Pyrazolyl-substituted polyconjugated molecules for optoelectronic applications

Article abstract of DOI:10.1016/j.dyepig.2009.10.007

Pyrazolyl-substituted diethylene derivative and its analogue containing additional cyano groups were synthesized and investigated as potential multifunctional materials for organic light emitting diodes. The influence of the electron affinitive cyano groups on the ionization potential (I_p) of the films as well as on the photoluminescence (PL) spectrum, PL quantum yield (η) and PL decay time of the dilute solutions and thin films of the pyrazole derivatives was studied. PL measurements revealed that highly luminescent (η?=?0.88) cyano-free pyrazole derivative in solution became non-emissive (η?=?0.01) by attaching cyano groups as a result of these groups-induced torsional deactivation. However in the solid state, the steric and electrostatic effects of the bulky and polar cyano groups prevented tight packing of the pyrazole derivative molecules, thus significantly reducing migration-induced quenching of the excitons at the defects. Incorporation of the cyano groups resulted in the two-fold enhancement of the PL quantum yield in the film of the pyrazole derivative as compared to that of the cyano-free film. The I_p of 5.90?eV estimated for the cyano groups-containing compound was found to be higher as compared to the I_p of 5.46?eV for cyano-free analogue, what in conjunction with the PL data for the films indicates increased electron affinity in the pyrazole derivative with additional cyano groups.

Full text of DOI:10.1016/j.dyepig.2009.10.007

Dyes and Pigments 85 (2010) 79e85
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Pyrazolyl-substituted polyconjugated molecules for optoelectronic applications
a
b
,
b
b
c
*
ꢀ
_
ꢀ_
E. Arbaciauskiene , K. Kazlauskas , A. Miasojedovas , S. Jursenas , V. Jankauskas ,
d
a
a,
**
ꢀ
ꢀ
W. Holzer , V. Getautis , A. Sackus
Department of Organic Chemistry, Kaunas University of Technology, Radvilenų pl. 19, LT-50270, Kaunas, Lithuania
a
_
b
c
_
Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222, Vilnius, Lithuania
_
Department of Solid State Electronics, Vilnius University, Sauletekio 9-III, LT-10222, Vilnius, Lithuania
d Department of Drug and Natural Product Synthesis, Faculty of Life Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyrazolyl-substituted diethylene derivative and its analogue containing additional cyano groups were
synthesized and investigated as potential multifunctional materials for organic light emitting diodes. The
influence of the electron affinitive cyano groups on the ionization potential (Ip) of the films as well as on
Received 17 June 2009
Received in revised form
13 October 2009
Accepted 17 October 2009
Available online 28 October 2009
the photoluminescence (PL) spectrum, PL quantum yield (
thin films of the pyrazole derivatives was studied. PL measurements revealed that highly luminescent
¼ 0.88) cyano-free pyrazole derivative in solution became non-emissive ( ¼ 0.01) by attaching cyano
h) and PL decay time of the dilute solutions and
(
h
h
groups as a result of these groups-induced torsional deactivation. However in the solid state, the steric
and electrostatic effects of the bulky and polar cyano groups prevented tight packing of the pyrazole
derivative molecules, thus significantly reducing migration-induced quenching of the excitons at the
defects. Incorporation of the cyano groups resulted in the two-fold enhancement of the PL quantum yield
in the film of the pyrazole derivative as compared to that of the cyano-free film. The Ip of 5.90 eV
estimated for the cyano groups-containing compound was found to be higher as compared to the Ip of
5.46 eV for cyano-free analogue, what in conjunction with the PL data for the films indicates increased
electron affinity in the pyrazole derivative with additional cyano groups.
Keywords:
Pyrazole
Cyano groups
Fluorescence
Ionization potential
Charge carrier mobility
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
of the polyconjugated molecules serve not only as a “conjugated
bridge”, it also improves electroluminescent properties by reducing
Low-molecular-weight functional dyes exhibiting both emission
and charge transport are of interest for the application in opto-
electronic devices such as organic light emitting diodes (OLEDs) [1].
Although much progress has been achieved on OLEDs, there are still
some challenges of improving the performance and durability of
OLEDs for displays and lighting applications. One of the key issues
for developing the next generation high-performance OLEDs is the
design and synthesis of multifunctional materials with both light
emission and charge transport properties. The molecules consisting
of polyconjugated structure end-capped with heteroaromatic or
polycyclic aromatic chromophores are known to be highly fluo-
rescent [2e4]. Vinylene linkage that is often employed in the design
steric interactions between aromatic rings, and thus, decreasing the
free rotation between units [5]. Pyrazoline derivatives are known to
exhibit efficient blue emission and hole-transport performance,
and therefore, might serve as the multifunctional materials for light
emitting devices [6]. As distinct from pyrazoline, aromatic pyrazole
is much less studied and exploited as electroactive moiety.
Introduction of cyano groups into the vinylene linkages of the
polyconjugated materials has an advantage of increasing electron
affinity, and therefore, greatly improving electron injection from
the metal contact of an OLED [7]. Incorporation of cyano moieties
into fluorescent compounds also facilitates formation of highly
fluorescent organic nanoparticles, which have potential applica-
tions in on/off fluorescence sensors [8].
