Novel dipyrazolopyridine derivatives as deep blue emitters for polymer based organic light emitting diodes

Article abstract of DOI:10.1016/j.saa.2012.04.058

Paper reports synthesis and spectroscopic properties of four newly synthesized dipyrazolo[3,4-b; 3′,4′-e]pyridine (DPP) derivatives. The spectroscopic studies are supplemented by quantum-chemical calculations using DFT/TDDFT/PCM method at the B3LYP/6-31+G

Full text of DOI:10.1016/j.saa.2012.04.058

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 610–613
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
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Short Communication
Novel dipyrazolopyridine derivatives as deep blue emitters for polymer
based organic light emitting diodes
E. Gondek a, J. Nizioł b, A. Danel c, B. Jarosz c, P. Ga˛siorski d, A.V. Kityk d,
⇑
a Institute of Physics, Technical University of Kraków, Podchoraßzych 1, 30-084 Kraków, Poland
b AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Reymonta 19, 30-059 Kraków, Poland
c University of Agriculture, Balicka 122, 30-149 Kraków, Poland
d Faculty of Electrical Engineering, Czeßstochowa University of Technology, Al. Armii Krajowej 17, 42-200 Czeßstochowa, Poland
a r t i c l e i n f o
a b s t r a c t
Paper reports synthesis and spectroscopic properties of four newly synthesized dipyrazolo[3,4-b; 30,40-
e]pyridine (DPP) derivatives. The spectroscopic studies are supplemented by quantum-chemical calcula-
tions using DFT/TDDFT/PCM method at the B3LYP/6-31+G(d,p) level of theory. The optical absorption and
fluorescence emission processes appear to be weakly dependent on attached side phenyl and/or methyl
groups exhibiting in cyclohexane solution the first absorption band (000 transition) in the region of 386–
401 nm and the fluorescence band (000 transition) in the range of 412–425 nm. The electroluminescence
devices (OLEDs) with an active PVK layer doped by DPP dyes have been designed. All the devices exhibit
deep blue electroluminescence with the emission maximum being rather weakly dependent on the type
of the fluorescent dopant. The obtained results demonstrate that a series of newly synthesized DPP dyes
may be considered as perspective blue fluorescent emitters for electroluminescent applications.
Ó 2012 Elsevier B.V. All rights reserved.
Article history:
Received 13 February 2012
Received in revised form 31 March 2012
Accepted 10 April 2012
Available online 23 April 2012
Keywords:
Dipyrazolopyridine dyes
Optical absorption
Fluorescence
Electroluminescence
DFT/TDDFT/PCM calculations
Introduction
of the fabricated devices. This drawback was eliminated by further
modifications of PQ derivatives [9–11]. Relevant molecules incor-
Heterocyclic dyes have attracted a considerable interest in the
past decade due to a number of optoelectronic applications among
which organic light emitting diodes and electroluminescent dis-
plays (OLEDs) may be considered as dominant. Relevant technolo-
gies have already been transferred from laboratories to the
industry [1,2] and due to obvious advantages OLEDs over the tradi-
tional LCD/LED displays they are expected to become competitive
on the world markets in nearest perspective. Despite spectacular
commercial success of OLEDs, there is still an aspiration to improve
their electroluminescent characteristics such as quantum effi-
ciency, color purity and stability as being required by full-color dis-
play technology [1,3,4]. A key element in OLED structure is a dye
doped polymer layer. Among variety of potentially suitable fluores-
cent molecules, a special attention were paid to pyrazoloqinoline
(PQ) dyes. The core molecule was synthesized long time ago and
since then a lot of its derivatives have been studied in different
applications [5]. At the beginning of the last decade due to remark-
able fluorescence found for a number of PQ derivatives [6] these
heterocyclic organic dyes have been successfully employed in
many OLED structures [7,8]. However, their crystallization inside
the active layer decreased significantly the efficiency and lifetime
porated in OLED architecture featured in efficient electrolumines-
cence, preferably in blue, blue-green or green spectral regions
[12–15].
In the current work we further developed the concept started in
PQs and proposed four new dipyrazolopyridine (DPP) chromoph-
ores. Several other DPP derivatives have been originally synthe-
sized by Danel and coworkers [16]. The core of newly
synthesized dyes represents dipyrazolo[3,4-b; 30,40-e]pyridine mol-
ecule consisting of pyridine substituted with two pyrazole rings on
both sides opposite to the hetereoatom as shown in the scheme
below.
