Spectral emission properties of 4-aryloxy-3-methyl-1-phenyl-1H-pyrazolo[3,4-b]quinolines

Article abstract of DOI:10.1016/j.saa.2009.02.026

Spectral emission properties of novel 4-aryloxy-1H-pyrazolo[3,4-b]quinolines were investigated. All of the compounds exhibit strong blue luminescence in the solution and in the solid state as well. Pyrazoloquinolines were used as dopants in PVK matrices i

Full text of DOI:10.1016/j.saa.2009.02.026

Spectrochimica Acta Part A 73 (2009) 281–285
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectral emission properties of
4-aryloxy-3-methyl-1-phenyl-1H-pyrazolo[3,4-b]quinolines
M. Pokladkoa, E. Gondeka, J. Sanetraa, J. Nizioła, A. Danelb,c, I.V. Kitykd,∗, Ali H. Reshake
a Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, al.Mickiewicza 30, 30-059 Kraków, Poland
b Department of Chemistry, The H.Kołła˛taj University of Agriculture, ul.Balicka 122, 30-141 Krakow, Poland
c Institute of Physics, Cracow University of Technology, ul.Podchora˛z˙ ych 1, 30-084 Krakow, Poland
d Electrical Engineering Department, Cze˛stochowa Technological University, Al. Armii Krajowej 17/19, Cze˛stochowa, Poland
e Institute of Physical Biology, South Bohemia University, Institute of System Biology and Ecology, Academy of Sciences, Nove Hrady 37333, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Spectral emission properties of novel 4-aryloxy-1H-pyrazolo[3,4-b]quinolines were investigated. All of
the compounds exhibit strong blue luminescence in the solution and in the solid state as well. Pyrazolo-
quinolines were used as dopants in PVK matrices in electroluminescent devices with ITO/PVK:PQ/Ca/Al
light emitting diode configuration.
Received 1 November 2008
Received in revised form 16 February 2009
Accepted 18 February 2009
© 2009 Elsevier B.V. All rights reserved.
Keywords:
4-Aryloxy-1H-pyrazolo[3,4-b]quinolines
Blue electroluminescence
OLED
PVK
1. Introduction
1H-pyrazolo[3,4-b]quinolines PAQs are intense fluorescent
compounds. The first PAQ was synthesized by Tomasik in 1928
Since the pioneering works of Pope et al. in the 1960s, enormous
progress has occurred as far as the application of simple organic
molecules in organic electroluminescent devices is concerned [1].
At this time the drive voltage was in the order of 100 V or above
to achieve a significant light output so that these devices based
on organic crystals (e.g. anthracene) could not compete with their
inorganic counterparts. The real breakthrough occurred in 1987
when Tang fabricated double-layer EL cell with arylamine as hole-
transporting layer and AlQ3 as emitting and electron-transporting
layer [2]. The driving voltage was below 10 V and the maximally
achieved brightness was about Cd/m2. This compound is a typical
example of heterocyclic metal chelate and most of all long-lived
devices include it as the ETL.
[13]. In 1962 Wolfrum published a paper on the application of
pyrazoloquinolines as optical brighteners [14]. The same proper-
ties were underlined in Brack’s paper and its patent application
[15]. The first systematic investigation of photophysical properties
of PAQs were done by Rotkiewicz et al. [16]. Pyrazoloquinolines are
blue emitting fluorophores. They emit predominantly in blue and
blue-green area of the spectra. The introduction of electro donat-
ing or electro accepting groups in carbocyclic ring of the quinoline
cause hypso- or bathochromic shift (ca. 10–15 nm) [17]. In the case
of strong electrodonating group, like NAr2 or NR2 a strong red
shift can be observed (ca. 80 nm) and green luminophores can be
prepared. Pyrazoloquinolines were applied as crown-ether based
sensors too [18]. The PAQs were applied in organic electrolumines-
cent devices in the form of vacuum evaporated films or dopants in
PVK (poly-9-N-vinylcarbazole) or PMPS (polyphenylmethyl silane)
matrices.
The other heterocyclic systems include derivatives of pyridine
[3], benzoxazole [4], oxadiazole [5], thiophene [6], carbazole [7],
quinoxaline [8] or triazole derivatives [9].
One of the most widely used materials in OLEDs is AlQ3 and
its modifications. Besides it quinoline oligomers [10] or polymers
[11] are used as thermally stable electron transporting and emissive
materials. Pyrazole in the form of pyrazoline derivatives are used
as hole-transporting materials as simple molecule or incorporated
into polymer backbone [12].
2. Experimental
2.1. Material synthesis
PVK was purchased from Aldrich Chemicals. The employed
organic compounds and their abbreviations are shown in Fig. 1.
The synthesis of the luminophors is described in Scheme 1.
Anthranilic 1 acid was converted by reaction with diketene
dimer into 2-acetonylo-3,1-benzoxazinone derivative 2. Reaction
∗
Corresponding author. Tel.: +48 601 504268.
E-mail address: Iwan.Kityk@polsl.pl (I.V. Kityk).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.02.026

