Table 1 Physical and chemical data for 2–7
Compound
l
abs/nma
l
em/nm (Wf (%))a
t/ns
108krb/s21
108knrb/s21
(Tg/Td)/uCc
Eox/mVd
HOMO/LUMO/eVe
2
3
4
5
6
7
365, 317
428 (17)
428 (20)
463 (45)
609 (64)
610 (66)
612 (63)
1.4
1.2
1.8
0.9
1.0
1.1
1.2
1.7
2.5
7.0
6.7
5.7
5.9
6.8
3.1
3.9
3.4
3.3
123/439
141/404
146/457
182/501
195/549
202/574
527
545
405
484
492
406
5.33/2.06
5.35/2.08
5.21/2.32
5.28/3.05
5.29/3.06
5.21/2.97
370, 320
408, 328
495, 318
496, 318
493, 408, 323
a Measured in toluene solutions. Quantum yield (Wf, (%)) was measured relative to Coumarin-1 or Nile Red. Corrections due to the change in
solvent refractive indices were applied. b kr ~ Wf/t; Wf ~ kr/(kr 1 knr). c Tg and Td were obtained from DSC and TGA measurements, respec-
tively. Eox was measured in CH2Cl2. All the potentials are reported relative to ferrocene that was used as internal standard in each experi-
ment. Ferrocene oxidation potential was located at 1517 mV, with DEp ~ 108 mV, relative to Ag/AgNO3 non-aqueous reference electrode.
d
The concentration of the compound was 2.5 6 1024 M and the scan rate was 100 mV s21 e HOMO and LUMO were calculated from CV
.
data and absorption spectra.
Fig. 1 Absorption (5) and emission (2 and 5) spectra.
Fig. 2 EL spectra of the devices I and II, and PL spectra for 5.
of the naphthyl units at 318 nm is approximately 1.5 times greater
than that observed on direct excitation of 4,7-dithienylben-
zo[2,1,3]thiadiazole at 495 nm. However, excitation of 2, the
model compound for 5 without the energy acceptor, at 365 nm
results a 428 nm emission. The residual emission from the energy
donor in compounds 5–7 is negligible. The excited state life times
for the dyads are slightly smaller than the donor molecules. Such
intramolecular energy transfer have also been reported for small
molecules13 and polymers.14
Despite the high molecular weight, compound 5 can be vacuum
deposited as a thin film with retention of film morphology even
upon heating at 110 uC for w24 h. Devices of different
configurations were fabricated for compound 5: (I) ITO/5
(40 nm)/TPBI (40 nm)/Mg:Ag; (II) ITO/5 (40 nm)/Alq3 (40 nm)/
Mg:Ag; (III) ITO/NPB (40 nm)/5 (10 nm)/TPBI (40 nm)/Mg:Ag
Notes and references
1 Formostrecentreviews,see:H.Imahori,J.Phys.Chem.B,2004,108,6130;
M.S.Choi,T.Yamazaki,I.YamazakiandT.Aida,Angew. Chem.,Int. Ed.
Engl., 2004, 43, 150; T. Weil, E. Reuthner, C. Beer and K. Mullen, Chem.
Eur. J., 2004, 10, 1398; V. Balzani, P. Ceroni, M. Maestri, C. Saudan and
V. Vicinelli, Top. Curr. Chem., 2003, 228, 159.
2 P. Furuta, J. Brooks, M. E. Thompson and J. M. J. Fre´chet, J. Am.
Chem. Soc., 2003, 125, 13165.
3 M. A. Wolak, B. B. Jang, L. C. Palilis and Z. H. Kafafi, J. Phys. Chem.
B, 2004, 108, 5492.
4 C. Y. Jiang, W. Yang, J. B. Peng, S. Xiao and Y. Cao, Adv. Mater.,
2004, 16, 537; E. B. Namdas, A. Ruseckas, I. D. W. Samuel, S. C. Lo
and P. L. Burn, J. Phys. Chem. B, 2004, 108, 1570; J.-P. Duan, P.-P. Sun
and C.-H. Cheng, Adv. Mater., 2003, 15, 224.
