Highly Luminescence Anthracene Derivatives as Promising Materials for OLED Applications

Article abstract of DOI:10.1002/ejoc.201600532

Novel symmetrical anthracene derivatives with bulky carbazolyl-fluorene, diphenylamino-fluorene, or carbazolyl-carbazole units connected to the anthracene frame through an ethynyl bridge were synthesized in excellent yield by using Sonogashira cross-coupling. The ethynyl bridge in the anthracene dyes increases π-electron conjugation and the bulky substituents considerably attenuate intermolecular interactions. The dyes possess high thermal stability, tremendous solubility in common organic solvents, and especially high photoluminescence quantum yield (Φ_f) in solution in the range of 77–98 %. OLED devices were fabricated. The AFM images of thin films and blends prepared from all compounds show a uniform and flat surface, indicating excellent film-forming properties. Devices incorporating the anthracene derivative with diphenylamino-fluorene end-capping groups exhibited the highest value of current density (J). All of the fabricated OLED devices emitted yellowish-orange light under applied voltage.

Full text of DOI:10.1002/ejoc.201600532

DOI: 10.1002/ejoc.201600532
Full Paper
Luminescence
Highly Luminescence Anthracene Derivatives as Promising
Materials for OLED Applications
[
a]
[a]
[a,b]
Marzena Grucela,[b]
Aneta Slodek, Michal Filapek, Ewa Schab-Balcerzak,
[
a]
[b]
[c]
[c]
Sonia Kotowicz, Henryk Janeczek, Karolina Smolarek, Sebastian Mackowski,
[a]
[d]
[a]
Jan Grzegorz Malecki, Agnieszka Jedrzejowska, Grazyna Szafraniec-Gorol,
[e]
[a]
[a]
[a]
Anna Chrobok, Beata Marcol, Stanislaw Krompiec, and Marek Matussek*
Abstract: Novel symmetrical anthracene derivatives with bulky cence quantum yield (Φ ) in solution in the range of 77–98 %.
f
carbazolyl-fluorene, diphenylamino-fluorene, or carbazolyl- OLED devices were fabricated. The AFM images of thin films
carbazole units connected to the anthracene frame through an and blends prepared from all compounds show a uniform and
ethynyl bridge were synthesized in excellent yield by using So- flat surface, indicating excellent film-forming properties. Devi-
nogashira cross-coupling. The ethynyl bridge in the anthracene ces incorporating the anthracene derivative with diphenyl-
dyes increases π-electron conjugation and the bulky substitu- amino-fluorene end-capping groups exhibited the highest
ents considerably attenuate intermolecular interactions. The value of current density (J). All of the fabricated OLED devices
dyes possess high thermal stability, tremendous solubility in emitted yellowish-orange light under applied voltage.
common organic solvents, and especially high photolumines-
Introduction
Research on the construction of the organic light-emitting dio-
des (OLEDs) published in the second half of the twentieth cen-
tury by Tang and VanSlyke became the prototype for current
electroluminescent materials. The dynamic development of
organic electronics, particularly organic light-emitting diodes, is
driven by their increasingly impressive performance as a new
generation of flexible and low-cost flat-panel displays.
immense achievements in the field of OLEDs can be ascribed
to their high efficiency and thermal stability, low power con-
A number of blue OLEDs based on small molecules with ex-
cellent electroluminescence (EL) performance, high efficiency
[
5–7]
and thermal stability have been reported.
In spite of this
progress, organic EL devices based on blue-emitters exhibit life-
times that are rather too short to be marketable; therefore, fur-
ther efforts in this field with respect to increasing efficiency and
color purity have to be made. Indeed, the green and red emit-
ters demonstrate superior color purity, lifetime and efficiency
[
1]
[
2,3]
The
[
8]
compared with blue emitters. Luminous efficiency of green
and red anthracene-based emitters have been reported to be
[
9,10]
[
4]
as high as 20 and 8.5 cd/A, respectively.
sumption, realistic color reproduction and high transparency.
