Novel families of hole-transporting monomers and polymers

Article abstract of DOI:10.1246/cl.2004.1336

The novel families of hole transporting monomers and polymers containing hydrazone moieties are reported. The polymers were prepared in polyaddition reaction of hydrazone-containing diepoxides with 4,4′-thiobisbenzenethiol. The hole drift mobilities in th

Full text of DOI:10.1246/cl.2004.1336

1336
Chemistry Letters Vol.33, No.10 (2004)
Novel Families of Hole-Transporting Monomers and Polymers
Vytautas Getautis,ꢀ Juozas Vidas Grazulevicius, Tadas Malinauskas, Vygintas Jankauskas,y
Zbig Tokarski,yy and Nusrallah Jubranyy
Faculty of Chemical Technology, Kaunas University of Technology, Kaunas, LT 50270, Lithuania
yDepartment of Solid State Electronics, Vilnius University, Vilnius, LT 10222, Lithuania
yySamsung Information Systems America, Woodbury, MN 55125, U.S.A.
(Received June 14, 2004; CL-040676)
The novel families of hole transporting monomers and poly-
crystalline materials.
mers containing hydrazone moieties are reported. The polymers
were prepared in polyaddition reaction of hydrazone-containing
diepoxides with 4,40-thiobisbenzenethiol. The hole drift mobili-
ties in the newly synthesized polymers exceed 10ꢁ4 cm2 Vꢁ1sꢁ1
and in the compositions of the monomers with polymeric binder
exceed 10ꢁ6 cm2 Vꢁ1sꢁ1 at an electric field of 106 V cmꢁ1 as
characterized by the xerographic time of flight technique.
In this work we briefly report on the synthesis, characteriza-
tion and photoconductive properties of novel photoconductive
monomers and polymers containing hydrazone moiety. The
newly synthesized polymers exhibit high hole drift mobilities
and excellent film-forming properties.
Synthesis of the monomers containing hydrazone moieties
was carried out by three step reaction. The first step was Vils-
meyer formylation of 9-ethylcarbazole or triphenylamine using
POCl3/DMF complex to get diformyl compounds. The second
step was the condensation of the diformyl compounds with phe-
nylhydrazine to obtain dihydrazones of diformyl compounds. By
interaction of the dihydrazones with epichlorohydrin in the pres-
ence of KOH 9-ethylcarbazole-3,6-dicarbaldehyde bis(N-2,3-
epoxypropyl-N-phenylhydrazone) (1a) and triphenylamine-4,40-
dicarbaldehyde bis(N-2,3-epoxypropyl-N-phenylhydrazone) (1b)
were obtained. The synthesized diepoxides 1a, 1b were purified
by column chromatography followed by crystallisation from
toluene to obtain pure and well defined compounds. 1a, 1b were
characterized by 1H NMR, IR and elemental analysis.3,4 In
the additional step, polyaddition of diepoxypropylhydrazones
1a, 1b with 4,40-thiobisbenzenethiol in THF was carried out in
the presence of triethylamine at the reflux temperature of THF
(Scheme 1). Polymers 2a, 2b possessing hydrazone moieties
were isolated with 50–80% yield.
Polyaddition reaction was carried out for 4 and 60 h, and
the polymers with different molecular weight were isolated.
The average molecular weights and their distribution of the
polymers 2a, 2b detected by GPC are presented in Table 1.
Longer duration of the reaction obviously increases molecular
weight of the polymers. DSC results show a slight increase in
glass transition temperature (Tg), from 127 to 131 ꢂC for polymer
2a and from 122 to 125 ꢂC for polymer 2b, with increasing
molecular weight. A change of chromophore, from carbazole
to triphenylamine, leads to the decrease of Tg by ca. 5–6 degrees.
This is apparently due to less tight packing of the polymer chains
in the compound 2b.