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M. Pokladko et al. / Spectrochimica Acta Part A 73 (2009) 281–285
The reaction mixture was diluted with water; the precipitate was
filtered off, dried and crystallized.
2.1.2. 4-(2-tert-Butylphenoxy)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]quinoline
6a
Light yellow needless, 51%, mp. 172–3 ◦C (toluene).
1H NMR (300 MHz, CDCl3, ı ppm): 1.68 (s, 9H, tert-Bu); 2.46 (s,
3H, 3-Me); 6.31 (d, J = 7.9 Hz, 1H); 6.96 (t, J = 7.6 Hz, 1H); 7.04 (t,
J = 7.6 Hz, 1H); 7.28 (t, J = 7.4 Hz, 1H); 7.33 (t, J = 7.5, 1H, 6-H); 7.51
(d, J = 7.8, 1H); 7.54 (t, J = 7.9 Hz, 3,5-H1Ph); 7.72 (t, J = 7.2 Hz, 1H, 7-
H); 7.99 (d, J = 8.4 Hz, 1H, 5-H); 8.17 (d, J = 8.7 Hz, 1H, 8-H); 8.48 (d,
J = 7.9 Hz, 2H, 2,6-H1Ph).
13C NMR (300 MHz, CDCl3, ı ppm): 14.89; 30.13; 35.19; 111.08;
114.8; 118.61; 120.44; 122.86; 123.07; 123.39; 123.89; 125.12;
127.51; 127.74; 129.03; 129.05; 130.89; 137.25; 139.84; 142.14;
150.89; 152.59; 153.27; 157.91.
Anal.cald for C27H25N3O: C 79.58; H 6.18; N 10.31. Found C 79.51;
H 6.12; N 10.03.
2.1.3. 3-Methyl-4-(naphtalen-1-yloxy)-1-phenyl-1H-
pyrazolo[3,4-b]quinoline
6b
Light yellow crystals, 55%, mp. 167 ◦C.
1H NMR (300 MHz, CDCl3, ı ppm): 2.36 (s, 3H, Me); 6.32 (d,
J = 7.7 Hz, 1 H); 7.16 (t, J = 7.9 Hz, 1 H); 7.24–7.33 (m, 2H); 7.55 (t,
J = 8.1 Hz, 3H); 7.65 (t, J = 7.3 Hz, 1H); 7.69 (t, J = 7.4 Hz, 1H); 7.74 (t,
J = 7.6 Hz, 1H); 7.94 (d, J = 8.1 Hz, 1H); 8.04 (d, J = 8.5 Hz, 1H, 5-H);
8.19 (d, J = 8.7 Hz, 1H, 8-H); 8.50 (d, J = 8.0 Hz, 2H, 2,6-H1Ph); 8.67 (d,
J = 8.15 Hz, 1H).
13C NMR (300 MHz, CDCl3, ı ppm): 14.50; 108.33; 111.11; 118.85;
120.33; 121.52; 122.70; 122.88; 124.12; 124.69; 125.12; 125.61;
126.47; 127.87; 128.99; 129.04; 131.12; 134.84; 139.85; 142.23;
150.76; 152.64; 153.67; 155.27.
Anal. calcd. for C27H19 N3O: C 80.78; H 4.77; N 10.46. Found C
80.56; H 4.34; N 10.32.
2.1.4. 4-(Biphenyl-4-yloxy)-3-methyl-1-phenyl-1H-pyrazolo[3,4-
b]quinoline
6c
Yellow needless, 49%, mp. 171–2 ◦C (toluene).
1H NMR (300 MHz, CDCl3, ı ppm): 2.40 (s, 3H, Me); 6.95
(d, J = 8.6 Hz, 2H); 7.23–7.42 (m, 5H); 7.50–7.55 (m, 6H); 7.75 (t,
J = 7.6 Hz, 1H, 7-H); 8.10 (d, J = 8.4 Hz, 1H, 5-H); 8.17 (d, J = 8.7 Hz,
1H, 8-H); 8.47 (d, J = 8.1 Hz, 2H, 2,6-H1Ph).
Fig. 1. Chemical formula of Mol. 1–5.
13C NMR (300 MHz, CDCl3, ı ppm): 14.84; 110.95; 115.83; 118.83;
120.33; 122.99; 124.05; 125.10; 126.77; 127.13; 128.64; 128.77;
128.94; 129.00; 131.06; 136.06; 139.55; 139.80; 140.05; 142.11;
150.72; 152.58; 153.25; 158.71.
of 2 with phenylhydrazine gave 2-(5-methyl-2-phenyl-2H-pyrazol-
3-ylamino)-benzoic acid 3. This acid can be converted to
4-hydroxyderivative 4 by heating it in polyphosphoric acid PPA
or immediately to 4-chloro-3-methyl-1-phenyl-1H-pyrazolo[3,4-
b]quinoline 5 by heating with POCl3. Both 4 and 5 are the
source of many fluorescent and biologically active substances.
The synthetic details of products 2,3,4 and 5 are described
in literature [14]. 4-Chloro-3-methyl-1-phenyl-1H-pyrazolo[3,4-
b]quinoline was transformed into aryloxy derivatives by heating
with hydroxy aromatic compounds ArOH in EtOH/KOH solution.
1H NMR spectra were recorded on Mercury-Vx 300 MHz Varian.
Melting points were measured on Melt-temp apparatus.
Chemicals were purchased from commercial suppliers (Merck
or Aldrich). Solvents were purchased from POCh (Polish Chemical
Company).
Anal. calcd. for C29H21N3O: C 81.48; H 4.95; N 9.83. Found C
81.27; H 4.67; N 9.74.
2.1.5. 4-(Biphenyl-2-yloxy)-3-methyl-1-phenyl-1H-pyrazolo[3,4-
b]quinoline
6d
Light yellow crystals, 56%, mp. 185–7 ◦C (toluene).
1H NMR (300 MHz, CDCl3, ı ppm): 2.36 (s, 3H, Me); 6.45 (d,
J = 8.1 Hz, 1H); 7.11 (t, J = 7.7 Hz, 1H); 7.17 (t, J = 7.4 Hz, 1H); 7.24–7.28
(m, 1H); 7.33–7.41 (m, 2H); 7.47–7.54 (m, 5H); 7.73 (t, J = 7.44 Hz,
1H); 7.79 (d, J = 7.50 Hz, 2H); 8.07 (d, J = 8.4 Hz, 1H, 5-H); 8.14 (d,
J = 8.7 Hz, 8-H, 1H); 8.43 (d, J = 8.0 Hz, 2H, 2,6-H1Ph).
13C NMR (300 MHz, CDCl3, ı ppm): 14.57; 110.61; 114.79; 118.78;
120.37; 123.01; 123.35; 123.98; 125.08; 127.55; 128.31; 128.90;
128.98; 129.45; 130.76; 130.98; 131.51; 137.41; 139.76; 142.02;
150.62; 152.57; 153.59; 156.00.
2.1.1. Pyrazolo[3,4-b]quinoline 6a–e: general procedure
Pyrazoloquinoline 5 (0.01 mol) and appropriate ArOH deriva-
tive (0.015 mol) were boiled in EtOH (50 mL) with KOH (0.015 mol).

