High hole mobilities in carbazole-based glass-forming hydrazones

Article abstract of DOI:10.1039/b209732j

The properties of a series of carbazole-based dihydrazones are reported. The dependence of their thermal and glass-forming properties on their chemical structure is discussed. The hydrazones having phenyl substituents at the N atom of the hydrazine moiety form glasses and their amorphous films on the glass or polyester substrates can be prepared by casting from solutions. The ionization potentials of the synthesized hydrazones measured by the electron photoemission technique range from 5.24 to 5.50 eV. Hole drift mobilities of some newly synthesized carbazole-based dihydrazones appproach 10^-2 cm² V^-1 s^-1 at an electric field of 6.4 × 10⁵ V cm^-1, at 22 °C.

Full text of DOI:10.1039/b209732j

View Article Online / Journal Homepage / Table of Contents for this issue
High hole mobilities in carbazole-based glass-forming hydrazones
Jolita Ostrauskaite,a Viktoras Voska,a Jacek Antulis,b Valentas Gaidelis,b
Vygintas Jankauskasb and Juozas V. Grazulevicius*a
aDepartment of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19,
LT 3028 Kaunas, Lithuania. E-mail: juozas.grazulevicius@ctf.ktu.lt
bDepartment of Solid State Electronics, Vilnius University, Sauletekio al. 9, LT 2040 Vilnius,
Lithuania
Received 3rd October 2002, Accepted 9th October 2002
First published as an Advance Article on the web 1st November 2002
The properties of a series of carbazole-based dihydrazones are reported. The dependence of their thermal and
glass-forming properties on their chemical structure is discussed. The hydrazones having phenyl substituents at
the N atom of the hydrazine moiety form glasses and their amorphous films on the glass or polyester substrates
can be prepared by casting from solutions. The ionization potentials of the synthesized hydrazones measured
by the electron photoemission technique range from 5.24 to 5.50 eV. Hole drift mobilities of some newly
synthesized carbazole-based dihydrazones appproach 1022 cm2 V21 s21 at an electric field of 6.46105 V cm21
at 22 uC.
,
Aromatic hydrazones of the general formula RCHLN–NR2,
where R are aromatic or heteroaromatic groups, are widely
studied as organic hole transport materials.1–4 Hydrazones
as well as aromatic amines, such as N,N’-diphenyl-N,N’-bis-
(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine (TPD)5 and 1,1-
bis(di-4-tolylaminophenyl)cyclohexane (TAPC),6 are widely
used in electrophotography. Most of the modern organic elec-
trophotographic photoreceptors have a dual-layer configura-
tion. The main advantage of this configuration is the possibility
of separate optimization of the two layers. The charge genera-
tion layer usually consists of a dye such as titanyl phthalocya-
nine dispersed in a polymer binder, e.g. poly(vinylbutyral). The
charge transport layer is usually prepared by embedding an
organic hole transport material into a polymer matrix, e.g.
polycarbonate. A charge transport layer has to contain a large
amount (up to 50%) of the active material to ensure effective
transport of holes. Introduction of such a large amount of
low-molar-mass charge transport compound into the polymer
matrix can lead to crystallization. To prevent this problem,
charge transporting compounds which do not readily crystal-
lize are needed. Electrophotographic photoreceptors with hydra-
zone molecules, dispersed in polymer binders, show excellent
performance.7–10 Among the great number of studies devoted
to low-molar-mass aromatic hydrazones only a few glass-
forming compounds are reported.11 Carbazole-containing
hydrazones described in the literature exhibit relatively high
hole drift mobilities.12,13 Most of carbazole-based hydrazones
reported contain only one hydrazine moiety per molecule. Such
molecules are rather small and their inclination to crystallize
is rather high.12–14 In this work we report on the properties
of carbazole-based dihydrazones. Designing these molecules
we expected that an increase of the molar mass of the
carbazole-based hydrazones would reinforce the inclination of
the molecules to form glasses and the enlarged system of
conjugated p-electrons would allow enhanced charge-carrier
mobilities. For the comparison of the properties some new
carbazole-based monohydrazones have also been synthesized
and studied.
in Fig. 1. The first step was the formylation of 9-substituted
carbazoles 1a–c using the Vilsmeier reaction15 to get mono- and
diformyl compounds 2a–c’. The second step was condensation
of aldehydes 2a–c’ with differently substituted hydrazines.
All final products, hydrazones 3a–c’ and 4a–c’, were purified
by column chromatography. After column chromatography
all the hydrazones except 3c’ and 4c’ were recrystallized
from ethanol or methanol to obtain pure and well defined com-
pounds. Compounds 3c’ and 4c’ were isolated as amorphous
powders after column chromatography. All carbazole-based
1
hydrazones were characterized by IR, H NMR spectroscopy,
mass spectrometry, and elemental analysis (see Experimental
section). They are soluble in common organic solvents such as
chloroform, acetone, and THF.
