Dimethyldiphenylamino-substituted carbazoles as electronically active molecular materials

Article abstract of DOI:10.1016/j.dyepig.2012.09.021

Synthesis and properties of new dimethyldiphenylamino-substituted carbazoles are reported. A comparative experimental analysis of 2-, 2,7-substituted carbazole derivatives and 3-, 3,6-substituted analogues is performed. Disubstituted carbazole derivatives exhibit more than 2-fold higher glass transition temperatures (99-100 °C) as compared to the monosubstituted compounds (38-40 °C). Dilute solutions of the dimethyldiphenylamino- substituted carbazole compounds are found to emit in the blue region with the fluorescence quantum yield of up to 0.45. More symmetrical molecular geometry of the 2-, 2,7-substituted carbazole compounds favors closer molecular packing in the neat films thereby facilitating exciton migration and migration induced exciton quenching, which results in the significantly reduced excited state decay times (down to sub-nanoseconds) and fluorescence quantum yields (down to 0.04). The 2-, 2,7-substituted carbazole compounds are shown to demonstrate the superiority over 3-, 3,6-substituted compounds by giving rise to extended π-conjugation and expressing over one order of magnitude higher radiative decay rates. The synthesized compounds are electrochemically stable: their cyclic voltammograms show reversible oxidation behavior. The ionization energies of the synthesized compounds are in the range of 5.16-5.28 eV. Hole drift mobilities of the molecular mixtures of dimethyldiphenylamino-substituted carbazoles with bisphenol Z polycarbonate reach 10^-5 cm² V^-1 s^-1 at high electric fields.

Full text of DOI:10.1016/j.dyepig.2012.09.021

Dyes and Pigments 96 (2013) 574e580
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Dimethyldiphenylamino-substituted carbazoles as electronically active molecular
materials
,
,
a
a
c
A. Tomkeviciene a b, G. Puckyte b, J.V. Grazulevicius b , K. Kazlauskas , S. Jursenas , V. Jankauskas
*
a
_
Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania
b Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania
c
_
Department of Solid State Electronics, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis and properties of new dimethyldiphenylamino-substituted carbazoles are reported.
A comparative experimental analysis of 2-, 2,7-substituted carbazole derivatives and 3-, 3,6-substituted
analogues is performed. Disubstituted carbazole derivatives exhibit more than 2-fold higher glass
transition temperatures (99e100 ꢀC) as compared to the monosubstituted compounds (38e40 ꢀC). Dilute
solutions of the dimethyldiphenylamino-substituted carbazole compounds are found to emit in the blue
region with the fluorescence quantum yield of up to 0.45. More symmetrical molecular geometry of the
2-, 2,7-substituted carbazole compounds favors closer molecular packing in the neat films thereby
facilitating exciton migration and migration induced exciton quenching, which results in the significantly
reduced excited state decay times (down to sub-nanoseconds) and fluorescence quantum yields (down
to 0.04). The 2-, 2,7-substituted carbazole compounds are shown to demonstrate the superiority over 3-,
Received 17 April 2012
Received in revised form
27 September 2012
Accepted 28 September 2012
Available online 12 October 2012
Keywords:
Carbazole
Dimethyldiphenylamino
Glass-forming
3,6-substituted compounds by giving rise to extended
p-conjugation and expressing over one order of
Thermal stability
Ionization energy
Charge mobility
magnitude higher radiative decay rates. The synthesized compounds are electrochemically stable: their
cyclic voltammograms show reversible oxidation behavior. The ionization energies of the synthesized
compounds are in the range of 5.16e5.28 eV. Hole drift mobilities of the molecular mixtures of
dimethyldiphenylamino-substituted carbazoles with bisphenol Z polycarbonate reach 10ꢁ5 cm2 Vꢁ1 sꢁ1
at high electric fields.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
efficient synthesis procedure of the former compounds. As noted in
the review [6], while the class of 3,6-substituted carbazole deriv-
Different classes of organic electronically active materials are
nowadays widely studied and used as the components of opto-
electronic and electronic devices [1e5]. Among electronically active
materials carbazole-based compounds represent one of the most
widely used and studied class of substances [6e9]. Carbazole
compounds have drawn attention due to their versatility in func-
tionalization, variety of linking topologies, good chemical and
environmental stability, excellent charge transport ability etc.
Nowadays carbazole-based polymers, oligomers and low-molar-
mass compounds are successfully utilized in organic light emit-
ting diodes, organic solar cells, biosensors, organic-thin-film tran-
sistors and other devices [6,10e16]. It is worth mentioning that
much less research has been carried out on low-molar-mass 2,7-
substituted carbazole derivatives as compared to 3,6-substituted
derivatives. The main obstacle until recently was the lack of an
atives was found to be very interesting for electrochemical and
phosphorescence applications, the class of 2,7-substituted carba-
zoles showed promising optical properties in the visible range. It
was also recognized that the major intrinsic difference between
these two families of carbazole derivatives is the effective conju-
gation length, being larger for 2,7-carbazole derivatives. These
findings stimulated design of new 2,7-substituted carbazole
derivatives with improved properties.
