Synthesis of a new conjugated 2,2?-(1,4-phenylenedivinylene)bis-quinoxaline and a series of styryl derivatives of quinoxaline and quinoline as promising electro- and photoluminescent materials

Article abstract of DOI:10.1007/s10593-009-0229-3

A new poly(p-phenylenedivinylene) oligomer 2,2'-(1,4-phenylenedivinylene) bisquinoxaline, a promising candidate in organic electroluminescent devices, was synthesised via a Knoevenagel condensation reaction of 2-methylquinoxaline with 1,4-benzenedicarbald

Full text of DOI:10.1007/s10593-009-0229-3

Chemistry of Heterocyclic Compounds, Vol. 45, No. 1, 2009
SYNTHESIS OF A NEW CONJUGATED
2,2'-(1,4-PHENYLENEDIVINYLENE)BIS-
QUINOXALINE AND A SERIES OF STYRYL
DERIVATIVES OF QUINOXALINE AND
QUINOLINE AS PROMISING ELECTRO-
AND PHOTOLUMINESCENT MATERIALS
B. Bachowska1*, G. Matusiak1
A new poly(p-phenylenedivinylene) oligomer 2,2'-(1,4-phenylenedivinylene)bisquinoxaline, a promising
candidate in organic electroluminescent devices, was synthesised via a Knoevenagel condensation
reaction of 2-methylquinoxaline with 1,4-benzenedicarbaldehyde. The same synthetic strategy was
applied to obtain substituted styryl dyes, derivatives of quinoxaline, whereas styryl derivatives of
quinoline were prepared by "Anil Synthesis". The considered dyes are of interest as fluorescence
emitting materials.
Keywords: "Anil Synthesis", Knoevenagel condensation, luminophores, poly(p-phenylenedivinylene)
oligomer.
Organic compounds used as electroluminescent materials can be divided into two major classes, the first
one is based on low molecular weight compounds like fluorescent dyes and metal chelates, and the second one
involves polymeric materials [1]. The nitrogen/oxygen-containing π-electron deficient heterocycles such as
oxadiazoles, triazoles, triazines, pyridines, pyrimidines, quinolines, and quinoxalines are potential candidates for
ETL (electron-transporting layer) materials [2]. Polyquinoxalines (PQxs), for example, combine advantageous
features such as high electron affinity, good thermal stability, and good processability with sufficiently low
refractive indices [3]. Due to their chemical structure PQxs have been utilized as electron
n
PPV
_______
* To whom correspondence should be addressed; e-mail: b.bachowska@ajd.czest.pl.
1Institute of Chemistry and Environmental Protection, Jan Długosz University, Częstochowa 42-200, Poland.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 95-100, January, 2009. Original
article received April 02, 2007.
80
0009-3122/09/4501-0080©2009 Springer Science+Business Media, Inc.

