9-phenylcarbazole-based hydrazone twin compounds as P-type organic semiconductors

Article abstract of DOI:10.1080/15421406.2011.538605

Hole transporting glass-forming low-molar-mass hydrazones containing 9-phenylcarbazole moiety were synthesized and characterized by NMR-, infrared- and mass spectrometries. The thermal, optical and photoelectrical properties of the synthesized compounds a

Full text of DOI:10.1080/15421406.2011.538605

Mol. Cryst. Liq. Cryst., Vol. 536: pp. 182=[414]–191=[423], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.538605
9-Phenylcarbazole–Based Hydrazone Twin
Compounds as P-Type Organic Semiconductors
RAMUNAS LYGAITIS,1 JUOZAS VIDAS
GRAZULEVICIUS,1 AND VYGINTAS JANKAUSKAS2
1Department of Organic Technology, Kaunas University of Technology,
Radvilenu Plentas, Kaunas, Lithuania
2Department of Solid State Electronics, Vilnius University,
Sauletekio Aleja, Vilnius, Lithuania
Hole transporting glass-forming low-molar-mass hydrazones containing
9-phenylcarbazole moiety were synthesized and characterized by NMR-, infrared-
and mass spectrometries. The thermal, optical and photoelectrical properties of
the synthesized compounds are reported. Compounds containing 1,3-benzendithiol
linking bridge were isolated as amorphous materials therefore differential scanning
calorimetry experiments revealed glass transitions approximately at 60ꢀC. The ioni-
zation potentials of the synthesized compounds were measured by electron photoe-
mission technique in air range from 5.23 eV to 5.4 eV. Hole drift mobilities in the
films of the solid solutions of some of the synthesized hydrazones in bisphenol Z
polycarbonate measured by the time-of-flight technique exceed 10ꢁ5 cm2 Vꢁ1 sꢁ1
at high electric fields.
Keywords Carbazole; glass transition; hole transport; hydrazone
Introduction
Aromatic hydrazones which are usually prepared by condensation of formyl
derivatives with aromatic hydrazines, are widely studied and used as organic hole
transport materials for electrophotographic photoreceptors [1,3]. Among hole-
transporting hydrazones carbazole-containing derivatives prevail [4,5]. Carbazole
is a relatively cheap commercial compound widely used for the synthesis of electro-
active compounds [6]. Most of the modern organic electrophotographic photorecep-
tors have a dual-layer configuration. A charge generation layer usually consists of a
dye such as titanyl phthalocyanine dispersed in a polymer binder, e.g., poly(vinyl-
butyral). A charge transport layer is usually prepared by inbeding an organic hole
transport material into a host polymer, e.g., polycarbonate. This layer has to contain
a large amount (50% or even more) of the active material to ensure effective charge
transport. Introduction of such a large amount of low-molar-mass charge transport
compound into a polymer matrix can lead to crystallization. To prevent this problem
Address correspondence to Juozas Vidas Grazulevicius, Department of Organic Tech-
nology, Kaunas University of Technology, Radvilenu Plentas 19, LT-3028, Kaunas,
Lithuania. E-mail: Juozas.Grazulevicius@ktu.lt
182=[414]

9-Phenylcarbazole–Based Hydrazone Twin Compounds
183=[415]
charge transporting compounds which do not readily crystallize are required.
Increasing of molecular size is one of the methods of preventing crystallization.
In this article we report on the synthesis and properties of 9-phenylcarbazole-based
twin hydrazones which are stable in a solid amorphous state and can be used for the
preparation of electrophotographic photoreceptors with enhanced morphological
stability.
Experimental
Materials
Carbazole was purchased from ‘‘Reakhim’’ (Russia). 1-Bromo-4-butylbenzene,
iodobenzene, epichlorohydrin, N-phenylhydrazine, 4,40-thiobisbenzenethiol, 1,3-
benzenedithiol, phosphorous oxychloride were purchased from ‘‘Aldrich’’ and used
as received.