In this paper we report on the synthesis of new carbazole
derivatives having methyl-substituted amino groups at 2 or 2,7
positions of the carbazole moiety with potential application in
organic optoelectronics. We chose para position of the methyl
group in the diphenylamino moiety taking into account our
previous study, which indicated that this position of the methoxy
substituents in diphenylamino moiety attached to the 3 or 3,6
positions of the carbazole moiety offers the most evident positive
effect [17]. In addition to the 2- and 2,7-substituted carbazole
derivatives we have also synthesized analogous 3- and 3,6-
* Corresponding author. Tel.: þ370 37 300193; fax: þ370 37 300152.
E-mail address: juozas.grazulevicius@ktu.lt (J.V. Grazulevicius).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.09.021

A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580
575
substitued compounds, and thus, present a comparative study of
their thermal, optical, photophysical, electrochemical, and photo-
electrical properties.
Oxidation potentials were obtained as an average value between
red=ox
each anodic and corresponding cathodic potential:
E
¼
1=2
1=2ðEpc þ EpaÞ. HOMO energy levels were estimated on the basis of
the reference energy level of ferrocene (4.8 eV below the vacuum
level [25]) according to HOMO ¼ 4.8 þ (E1=2 ꢁ E1Fc=2) eV below the
vacuum level. The optical band gaps ðEgopÞ were estimated from the
edges of electronic absorption spectra, while LUMO values were
2. Experimental
2.1. Instrumentation
estimated using equation LUMO ¼ HOMO ꢁ Egop
.
1H and 13C NMR spectra were recordered using Varrian Unity
Inova [300 MHz (1H), 75.4 MHz (13C)] spectrometer at room
temperature. All the data are given as chemical shifts in
d (ppm),
2.2. Materials
(CH3)4Si (TMS, 0 ppm) was used as an internal standard. The course
of the reactions products were monitored by TLC Silica gel 60 F254
plates and developed with I2 or UV light. Silica gel (grade 60, 63e
Compounds 4-bromo-20-nitrobiphenyl [26], 2-bromocarbazole
[26], 2-bromo-9-ethylcarbazole [27],
4,40-dibromo-2-
nitrobiphenyl [28], 2,7-dibromocarbazole [28], 2,7-dibromo-9-
ethylcarbazole [12], 3-iodocarbazole [29], 3-iodo-9-ethylcarbazole
[30], 3,6-diiodocarbazole [29], 3,6-diiodo-9-ethylcarbazole [31]
were prepared according to the published procedures.
2-Di(4-methylphenyl)amino-9-ethylcarbazole (1). Tris-(dibenzy-
lideneacetone)dipalladium(0) (Pd2(dba)3, 0.2 g, 0.2 mmol) and tri-
tert-butylphosphine ((t-Bu)3P, 0.04 g, 0.2 mmol) were dissolved
under argon in 20 ml of dry toluene and stirred for 10 min at room
temperature (preformation of the catalyst). Then the mixture of 2-
bromo-9-ethylcarbazole (1.5 g, 5.5 mmol), 4,40-dimethyldiphenyl-
amine (1.3 g, 6.6 mmol) and sodium tert-butoxide (3.1 g,
32.8 mmol) in 40 ml of dry toluene were added. The reaction
mixture was heated at 90 ꢀC for 24 h. After recooling, the reaction
mixture was diluted with ethyl acetate and the organic phase was
washed with water and brine. After being dried over MgSO4 and
filtered, the solvent was removed and the residue was purified by
column chromatography using hexane/acetone (6/1) as an eluent. It
was recrystallized from methanol. The yield of white crystals was
70% (1.5 g). Mp ¼ 153e154 ꢀC.
ꢀ
200 mesh, 60 A, Fluka) was used for column chromatography.
Melting points were determined using Electrothermal Mel-Temp
melting point apparatus. Mass (MS) spectra were recorded on
a Waters ZQ (Waters, Milford, MA). Elemental analysis was per-
formed with an Exeter Analytical CE-440 Elemental. Infrared (IR)
spectra were recorded using Perkin Elmer Spectrum GX spectrom-
eter. The spectra of solid compounds were performed in KBr pellets.
Differential scanning calorimetry (DSC) measurements were carried
out using PerkineElmer DSC-7 series thermal analyzer at a heating
rate 10 ꢀC/min under nitrogen flow. Thermogravimetric analysis
(TGA) was performed on a Mettler Toledo TGA/SDTA 851e.
Absorption spectra of the dilute tetrahydrofuran (THF) solutions
were recorded on UVeViseNIR spectrophotometer Lambda 950
(Perkin Elmer). Fluorescence of the dilute THF solutions and the
neat films of the investigated compounds was excited by 365 nm
wavelength light emitting diode (Nichia NSHU590-B) and
measured using back-thinned CCD spectrophotometer PMA-11
(Hamamatsu). For these measurements, the dilute solutions of
the investigated compounds were prepared by dissolving them in
a spectral grade THF at 1 ꢂ 10ꢁ5 M concentration. Neat films of the
investigated compounds were prepared from the 1 ꢂ 10ꢁ3 M THF
solutions on the quartz substrates by drop-casting in an ambient
air. Fluorescence quantum yields (Ff) of the compound solutions
were estimated by comparing wavelength-integrated fluorescence
intensity of the compound solutions with that of the reference.