The structures of that type should provide a better and more
symmetrical charge distribution than in the case of PQs. Moreover,
a rule-of-thumb, established during developing the PQ chromoph-
ores, predicts that the presence of aryl moieties attached to PQ
rings prevents the molecule from unwanted crystallization. This
concept is also applied here to DPP derivatives. As one may see
in the scheme above, the studied molecules, abbreviated as DPP1
through DPP4, were built up of DPP core surrounded by aryl and/
or methyl ligands in different geometries. Particular arrangement
of aryl and methyl groups attached to pyrazole does not vary sig-
nificantly the molecule’s overall dipole moment [17], which re-
mains rather small, ranging from c.a. 2.0 to 2.5 D. We expected,
that this property would enable solubility of DPP derivatives in
⇑
Corresponding author. Tel.: +48 34 3250831.
E-mail addresses: kityk@ap.univie.ac.at, andriy.kityk@univie.ac.at (A.V. Kityk).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.058

E. Gondek et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 610–613
611
the case of less polar solvents, what would be a benefit from prac-
tical point of view.
In this brief report we give the optical spectroscopy character-
ization of DPP dyes and demonstrate their applicability in polymer
based electroluminescent devices as dopant emitters in deep blue
region of the visible spectra. The optical absorption, fluorescence
and electroluminescence spectra are supplemented by the quan-
tum-chemical analysis based on DFT/TDDFT/PCM calculations.
Synthesis
4-tert-Butylphenyl-3,5-dimethyl-1,7-diphenyl-bis-pyrazolo[3,4-b;
30,40-e]pyridine (DPP1)
A
mixture of 5-amino-3-methyl-1-phenylpyrazole (3.4 g;
0.02 mol), p-tert-butylbenzaldehyde (1.62 g; 0.01 mol) and 10 ml
of sulfolane were heated together in a round bottomed flask
(50 ml) equipped with air condenser at 140–150 °C for 45 min
and at 240–250 °C for 2 h. After cooling the reaction mixture was
diluted with ethanol (25 ml) and the precipitate was filtered off.
The compound was purified on column packed with silica gel
(Merck 60, 70–230 mesh) using toluene as eluent. White crystals,
42%, mp. 231–2 °C (toluene). Anal. calcd. for: C31H29N5: C 78.95;
H 6.20%; N 14.85%. Found: C 78.65; H 6.01; N 14.82%. 1H
NMR(CDCl3, d ppm): 8.40(d, J = 8.1 Hz, 4 H); 7.50(m, 4 H); 7.33(s,
4 H); 7.20(m, 2 H); 2.05(s, 6 H); 1.33(s, 9 H).
Fig. 1. Steady state optical absorbance (blue color) and normalized fluorescence
(green color) spectra of DPP dyes: (a) DPP1; (b) DPP2; (c) DPP3; (d) DPP4. Vertical
broken bars correspond to spectral positions of 000 vibronic band (optical
absorption, blue online color) and 000 vibronic band (fluorescence emission, green
online color) being determined from the measured spectra by second derivative
method. Solid vertical bars (red color) are the excitation spectra calculated by
TDDFT/PCM method (CHX solution) with B3LYP xc-potential. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
5-Methyl-1,3,7-triphenyl-bis-pyrazolo[3,4-b; 30,40-e]pyridine (DPP2)
Equimolar amounts (0.01 mol) of 5-methyl-2-phenylpyrazol-3-
amine and 5-chloro-1,3-diphenyl-pyrazole-4-carbaldehyde were
heated together at 140–180 °C for 60 min. After cooling the reac-
tion mixture was digested with ethanol (25 mL), boiled and fil-
tered. The precipitate was purified on column packed with silica
gel (Merck 60, 70–230 mesh) using toluene as eluent. Light yellow
crystals, 33%, mp. 215–216 °C (toluene). Anal. calcd. for: C26H19N5:
C 77.79; H 4.77; N 17.44. Found C 77.47; H 5.03; N 17.51. 1H
NMR(CDCl3, d ppm): 2.72(s, 3 H, Me); 7.26–7.36(m, 2 H); 7.48–
7.62(m, 6 H); 8.08(d, 2 H, J = 6.9 Hz); 8.40(d, 2 H, J = 6.9 Hz);
8.40(d, 2 H, J = 7.6 Hz); 8.50(d, 2 H, J = 7.6 Hz); 8.61(s, 1 H).
of the optical absorption spectra is characterized by two partially
superimposed bands being at room temperature practically
unstructured due to strongly overlapped broad vibronic bands.