To this end, pyrazolyl-substituted diethylene derivative 1,4-bis
{(E)-2-[(1-biphenylyl-3-hexyloxy)pyrazol-4-yl]ethenyl}benzene
(P1) and its analogue with cyano moieties 1,4-bis{(E)-1-nitrile-2-
[(1-biphenylyl-3-hexyloxy)pyrazol-4-yl]ethenyl}benzene (P2) have
been synthesized and characterized. Here, the photophysical and
* Corresponding author. Tel.: þ370 52 366032; fax: þ370 52 366059.
** Corresponding author. Tel.: þ370 37 300194; fax: þ370 37 451432.
E-mail addresses: karolis.kazlauskas@ff.vu.lt (K. Kazlauskas), algirdas.sackus@
ꢀ
ꢀ
ktu.lt (A. Sackus).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.10.007

ꢀ _
E. Arbaciauskiene et al. / Dyes and Pigments 85 (2010) 79e85
80
photoelectrical properties of the synthesized pyrazole derivatives
are reported.
2.2.1. 4-Bromo-1-(4-bromophenyl)-3-hexyloxy-1H-pyrazole
(4, C15H18Br2N2O)
The solution of 3 (2.23 g, 7 mmol) in dry DMF (20 ml) was cooled
to 0 ꢂC under inert atmosphere and NaH (60% dispersion in mineral
oil, 280 mg, 7 mmol) was added portion wise. After mixing for
15 min hexyl bromide (1.2 ml, 8.4 mmol) was added drop wise. The
mixture was stirred at 60 ꢂC for 3 h (TLC control, eluent: ethyl
acetate:n-hexane ¼ 1:10). 10 ml of water were added and the
mixture was extracted with ethyl acetate. The organic layers were
combined, washed with brine, dried over Na2SO4, filtrated, the
solvent was evaporated. The residue was purified by column
chromatography (eluent: ethyl acetate:n-hexane ¼ 1:10) to give
2. Experimental details
2.1. Measurements
The 1H and 13C NMR spectra were measured on Varian Unity
Inova spectrometer (1He300 MHz, 13Ce75 MHz) or on Bruker
Avance 500 spectrometer (1He500 MHz, 13Ce125 MHz) in CDCl3.
The IR spectra of the samples in KBr pellets were recorded on
a Perkin Elmer Spectrum GX FT-IR System spectrometer. The
absorption spectra of the sample THF solutions (10ꢀ4 M) placed in
a 1 mm path length microcell were recorded on a Perkin Elmer
Lambda 35 UV/VIS spectrometer. Photoluminescence (PL) of the
sample solutions and thin films was excited by 365 nm wave-
length light emitting diode (Nichia NSHU590-B) and measured
using back-thinned CCD spectrometer (Hamamatsu PMA-11). The
pure 4 as a liquid. Yield: 95% (2.67 g); IR (KBr; n
, cmꢀ1): 3143
(CHarom), 2954, 2930, 2858 (CHaliph), 1593, 1549, 1512, 1494, 1470,
1424, 1392, 1368, 1309 (C]C, CeN), 1219, 1098 (CeOeC), 824
(CH ] CH of disubstituted benzene); 1H NMR (300 MHz, CDCl3;
d,
ppm): 7.74 (s, 1H, 5-H), 7.54e7.50 (m, 2H, Ph 2,6-H), 7.45e7.41 (m,
2H, Ph 3,5-H), 4.30 (t, J ¼ 6.6 Hz, 2H, OCH2), 1.87e1.78 (m, 2H,
OCH2CH2), 1.51e1.42 (m, 2H, O(CH2)2CH2), 1.40e1.32 (m, 4H,
luminescence quantum yield (h) of the solutions was estimated by
comparing wavelength-integrated PL intensity of the solution
with that of the reference. Quinine sulfate dissolved in 0.1 M
(CH2)2CH3), 0.91 (t, J ¼ 7.0 Hz, 3H, CH3); 13C NMR (75 MHz, CDCl3;
d,
ppm): 161.01 (C-3), 138.73 (Ph C-1), 132.80 (Ph C-3,5), 127.39 (C-5),
118.87 (Ph C-2,6), 118.55 (Ph C-4), 83.18 (C-4), 69.86 (OCH2), 31.47
(CH2CH2CH3), 28.92 (OCH2CH2), 25.49 (OCH2CH2CH2), 22.58
(CH2CH3), 14.02 (CH3). APCI-MS: m/z (%) ¼ 405 [M þ H þ 4]þ (48),
403 [M þ H þ 2]þ (100), 401 [M þ H]þ (48); Anal. Calcd for
C15H18Br2N2O: C, 44.80; H, 4.51; N, 6.97. Found, %: C, 44.40; H, 4.06;
N, 6.72.
H2SO4 and exhibiting
h
¼ 53 ꢁ 2.3% at an excitation wavelength of
366 nm has been used as a reference [9]. Optical densities of the
reference and the sample solutions were ensured to be below 0.05
to avoid reabsorption effects. Estimated quantum yield was veri-
fied by using an alternative method of an integrating sphere
(Sphere Optics) coupled to the CCD spectrometer [10]. The inte-
grated sphere method was also employed in the quantum yield
estimations of the thin films. PL transients of the sample solutions
and thin films were measured using time-correlated single photon
counting system (PicoQuant PicoHarp 300). Pulsed excitation at
1 MHz repetition rate was provided by picosecond diode laser
with the pulse duration of 70 ps and the emission wavelength of
375 nm.