The thermal properties of 9-alkyl- and 9-phenylcarbazolyl-
hydrazones were examined by differential scanning calorimetry
(DSC) and thermogravimetric (TG) analysis. The data from
the TG investigations of the selected samples are presented in
Table 1. These data show the relatively high thermal stability of
all the molecules synthesized. A 5% weight loss for compounds
3a, 3c, 4a and 4b, having diphenyl substituted hydrazine
N atoms, is observed at 260 uC, while compounds 4a’ and
4b’, containing a methylphenyl substituted N atom, show a
corresponding weight loss at 290 uC. The 5% weight loss
temperature (Tdec 2 5%) for carbazole-based monohydrazones
3a,c is the same as for dihydrazones 4a,b, i.e. 260 uC. It is
evident, that the substituents at the hydrazine N atom affect the
5% weight loss temperature and consequently the thermal
stability of the carbazole-containing hydrazone compounds.
Neither the introduction of the second hydrazine moiety into
the molecule nor a change of the substituent at the carbazole
N atom influence the 5% weight loss temperature of the
hydrazone compounds synthesized. This observation indirectly
shows that the thermal degradation of these compounds starts
from the hydrazone group.
Two stages are characteristic of the thermal degradation of
the carbazolylhydrazones synthesized. As an illustration of this
the TG curves of the hydrazones having different substituents
at the hydrazine N atom (4a and 4a’) are shown in Fig. 2. The
first step is apparently due to breaking of the azo bond of the
hydrazine moiety. The weight loss percentage in this stage for
all hydrazones corresponds to the share of hydrazine moiety in
the hydrazone molecules. The onset of the second step for
Results and discussion
We have synthesized a series of 9-alkyl- and 9-phenylcarba-
zolylhydrazones by two-step reaction, as schematically shown
DOI: 10.1039/b209732j
J. Mater. Chem., 2002, 12, 3469–3474
This journal is # The Royal Society of Chemistry 2002
3469

View Article Online
Fig. 1 Scheme of the synthesis of the 9-substituted carbazolylhydrazones.
Table 1 TG data for selected 9-substituted carbazolylhydrazones
Table 2 DSC data for the 9-substituted carbazolylhydrazones
Compound
3a
3c
4a
4a’
4b
4b’
Compound
Tg/uC
Tm/uC
Tcr/uC
Tdec 2 5%/uC
260
260
260
290
260
290
3a
3c
55
51
88
79
64
45
89
91
161a
214
200a
232
135a
207
252a
—
—
175
—
133
—
101
—
4a
4a’
4b
4b’
4c
hydrazones 4a and 4a’ is observed at approximately the same
temperature while the 5% weight loss temperature differs by
30 uC.
The temperatures of the thermal transitions of the selected
hydrazones are summarized in Table 2. 9-Alkylcarbazole-
based hydrazones having methyl substituents at the hydrazine
N atom (4a’ and 4b’) are more inclined to crystallization than
the corresponding diphenyl substituted hydrazones (4a and 4b).
The compounds with diphenyl substituted hydrazine N
atoms (4a and 4b) neither melt nor crystallize in the second
4c’
—
a1st heating only.
heating run of the DSC experiment, however hydrazones
having methyl substituents show crystallization and a follow-
ing melting in the second DSC heating run. An illustration of
the DSC thermograms of carbazolylhydrazones having differ-
ent substituents at the hydrazine N atom (4a and 4a’) is
presented in Fig. 3. The first DSC run of diphenyl-substituted
hydrazone 4a reveals melting at 200 uC but after slow cooling
the second heating run reveals only a glass transition at 88 uC.
Methylphenyl-substituted hydrazone 4a’ melts at 236 uC as is
shown by the DSC curve of the first heating run. The DSC
curve of 4a’ obtained in the second heating run shows not only
a glass transition at 79 uC but also crystallization at 133 uC and
then melting at 232 uC. 9-Phenylcarbazole-based hydrazones
seem to be more inclined to form glasses than 9-alkylcarbazole-
based hydrazones. The DSC curves of 4c’ and 4c obtained after
heating the samples to the melt and slow cooling do not show
any crystallization peaks. Apparently the incorporation of the
rigid phenyl group into the molecules hinders the packing of
molecules and increases the stability of the amorphous state.
Incorporation of the phenyl group also markedly increases the
Tg of the compounds (see Table 2). Thus the Tg of compound
4c is by 25 uC higher than that of compound 4b although the
Fig. 2 TG curves of hydrazones 4a and 4a’ at a heating rate of
10 K min21, N2 atmosphere.
3470
J. Mater. Chem., 2002, 12, 3469–3474

View Article Online
by the bathochromically and hyperchromically shifted K bands
attributed to pAp* transitions.16 This observation shows that
the molecules of the dihydrazones are strongly p-conjugated
through the lone electron pairs of the nitrogen atoms and that
the p-electrons are delocalized in these molecules.