Anthracene and its derivatives have been extensively investi-
gated for many years especially as blue-light emitters. The de-
rivatives have attracted attention as small organic molecules
possessing unconventional physical properties, such as out-
standing electroluminescence and photoluminescence (PL),
high thermal stability, and extensive conjugated π-electron
The emitting materials possess suitable emission with a wide
range of colors, high thermal stability and tremendous effi-
ciency, which are desirable characteristics of OLEDs for the de-
velopment for flat-panel displays.
[
a] Faculty of Mathematics, Physics and Chemistry, Institute of Chemistry,
University of Silesia,
[
11]
structure. In addition, anthracene derivatives are stable com-
pounds that are amenable for substitution at the 9- and 10-
positions. Modifications of anthracene with bulky groups along
with a great variety of electron-donating (D) and electron-with-
drawing (A) substituents is of particular importance for sup-
pressing aggregation by steric hindrance, improvement the EL
Szkolna 9, Katowice, 40-007, Poland
E-mail: matussekmarek@gmail.com
http://inorganic.us.edu.pl/
[
[
b] Centre of Polymer and Carbon Materials, Polish Academy of Sciences,
M. Curie-Sklodowska 34, Zabrze, 41-819, Poland
c] Institute of Physics, Faculty of Physics, Astronomy and Informatics,
Nicolaus Copernicus University,
[12]
performance, and enhancement of π-conjugated structure.
Grudziadzka 5, Torun, 87-100, Poland
[
13,14]
[15,16]
[17]
Fluorene,
carbazole,
naphthalene,
and tri(phenyl)-
derivatives are often selected as blue-emitting build-
[
[
d] Faculty of Mathematics, Physics and Chemistry, Institute of Physics,
University of Silesia,
Uniwersytecka 4, Katowice, 40-007, Poland
e] Department of Chemical Organic Technology and Petrochemistry, Silesian
University of Technology,
[
18]
silane
ing blocks for synthesis of anthracene-based emitters. A num-
ber of D-A-D, D-π-A, D-π-A-π-D compounds based on the
[
19–22]
anthracene frame have been reported.
Recently, D-A-D
Krzywoustego 4, Gliwice, 44-100, Poland
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600532.
molecules based on anthracene with bulky moieties, namely
9,10-bis(9′,9′-diethyl-7′-diphenylamino-fluoren-2-yl)anthracene
Eur. J. Org. Chem. 0000, 0–0
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Figure 1. Structure of dyes A1–A3.
(
BDDFA) were synthesized and studied as blue-light emitting dures with minor modifications, which resulted in higher yields.
material in OLED, showing high quantum efficiency of 3.3 % In the first step, anthracene was brominated at the 9- and 10-
2
[23]
with maximum brightness of 4200 cd/m .
A maximum cur- positions to give 1 in 92 % yield. The latter was then converted
rent efficiency of 3.6 cd/A was achieved for an EL device with into protected (ethynyl)anthracene derivative 2 through Sono-
a D-A-D framework based on the anthracene scaffold combined gashira coupling reaction with trimethylsilylacetylene (TMSA) in
with naphthalene moieties. It was observed that the introduc- the presence of CuI and Pd(PPh ) . Iodoarenes 5 and 6 were
3
4
tion of substituents in the 2-position of the anthracene leads to obtained from commercially available fluorene upon iodination,
a non-coplanar structure and suppression of the intermolecular followed by alkylation under conditions of phase-transfer catal-
interactions, which results in high efficiency and color purity for ysis (PTC) to afford 2,7-diiodo-9,9-dibutylfluorene 4 in 75 %
[
24]
blue-light emitting materials.
yield. Ullmann condensation of 4 with either carbazole or di-
In this study, we present the synthesis and comprehensive phenylamine, resulted in the formation of target intermediates
characterization of orange-emitting materials based on sym- 5 and 6, respectively. Iodoarene 9 was synthesized by following
metrically substituted anthracene of D-π-A-π-D type, 9,10- procedures similar to those used for 5 and 6. Briefly, iodination
bis{[7-(9H-carbazol-9-yl)-9,9-dibutylfluoren-2-yl]ethynylanthra- of carbazole gave 7, then N-alkylation afforded 9-butyl-3,6-di-
cene (A1), 9,10-bis{[7-(N,N-diphenylamino)-9,9-dibutylfluoren-2- iodocarbazole 8 in 83 % yield, and finally Ullmann condensation
yl]ethynyl}anthracene (A2), and 9,10-bis{[6-(9H-carbazol-9-yl)-9- of 8 provided 9 in 63 % yield.
