Glass-forming hole-transporting triphenylamine-based hydrazones with reactive functional groups

Article abstract of DOI:10.1080/15421400601150361

Synthesis and the properties of a series of triphenylamine-based hydrazones are reported. The dependence of their thermal and glass-forming properties on their chemical structure is discussed. The ionization potentials of the synthesized hydrazones measured by the electron photoemission technique range from 5.16 to 5.32 eV. Hole-drift mobilities of some derivatives molecularly dispersed in bisphenol Z polycarbonate exceed 10-4cm2V-1s-1 at high electric fields (106V/cm),at room temperature.

Full text of DOI:10.1080/15421400601150361

Mol. Cryst. Liq. Cryst., Vol. 466, pp. 85–100, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601150361
Glass-Forming Hole-Transporting
Triphenylamine-Based Hydrazones
with Reactive Functional Groups
Ausra Matoliukstyte
Juozas Vidas Grazulevicius
Department of Organic Technology, Kaunas University of Technology,
Kaunas, Lithuania
Vygintas Jankauskas
Department of Solid State Electronics, Vilnius University, Vilnius,
Lithuania
Synthesis and the properties of a series of triphenylamine-based hydrazones are
reported. The dependence of their thermal and glass-forming properties on their
chemical structure is discussed. The ionization potentials of the synthesized
hydrazones measured by the electron photoemission technique range from 5.16
to 5.32 eV. Hole-drift mobilities of some derivatives molecularly dispersed in
ꢀ4
2
ꢀ1 ꢀ1
6
bisphenol Z polycarbonate exceed 10 cm V
at room temperature.
s
at high electric fields (10 V=cm),
Keywords: Hole-drift mobility; Hydrazone; Ionization potential; Triphenylamine
INTRODUCTION
Aromatic hydrazones of the general formula RCH=NꢀNR , where R
2
are aromatic, heteroaromatic, or aliphatic groups, are widely studied
as organic hole-transport materials [1–3]. Hydrazones as well as
0
0
aromatic amines, such as N,N -diphenyl-N,N -bis(3-methylphenyl)-
(
0
0
1,1 -biphenyl)-4,4 -diamine (TPD) [4] and 1,1-bis(di-4-tolylaminophenyl)
cyclohexane (TAPC) [5], are widely used in electrophotography. Most
of the modern organic electrophotographic photoreceptors have a
Address correspondence to Juozas Vidas Grazulevicius, Department of Organic
Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas,
Lithuania. E-mail: Juozas.Grazulevicius@ktu.lt
85

8
6
A. Matoliukstyte et al.
dual-layer configuration. The main advantage of this configuration is
the possibility of separate optimization of the two layers. The charge-
generation 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. Introduc-
tion of such a large amount of low-molar-mass charge-transport com-
pound into the polymer matrix can lead to crystallization.
Morphological stability of charge-transported layers can be increased
either by using charge-transport compounds that do not readily crys-
tallize (molecular glasses) or using (photo)polymerizable or (photo)-
cross-linkable charge-transport materials.
Triphenylamine-containing hydrazones described in the literature
exhibit relatively high hole-drift mobilities [6]. Most of triphenylamine-
based hydrazones reported contain only one hydrazine moiety per
molecule. Such molecules are rather small, and their inclination to
crystallize is rather high. In this work, we report on the properties
of triphenylamine-based dihydrazones with the functional hydroxyl
and epoxypropyl groups. When we designed these molecules, we
expected that an increase of the molar mass of the triphenylamine-
based hydrazones would reinforce the inclination of the molecules to
form glasses and that the enlarged system of conjugated p-electrons
would allow enhanced charge-carrier mobilities. The presence of
reactive functional groups should make them useful in prepartion of
polymeric or cross-linked charge-transport layers.
EXPERIMENTAL
Materials
The starting compounds 4-iodoanisole, triphenylamine, boron tribromide,
N,N-diphenylhydrazine hydrochloride, N-methyl-N-phenylhydrazine,
0
epichlorohydrin, and 4,4 -thiobisbenzenethiol were purchased from
Sigma-Aldrich. 3-Bromomethyl-3-methyloxetane was bought from Che-
mada Fine Chemicals (Israel). All the starting compounds were used
as received. All the required auxiliary chemicals [i.e., 18-crown-6, potass-
ium carbonate, copper powder, phosphorus oxychloride, sodium sulphate,
benzyltrimethylammonium chloride, magnesium sulphate, sodium
hydride, and triethylamine (TEA)] were purchased from Aldrich. Organic
solvents were purified and dried by standard methods [7]. Silica gel was
used for column chromatography.