2.2.2. 1-(4-Bromophenyl)-3-hexyloxy-1H-pyrazole-4-carbaldehyde
(5, C16H19BrN2O2)
To a solution of 4 (804 mg, 2 mmol) in 16 ml of absolute THF
under inert atmosphere at ꢀ78 ꢂC n-BuLi (2.5 M in hexane, 0.8 ml,
2 mmol) was added drop wise and dry DMF (0.23 ml, 3 mmol) was
added. The mixture was gradually warmed up to room temperature
and stirred for 30 min (TLC control, eluent: ethyl acetate:n-
hexane ¼ 1:10). 5 ml of water were added and extraction was done
with ethyl acetate. The organic layers were combined, washed with
brine, dried over Na2SO4, filtrated, the solvent was evaporated. The
residue was purified by column chromatography (eluent: ethyl
acetate:n-hexane ¼ 1:10) to give pure 5 as a liquid. Yield: 50%
The ionization potentials (Ip) of the sample films were estimated
by performing electron photoemission spectroscopy (PES) in air, as
it is described in Refs. [11,12]. The charge carrier mobilities were
measured by the xerographic time-of-flight technique (XTOF)
[13,14]. The samples for the mobility measurements were prepared
by casting the materials dispersed in bisphenol Z polycarbonate
(PC-Z) (Iupilon Z-200 from Mitsubishi Gas Chemical Co.) on poly-
ester film with conductive Al layer serving as the sample substrate.
In XTOF measurements, the electric field inside the sample was
created by applying positive corona charging. Charge carriers at the
sample surface were generated by exciting the sample with the
pulses of nitrogen laser (pulse duration 1 ns, wavelength 337 nm).
Excitation induced decrease of the surface potential by 1e5% was
achieved in respect of the initial potential before excitation. The
capacitance probe connected to a wide-frequency-band electrom-
eter was employed to monitor the time evolution of the surface
potential decrease rate dU/dt after the excitation, which was used to
extract charge carrier transit time, and consequently, to deduce
(351 mg); IR (KBr; n
, cmꢀ1): 3127, 3101 (CHarom), 2955, 2930, 2857
(CHaliph), 1669 (C]O), 1562, 1508, 1374, (C]C, CeN), 1224, 1209
(CeOeC), 821 (CH ] CH of disubstituted benzene); 1H NMR
(300 MHz, CDCl3; d, ppm): 9.85 (s, 1H, CHO), 8.21 (s, 1H, 5-H),
7.59e7.50 (m, 4H, Ph-H), 4.37 (t, J ¼ 6.6 Hz, 2H, OCH2),1.89e1.80 (m,
2H, OCH2CH2), 1.52e1.43 (m, 2H, O(CH2)2CH2), 1.38e1.33 (m, 4H, O
(CH2)3(CH2)2), 0.91 (t, J ¼ 7.0 Hz, 3H, CH3); 13C NMR (75 MHz, CDCl3;
d, ppm): 183.33 (CHO), 164.03 (C-3), 137.98 (Ph C-1), 132.54 (Ph C-
3,5), 129.01 (C-5), 120.36 (Ph C-4), 120.07 (Ph C-2,6), 111.68 (C-4),
69.70 (OCH2), 31.45 (CH2CH2CH3), 28.83 (OCH2CH2), 25.49
(OCH2CH2CH2), 22.51 (CH2CH3),13.99 (CH3); APCI-MS: m/z (%) ¼ 375
[M þ Na þ 2]þ (47), 373 [M þ Na]þ (48), 295 [M-Br þ Na]þ (100);
Anal. Calcd for C15H18Br2N2O: C, 44.80; H, 4.51; N, 6.97. Found, %: C,
55.11; H, 5.81; N, 8.07.
carrier drift mobility (m).
The course of the reactions was monitored by TLC on Merck TLC
aluminium Silica gel 60 F254 sheets and developed with I2 or UV
ꢁ
light. Silica gel (Merck, grade 9385, 230e400 mesh, pore size 60 A)
2.2.3. 1-(Biphenyl-4-yl)-3-hexyloxy-1H-pyrazole-4-carbaldehyde
(6, C22H24N2O2)
was used for column chromatography. Elemental analyses were
performed with an Exeter Analytical CE-440 Elemental Analyzer.
Melting points were determined in capillary tubes on capillary
melting point apparatus Electrothermal MEL-TEMPÒ.
To a solution of pyrazole 5 (350 mg, 1 mmol) in 1,4-dioxane
(10 ml) under argon atmosphere anhydrous K3PO4 (636 mg,
3 mmol), phenylboronic acid (366 mg, 3 mmol) and Pd(PPh3)4
(92 mg, 0.08 mmol) were added. After refluxing for 3 h under argon
atmosphere (TLC control, eluent: ethyl acetate:n-hexane ¼ 1:10)
the mixture was diluted with water (10 ml) and extracted with ethyl
acetate. The combined organic layers were washed with brine, dried
over sodium sulfate, filtrated, the solvent was evaporated. The
2.2. Materials
4-Bromo-1-(4-bromophenyl)-3-hydroxy-1H-pyrazole (3) was
synthesized according the procedure described in Ref. [15].

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E. Arbaciauskiene et al. / Dyes and Pigments 85 (2010) 79e85
81
product was purified by column chromatography (eluent: ethyl
acetate:n-hexane ¼ 1:20) to give 6 as a liquid. Yield 94% (327 mg), IR
chloroform. The combined organic layers were washed with brine,
dried over sodium sulfate, filtrated, the solvent was evaporated.
The product was crystallized from chloroform:diethyl ether ¼ 1:2.
