Hole-transporting carbazole-based imines

Article abstract of DOI:10.1080/15421406.2011.538606

Carbazole-based imines were synthesized by the reactions of 3-amino-9-ethylcarbazole with heteroaromatic aldehydes (3-formyl-9- ethylcarbazole or 3-formyl-10-(2-ethylhexyl)phenothiazine) or with 4,4′-bis(dimethylamino)benzophenone. The characterization of the derivatives synthesized by 1H NMR and IR spectroscopy as well as by MS spectrometry is presented. The electron photoemission spectra of thin layers of the materials were recorded and the ionization potentials of ca 5.3eV were established. Hole drift mobilities in the layers of bisphenol Z polycarbonate containing 50% of the electro-active material reached 4×10 ^-6cm²/Vs at an electric field of 1.4×10 ⁶V/cm. Copyright Taylor & Francis Group, LLC.

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Hole-Transporting Carbazole-Based
Imines
Jurate Simokaitiene a , Saulius Grigalevicius a , Juozas Vidas
Grazulevicius a , Vygintas Jankauskas b & Jonas Sidaravicius c
a Department of Organic Technology , Kaunas University of
Technology, Radvilenu plentas, LT , Kaunas , Lithuania
b Department of Solid State Electronics , Vilnius University,
Sauletekio aleja, LT , Vilnius , Lithuania
c Department of Polygraphic Machines , Vilnius Gediminas Technical
University LT , Vilnius , Lithuania
Published online: 03 Mar 2011.
To cite this article: Jurate Simokaitiene , Saulius Grigalevicius , Juozas Vidas Grazulevicius , Vygintas
Jankauskas & Jonas Sidaravicius (2011) Hole-Transporting Carbazole-Based Imines, Molecular Crystals
and Liquid Crystals, 536:1, 192/[424]-199/[431], DOI: 10.1080/15421406.2011.538606
To link to this article: http://dx.doi.org/10.1080/15421406.2011.538606
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Mol. Cryst. Liq. Cryst., Vol. 536: pp. 192=[424]–199=[431], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.538606
Hole-Transporting Carbazole-Based Imines
JURATE SIMOKAITIENE,1 SAULIUS
GRIGALEVICIUS,1 JUOZAS VIDAS
GRAZULEVICIUS,1 VYGINTAS JANKAUSKAS,2 AND
JONAS SIDARAVICIUS3
1Department of Organic Technology, Kaunas University of Technology,
Radvilenu plentas, LT, Kaunas, Lithuania
2Department of Solid State Electronics, Vilnius University, Sauletekio
aleja, LT, Vilnius, Lithuania
3Department of Polygraphic Machines, Vilnius Gediminas Technical
University LT, Vilnius, Lithuania
Carbazole-based imines were synthesized by the reactions of 3-amino-9-
ethylcarbazole with heteroaromatic aldehydes (3-formyl-9-ethylcarbazole or
3-formyl-10-(2-ethylhexyl)phenothiazine) or with 4,40-bis(dimethylamino)benzo-
1
phenone. The characterization of the derivatives synthesized by H NMR and IR
spectroscopy as well as by MS spectrometry is presented. The electron photoemis-
sion spectra of thin layers of the materials were recorded and the ionization poten-
tials of ca 5.3 eV were established. Hole drift mobilities in the layers of bisphenol Z
polycarbonate containing 50% of the electro-active material reached 4 ꢀ 10ꢁ6 cm2=
Vs at an electric field of 1.4 ꢀ 106 V=cm.
Keywords Carbazole; hole drift mobility; imine; ionization potential; molecular
glass
Introduction
Low molecular weight organic compounds that form glasses are called molecular
glasses or amorphous molecular materials. Electroactive molecular glasses show
excellent processability, transparency, isotropic and homogeneous properties and
receive growing attention both in terms of academic interest and technological appli-
cations [1]. Glass-forming aromatic amines, heterocyclic compounds, hydrazones,
enamines belong to the class of hole-transporting materials and are known for vari-
ous applications such as electrophotographic photoreceptors, organic light-emitting
diodes, photovoltaic cells [2–8]. Aromatic imines were also reported as potential elec-
troactive materials [9,10], however their charge-transporting properties, to our
knowledge, were not studied.
Address correspondence to Juozas Vidas Grazulevicius, Department of Organic Tech-
nology, Kaunas University of Technology, Radvilenu plentas 19, LT-50254, Kaunas,
Lithuania. E-mail: juozas.grazulevicius@ktu.lt
192=[424]