Triphenylamine-Based Hydrazones
87
N-(4-Methoxyphenyl)-N-phenylbenzenamine (1)
Diphenylamine (10 g, 59 mmol), 4-iodoanisole (18.02 g, 77 mmol),
powdered potassium carbonate (27.6 g, 200 mmol), copper powder
(
5
7.5 g, 118 mmol), and 18-crown-6 (0.56 g, 2.1 mmol) were refluxed in
0 ml of o-dichlorobenzene under nitrogen for 36 h. Copper and
inorganic salts were filtered off. The solvent was removed by distil-
lation. The product was crystallized from methanol, filtered, and
ꢁ
dried. Yield: 13.42 g (83%) of C H NO (mp: 104 C). FW ¼ 275 g=mol.
1
9
17
þ
þ
ꢀ1
MS(APCI , 20 V), m=z (%): 276 ([M þ H] , 100). IR (in KBr) n=cm
:
3
(
053, 3003 (CH ), 2909 (CH
), 1585, 1507, 1490 (C=C ), 1246
ar
aliphatic ar
1
CꢀOꢀC). H NMR (300 MHz, CDCl ), d (ppm): 3.65 (s, 3H, ꢀOCH ),
3
3
6
.67–7.06 (m, 14H, ꢀCH ).
ar
Di(4-formylphenyl)-4-methoxyphenylamine (2)
POCl (10.11 ml, 110 mmol) was added dropwise to dry dimethylfor-
3
ꢁ
mamide (10.13 ml, 131 mmol) at 0 C under an N atmosphere. Then
2
the solution of N-(4-methoxyphenyl)-N-phenylbenzenamine (1) (3 g,
1
1 mmol) in 15 ml of o-dichlorobenzene was added stepwise to the
ꢁ
reaction flask. The reaction mixture was stirred at 85 C until the
starting compound was reacted. Then the reaction mixture was
cooled down to the room temperature, poured into ice water, and
neutralized with sodium acetate until pH ¼ 6–8. The crude product
was extracted with chloroform several times. The chloroform solution
was washed with water, dried with anhydrous sodium sulphate,
and filtered, and the solvent was removed. The product was purified
by silica-gel column chromatography using an eluent mixture of hexane
and acetone in a volume ratio of 3:1. Yield: 2.41 g (67%) of C H NO
21
þ
17
3
þ
(
(
FW ¼ 331 g=mol). MS(APCI , 20 V), m=z (%): 332 ([M þ H] , 100). IR
ꢀ1
in KBr) n=cm : 3062, 3008 (CH ), 2835 (CH
), 1687 (CHO),
aliphatic
ar
1
1
583, 1561 (C=C ), 1247 (CꢀOꢀC). H NMR (300 MHz, CDCl ), d
ar
3
(
ppm): 3.70 (s, 3H, ꢀOCH ), 6.77–7.17 (m, 8H, ꢀCH ), 7.80–7.84
3
ar
(
m, 4H, ꢀCH ), 9.66 (s, 2H, ꢀCHO).
ar
Di(4-formylphenyl)-4-hydroxyphenylamine (3)
Di(4-formylphenyl)-4-methoxyphenylamine(2) (2 g, 6 mmol) was
dissolved in dry dichloromethane (45 ml) at room temperature and
ꢁ
cooled to ꢀ80 C (acetone=dry ice). Then 30 ml of 1 M solution of boron
tribromide (30 mmol) in dichloromethane was added slowly. The
ꢁ
homogeneous mixture obtained was stirred at ꢀ80 C for 30 min and
was then allowed to warm slowly to room temperature overnight.
The reaction mixture was poured into water (200 ml), stirred for
3
0 min, and extracted with ethyl acetate. The combined organic phase

