Synthesis and electronic spectra of new low-molecular weight compounds with possible application in electroluminescent layers

Article abstract of DOI:10.2478/s11532-011-0098-3

Two new low-molecular weight compounds - (Z)-4-(4-(dimethylamino) benzylidene)-1-(9-ethyl-9H-carbazol-3-yl)-2-phenyl-1Himidazol- 5(4H)-one and 2-(6-hydroxyhexyl)-6-(pyrrolidin-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) - with possible application in or

Full text of DOI:10.2478/s11532-011-0098-3

Cent. Eur. J. Chem. • 9(6) • 2011 • 1126-1132
DOI: 10.2478/s11532-011-0098-3
Central European Journal of Chemistry
Synthesis and electronic spectra of new
low-molecular weight compounds with possible
application in electroluminescent layers
Research Article
Georgi H. Dobrikov1, Georgi M. Dobrikov2*, Mariya Aleksandrova1
1Department of Microelectronics,
Technical University of Sofia, Sofia 1000, Bulgaria
2Institute of Organic Chemistry with Centre of Phytochemistry,
Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Received 24 June 2011; Accepted 14 August 2011
Abstract:
Two new low-molecular weight compounds – (Z)-4-(4-(dimethylamino)benzylidene)-1-(9-ethyl-9H-carbazol-3-yl)-2-phenyl-1H-
imidazol-5(4H)-one and 2-(6-hydroxyhexyl)-6-(pyrrolidin-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) – with possible application
in organic light-emitting devices were synthesized. Their photophysical properties in solution and in polymer films were investigated.
The determined relative fluorescence quantum yields in solution for both compounds were 0.003 and 0.51, while those in poly(methyl
methacrylate) films were around 0.10 and 1.0, respectively. For 1H-imidazol-5(4H)-one derivative, single-layer organic displays
with one emitting layer were prepared by spin-coating technology. The applied voltage was 40 V (AC) with 1-3 KHz frequency.
The emission maximum of the experimental AC display structures was at 600-630 nm. For displays with 2-(6-hydroxyhexyl)-6-
(pyrrolidin-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) the applied voltage was 60 V (AC) with 6-9 KHz frequency, but its future
success will require more appropriate binding polymers. Based on the obtained experimental results, it is concluded that the investigated
compounds could be applied for preparation of color electroluminescent structures.
Keywords: Low-molecular weight compounds • 1,8-naphthalimides • Imidazolones • Fluorescence • Electroluminescent layers
© Versita Sp. z o.o.
local conditions during the work of the device (high
temperatures, electric fields, etc.).
1. Introduction
An important prerequisite for the application of these
materials in optoelectronic devices is the determination
of their fluorescence quantum yields in solution and the
study of their photoluminescent (PL) properties in solid
state.
In the present paper, the synthesis and the main
photophysical characteristics of two new LMWC are
reported and their experimental electroluminescent
structures were prepared.
Over the past few years, an increasing interest has been
directed toward the preparation and the characterization
of organic materials with possible application in display
techniques and in optoelectronic devices [1]. Currently
existing organic light-emitting materials are divided into
two main classes: i) high-molecular weight compounds
(conjugated polymers) and ii) low molecular weight
materials (LMWC) [2,3]. The main problem arising during
preparation of organic electroluminescent (EL) devices
with conjugated polymers is the formation of a high
energy barrier at the electrode/polymer interfaces [4].