(KBr; n
, cmꢀ1): 3119, 3087 (CHarom), 2954, 2934, 2850 (CHaliph),1674
(C]O), 1563, 1499, 1367, 1354 (C]C, CeN), 1222, 1202 (CeOeC),
Yield of P2 was 45% (147 mg), mp 235e236 ꢂC. IR (KBr; , cmꢀ1):
n
840, 768, 701 (CH ] CH of mono- and disubstituted benzenes); 1H
3033 (CHarom), 2953, 2929, 2856 (CHaliph), 2210 (C^N), 1598, 1562,
1523, 1491, 1467, 1377, 1340 (C]C, CeN), 1229 (CeOeC), 837, 762,
697 (CH ] CH of mono- and disubstituted benzenes). 1H NMR
NMR (300 MHz, CDCl3; d, ppm): 9.87 (s, 1H, CHO), 8.28 (s, 1H, 5-H),
7.73e7.66 (m, 4H, N-Ph-H), 7.61e7.59 (m, 2H, N-Ph-Ph 2,6-H),
7.49e7.44 (m, 2H, N-Ph-Ph 3,5-H), 7.40e7.38 (N-Ph-Ph 4-H), 4.41 (t,
J ¼ 6.6 Hz, 2H, OCH2), 1.87 (m, 2H, OCH2CH2), 1.55e1.45 (m, 2H, O
(CH2)2CH2),1.40e1.35 (m, 4H, O(CH2)3(CH2)2), 0.94 (t, J ¼ 7.0 Hz, 3H,
(500 MHz, CDCl3; d, ppm): 8.81 (s, 2H, Pyr-5-H), 7.76 (m, 4H, N-Ph
3,5-H), 7.69 (s, 4H, CH ] CH-Ph-H), 7.68 (m, 4H, N-Ph 2,6-H), 7.62
(m, 4H, N-Ph-Ph 2,6-H), 7.53 (s, 2H, Pyr-CH), 7.47 (m, 4H, N-Ph-Ph
3,5-H), 7.37 (m, 2H, N-Ph-Ph 4-H), 4.42 (t, J ¼ 6.6 Hz, 4H, OeCH2),
1.89 (m, 4H, OCH2CH2), 1.52 (m, 4H, OCH2CH2CH2), 1.40 (m, 4H,
CH2CH2CH3), 1.39 (m, 4H, CH2CH3), 0.94 (t, J ¼ 7.0 Hz, 6H, CH3);
CH3); 13C NMR (75 MHz, CDCl3;
d, ppm): 183.39 (CHO), 164.12 (C-3),
140.08, 139.77, 138.18, 129.04, 128.88, 128.14, 127.68, 126.86, 119.04
(Ph C-2,6), 111.52 (C-4), 69.70 (OCH2), 31.48(CH2CH2CH3), 28.93
(OCH2CH2), 25.56 (OCH2CH2CH2), 22.54 (CH2CH3), 14.00 (CH3);
APCI-MS: m/z (%) ¼ 371 [M þ Na]þ (100); Anal. Calcd for
4C15H18Br2N2O*3H2O: C, 73.00; H, 7.10; N, 7.74. Found, %: C, 72.80;
H, 6.94; N, 7.52.
13C NMR (125 MHz, CDCl3;
d, ppm):163.6 (Pyr-C-3), 140.0 (N-Ph-
Ph-C-1), 139.4 (N-Ph-C-1), 138.5 (N-Ph-C-4), 134.1 (CH-Ph-C-1,4),
130.4 (Pyr-CH), 128.9 (N-Ph-Ph-C-3,5), 128.1 (N-Ph-C-2,6), 127.6
(N-Ph-Ph-C-4), 127.0 (N-Ph-Ph-C-2,6), 126.4 (Pyr-C-5), 125.8 (C-
CN-Ph-C-2,3,5,6), 119.1 (C^N), 118.8 (N-Ph-C-3,5), 105.7 (Pyr-C-4),
105.6 (ChNeC), 69.8 (OeCH2), 31.6 (CH2CH2CH3), 29.0 (OCH2CH2),
25.7 (OCH2CH2CH2), 22.6 (CH2CH3), 14.1 (CH3); APCI-MS: m/z (%)
817 [M þ H]þ (10), 492 [M-2C6H5e2CN þ H]þ (50), 406 [M-
2.2.4. 1,4-Bis{(E)-2-[(1-biphenylyl-3-hexyloxy)pyrazol-4-yl]
ethenyl}benzene (P1, C52H54N4O2)
To the suspension of 6 (348 mg, 1 mmol) and [1,4-phenylenebis
(methylen)]-bis[triphenylphosphonium] dibromide (315 mg,
0.4 mmol) in abs. THF (10 ml) under argon atmosphere NaOMe
(408 mg, 6 mmol) solution in abs. MeOH (8 ml) was added. Reaction
mixture was stirred overnight at r.t. under argon atmosphere (TLC
control, eluent: dichloromethane:diethyl ether:n-hexane ¼ 6:2:16).
Reaction mixture was poured into water (15 ml) and extracted with
chloroform. The combined organic layers were washed with brine,
dried over sodium sulfate, filtrated, the solvent was evaporated. The
residue was refluxed with I2 (25 mg, 0.1 mmol) in 10 ml of chlo-
roform for 30 min. After cooling to room temperature the mixture
was washed with sat. Na2S2O3 solution. The organic layer was
washed with brine, dried over sodium sulfate, filtrated, the solvent
was evaporated. The product was crystallized from chloroform:
diethyl ether ¼ 1:2. Yield of P1 was 54% (166 mg), mp 213e218 ꢂC.