Hole-Transporting Carbazole-Based Imines
193=[425]
For the purpose of developing of electronically active molecular glasses, we have
reported previously several series of carbazole and [3,30]bicarbazole-based deriva-
tives [11–14]. In the present study we report on the synthesis and optical, photophy-
sical as well as photoelectrical properties properties of new carbazole based imines.
Experimental
Instrumentation
1H NMR spectra were recorded using Varian Unity Inova and JOEL JNM- F ꢀ 100
apparatus. Mass spectra were obtained on a Waters ZQ 2000 spectrometer. FTIR
spectra were recorded using Perkin Elmer FT-IR System. UV spectra were measured
with a Spectronic GenesysTM 8 spectrometer. Fluorescence (FL) spectra were
recorded with a Hitachi MPF-4 spectrometer.
The ionization potentials (Ip) of the materials were measured by the electron
photoemission in air method [15]. The details of the Ip measurements were reported
earlier [16].
Hole drift mobilities were measured by the xerographic time of flight technique
[17,18]. The samples for the charge carrier mobility measurements were prepared by
procedure as described earlier [19].
Materials
10H-phenothiazine (1), 3-amino-9-ethylcarbazole (4), 3-formyl-9-ethylcarbazole (5),
4,40-bis(dimethylamino)benzophenone, (Michler’s ketone, 6), 1,4-diazabicyclo[2.2.2]-
octane, POCl3 and TiCl4 were purchased from Aldrich and used as received.
10-(2-Ethylhexyl)phenothiazine (2) was prepared by the similar procedure as
described in literature [20]. 10H-phenothiazine (20 g, 0.1 mol), 1-bromo-2-ethylhex-
ane (38.6 g, 0.2 mol) and tetrabutylammonium hydrogen sulphate (0.2 g, 0.6 mmol)
were heated to reflux in 300 ml of dry toluene and then powdered potassium hydrox-
ide (8.4 g, 0.15 mol) was added. The reaction mixture was refluxed for 12 h. After the
reaction the cooled mixture was filtered, washed thoroughly with water, dried over
anhydrous sodium sulphate and the solvent was removed by evaporation. The resi-
due was fractionally distilled at high vacuum. The yield of 2 (yellowish oil) was 24 g.
1H NMR (CDCl3, 300 MHz) d, ppm: 0.93 (t, J ¼ 6.8 Hz, 3H, CH3); 0.93 (t,
J ¼ 7.4 Hz, 3H, CH3); 1.30–1.55 (m, 8H, CH2); 1.92–2.08 (m, 1H, CH); 3.79 (d,
2H, J ¼ 6.8 Hz,CH2-N); 6.89–7.26 (m, 8H, Ar). IR (KBr), cmꢁ1: 3066; 2958, 2927,
2857; 1594, 1571; 1285. MS(APCIþ, 20 V), m=z, %: 312 ([MþH]þ, 95).
3-Formyl-10-(2-ethylhexyl)phenothiazine (3) was synthesized from compound 2
by Vilsmeier formylation reaction [21]. POCl3 (1.47 ml, 16 mmol) was added drop
wise to dry DMF (3.7 ml, 48 mmol) at 0ꢂC under nitrogen atmosphere. 10-(2-
Ethylhexyl)phenothiazine (4.98 g, 16 mmol) was added to the reaction flask. The
reaction mixture was stirred at 85ꢂC until the starting compound fully reacted. Then
the mixture was cooled down to the room temperature, poured into ice water and
neutralized with sodium acetate till pH ¼ 6–8. The precipitated product was filtered
off. The crude product separated by filtration was further purified by silica gel col-
umn chromatography using hexane=ethyl acetate (vol. ratio 3:1) as an eluent. The
1
yield of 3 (yellow amorphous resin) was 0.74 g. H NMR (CDCl3, 300 MHz) d,
ppm: 0.62–0.95 (m, 6H, CH3); 1.00–1.44 (m, 8H, CH2); 1.70–2.10 (m, 1H, CH);
3.94 (d, 2H, J ¼ 7,3 Hz, CH2-N); 7.10–8.50 (m, 7H, ar); 9.97 (s, 1H, CHO). IR