The light emission of such EL structures is dependent
on the work function of the electrodes. In contrast with
2. Experimental procedure
conjugated polymers, the light emission of LMWC is a 2.1. General
result of intramolecular n-π* charge transfer excited Flash column chromatography was carried out using
states caused by applied voltage with frequencies up silica gel 60 (0.040–0.063 mm, 230-400 mesh ASTM,
to 1 kHz [5]. A significant problem in the development Merck). Commercially available solvents for reactions,
of LMWC is their low chemical stability against extreme flash column chromatography and film preparation
* E-mail: gmdob@orgchm.bas.bg
1126

G. H. Dobrikov, G.i M. Dobrikov, M. Aleksandrova
were used after distillation – hexane, glacial acetic acid 1H and 13C NMR data are reported as follows:
(AcOH), dichloromethane (DCM), methanol (MeOH), chemical shift, multiplicity (s = singlet, d = doublet,
ethanol (EtOH), diethyl ether (Et2O), triethylamine (Et3N). t = triplet, q = quartet, br = broad, m = multiplet),
Commercially available reagents for synthesis were integration, identification, and coupling constants
purchased from Sigma-Aldrich - anhydrous potassium (in Hz). For numbering of the atoms see Figs. 1 and 2.
acetate (KOAc), acetic anhydride (Ac2O), hippuric acid, Mass spectra (MS) were measured with a Thermo
4-(dimethylamino)-benzaldehyde, 9-ethyl-9H-carbazol- Scientific High Resolution Magnetic Sector MS DFS
3-amine, citric acid, pyrrolidine, dimethylformamide by electron ionization (EI), and were reported as
(DMF). Melting points were determined in capillary fragmentation in m/z with relative intensities (%).
tubes on an Electrothermal MEL-TEMP 1102D-230 Elemental analysis was performed by the Microanalytical
VAC apparatus without corrections. NMR spectra Laboratory for Elemental Analysis of the Institute of
were recorded at 300 K on a Bruker Avance DRX-250 Organic Chemistry, Bulgarian Academy of Sciences.
(250.13 for 1H and 62.90 MHz for 13C) and at 293 K on
a Bruker Avance II+ 600 (600.13 for 1H and 150.92 MHz
2.2. Synthesis of compound 5
Compound 5 was obtained in two steps (Scheme 1).
and for 13C NMR) spectrometers with TMS as internal
1
standard for chemical shifts (δ, ppm). H spectra were
calibrated to the signal of TMS (δ=0.0000). 13C spectra
were calibrated to the signal of the solvent (CDCl3,
δ=77.0000). The following additional NMR techniques
were used: Distortionless Enhancement by Polarization
Transfer at 135 degree of impulse flip angle (DEPT135),
Two-dimensional Correlation Spectroscopy (COSY),
Heteronuclear Single Quantum Coherence (HSQC)
and Heteronuclear Multiple Bond Correlation (HMBC).
The first step was the preparation of the intermediate
(Z)-4-(4-(dimethylamino)benzylidene)-2-phenyloxazol-
5(4H)-one (3) by implementation of the classical
Erlenmeyer condensation [6] – a reaction between
hippuric acid (1) and 4-(dimethylamino)-benzaldehyde
(2) in refluxing acetic anhydride (Ac2O) in the presence
of anhydrous potassium acetate (KOAc). At the second
step, the purified intermediate 3 was refluxed with
9-ethyl-9H-carbazol-3-amine (4) and anhydrous KOAc
Scheme 1. Two-step synthesis of compound 5.
1127

Synthesis and electronic spectra of new low-molecular weight
compounds with possible application in electroluminescent layers
2.3. Synthesis of compound 10
in acetic acid (AcOH) in order to obtain the desired
compound 5 as pure Z isomer [7].
Thepreparation ofcompound 5includes thefollowing
steps (Scheme 1):
Step 1: Preparation of the intermediate (Z)-4-(4-
(dimethylamino)benzylidene)-2-phenyloxazol-5(4H)-
one (3):
A mixture of hippuric acid (1) (2.00 g, 11.2 mmol),
4-(dimethylamino)-benzaldehyde (2) (1.67 g, 11.2 mmol)
and anhydrous KOAc (1.10 g, 11.2 mmol) was refluxed
in 15 mL acetic anhydride for 1 h. After cooling, the
volatile was evaporated at 60°C in vacuo and the crude
intermediate 3 was obtained. This solid was washed with
15 mL hexane in ultrasonic bath and recrystallized from
EtOH:water = 1:1. After drying in vacuo, the oxazolone 3
was used as starting material for the next synthetic step
without characterization.