butylcarbazol-3-yl]ethynyl}anthracene (A3) (Figure 1). In these
The desired anthracene derivatives A1–A3 were prepared by
D-π-A-π-D molecules, anthracene, acting as the acceptor, is prior deprotection of 9,10-bis[(trimethylsilyl)ethynyl]anthracene
connected with donating moieties, such as carbazole or fluor- 2 by treatment with tetrabutylammonium fluoride (TBAF), and
ene, through triple bonds (C≡ C). The use of the triple bond as followed by Pd-catalyzed Sonogashira cross-coupling reaction
the conjugated bridge between building blocks (D, A) ensures in situ with the respective iodoarene 5, 6, or 9 (Scheme 2). The
lack of torsion, providing planarity of the molecule, which sig- synthesis of a compound similar to A1 but with a longer alkyl
nificantly increases π-electron conjugation. In addition, the chain (n-hexyl) at the 9,9′-position of the fluorene has recently
bulky carbazolyl-fluorene, diphenylamino-fluorene, and carb- been described and examined as a dye applied in two-photon
[
25]
azolyl-carbazole groups in novel orange emitters, significantly excited fluorescence. However, 9,10-bis{[7-(9H-carbazol-9-yl)-
suppress intermolecular interactions and yield high quantum 9,9-dihexyl-9H-fluoren-2-yl]ethynyl}anthracene was prepared by
efficiency and high stability. The synthesis of novel compounds using a different synthetic route with 67 % yield.
A1–A3 was developed, and a detailed electrochemical, photo-
physical, theoretical, and electrical characterization of the dyes
was undertaken.
The target compounds were obtained as orange (A1), red
(
A2), and brown (A3) solids (Figure 2) with high yields in the
range of 82–88 %. Our design of D-π-A-π-D type molecules
based on anthracene derivatives bearing fluorene, carbazole,
and diphenylamine moieties with high π-electron linkage was
developed to achieve highly π-conjugated systems.
Results and Discussion
In addition, triple (C≡ C) bonds as linkage between building
blocks (D and A) prevents torsions, and primarily allows for a
Synthesis
The structures of designed anthracene derivatives A1–A3 are free rotation of D and A fragments leading to a planarity of the
[
26]
presented in Figure 1. Scheme 1 illustrates the routes used for molecule (Figure 3).
The obtained compounds A1–A3 were
1
the synthesis of intermediates 1–9, which were required for the fully characterized by standard spectroscopic methods ( H and
1
3
preparation of target compounds A1–A3 (Scheme 2). The key
C NMR, mass spectrometry). The chemical structures of the
intermediates were synthesized according to well-known proce- synthesized compounds were also confirmed based on elemen-
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Scheme 1. Synthesis of compounds 1–9. Reagents and conditions: (a) Br
AcOH/H O/H SO , 70 °C, 6 h; (d) 50 % NaOH, TBAB, n-C
phenanthroline, DMF, reflux, 30 h; (f) KI, KIO , AcOH, reflux, 15 min.
2
, CHCl
3
, room temp., 4 h; (b) TMSA, Pd(PPh
3
)
4
, CuI, THF/TEA, 60 °C, 24 h; (c) I
2
, H
5 6
IO ,
2
2
4
H Br, DMSO, room temp., 8 h; (e) carbazole (for 5, 9) or diphenylamine (for 6), CuI, K CO , 1,10-
4 9 2 3
3
Scheme 2. Synthetic route for the preparation of A1–A3.
tal analysis, which showed good agreement between the calcu- observed for other thermostable materials.[27] To verify this as-
lated and experimentally determined carbon, nitrogen, and sumption, DSC measurements were performed to assess the
hydrogen content. However, a deficiency in the carbon content purity of the compounds. As expected, high purity of anthra-
of about 1 % was found, which is likely a result of the difficulties cene derivatives A1 and A2 (above 98 %) was confirmed by
in burning these high-temperature compounds, which was also using this technique (see the Experimental Section). Moreover,
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ties, including the ability to form uniform, transparent amor-
phous thin layers by vapor deposition and spin-coating meth-
[
29]
ods, in contrast to single crystals and liquid crystals.