The first absorption maximum only weakly depends on the posi-
tion of side methyl and/or phenyl groups and is centered for the
studied DPP dyes in the spectral region of 370–385 nm. Similar
comment may be addressed also to the absorption threshold posi-
tion which in these dyes is slightly different depending on the posi-
tions of the attached radicals, namely it corresponds to the
excitation energies in the range of 2.95–3.06 eV. The first absorp-
tion band exhibits smooth shape thus the position of its 000 vibron-
ic band, associated particularly with the HOMO ? LUMO
transition, cannot be precisely determined from the measured
absorption spectra. Its rough evaluation, applying the second
derivative method, gives the HOMO–LUMO gap for the excitation
processes in the range 3.09–3.21 eV (386–401 nm). Relevant spec-
tral positions corresponding to 000 vibronic band are listed in Table
1 and marked by blue dash vertical bars for each particular dye in
Fig. 1.
1,3,4,5,7-Pentaphenyl-bis-pyrazolo[3,4-b; 30,40-e]pyridine (DPP3)
Light yellow crystals, 23%, mp. 284–286 °C (toluene). Anal.
calcd. for: C37H25N5: C 82.35; H 4.67; N 12.98. Found C 82.41; H
4.54; N 13.06. 1H NMR(DMSO-d6, d ppm): 6.79–6.89(m, 4 H);
6.99–7.18(m, 11 H); 7.32–7.37(m, 2 H), 7.55-7.61(m, 4 H); 8.52–
8.59(m, 4 H).
1,7-Dimethyl-3,5-diphenyl-bis-pyrazolo[3,4-b; 30,40-e]pyridine (PP4)
In the following we compare the measured optical absorption
spectra with the excitation spectra evaluated by DFT/TDDFT meth-
od at B3LYP/6-31+G(d,p) level of theory. The calculations have
been carried out within the quantum chemical package of pro-
grams Gaussian-09 [18]. To account the solvation the DFT/TDDFT
methods were combined with polarizable continuum model
(PCM) applying the linear-response approach. Fig. 2 shows the
equilibrium molecular geometry of DPP dyes optimized by DFT/
PCM method in CHX solution. All the DPP dyes are characterized
by the planar symmetry of its dipyrazolopyridine skeleton whereas
the phenyl rings take twisted orientation with different torsion an-
gles. The first absorption band is mainly formed by the HOMO–
LUMO transition, relevant molecular orbitals (calculated by the
program Gabedit, version 2.3.6 [19]) and their energies (see la-
beled) are shown in Fig. 2. The excitation process is evidently
Yellow powder, 31%, 143 °C (toluene). Anal. calcd. for C21H17N5:
C 74.32; H 5.05; N 20.63. Found 74.36; H 5.14; N 20.72. 1H
NMR(CDCl3, d ppm): 4.21(s, 6 H, N-Me); 7.42–7.57(m, 6 H);
7.96(d, 4 H, J = 6.9 Hz); 8.83 (1 H).
Results and discussion
The optical absorption and fluorescence spectra were measured
using spectrophotometer Oriel MS125 equipped with CCD array.
The measurements were performed by means of a standard 1 cm
path length quartz cuvette for absorption spectrometry. Fig. 1
shows the steady state optical absorbance and normalized fluores-
cence spectra of DPP dyes being recorded in cyclohexane (CHX)
solution (mass concentration of about 0.1%). The low-energy part

612
E. Gondek et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 610–613
The spectral position of the first absorption band (000-transition)
is predicted in most cases with an excellent accuracy, better than
0.02 eV, and only in the case of DPP4 dye the discrepancy between
the experiment and theory is somewhat larger, ꢂ0.09 eV.
The fluorescence emission has been excited by means of mer-
cury lamp at the wavelength kex = 365 nm. Relevant spectra were
recorded in CHX solution (mass concentration of about 0.1%). The
normalized fluorescence spectra for DPP dyes are shown in Fig. 1.