Quinine sulfate in 0.1 M H2SO4 with Ff ¼ 0.53 ꢃ 0.023 was used as
a reference [18]. Optical densities of the reference and the sample
solutions were kept below 0.05 to avoid reabsorption effects.
Estimated quantum yields of the compound solutions were verified
using an integrating sphere method [19]. The latter method was
also employed in the evaluation of Ff of the compounds neat films.
Fluorescence transients of the samples were measured using
a time-correlated single photon counting system PicoHarp 300
(PicoQuant) utilizing semiconductor diode laser (repetition rate
1 MHz, pulse duration 70 ps, emission wavelength 375 nm) as an
excitation source.
ꢀ
ꢁ
ꢀ
ꢁ
MS APClþ; 20V ; m=zð%Þ : 391 ½M þ Hꢄþ; 100 :
Elemental analysis. Calcd. for C28H26N2 (%): C 86.12, H 6.17, N
7.17. Found (%): C 86.19, H 6.28, N 7.12.
1H NMR (300 MHz, CDCl3,
d
, ppm): 1.39 (t, J ¼ 7.3 Hz, 3H, CH3), 2.39
(s,6H, CH3), 4.24 (q, J¼ 6.9Hz,2H,NCH2),6.99(dd,J¼ 2.2Hz, J¼ 8.4Hz,
1H, Ar), 7.10e7.16 (m, 9H, Ar), 7.23 (t, J ¼ 8.0 Hz,1H, Ar), 7.38e7.45 (m,
2H, Ar), 7.96 (d, J ¼ 8.4 Hz, 1H, Ar), 8.04 (d, J ¼ 7.7 Hz, 1H, Ar).
13C NMR (75.4 MHz, CDCl3,
d, ppm): 14.0 (CH3), 21.1 (CH3), 37.6
(CH2), 104.0, 108.5, 116.8,118.5,119.1, 120.0, 121.1, 123.3, 124.3,124.9,
130.0, 132.2, 141.3, 146.3, 147.0.
IR. nmax in cmꢁ1 (KBr): (CeH Ar) 3050, 3019, (CeH) 2978, 2935,
(C]C Ar) 1507, 1495, (CeN) 1226.
2,7-Di[di(4-methylphenyl)amino]-9-ethylcarbazole
(2)
was
prepared according to the procedure similar to that described for 1.
0.1 g (0.11 mmol) of Pd2(dba)3, 0.02 g (0.11 mmol) of (t-Bu)3P, 1 g
(2.8 mmol) of 2,7-dibromo-9-ethylcarbazole, 1.2 g (5.9 mmol) of
4,40-dimethyldiphenylamine, 1.6 g (17 mmol) of sodium tert-
butoxide, and 25 ml of dry toluene were used. The product was
purified by column chromatography using hexane/acetone (10/1)
as an eluent. The yield of yellow crystals was 62% (1.03 g).
Mp ¼ 258e259 ꢀC.
The ionization energy (EI) of the films of the synthesized
compounds was measured by the electron photoemission in air
method as described elsewhere [20,21]. The samples for the
measurements were prepared by dissolving materials in THF and by
coating on Al plates pre-coated with w0.5
mm thick methyl-
methacrylate and methacrylic acid copolymer adhesive layer. The
measurement error is evaluated as 0.03 eV. Hole drift mobility was
measured by xerographic time of flight technique [22,23]. The
samples for the charge carrier mobility measurements were
prepared as described elsewhere [24]. CV measurements were
carried out by a three-electrode assembly cell from Bio-Logic SAS
ꢀ
ꢁ
ꢀ
ꢁ
MS APClþ; 20V ; m=zð%Þ : 586 ½M þ Hꢄþ; 100 :
Elemental analysis. Calcd. for C42H39N3 (%): C 86.12, H 6.71, N
7.17. Found (%): C 85.99, H 6.81, N 6.98.
and
a micro-AUTOLAB Type III potentiostat-galvanostat. The
measurements were carried out at a glassy carbon electrode in
dichloromethane solutions containing 0.1 M tetrabutylammonium
perchlorate as electrolyte and Ag/AgNO3 as the reference electrode.
1H NMR (300 MHz, CDCl3,
d
, ppm): 1.27 (t, J ¼ 7.3 Hz, 3H, CH3),
2.37 (s, 12H, CH3), 4.10 (q, J ¼ 6.9 Hz, 2H, NCH2), 6.90e7.29 (m, 20H,
Ar), 7.84 (d, J ¼ 8.3 Hz, 2H, Ar).

576
A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580
13C NMR (75.4 MHz, CDCl3,
d
, ppm): 14.0 (CH3), 21.1 (CH3), 37.3
13C NMR (75.4 MHz, CDCl3,
d, ppm): 13.6 (CH3), 21.0 (CH3), 37.3
(CH2), 104.4, 117.1, 120.4, 120.5, 124.0, 124.1, 130.0, 131.9, 141.6, 146.2.
(CH2), 109.5, 118.8, 122.8, 123.0, 123.7, 125.3, 129.9, 131.1, 134.9,
140.2.