2C6H5e2CNe2C3H6 þ H]þ (100); Anal. Calcd for C54H52N6O2
ꢃ
3H2O: C, 74.46; H, 6.71; N, 9.65. Found, %: C, 74.45; H, 6.30; N,
9.24.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic strategy (Scheme 1) displays the chemical reac-
tion scheme that was used to synthesize the key compound 1-
(biphenyl-4-yl)-3-hexyloxy-1H-pyrazole-4-carbaldehyde (6) and is
based on these concepts.
Thus, dihalopyrazole synthon 3 was prepared via 3-hydroxy-4-
phenyl-1H-pyrazole (2) starting from the commercially available
1-phenylpyrazolidin-3-one following the previously reported
IR (KBr; n
, cmꢀ1): 3034 (CHarom), 2952, 2927, 2856 (CHaliph), 1607,
1565, 1525, 1491, 1400, 1363, 1356 (C]C, CeN), 1247, 1204
(CeOeC), 837, 762, 696 (CH ] CH of mono- and disubstituted
benzenes). 1H NMR (500 MHz, CDCl3;
d, ppm): 7.86 (s, 2H, Pyr 3-H),
O
HO
HO Br
FeCl3
Br2
7.67 (m, 4H, N-Ph 3,5-H), 7.65 (m, 4H, N-Ph 2,6-H), 7.61 (m, 4H, N-
Ph-Ph 2,6-H), 7.46 (m, 4H, N-Ph-Ph 3,5-H), 7.44 (s, 4H, CH-Ph-H),
7.36 (m, 2H, N-Ph-Ph 4-H), 7.14 (d, J ¼ 16.3 Hz, 2H, Pyr-CH ] CH),
6.95 (d, J ¼ 16.3 Hz, 2H, Pyr-CH), 4.42 (t, J ¼ 6.6 Hz, 4H, OeCH2), 1.91
(m, 4H, OCH2CH2), 1.57 (m, 4H, OCH2CH2CH2), 1.42 (m, 4H,
HN
N
N
N
N
N
HCl (aq.), EtOH
reflux
CCl4
reflux
Br
3, 80%
CH2CH2CH3), 1.40 (m, 4H, CH2CH3), 0.95 (t, J ¼ 7.0 Hz, 3H, CH3); 13
C
1
2, 70%
NMR (125 MHz, CDCl3; d, ppm):162.3 (Pyr-C-3), 140.2 (N-Ph-Ph-C-
1), 139.1 (N-Ph-C-4), 137.9 (N-Ph-C-1), 136.8 (CH-Ph-C-1,4), 128.8
(N-Ph-Ph-C-3,5), 128.0 (N-Ph-C-2,6), 127.5 (Pyr-4-CH ] C), 127.3
(N-Ph-Ph-C-4), 126.8 (N-Ph-Ph-C-2,6), 126.2 (CH-Ph-C), 124.7
(Pyr-C-5), 117.7 (N-Ph-C-3,5), 116.6 (Pyr-4-CH), 108.8 (Pyr-4-C), 69.2
(OeC), 31.7 (CH2CH2CH3), 29.1 (OCH2CH2), 25.8 (OCH2CH2CH2),
22.6 (CH2CH3), 14.1 (CH3); APCI-MS: m/z (%) ¼ 767 [M þ H]þ (50),
451 [Me2 ꢃ C6H5eC11H26O2 þ Na]þ, (100); Anal. Calcd for
C52H54N4O2 ꢃ H2O: C, 79.56; H, 7.17; N, 7.14. Found, %: C, 79.57; H,
7.17; N, 6.90.
C6H13Br
NaH, DMF
60 oC
O
O
N
O
N
O
O
Br
PhB(OH)2
DMF
N
N
2.2.5. 1,4-Bis{(E)-1-nitrile-2-[(1-biphenylyl-3-hexyloxy)pyrazol-
4-yl]ethenyl}benzene (P2, C54H52N6O2)
N
N
Pd(PPh3)4, K3PO4
dioxane, reflux
n-BuLi, THF
-78 oC
To the suspension of 6 (348 mg, 1 mmol) and 1,4-phenyl-
enediacetonitrile (63 mg, 0.4 mmol) in dry chloroform (15 ml) and
abs. MeOH (4 ml) under argon atmosphere NaOMe (408 mg,
6 mmol) solution in abs. MeOH (4 ml) was added. Reaction mixture
was stirred overnight at r.t. under argon atmosphere (TLC control,
eluent: dichloromethane:diethyl ether:n-hexane ¼ 6:2:16). Reac-
tion mixture was poured into water (15 ml) and extracted with
Br
Br
4, 95%
6, 94%
5, 50%
Scheme 1. Synthesis route to 1-(biphenyl-4-yl)-3-hexyloxy-1H-pyrazole-4-carbalde-
hyde (6).