transporting materials in OLEDs (organic light emitting diodes) [4, 5]. Another fundamental work concerning
the evolution of OLEDs was published by Friend et al. [6]. They used a highly fluorescent conjugated polymer:
poly(p-phenylenevinylene) (PPV) as the active material in a single-layer OLED.
Although PPV itself is insoluble and difficult to process, Friend et al. [6] found a way to build up
PPV-OLEDs via the thermoconversion of a processable precursor polymer. Today growing attention is paid to
modification of conjugated molecules by changing their central cores in order to investigate the structural
influence on the LED properties [7]. In this context Liang and coworkers have tested PPV oligomers containing
8-substituted quinolines [8, 9].
In recent years, the technical importance of stilbenyl and styryl heterocycles has rapidly increased as a
result of the wide application of products, especially those serving as fluorescent whitening agents [10, 11].
Styrylpyridinium salts, for example, have interesting physical properties enabling their use in nonlinear
optics and in physiology/biochemistry areas [12], whereas styrylquinolinium methyl sulfate is useful in
photography as an optical sensitizer [13].
In the present work we have performed a one-pot synthesis of a new PPV oligomer: 2,2'-(1,4-
phenylenedivinylene)bisquinoxaline (1) from 2-methylquinoxaline (2) serving as a starting material. The reason
for using quinoxaline as a building block is that polyquinoxalines are excellent electroluminescent and electron-
transporting materials [4, 5]. We have also examined the synthesis and properties of styryl derivatives of
quinoxaline and quinoline.
It is well known that the methyl group in position 2 of the quinoline and quinoxaline ring can be
activated by the presence of a nitrogen atom, and then it undergoes condensation with an aldehyde catalyzed by
an acid or base [14, 15]; in our experiments we chose the acid catalyst. The reaction of 2-methylquinoxaline (2)
with 1,4-benzenedicarbaldehyde (3) in acetic anhydride affords the condensation product 1.
N
8
Ac2O
2
OHC
CHO
+
N
125°C, N2
N
Me
3'
5
N
N
3
2
3
5'
N
1
8'
for 2
route a
N
N
R
8
OHC
Ac2O
OMe
+
N
2'
3'
Ac2O
125°C, N2
Me
for 7
5
2, 7
8
N
OMe
3
4
125°C, N2
route b
8
8
N
2'
3'
5
N
OMe
N
+
2'
3'
5
N
OMe
2'
Me
3'
5
OMe
6
2 R = H, 7 R = Me
81

1
1
Compound 1 was fully characterized by H NMR spectroscopy and elemental analysis. In the H NMR
spectrum it exhibits two doublets at 6.97 and 7.11 ppm with a three-bond coupling constant 3JH,H = 16.4 Hz arising
from trans carbon−carbon double bonds, indicating the presence of trans-configuration in the vinylene units.
For the synthesis of styrylquinoxalines 4–6 the same synthetic strategy (Knoevenagel condensation) as
for PPV oligomer was used. The styrylquinoxalines 4–6 were prepared by condensation of 2-methyl-quinoxaline
2 (route a) and 2,3-dimethylquinoxaline (7) (route b) with 4-methoxybenzaldehyde (8).
1
The structures of resulting compounds were confirmed by H NMR spectroscopy and elemental
analysis. The spin-spin coupling constants for the olefinic protons of compounds 4–6 (15.6–16.3 Hz) indicate
the presence of trans-double bond.
The variously substituted N,N-dimethylaminostyrylquinoline derivatives 9 and 10 were obtained by
"Anil Synthesis" [16]. In this procedure 2,4,6-trimethylquinoline (11) (prepared by the reaction of p-toluidine
with acetylacetone) and Schiff’s base 12 in the ratio 1 equivalent per one reacting methyl group of 11 were
stirred at room temperature for 1.5 h.
Products 9 and 10 were isolated by separation of the reaction mixture by column and TLC preparative
chromatography. Their structures were confirmed by 1H NMR and elemental analysis results.
All considered products exhibited intense luminescence in CHCl3 and DMF solution under UV
excitation (365 nm). The color of luminescence is pale blue for compounds 1, 4–6 and yellow-green for
compounds 9 and 10. This strong fluorescence suggests that these materials are suitable candidates for
fabricating a light-emitting devices. Further research on their emissive applications is in progress.
Me
Me
N
CH
NMe2
3
+
N
Me
12
11
DMF, r. t.
t
KOBu-
8
N
Me
8
Me
N
Me
5
2'
3'
3
5
+
NMe2
3
10
Me
9
2'
3'
Me2N
EXPERIMENTAL
Melting points were determined on a Boetius apparatus. The TLC was performed on 60F 254 silica gel
1
precoated DC aluminium sheets (Merck). H NMR spectra were recorded on a 200 MHz Bruker spectrometer
using TMS as an internal standard. Mass spectra were measured on a AMD-604 instrument at 70 eV. The results
of elemental analyses agreed favorably with the calculated data.
2,2'-(1,4-Phenylenedivinylene)bisquinoxaline (1). A mixture of 2-methylquinoxaline 2 (1.44 g,
10 mmol), 1,4-benzenedicarbaldehyde (3) (0.67 g, 5 mmol), and acetic anhydride (10 ml) was stirred and heated
82