9-(4-Butylphenyl) carbazole-3-carbaldehyde (2a). 7 g (0.023 mol) of 9-(4-
butylphenyl)-carbazole (was prepared according the procedure described in [7])
(1a) were dissolved in DMF and cooled to 0ꢀC. Then 11 ml (0.117 mol) of POCl3
were added dropwise under stirring (N2 atmosphere). The reaction mixture was
stirred at 80–85ꢀC for 72 h, and then poured into the beaker with crushed ice and
neutralized with solution of sodium hydroxide. The obtained precipitate was
washed using the surplus of water. The crude product after drying was purified by
silica gel column chromatography (ethyl acetate=n-hexane ¼ 1:3) to yield 5.85 g
1
(78%) of brownish crystalline material (m.p.: 95ꢀC, FW ¼ 327 g=mol). A H NMR
spectrum yielded the following chemical shifts (100 MHz, CDCl3, d, ppm): 10.19
(s, 1H, CHO), 8.95 (d, 1H, 4-HHt), 8.08 (d, 1H, 5-HHt), 8.0 (d, 1H, 2-HHt),
7.37–7.61 (m, 8H, Ar, Ht), 2.8 (t, 2H, J ¼ 6.5 Hz, PhCH2), 1.23–1.9 (m, 4H,
CH3CH2CH2), 0.99 (t, 3H, J ¼ 6.75 Hz, CH3CH2-). An infrared absorption
spectrum yielded the following peaks (KBr windows), (in cmꢁ1): n (aliphatic CH)
=
=
2940, 2830; n (aldehydic CH) 2720; n (C O) 1650; n (C C in Ar) 1600, 1580; n
(CꢁN) 1250, 1160; c (arene CꢁH) 780. MS (APCIþ, 20 V), m=z: 328 ([M þ H]þ).
9-Phenylcarbazole-3-carbaldehyde (2b). was synthesized by Vilsmeier reaction using
POCl3=DMF complex under nitrogen atmosphere. 7 g (0.029 mol) of 9-
phenylcarbazole were dissolved in DMF and cooled to 0ꢀC. Then 10.6 ml
(0.116 mol) of POCl3 were added dropwise under stirring (N2 atmosphere). The
reaction mixture was stirred at 85ꢀC for 72 h, then poured into the ice water and
neutralized with aqueous solution of sodium hydroxide (30%). The crude product
was filtered and washed with the surplus of water. The crude product was purified
by silicagel column chromatography (eluent: ethylacetate-n-hexane, 1:6). Yield of
1
yellowish crystals 5.6 g (70%, m.p.: 108–109ꢀC). A H NMR spectrum yielded the
following chemical shifts (100 MHz, CDCl3, d, ppm): 10.12 (s, 1H, CHO), 8.65 (s,
1H, 4-HHt), 8.20 (d, 1H, 5-HHt), 7.30–7.75 (m, 9H, Ar, Ht), 7.93 (d, 1H, 2-HHt).
An infrared absorption spectrum yielded the following peaks (KBr windows), (in
cmꢁ1): n (arene CꢁH) 3052; n (C O) 1700; n (C C in Ar) 1600, 1500; n (CꢁN)
=
=
1280, c (arene CꢁH) 744.
9-(4-Butylphenyl) carbazole-3-carbaldehyde N-phenylhydrazone (3a). 8.82 g
(0.027 mol) of 9-(4-butylphenyl) carbazole-3-carbaldehyde was dissolved in 200 ml

184=[416]
R. Lygaitis et al.
of methanol under mild heating. The solution of 4.38 g (0.04 mol) N-phenylhydrazine
in methanol was added to the cooled reaction mixture. 9-(4-Butyl phenyl)
carbazole-3-carbaldehyde reacted completely after 4 hours at room temperature.
