Synthesis of hole-transporting hydrazone dendrimers

Article abstract of DOI:10.1246/cl.2000.1298

Hole-transporting aromatic hydrazone dendrimers were synthesized by convergent method. Structures of dendrimers were characterized by NMR, IR, GC/MS (FAB⁺), and elemental analysis.

Full text of DOI:10.1246/cl.2000.1298

1
298
Chemistry Letters 2000
Synthesis of Hole-Transporting Hydrazone Dendrimers
†
Haehyun Nam, Dae Ho Kang, Jai Kyeong Kim, and Soo Young Park*
School of Material Science and Engineering, Seoul National University,
San 56-1, Shillim-dong, Kwanak-gu, Seoul 151-742, Korea
Electronic Materials and Devices Research Center, Korea Institute of Science and Technology,
P.O. Box 131, Cheongryang 136-791, Seoul, Korea
†
(Received July 31, 2000; CL-000729)
Hole-transporting aromatic hydrazone dendrimers were
available first generation dendron 1, second and third genera-
tion dendrons (6 and 8, respectively) were successfully syn-
thesized. Core group 9 was synthesized by Vilsmeyer–Haack
formylation of triphenylamine in a single step. Condensation
reaction of 9 with dendrons 1 and 6 yielded first and second
generation dendrimers 10 and 11 in 99% and 16% yields,
respectively. However, reaction of 9 and 8 did not give third
generation dendrimer presumably due to the equilibrium
nature of condensation reaction and the severe steric hin-
drance of 8.
synthesized by convergent method. Structures of dendrimers
+
were characterized by NMR, IR, GC/MS (FAB ), and elemen-
tal analysis.
Aromatic hydrazone molecules dispersed in a binder poly-
1
2
mer or attached to the polymer chain as a pendant group are
used as the main constituent of electrophotographic devices due
1
,2
to their excellent hole-transporting property.
To further
enhance the hole-transporting ability, new material system
which does not involve inactive polymeric components and
thus provides higher concentration of hydrazone moiety is
needed. Dendritic molecular architecture is considered useful
for this purpose because both the compact globular structure
Structures of hydrazine dendrimers 10 and 11 were charac-
terized by NMR, FT-IR, elemental analysis, and GC/MS.
4
Aromatic hydrazone dendrimers 10 and 11 were very soluble in
common organic solvents such as chlorobenzene, chloroform,
and DMF. This contrasts to the rather limited solubility of lin-
ear hydrazone molecules originating from their high crystallini-
ty. Aromatic hydrazone dendrimers 10 and 11 were amorphous
materials which formed excellent optical films by spin-coating.
These films were clear, transparent, homogeneous, and
mechanically tough. Glass transition temperature of 11 was
measured to be 164 °C, however that of 10 was not detected by
DSC.
3
and good processibility are normally guaranteed. In this work,
we report novel spin-coatable dendrimers consisting of
branched aromatic hydrazone moieties. Dendrimer syntheses
were carried out by the convergent method as outlined in
Scheme 1. Protecting of hydrazine group by phthalic anhydride
and the Vilsmeyer formylation of aromatic ring were repeatedly
applied for these convergent syntheses. From the commercially
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1299
UV–visible absorption maxima were observed at 400 nm
and 418 nm for the dendrimers 10 and 11 in chloroform solu-
tions. Cut-off wavelengths of these dendrimers were 444 nm
and 487 nm corresponding to the HOMO to LUMO excitation
energies of 2.8 eV and 2.5 eV for 10 and 11, respectively. Red-
shifted absorption of 11 over that of 10 suggests that the den-
dritic structure provides partial conjugation of hydrazone
branches. According to this conjugation effect, ionization
potential (IP) of 11 was also measured to be higher than that of
PVK. Compared to the turn-on voltage of control device (5.0
V), those of multi layer devices using 10 and 11 dendrimer lay-
ers were significantly lowered to be 2.5 V and 4.5 V, which
suggests good hole-injecting and transporting properties of aro-
matic hydrazone dendrimers. Much lower turn-on voltage of the
device containing the layer of 10 over those of 11 and control is
considered to originate from the lower IP value (5.1 eV) of 10
which matches better to the Fermi level of ITO anode whose
work function is 4.8 eV.
1
0 by 0.1 eV. Cyclic voltametry measurements were carried
out with dendrimers coated on an ITO electrode in a 0.1 M
References and Notes
tetrabutylammonium tetrafluoroborate (TBABF ) in acetoni-
1
2
3
4
J. Mort and G. Pfister, “Electronic Properties of Polymers,”
Wiley, New York (1982).
M. Döbler, C. Weder, P. Neuenschwander, and U. W.
Suter, Macromolecules, 31, 6184 (1998).
G. R. Newkome, C. N. Moorefield, and F. Vögtle,
4
trile at a constant scan rate of 20 mV/s using a three-electrode
cell and potentiostat assembly. Actual IP values of 10 and 11
measured by cyclic voltametry were 5.1 eV and 5.2 eV,
respectively. To attest the hole transporting properties of 10
and 11, we fabricated multilayer EL devices ITO/dendrimer
“Dendritic Molecules,” VCH, Weinheim (1996).
Structure characterization of 10 and 11. 10 ; H NMR
1
(
10,11)(60nm)/Alq (90nm)/Al and also ITO/PVK
3
(
60nm)/Alq (90nm)/Al as a control. All these devices showed
(CDCl ) δ 7.49 (d, 6H), 7.31 (t, 12H), 7.08 (m, 28H); IR
3
3
–
1
strong green emission from Alq layer. At 10 V, brightness of
devices with hole transporting layer of 10, 11 and PVK were
2
quantum efficiencies were 2.93 × 10 %, 1.28 × 10 %, and
1
efficiencies and the luminance properties of the synthesized
dendrimers were higher than, or at least comparable to that of
(KBr window, cm ) 1597 (ν C=N); Anal. Found : C,
3
+
82.26; H, 5.137; N, 11.80%; M , 828. Calcd for C H N :
5
7
45
7
.8 × 10–2 µW, 1.3 × 10 µW, and 2.9 × 10 µW, and the
–3
–3
C, 82.68; H, 5.48; N, 11.84%; M, 828. 11 ; H NMR
1
–
2
–2
(CDCl ) δ 8.60 (d, 6H), 7.66 (m, 24H), 7.43 (t, 24H), 7.18
3
–
2
–1
.35 × 10 %, respectively. We could see that the quantum
(m, 51H) ; IR (KBr window, cm ) 1590 (ν C=N); Anal.
Found: C, 79.12; H, 4.832; N, 13.76%; M , 1993. Calcd
+
for C135H105N19 : C,81.34; H, 5.31; N, 13.35%; M, 1993.