Amorphous thin films on the substrate of different mor-
phological stability can be prepared by the casting technique
from all carbazolyl hydrazones reported in this presentation.
The stabilities of the films with thicknesses of 0.5–1 mm were
sufficient for ionization potential (Ip) measurements. The
values of Ip are given in Table 3. The Ip of the dihydrazones
4a and 4b, having a diphenyl-substituted N atom of the
hydrazine moiety, is by about 0.1 eV higher than the Ip of
dihydrazones 4a’ and 4b’, which bear a methyl substituent at
the hydrazine N atom. The 9-phenyl-9H-carbazolyl-based
hydrazones exhibit higher Ip than the 9-alkyl-substituted
compounds. Thus ionization potentials of carbazole-based
hydrazones can be varied by changing the substituents at the
nitrogen atoms.
For the measurements of the electrophotographic para-
meters and hole drift mobilities, amorphous films of 2–10 mm
thickness of the pure hydrazones or of their solid solutions in
polycarbonate (PC) were prepared. The morphological stabi-
lity of the glasses of 3c’, 4a, 4b and 4c’ allowed the preparation
of amorphous films of the pure compounds and of their
mixtures with PC. The solubility of compound 4a’ was insuf-
ficient to prepare layers of the necessary thickness. The
morphological stability of the glasses of hydrazones 3c, 4c,
4b’ was not high enough to prepare amorphous films of the
pure compounds and the measurements of the electrohoto-
graphic parameters and hole mobilities were carried out using
their solid solutions in PC.
The electrophotographic parameters of the films of carba-
zolylhydrazones and/or of their solid solutions in PC are
presented in Table 4.
Fig. 3 DSC curves of hydrazones 4a and 4a’ at a heating rate of
10 K min21, N2 atmosphere.
The photosensivity (S1/2
) of the carbazolylhydrazones
molar masses of these compounds are very similar. The
comparison of the values of Tg of N,N-diphenyl-substituted
hydrazones (4a, 4b) and N-methyl-N-phenyl-substituted hydra-
zones (4a’, 4b’) reveals a similar effect. The Tg’s of the
disubstituted carbazolylhydrazones (4a, 4c) are markedly
higher than those of their monosubstituted counterparts (3a,
3c).
The synthesized hydrazones absorb light in the 240–450 nm
region. As an example, the UV–vis absorption spectra of
hydrazones 4a, 4a’ are shown in Fig. 4. For a comparison the
spectrum of 9-ethyl-9H-carbazole is given in Fig. 4. The spectra
of the hydrazones 4a, 4a’ exhibit a strong bathochromic shift
with respect to the spectrum of 9-ethyl-9H-carbazole. In the
UV spectra of the hydrazones, B and R bands are submerged
reported in this presentation is similar to that of known
compounds having carbazole moieties.17 It decreases several
times after the introduction of 50 wt.% PC. Nevertheless it is
high enough for hole drift mobility measurements. The low
UR/U0 ratio shows that the large majority of holes generated
at the surface of the layers traverse through their volume
and reach the opposite side. This observation shows that hole
transport is effective in these materials.
The results of the time-of-flight measurements corroborate
the data from the electrophotographic research. Fig. 5 shows
the electric field dependences of the hole drift mobilities of the
Table 3 Ionization potentials of the films of the 9-substituted
carbazolylhydrazones
Compound 3a
3c
3c’
4a
4a’
4b
4b’
4c
4c’
Ip/eV 5.40 5.47 5.47 5.35 5.27 5.37 5.24 5.46 5.50
Table 4 Electrophotographic parameters of the 9-substituted carba-
zolylhydrazones (steady light 1017 quanta s21 m22, l ~ 295 nm, 20 uC)
Compound
d/mm
U0/V
UR/V
UR/U0
S1/2/m2 J21
3c1 PC
3c’
3c’ 1 PC
4a
4b
4b’1 PC
4c
4c1 PC
4c’
8.3
6.3
3.8
4.5
3.6
3.6
8.5
3.2
4.5
3.8
798
428
500
229
336
372
200
248
408
436
72
56
18
3.9
6
16
12
10
8
0.089
0.028
0.036
0.017
0.0018
0.043
0.06
0.04
0.02
0.023
0.45
3.0
0.56
6.9
7.9
2.7
2.7
1.7
7.7
1.7
Fig. 4 UV–vis absorption spectra (CHCl3 solutions) of hydrazones 4a
and 4a’, and 9-ethyl-9H-carbazole.