The precipitated product was filtered and washed with a big amount of cold
methanol. After the drying it was obtained 9.19 g (81%) of yellowish crystals (m.p.:
120–122ꢀC, FW ¼ 417 g=mol). An infrared absorption spectrum yielded the
following peaks (KBr windows), (in cmꢁ1): n (NH) 3331; n (arene CꢁH) 3030; n
=
(aliphatic CH) 2927, 2860; n (C C in Ar) 1600, 1514; n (CꢁN) 1257, 1228; c
(arene CꢁH) 745. MS (APCIþ, 20 V), m=z: 418 ([M þ H]þ), 326.
9-Phenylcarbazole-3-carbaldehyde N-phenylhydrazone (3b). 4 g (0.0147 mol) of
9-phenylcarbazole-3-carbaldehyde were dissolved in 300 ml of methanol under
mild heating. The solution of 2.5 g (0.023 mol) N-phenylhydrazine in methanol
was added to the cooled reaction mixture. Starting compound 9-phenylcarbazole-
3-carbaldehyde was reacted completely after 3 hours at room temperature. The
reaction mixture was concentrated and then the flask was placed to the freezer for
crystallization. The product was filtered, washed with a large amount of cold
methanol and dried. Yield of yellowish crystals 3.62 g (68%) (m.p.: 84–88ꢀC,
FW ¼ 361 g=mol). An infrared absorption spectrum yielded the following peaks
(KBr windows), (in cmꢁ1): n (NH) 3308; n (arene CꢁH) 3030; n (C C in Ar)
=
1600, 1514; n (CꢁN) 1257, 1228; c (arene CꢁH) 745. MS (APCIþ, 20 V), m=z:
362 ([M þ H]þ), 270.
9-(4-Butylphenyl) carbazole-3-carbaldehyde N-(2,3-epoxypropyl)-N-phenylhydrazone (4a).
6 g (0.014 mol) of 9-phenylcarbazole-3-carbaldehyde N-phenylhydrazone was
dissolved in 20 g of 3-chloro-1,2-epoxypropane. 1.8 g (0.032 mol) of KOH was
added to the reaction mixture in four portions. Also 0.8 g of Na2SO4 was added
during the first addition of KOH. The reaction mixture was stirred for 25 hours
at room temperature. The crude product was extracted with diethylether. The
solvent was removed by rotary evaporation and 3-chloro-1,2-epoxypropane was
removed by vacuum distillation. The crude product was purified by silicagel
column chromatography (eluent: ethylacetate-n-hexane, 1:10). Yield of yellowish
amorphous material 66% (4.47 g) (m.p.: 132–132.5ꢀC, FW ¼ 473 g=mol). A 1H
NMR spectrum yielded the following chemical shifts (100 MHz, CDCl3, d, ppm):
8.423 (d, J ¼ 1.3 Hz, 1H, 4-HHt), 8.2 (d, J ¼ 7.6 Hz, 1H, 5-HHt), 7.93 (s, 1H,
=
CH N), 7.87 (dd, J1 ¼ 1.6 Hz, J2 ¼ 8.5 Hz, 1H, 2-HHt), 7.5–7.25 (m, 11H, Ht, Ph),
6.98 (t, J ¼ 7.2 Hz, 1H, 4-HPh), 4.41 (dd, J1 ¼ 1.9 Hz, J2 ¼ 16.4 Hz, 1H, one NCH2
proton), 4.06 (dd, J1 ¼ 3.8 Hz, J2 ¼ 16.4 Hz, 1H, another NCH2 proton), 3.3 (m,
1H, CHO), 2.91 (t, J ¼ 4.7 Hz, 1H, CH2O one proton), 2.81–2.68 (m, 3H, CH2O
another proton, PhCH2), 1.74 (p, 2H, CH2CH2CH2CH3); 1.56–1.39 (m, 2H,
CH2CH3); 1.02 (t, J ¼ 7.3 Hz, 3H, CH2CH3). An infrared absorption spectrum
yielded the following peaks (KBr windows), (in cmꢁ1): n (arene CꢁH) 3055; n
=
(aliphatic CH) 2927, 2856; n (C C in Ar) 1595, 1515; n (CꢁN) 1230; c (arene
CꢁH) 748. MS (APCIþ, 20 V), m=z: 474 ([M þ H]þ).
