Light-Emitting Carbazole DeriVatiVes
Table 1. Physical Data for the Compounds
J. Am. Chem. Soc., Vol. 123, No. 38, 2001 9407
Tg/Tm,a
°C
Td,b
°C
λ
max (ꢀmax, 10-3 M-1 cm-1),c
λ
em (Φf),d
nm
λ
em,,e
nm
Eox (∆Ep),f
HOMO/LUMO,g
eV
compd
nm
mV
9
120/na
123/na
174/na
490 351 (19.0), 278 (52.2), 257 (47.3)
509 351 (18.4), 306 (41.9), 271 (47.6)
510 408 (21.2), 328 (51.1), 274 (63.2)
497 (0.05) 463
486 (0.07) 454
548 (0.03) 523
539 (0.12) 512
515 (0.18) 509
127 (78), 518 (76), 1164 (i)
189 (68), 538 (75), 1145 (i)
116 (63), 448 (74), 1012 (i)
167 (74), 457 (69), 1002 (i)
241 (84), 469 (74), 997 (i)
159 (87), 455 (78), 1018 (i)
125 (76), 394 (75), 932 (63), 1219 (i)
17 (72), 191 (73), 637 (73), 745 (70)
115 (72), 477 (76), 1071 (i)
111 (71), 511 (73), 818 (76), 986 (i)
63 (77), 348 (72), 769 (63), 885 (75)
314 (68)
4.93/2.00
4.99/2.07
4.91/2.33
4.97/2.21
5.04/2.40
4.96/2.35
4.92/1.86
4.82/1.86
4.92/2.22
4.91/2.24
4.86/2.24
10
11
12
180/355 463 408 (21.0), 318 (55.0), 274 (61.3)
13
194/na
185/na
132/na
152/na
174/na
184/na
183/na
60/175
555 385 (40.0), 318 (68.3), 173 (94.1)
621 406 (33.3), 320 (83.8), 275 (104.0) 537 (0.08) 511
14
15
566 352 (62.7), 326 (61.6)
532 309 (88.8), 286 (87.0)
554 383 (16.7), 316 (47.5), 274 (62.9)
513 415 (26.6), 319 (71.7), 274 (78.1)
455 420 (23.7), 319 (64.2), 274 (71.0)
382 311, 353
439 (0.08) 461
450 (0.12) 469
548 (0.05) 519
543 (0.11) 529
553 (0.19) 541
16
18
19
20
TPD
NPD
ITO
TPBI
Alq3
Mg:Ag
100/265 479 271, 342
342 (66)
4.70 (EF)/na
6.20/2.70
6.09/2.95
na/3.70 (EF)
1.80
a Obtained from DSC measurements; na, Tm not detected. b Obtained from TGA measurements. c Measured in CH2Cl2 solution. d Measured in
CH2Cl2 solution; Φf: fluorescence quantum efficiency. e Film samples. f Measured in CH2Cl2. All Eox data are reported relative to ferrocene, which
has an Eox at 226 mV relative to Ag/Ag+ and the anodic peak-cathodic peak separation (∆Ep) is 90 mV; i , irreversible process. The concentration
of the complexes used in this experiment was 2.5 × 10-4 M and the scan rate was 100 mV s-1
.
g na, not available.
of palladium-catalyzed N-arylation reactions has opened up the
possibility of producing a wide variety of arylamines. Such
N-arylations have been successfully achieved with the use of
Pd(OAc)2/(t-Bu)3P, Pd(OAc)2/DPPF, or Pd(dba)2/(t-Bu)3P (dba
) dibenzylideneacetone) as the catalyst combinations in the
presence of sodium tert-butoxide. Hartwig and co-workers have
found that the latter catalyst reduces the reaction time and
temperature substantially and improves the yield of C-N
coupling product.16b,c For the preparation of the 3,6-diaminocar-
bazoles in this study, we screened all the above three catalysts
and found that they worked equally well. However, the use of
Pd(dba)2/(t-Bu)3P catalyst16b,c,17 significantly reduces the reaction
time and temperature. As much as 95% isolated yields based
on 3,6-dibromocarbazoles could be achieved in these reactions.
Such palladium-catalyzed amination reactions at the 3,6-
positions of carbazoles have not yet been reported, even though
nickel-catalyzed Grignard coupling reactions18 and Sogonashira
coupling reactions19 of 3,6-dibromocarbazoles have been real-
ized. The incorporation of an aromatic ring at the central
nitrogen atom of the carbazole molecule was also effectively
catalyzed by Pd(dba)2/(t-Bu)3P. The desired 3,6-dibromocar-
bazoles can be readily synthesized from the reaction of the
corresponding carbazoles with 2 equiv of NBS in dimethylfor-
mamide.