at 125°C for 40 h under N2. The reaction mixture was then poured into ice water (ca 100 ml) and stirred
overnight. The dark-brown solid obtained was filtered off, washed with water, and dried to give compound 1
(1.44 g, 75%) as an orange powder from DMF. Mp 268–270°C. 1H NMR (DMSO-d6), δ, ppm (J, Hz): 9.25 (2H,
s, H-3,3'); 8.11 (2H, d, J = 8.6, H-5, 5'); 7.93–7.75 (6H, m, H-6, 6', 7,7', 8,8'); 7.60 (4H, s, H Ar); 7.11 (d,
J = 16.4, 2H trans-olefinic); 6.97 (d, J = 16.4, 2H trans-olefinic). Mass spectrum, m/z (I, %): 386 [M+] (100),
257 [M+−C8H5N2] (25). Found, %: C 80.64; H 4.74; N 14.31. C26H18N4. Calculated, %: C 80.81; H 4.69;
N 14.49.
2-(p-Methoxystyryl)quinoxaline (4). A mixture of 2-methylquinoxaline 2 (1.44 g, 10 mmol),
p-methoxybenzaldehyde 8 (1.36 g, 10 mmol), and acetic anhydride (15 ml) was stirred and heated at 125°C for
40 h under N2. It was poured into ice water (100 ml) and stirred for 2 h. After extraction with dichloromethane
(3×30 ml) the organic layer was washed with water and dried over anhydrous Na2SO4. The solvent was then
evaporated and the residue was purified by column chromatography using hexane−ethyl acetate (5:4) as an
1
eluent to give compound 4 (1.52 g, 58%) as bright-red needles. Mp 118–119°C. H NMR (CDCl3), δ, ppm (J,
Hz): 9.03 (s, H-3); 8.15 (d, J = 8.6, H-5); 7.92–7.77 (3H, m, H-6,7,8); 7.45 (d, J = 8.8, 2H-2'); 7.13 (d, J = 16.3,
1H trans-olefinic); 6.98 (d, J = 16.3, 1H trans-olefinic); 6.91 (d, J = 8.8, 2H-3'); 3.82 (s, OCH3). Mass spectrum,
m/z (I, %): 262 [M+] (86.5), 231 [M+−OCH3] (18). Found, %: C 77.59; H 5.43; N 10.41. C17H14N2O. Calculated,
%: C 77.84; H 5.38; N 10.68.
2,3-Di(p-methoxystyryl)quinoxaline (5) and 2-(p-methoxystyryl)-3-methylquinoxaline (6). A mixture of
compound 7 (1.58 g, 10 mmol), compound 8 (2.72 g, 20 mmol), and acetic anhydride (20 ml) was stirred and heated
at 125°C for 40 h under N2. It was poured into ice water (100 ml) and stirred overnight. After extraction with
dichloromethane (3×30 ml) the organic layer was washed with water and dried over anhydrous Na2SO4. Evaporation
of the solvent left an oil, which was separated by column chromatography using dichloromethane as an eluent to give
compound 5 (1.3 g, 31.4%) as bright-orange crystals (from trichloromethane−ethanol, 1:1) and compound 6 (0.95 g,
34.4%) as yellow plates (from trichloromethane−ethanol, 1:1).
Compound 5. Mp 130°C. 1H NMR (CDCl3), δ, ppm (J, Hz): 7.87 (2H, d, J = 8.6, H-5, 8); 7.72 (2H, dd,
J = 8.6, J = 6.9, H-6,7); 7.28 (d, J = 8.6, 4H-2'); 7.10 (d, J = 16.0, 2H trans-olefinic); 7.05 (d, J = 16.0, 2H