8
8
A. Matoliukstyte et al.
was dried over MgSO , filtered, and concentrated under vacuum. The
4
product was purified by silica-gel column chromatography using an
eluent mixture of hexane and acetone in a volume ratio of 4:1. Yield:
þ
1
.55 g (81%) of C H NO (FW ¼ 317 g=mol). MS(APCI , 20 V), m=z
2
0
15
3
þ
ꢀ1
(
(
(
%): 318 ([M þ H] , 100). IR (in KBr) n=cm : 3279 (OH), 3065, 3030
1
CH ), 2823 (CH
), 1691 (CHO), 1585, 1504 (C=C ). H NMR
ar
ar
aliphatic
300 MHz, CDCl ), d (ppm): 4.95 (s, 1H, ꢀOH), 6.95–7.30 (m, 8H,
3
ꢀ
CH ), 7.76–7.80 (m, 4H, ꢀCH ), 9.88 (s, 2H, ꢀCHO).
ar
ar
Hydrazones 4 and 5 (1–2 g) were prepared by condensation of
dialdehyde 3 and the derivatives of hydrazine by the following general
procedure.
A solution of N, N-diphenylhydrazine hydrochloride (or N-methyl-
N-phenylhydrazine) in methanol was added dropwise to a solution of
di(4-formylphenyl)-4-hydroxyphenylamine (3) in methanol by stirring
in a molar ratio of 2 mol of hydrazine to 1 mol of formyl group. The
reaction mixture was refluxed for 1 h. Then it was cooled down to
the room temperature. The product was purified by column chromato-
graphy (silica gel, eluent acetone–hexane, 1:2). The compound 5 was
recrystallized from methanol. The compound 4 was isolated as
amorphous powders after column chromatography.
Di((4-(diphenylamino)iminomethyl)phenyl)-4-
hydroxyphenylamine (4)
þ
Yield: 1.56 g (76%) of C H N O (FW ¼ 649 g=mol). MS(APCI ,
4
4
35
5
þ
ꢀ1
2
3
(
0 V), m=z (%): 650 ([M þ H] , 27). IR (in KBr) n=cm : 3424 (OH),
060, 3033 (CH ), 2953, 2922 (CH
), 1596 (C=N), 1506, 1493
aliphatic
ar
1
C=Car). H NMR (300 MHz, CDCl ), d (ppm): 4.94 (s, 1H, ꢀOH),
3
7
.03–7.49 (m, 32H, aromatic protons and 2H, PhꢀCH=N-).
Di((4-(methylphenylamino)iminomethyl)phenyl)-4-
hydroxyphenylamine (5)
þ
Yield: 1 g (68%) of C H N O (FW ¼ 525 g=mol). MS(APCI , 20 V),
3
4
31
5
þ
ꢀ1
m=z (%): 526 ([M þ H] , 17). IR (in KBr) n=cm : 3320 (OH), 3065,
3
(
5
023 (CH ), 2950, 2904, 2882 (CH
), 1593 (C=N), 1536, 1493
ar
1
aliphatic
C=Car). H NMR (300 MHz, CDCl ), d (ppm): 3.40 (s, 6H, ꢀCH ),
3
3
.01 (s, 1H, ꢀOH), 6.95–7.34 (m, 22H, aromatic protons and 2H,
PhꢀCH=Nꢀ).
The synthesis of hydrazones bearing epoxy groups 6 and 7 was
carried out according to the following general procedure.
The corresponding hydrazone (2 mmol), 20 ml of epichlorohydrin,
and 0.02 g of benzyltrimethylammonium chloride were placed in a
ꢁ
1
00 ml, round-bottomed flask. The mixture was heated at 80 C
for 2 h. The excess of epychlorohydrin was removed at reduced