Compound 10 was synthesized in two steps, as well
(Scheme 2). The first step included preparation by
a known procedure [8] of the intermediate 8, which
does not possess fluorescent properties. The reaction
takes place in refluxing absolute ethanol and does not
affect the bromine atom. In the second step the desired
compound 10 was obtained with excellent yield through
non-catalytic N-arylation of pyrrolidine with compound 8
in boiling DMF [9].
Step 1: Preparation of intermediate 6-bromo-2-(6-
hydroxyhexyl)-1H benzo[de]isoquinoline-1,3(2H)-dione
(8).
A mixture of 6 (1.50 g, 5.41 mmol) and 7 (0.70 g,
5.95 mmol) was refluxed for 4 h in 40 mL absolute EtOH.
After cooling, the reaction mixture was poured into
250 mL 5% aqueous citric acid and stirred overnight.
The obtained white precipitate of the crude product 8
was filtered, washed with water and dried in vacuo. After
purification with flash column chromatography (60 g
silica gel, phase hexane:Et2O = 1:2), the pure product 8
was obtained as white crystals (1.05 g, 51% yield). M.p.
93-94 ºC. 1H-NMR (CDCl3, 600.13 MHz): δ 8.65 (dd, 1H,
10-H), 8.56 (dd, 1H, 8-H), 8.40 (m, 1H, 4-H), 8.04 (m, 1H,
5-H), 7.84 (m, 1H, 9-H), 4.17 (m, 2H, 12-H), 3.65 (q, 2H,
17-H), 1.75 (m, 2H, 13-H), 1.60 (m, 2H, 16-H), 1.41-1.49
(m, 4H, 14-H, 15-H). 13C-NMR (CDCl3, 150.92 MHz): δ
163.63 (s, 1C, C=O), 163.61 (s, 1C, C=O), 133.24 (d,
1C, 8-C), 132.02 (d, 1C, 10-C), 131.21 (d, 1C, 4-C),
131.07 (d, 1C, 5-C), 130.58 (s, 1C, 7-C), 130.23 (s, 1C,
6-C), 128.95 (s, 1C, 7a-C), 128.06 (d, 1C, 9-C), 123.05
(s, 1C, 11-C), 122.19 (s, 1C, 3-C), 62.74 (t, 1C, 17-C),
40.33 (t, 1C, 12-C), 32.56 (t, 1C, 16-C), 27.93 (t, 1C,
13-C), 26.64* (t, 1C, 14-C or 15-C), 25.24* (t, 1C, 14-C
or 15-C). Anal. calcd. for C18H18BrNO3 (Mw =376.24):
C, 57.46; H, 4.82; Br, 21.24; N, 3.72; O, 12.76. Found:
C, 57.40; H, 4.88; Br, 21.30; N, 3.71. MS (EI) m/z (rel.
int.): 376.47 (M+●, 90), 346.27 (16), 304.08 (10), 302.92
(15), 302.06 (11), 290.98 (62), 290.12 (87), 289.04 (78),
277.91 (75), 277.10 (100), 276.04 (99), 260.04 (39),
259.03 (56), 257.98 (30), 232.84 (36), 232.06 (38),
230.04 (17), 197.05 (9), 151.08 (10), 126.04 (15).
Step 2: Preparation of the target product 2-(6-
hydroxyhexyl)-6-(pyrrolidin-1-yl)-1H-benzo[de]
isoquinoline-1,3(2H)-dione (10).
Step 2: Preparation of the target product: (Z)-4-(4-
(dimethylamino)benzylidene)-1-(9-ethyl-9H-carbazol-3-
yl)-2-phenyl-1H-imidazol-5(4H)-one (5):
A mixture of oxazolone (3) (1.46 g, 5.0 mmol),
9-ethyl-9H-carbazol-3-amine (4) (4.21 g, 20.0 mmol)
and KOAc (0.49 g, 5.0 mmol) was refluxed in glacial
acetic acid (20 mL) for 7 h. After cooling, the volatile
was evaporated at 60°C in vacuo. The residue was
washed with 50 mL hexane in ultrasonic bath and dried
in vacuo to obtain the crude product 5. After twofold
flash column chromatography (190 g silica gel, phase:
DCM and DCM:Et2O = 20:1), 1.07 g of pure compound
5 was obtained as orange crystals (44% yield). M.p.