9-Phenylcarbazole-3-carbaldehyde N-(2,3-epoxypropyl)-N-phenylhydrazone (4b).
3.5 g (0.0097 mol) of compound 9-phenylcarbazole-3-carbaldehyde N-phenyl-
hydrazone was dissolved in 10 g of 3-chloro-1,2-epoxypropane. 1.8 g (0.032 mol) of
KOH was added to the reaction mixture in four portions. Na2SO4 (0.5 g) was
added during the first addition of KOH. The reaction mixture was stirred for 20

9-Phenylcarbazole–Based Hydrazone Twin Compounds
185=[417]
hours at room temperature. The crude product was extracted with diethylether. The
solvent was removed by rotary evaporation and 3-chloro-1,2-epoxypropane was
removed by vacuum distillation. The crude product was purified by silicagel
column chromatography (eluent: ethylacetate=n-hexane, 1:10). Yield of yellowish
1
crystals 2.7 g (67%) (FW ¼ 417 g=mol). A H NMR spectrum yielded the following
chemical shifts (250 MHz, CDCl3, d, ppm): 8.423 (d, J ¼ 1.3 Hz, 1H, 4-HHt), 8.2
=
(d, 1H, J ¼ 6.9 Hz, 5-HHt), 7.9 (s, 1H, CH N), 7.87 (dd, 1H, J1 ¼ 1.6 Hz,
J2 ¼ 8.4 Hz, 2-HHt), 7.68–7.24 (m, 12H, Ht, Ph), 6.98 (t, J ¼ 6.6 Hz, 1H, 4-HPh),
4.42 (dd, 1H, J1 ¼ 1.6 Hz, J2 ¼ 16.7 Hz, one NCH2 proton), 4.05 (dd, 1H,
J1 ¼ 4.4 Hz, J2 ¼ 15.8 Hz, another NCH2 proton), 3.3 (m, 1H, CHO), 2.91 (t, 1H,
J ¼ 4.4 Hz, CH2O one proton), 2.71 (dd, 1H, J1 ¼ 5 Hz, J2 ¼ 2.5 Hz, CH2O onother
proton). An infrared absorption spectrum yielded the following peaks (KBr
windows), (in cmꢁ1): n (arene CꢁH) 3055; n (C C in Ar) 1596, 1499; n (CꢁN)
=
1232; c (arene CꢁH) 747. MS (APCIþ, 20 V), m=z: 418 ([M þ H]þ).
Di(4-{[9-(4-butylphenyl)
carbazole-3-methylene
1-phenylhydrazino]-2-
hydroxypropyl}-thiophenyl)sulphur (5a). 3 drops of TEA were slowly added to the
solution of 1.5 g (3.17 mmol) of 9-(4-butylphenyl) carbazole-3-carbaldehyde
N-(2,3-epoxypropyl)-N-phenylhydrazone (4a) and 0.37 g (1.5 mmol) of
4,40-thiobisbenzenethiol in 7 ml of 2-butanone, while the temperature of the
reaction mixture was maintained below 30ꢀC. The reaction mixture than was
stored over night at room temperature. The product precipitated when reaction
was complete. The crude product (0.77 g) was washed with acetone. The fine
amorphous powder was purified by silica gel chromatography eluting with THF.