4c’1 PC
10
J. Mater. Chem., 2002, 12, 3469–3474
3471

View Article Online
carbazoles 1a–c with POCl3 by the following general
procedure.
POCl3 (1 equiv) was added dropwise to dry dimethylforma-
mide (1.2 equiv) at 0 uC under an N2 atmosphere. The solution
(ca. 2 wt.%) of carbazole derivatives in dry o-dichlorobenzene
was added stepwise during ca. 15 min to the reaction flask. The
reaction mixture was stirred at 85 uC until the starting
compound was reacted (TLC monitoring). Then the reaction
mixture was cooled to room temperature, poured in to ice
water and neutralized with sodium acetate to pH ~ 6–8. The
aqueous solution was extracted with chloroform several times.
The chloroform solution was washed with water, dried with
anhydrous sodium sulfate, filtered and the solvent removed.
The crude product was purified by column chromatography
(silica gel, eluent: acetone–hexane, 1 : 3).
9-Ethyl-9H-carbazole-3,6-dicarbaldehyde (2a’). Yield 78%,
(C16H13NO2 ~ 251.3 g mol21). IR (KBr), n/cm21: 1684
¯
Fig. 5 Electric field dependence of the hole drift mobility of the
amorphous films of the 9-substituted carbazolylhydrazones and their
solid solutions in PC, at 22–25 uC.
(CHO), 1598, 1489 (ar. CLC). MS (m/z): 251 (M1), 236, 222,
207, 178. H NMR (CDCl3), d (ppm): 1.53 (t, 3H, alk.), 4.45
1
(m, 2H, alk.), 7.53 (d, 2H, ar.), 8.07 (d, 2H, ar.), 8.63 (s, 2H,
ar.), 10.12 (s, 2H, CHO). Anal. Calcd for C16H13NO2: C, 76.48;
H, 5.21; N, 5.57, Found: C, 76.61; H, 5.30; N, 5.43%.
amorphous films of hydrazones 3c’, 4a, 4b, 4c and 4c’ and of the
solid solutions in PC of 3c, 3c’, 4b’, 4c and 4c’. The highest hole
drift mobilities were observed in glasses 4a, 4b and 4c. The hole
drift mobility in 4a reaches 1022 cm2 V21 s21 at an electric field
of 6.4 6 105 V cm21, at 22 uC. To our knowledge this is one
of the highest charge mobilities in disordered organic solids
reported up to now. The dihydrazone 4b, with a longer alkyl
substituent, exhibits a little lower hole drift mobility than those
of hydrazones 4a and 4c, apparently due to the lower con-
centration of the functional units. Hydrazones containing two
hydrazine moieties per molecule (4a, 4c and 4c’) show higher
hole drift mobilities than mono-substituted hydrazones 3a,
3c and 3c’. This is apparently due to the larger systems
of conjugated p-electrons in 4c and 4c’. A similar reason
apparently predetermines the higher hole drift mobility in 4c
relative to 4c’. The hole mobilities of PC doped with 4c and 4c’
markedly exceed 1024 cm2 V21 s21 at an electric field of 6.4 6
105 V cm21. These are also very high charge mobilities, as for
molecularly doped polymer.
9-Hexyl-9H-carbazole-3,6-dicarbaldehyde (2b). Yield 35%,
(C20H21NO2 ~ 307.5 g mol21). IR (KBr), n/cm21: 2953, 2858
¯
(alk. C–H), 1685 (CHO), 1593, 1487 (ar. CLC). MS (m/z): 307
(M1), 279, 263, 248, 236, 220. 1H NMR (CDCl3), d (ppm): 0.85
(t, 3H, alk.), 1.30 (m, 6H, alk.), 1.88 (m, 2H, alk.), 4.48 (t, 2H,
alk.), 7.52 (d, 2H, ar.), 8.08 (d, 2H, ar.), 8.63 (d, 2H, ar.), 10.10
(s, 2H, CHO). Anal. Calcd for C20H21NO2: C, 78.15; H, 6.89;
N, 4.56. Found: 78.29; H, 7.02; N, 4.62%.
9-Phenyl-9H-carbazole-3-carbaldehyde (2c). Yield 51%,
(C19H13NO ~ 271.3 g mol21). IR (KBr), n/cm21: 3060 (ar.
¯
C–H), 1700 (CHO), 1600, 1500 (ar. CLC), 1280 (C–N). MS
1
(m/z): 271 (M1), 242. H NMR (CDCl3), d (ppm): 7.30–7.75
(m, 9H, ar.), 7.93 (d, 1H, ar.), 8.20 (d, 1H, ar.), 8.65 (s, 1H, ar.),
10.12 (s, 1H, CHO). Anal. Calcd for C19H13NO: C, 84.11; H,
4.83; N, 5.16. Found: C, 84.34; H, 4.98; N, 5.24%.