5 K. R. Justin Thomas, J. T. Lin, Y.-T. Tao and C.-H. Chuen, Adv.
Mater., 2002, 11, 822; K. R. Justin Thomas, J. T. Lin, M. Velusamy,
Y.-T. Tao and C.-H. Chuen, Adv. Func. Mater., 2004, 14, 83.
6 Y. H. Niu, Q. Hou and Y. Cao, Appl. Phys. Lett., 2003, 82, 2163;
M. K. Fung, S. L. Lai, S. W. Tong, S. N. Bao, C. S. Lee, W. W. Wu,
M. Inbasekaran, J. J. O’Brien and S. T. Lee, J. Appl. Phys., 2003, 94, 5763.
7 A. J. Berresheim, M. Mu¨ller and K. Mu¨llen, Chem. Rev., 1999, 99, 1747;
I.-Y. Wu, J. T. Lin, Y.-T. Tao and E. Balasubramaniam, Adv. Mater.,
2000, 12, 668.
(TPBI ~ 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; Alq3
~
tris(8-hydroxyquinolinolato)aluminum (III); NPB ~ 1,4-bis(1-
naphthylphenylamino)-biphenyl). The EL spectra of the devices I
and III are similar to the PL spectra of the film (Fig. 2), indicating
light emission from the compound. In contrast, there is significant
emission from Alq3 in device II. It is likely that the smaller HOMO
energy gap between 5 and Alq3 (HOMO: 5, 5.28 eV; TPBI, 6.20 eV;
Alq3, 6.09 eV) results in leakage of holes from 5 into the Alq3 layer.
Device I produced a bright red emission with CIE coordinates 0.60,
0.39, maximum brightness of 5320 cd m22 and maximum external
quantum efficiency 1.05%. A similar performance was observed for
the device III. An attempt was also made using 5 as the electron
transport layer. However, the performance of this device was poor,
possibly due to the poor electron transport rate of 5.
In summary, we have synthesized star-shaped donor–acceptor–
donor molecules possessing high glass transition temperatures and
efficient antenna effect. These materials serve as high Tg EL material.
Orange-emittingELdeviceshavebeenfabricated, usingcompound5
as a hole-transporting/emitting layer, or as an emitting layer.
We thank Professor Sunny I. Chan for his critical comments.
8 J. F. Hartwig, Angew. Chem. Int. Ed., 1998, 37, 2046.
9 J. K. Stille, Angew. Chem. Int. Ed., 1986, 25, 508.
10 K. Naito and A. Miura, J. Phys. Chem., 1993, 97, 6240; K. Naito, Chem.
Mater., 1994, 6, 2343.
11 B.Valeur,MolecularFluorescence;Wiley-VCH,Weiheim,Germany,2002.
12 The distances were calculated by the AM1 method. The benzene centroid
of the benzothiadiazole core was taken as the center of the acceptor, and
the nitrogen atom of the peripheral amine was taken as the center of the
˚
˚
˚
donor. The calculated distances: 5 (8.098 A; 7.987 A); 6 (8.153 A;
˚
8.210 A); 7 (7.732 A; 7.950 A).
˚
˚
13 J. Jacob, S. Sax, T. Piok, E. J. W. List, A. C. Grimsdale and K. Mu¨llen,
J. Am. Chem. Soc., 2004, 126, 6987.
14 C. Ego, D. Marsitzky, S. Becker, J. Zhang, A. C. Grimsdale, K. Mu¨llen,
J. D. Mackenzie, C. Silva and R. H. Friend, J. Am. Chem. Soc., 2003,
125, 437.
C h e m . C o m m u n . , 2 0 0 4 , 2 3 2 8 – 2 3 2 9
2 3 2 9