285-286 ºC. 1H-NMR (CDCl3, 250 MHz): δ 8.27 (d,
2H, J = 8.9 Hz), 8.03 (dt, 1H, J = 7.8, 1.0 Hz), 7.98 (m,
1H), 7.63-7.65 (m, 1H), 7.59-7.62 (m, 1H), 7.39-7.55
(overlapping m, 3H), 7.36 (s, 1H), 7.31 (m, 1H), 7.19-
7.27 (overlapping m, 4H), 6.77 (d, 2H, 3`-H, 5`-H, J =
8.9 Hz), 4.39 (q, 2H, 23-H, J = 7.2 Hz), 3.10 (s, 6H,
NMe2), 1.45 (t, 3H, 24-H, J = 7.2 Hz). 13C-NMR (CDCl3,
62.9 MHz): δ 171.41 (s, 1C, C-imidazolone), 157.55
(s, 1C, C-imidazolone), 151.68 (d, 1C, 4`-C), 140.43
(s, 1C, C-N from carbazole), 139.29 (s, 1C, C-N from
carbazole), 134.78 (d, 2C), 130.63 (d, 1C), 130.43 (d,
1C), 129.60 (s, 1C, C-N from carbazole), 129.04 (d,
2C), 128.13 (d, 2C), 126.47 (s, 1C), 126.20 (d, 1C),
125.14 (d, 1C), 123.44 (s, 1C), 122.67 (s, 1C), 122.56
(s, 1C), 120.84 (d, 1C), 119.84 (d, 1C), 119.14 (d, 1C),
111.76 (d, 2C), 109.00 (d, 1C), 108.68 (d, 1C), 40.07
(q, 2C, NMe2), 37.71 (t, 1C, 23-C), 13.84 (q, 1C, 24-C).
Anal. calcd. for C32H28N4O (Mw = 376.24): C, 79.31; H,
5.82; N, 11.56; O, 3.30 %. Found: C, 79.38; H, 5.80; N,
11.49 %. MS (EI) m/z (rel. int.): 485 (47), 484 (M+●, 100),
298 (15), 297 (56), 242 (15), 194 (16), 179 (40), 159
(49), 105 (24).
A mixture of 8 (0.554 g, 1.47 mmol) and pyrrolidine
(9) (1.22 mL, 14.72 mmol) was refluxed for 1.5 h
in 25 mL dry DMF. The orange solution was cooled,
poured into 200 mL saturated aqueous citric acid and
stirred overnight. The obtained orange precipitate of
the crude product 10 was filtered, washed with water
and dried in vacuo. After purification with column
1128

G. H. Dobrikov, G.i M. Dobrikov, M. Aleksandrova
chromatography (40 g silica gel; phase A: DCM, phase 55 spectrofluorimeter, respectively. The brightness of
B: Et2O:MeOH:Et3N = 50:1:1), the pure compound 10 the light emissions of the EL structures was measured
was obtained as orange-yellow crystals (0.57 g, 96% by an LS-110-Konica Minolta luminance meter. The
1
yield). M.p. 133-134ºC. H-NMR (CDCl3, 600.13 MHz): fluorescence quantum yield (QF) was determined
δ 8.57 (m, 2H, 8-H, 10-H), 8.41 (d, 1H, 4-H, J = 8.7 Hz), relative to that of 3-p-methoxyphenylmethylene-1(3H)
7.52 (dd, 1H, 9-H, J = 8.4, 7.5 Hz), 6.80 (d, 1H, 5-H, isobenzofuranone (QF = 0.12 in ethanol) [10]. Detailed
J = 8.7 Hz), 4.18 (m, 2H, 12-H), 3.78 (m, 4H, 18-H), information about quantum yields’ determination may be
3.63 (t, 2H, 17-H, J = 6.5 Hz), 2.10 (m, 4H, 19-H), found elsewhere [11]. The emission spectra in polymer
1.76 (qt, 2H, 13-H, J = 7.5 Hz), 1.59 (qt, 2H, 16-H, films were measured in reflection mode. The solvents
J=6.9Hz),1.41-1.49(m,4H,14-H,15-H).13C-NMR(CDCl3, used were of fluorescence grade.