IR. nmax in cmꢁ1 (KBr): (CeH Ar) 3054, 3021, (CeH) 2979, 2937,
(C]C Ar) 1506, 1457, (CeN) 1272.
n
IR. nmax in cmꢁ1 (KBr): (CeH Ar) 3053, 3022, (CeH) 2973, 2917,
(C]C Ar) 1507, 1483, (CeN) 1274.
n
3-Di(4-methylphenyl)amino-9-ethylcarbazole (1A) was obtained
by an improved Ullmann coupling reaction. 1 g (3.1 mmol) of 3-
iodocarbazole, 0.74 g (3.7 mmol) of 4,40-dimethyldiphenylamine,
3.5 g (24.9 mmol) of powdered anhydrous potassium carbonate,
0.79 g (12.5 mmol) of copper powder, and 0.11 g (0.4 mmol) of 18-
crown-6 were refluxed in o-dichlorobenzene (10 ml) under argon
for 24 h. Then copper and inorganic salts were removed by filtration
of the hot reaction mixture. The solvent was removed under
reduced pressure and the crude product was purified by column
chromatography using hexane/acetone (10/1) as an eluent. The
yield of yellowish powder was 80% (0.97 g).
3. Results and discussion
2- and 2,7-substituted carbazole compounds 1 and 2 were
synthesized as described in Scheme 1 by a palladium-catalyzed
aromatic CeN coupling reaction of 2-bromo-9-ethylcarbazole or
2,7-dibromo-9-ethylcarbazole with 4,40-dimethyldiphenylamine.
For the comparison of the properties 3- and 3,6-substituted
analogues of 1 and 2, i.e. compounds 1A and 2A were synthesized
by Ullmann coupling reaction of 3-iodo-9-ethylcarbazole or 3,6-
ꢀ
ꢁ
ꢀ
ꢁ
diiodo-9-ethylcarbazole
(Scheme 1).
with
4,40-dimethyldiphenylamine
MS APClþ; 20V ; m=zð%Þ : 391 ½M þ Hꢄþ; 100 :
The chemical structures of the synthesized compounds were
confirmed by 1H and 13C NMR, IR spectroscopies, mass spectrom-
etry and elemental analysis.
Elemental analysis. Calcd. for C28H26N2 (%): C 86.12, H 6.17, N
7.17. Found (%): C 86.01, H 6.42, N 7.01.
1H NMR (300 MHz, CDCl3,
d
, ppm): 1.51 (t, J ¼ 7.3 Hz, 3H, CH3),
The glass-forming capability and thermal stability of the mate-
rials were estimated by DSC and TGA, respectively. The thermal
characteristics of the carbazole derivatives 1, 2, 1A and 2A are
summarized in Table 1. The values of 5% weight loss temperature
(TID) of the synthesized compounds range from 321 to 411 ꢀC.
Compounds 2 and 2A having two dimethyldiphenylamino groups
exhibit higher 5% weight loss temperatures than their mono-
substituted counterparts 1 and 1A. 2- and 2,7-substituted carba-
zole compounds 1 and 2 show superior thermal stability than the
corresponding 3- and 3,6-substituted analogues 1A and 2A.
All the synthesized compounds are capable of glass formation
with glass transition temperatures (Tg) ranging from 38 to 100 ꢀC.
Endothermic melting peaks were observed for the synthesized
compounds, except for 1A, in the first DSC heating scans. In the
second DSC heating scans, compound 2 and its 3,6-disubstituted
analogue 2A revealed not only a glass transition but also crystalli-
zation and melting (Fig. 1). Meanwhile, no crystallization exotherm
and melting endotherm were observed in the 2nd heating scan of
compound 1. The material remained in the glassy state after
melting and subsequent cooling. Compound 1A was isolated as an
amorphous substance. When its sample was heated, a glass tran-
sition was observed at 39 ꢀC and no peaks due to crystallization and
melting appeared. Cooling down and the following repeated
heating revealed only a glass transition. The amorphous nature and
low glass transition temperature of 1A can apparently be explained
by the relatively loose packing of the molecules.
2.37 (s, 6H, CH3), 4.40 (q, J ¼ 6.9 Hz, 2H, NCH2), 7.05e7.12 (m, 8H,
Ar), 7.23 (t, J ¼ 8.0 Hz, 1H, Ar), 7.31e7.53 (m, 4H, Ar), 7.95 (s, 1H, Ar),
8.00 (d, J ¼ 8.0 Hz, 1H, Ar).
13C NMR (75.4 MHz, CDCl3,
d, ppm): 14.2 (CH3), 21.0 (CH3), 37.9
(CH2), 108.8, 109.4, 118.6, 118.9, 120.9, 123.0, 124.0, 125.5, 126.0,
129.9, 131.1, 137.2, 140.3, 140.7, 146.8.