It was obtained 0.43 g (24%) of yellowish powder (m.p.: 200–202ꢀC, FW ¼ 1196 g=
mol). A 1H NMR spectrum yielded the following chemical shifts (250 MHz,
DMSO-d6, d, ppm): 8.34 (s, 2H, 4,40-HHt); 8.26 (d, J ¼ 7.7 Hz, 2H, 5,50-HHt); 7.98
0
=
(s, 2H, 2CH N); 7.74 (d, J ¼ 9.2 Hz, 2H, 2,2 -HHt); 7.52–7.08 (m, 32H, Ht, Ar);
6.83 (t, J ¼ 7.3 Hz, 2H, 4,40-HPh); 5.54 (d, J ¼ 4.4 Hz, 2H, OH); 4.25–3.98 (m, 6H,
NCH2CH); 3.20–3.12 (m, 4H, CH2S); 2.69 (t, J ¼ 7.5 Hz, 4H, PhCH2CH2); 1.63
(p, 4H, CH2CH2CH2CH3); 1.44–1.27 (m, 4H, CH2CH3); 0.92 (t, J ¼ 7.3 Hz, 6H,
CH2CH3). An infrared absorption spectrum yielded the following peaks (KBr
windows), (in cmꢁ1): n (br, OH) 3419; n (arene CꢁH) 3035; n (aliphatic CH)
=
2926, 2856; n (C C in Ar) 1594, 1515; n (CꢁN) 1232; c (arene CꢁH) 746.
Di{4-[(9-phenylcarbazole-3-methylene
1-phenylhydrazino)-2-hydroxypropyl]-
thiophenyl}sulphur (5b). 3 drops of TEA were slowly added to the solution of
1.2 g (2.87 mmol) of 9-phenylcarbazole-3-carbaldehyde N-(2,3-epoxypropyl)-N-
phenylhydrazone (4b) and 0.343 g (1.37 mmol) of 4,40-thiobisbenzenethiol in 5 ml
of 2-butanone, while the temperature of the reaction mixture was maintained
below 30ꢀC. The reaction mixture than was stored over night at room
temperature. The product precipitated when reaction was complete. The crude
product was washed with plenty of acetone. The fine amorphous powder was
dried in the vacuum oven. It was obtained 0.9 g (60%) of yellowish powder (m.p.:
213–215ꢀC, FW ¼ 1084 g=mol). A 1H NMR spectrum yielded the following
chemical shifts (250 MHz, DMSO-d6, d, ppm): 8.35 (s, 2H, 4,40-HHt); 8.27 (d,
0
=
J ¼ 7.7 Hz, 2H, 5,5 -HHt); 7.99 (s, 2H, 2CH N); 7.77 (dd, J1 ¼ 8.7 Hz, J2 ¼ 1.4 Hz,
2H, 2,20-HHt); 7.73–7.08 (m, 34H, Ht, Ar); 6.83 (t, J ¼ 7.2 Hz, 2H, 4,40-HPh); 5.54
(d, J ¼ 4.6 Hz, 2H, OH); 4.25–4.00 (m, 6H, NCH2CH); 3.22–3.11 (m, 4H, CH2S).

186=[418]
R. Lygaitis et al.
An infrared absorption spectrum yielded the following peaks (KBr windows), (in
cmꢁ1): n (br, OH) 3503; n (arene CꢁH) 3055; n (C C in Ar) 1596, 1499; n (CꢁN)
=
1232; c (arene CꢁH) 744.
1,3-Bis{4-[9-(4-butylphenyl)
carbazole-3-methylene
1-phenylhydrazino]
(3-
hydroxy)thiabutyl}benzene (6a). 3 drops of TEA were slowly added to the
solution of 1.5 g (3.17 mmol) of 9-(4-butylphenyl) carbazole-3-carbaldehyde N-(2,3-
epoxypropyl)-N-phenylhydrazone (4a) and 0.21 g (1.5 mmol) of 1,3-benzendithiol in
10 ml of 2-butanone, while the temperature of the reaction mixture was maintained
below 30ꢀC. The reaction mixture than was stored over night at room temperature.