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283
Scheme 1. (a) Diketene dimer/CCl4; (b) phenylhdrazine/CH3COOH; (c) PPA, 100 ◦C; (d) POCl3; and (e) EtOH, ArOH, KOH, PQ-Cl.
Anal. calcd. for C29H21N3O: C 81.48; H 4.95; N 9.83. Found C
81.36; H. 4.76; N 9.76.
ITO/PVK:PQ/Ca/Al. This one correlates with the lower first absorp-
tion maximum presented in Fig. 2.
The EL spectra show that the Mol. 1 has an increase of the bright-
ness at the lowest voltages. So this chromophore is promising with
respect to other compounds. At the same time chromophore Mol.
4 has the lowest brightness for the same voltages. So one can say
about the correlation between the absorption strength of the first
absoprtion maxima and the efficiencies of the EL (Figs. 4 and 5,
Table 1).
Comparing the spectral features of the EL and PL one can see
that for the EL the spectral positions of the principal maxima are
not so dispersed as for the case of the PL. In every case one can
see that the brightness is very sensitive to the spectral positions
2.1.6. 3-Methyl-1-phenyl-4-(pyridin-3-yloxy)-1H-pyrazolo[3,4-
b]quinoline
6e
Light yellow crystals, 42%, mp. 162–3 ◦C (toluene).
1H NMR (300 MHz, CDCl3, ı ppm): 2.38 (s, 3H, Me); 7.04 (d,
J = 8.5 Hz, 1H); 7.21 (dd, J = 8.8 Hz, 1H); 7.29 (t, J = 7.4 Hz, 1H, 4-H1Ph);
7.41 (t, J = 7.6 Hz, 1H, 6-H); 7.55 (t, J = 7.5 Hz, 2H, 3.5-H1Ph); 7.78 (t,
J = 7.7 Hz, 1H, 7-H); 8.02 (d, J = 8.6 Hz, 1H, 5-H); 8.19 (d, J = 8.7 Hz, 1H,
8-H); 8.38 (d, J = 4.7 Hz, 1H); 8.45 (d, J = 8.6 Hz, 2H, 2,6-H1Ph); 8.48
(d, J = 2.8 Hz, 1H).
13C NMR (300 MHz, CDCl3, ı ppm): 14.78; 110.56; 118.24;
120.36; 121.94; 122.44; 124.28; 124.28; 124.43; 125.25; 129.02;
129.12; 131.19.138.68; 139.67; 141.48; 144.33; 150.69; 152.06;
152.43; 155.49.
Anal. calcd. for C22H16 N4O: C 74.99; H 4.58; N 15.89. Found C
74.78; H 4.41; N 15.72.
3. Results and discussion
The absorption spectra presented in Fig. 2 are typical for pyra-
zoloquinolines [19,20]. They possess two principal bands—the first
one at 390 nm, and the second one at 275 nm. The principal
differences occur in relative absorption strengths on the longer
wavelength bands with respect to the second higher energy band.
Particular presents Mol. 4, which has substantially stronger absorp-
tion compared to another one. Moreover, for the Mol. 4 we have long
wavelength spectral shift of the second band up to 300 nm.
From Fig. 3 one can see that the PL for the molecule is red
shifteds with respect to the other ones. This one correlates with
the maximally achieved brightness of the LED in the geometry
Fig. 2. Typical absorption spectra for the investigated chromophore in THF solvant.