150.92 MHz): δ 164.92 (s, 1C, 1-C), 164.14 (s, 1C, 2-C),
The poly(methyl methacrylate) (PMMA) films were
152.60 (s, 1C, 6-C), 133.40 (d, 1C, 4-C), 131.89 (d, 1C, prepared as follows: 300 mg PMMA was dissolved in
8-C), 131.14 (s, 1C, 7a-C), 131.03 (d, 1C, 10-C), 123.00 10-4 M dichloromethane solution of the investigated
(s and d, 2C, 7-C, 9-C), 122.52 (s, 1C, 11-C), 110.65 compound 5. The solution was poured in Petri dishes
(s, 1C, 3-C), 108.45 (d, 1C, 5-C), 62.68 (t, 1C, 17-C), and the solvent was evaporated in air for about
53.14 (t, 2C, 18-C), 39.81 (t, 1C, 12-C), 32.58 (t, 1C, 36 hours. The thickness of the resulting films was uniform
16-C), 27.99 (t, 1C, 13-C), 26.54* (t, 1C, 14-C or 15-C) and the concentration of 5 was about 0.5 – 0.7% weight.
26.06 (t, 2C, 19-C), 25.15* (t, 1C, 14-C or 15-C). Anal. The role of PMMA is to increase the adhesion of films on
calcd. for C22H26N2O3 (Mw = 366.45): C, 72.11; H, 7.15; the glass substrates and to promote linear orientation of
N, 7.64; O, 13.10. Found: C, 72.16; H, 7.11; N, 7.69. MS LMWC molecules. PMMA also decreases the threshold
(EI) m/z (rel. int.): 366.28 (M+●, 73), 349.25 (24), 279.22 voltage of the emitting layer.
(34), 266.18 (100), 223.14 (10).
For the preparation of electroluminescent structures,
Compounds 5 and 10 were carefully purified by theabovedescribedhomogeneousmixtureofcompound
column chromatography and were unambiguously 5 and PMMA in dichloromethane was deposited over
characterized by 1D and 2D nuclear magnetic cleaned glass substrates through a spin-coating method.
resonance (NMR) experiments, mass spectroscopy For deposition of the electrodes of EL structures, the
(MS), elemental analysis and melting points. Analytical vacuum installation A-400 VL was used. The aluminum
data show that both compounds possess satisfactory electrodes were deposited by vacuum evaporation and
purity (over 99.5%), permitting investigation of their the indium-tin oxide (ITO) transparent electrode was
fluorescent properties.
deposited by RF sputtering.
2.4.
Steady state UV-Vis absorbance,
fluorescence measurements and film’s
preparation
3. Results and Discussion
The new structures selected for synthesis are
representatives of two classes of fluorescent organic
compounds, namely 1H-imidazol-5(4H)-ones and 1,8-
naphthalimides. Their preparation allows the usage of
Absorbance spectra and corrected fluorescence spectra
were acquired separately on a Perkin Elmer Lambda
25 UV-VIS spectrophotometer and a Perkin Elmer LS
Scheme 2. Two-step synthesis of compound 10.
1129

Synthesis and electronic spectra of new low-molecular weight
compounds with possible application in electroluminescent layers
cheap and commercially available starting materials.
Both classes of compounds possess high chemical
stability in combination with appropriate fluorescent
properties and were a subject of our previous studies
[12,13].