IR. nmax in cmꢁ1 (KBr): (CeH Ar) 3049, 3022, (CeH) 2976, 2920,
(C]C Ar) 1507, 1482, (CeN) 1226.
n
3,6-Di[di(4-methylphenyl)amino]-9-ethylcarbazole (2A) was
prepared according to the procedure similar to that described for
1A. 0.5
g (1.1 mmol) of 3,6-diiodo-9-ethylcarbazole, 0.48 g
(2.5 mmol) of 4,40-dimethyldiphenylamine, 1.2 g (8.8 mmol) of
potassium carbonate, 0.28 g (4.4 mmol) of copper powder, 0.2 g
(0.7 mmol) of 18-crown-6, and 10 ml of o-dichlorobenzene were
used. The product was purified by column chromatography using
hexane/acetone (10/1) as an eluent. The yield of yellow crystals was
64% (0.42 g). Mp ¼ 223e224 ꢀC.
ꢀ
ꢁ
ꢀ
ꢁ
MS APClþ; 20V ; m=zð%Þ : 586 ½M þ Hꢄþ; 100 :
Elemental analysis. Calcd. for C42H39N3 (%): C 86.12, H 6.71, N
7.17. Found (%): C 86.16, H 6.80, N 7.21.
1H NMR (300 MHz, CDCl3,
d
, ppm): 1.51 (t, J ¼ 7.3 Hz, 3H, CH3),
2.34 (s, 12H, CH3), 4.37 (q, J ¼ 6.8 Hz, 2H, NCH2), 6.99e7.08 (m, 16H,
Ar), 7.28-7.36 (m, 4H, Ar), 7.76 (s, 2H, Ar).
N
N
N
N
Br
Br
Br
I
I
I
H
H
N
Cu, K CO ,
3
Pd (dba) , P(t-Bu) ,
2
2
3
3
N
18-crown-6, o-DCB
t-BuONa, Toluene
N
N
N
N
N
N
N
N
N
N
2A
1A
2
1
Scheme 1. Synthesis of 2(3) and 2,7(3,6)-substituted carbazole compounds.

A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580
577
Table 1
1
Thermal characteristics of compounds 1, 2 and 1A, 2A.
a
4x104
Compound
Tg, [ꢀC]
Tm, [ꢀC]
Tcr, [ꢀC]
TID, [ꢀC]
1
2
1A
2A
40
100
38
151a
256
eb
e
342
411
321
373
1A
3x104
2x104
1A
168
e
Φ
f =0.12
99
223
145
1
a
f =0.26
Φ
Melting point was only detected during the first heating.
Obtained as an amorphous materials.
b
1
1x104
0
Tg of the synthesized compounds is substitution pattern inde-
0
1
pendent, i.e. 2- and 2,7-substituted carbazole derivatives 1 and 2
have nearly the same Tg value as the corresponding 3- and 3,6-
substituted counterparts 1A and 2A. However, the glass-transition
temperatures of the disubstituted derivatives 2 and 2A are more
than twice higher than those of the monosubstituted counterparts.
Absorption and fluorescence spectra of the dilute THF solutions
of compounds 1, 1A, 2 and 2A are shown in Fig. 2. The details of the
photophysical properties of the compounds are summarized in
Table 2. The lowest-energy absorption bands of the mono- as well
as disubstituted carbazole derivatives are located at 355e405 nm,
that is, shifted to longer wavelengths in respect to the absorption
of single carbazole moiety (345 nm) [32] implying extended
conjugation. Fluorescence spectra of all the derivatives display
maxima at 400e437 nm, which again, if compared with the fluo-
rescence maximum of carbazole moiety (365 nm), are considerably
red-shifted indicating extention of electron wavefunctions from the
carbazole to diphenylamino groups. Obviously, an oscillator
strength of the lowest-energy absorption bands of the disubsti-
tuted carbazole compounds 2, 2A is nearly twice as large as that of
the monosubstituted compounds 1, 1A as a result of a double
number of diphenylamino chromophores. Note, however, that this
effect is more pronounced for 2-, 2,7-substituted carbazole
compounds as compared to that of 3-, 3,6-substituted analogues. In
agreement with density functional theory calculations of the
similar 2,7- and 3,6-disubstituted carbazole compounds [33], the
absorption edge of the compounds 1A and 2A is formed by weakly-
allowed S0 / S1 transition, which is forbidden for the compounds 1
and 2. As opposed to this, oscillator strength of the S0 / S2 tran-
sition for 2-, 2,7-substituted carbazole compounds is predicted to
be several times larger than that for 3-, 3,6-substituted analogues.
The theoretical predictions well correspond to our experimental
results and also explain slightly red shifted absorption of the
compounds 1A and 2A in respect to that of the compounds 1 and 2.
The similar behavior was also observed in the fluorescence spectra
of the mono- and disubstituted carbazole compounds. The fluo-
rescence bands of the disubstituted compounds 2, 2A experienced
b
4x104
3x104
2x104
2A
2A
Φ
f =0.11
2
2
Φ
f =0.45
1x104
0
0
300
400
Wavelength (nm)
500
Fig. 2. Absorption (solid line) and fluorescence (dashed line) spectra of dilute THF
solutions (10ꢁ5 M) of a) 2- and 3-monosubstituted carbazole compounds 1 and 1A, and
b) 2,7- and 3,6-disubstituted carbazole compounds 2 and 2A. Fluorescence quantum
yields indicated.
red shift in respect to those of the monosubstituted counterparts 1,
1A. The fluorescence quantum yields of 3- and 3,6-substituted
derivatives (1A and 2A) were rather low and close (Ff ¼ 0.11e
0.12), whereas fluorescence quantum yields of 2- and 2,7-
substituted compounds were estimated to be somewhat higher,
i.e. 0.26 for the monosubstituted carbazole compound 1 and 0.45
for the disubstituted compound 2. Interestingly, irrespective of the
substitution pattern, the disubstituted carbazole derivatives
demonstrated twice as small Stokes shift (w0.2 eV) as compared to
that of the monosubstituted derivatives (0.39 eV).