9-Phenyl-9H-carbazole-3,6-dicarbaldehyde (2c’). Yield 43%,
Conclusions
(C20H13NO2 ~ 299.3 g mol21). IR (KBr), n/cm21: 3080 (ar.
¯
C–H), 1700 (CHO), 1600, 1500 (ar. CLC), 1290 (C–N). MS
A series of carbazole-based dihydrazones were synthesized.
The hydrazones having phenyl substituents at the N atom of
the hydrazine moiety form glasses and amorphous films of
them can be prepared on the substrates by casting from
solutions. The ionization potentials of the synthesized hydra-
zones measured by electron photoemission range from 5.24
to 5.50 eV. The hole drift mobilities of some carbazole-based
dihydrazones appproach 1022 cm2 V21 s21 at an electric field
of 6.4 6 105 V cm21, at 22 uC.
1
(m/z): 299 (M1), 269, 240. H NMR (CDCl3) d (ppm): 7.30–
7.75 (m, 7H, ar.), 8.03 (d, 2H, ar.), 8.70 (s, 2H, ar.), 10.15 (s,
2H, CHO). Anal. Calcd for C20H13NO2: C, 80,25; H, 4.38; N,
4.68. Found: C, 80.40; H, 4.45; N, 4.74%.
1–2 g of hydrazones 3a–c’ and 4a–c’ were prepared by
condensation of aldehydes 2a–c’ and the derivatives of
hydrazine by the following general procedure.
A solution of N,N-diphenylhydrazine hydrochloride (or
N-methyl-N-phenylhydrazine) in ethanol or methanol was
added dropwise to a solution of mono- or diformyl compound
in ethanol or methanol by stirring. Molar ratio, 2 mol of
hydrazine : 1 formyl group. The reaction mixture was refluxed
until all aldehyde was reacted (TLC monitoring). Then the
reaction mixture was cooled to the room temperature. The
precipitated product was filtered and purified by column
chromatography (silica gel, eluent chloroform–hexane, 1 : 2).
followed by recrystallization from ethanol or methanol.
Compounds 3c’ and 4c’ was isolated as amorphous powders
after column chromatography.
Experimental
Materials
The starting compounds: 9-ethyl-9H-carbazole (1a), 9-phenyl-
9H-carbazole (1c), 9-ethyl-9H-carbazole-3-carbaldehyde (2a),
and all the required chemicals: 9H-carbazole, 1-bromohexane,
potassium hydroxide, tetrabutylammonium hydrosulfate, phos-
phorus oxychloride, sodium acetate, N,N-diphenylhydrazine
chloride and N-methyl-N-phenylhydrazine were purchased
from Aldrich and purified using standard procedures. Organic
solvents were purified and dried as usual. Silica gel was used for
column chromatography.
9-Ethyl-9H-carbazole-3-carbaldehyde N,N-diphenylhydrazone
(3a). Yield 43%, (C27H23N3 ~ 389.5 g mol21), mp 160–161 uC.
N-hexylcarbazole (1b) was synthesized as described in the
literature.18 1–2 g of mono- and diformyl compounds 2a’–c’
were synthesized by the Vilsmeier reaction of N-substituted
IR (KBr), n/cm21: 3050 (ar.C–H), 2970, 2925 (alk. C–H), 1630
¯
(CLN), 1598, 1500 (ar. CLC), 1300 (C–N). MS (m/z): 389 (M1),
360, 283, 205, 194, 179 168, 165, 77. 1H NMR (CDCl3), d
3472
J. Mater. Chem., 2002, 12, 3469–3474

View Article Online
(ppm): 1.41 (t, 3H, alk.), 4.33 (m, 2H, alk.), 7.10–7.50 (m, 15H,
ar.), 7.85 (d, 1H, ar.), 8.05 (d, 1H, ar.), 8.20 (s, 1H, methine).
Anal. Calcd for C27H23N3: C, 83.26; H, 5.95; N, 10.79. Found:
C, 83.48; H, 6.04; N, 10.93%.
250–252 uC. IR (KBr), n/cm21: 3050 (ar. C–H), 1590, 1480 (ar.
¯
CLC), 1300 (C–N). MS (m/z): 631 (M1), 464, 268, 242. 1H
NMR (CDCl3), d (ppm): 7.15–7.65 (m, 27H, ar.), 7.70–7.80 (m,
4H, ar.), 8.30 (s, 2H, methine). Anal. Calcd for C44H33N5: C,
83.65; H, 5.26; N, 11.09. Found: C, 83.94; H, 5.34; N, 11.12%.
9-Phenyl-9H-carbazole-3-carbaldehyde N,N-diphenylhydrazone
(3c). Yield 77%, (C31H23N3 ~ 437.5 g mol21), mp 213–214 uC.