Triphenylamine-Based Hydrazones
89
pressure, and the resin obtained was dissolved in CH Cl , washed sev-
2
2
eral times with water, and dried over magnesium sulphate. The solvent
was evaporated under vacuum, and the product was purified by silica-
gel column chromatography (silica gel, eluent hexane–chloroform, 4:1),
followed by recrystallization of compound 7 from methanol. Compound
6
was isolated as amorphous powder after column chromatography.
Di((4-(diphenylamino)iminomethyl)phenyl)-4-
(2,3-epoxypropyl)oxyphenylamine (6)
þ
Yield: 0.67 g (62%) of C H N O (FW=705 g=mol). MS(APCI ,
4
7
39
5
2
þ
ꢀ1
2
0 V), m=z (%): 706 ([M þ H] , 66). IR (in KBr) n=cm : 3059, 3045
(
CH ), 2967, 2825 (CH
), 1599 (C=N), 1506, 1493 (C=C ), 1245
ar
aliphatic
ar
1
(
CꢀOꢀC). H NMR (300 MHz, CDCl ), d (ppm): 2.80 (dd, 1H, in oxi-
3
rane ring), 2.82 (dd, 1H, in oxirane ring), 3.40 (m, 1H, ꢀOCHꢀ),
3
6
.97 (dd, 1H, ortho to ꢀOCH ꢀ), 4.00 (dd, 1H, para to ꢀOCH ꢀ),
2
2
.90–7.54 (m, 32H, aromatic protons and 2H, PhꢀCH=Nꢀ).
Di((4-(methylphenylamino)iminomethyl)phenyl)-4-
(2,3-epoxypropyl) Oxyphenylamine (7)
þ
Yield: 0.5 g (43%) of C H N O (FW ¼ 581 g=mol). MS(APCI ,
3
7
35
5
2
þ
ꢀ1
2
(
1
0 V), m=z (%): 582 ([M þ H] , 84). IR (in KBr) n=cm : 3055, 3032
CH ), 2950, 2904, 2882 (CH
), 1600 (C=N), 1536, 1493 (C=C ),
ar
aliphatic
ar
1
240 (CꢀOꢀC). H NMR (300 MHz, CDCl ), d (ppm): 2.82 (dd, 1H, in oxi-
3
rane ring), 2.90 (dd, 1H, in oxirane ring), 3.40 (m, 1H, ꢀOCHꢀ), 3.51 (s,
3
H, ꢀCH ), 4.0 (dd, 1H, ortho to ꢀOCH ꢀ), 4.2 (dd, 1H, para to
3
2
ꢀ
OCH ꢀ), 6.93–7.62 (m, 22H, aromatic protons and 2H, PhꢀCH=Nꢀ).
2
Di((4-(diphenylamino)iminomethyl)phenyl)-4-
((3-methyloxetan-3-yl)methoxy) Phenylamine (8)
Compound 4 (0.2 g, 0.3 mmol) was dissolved in 10 ml of ethylmethyl-
ketone, and 3-bromomethyl-3-methyloxetane (5ml) was added dropwise.
After adding of 0.09 g (3.6 mmol) of NaH, the reaction was carried out
ꢁ
at 60 C for 2 h. The excess of 3-bromomethyl-3-methyloxetane was
removed at reduced pressure, and the resin obtained was dissolved
in ethyl acetate, washed several times with water, and dried over
magnesium sulphate. The solvent was evaporated under vacuum,
and the product was purified by silica-gel column chromatography
using an eluent mixture of hexane and acetone in a volume ratio
of 3:1. Yield: 0.17 g (73%) of C H N O (FW ¼ 733 g=mol).
4
9
43
5
2
þ
þ
ꢀ1
MS(APCI , 20 V), m=z (%): 734 ([M þ H] , 56). IR (in KBr) n=cm
:
3
1
1
049, 3033 (CH ), 2977, 2845, 2840 (CH
), 1583 (C=N), 1500,
ar
aliphatic
1
495 (C=C ), 1243 (CꢀOꢀC). H NMR (300 MHz, CDCl ), d (ppm):
ar
3
.49 (s, 3H, ꢀCH ), 4.06 (s, 2H, ꢀOCH ), 4.50 (ds, 2H, in oxetane
3
2