The UV-Vis absorbance and fluorescence spectra
of compounds 5 and 10 in solution (CHCl3) at room
temperature are shown on Figs. 1 and 2 respectively.
In order to investigate the influence of medium
rigidity on the fluorescence quantum yields of 5 and
10, the compounds of interest were incorporated in
PMMA matrix at room temperature. The corresponding
absorbance and fluorescence spectra are shown on
Figs. 3 and 4.
Figure 1. Absorbance and fluorescence spectra of 5 in CHCl3.
An increase of the fluorescence intensity was
observed in PMMA films. The approximate values of the
fluorescence quantum yields of compounds 5 and 10
became 0.1 and 1, respectively (Table 1). The significant
increase of the fluorescent quantum yield of 5 in PMMA
film (33 fold) encouraged us in our choice to investigate
its possible electroluminescent (EL) properties in thin
films. Since 5 has a significantly higher melting point,
we presumed that it would be more resistant than
compound 10 to the expected extreme conditions
during the electroluminescent experiments (harsh rise
of the temperature, strong electric fields, etc.). We did
not observe visible decomposition or crystal structure
changes of 5 at temperatures lower than the melting
point. These results suggested the viability of compound
5 as a model structure for our electroluminescent
experiments.
Figure 2. Absorbance and fluorescence spectra of 10 in CHCl3.
The experimental EL structure was prepared with
an active EL film of compound 5 with mean thickness
300 nm. The film was deposited by a spin-coating
method on an ITO covered substrate at 3200 rpm
via addition of PMMA in solution for improving film
adhesion. A vacuum evaporated aluminum layer with
200 nm mean thickness was used as the top electrode.
The structure was supplied by 40 V AC with 1-3 KHz
frequency. Fig. 5 shows the scheme of the prepared
structure, while Fig. 6 show the obtained EL spectra.
The highest intensity of the emitted light was detected
at frequencies of the applied voltage between 1.5
and 2 KHz. The electroluminescence maximum of the
structure is in the visible region of the spectrum at 600-
630 nm. The intensity of the light emission depends on
the frequency of the applied voltage. The change of the
frequencies from 1 to 3 KHz leads to a bathochromic
shift of the maximum by about 30 nm and to a lowering
of the emission intensity, as well. The investigated
structures produce light emission with a brightness of
150 cd m-2 (at frequency 1.5-2 KHz). The measurements
were performed at permanent work of the structure for
Figure 3. Absorbance and fluorescence spectra of compound 5
deposited in PMMA film.
Figure 4. Absorbance and fluorescence spectra of compound 10
deposited in PMMA film.
1130

G. H. Dobrikov, G.i M. Dobrikov, M. Aleksandrova
Figure 5. Experimental electroluminescent structure and its layers.
Figure 6. Electroluminescent spectra of EL structure with compound 5 incorporated in light emitting layer at three different AC frequencies:
a) at 1 KHz; b) at 1.5-2 KHz; c) at 3 KHz.
Table 1. Absorbance and fluorescence maxima and fluorescence quantum yields of compounds 5 and 10 in CHCl3 solution and PMMA film.
Absorbance
maximum [nm]
Fluorescence maximum
[nm]
Fluorescence quantum
yield Qfl
Compound
In CHCl3 solution
5
465
445
525
505
0.003
0.510
10
In PMMA film
5
455
435
555
525
around 0.10
around 1.00
10
8 hours. The electroluminescent measurements were 6-9 KHz). After about 1 minute work, electrical break
repeated at every 30 minutes. The results showed that occurred, causing mechanical destruction of the glass
the parameters of the investigated structures remain substrate and visible decomposition of the organic layer.
stable during the experiment. It should be pointed out Only for a few seconds before the electrical break, a
that the maximum of electroluminescence of compound bright yellow electroluminescence was observed but its
5 is batochromically shifted in comparison to its duration was insufficient for measurement of EL spectra
fluorescent maximum in PMMA film by about 50 nm. or brightness of the light emission.