Organic charge-transporting materials are used in electro-
photographic photoreceptors, light-emitting diodes, photovoltaic
devices, and other optoelectronic devices.1 Rapid charge trans-
porting ability, high photosensitivity, simple synthesis, and
low price are the advantages of the hydrazones against others
charge transporting materials (TM).2 Low molecular weight
TM containing hydrazone moieties are usually crystalline mate-
rials, are not capable of forming thin neat homogenous layers,
and must be used in combination with polymeric hosts. The pres-
ence of a large proportion of polymer host in the compositions,
usually reaching 50% of the total composition mass, leads to the
considerable decrease of charge carrier mobility. Even in such
compositions, the possibility of the TM crystallization remains
and this causes problems during electrophotographic layer
preparation and extended printing. From this point of view
the photoconducting polymers or oligomers are superior to
O
O
HS
S
SH
N
N
N
Ar
N
(C2H5)3N
1a,b
S
S
S
*
*
n
OH
HO
The absorption spectra of the monomers 1a, 1b and poly-
mers 2a, 2b are given in Figure 1. The absorption spectrum of
the polymer 2b is bathochromically shifted with respect to the
N
N
N
Ar
N
2a,b
Table 1. Molecular mass and DSC data of polymers
Mw Tg, Reaction Yield,
/Mn /ꢂC time,/h
/%
Polymer
Mn
Mw
a Ar =
,
b Ar =
N
N
2a
6700 26000 3.9 127
8500 74000 8.7 131
4
60
52
82
2b
7100 29000 4.1 122
9100 63000 6.9 125
4
60
50
77
Scheme 1. Synthesis rout to the polymers 2a, 2b containing
hydrazone moieties from monomers 1a, 1b.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.10 (2004)
1337
10-2
10-3
spectrum of polymer 2a this is the consequence of the increased
conjugated ꢀ-electron system of the triphenylamine in compar-
ison with carbazolyl chromophore. On the other hand difference
in ꢀ-electron conjugation between corresponding monomers
and polymers is not significant which proves that conjugated
ꢀ-electron systems remain intact during polyaddition reaction.
t=25oC
1x10-4
1x10-5
10-6
1a
0,8
µ
1b
2a
2b
1a+PVB, 1:1
2a
10-7
0,6
0,4
0,2
1b+PVB, 1:1
2b
10-8
0
200
400
600
E1/2 (V/cm)1/2
800
1000
1200
1400
Figure 2. Field dependencies of the hole-drift mobilities.
films of 2a, 2b is by ca. three orders of magnitude higher than
in a classical polymeric photoconductor poly(N-vinylcarba-
zole).8 In addition, the hole mobility in polymer 2b containing
triphenylamine moiety is by ca. one order of magnitude higher
than in polymer 2a containing carbazolyl group.
0,0
250 275 300 325 350 375 400 425
Wavelength [nm]
More representatives of this family of hole-transporting
polymers will be described in our following publications.
Figure 1. UV absorption spectra of the monomers 1a, 1b and
polymers 2a, 2b in THF solutions.
We thank Dr. G. Lattermann (Bayreuth University,
Germany) for assistance with the GPC analysis.
The ionization potential (Ip) was measured by electron pho-
toemission in air, similar to the method described.5 The Ip value
of 5.37 eV is for monomer 1a and 5.40 eV for polymer 2a. Re-
placement of the 9-ethylcarbazolyl group by the triphenylamino
group leads to the decrease of Ip to 5.34 eV for monomer 1b and
5.30 eV for polymer 2b accordingly.
References and Notes
1
2
Y. Shirota, J. Mater. Chem., 10, 1 (2000).
J. V. Grazulevicius and P. Strohriegl, ‘‘Handbook of
AdvancedElectronic and Photonic Materials and Devices,’’
ed. by H. S. Nalwa, Academic Press (2000), Vol. 10,
p 233.