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Table 1
Principal parameters for the investigated chromospheres.
Molecule
ꢀELmax (nm)
ꢀPLmax (nm)
Mol. 1
Mol. 2
Mol. 3
Mol. 4
Mol. 5
457
447
454
446
449
455
455
456
455
456
Fig. 3. PL spectra of the investigated chromophore.
Fig. 6. Spectral features of the investigated dye chromophores.
It is particularly very interesting due to substantial difference of
the first absorption maxima of the Mol. 4 with respect to another
molecule. One of the possible explanations of such behaviour may
be caused by existence of several molecular conformations forming
the relatively broad spectral emission features.
4. Conclusions
4-Aryloxy-1H-pyrazolo[3,4-b]quinolines synthesized from 4-
chloro-1H-pyrazolo[3,4-b]quinolines by nucleophilic substitution
with potassium arylates were used as dye chromophores for the
LED in single layered ITO/PVK:PQ/Ca/Al configuration. The LED has
demonstrated maximal values of the brightness about 50 Cd/cm2
in the spectral range 450–480 nm. Comparing the spectral features
of the EL and PL one can see that for the EL the spectral positions of
the principal maxima are not so dispersed as for the case of the PL.
In every case one can see that the brightness is very sensitive to the
spectral positions of the corresponding spectral bands. More strik-
ing is fact that first EL spectral maximum for the Mol. 1 (457 nm)
exists at higher wavelengths than for the PL (455 nm). For all the
other chromophores, these spectral shifts show opposite behav-
iors. It is particularly very interesting due to substantial difference
of the first absorption maxima of the Mol. 4 with respect to another
molecule.
Fig. 4. Brightness features versus the applied voltages for different dye chro-
mophores incorporated into the ITO/Mol N + PVK/Ca/Al LED architecture.
of the corresponding spectral bands. More striking is the fact that
first EL spectral maximum for the Mol. 1 (457 nm) exists at higher
wavelengths than for the PL (455 nm) (see Figs. 4 and 6). For all the
other chromophores, these spectral shifts show opposite behaviors.
Acknowledgment
This work was supported by grant 66/N/Singapore/2007/0.
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