Fluorescence spectra of the thin films of compounds 1, 1A, 2 and
2A are depicted in Fig. 3. Essentially, the spectral shapes of the
compounds are found to be very similar to those observed for dilute
solutions. However,
a closer inspection of the spectrum of
compound 2 allows to detect additional emission band in the long-
wavelength tail (>500 nm) of the spectrum. As compared to the
solution spectra, the maxima of the fluorescence spectra of the
films are slightly red-shifted by ca. 4e10 nm as a result of the
intermolecular interaction in the solid state, which also causes
lowering (in most cases) of the fluorescence quantum yields. The
solid state enables excitation migration over molecules via hopping
process by permitting excitons to reach nonradiative decay sites
(lattice defects, distortions etc), and thus, degrade their radiative
properties [8,34]. The drop of Ff in the films of compounds 1A and
2A is very small (up to 3%), whereas for compounds 1 and 2 the
drop of Ff is much larger (up to 41%). A similar Ff behavior was also
observed for 2,7- and 3,6-substituted carbazole trimers [33]. The
largest Ff drop of 41% experiencing disubstituted carbazole
compound 2 also possesses the distinct long-wavelength emission
tail as discussed above, which most likely originates from non-
emissive excimer-like states. The most elongated and symmet-
rical molecular geometry of the 2,7-substituted carbazole deriva-
tive 2 obviously facilitates tight arrangement of the molecules in
the solid film, which enhances excitonic effects. The latter result
precludes utilization of compound 2 as a blue emitter in the neat
form, however, it does not rule out the alternative possibility of the
compound usage as a blue dopant.
Tm = 256 oC
2
1st heating
Tg = 100 oC
2
nd heating
Tcr = 168 oC
0
25 50 75 100 125 150 175 200 225 250
Temperature, [oC]
Fig. 1. DSC curves of compound 2 (heating rate 10 ꢀC minꢁ1, N2 atmosphere).

578
A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580
Table 2
Photophysical properties of 10ꢁ5 M THF solutions and thin films of 2(3)- and 2,7(3,6)-substituted carbazole compounds 1(1A) and 2(2A), respectively.
Compd.
Dilute solution
Thin film
labs, [nm]
ε, [Mꢁ1 cmꢁ1
]
lmf ax, [nm]
Ff
s, [ns]
kr,
knr
,
Stokes
shift, [eV]
lmf ax, [nm]
Ff
s a
, [ns]
[ꢂ 108 sꢁ1
]
[ꢂ 108 sꢁ1
]
1
268, 317, 355
305, 375
31400, 20300,
20500
400
0.26
1.4
5.6
1.4
7.0
1.86
0.21
3.21
0.16
5.29
0.39
0.39
0.18
0.22
410
429
412
444
0.17
0.4 (52%)
1.2 (26%)
7.2 (22%)
0.26 (6%)
2.6 (21%)
6.8 (73%)
0.1 (51%)
0.8 (9%)
1A
2
32600, 2200
425
407
437
0.12
0.45
0.11
1.57
3.93
1.27
0.09
0.04
0.12
276, 311, 384
308, 361, 405
38500, 28700,
40700
6.4 (40%)
1.2 (39%)
6.6 (61%)
2A
46500, 7100,
2000
a
Fractional intensities of each fluorescence decay component are indicated in brackets.