When
crystalline samples of anthracene derivatives A1 and A2, ob-
tained after purification, were heated, an endothermic peak due
to melting was observed. When the isotropic liquid was cooled,
in their 2nd heating DSC scan, the thermograms display an en-
dothermic baseline shift owing a glass transition (T ) phenome-
non. Additionally, in the case of A2, upon further heating be-
g
Figure 2. Form (powders) of the target compounds.
yond T an exothermal crystallization was found at 182 °C, and
subsequent melting was observed at 284 °C. Figure 4 shows
DSC thermograms of A2.
g
all compounds exhibit excellent solubility in common organic
solvents, such as acetone, dichloromethane, chloroform, ethyl
acetate or tetrahydrofuran, which allows the easy fabrication of
devices.
g c
Figure 4. DSC thermogram of A2. T – glass transition temperature; T – crys-
tallization temperature; T – melting point temperature.
m
In the case of A3, without dibutylfluorene units, only T was
g
Figure 3. The approach of using a linkage, which influences on the geometry
of molecules.
observed during heating scans. The highest T was observed
g
for A1, with dibutylfluorene end-capped with carbazole units.
The TGA results showed that all molecules retain high thermal
stability, with decomposition temperatures corresponding to
5
% weight loss (T5 %) over 400 °C.
Thermal Properties
The thermal properties of the investigated compounds were
evaluated by differential scanning calorimetry (DSC) and ther- Photophysical Properties
mogravimetric analysis (TGA) under nitrogen atmosphere; the
obtained results are shown in Table 1.
The photophysical properties of anthracene derivatives were in-
vestigated by UV/Vis spectroscopy and photoluminescence (PL)
measurements carried out in chloroform solution (Figure 5), and
in solid state, both as a film and blended with poly(9-vinylcarb-
azole) (PVK). The obtained data are collected in Table 2 and UV/
Vis spectra of compound A1 are presented in Figure 6.
Table 1. Thermal properties of the anthracene derivatives.
TGA
DSC
[
a]
[a]
Char yield[b]
[%]
T
5 %
T
[°C]
10 %
T
[°C]
g
T
m
[°C]
[
°C]
Electronic spectra of the studied anthracene derivatives (ex-
cept for A2) exhibited similar characteristics with several bands
between 250 and 600 nm (Figure 5, a). The first, most intense
band is observed below 300 nm. Differences in absorption in
the range 300–400 nm are seen for A2 and the other two com-
pounds. Compound A2 exhibited two bands with maxima
A1
A2
A3
434
402
478
460
433
512
58
48
70
185
124
133
301
286
–
[
a] T5 %. T10 %: temperatures at 5, 10 % weight loss, respectively. [b] Residual
weight when heated to 800 °C in nitrogen.
Based on the DSC results, it was found that the investigated (λmax) at 316 and 372 nm, whereas A1 and A3 showed one
anthracene derivatives can form stable amorphous glasses band with λmax at 342 and 322 nm, respectively. The pro-
above room temperature; thus, they belong to a class of organic nounced differences result from the chemical structure of the
functional materials referred to as molecular glasses, which has compounds. Compounds A1 and A3 are similar because they
attracted increasing attention since the development of organic are both end-capped with carbazole derivative, whereas A2
[
28]
optoelectronic devices.
They possess many valuable proper- contains diphenylamine units as end group. For the UV/Vis
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Figure 6. The UV/Vis spectra of A1 in chloroform solution (c = 10–5 mol/L)
and in solid state together with PVK spectrum in film.
ment by the introduction of the anthracene unit. Above 400
nm, compounds A1–A3 all have similar vibration patterns of
absorption band connected with the presence of the anthra-
cene structure. The absorption spectra of the anthracene deriv-
atives in the solid state revealed a slight bathochromic shift of
the absorption band maximum with respect to that in solution.
The emission spectra of studied compounds were measured
in chloroform solution under different excitation wavelengths
(Figure 5, b). The influence of the excitation wavelength (λex)
on PL properties is exemplified for compound A1 in Figure 7.