194=[426]
J. Simokaitiene et al.
(KBr), cmꢁ1: 3053; 2957, 2926, 2849; 1687; 1586; 1230. MS(APCIþ, 20 V), m=z, %:
308 ([MþH]þ, 100).
3-(9-Ethylcarbazol-3-yl-iminomethyl)-9-ethylcarbazole (7). 3-Formyl-9-ethylcar-
bazole (5) (1.0 g, 4.48 mmol) and 3-amino-9-ethylcarbazole (4) (1.4 g, 96.7 mmol)
were dissolved in 10 ml of ethanol. The mixture was refluxed under N2 for 24 h. Then
the reaction mixture was cooled down, the product precipitated out and was recov-
ered by filtration. The yield of 7 (of yellow amorphous material) was 1.6 g. 1H NMR
(CDCl3, 100 MHz) d, ppm: 0.36 (t, 6H, CH3); 4.10–4.34 (m, 4H, CH2); 7.11–8.64 (m,
14H, ar); 8.75 (s, 1H, N CH). IR (KBr), cmꢁ1: 3048, 2967, 2863, 1627; 1614; 1597,
=
1472, 1447, 1281, 1238, 1153, 1132, 1121, 842, 819, 803, 783, 766, 744, 726.
MS(APCIþ, 20 V), m=z, %: 416 ([MþH]þ, 100).
3-(9-Ethylcarbazol-3-yl-iminomethyl)-10-(2-ethylhexyl)phenothiazine (8). 3-
Formyl-10-(2-ethylhexyl)phenothiazine (3) (0.89 g, 2.6 mmol) and 3-amino-9-ethyl-
carbazole (4) (0.82 g, 3.9 mmol) were dissolved in 30 ml of methanol. The reaction
mixture was refluxed under N2 for 2 h. Then the solvent was distilled under reduced
pressure and the crude product was purified by silica gel column chromatography
using hexane=ethyl acetate (vol. ratio 7:1) as an eluent. The yield of 8 (yellow
1
amorphous material) was 0.4 g. H NMR (CDCl3, 100 MHz) d, ppm: 0.86–1.02
(m, 6H, CH3); 1.41–1.61 (m, 8H, CH2); 1.73–1.90 (m, 1H, CH); 1.95–2.13 (t, 3H,
N-CH2-CH3); 3.80–3.88 (d, 2H, N-CH2-CH; 4.36–4.46 (q, 2H, N-CH2-CH3);
6.95–8.23 (m, 14H, ar); 8.58 (s, 1H, N CH). IR (KBr), cmꢁ1: 3058, 2959, 2826,
=
2857, 1633, 1612, 1598, 1573, 1460, 1333, 1249, 1238, 1227, 1162, 1334, 1123,
1102, 780, 764, 747, 727. MS(APCIþ, 20 V), m=z, %: 532 ([MþH]þ, 100).
N-[Bis(4-dimethylaminophenyl)methylidene]-N-(9-ethylcarbazol-3-yl)amine (9).
3-Amino-9-ethylcarbazole (4) (0.6 g, 2.85 mmol), 4,40-bis(dimethylamino)benzophe-
none (6) (1.54 g, 5.7 mmol) and 1,4-diazabicyclo[2.2.2]octane (0.96 g, 8.56 mmol)
were dissolved in 40 ml of 1,2-dichlorobenzene. Then TiCl4 (0.86 g, 4.55 mmol) was
added drop-wise to the solution. The reaction mixture was stirred at 130ꢂC under N2
for 24 h. Then the reaction mixture was cooled down to the room temperature,
1,4-diazabicyclo[2.2.2]octane precipitated out and was removed by filtration. Then
hexane was added to the solution and excess of 6 precipitated out. When the precipi-
tate was filtered off, the solvents were distilled under reduced pressure and the crude
product was purified by silica gel column using hexane=acetone (vol. ratio 1:6) as an
1
eluent. The yield of 9 (orange amorphous material) was 0.32 g. H NMR (CDCl3,
100 MHz) d, ppm: 1.38–1.48 (t, 3H, CH3); 2.94 (s, 6H, CH3); 3.18 (s, 6H, CH3);
4.33–4.38 (q, 2H, CH2) 6.53–8.10 (m, 15H, ar). IR (KBr), cmꢁ1: 3447, 3041, 2888,
2859, 2807, 1869, 1737, 1712, 1609, 1584, 1360, 1276, 12428, 1211, 1190, 1182,
1168, 1137, 1120, 823, 811, 782, 749, 685. MS(APCIþ, 20V), m=z, %: 461 ([MþH]þ,
100).
Results and Discussion
Carbazole-based imines (7–9) were prepared as described in Scheme 1 via the nucleo-
philic addition reaction of the commercially available 3-amino-9-ethylcarbazole (4)
with an excess of the corresponding aldehyde (3 or 5) or ketone 6. The reactions
of amine with the aldehydes proceeded without catalyst. TiCl4 and 1,4-diazabicy-
clo[2.2.2]octane were used for the acceleration of the reaction between 4 and 4,
40-bis(dimethylamino)benzophenone (6). The starting aldehyde 3-formyl-10-(2-ethyl-
hexyl)phenothiazine (3) was prepared from 10H-phenothiazine (1) by two step