9
0
A. Matoliukstyte et al.
ring), 4.68 (ds, 2H, in oxetane ring), 6.91–7.50 (m, 32H, aromatic pro-
tons and 2H, PhꢀCH=Nꢀ).
Di(4-(4-(di((4-(diphenylamino)iminomethyl)phenyl)amino)-
phenyl)oxy-2-hydroxypropylthio)phenylsulfide (9)
0
Compound 6 (0.5 g, 0.77 mmol) and 0.1 g (0.38 mmol) of 4,4 -thiobisben-
zenethiol were dissolved in 10 ml of ethylmethylketone. The reaction
ꢁ
was catalyzed by adding of 0.2 ml of TEA at 80 C. After 24 h, the reac-
tion was terminated by precipitation into methanol. The precipitated
product was filtered off and purified by silica-gel column chromato-
graphy using an eluent mixture of hexane and chloroform in a volume
ratio of 1:2. Yield: 0.41 g (64%) of C H N O S . IR (in KBr),
1
06 88 10
4 3
ꢀ
1
n=cm : 3426 (OH), 3065, 3034 (CHar.), 2923, 2882, 2867 (CHaliphatic),
1
1
586 (C=N), 1593, 1512, 1482 (C=Car.). H NMR (300 MHz, CDCl ), d
3
(
4
ppm): 2.69–2.85 (m, 2H, ꢀCH ꢀCHꢀCH ꢀ), 3.30 (m, 4H, ꢀSCH ),
2
2
2
.07 (s, 4H, ꢀOCH ), 4.14 (s, 2H, ꢀOH), 6.84–7.52 (m, 72H, aromatic
2
protons and 4H, PhꢀCH=Nꢀ).
Measurements
1
H NMR spectra of deuterated chloroform solutions were taken on a
1
Gemini-2000 [300-MHz ( H)] spectrometer. IR spectra were recor-
dered using a Perkin-Elmer FT-IR system. The spectra of solid com-
pounds were performed in a form of KBr pellets. Electron-impact
mass spectra were obtained on a Waters 2Q 2000. The UV spectra
TM
ꢀ5
were recorded on a Spectronic Genesys
8 spectrometer. A 10
M
solution of an investigated compound in THF was placed into a
microcell with an internal width of 1 mm.
Differential scanning calorimetry (DSC) measurements were car-
ꢁ
ꢀ1
ried out with a Mettler DSC-821e at 10 C min heating rate under
a nitrogen atmosphere. The glass-transition temperatures (T ) were
g
determined from second heating scans. Thermogravimetric analysis
ꢁ
ꢀ1
(
TGA) was performed on a Mettler TGA=SDTA 851e at a 10 C min
heating rate under a nitrogen atmosphere.
The ionization potentials (I ) of the films of the synthesized com-
p
pounds were measured by electron photoemission in the air method
as described before [8,9]. The standard error in the mean to 95% con-
fidence for the values of ionization potential was 0.04 eV.
Hole-drift mobilities were measured by the xerographic time of
flight (XTOF) technique [10,11]. Positive corona charging created an
electric field inside the charge transport layer. Charge carriers were
generated at the layer surface by illumination with pulses of nitrogen
laser (pulse duration was 2 ns, wavelength 337 nm). The layer surface

Triphenylamine-Based Hydrazones
91
potential decrease as a result of pulse illumination was up to 1–5% of
the initial potential before illumination. The capacitance probe that
was connected to the wide-frequency-band electrometer measured
the rate of the surface potential decrease, dU=dt. The transit time tt
was determined by the kink on the transient curve in a linear scale.
2
The drift mobility was calculated by the formula l ¼ d =U0tt, where
d is the layer thickness and U is the surface potential at the moment
0
of illumination. The samples for the measurements were prepared by
casting the solutions of the molecular mixtures of the synthesized com-
pounds with bispenol Z polycarbonate at mass proportion 1:1 or 1:1.6
in THF. The substrates were polyester films with an Al layer.
RESULTS AND DISCUSSION
Compounds 4 and 5 were prepared as described in Scheme 1 by a mul-
tistep synthetic route involving Ullmann coupling of diphenylamine
with 4-iodoanisole, Vilsmeyer formylation of N-(4-methoxyphenyl)-N-
phenylbenzenamine 1 with POCl =DMF complex, demethylation of
3
methoxy group of diformyl compound 2, and condensation of com-
pound 3 with differently substituted hydrazines.
SCHEME 1 Reagents and conditions: (a) 4-iodoanisole, Cu, 18-crown-6,
K CO , o-dichlorobenzene, reflux, 36 h; (b) POCl =DMF complex, o-dichloro-
2
3
3
o
benzene, 85 C, 24 h; (c) BBr
3
, CH
2
Cl
2
, 24 h; (d) methanol, N,N-diphenylhydra-
zine hydrochloride for 4 and N-methyl-N-phenylhydrazine for 5, reflux, 1 h.