Changes in the emission intensity with an increase of λex
from 300 to 500 nm were observed. The investigated anthra-
cene derivatives under different excitation wavelengths exhib-
ited a single emission band with vibrionic pattern in the case
of A1 and A3, and the maximum of the emission is located in
the green spectral region. The most intense emission of light
was found under excitation at λex = 490 nm. Compounds A1–
A3 emitted light in chloroform solution with very high PL quan-
Figure 5. (a) The UV/Vis and (b) PL spectra of A1–A3 in chloroform solution
c = 10–5 mol/L).
(
tum yields (Φ ) in the range of 98–77 % (Table 2), suggesting
f
properties of intermediate compounds 5 (corresponds A1) and that they would have highly efficient electroluminescent prop-
(corresponds A2), the same differences are found. In the UV/ erties in OLED devices. Compound A3, which does not contain
Vis spectrum of 5 above 300 nm, one absorption with λmax at dibutylfluorene units, exhibited the lowest Φ . Probably, the
6
f
326 nm is detected, in contrast to 6, which exhibited two λmax molecular structure of the anthracene derivatives possesses a
at 311 and 357 nm. The presence of these bands is also seen relatively high rigidity, which promotes a high PL efficiency.
in the final compounds A2 and A1; however, their maxima are Considering the rigidity of these compounds, it seems that the
slightly shifted because of changes in their electronic environ- highest and the lowest Φ should be observed in the case of
f
Table 2. UV/Vis and PL data of investigated compounds in solution, film, and blend.
UV/Vis (ε [L·mol–1 cm–1])
max [nm]
PL λem [nm]
ex = 490 nm
Φ
f
[d]
Medium
λ
λ
A1
A2
A3
solution[a]
film
276 (91,136); 292 (49,530); 342 (51512); 466 (60,427); 490 (55,474)
368; 472; 500
511; 544
529; 560
521; 553
548
0.93
0.10
0.01
0.98
0.11
0.15
0.77
0.17
0.11
[
b]
[c]
blend
330; 345; 473; 500
[
a]
262[c]; 280 (144,273); 316 (92,955); 372 (93,923); 480 (143,305); 502 (133,622)
314; 375; 473; 536
solution
film
556; 586[c]
562
[
b]
blend
331; 343; 372; 484
[
a]
solution
film
276 (102,984); 322 (39,766); 465 (38,746); 489 (37,727)
517; 550
640
589
[
c]
330; 477; 504; 554
330; 345; 476; 500
[
b]
[c]
blend
[
a] CHCl
3
; c = 10–5 mol/L. [b] 15 wt.-% content of compound in PVK. [c] The second-derivatives method was used to find the position. [d] Φ
www.eurjoc.org
f
– quantum yield.
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for anthracene derivative A3, without dibutylfluorene units.
Compound A2 showed the same position of λem in solution as
in the solid state. However, the emission quantum yield of the
samples in the solid state was much lower than those in solu-
tion, and the highest Φ of about 17 % was found for A3
f
(
Table 2).
Electrochemical Properties
The electrochemical properties of A1–A3 were investigated in
CH Cl solution by means of cyclic voltammetry (CV). Repre-
2
2
sentative cyclic voltammograms are presented in Figure 8. The
electrochemical oxidation and reduction onset potentials were
Figure 7. (a) The effect of λex on PL properties of compound A1 in chloroform
solution (c = 10–5 mol/L) and (b) PL spectra of A3 in solution and in the solid
state.
compounds A3 and A2, respectively. Dynamic intramolecular
rotation of diphenylamine derivatives in A2 may cause nonradi-
ative reduction of its excited states leading to lower Φ . In the
f
presented compounds the opposite trend was found. However,
to confirm the molecule rigidity, calculations of the bond rota-
tional barrier and comparison with results from X-ray measure-
ments are necessary. Moreover, other phenomena may influ-
ence the PL intensity, such as photoinduced electron transfer
(
PET) and intersystem crossing (ISC).