Polymers 2a, 2b are soluble in common organic solvents
such as chloroform, THF, dioxane etc. This really good solubil-
ity is mainly due to the flexible linking fragments between chro-
mophores. Clear, transparent and homogeneous films of poly-
mers 2a, 2b were obtained by the casting technique. The hole-
drift mobility for synthesized monomers 1a, 1b polymers 2a,
2b was measured by time of flight technique.6,7 Positive corona
charging created electric field inside the TM layer. Charge car-
riers were generated at the layer surface by illumination with
pulses of nitrogen laser (pulse duration was 2 ns, wavelength
337 nm). The layer surface potential decrease as a result of pulse
illumination was up to 1–5% of initial potential before illumina-
tion. The capacitance probe that was connected to the wide fre-
quency band electrometer measured the speed of the surface po-
tential decrease dU=dt. The transit time tt was determined by the
kink on the curve of the dU=dt transient in linear or double log-
arithmic scale. The drift mobility was calculated by the formula
(ꢁ ¼ d2=U0tt, where d is the layer thickness and U0 is the sur-
face potential at the moment of illumination. Figure 2 shows
the room temperature dependencies of hole-drift mobility on
electric field in monomers 1a, 1b with polymeric binding
material polyvinylbutiral (PVB) and polymers 2a, 2b. The
hole-drift mobilities in amorphous films of 2a, 2b exceeds
10ꢁ4 cm2 Vꢁ1sꢁ1 at an electric field of 106 V cmꢁ1. This is a
rather high mobility as for amorphous polymeric TM, it is by
ca. one order of magnitude higher than in the compositions of
1a, 1b with PVB. This improvement in results over monomers
is mainly due to the elimination of polymeric binding material
from the composition. The hole-drift mobilities in amorphous
3
1a: mp 119–120 ꢂC (from toluene) 1H NMR (300 MHz,
CDCl3), ꢂ: 8.38 (split s, 2H); 7.9–7.88 (m, 4H); 7.49–7.43
(m, 4H); 7.40–7.32 (m, 6H); 6.96 (t, 2H, J ¼ 7:2 Hz);
4.42–4.29 (m, 6H); 4.06–3.97 (dd, 2H, (HA), JAX ¼ 4:5 Hz,
JAB ¼ 16:4 Hz); 3.31 (m, 2H); 2.90–2.85 (dd, 2H, (HA),
JAX ¼ 3:9 Hz); 2.70–2.65 (dd, 2H, (HB), JBX ¼ 2:7 Hz;
JAB ¼ 5:1 Hz); 1.43 (t, J ¼ 7:2 Hz). Anal. Calcd for
C41H46N6O2: C 75.20; H 7.08; N 12.83%. Found, C 75.01;
H 6.91; N 12.68%.
4
1b: mp 163.5–165 ꢂC (from toluene) 1H NMR (300 MHz,
CDCl3), ꢂ: 7.63 (s, 2H); 7.62–7.56 (m, 4H); 7.43–7.02 (m,
17H); 6.94 (t, 2H, J ¼ 7:1 Hz); 4.40–4.30 (dd, 2H, (HA),
JAX ¼ 2:1 Hz, JAB ¼ 16:5 Hz); 4.02–3.92 (dd, 2H, (HB),
JBX ¼ 4:2 Hz); 3.26 (m, 2H); 2.84 (dd, 2H, (HA),
JAX ¼ 4:2 Hz, JAB ¼ 5:1 Hz); 2.65–2.60 (dd, (HB), JBX
¼
2:7 Hz). Anal. Calcd for C38H35N5O2: C 76.87; H 5.94; N
11.80%. Found, C 76.71; H 5.91; N 11.70%.
E. Miyamoto, Y. Yamaguchi, and M. Yokoyama, Electro-
photography, 28, 364 (1989).
E. Montrimas, V. Gaidelis, and A. Pazera, Liet. Fiz. Z., 6,
569 (1966).
A. Y. C. Chan and C. Juhasz, Int. J. Electron., 62, 625
(1987).
P. M. Borsenberger and D. S. Weiss, ‘‘Organic photorecep-
tors for Imaging Systems,’’ Marcel Dekker, Inc. (1993),
p 411.
5
6
7
8
Published on the web (Advance View) September 18, 2004; DOI 10.1246/cl.2004.1336