Hole-Transporting Carbazole-Based Imines
195=[427]
Scheme 1. The preparation of carbazole-based imines (7–9).
synthesis. The first step was alkylation of 1 with 2-ethylhexylbromide under basic
conditions to get alkyl substituted derivative 2. Vilsmeier reaction [21] of compound
2 with phosphoryl chloride afforded 3-formyl-10-(2-ethylhexyl)phenothiazine (3).
All the newly synthesized compounds were identified by mass spectrometry, IR
and 1H NMR spectrometries. The data were found to be in good agreement with the
proposed structures. Carbazole-based imines (7–9) were readily soluble in common
organic solvents at the room temperature. Transparent thin amorphous films of
these materials could be prepared by casting or spin coating from solutions.
UV absorption and FL spectra of dilute THF solutions of the compounds
synthesized were recorded. The wavelengths of the absorption and emission
maxima (kmax) are collected in Table 1. The characteristics of 9-ethylcarbazole
(EtCz) are presented for the comparison in Table 1. The electronic absorption
energy of compounds 7–9 is comparable, and the kmax values are in the range of
242–389 nm. Their FL emission maxima appear in the region of wavelengths from
418 to 436 nm.
The lowest energy absorption maxima of compounds 7–9 are red shifted with
respect of the corresponding maxima of EtCz or of 9,90-dialkyl-[3,30]bicarbazolyl
Table 1. UV absorption and FL emission maxima of 7–9 and EtCz
Compound
FLa: kmax (nm)
UVa: kmax (nm)
7
8
424
436
418
349, 367
245, 296, 349
262, 294, 389
242, 324, 352
9
EtCz
259, 290, 329, 342
aConcentration 10ꢁ5 mol lꢁ1. Excitation wavelength 295 nm.