After evaporation of the solvent the residue was subjected by chromatography (silica
gel, Aldrich) using 1:5 ethyl acetate:hexane and only ethyl acetate for the final eluting
of the product. It was obtained 1.3 g (80%) of yellow amorphous material
(FW ¼ 1088 g=mol). A 1H NMR spectrum yielded the following chemical shifts
(250 MHz, CDCl3, d, ppm): 8.23 (dd, J1 ¼ 4.2 Hz, J2 ¼ 1.2 Hz, 2H, 4,40-HHt); 8.13
0
=
(d, J ¼ 7.6 Hz, 2H, 5,5 -HHt); 7.77 (d, J ¼ 4.7 Hz, 2H, 2CH N); 7.74–7.67 (m, 2H,
2,20-HHt), 7.74–7.17 (m, 30H, Ht, Ph); 6.99 (t, J ¼ 6.6 Hz, 2H, 4,40-HPh), 4.26 (m,
2H, CHOH); 4.10–3.92 (m, 4H, NCH2CH); 3.26–3.04 (m, 6H, OH, CH2S); 2.72
(t, J ¼ 7.7 Hz, 4H, PhCH2CH2); 1.69 (p, 4H, CH2CH2CH2CH3); 1.50–1,38 (m, 4H,
CH2CH3); 0.99 (t, J ¼ 7.2 Hz, 6H, CH2CH3). An infrared absorption spectrum
yielded the following peaks (KBr windows), (in cmꢁ1): n (br, OH) 3525; n (arene
=
CꢁH) 3032; n (aliphatic CH) 2954, 2927, 2856; n (C C in Ar) 1595, 1515, 1497; n
(CꢁN) 1232; c (arene CꢁH) 745.
1,3-Bis[4-(9-phenylcarbazole-3-methylene 1-phenylhydrazino)(3-hydroxy)-thiabutyl]
benzene (6b). 3 drops of TEA were slowly added to the solution of 1 g
(2.39 mmol)
of
9-phenylcarbazole-3-carbaldehyde
N-(2,3-epoxypropyl)-N-
phenylhydrazone (4b) and 0.16 g (1.14 mmol) of 1,3-benzendithiol in 5 ml of
2-butanone, while the temperature of the reaction mixture was maintained below
30ꢀC. The reaction mixture than was stored over night at room temperature.
After evaporation of the solvent the residue was subjected by chromatography
(silica gel) using 1:5 ethyl acetate : hexane and only ethyl acetate for the final
eluting of the product. It was obtained 1 g (90%) of yellow amorphous material
(FW ¼ 976 g=mol). A 1H NMR spectrum yielded the following chemical shifts
(250 MHz, CDCl3, d, ppm): 8.226 (dd, J1 ¼ 4.6 Hz, J2 ¼ 1.2 Hz, 2H, 4,40-HHt); 8.13
0
=
(d, J ¼ 7.6 Hz, 2H, 5,5 -HHt); 7.77 (d, J ¼ 4.7 Hz, 2H, 2CH N); 7.74–7.67 (m, 2H,
2,20-HHt), 7.63–7.18 (m, 30H, Ht, Ph); 6.99 (t, J ¼ 6.7 Hz, 2H, 4,40-HPh), 4.25 (m,
2H, CHOH); 4.17–3.86 (m, 4H, NCH2CH); 3.26–3.05 (m, 6H, OH, CH2S). An
infrared absorption spectrum yielded the following peaks (KBr windows), (in
cmꢁ1): n (OH, br) 3416; n (arene CꢁH) 3055; n (C C in Ar) 1595, 1499; n (CꢁN)
=
1231; c (arene CꢁH) 746.
Methods
1H NMR spectra were obtained on a Bruker AC 250 (250 MHz) instrument. Mass
(MS) spectra were obtained on a Waters ZQ (Waters, Milford, USA). IR-
spectroscopy was performed on Specord 75 IR and Perkin Elmer Spectrum GX
spectrophotometers, using KBr pellets. Differential scanning calorimetry (DSC)
measurements were performed on Perkin-Elmer DSC-7. Thermogravimetric analysis

9-Phenylcarbazole–Based Hydrazone Twin Compounds
187=[419]
Scheme 1. Synthetic route to 9-phenylcarbazole–based hydrazone twin compounds.