9
2
A. Matoliukstyte et al.
SCHEME 2 Reagents and conditions: (a) benzyltrimethylammonium chloride,
ꢁ
epichlorohydrine for 6 and 7, 80 C, 2 h; (b) 3-bromomethyl-3-methyloxetane,
ꢁ
NaH, ethylmethylketone, 60 C, 2 h.
Hydrazones containing reactive epoxy groups (6–8) were synthesized
by alkylation of the corresponding hydroxyl substituted hydrazones
(
4,5) with epychlorohydrin. Hydrazone with reactive oxetanyl group
was prepared by alkylation of compound 4 with 3-bromomethyl-
3
-methyloxetane in the presence of NaH (Scheme 2).
With the purpose of demonstrating how the reactive functional
groups can be employed in further synthesis, we have synthesized
the twin-like molecule 9. The increase of molecular size and especially
the presence of hydroxyl groups capable of hydrogen bonding can
improve morphological stability of molecular glasses [2,3]. Compound
0
9
was synthesized by the condensation of hydrazone 6 with 4,4 -
thiobisbenzenethiol in the presence of TEA as a catalyst (Scheme 3).
The structures of the newly synthesized triphenylamine-based
1
hydrazones were confirmed by IR, H NMR, and mass spectrometries.
The synthesized compound can be used for the preparation of cross-
linked charge-transport layers. Compound 9 can be cross-linked with
polyisocianates as cross-linking agents, as was reporterd earlier for
similar systems [12]. Both monomers 6–8 and compound 9 can also
be incorporated into a polymer network by the methods used for hard-
ening epoxy resins.
Thermal properties of the triphenylamine-based dihydrazones
were examined by DSC and TGA under a nitrogen atmosphere. The
triphenylamine-based hydrazones having diphenyl substituents at

Triphenylamine-Based Hydrazones
93
0
SCHEME 3 Reagents and conditions: (a) 4,4 -thiobisbenzenethiol, TEA,
ethylmethylketone, 60 C, 24 h.
ꢁ
the hydrazine N atom (4, 6, 8, and 9) were obtained as amorphous
substances as confirmed by DSC. When the samples of 4, 6, 8, and 9
ꢁ
were heated, the glass transitions were observed in range 64–109 C,
and no peaks due to crystallization and melting appeared. Cooling
down and repeated heating revealed only the glass transitions again.
The methylphenyl-substituted hydrazones 5 and 7, which were iso-
lated as crystalline compounds, show melting only in the first DSC
heating scan and form glasses upon cooling from the melt. The second
ꢁ
ꢁ
heating scan reveals only glass transition at 87 C for 5 and 65 C for 7
and no signals due to crystallization and melting. DSC curves of com-
pound 5 are given in Fig. 1 for an illustration.
The glass-transition temperatures (T ), melting temperatures (T ),
g
m
and temperatures at which initial loss of mass (5%) was observed (TID)
are summarized in Table 1. Hydrazones having hydroxyl groups (4,5)
ꢁ
show T by more than 20 C higher than the corresponding alkylated
g
derivatives (6–8). This observation can hardly by explained by intro-
duction of short alkyl chains and epoxide or oxetane cycles into the
molecules of 6–8. Apparently, hydrogen bonding predetermines higher
T of compounds 4 and 5. Compound 9 shows the highest T among the
g
g
compounds synthesized. This observation can also by explained by
hydrogen bonding as well by increased molecular size. The comparison
of the values of T of N,N-diphenyl-substituted hydrazones (4, 6) and
g
N-methyl-N-phenyl-substituted hydrazones (5, 7) shows that the Tg
ꢁ
values of the former compounds are higher only by 2 C than those of
the later derivatives.
The data presented in Table 1 show that the thermal stability of the
synthesized hydrazones is comparable with that of other aromatic
hydrazones reported earlier [13]. A 5% mass loss for compounds 4, 6,