In the PL spectra of the blends, the emission band coming
from the anthracene derivative dominates the spectra. The
emission spectra of A1–A3 in blend with PVK revealed that the
λem was bathochromically shifted with respect to the chloro-
form solution. The same tendency, that is, shift of λem to lower
energetic region, was found for the compounds in the form of
film on glass substrate (Table 2 and Figure 7, b). This redshift
of the λem is probably due to molecular packing. The significant
shift of λem from the green to the orange region was observed
Figure 8. Voltammograms of (a) A1, (b) A2, and (c) A3 during reduction and
oxidation processes. GC as working electrode; sweep rate ν = 100 mV/s for
CV or 2.5 mV/s for DPV measurements, 0.1
4 6 2 2
M Bu NPF in CH Cl .
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Table 3. Electrochemical data for A1–A3.
E
1red
E
[eV]
2red
E
[eV]
3red
E
1ox
E
2ox
HOMO[a] (CV) LUMO[b] (CV) HOMO (DFT) LUMO (DFT)
E
g
(CV)[c]
[eV]
E
g
(opt)
[eV]
g
E (DFT)
[eV]
[
eV]
[eV] [eV]
A1
A2
A3
–1.74 –2.03 –2.16 0.56 0.77
–1.84 –2.12 –2.28 0.31 0.67
–1.82 –2.04 –2.32 0.45 0.72
–5.66
–5.41
–5.55
–3.36
–3.26
–3.28
–5.23
–5.02
–5.59
–2.75
–2.69
–2.73
2.30
2.15
2.27
2.40
2.29
2.36
2.48
2.33
2.86
[
g 2 2 4 6
a] HOMO = –5.1 – Ecv ox. [b] LUMO = –5.1 – Ecv red. [c] E = Eox (onset) – Ered (onset); 1 mM in CH Cl with 0.1M Bu NPF .
used to estimate the HOMO and LUMO energies (or rather, ioni- with 15 wt.-% A1–A3. All studied compounds have appropriate
zation potentials and electron affinities) of the materials (as- HOMO and LUMO levels considering the PVK matrix. The HOMO
[
30]
suming that the IP of ferrocene was –5.1 eV). The calculated of the guest (in the case of compounds A1–A3) is higher than
HOMO and LUMO levels together with electrochemical energy the HOMO level of PVK (PVK HOMO = –5.8 eV) and the LUMO
band gap (E ) are presented in Table 3. All compounds have level of anthracene derivatives A1–A3 is lower than the LUMO
g
similar properties during the reduction processes. The first re- of the matrix (PVK LUMO = –2.2 eV). Thus, one of the effective
duction step (E1red) (in the range from –1.74 to –1.84 V) is re- OLED functioning conditions is fulfilled for all investigated com-
versible. This is undoubtedly caused by reduction of the anthra- pounds (Table 3).
cene moiety. However, E1red values differ slightly depending on
The electroluminescent properties of the synthesized anthra-
donor properties of the substituents. Further reduction of the cene derivatives were tested in devices with a configuration
potential allows for registration of two more reduction degrees ITO/PEDOT:PSS/compound/Al and ITO/PEDOT:PSS/blend/Al
(
E2red and E3red). For A1 and A3, E2red is almost equal in the (thickness of active layer are given in Table 4). The morphol-
value (i.e., ca. –2.04 V). This observation suggests the reduction
of terminal carbazole moieties. In the case of A2, E2red is slightly
lower (occurs at –2.12 V), which is a consequence of the strong
donating character of the diphenylamine end-capping group.
On the other hand, differences during the oxidation process
are significant because oxidation takes place at the π-excess
moiety. Thus, higher electron-excising ring character results in
higher HOMO energy level (as a consequence, smaller Eox value
is observed). Indeed, in the case of A2 (possessing strong donor
Table 4. J–V characteristic parameters of devices based on anthracene deriva-
tives together with electroluminescence wavelength maximum (λEL) and the
thickness and surface roughness (RMS) of the active layer.
Active
layer
J
V
on
λ
[nm]
EL
Thickness RMS
2
[mA/cm ] [V]
[nm]
[nm]
A1
A2
A3
film
blend
film
blend
film
blend
0.9
9.8
2.2
78.5
0.9
5.4
2.6
1.8
2.6
–
125
1.498
0.926
0.676
1.099
2.143
1.500
536; 550 125
–
580
–
620
97
115
40
–
0
diphenylamine moieties), E1ox occurs at the lowest potential:
.31 V. As can be seen in Figure 8, b, E2ox = 0.67 V. It is worth
135
noting that, in both cases, the oxidation is fully reversible. How-
ever, the charge accompanying E1ox is twice as high as meas-
ured for E2ox. The first oxidation step probably occurs at the
diphenylamine moiety (A2 possesses two Ph N moieties per
2
molecule) and the second at the anthracene moiety. For A1 and
A3, only E1ox is reversible. Additionally, in the case of A3 (having
carbazole end-capping groups), polymerization at potentials
higher than E2ox was observed.