(TGA) was fulfilled using NETZSCH STA 409 thermogravimeter at a heating rate
10 K=min under N2. Melting points were measured on a Buchi 510 melting point
apparatus. The ionisation potentials (Ip) of the films of the synthesized compounds
were measured by the electron photoemission in air method as described [8]. Stan-
dard error in the mean to 95% confidence for values of ionisation potential were
up to 0.04 eV. Hole drift mobilities were measured by a xerographic time-of-flight
method [9].
Synthesis and Characterization
The charge transport materials 5a,b and 6a,b were prepared by multi-step synthetic
route involving formylation of 9-phenylcarbazole or 9-(4-butylphenyl)carbazole (1a)
(prepared by Ullmann coupling as described in [7]) followed by the synthesis of the
corresponding phenylhydrazones (3a,b) and their reaction with 3-chloro-1,2-epoxy-
propane. The last step i.e., coupling of oxiranes 4a,b with 4,40-thiobisbenzenethiol
and 1,3-benzenedithiol, respectively, at the room temperature in 2-butanone in the
presence of triethylamine gave compounds 5a,b and 6a,b, respectively (Scheme 1).
All final products 5a,b and 6a,b were purified by column chromatography. The
1
structures of the synthesized compounds were confirmed by H NMR, IR spectro-
scopies and mass spectrometry.
Thermal Properties
Thermal properties of the synthesized compounds were examined by DSC and TGA.
The data obtained are presented in Table 1. TGA experiments in nitrogen atmos-
phere revealed that all of the synthesized molecules exhibit moderate thermal stab-
ility. The temperatures of the onset of their thermal degradation are up to 254ꢀC.
Figure 1 shows TG and differential thermogravimetry (DTG) curves of
compound 4b.
Three peaks can be observed in the DTG curve, which means that three stages
can be distinguished in the process of thermal degradation of 4b. The first stage of
thermal degradation of 4b starts at around 218ꢀC, reaches maximum weight loss rate

188=[420]
R. Lygaitis et al.
Table 1. Thermal characteristics of 4a,b–6a,b
Compound
Tm, [ꢀC]1
Tg, [ꢀC]1
TID, [ꢀC]2
4a
4b
5a
5b
6a
6b
134a
—
198
207
—
32a
43a
72
88
62
220
218
242
254
225
215
—
65
1Determined by DSC, scan rate 20ꢀC=min, N2 atmosphere.
2Onset of decomposition determined by TGA, heating rate 10ꢀC=min, N2
atmosphere.
aHeating rate 10ꢀC=min.
at ca. 255ꢀC and ends at 270ꢀC. The weight loss of 17% observed at the end of the
first stage of the thermal degradation of 4b corresponds approximately to the mass
fraction of epoxypropyl residue. The second stage ends at ca. 470ꢀC. At this tem-
perature, the compound apparently loses the hydrazone functionality and phenyl
ring at the nitrogen atom of the carbazole moiety. The third stage ends at ca.
580ꢀC with the degradation of the carbazolyl group. All the hydrazone compounds
described in this article showed glass–transitions with Tg ranging from 32ꢀC to 88ꢀC.
Endothermic melting peaks were observed for 4a and 5a,b in the 1st heating of DSC
scans (Figure 2). The other compounds have been isolated as amorphous materials
and revealed no recognizable melting peaks in the temperature range used. Com-
pound 4a with butyl substituted phenylcarbazole moiety exhibits Tg by ꢂ11ꢀC lower
than compound 4b having no substituent at the phenyl group of phenylcarbazole
Figure 1. TG curve of 4b obtained at a heating rate of 10ꢀC=min, N2 atmosphere (arrows
show the end of thermal degradation stage).