9
4
A. Matoliukstyte et al.
FIGURE 1 DSC curves of dihydrazone 5, at the heating=cooling rate of
ꢁ
1
0 C=min, N
2
atmosphere.
and 8, having diphenyl-substituted hydrazine nitrogen atoms, is
ꢁ
observed at 255 C, whereas compounds 5 and 7, containing methylphe-
nyl-substituted nitrogen atoms, show a corresponding mass loss at
ꢁ
2
92 C. It is evident that the substituents at the hydrazine N atom affect
the 5% mass loss temperature and consequently the thermal stability
of the synthesized compounds. This observation corroborates the ear-
lier observation that the thermal degradation of aromatic hydrazones
starts from the degradation of the hydrazone moiety [13]. The nature
TABLE 1 Thermal Characteristics of the Triphenylamine-Based
Dihydrazones
ꢁ
ꢁ
ꢁ
Compound
T
g
( C)
T
m
( C)
T
ID( C)
a
4
5
6
7
8
9
89
87
67
65
64
—
255
292
255
292
255
265
b
a
b
a
a
194
—
162
—
109
—
a
Obtained as an amorphous material.
First heating only.
b

Triphenylamine-Based Hydrazones
95
ꢀ5
ꢀ1
)
FIGURE 2 UV-vis absorption spectra of dilute THF solutions (10 mol l
of 4 (solid line), 5 (dashed line), and triphenylamine (dotted line).
of the substituents at the phenyl group of the tripenylamine moiety
seems not to have influence on the thermal stability of the materials.
The 5% mass-loss temperatures for hydroxyl-, epoxypropyl-, and oxeta-
ꢁ
nyl-substituted derivatives 4, 6, and 8 is the same (i.e., 255 C).
The synthesized hydrazones absorb electromagnetic radiation in the
2
40 to 450-nm region. The absorption spectra of all compounds 4–9 are
similar, due to their similar structure. As an example, the UV-vis
absorption spectra of hydrazones 4 and 5 are shown in Fig. 2. For
comparison, the spectrum of triphenylamine is given. The spectra of
the hydrazones 4 and 5 exhibit a strong bathochromic shift with respect
to the spectrum of triphenylamine. This shows that the p-electron
conjugation system in compounds 4 and 5 is extended. Moreover,
UV spectra of diphenyl-substituted hydrazones exhibit higher values
of molar extinction coefficient and small bathochromic shifts with
respect of the spectra of methylphenyl-substituted hydrazones.
The fluorescence emission spectra of dilute THF solutions of com-
pounds 4 and 5 are shown in Fig. 3. The bands of fluorescence spectra
of hydrazones are shifted toward the long wavelength region in
comparison with those of triphenylamine. The emission from
diphenyl-substituted hydrazones is bathochromically shifted relative
to that from methylphenyl-substituted hydrazones. This observation
is consistent with the UV spectroscopy data.

9
6
A. Matoliukstyte et al.
FIGURE 3 Normalized fluorescence spectra of dilute THF solutions
ꢀ
5
ꢀ1
(
(
10 mol l ) of hydrazones 4 (solid line), 5 (dashed line), and triphenylamine
dotted line). For 4 and 5 kmax ¼ 310 nm; for triphenylamine kmax ¼ 290 nm.
The red shift of UV-Vis absorption and fluorescence spectra can be
interpreted as the extended conjugation of p-electrons. Therefore, the
new derivatives should show lower values of ionization potential (Ip)
than that of triphenylamine derivatives.
The newly synthesized triphenylamine-based hydrazones are
soluble in common organic solvents such as chloroform, acetone, and
tetrahydrofuran. Amorphous thin films on the substrate can be pre-
pared by casting or spin-coating techniques from all hydrazones
reported in this presentation. The stabilities of the films with thick-
nesses of 0.5 to 1 lm were fully sufficient for ionization potential (Ip)
measurements. Figure 4 shows electron photoemission spectra of the
amorphous films of compounds 4–9. The intersection point of the
straight lines drown with the absciss axis gives the values of the ioni-
zation potential, which are summarized in Table 2. The I values of the
p
synthesized hydrazones are more than 0.3 eV lower than those of
4
-methyltriphenylamine, which was reported to be 5.64 eV [14].
The typical time-of-flight transients for the molecular mixtures of
the synthesized hydrazones with bisphenol Z polycarbonate (PC-Z)
are presented in Fig. 5. Nondispersive hole transport is characteristic
of most of the investigated molecular mixtures. The transit time t was
t