Fig. 4 illustrates fluorescence transients of the dilute THF solu-
tions of 2- and 3-monosubstituted carbazole compounds 1 and 1A,
and 2,7- and 3,6-disubstituted carbazole compounds 2 and 2A. The
transients follow single exponential decay profile with the decay
enhancement of the radiative relaxation rate, which occurs upon
substitution pattern modification. In the case of monosubstituted
compounds, change of the substitution pattern from 3- to 2-
(compounds 1A and 1) results in nearly 10-fold increase of kr,
whereas in the case of disubstituted derivatives, alteration of the
substitution pattern from 3,6- to 2,7- (compounds 2A and 2) boosts
kr up to 20 times. Note that the increase of kr perfectly correlates
with the increase of extinction coefficient (ε) of the lowest-energy
absorption bands, thus again confirming importance of the radia-
tive processes upon variation of the substitution pattern. The
remarkable enhancement of radiative relaxation rate for 2-, 2,7-
substituted carbazole compounds caused by the 2e4-fold
increased fluorescence quantum yield and 4e5-fold shortened
excited-state relaxation time demonstrates an advantage of this
time constants (s) of 1.4 ns for compounds 1 and 2, and 5.6 and
7.0 ns for compounds 1A and 2A, respectively. To reveal the
contributions of the competing radiative and nonradiative excited-
state relaxation mechanisms, the radiative and nonradiative
relaxation rates, kr and knr, respectively, were evaluated for all the
studied compounds (see Table 2). As expected from the modest
fluorescence quantum yields (Ff < 0.5) of the compounds, the
nonradiative decay unambiguously dominates in excited-state
relaxation processes, and it changes only slightly with the
number of diphenylamino substituents (compare 1 to 2 and 1A to
2A). However, the flexible dimethyldiphenylamino substituents at
2-, 2,7- positions, in contrast to the more rigid carbazolyl substit-
uents [33], enhance knr for ca 3 times as compared to that observed
for 3-, 3,6-substituted derivatives (cf. knr of 1 with that of 1A and of
2 with that of 2A). The most important result is the tremendous
1
a
1A film
Φ
=0.09
f
1 film
=0.17
Φ
f
0
1
b
2A film
=0.12
Φ
f
2 film
=0.04
Φ
f
0
400
500
Wavelength (nm)
600
Fig. 4. Fluorescence transients of dilute THF solutions (10ꢁ5 M) of a) 2(3)-mono-
substituted carbazole compounds 1(1A) and b) 2,7(3,6)-disubstituted carbazole
compounds 2(2A) measured at fluorescence band maxima. Lines indicate single-
exponential fits of the experimental data.
Fig. 3. Fluorescence spectra of the neat films of a) 2- and 3-monosubstituted carbazole
compounds 1 and 1A, and b) 2,7- and 3,6-disubstituted carbazole compounds 2 and
2A. Fluorescence quantum yields indicated.

A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580
579
Table 3
substitution pattern against 3-, 3,6-substitution pattern. As
revealed by the comparative experimental and theoretical analysis
of the similar carbazole derivatives, i.e. 2,7- and 3,6-substituted
carbazole trimers [33], the 2,7-substitution pattern facilitates
HOMO, LUMO, EI, band gap energies, and electrochemical characteristics of 1, 2 and
1A, 2A.a
Compound
E1/2, [V]
Egopb, [eV]
EIc, [eV]
EHOMOd, [eV]
ELUMOe, [eV]
1
2
1A
2A
0.44
0.25
0.35
0.27
3.20
3.06
3.00
2.89
5.28
5.16
5.28
5.17
ꢁ5.00
ꢁ4.81
ꢁ4.91
ꢁ4.83
ꢁ1.80
ꢁ1.75
ꢁ1.91
ꢁ1.94
extension of the
central carbazole moiety, whereas the 3,6-substitution pattern
restricts the -conjugation within only one side substituent and the
central carbazole group. This substantial feature implies twice as
large transition dipole moment in the 2,7-substituted carbazole
compounds and originates from the more elongated and more
symmetrical shape of the molecules.
p-conjugation over both side-substituents via
p
a
The CV measurements were carried out at
a glassy carbon electrode in
dichloromethane solutions containing 0.1 M tetrabutylammonium perchlorate as
electrolyte and Ag/AgNO3 as the reference electrode. Each measurement was cali-
brated with ferrocene (Fc).
The optical band gaps Egopt estimated from the edges of electronic absorption
b
As opposed to the monoexponential fluorescence transients of
the compound solutions, fluorescence decay profiles of the
compound thin films (not shown) exhibit highly non-exponential
behavior. This behavior accompanied by the red shifted fluores-
cence bands in the solid films is a signature of dispersive exciton
hopping through the localized states in the disordered media [35]
and can be attributed to the spectral diffusion phenomenon [8].
The fluorescence decay profiles were fitted by using two- or three-
component exponential decay models to disclose the main
contributing component (see Table 2). All the compounds feature
fast (sub-nanosecond) initial excited state decay in the condensed
phase, which is mainly caused by the excitation migration facili-
tating excitation quenching at nonradiative decay sites. However, it
is worth mentioning that the major decay components (with frac-
tional intensities >50%) of the 3- and 3,6-substituted compounds
1A and 2A are 6.8 ns and 6.6 ns, respectively, which are similar to
those measured in the compound solutions (5.6 ns and 7.0 ns).
Conversely, the dominant excited state decay components of the 2-
and 2,7-substituted carbazole compounds 1 and 2 are 0.4 ns and
0.1 ns, respectively, which are up to an order of magnitude faster
than the decay time constants of the compound solutions. The
substantial difference of the dominant decay components again
points out more favorable molecular packing in the films of 2-, 2,7-
substituted carbazole compounds 1 and 2, which enhances exciton
migration and migration induced exciton quenching at non-
radiative decay centers.
spectra.
c
Ionization energy was measured by the photoemission in air method from films.
d
e
EHOMO ¼ ꢁ4:8 þ ðE1=2 ꢁ E1Fc=2Þ, when E
¼ 0.24 V.
Fc
1=2
ELUMO ¼ EHOMO ꢁ Egopt
.
triphenylamino substituted derivatives of oxadiazole which
showed anodic electropolymerization [36], the repeated scans for
the synthesized compounds revealed no any signs of electro-
polymerization. This was apparently due to the presence of methyl
groups in the para positions of the diphenylamino moieties. We
assume that diphenylamino substituent is able to prevent electro-
chemical coupling by stabilization of the radical cations of 1A thus
preventing it from electrodimerization.
