 °C higher
than that of 5c and the parent complex BPh2(2-PI).
Single-crystal X-ray diffraction revealed that com-
pounds 5a and 5b are isomorphous. The structure of
compound 5a is shown in Figure 1. The boron atom in
5a has a typical tetrahedral geometry. The F-2-PI ligand
is bidentately chelated to the boron center via its
pyridine and indole nitrogens. The other two boron sites

5-Substituted 2-(2′-Pyridyl)indoles
Organometallics, Vol. 21, No. 22, 2002 4747
the emission of complex 5c with an electron-donating
substituent is red-shifted by 16 nm to 532 nm. The free
ligands display a similar trend of emission energy
change. These results are consistent with our earlier
ab initio molecular orbital calculations5 on BPh2(2-PI),
which showed that the HOMO is a π-orbital with atomic
orbital contributions almost exclusively from the indole
moiety, while the LUMO is a π*-orbital with atomic
orbital contributions mostly from the pyridyl ring. Con-
sequently, the electron-withdrawing fluorine or chlorine
on the indole ring stabilizes or decreases the HOMO
π-level,13 hence increasing the LUMO(π*)-HOMO(π)
energy gap, while the electron-donating methoxy group
destabilizes or increases the HOMO (π) energy level,13
thus decreasing the LUMO(π*)-HOMO(π) energy gap
and resulting in a red shift of the emission energy. The
∼30 nm blue shift of the BPh2(2-PazI) emission relative
to BPh2(2-PI), reported earlier by our group, is another
example of the effect of an electronegative element
(nitrogen) on the HOMO energy level.5
Fluorine is more electronegative than chlorine, and
hence one would expect a greater blue shift of the
emission energy for complex 5a than for 5b. The cause
for the similar emission energies of 5a and 5b and the
free ligands 4a and 4b is likely related to the interac-
tions of the lone pair of the fluoro and chloro group with
the π-orbitals of the indole ring, which can push the
HOMO level up, hence decreasing the band gap. To
confirm the involvement of lone-pair electrons, we
performed ab initio molecular orbital calculations for
compounds 5a and 5b. The geometric parameters from
X-ray diffraction analysis were used for the calculation.
The calculations were performed in a cc-pVDZ (correla-
tion consistent double-ú valence basis14) on an RHF
(restricted Hartree-Fock) level of computation. The
Gaussian suite of programs15 was employed. The orbital
diagrams were generated with the Molekel program.16
All contour values are (0.03 au. As shown in Figure 4,
the HOMO, second HOMO, LUMO, and second LUMO
are all π-type orbitals of the X-2-PI ligand and are very
F igu r e 3. Top: Emission spectra of the free ligands 4a
(solid line), 4b (empty triangles), and 4c (empty circles).
Bottom: Emission spectra of 5a (solid line), 5b (crosses),
and 5c (empty squares) in CH2Cl2 solution.
P h otolu m in escen t (P L) a n d Electr olu m in escen t
(EL) P r op er ties of Com p ou n d s 5a -c. Upon irradia-
tion by UV light, compounds 5a -c produce a bright
blue/green color both in solution and in the solid state.
As shown in Figure 3, the emission maximum of
compounds 5a -c in CH2Cl2 solution is at 490, 487, and
532 nm, respectively. These emissions are attributed to
a π* f π transition of the substituted 2-PI ligands, on
the basis of previous ab initio molecular orbital calcula-
tions on BPh2(2-PI) and BPh2(2-PazI). The free ligands
4a -c all display weak UV/blue emissions. The emission
maximum of the free ligands 4a and 4b is very similar,
λmax ) 373 nm. In contrast, the free ligand 4c emits at
λmax ) 411 nm. The red shift of the emission energy of
the complexes as compared to that of the free ligands
is due to the deprotonation of the free ligand, which
causes a narrowing of the band gap. This observation
is consistent with our previous studies on luminescent
di(2-pyridyl)amine and 7-azaindole complexes, where
the deprotonated ligands display a dramatic red shift
in emission energy as compared to the neutral ligands
due to narrowing of the π*-π gap.11
(11) (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.
(12) (a) Drago, R. S. Physical Methods in Chemistry; W. B. Saunders
Company: Philadelphia, 1977; Chapter 5. (b) Masetti, F.; Mazzucato,
U.; Galiazzo, G. J . Lumin. 1971, 4, 8. (c) Werner, T. C.; Hawkins, W.;
Facci, J .; Torrisi, R.; Trembath, T. J . Phys. Chem. 1978, 82, 298. (d)
Bluemer, G. P.; Zander, M.; Ruetgerswerke, A. G. Z. Naturforsch. A
1979, 34A, 909.
(13) Carey, F. A.; Sundberg, R. J . Advanced Organic Chemistry, Part
A: Structure and Mechanisms, 3rd ed.; Plenum Press: New York, 1990;
Chapter 10.
(14) Wong, D. E.; Dunning, T. H., J r. J . Chem. Phys. 1993, 98, 1358.
(15) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; 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.; J ohnson, 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.: Pitts-
burgh, PA, 1998.
We have reported previously that BPh2(2-PI) has a
bright green luminescence at 516 nm both in solution
and in the solid state. Compared with BPh2(2-PI), the
emission of complexes 5a ,b, with electron-withdrawing
substituents, is blue-shifted by 25 nm to 490 nm, and
(16) Flkiger, P.; Lthi, H. P.; Portmann, S.; Weber, J . MOLEKEL,
4.1; Swiss Center for Scientific Computing: Manno (Switzerland),
2000-2001.