Triphenylamine-Based Hydrazones
97
FIGURE 4 Electron photoemission spectra in air of the films of hydrazones 4–9.
obtained from time-of-flight transients in a linear plot from straight
signal decay moment. It was impossible to obtain transit time of
PC-Z doped with 4 because of the higher dark conductivity; however,
the observed signals suggested the fast hole transfer.
The room-temperature electric field dependencies of hole-drift
mobility (l) values for the solid solutions of the synthesized
hydrazones in PC-Z are given in Fig. 6. The linear dependencies of
the hole-drift mobilities on the square root of the electric field E are
observed. In all the cases, drift mobility may be well approximated by
pffiffiffiffi
the formula l ¼ l expða EÞ; here a; is Pool–Frenkel parameter [1].
0
Such dependencies of charge mobility on electrical fields are
characteristic of many organic photoconductors and are predicted
TABLE 2 Ionization Potentials of the Films
of the Triphenylamine-Based Dihydrazones
Compound
Ip (eV)
4
5
6
7
8
9
5.26
5.16
5.32
5.30
5.25
5.30

9
8
A. Matoliukstyte et al.
FIGURE 5 Typical time-of-flight transients for 6 þ PC-Z at various surface
potentials U . The arrow marks transit time t
0
t
.
by the B a¨ ssler–Borsenberger model [1]. The solid solutions of
triphenylamine-based hydrazones 5–9 in PC-Z (mass proportion
1
1
:1.6 or 1:1) demonstrated hole-drift mobility values in a range from
ꢀ
5
ꢀ4
2
4
.8ꢂ10
to 1.1ꢂ10 cm =Vs at electric field E ¼ 6.4 ꢃ 10 V=cm, at
FIGURE 6 Electric field dependencies of the hole-drift mobilities (l) for charge
transport layers of compounds 5–9 doped in PC-Z (mass proportion 1:1.6 or 1:1).

Triphenylamine-Based Hydrazones
99
TABLE 3 Hole-Mobility Data for Charge Transport Layers of Compounds
5–9 Doped in PC-Z
ꢄ
2
ꢄꢄ
2
1=2
Composition
d (lm)
l
0
(cm =Vs)
l
(cm =Vs)
a (cm=V)
ꢀ
7
ꢀ5
Al þ [5 þ (PC-Z), 1:1.6]
Al þ [6 þ (PC-Z), 1:1]
Al þ [7 þ (PC-Z), 1:1]
Al þ [8 þ (PC-Z), 1:1]
Al þ [9 þ (PC-Z), 1:1]
12
6
10
7
6ꢂ10
1.8ꢂ10
8.6ꢂ10
1.1ꢂ10
2.9ꢂ10
2.7ꢂ10
0.0043
0.0041
0.0048
0.0039
0.0038
ꢀ
6
ꢀ5
ꢀ4
ꢀ5
ꢀ5
3.2ꢂ10
2.4ꢂ10
1.3ꢂ10
1.4ꢂ10
ꢀ6
ꢀ6
ꢀ6
5
ꢄ
ꢄꢄ
l
l
0
: zero field mobility.
: hole-drift mobility values at electric field E ¼ 6.4ꢂ10 V=cm.
4
room temperature (Table 3). These are high charge mobilities, as for
molecularly doped polymers.
In conclusion, new glass-forming triphenylamine-based hydrazones
with reactive functional groups were synthesized and investigated as
potential charge-transport materials for optoelectronics. The thermal,
optical, and photoelectrical properties of the synthesized compounds
were studied. The synthesized triphenylamine-based hydrazones form
glasses with the glass-transition temperatures in the range of 64–
ꢁ
1
09 C as characterized by DSC. The electron photoemission spectra
of the synthesized compounds have been recorded, and the ionization
potentials of 5.16–5.32 eV have been determined. Room-temperature
hole-drift mobilities of some synthesized hydrazones molecularly
ꢀ
4
2
ꢀ1 ꢀ1
dispersed in bisphenol Z polycarbonate exceed 10 cm V
s
at
6
high electric fields (10 V=cm). The synthesis and properties of poly-
meric materials based on the newly synthesized monomers will be
included in a future publication.
ACKNOWLEDGMENTS
Financial support of this research by the Lithuanian Science and
Studies Foundation is gratefully acknowledged. Habil. dr. V. Gaidelis
from the Department of Solid State Electronics, Vilnius University,
is thanked for the ionization potential measurements.
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(
1