Based on this experimental result, it could be concluded
that the energy transitions taking place in the molecules layerdeposition,especiallyfindinganappropriatebinding
of compound 5 are not the same for both processes.
polymer (with different electrophysical parameters than
Experiments aimed at improving the technology of
In order to compare results for compound 5, PMMA), are currently in progress, and results will be the
an experimental EL structure was prepared with subject of further publications.
compound 10, as well, applying the technology
described above. Unfortunately, the experiment with
this structure was not successful. Despite its higher
quantum yield, the structure demonstrated instability
4. Conclusions
of its electroluminescent parameters after voltage was
supplied (the experimental established parameters
for causing of EL for this structure were 60 V AC and
Two new and stable low-molecular weight compounds
(Z)-4-(4-(dimethylamino)benzylidene)-1-(9-ethyl-9H-
carbazol-3-yl)-2-phenyl-1H-imidazol-5(4H)-one (5) and
1131

Synthesis and electronic spectra of new low-molecular weight
compounds with possible application in electroluminescent layers
2-(6-hydroxyhexyl)-6-(pyrrolidin-1-yl)-1H-benzo[de]
isoquinoline-1,3(2H)-dione (10) were synthesized
and their main photoluminescence properties were
investigated.ThepreparedexperimentalELstructurewith
compound 5 showed encouraging electroluminescent
properties (especially stability of the light emission for
a long period of time – 8 hours) when high frequency
AC voltage was supplied and could be applied as an
alternative to OLEDs with polymeric light emitting layers.
Compound 10 is of great importance for us because of
the demonstrated bright electroluminescence – even for
a short period of time – and the efforts for its successful
application in EL-layers will be continued.
Acknowledgements
This study was supported by contracts DO-1/377-3,
DOO2-358 with the Ministry of Education and Science
of Bulgaria.
References
[1]
[2]
[8]
[9]
M.M. Kravtsiv, A.P. Kovalskiy, O.I. Shpotyuk, Adv.
Optoelectron. Lasers 2, 194 (2003)
S.W. Kim, B.H. Hwang, J.H. Lee, J.I. Kang,
K.W. Min, W.Y. Kim, Curr. Appl. Phys. 2, 335
(2002)
I. Grabtchev, T. Konstantinov, S. Guittonneau,
P. Meallier, Dyes and Pigments 35, 361 (1997)
J. Gan, H. Tian, Z. Wang, K. Chen, J. Hill,
P.A. Lane, M.D. Rahn, A.M. Fox, D.D.C. Bradley, J.
Organometal. Chem. 645, 168 (2002)
[3]
[4]
[5]
[10]
[11]
[12]
J. Davenas, S. Besbes, H. ben Ouada, M. Majdoub,
Mater. Sci. Eng. C 21, 259 (2002)
S. Braun, W. Osikowicz, Y. Wang, W.R. Salaneck,
Org. Electron. 8, 14 (2007)
E.F. Schubert, T. Gessmann, J.K. Kim, In: K. Mullen,
U. Scherf (Eds.), Organic Light Emitting Devices.
Synthesis, Properties and Applications (WILEY-
VCH Verlag GmbH & Co. KGaA, Weinheim, 2006)
A.K. Mukerjee, Heterocycles 26, 1077 (1987)
V.O. Topuzyan, A.A. Oganesyan, G.A. Panosyan,
Russ. J. Org. Chem (Eng.) 40, 1644 (2004)
P. Nikolov, F. Fratev, St. Minchev, Z. Naturforsch.
A 38, 200 (1983)
A.T.R. Williams, S.A. Winfield, J.N. Miller, Analyst
108, 1067 (1983)
I. Petkova, G. Dobrikov, N. Banerji, G. Duvanel,
R. Perez, V. Dimitrov, P. Nikolov, E. Vauthey, J.
Phys. Chem. A 114, 10 (2010)
P. Angelova, N. Kuchukova, G. Dobrikov,
I. Timtcheva, K. Kostova, I. Petkova, E. Vauthey,
J. Mol. Struc. 993, 185 (2011)
[13]
[6]
[7]
1132