ꢀ _
E. Arbaciauskiene et al. / Dyes and Pigments 85 (2010) 79e85
82
methods [15]. Then, the hydroxyl group of 3 was alkylated (hex-
yloxy group introduction) to lengthen the side chains of target
polyconjugated structures P1 and P2 for the better solubility. The
key step - formylation of intermediate 4 was performed with the
addition of 1 equiv of n-BuLi (2.5 M solution in hexanes) in THF at
ꢀ78 ꢂC which was quenched with DMF. As the result only
selectively monoformylated pyrazole 5 formed in the reaction
mixture. The highest yield of 5 was achieved by rapidly adding
n-BuLi - and immediately after that quenching with DMF to avoid
the decomposition of the formed aryllithium intermediate. The
structure of the carbaldehyde 5 was analyzed and identified
following the 1H, 13C NMR, IR, MS spectroscopy and elemental
analysis data. The 1H NMR spectrum of 5 displays the signal of the
aldehyde proton at 9.85 ppm. The 13C NMR spectrum of 5 as
compared with the spectrum of 4 displays the additional signal at
183.30 ppm, which belongs to the aldehyde group, while the signal
of C-4 at 83.18 ppm in the spectrum of 4 is changed to 111.77 ppm
in the spectrum of 5 that prove the selective formylation of the
heterocyclic ring to C-4 position (Fig. 1).
PPh3Br
N
N
O
BrPh3P
N
N
O
NaOMe, THF
MeOH, r.t.
P1, 54%
6
CN
N
O
NC
N
NC
N
CN
O
N
NaOMe, CHCl3
MeOH, r.t.
P2, 45%
Scheme 2. Synthesis route to the polyconjugated molecules P1 and P2.
To extend the
p-conjugated system compound 5 was further
coupled with the phenylboronic acid under Suzuki reaction
conditions to give the pyrazolecarbaldehyde derivative 6 in a high
yield. The target compound P1 was synthesized using well known
Wittig reaction conditions from carbaldehyde 6 and corresponding
bis(triphenylphosphonium) salt (Scheme 2). As it was observed
from the crude sample of P1 according to the TLC and 1H NMR
spectral data an inseparable mixture of cis/trans-olefin geometries
was formed. The isomers cisecis and cis-trans were converted to the
transetrans isomer following the previously reported methods [16],
as the presences of cis isomers are detrimental to the emission
properties of polyconjugated molecules [17].
the introduction of an additional electron-withdrawing function in
this molecule could dramatically affect its properties as a lumines-
cent material.
The structures of P1 and P2 were analyzed and identified
following the 1H, 13C NMR, IR, MS spectroscopy and elemental
analysis data.
3.2. Photophysical and photoelectrical properties of P1 and P2
Fig. 2 displays absorption and PL spectra of the dilute solutions
of compounds P1 and P2 and PL spectra of their thin films. The
lowest energy absorption bands of the two pyrazolyl-substituted
diethylenes P1 and P2 in dilute solutions exhibit maxima at 383 nm
The synthesis of the same skeletal structure P2, but consisting of
two CN groups, was fulfilled by the Knoevenagel reaction of car-
baldehyde 6 and 1,4-phenylenediacetonitrile. We anticipated that
Ph C-3,5
Ph C-2,6
O
Br
N
N
C-5
Ph C-4
Br
C-3
Ph C-1
C-4
Ph C-3,5
Ph C-2,6
O
O
N
N
C-5
CHO
Ph C-4
Br
C-3
Ph C-1
C-4
100
150
ppm
Fig. 1. The aromatic parts of 13C NMR spectral data of compounds 4 (above) and 5 (below) (75 MHz, CDCl3).

ꢀ
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E. Arbaciauskiene et al. / Dyes and Pigments 85 (2010) 79e85
83
a significant role in the emission quenching. However, emission
enhancement (in the case of P2) followed by transition from the
liquid to solid phase is untypical and need to be clarified. In a solid
state cyano-substituted P2 molecule torsions and so the torsion
induced nonradiative deactivation processes are suppressed due to
the rigid environment [19,20]. Moreover, steric and electrostatic
effects induced by the introduction of bulky and polar substituents
prevent tight packing of the molecules in the solids, what signifi-
cantly reduces excitation migration, and thus, lowers exciton
migration-related quenching. This explains much higher (by
a factor of 20) fluorescence efficiency in the films of P2 as compared
to that in solutions and by a factor of two higher efficiency as
compared to that of the films of P1 composed of highly fluorescent
planar molecules (see Fig. 2). The enhancement of fluorescence
efficiency for the compounds featuring cyano groups in a solid state
as compared to that in solution was also reported in numerous
papers [8,19,21].
P1
=0.11
Film
=0.88
Sol.
P2
PL transients measured in the dilute solutions and thin films of
P1 and P2 are depicted in Fig. 3. The solution of P1 exhibits single
exponential decay with decay time constant (
s) of about 0.9 ns.
Exponential decay accompanied with close to unity fluorescence
quantum yield in planar P1 molecules clearly indicates the
domination of radiative relaxation. The PL decay of P2 solution is
=0.20
Film
found to be considerably faster (s < 0.1 ns) as compared to that of
=0.01
Sol.
P1. Since the decay is faster than the instrument response function
(IRF), PL transient closely follows the IRF and inhibits more precise
estimation of the time constant. Short PL decay time of P2 in
solution is consistent with very low emission yield and evidences
the significance of nonradiative relaxation.
300
400
500
600
700
Wavelength (nm)
Fig. 2. Absorbance and normalized PL spectra of 10ꢀ5 M THF solutions (thin lines) as
well as PL spectra of thin films (thick lines) of P1 and P2. PL quantum yield (h) values
are indicated.