trans-olefinic); 6.93 (d, J = 8.6, 4H-3'); 3.85 (s, 2OCH3). Mass spectrum, m/z (I, %): 153
[M+−2C6H4OCH3−C2H3] (100). Found, %: C 78.95; H 5.71; N 7.04. C26H22N2O2. Calculated, %: C 79.16; H 5.62;
N 7.10.
1
Compound 6. Mp 138°C. HNMR (CDCl3), δ, ppm (J, Hz): 7.87-7.77 (3H, m, H-5,7,8); 7.56 (dd,
J = 8.6, J = 6.8, H-6); 7.43 (d, J = 8.6, 2H-2'); 7.09 (d, J =15.6, 1H trans-olefinic); 6.95–6.90 (3H, m, 2H-3' and
1H olefinic); 3.83 (s, OCH3); 2.65 (s, CH3). Mass spectrum, m/z (I, %): 276 [M]+ (7), 277 [M++1] (100). Found,
%: C 78.03; H 5.97; N 10.03. C18H16N2O. Calculated, %: C 78.23; H 5.83; N 10.13.
4,6-Dimethyl-2-(p-N,N-dimethylaminostyryl)quinoline (9) and 2,6-Dimethyl-4-(p-N,N-dimethyl-
aminostyryl)quinoline (10). A mixture of 2,4,6-trimethylquinoline 11 (1.71 g, 10 mmol) and (p-N,N-dimethyl-
aminobenzylidene)aniline 12 (6.72 g, 30 mmol) in anhydrous DMF (70 ml) was stirred while potassium
tert-butoxide (3.36 g, 30 mmol) was slowly added. After addition, the color of the mixture was changed from
yellow to brown and tthe emperature increased to 30°C. After 1.5-h stirring followed by cooling the reaction
mixture was poured into water and acidified with 10% HCl. The orange precipitate was filtered off, dried, and
separated by column chromatography using dichloromethane−acetone (5:1) as an eluent. The main fraction of
noncrystalline material, 1.98 g showing two spots in TLC, was separated by preparative TLC chromatography
using dichloromethane−methanol (9:1) as an eluent. Two products were obtained: 9 (1.12 g, 37,3%) as red
crystals (benzene) and 10 (0.60 g, 20%) as violet crystals (ethanol).
Compound 9. Mp 158–160°C. 1H NMR (CDCl3), δ, ppm (J, Hz): 7.82 (d, J = 15.8, 1H trans-olefinic);
7.74 (s, H-5); 7.51 (d, J = 8.5, H-8); 7.41 (s, H-3); 7.25 (d, J = 8.5, H-7); 7.10–7.03 (3H, m, 2H-2', and 1H
olefinic); 6.60 (d, J = 8.7, 2H-3'); 2.90 (s, N(CH3)2); 2.73 (s, CH3); 2.35 (s, CH3). Mass spectrum, m/z (I, %): 303
[M++1] (22.6). Found, %: C 83.17; H 7.49; N 9.09. C21H22N2. Calculated, %: C 83.40; H 7.33; N 9.26.
83

1
Compound 10. Mp 209–210°C. H NMR (CDCl3), δ, ppm (J, Hz): 7.76 (s, H-5); 7.55 (d, J = 15.6, 1H
trans-olefinic); 7.48 (d, J = 8.5, H-8); 7.30 (s, H-3); 7.23 (d, J = 8,45, H-7); 7.18 (d, J = 15.6, 1H trans-olefinic); 7.03
(d, J = 8.5, 2H-2'); 6.74 (d, J = 8.5, 2H-3'); 2.99 (s, N(CH3)2); 2.67 (s, CH3); 2.42 (s, CH3). Mass spectrum, m/z (I,
%): 302 [M+] (12), 303 [M++1] (100), 172 [M+ – C6H4N(CH3)2] (15). Found, %: C 83.37; H 7.21; N 9.05. C21H22N2.
Calculated, %: C 83.40; H 7.33; N 9.26.
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