Preparation of pyrene- and naphthalene-containing unsym-
metric diamine 18 was achieved by sequential amination of 6
and the monosubstituted intermediate 17. 17 was isolated in
45% yield in a 1:1 reaction between 3,6-dibromo-9-ethylcar-
bazole (6) and N-(1-naphthyl)phenylamine. This unsymmetric
diamine 18 was designed mainly to uncover the role of pyrene
in this series of hole-transporting materials. A variation of the
N-substituent (9-substituent), from ethyl to phenyl or fused
aromatics (fluorene), was also introduced in these carbazoles
to evaluate the role of N-substituent on tuning the hole-
transporting and emitting properties.
Thermal Properties. The thermal properties of the new
carbazole compounds were determined by DSC and DTA
measurements (Table 1). Except for 12, all the compounds
exhibited a glass transition in the first heating cycle and no
melting endotherms and crystallization exotherms were noticed.
In the case of 12, a melting isotherm was observed during the
first heating cycle, but rapid cooling led to the formation of a
glassy state, which persisted in the subsequent heating cycles.
The glass transition temperatures (Tg) of these compounds
appear to be rather high among commonly used hole-transport
materials, such as 1,4-bis(1-naphthylphenylamino)-biphenyl (R-
NPD, Tg ) 100 °C) and 1,4-bis(phenyl-m-tolylamino)biphenyl
(TPD, Tg ) 60 °C),6b and many starburst arylamines.5a
The nonplanar and star-shaped nature of the carbazole
compounds may be responsible for easy glass formation.
Replacement of a diphenylamino group by a rigid carbazolyl
group was found to result in a dramatic increase in the glass
transition temperature of triphenylamine-based starburst com-
pounds.5a For the carbazole compounds in this study, the role
of carbazole in raising Tg is evident when comparing 9 (120
°C) and 10 (123 °C) with NPD (100 °C) or 16 (152 °C) with 9
and 10. The glass transition temperatures (Tg) of the pyrene-
incorporated amines 11-14 and 18-20 range from 174 to 194
°C, more than 50 °C higher than those of naphthylamine-
substituted carbazoles 9 and 10, which are 120 and 123 °C,
respectively. The role of pyrene in raising Tg is clearly
demonstrated from the steep rise of Tg in going from 9 (120
°C) to 11 (174 °C) or from 10 (123 °C) to 18 (174 °C) and 12
(180 °C). Apparently bulkier pyrene substituents hamper the
intramolecular rotations of the molecules. With the same
peripheral substituent diarylamine, replacement of N-ethyl
substituent by an N-aryl group leads to a small enhancement in
the thermal stabilities (9 f 10 and 11 f 12). However, the
p-cyanophenyl group excerts a larger effect (12 f 13). This
increase in glass transition temperature may be attributed to the
polar nature of the cyano moiety.20 The vapor-deposited film
of 11 (Tg ) 174 °C) was subjected to heating at 150 °C and
scrutinized by polarized microscope periodically. The film
remained amorphous even after 20 h. Similar treatment on NPD
film (Tg ∼ 100 °C, Tc ∼ 200 °C) led to crystallization within 3
h. Such an outcome further points to the correlation between a
high Tg and temporal stability of the film. This should be
beneficial to the devices’ performance.
(15) (a) Aalten, H. L.; van Koten, G.; Grove, D. M. Tetrahedron 1989,
45, 5565. (b) Paine, A. J. J. Am. Chem. Soc. 1987, 109, 1496. (c) Lindley,
L. Tetrahedron 1984, 40, 1433. (d) Weingarten, H. J. Org. Chem. 1964,
29, 977.
(16) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805. (b) Hartwig, J. F. Angew. Chem., Int. Ed. Engl.
1998, 37, 2047. (c) Hartwig, J. F. Synlett 1997, 329.
(17) Nishiyama, M.; Yamamoto, T.; Koie, Y. Tetrahedron Lett. 1998,
39, 617.
(18) Park, M.; Buck, J. R.; Rizzo, C. J. Tetrahedron 1998, 54, 12707.
(19) Campbell, I. B. In Organocopper Reagents; Taylor, R. J. K., Ed.;
Oxford University Press: Oxford, U.K., 1994; Chapter 10.
(20) Wang, S.; Oldham, Jr.; W. J.; Hudack, Jr.; R. A.; Bazan, G. C. J.
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