105
and 413 nm, respectively. Presence of CN- groups in P2 causes
a bathochromic shift of the absorption band as compared to that of
cyano-free compound P1. The similar shift amounting to 20e30 nm
was also observed for CN-PPV oligomers (with cyano groups at
Solutions
104
P1
a
positions), where it was attributed to the electron-withdrawing
= 0.87 ns
nature of cyano groups [18]. The PL spectrum of P1 solution is
Stokes shifted by 47 nm in respect of the absorption band and
103
feature high emission quantum yield (
of compound P2 bearing cyano groups, Stokes shift is found to be
slightly larger (57 nm), and more importantly, a huge decrease in
h
¼ 0.88), whereas in the case
P2
102
< 0.1 ns
IRF
emission quantum yield (
h
¼ 0.01) is observed. Furthermore, high
PL quantum yield of the isolated molecules of P1 in the solution is
reduced down to 0.11 (by a factor of 8) in the solid state. In sharp
contrast to this, very weak fluorescence of P2 solution experiences
101
Thin films
boost in efficiency up to
condensation (Fig. 2).
It is known that cyano groups attached into vinylene linkage
with phenyl rings facilitate intramolecular torsions around the
vinylene double bonds due to the steric hindrance resulting in
fluorescence quenching in the isolated molecules [19]. This is
h
¼ 0.20 (by a factor of 20) upon
104
P2
103
P1
in agreement with our observations of very low h obtained for P2 in
a dilute solution. As opposed to that, the isolated molecules of
analogous cyano-free compound P1 feature coplanar structure,
which determines high emission yield of P1 in a solution.
The reduction of fluorescence efficiency in the solid state is
common for majority of fluorophores and is due to excitation
migration-related quenching in the solid state. In the film of P1 the
molecule coplanarity enhances exciton coupling, which promotes
the nonradiative decay mainly due to excitation migration to
intrinsic defects or impurity-related quenching sites [19]. Since the
films were prepared in ambient conditions, oxygen is likely to play
102
101
0
5
10
Time (ns)
Fig. 3. PL transients of P1 and P2 in 10ꢀ5 M THF solutions and in the thin films
(points). Line indicates single exponential fit to the experimental data. IRF
instrument response function.
a
e

ꢀ _
E. Arbaciauskiene et al. / Dyes and Pigments 85 (2010) 79e85
84
10-6
10-7
10-8
The PL transient in the film of P1 is strongly non-exponential,
which is in agreement with migration-related exciton quenching.
As opposed to the PL decays in the solutions, the decay of the P2
film was found to be slower than that of P1 film and much slower
than that of P2 solution supporting the assumption on suppressed
exciton migration and nonradiative deactivation in less tightly
packed film of P2 with cyano groups.
Evidently, the PL transients in the films and solutions qualita-
tively correlate with the PL quantum yield data pointing out
significance of cyano substituents of the pyrazole-based poly-
conjugated molecules on the excitation deactivation pathways via
molecular torsions in a liquid phase or reduction of exciton
migration-related quenching in condensed phase.
The ionization potentials of the P1 and P2 were found to be
5.46 eV and 5.90 eV, respectively. Assuming the bandgaps of P1 and
P2 films to be roughly the same (see Fig. 2) the larger Ip value of P2
achieved upon introduction of cyano groups into the vinylene
linkages resulted in the increased electron affinity of P2 by more
than 0.4 eV, as compared to that of P1. This is consistent with the
higher electron affinities reported for cyano-substituted PPV
polymers [7].
P1:(PC-Z) film,
mass ratio 1:3
d=8.5 m
0 exp( E )
0.0041(cm/V)1/2
0
0
200 400 600 800 1000 1200
E
1/2 (V/cm)1/2
Fig. 5. Hole drift mobility (mh) as a function of the square root of electric field for P1
dispersed in PC-Z (mass ratio 1:3). Line is exponential approximation of the experi-
mental data points. m0 - mobility at E ¼ 0.
Transparent and homogeneous film of P1 molecularly dispersed
in polymer host of bisphenol Z polycarbonate (PC-Z) (with P1 to
PC-Z mass ratio of 1:3) was prepared by the casting technique and
used in XTOF measurements. Unfortunately, the solubility of the
compound P2 was insufficient to prepare the film of the required
thickness for the estimation of charge carrier mobility. Fig. 4 shows
the transients of the surface potential decrease rate dU/dt in P1:
PC-Z film measured at different applied voltages. The transit time tt
of charged carriers was determined from the kink in the dU/dt
transient plotted in a double logarithmic scale and then used to
obtained mh value is considered to be rather moderate as compared
to the typical values of mh for charge transporting materials
dispersed in polymers [22].
4. Conclusions
Pyrazolyl-substituted diethylene derivative and the analogous
compound with the additional cyano groups attached at the
vinylene linkages were synthesized and characterized. The ioniza-
tion potential measurements indicated that introduction of cyano
groups into the vinylene linkages increases the Ip from 5.46 eV to
5.90 eV, what in conjuction with PL data also leads to increased
electron affinity for pyrazolyl-substituted diethylenes. The hole
drift mobility estimated by xerographic time-of-flight technique for
the solid solution of cyano-free derivative dispersed in poly-
carbonate (mass ratio 1:3) was found to be 3.5 ꢃ 10ꢀ7 cm2 Vꢀ1 sꢀ1
deduce the drift mobility (
m) using the formula
m
¼ d2/U0tt, where
d is the sample thickness and U0 is the surface potential at the
moment of the excitation.
The room temperature electric field dependence of the hole drift
mobility (mh) for the solid solution of P1 in PC-Z is presented in
Fig. 5. The exponential increase of mh with the square root of the
electric field was observed. Such behavior is commonly observed
for organic charge transporting materials in amorphous state due to
a disorder [12]. mh of P1 dispersed in PC-Z was found to attain the
value of 3.5 ꢃ10ꢀ7 cm2 Vꢀ1 sꢀ1 at an electric field of 1 MV cmꢀ1. The
at an electric field of 1 MV cmꢀ1
.