196=[428]
J. Simokaitiene et al.
[22]. The highest bothochromic shift was observed for the phenothiazinyl-containing
derivative 8. FL spectrum of imine 7 containing two carbazolyl fragments and for
the comparison that of EtCz are presented in Figure 1. The spectrum of 7 is consider-
ably red shifted with respect of that of EtCz. The similar shifts were observed for 8
and 9. The red shifts of absorption and fluorescence spectra of the synthesized
compounds with respect of those of EtCz demonstrate that the new compounds
are p- conjugated through the lone electron pair at the nitrogen atom of imine frag-
ment and that p-electrons are delocalised over these molecules. In the same manner
with the red shift of UV absorption, which is interpreted as the extended conju-
gation, stabilizing the oxidized state of the compounds, they should show lower
values of ionization potential (Ip) than 9-alkylcarbazole or polymers containing
pendant carbazolyl groups.
Electron photoemission spectra of the layers of compounds (7–9) are presented
in Figure 2. The values of Ip of the layers of the compounds are very close and do not
exceed 5.35 eV. This value is by more tha 0.4 eV lower than that of derivatives having
electronically isolated carbazolyl groups [23].
Holes would be easily injected into the layers of the synthesized materials from a
charge generation layer or a conductive anode with Ip or work function close to
5.3 eV. The Ip values of charge generation materials, including those widely used
in electrophotographic photoreceptors such as titanyl phthalocyanines [24,25], pery-
lene pigments [26] and bisazo pigments [27] are in the range of 5.1–5.6 eV. The Ip of
the materials (7–9) is also close to that of indium-tin oxide (ꢃ4.9 eV) which is used as
anode in various optoelectronic devices [28]. The injection barrier of holes from an
electrode into the layers of 7–9 would be ca. 0.4 eV.
Compounds 7 and 8 having the best solubility in polymer host bisphenol Z poly-
carbonate (PC-Z) solutions were used for charge transport studies. Xerographic time
of flight (XTOF) measurements were used to characterize the magnitudes of hole
drift mobility (lh) for the compounds molecularly dispersed in PC-Z. Transit time
for the samples with the hole-transporting materials was determined by the braking
point on the dU=dt transient in double logarithmic scale (Fig. 3). The character of
Figure 1. FL spectra of THF dilute solutions of 7 (solid line) and of EtCz (dotted line).

Hole-Transporting Carbazole-Based Imines
197=[429]
Figure 2. Electron photoemission spectra of thin layers of compounds 7–9.
XTOF transients given in Figure 3 shows that dispersive charge transport is
characteristic of the molecular dispersion of 7 in PC-Z.
For the molecular dispersions of compounds 7, 8 in PC-Z the room temperature
hole mobilities show the linear dependencies on the square root of the electric field
(Fig. 4.). This is characteristic dependence for the majority of non-crystalline organic
semiconductors and can be attributed to the effects of disorder on charge transport
[8]. The lh value of 1.6 Â 10À8 cm2 VÀ1 sÀ1 was observed for the amorphous layer of
compound 7 dispersed in PC-Z at an electric field of 1.4 Â 106 V cmÀ1 at 25ꢀC. The
layer of the molecular dispersion of compound 8 in PC-Z showed higher lh of ca.
4 Â 10À6 cm2 VÀ1 sÀ1 at an electric field of 1.35 Â 106 V cmÀ1. The different charge
Figure 3. XTOF transients of 7 doped in PC-Z (50%), insert shows one of the transient curves
in linear plot.

198=[430]
J. Simokaitiene et al.
Figure 4. Electric field dependencies of hole drift mobility in charge transport layers of 7 and 8
molecularly doped in PC-Z (50%).
transporting properties can be predetermined by chemical structure of the com-
pounds as well as by distribution of the molecules in the polymer host.
In conclusion, we have synthesized series carbazole-based imines, the layers of
which demonstrate rather low ionization potentials of ca 5.3 eV. The molecular
dispersions of the synthesized materials in bisphenol Z polycarbonate show time-of-
flight hole mobilities reaching 4 ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1 at an electric field of 1.35 ꢀ
106 V cmꢁ1 Low values of ionization potentials and the moderate charge transport
properties of the synthesized materials show that they are potential hole-transporting
materials for the application in organic optoelectronic devices.
Acknowledgment
Financial support of this research by the Research Council of Lithuiania (grant No.
TAP68/2010) is gratefully acknowledged. Dr. V. Gaidelis is thanked for the help in
ionization potential measurements.
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