9-Phenyl-9H-carbazole-3,6-dicarbaldehyde bis(N-methyl-N-
phenylhydrazone) (4c’). Yield 68%, (C34H30N5 ~ 508.6 g mol21
IR (KBr), n/cm21: 3060 (ar. C–H), 1630 (CLN), 1600, 1500 (ar.
¯
)
CLC), 1300 (C–N). MS (m/z): 437 (M1), 360, 268, 241, 168. 1H
NMR (CDCl3) d (ppm): 7.10–7.30 (m, 7H, ar.), 7.30–7.50 (m,
9H, ar.), 7.50–7.70 (m, 4H, ar.), 7.83 (d, 1H, ar.), 8.13 (d, 1H,
ar.), 8.25 (s, 1H, methine). Anal. Calcd for C31H23N3: C, 85.10;
H, 5.30; N, 9.60. Found: C, 85.41; H, 5.41; N, 9.75%.
of amorphous powder. IR (KBr), n/cm21: 3050 (ar. C–H), 2910
¯
(alk.C–H),1590,1490(ar.CLC),1295(C–N).MS(m/z):508(M1),
402, 242. 1H NMR (CDCl3), d (ppm): 3.50 (s, 6H, methyl), 6.95
(t, 2H, ar.), 7.30–7.50 (m, 11H, ar.), 7.55–7.70 (m, 4H, ar.), 7.75 (s,
2H, ar.), 7.85 (d, 2H, ar.), 8.45 (s, 2H, methine). Anal. Calcd for
C34H30N5:C,80.28;H, 5.94;N, 13.77. Found:C,80.64;H, 6.05;N,
13.94%.
9-Phenyl-9H-carbazole-3-carbaldehyde N-methyl-N-phenyl-
hydrazone (3c’). Yield 83 %, (C26H21N3 ~ 375.5 g mol21).
IR (KBr) n/cm21: 3050 (ar. C–H), 2940 (alk. C–H), 1630
¯
Instrumentation
(CLN), 1600, 1500 (ar. CLC), 1300 (C–N). MS (m/z): 375 (M1),
284, 269. 1H NMR (CDCl3) d (ppm): 3.50 (s, 3H, methyl), 6.93
(t, 1H, ar.), 7.20–7.50 (m, 11H, ar.), 7.50–7.65 (m, 2H, ar.), 7.75
(s, 1H, ar.), 7.85 (d, 1H, ar.), 8.20 (d, 1H, ar.), 8.40 (s, 1H,
methine). Anal. Calcd for C26H21N3: C, 83.17; H, 5.64; N,
11.19. Found: C, 83.60; H, 5.71; N, 11.37%.
Nuclear magnetic resonance (1H NMR) spectra were recorded
using a Bruker AC 250 (250 Hz). IR and UV–vis absorption
spectra were recorded using a Bio-Rad Digilab FTS-40 and
UV–vis Hitachi U-3000 spectrophotometer, respectively. Elec-
tron impact mass spectra were obtained on a Finnigan MAT
8500 (70 eV) with a MAT 112 S, Varian. DSC measurements
were carried out with a PerkinElmer DSC-7 at 10 K min21
heating rate under N2 atmosphere. TG analysis was per-
formed on a Netzsch STA 409 with data acquisition system
414/1, at a 10 K min21 heating rate, under an N2 atmosphere.
9-Ethyl-9H-carbazole-3,6-dicarbaldehyde bis(N,N-diphenyl-
hydrazone) (4a). Yield 91%, (C40H33N5 ~ 583,7 g mol21),
199.5–200.5 uC. IR (KBr), n/cm21: 3059 (ar. C–H), 2975, 2931
¯
(alk. C–H), 1629 (CLN), 1590, 1494 (ar. CLC), 1293 (C–N). MS
(m/z): 583 (M1), 428, 414, 388, 359, 246, ,204, 168, 77. 1H
NMR (CDCl3), d (ppm): 1.38 (t, 3H, alk.), 4.31 (m, 2H, alk.),
7.16–7.46 (m, 24H, ar.), 7.83 (d, 2H, ar.), 8.20 (s, 2H, methine).
Anal. Calcd for C40H33N5: C, 82.30; H, 5.70; N, 12.00. Found:
83.14; H, 5.69; N, 12.21%.