Basic features of the fluorescent emission appear to be similar for
all the dyes. The asymmetric fluorescent bands are slightly struc-
tured due to vibronic coupling and are characterized by the emis-
sion maxima in the range 425–435 nm exhibiting thus rather weak
changes depending on lateral positions of substituted methyl and/
or phenyl groups. Here again, the spectral positions corresponding
to 000 electronic transition, as determined by the second derivative
method, are marked by green dash vertical bars in Fig. 1 and listed
in Table 1. Their comparison with the spectral positions of reverse
000 transitions (blue dash vertical bars) gives the Stokes shift, ꢂ17–
40 nm. Its relatively small magnitude may be an evidence that con-
formational reorganization of the solute in the excited state (solute
relaxation) is indeed weak. Making such conclusion we rely on the
solvatochromic Liptay–Mataga equation [21,22] according to
which the molecular reorganization (solvent relaxation) in weakly
polar solvents, as e.g. CHX, has practically no impact on the Stokes
shift.
Fig. 2. Equilibrium ground state molecular geometry, HOMO and LUMO orbitals
and their energies (see labeled) for DPP dyes as calculated by DFT/PCM method
(CHX solution) with B3LYP xc-potential. The orbitals have been calculated using the
program Gabedit [19]. Red and blue indicate opposite orbital phases. (For
interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
In order to explore the electroluminescence properties of DPP
dyes, relevant molecules were incorporated into the active layer
of a simple sandwich-type OLED structure, see insert in Fig. 3. It
consists of the glass substrate covered by indium tin oxide (ITO)
transparent electrode, 5 nm layer of poly(3,4-ethylene dioxythi-
ophene)-poly-(styrene sulfonate) (PEDOT:PSS), 80 nm of DPP
doped (1 wt%) polyvinylcarbazole (PVK) layer, 10 nm of Ca layer
and capping 100 nm protective aluminum layer. The active layer
was deposited on substrates by spin-coating in argon atmosphere
using PVK and DPP dissolved in tetrahydrofurane. Following such
route the EL devices of ITO/PEDOT:PSS/PVK:DPP/Ca/Al configura-
tion have been designed. Fig. 3 shows the electroluminescence
emission intensity versus the biasing voltage for these OLEDs.
Since the devices were not optimized, the turn-on voltage appears
to be higher than 12 V. Nevertheless this graph may serve for qual-
itative analysis to compare the efficiency of the studied dyes. The
lowest turn-on voltage and the strongest electroluminescence
intensity are observed in the case of DPP2 and DPP3 based devices.
The EL emission spectra, shown in Fig. 4, have been recorded at for-
ward-bias voltages of about 18–24 V. All the devices exhibit blue
electroluminescence with the emission maximum being rather
weakly dependent on the type of the fluorescent dopant. In com-
parison to the fluorescence spectra, the electroluminescence of
PVK:DPP based OLEDs exhibits either nearly the same spectral po-
sition of the emission band, as e.g. in the case of DPP1 or DPP4
dyes, or it is red shifted by about 10 or 25 nm for the PVK layer
doped by DPP2 or DPP3, respectively. However, in all the cases
the electroluminescence is characterized by the emission band
accompanied by a certain relocation of the electron density from
the electron-donating phenyl group to the electron-accepting
dipyrazolopyridine moiety. However, the dipole moments of both
ground or excited states are characterized either by moderate (as
e.g. for DPP1–DPP3 dyes), or rather small (DPP4 dye) values as
one can see in Table 1. This finding suggests that solvatochromic
effect on the absorption or emission energies is expected to be
not large for DPP molecules. The spectra measured in solvents of
different polarity may prove this assumption however relevant
investigations appear out of scope of actual study and will be pub-
lished elsewhere soon. Depending on lateral substituents the DFT
calculations give the HOMO energy in the range ꢀ5.80 ꢁ ꢀ5.74 eV
and the LUMO energy in the range ꢀ2.16 ꢁ ꢀ1.93 eV. Accordingly,
the HOMO–LUMO gap of 3.6 ꢁ 3.8 eV appears to be somewhat lar-
ger comparing to the one determined from the optical absorption
spectra (3.1–3.2 eV). It turns to be typical for DFT methods which
indeed deal with occupied and virtual molecular orbitals thus elec-
tron correlation effects, which accompany the excitation process
[20], obviously cannot be accounted there. For this reason its
time-dependent counterpart, TDDFT method, which typically
mixes several pairs of the ground state molecular orbitals due to
correlation effects, usually provides much more accurate evalua-
tion of the excitation energies and is more suitable for comparison
with optical spectra. The DPP derivatives are not exception in this
respect as one can see from Fig. 1 and Table 1. Here the TDDFT
excitation spectra are presented by the vertical solid red bars.