4748 Organometallics, Vol. 21, No. 22, 2002
Liu et al.
Ta ble 3. P h otolu m in escen ce Da ta for Com p lexes
5a -c a n d BP h 2(2-P I)2
emission
emission
λmax (nm) λmax (nm)
excitation
wavelength
(nm)
CH2Cl2,
298 K
solid,
quantum
yieldsa
complex
5a
298 K
490
487
516
532
503
500
516
530
376
376
376
396
0.33
0.22
0.29
0.036
5b
BPh2(2-PI)2
5c
a
Relative to 9,10-diphenylanthracene in CH2Cl2 at 298 K.
F igu r e 5. Diagram showing the EL device structure of
5a .
F igu r e 4. HOMO, second HOMO, LUMO, and second
LUMO of compounds 5a and 5b.
similar for both compounds in terms of appearance and
energy. The HOMO and second HOMO are near degen-
erate. Likewise, the LUMO and second LUMO are also
near degenerate. The HOMO and second HOMO for
both molecules consist of mostly atomic orbitals of the
indolyl ring. The lone-pair orbital of the halogen atom
has a significant contribution in the second HOMO. The
LUMO and second LUMO consist of atomic orbitals
from both pyridyl and indolyl rings with a dominant
pyridyl contribution. Perhaps, because the lone-pair
orbital of the fluorine atom can overlap relatively
efficiently with the π-orbitals of the indole ring due to
the compatible size and energy of 2p-orbitals from
carbon and fluorine atoms, compared to that of the
chlorine atom, the band gap increase due to the more
electronegative nature of the fluorine atom is partially
compensated, thus leading to the similar band gap for
compounds 5a and 5b.
In the solid state, the emission of 5a and 5b is found
at λ ) 503 and 500 nm, respectively. This slight red shift
may be caused by intermolecular interactions in the
solid state. There is almost no shift of the solid state
emission for complex 5c and BPh2(2-PI), which is found
at 516 and 530 nm, respectively. Substituent effects on
luminescence are well documented for a variety of
compounds.17 However, no systematic study on the
luminescence of boron compounds has been reported.
F igu r e 6. PL spectrum of 5a (solid circles) and EL
spectrum of the device (solid line).
Emission quantum yields were measured for com-
pounds 5a -c and BPh2(2-PI). Among all the com-
plexes, compound 5a has the highest quantum yield
(0.32), slightly higher than that of BPh2(2-PI) (0.29).
The quantum yield of compound 5b (0.22) is lower than
that of BPh2(2-PI), which is due to the heavy-atom
effect12 of the chlorine substituent, which leads to a
slight luminescence quenching. Compound 5c has a
low quantum yield (0.036), which is most likely due to
the thermal motion of the conformationally mobile
methoxy group, which results in energy loss via non-
radiative decay. The photoluminescent properties of
compounds 5a -c and BPh2(2-PI) are summarized in
Table 3.
Compounds 5a and 5b are suitable candidates as
emitters for EL devices because of their high emission
efficiency, high melting points, high volatility, and good
chemical stability. Since compound 5a has the highest
emission quantum yield, and its emission energy was
blue-shifted to blue/green color in comparison with
BPh2(2-PI), an EL device was fabricated with compound
5a as the emitting layer and the electron transport
layer. The EL device (Figure 5) consists of two layers, a
hole transport layer made of NPB and an emitting and
electron transport layer made of compound 5a . The EL
device emits blue-green light with an emission maxi-
(17) (a) Madej, S. L.; Okajima, S.; Lim, E. C. J . Chem. Phys. 1976,
65, 1219. (b) King, L. A. J . Chem. Soc., Perkin Trans. 1977, 7, 919. (c)
Tine, A.; Aaron, J . J . Can. J . Spectrosc. 1984, 29, 121. (d) Hotchandani,
S.; Testa, A. C. J . Lumin. 1994, 59, 59.

5-Substituted 2-(2′-Pyridyl)indoles
Organometallics, Vol. 21, No. 22, 2002 4749
shows that compound 5a is capable of functioning as
an emitter and an electron transport material as well.
The EL device is fairly bright, as shown by the lumi-
nance of 141 cd/m2 at 20 mA/cm2 and the external
efficiency of 0.82 cd/A at 20 mA/cm2.
In summary, three new blue/green luminescent boron
complexes based on 5-substituted 2-PI ligands have
been synthesized and characterized. These compounds
demonstrate that the emission of 2-PI boron chelate
complexes can be tuned by using appropriate substitu-
ent groups. An electron-withdrawing group such as
fluoride and chloride causes a blue shift of the emission
energy, while an electron-donating group such as meth-
oxy causes a red shift. The fluoro-substituted compound
5a appears to be the most promising as an emitter for
EL displays due to its high emission efficiency and high
melting point. Further substitution on the indole ring
by additional fluorine atoms could drive the emission
energy further into the blue region (∼450 nm), providing
new stable blue emitters, and such efforts are being
undertaken in our laboratory.
Ack n ow led gm en t. S.W. and Q.L. thank the Natu-
ral Sciences and Engineering Research Council of
Canada, and R.T. and M.M. thank the NSF (CHE-
9714998) and the Robert A. Welch Foundation (E-621)
for financial support.
F igu r e 7. J -V (top) and L-J (bottom) plots of the EL
device.
mum at 502 nm (Figure 6). The EL spectrum from the
device matches very well the PL spectrum of 5a in the
solid state, indicating that the observed EL originates
from compound 5a . The J -V and L-J characteristics
of the EL device are shown in Figure 7. The turn-on
voltage of the device is 11 V. Although the turn-on
voltage is relatively high in comparison to previously
reported EL devices based on boron chelate compounds,
due to the absence of an electron transport layer, it
Su p p or tin g In for m a tion Ava ila ble: X-ray diffraction
data for 5a -c, including tables of atomic coordinates, thermal
parameters, bond lengths and angles, and hydrogen param-
eters. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM020446J