Sample preparation
The samples for the ionization potential measurements were
prepared by casting films from THF solutions on Al plates,
pre-coated with y0.5 mm thick methylmethacrylate and a
methacrylic acid copolymer (MKM) adhesive layer. A 0.3%
solution of MKM in a 1 : 1 acetone–water mixture was used
for pre-coating. The function of this layer was not only to
improve adhesion, but also to eliminate the electron photo-
emission from the Al layer. No photoemission was detected
from the Al layer over-coated with MKM at illumination with
up to 6.25 eV quanta energy light. In addition, the MKM layer
is conductive enough to avoid charge accumulation on it
during measurements. The thickness of the film of hydrazone
compound was 0.5–1 mm. The ionization potentials were
measured by the electron photoemission in air method.19,20
The films for the measurements of the charge carrier mobility
were prepared by casting from 10–12% THF solutions of the
hydrazone compounds or their mixtures with PC at a mass
proportion of 1 : 1. The substrates were glass plates with a
conductive SnO2 layer or polyester film with an Al layer. After
preparation, the samples were dried at 80 uC for 1 h. The
thickness of the charge transporting layers varied in the range
of 2–10 mm.
9-Ethyl-9H-carbazole-3,6-dicarbaldehyde
bis(N-methyl-N-
phenylhydrazone) (4a’). Yield 35%, (C30H29N5 ~ 459.6 g mol21),
mp 229–230.5 uC. IR (KBr), n/cm21: 3052 (ar. C–H), 2973, 2930,
¯
2893 (alk. C–H), 1629 (C~N), 1593, 1500 (ar. C~C), 1296 (C–N).
MS (m/z): 459 (M1), 430, 352, 326, 296, 230, 164, 133, 106, 77. 1H
NMR (CDCl3), d (ppm): 1.45 (m, 3H, alk.), 3.45 (d, 6H, methyl),
4.35 (m, 2H, alk.), 6.90 (m, 2H, ar.), 7.26–7.48 (m, 10H, ar.), 7.70 (d,
2H, ar.), 7.90 (m, 2H, ar.), 8.40 (d, 2H, methine). Anal. Calcd for
C30H29N5: C, 78.40; H, 6.36; N, 15.24. Found: C, 78.91; H, 6.47; N,
15.43%.
9-Hexyl-9H-carbazole-3,6-dicarbaldehyde bis(N,N-diphenyl-
hydrazone) (4b). Yield 53%, (C44H41N5 ~ 639,8 g mol21),
mp 134–135 uC. IR (KBr), n/cm21: 3059, 3035 (ar. C–H), 2953,
¯
2926, 2856 (alk. C–H), 1628 (CLN), 1590, 1494 (ar. CLC), 1292
(C–N). MS (m/z): 637, 563, 470, 397, 317, 275, 168, 77. 1H
NMR (CDCl3), d (ppm): 0.84 (t, 3H, alk.), 1.29 (m, 6H, alk.),
1.83 (m, 2H, alk.), 4.24 (m, 2H, alk.), 7.16–7.46 (m, 2H, ar.),
7.85 (d, 2H, ar.), 8.20 (s, 2H, methine). Anal. Calcd for
C44H41N5: C, 82.59; H, 6.46; N, 10.59; Found: C, 82.94; H,
6.61; N, 10.78%.
Hole drift mobility measurements
The hole drift mobility was measured by a time-of-flight
method21 in an electrophotographic regime.22 The electric field
was created by positive corona charging. The charge carriers
were generated at the layer surface by illumination with pulses
from a nitrogen laser (pulse duration was 2 ns, l ~ 337 nm).
The layer surface potential decrease as a result of pulse
illumination was up to 1–5% of the initial potential before
illumination. A capacitance probe connected to the wide fre-
quency band electrometer was used to measure the rate of the
surface potential decrease dU?dt21 The transit time tt for was
determined by the kink on the curve of the dU?dt21 transient
on the linear or double logarithm plot. The drift mobility was
calculated by the formula m ~ d2?U021?tt21, where d is the layer
thickness, andd U0 is the surface potential at the moment of
illumination.
9-Hexyl-9H-carbazole-3,6-dicarbaldehyde bis(N-methyl-N-
phenylhydrazone) (4b’). Yield 84%, (C34H37N5
~ 515.7
g mol21), mp 203–205 uC. IR (KBr), n/cm21: 3055 (ar. C–H),
¯
2953, 2925, 2856 (alk. C–H), 1629 (CLN), 1595, 1500 (ar. CLC),
1293 (C–N). MS (m/z): 515 (M1), 444, 408, 382, 323, 258, 204,
178, 164, 77. 1H NMR (CDCl3), d (ppm): 0.85 (s, 3H, alk.), 1.30
(s, 6H, alk.), 1.85 (s, 2H, alk.), 3.45 (s, 6H, methyl), 4.36 (s, 2H,
alk.), 6.92 (d, 2H, ar.), 7.26–7.49 (m, 10H, ar.), 7.72 (s, 2H, ar.),
7.90 (d, 2H, ar.), 8.35 (s, 2H, methine). Anal. Calcd for
C34H37N5: C, 79.18; H, 7.23; N, 13.58. Found: 79.41; H, 7.34;
N, 13.71%.