Table 1
000
abs
00 0
The spectroscopy characteristics of DPP dyes. k is the spectral position of the first absorption band (000 transition); kfl is the spectral position of the fluorescence 000 band; lg
and
l
e are the ground and excited state dipole moments for the equilibrium ground state molecular geometry; kmax is the electroluminescence emission peak position for OLEDs
el
with configuration ITO/PEDOT:PSS/PVK:DPP/Ca/Al (DPP ꢃ DPP1-DPP4); X, Y and Z are the color coordinates of DPP emitters according to CIE(1931) standard; exp – experiment;
calc – calculated by DFT/TDDFT/PCM method at B3LYP/6-31+G(d,p) level of theory.
000
abs
000
fl
000
abs
max
el
Compound
lg (calc)
le (calc)
CIE(1931) (exp)
k
(calc)
k
(exp)
k
(exp)
k
(exp)
[nm]
[nm]
[nm]
[D]
[D]
[nm]
X
Y
Z
DPP1
DPP2
DPP3
DPP4
386
401
401
398
425
424
417
412
383.3
398.7
403.6
386.7
4.5
2.3
2.9
0.36
3.4
3.3
3.6
0.43
432
442
451
431
0.1489
0.1472
0.1526
0.1506
0.0832
0.1057
0.1214
0.0922
0.7679
0.7471
0.7260
0.7572

E. Gondek et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 610–613
613
method at the B3LYP/6-31+G(d,p) level of theory. The absorption
and fluorescence processes appear to be weakly dependent on the
position of side methyl radicals exhibiting the first absorption band
(000 transition) in the region of 386–401 nm and the fluorescence
band (000 transition) in the range of 412–425 nm. The quantum-
chemical analysis based on DFT/TDDFT/PCM with B3LYP xc-poten-
tial quite accurately predicts the excitation energies. The
absorption–emission cycle is accompanied by relatively small
Stokes shift (17–40 nm) what may be an evidence that the solute
relaxation in the excited state is indeed weak for all the DPP dyes.
We demonstrate also the OLEDs being constructed according to
the scheme with the polymer PVK layer doped by DPP fluorescence
emitters. All the devices exhibit deep blue electroluminescence with
the emission maximum being rather weakly dependent on the type
of the fluorescent dopant. Based on the electroluminescence spectra
the emission color is characterized by the color coordinates accord-
ing to the CIE(1931) standard. The obtained results show that a ser-
ies of newly synthesized DPP dyes may be considered as perspective
blue fluorescent emitters for electroluminescent applications.
Fig. 3. Luminance-voltage characteristics of the electroluminescence devices with
OLED structure ITO/PEDOT:PSS/PVK:DPP/Ca/Al (DPP ꢃ DPP1-DPP4). The insert
shows schematically the configuration of OLED.
Acknowledgement
The DFT/TDDFT calculations have been carried out in Wrocław
Centre for Networking and Supercomputing (http://www.
wcss.wroc.pl), Grant No. 160.
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being somewhat narrower than the fluorescence one. It means that
mutual coupling of dye molecules resulting in excimers formation,
which is likely presented in the CHX solution, appears considerably
reduced in DPP doped PVK layer due to essential dispersion of
guest dye molecules in the host polymer matrix. Based on the elec-
troluminescence spectra (Fig. 4) we give appropriate characteriza-
tion of the luminance color. Relevant color coordinates, X, Y and Z,
determined within the CIE(1931) standard are given in Table 1.
Evidently, the designed OLEDs emit light of deep blue color, the
most deeper blue emission exhibits DPP1 dye.
Conclusion
In conclusion we report here the synthesis aspects and spectro-
scopic properties of four newly synthesized dipyrazolopyridine
derivatives. The steady state optical absorption and fluorescence
spectra were recorded in CHX solution and compared with the re-
sults of quantum-chemical calculations using DFT/TDDFT/PCM
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