The PL measurements revealed high emission quantum yield
(
h
¼ 0.88) of the coplanar structure bearing molecules of the cyano-
free pyrazole derivative in a solution and very poor yield (
h
¼ 0.01)
100
of the analogous derivative with cyano groups caused by these
groups-induced intramolecular torsions. In the films prepared in
ambient conditions, the incorporation of cyano moieties was
shown to enhance PL efficiency of the pyrazole-based diethylene
derivative from 0.11 to 0.20 (by a factor of two). The enhancement
was attributed to the suppression of migration-induced quenching
of the excitons at the defects in the less tightly packed films of bulky
cyano groups-containing pyrazole derivatives.
P1:(PC-Z) film,
t
t
mass ratio 1:3
10-1
d=8.5 m
10-2
0.03
10-3
Acknowledgements
0.02
10-4
The research was supported by the Lithuanian State Science and
Studies Foundation. We gratefully acknowledge habil. dr. V. Gaidelis
for the help in ionization potential measurements.
0.01
552 V
10-5
0.00
0.00
0.02
Time (s)
0.04
References
10-5
10-4
10-3
Time (s)
10-2
10-1
100
[1] (a) Kim DU, Paik SH, Kim SH, Tsutsui T. Synth Met 2001;123:43;
(b) Kwok CC, Wong MS. Chem Mater 2002;14:3158;
ꢀ
(c) Malinauskas T, Stumbraite J, Getautis V, Gaidelis V, Jankauskas V, Juska G,
et al. Dyes Pigm 2009;81:131.
Fig. 4. XTOF transients in a double logarithmic scale for the P1 dispersed in PC-Z using
the mass ratio 1:3. Arrow indicates hole transit time at U0 ¼ 980 V. Inset shows
a typical transient curve in a linear scale.
[2] Seo HJ, Park HC, Lee SE, Park JW. Curr Appl Phys 2005;5:209.
[3] Kim BS, Joo S-H, Oh D, Wook Cha S, Choi DS, Lee CE, et al. Synth Met
2004;145:229.

ꢀ
_
E. Arbaciauskiene et al. / Dyes and Pigments 85 (2010) 79e85
85
[4] Sigalov M, Ben-Asuly A, Shapiro L, Ellern A, Khodorkovsky V. Tetrahedron Lett
[13] Montrimas E, Gaidelis V, Pazera A. Lithuanian J Phys 1966;6:569.
[14] Vaezi-Nejad SM. Int J Electronics 1987;62:361.
2000;41:8573.
[5] Tamoto N, Adachi C, Nagai K. Chem Mater 1997;9:1077.
[6] (a) Lu Z, Jiang Q, Zhu W, Xie M, Hou Y, Chen X, et al. Synth Met
2000;111e112:465;
[15] (a) Harries C, Loth G. Ber Dtsch Chem Ges 1896;29:513;
(b) Nedzelskyte E, Martynaitis V, Sackus A, Eller GA, Holzer W. Molbank
2007;3:M551.
(b) Sano T, Fujii Y, Nishio Y, Hamada Y. Jpn J Appl Phys 1995;34:3124;
(c) Zhang XH, Wu SK, Gao ZQ, Lee CS, Lee ST, Kwong HL. Thin Solid Films
2000;371:40.
[16] Olkhovik VK, Vasilevskii DA, Pap AA, Kalechyts GV, Matveienko YV, Baran AG,
et al. Archivoc 2008;9:69.
[17] Sarker AM, Gurel EE, Ding L, Styche E, Lahti PM, Karasz FE. Synth Met
€
[7] Greenham NC, Moratti SC, Bradley DDC, Friend RH, Holmes AB. Nature
1993;365:628.
2003;132:227.
[18] Hanack M. Thin Solid Films 2002;417:26.
[8] An BK, Kwon SK, Jung SD, Park SY. J Am Chem Soc 2002;124:14410.
[9] Adams MJ, Highfield JG, Kirkbright GF. Anal Chem 1977;49:1850.
[10] Mello JC, Wittmann HF, Friend RH. Adv Mater 1997;9:230.
[11] Miyamoto E, Yamaguchi Y, Yokoyama M. Electrophotography 1989;28:364.
[12] (a) Getautis V, Paliulis O, Paulauskaite I, Gaidelis V, Jankauskas V,
Sidaravicius J, et al. J Imag Sci Tech 2004;48:265;
[19] Oelkrug D, Tompert A, Egelhaaf H, Hanack M, Steinhuber E, Hohloch M, et al.
Synth Met 1996;83:231.
[20] Kim BK, Choi CH, Lee SH, Lee YS, Zong K, Yu SC. Colloids Surf A 2001;180:41.
[21] Palayangoda SS, Cai X, Adhikari RM, Neckers DC. Org Lett 2008;10:281.
[22] (a) Matoliukstyte A, Burbulis E, Grazulevicius JV, Gaidelis V, Jankauskas V.
Synth Met 2008;158:462;
(b) Simokaitiene J, Laukyte D, Grazulevicius JV, Kazlauskas K, Jursenas S,
Jankauskas V. Dyes Pigm 2009;81:235.
(b) Simokaitiene J, Grigalevicius S, Grazulevicius JV, Rutkaite R, Kazlauskas K,
Jursenas S, et al. J Optoelectron Adv Mater 2006;8:876.