9-Phenyl-9H-carbazole-3,6-dicarbaldehyde bis(N,N-diphenyl-
hydrazone) (4c). Yield 72 %, (C44H33N5 ~ 631.8 g mol21), mp
J. Mater. Chem., 2002, 12, 3469–3474
3473

View Article Online
6
P. M. Borsenberger, L. Pautmeier, R. Richert and H. Ba¨ssler,
J. Chem. Phys., 1991, 94, 8276–8281.
Measurements of electrographic parameters
The electrographic parameters of the layers were measured
using standard electrographic research procedures.23 The
photosensivity (S1/2/m2 J21) was defined using the half value
of the initial potential criteria. For this measurement the
layers were illuminated with light of the constant intensity
7
8
N. Mori, US Pat., 5 567 557, 1996, CAS No 123:241957.
N. Tatsya and U. Minoru, Jpn. Pat., 8 101 524, 1996, CAS
No125:100121.
V. Gaidelis, J. Gavutiene, V. Getautis, J. V. Grazulevicius,
V. Jankauskas, R. Kavaliunas, R. Lazauskaite, O. Paliulis,
M. Rossman, D. J. Sidaravicius, T. S. Smith and A.
Stanishauskaite, US Pat., 6 214 503, 2001, CAS No 134:273544.
9
(1017 quanta s21 m22). The residual potential (UR) was mea-
21
0
sured after illumination of a layer with (U0(dU?dt21
)
)
10 Y. Suzuki, US pat., 5 665 500, 1997, CAS No 125:22263.
11 H. Nam, D. H. Kang, J. K. Kim and S. Y. Park, Chem. Lett., 2000,
11, 1298–1299.
12 S. Nomura, K. Nishimura and Y. Shirota, Mol. Cryst. Liq. Cryst.,
1998, 315, 519–524.
1018 quanta s21 m22 units of light (U0 is the initial potential,
(dU?dt21)0 is the initial rate of potential photodecay) at a low
rate of surface potential decay.
13 A. Fujii, T. Shoda, S. Aramaki and T. Murayama, J. Imaging Sci.
Technol., 1999, 43, 430–436.
Acknowledgements
14 K. Naito and A. Miura, J. Phys. Chem., 1993, 97, 6240–6248.
15 A. Vilsmeier and A. Haack, Ber., 1927, 60, 119–122.
16 R. M. Silverstein, G. C. Bassler and T. C. Morrill, Spectroscopic
Identification of Organic Compounds, Wiley, New York, 1991,
p. 289–315.
Financial support from the State Science and Studies
Foundation of Lithuania is acknowledged. J. O. thanks Prof.
H.-W. Schmidt (University of Bayreuth, Germany) for access
to the laboratories of the Department of Macromolecular
Chemistry I in order to characterize the compounds reported
above.
17 V. Gaidelis, L. Jelenskis, E. Montrimas and D. Jurevicius,
Lithuanian J. Phys., 1984, 24, 80–89.
18 Ch. Beginn, J. V. Grazulevicius, P. Strohriegl, J. Simmerer and
D. Haarer, Macromol. Chem. Phys., 1994, 195, 2353–2370.
19 S. Grigalevicius, G. Blazys, J. Ostrauskaite, J. V. Grazulevicius,
V. Gaidelis, V. Jankauskas and E. Montrimas, Synth. Met., 2002,
128, 127–131.
References
1
P. Strohriegl and J. V. Grazulevicius, in Advanced Functional
Molecules and Polymers, ed. H. S. Nalwa, Gordon & Breach,
Chichester, 2001, vol. 3, p. 1–77.
20 M. Daskeviciene, V. Gaidelis, V. Getautis, V. Jankauskas,
O. Paliulis and J. Sidaravicius, Lithuanian J. Phys., 2001, 41,
521–526.
2
3
P. M. Borsenberger, Adv. Mater. Opt. Electron., 1992, 1, 73–80.
S. Nomura, K. Nishimura and Y. Shirota, Thin Solid Films, 1996,
273, 27–34.
S. Nomura, K. Nishimura and Y. Shirota, Mol. Cryst. Liq. Cryst.,
1998, 313, 247–252.
21 P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for
Imaging Systems, Marcel Dekker, New York, 1993, p. 273.
22 J. Kalade, E. Montrimas and V. Jankauskas, in Proc. ICPS’94:
The Physics and Chemistry of Imaging Systems, Rochester, 1994,
p. 747.
4
5
C. H. Chen, J. Shi and C. W. Tang, Macromol. Symp., 1998, 125,
1–48.
23 R. M. Schaffert, Electrophotography, The Focal Press, London
and New York, 1965, p. 203.
3474
J. Mater. Chem., 2002, 12, 3469–3474