9-Phenylcarbazole–Based Hydrazone Twin Compounds
189=[421]
Figure 2. DSC thermogram of 4a (scan rate 10ꢀC=min, N2 atmosphere).
moiety. The lower Tg’s due to alkyl chain also were observed for the hydrazone twin
compounds 5a and 6a. The twin compound bearing butyl chain in the phenyl ring
(5a) have by ca. 16ꢀC lower Tg comparing with the twin compound 5b having
un-substituted phenyl group. The linkage X in the twin compounds 5b and 6b
(Scheme 1) affects their Tg. If 4,40-thiobisbenzenedithiol is used for the preparation
of the twin compounds, the shape of the molecules obtained is expected to be linear
therefore Tg’s are higher compared to banana-shaped molecules.
Nonlinearity of the molecules as well as the presence of alkyl chains lead to the
increase of number of conformers and modes of packing of the molecules, therefore
leading to lower glass transition temperatures [10].
Optical and Photoelectrical Properties
Dilute solutions of all the compounds described in this article absorb electromag-
netic radiation under 400 nm. Long–wavelength absorption band maxima of dilute
solutions are between 341 and 347 nm. The data of UV absorption spectra of
compounds 4a,b–6a,b are summarized in Table 2.
UV spectra of the twin compounds show small bathochromic shift of 4–5 nm
with respect of the spectra of the corresponding oxiranes.
Table 2. Optical characteristics of synthesized compounds
Compound
k
max, [nm]
DEopt, [eV]
4a
5a
6a
4b
5b
6b
342
347
347
341
345
347
3.18
3.11
3.11
3.18
3.12
3.12

190=[422]
R. Lygaitis et al.
Table 3. Ionization potentials of phenylcarbazole–based hydrazones
Compound
4a
4b
5a
5b
6a
6b
Ip, [eV]
5.4
5.37
5.25
5.4
5.23
5.31
The values of Ip of the amorphous films of compounds 4a,b–6a,b are given in
Table 3. 4-Butylphenyl-3-carbazol–based hydrazone dimers exhibit lower Ip than
the corresponding 9-phenyl-substituted twin molecules. The values of optical band
gap and absorption maxima correlate with ionization potential data. From the point
of the view of Ip values the reported hydrazones are applicable as hole-transporting
materials in electrophotographic photoreceptors.
Time flight technique was used to study charge transport properties of phenyl-
carbazole based hydrazone twin molecules doped in PC–Z (50 wt.%). The linear
dependencies of l on the square root of electric field were observed for all the sys-
tems (Figure 3). The zero field hole drift mobilities and the hole mobilities at the
electric field of 6.4 ꢃ 105 V=cm are given in Table 4.
Figure 3. Electric field dependencies of hole drift mobilities for 50% solid solutions in PC–Z of
compounds 5a,b and 6a,b.
Table 4. Mobility data
Transport material, host polymer
l0, [cm2=Vs]
l, [cm2=Vs]
4a þ PC–Z, 1:1
4b þ PC–Z, 1:1
5a þ PC–Z, 1:1
5b þ PC–Z, 1:1
6a þ PC–Z, 1:1
6b þ PC–Z, 1:1
1.0 ꢄ 10ꢁ7
7.6 ꢄ 10ꢁ8
2 ꢄ 10ꢁ9
5.5 ꢄ 10ꢁ6
5 ꢄ 10ꢁ6
3.3 ꢄ 10ꢁ7
4.1 ꢄ 10ꢁ7
9.3 ꢄ 10ꢁ7
1.5 ꢄ 10ꢁ6
1.4 ꢄ 10ꢁ8
1.1 ꢄ 10ꢁ8
2 ꢄ 10ꢁ8

9-Phenylcarbazole–Based Hydrazone Twin Compounds
191=[423]
Hole mobility observed for hydrazones 5a,b and 6a,b doped in PC–Z ranged
from 10ꢁ7 cm2=Vs to 1.5 ꢃ 10ꢁ5 cm2=Vs at high electric fields.
Acknowledgment
Financial support of this research by the Research Council of Lithuania (grant No.
TAP67/2010) is gratefully acknowledged. Dr. V. Gaidelis is thanked for the help in
ionization potential measurements.
References
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