Taking HOMO energy level (ꢁ4.8 eV) of the ferrocene/ferroce-
nium redox system as a reference [37], HOMO energies for the
synthesized compounds were calculated. The obtained values are
given in Table 3. Note that EHOMO values may not represent absolute
solid-state or gas-phase ionization energies, however they can be
used to compare the different compounds. The EHOMO energies of
the monosubstituted carbazole compounds 1 and 1A are somewhat
lower than those of the disubstituted compounds 2 and 2A. Inter-
estingly, the EHOMO are only weakly affected by the positions of
dimethyldiphenylamino groups. EHOMO values together with optical
band gap energies ðEgopÞ were used to calculate the LUMO energy
levels. The LUMO energies were found to be dependent on the
linking topology of the substituents. The compounds 1 and 2 having
dimethyldiphenylamino groups at 2 and 2,7 positions of the
carbazole ring, respectively, showed slightly higher LUMO energy
levels than the corresponding compounds 1A and 2A with the
substituents at 3 and 3,6 positions.
To elucidate the energetic conditions for energy and electron
transfer in dilute solutions, the HOMO/LUMO values were esti-
mated by performing cyclic voltammetry (CV) measurements. The
cyclic voltammograms of the solutions of all the synthesized
compounds in dichloromethane show reversible oxidation
behavior. Fig. 5 illustrates CV curves of the 2-substituted carbazole
compound 1 and of its 3-substituted analogue 1A for the compar-
ison. The CV curve of the compound 1 is positively shifted as
compared to that of the compound 1A. In contrast to the
When considering the use of an organic materials for hole-
transporting applications it is important to have an under-
standing of their solid state ionization energies (EI). The EI were
measured by the electron photoemission in air method. The results
obtained are summarized in Table 3. Usually the photoemission
10-3
1x10-4
1x10-5
10-6
Al+[1+(PC-Z), 1:1]
MC+[2+(PC-Z), 1:1]
Al+[1A+(PC-Z), 1:1]
MC+[2A+(PC-Z), 1:1]
10-7
10-8
0
200
400
600
800 1000 1200 1400 1600
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
E
1/2, [V/cm]1/2
E vs Ag/Ag+, [V]
Fig. 6. Electric field dependencies of the hole drift mobilities (m) in charge transport
Fig. 5. Cyclic voltammograms of 1 (dashed line) and 1A (solid line) in argon-purged
layers of the synthesized compounds 1, 1A, 2 and 2A doped in PC-Z (mass proportion
1:1).
dichloromethane solution (scan rate of 50 mV sꢁ1).

580
A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580
Table 4
substituted analogues. The ionization energies and HOMO energy
levels of these compounds are affected by the number of dime-
thyldiphenylamino groups and unaffected by the linking topology
of these groups. Hole drift mobilities in the films of the molecular
mixtures of the synthesized compounds with bisphenol Z poly-
carbonate reach 10ꢁ5 cm2 Vꢁ1 sꢁ1 at high electric fields.
Hole mobility data for the molecular mixtures of compounds 1, 2 and 1A, 2A with
PC-Z.
Layer composition da, [
m
m] mo
,
mc
,
ad
,
b
[cm2 Vꢁ1 sꢁ1
]
[cm2 Vꢁ1 sꢁ1
]
[cm1/2 Vꢁ1/2
]
1 þ PC-Z, 1:1
2 þ PC-Z, 1:1
1A þ PC-Z, 1:1
2A þ PC-Z, 1:1
5.4
7.0
4.5
9.0
3.7 ꢂ 10ꢁ8
4.7 ꢂ 10ꢁ7
1.4 ꢂ 10ꢁ6
9.0 ꢂ 10ꢁ7
1.1 ꢂ 10ꢁ5
6.0 ꢂ 10ꢁ6
3.8 ꢂ 10ꢁ5
8.0 ꢂ 10ꢁ6
0.0071
0.0032
0.0042
0.0027
Acknowledgements
a
Layer thickness.
b
Mobility value at zero field strength.
A. Tomkeviciene acknowledges European Union Structural
Funds project ”Postdoctoral Fellowship Implementation in
Lithuania” for funding her postdoctoral fellowship. We thank Habil.
Dr. V. Gaidelis for the help in ionization potential measurements.
c
Mobility value at 6.4 ꢂ 105 V cmꢁ1 field strength.
The PooleeFrenkel parameter.
d
experiments are carried out in vacuum, since the sample surface
oxidation and gas adsorption distort measurement results. In our
case, the investigated materials were stable in respect to oxidation
and the measurements could be carried out in air. The measured EI
of the disubstituted carbazole compounds 2 and 2A were found to
be lower than those of the corresponding monosubstituted
compounds 1 and 1A. In agreement with the CV results, this
observation indicates that EI values, in close similarity to EHOMO
values, are affected by the number of dimethyldiphenylamino
groups, and relatively unaffected by the positions of these groups.
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