It can be seen in Table 3 that the energy gaps obtained from
independent methods (i.e., from cyclic voltammetry, UV/Vis
spectroscopy, and DFT calculations) are consistent in tendency.
This implies that band gaps, as well as both HOMO and LUMO
energy levels, can be tuned conveniently by changing the elec-
tron-donating ability of the donor in D-π-A-π-D dyes. However,
in each case, the band gaps determined for diluted solutions
by UV/Vis spectroscopy are higher than those obtained by us-
ing cyclic voltammetry techniques.
Electroluminescence Properties
The synthesized anthracene derivatives exhibited green emis-
sion in dilute solution (see the Supporting Information, Fig-
ure S7) with high quantum yield, which are expected to serve
as emitters in OLEDs. Two kinds of OLEDs were prepared: in the
first case, a neat film was used as the emitter layer, and in the
second device the active layer was obtained by doping PVK
Figure 9. AFM images (10 μm × 10 μm) of A2 (top) and 15 % doped PVK with
A2 (bottom).
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ogy />of the spin-coated films, which is another key factor for and flat surface, indicating excellent film-forming properties.
the OLED fabrication, was investigated by atomic force micros- The surface root-mean-square roughness (RMS) of these layers
copy (AFM). Figure 9 presents examples of AFM images for A2 were in the range of 0.676–2.143 nm.
layer and for a blend of A2 with PVK. The AFM images of thin
For the fabricated OLED devices, current density-voltage (J–
films and blends prepared from all compounds show a uniform V) characteristics were registered. Figure 10 presents an exam-
ple of the J–V characteristics for compound A3; the data ob-
tained for all devices are collected in Table 4.
The turn-on voltage (V ) of the devices based on compound
on
end-capped with diphenylamine units A2 was lower than those
of other anthracene derivatives. Additionally, the devices with
this compound achieved the highest value of current density
(J). OLEDs prepared by using neat film of anthracene derivatives
exhibited lower J values in relation to devices in which the ac-
tive layer was a blend. All fabricated devices emitted yellowish-
orange light under the applied voltage (Figure 10). For the devi-
ces based on PVK doped with anthracene derivatives, the elec-
troluminescence (EL) spectra were registered (Figure 11).
The EL spectra of devices were found to have emission with
maximum wavelength (λ ) between 550 and 620 nm. The EL
El
spectra measured for all three compounds correspond well to
the photoluminescence spectra and the bathochromic shifts of
λEl in the range of 15–31 nm compared with its PL spectra in
the blend were seen. In addition, the signal measured for the
diode with anthracene derivative A1 exhibited a bimodal char-
acter, similar to that of the PL band. As shown in Figure 11, in
Figure 10. J–V characteristics of the A3-based devices together with images
of the fabricated devices.
Figure 11. (a) Electroluminescence spectra of fabricated OLEDs for increasing and decreasing external voltages, (b) EL intensity as a function of voltage for
the investigated OLEDs and (c) EL intensity as a function of time for ITO/PEDOT:PSS/A3:PVK 15 % structure under an external voltage of 20 V.
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all cases, the emergence of emission was observed between 7 ure 12). In the case of A2, the HOMO comprises the whole
and 9V applied voltage. No variation of the EL spectra as a molecule, whereas the LUMO is localized on the -CC-anthra-
function of the external voltage for a given structure was found, cene-CC- fragment with some contribution of fluorene-(C H ) ,
4
9 2
which points towards the stability of the electronic properties. as seen in Figure 12 and Table S1.
The intensities of EL are the highest in the case of the ITO/
The calculated HOMO and LUMO energies are close to the
PEDOT:PSS/A2:PVK 15 % structure. In addition, in the case of values determined from electrochemical measurements, al-
the structure based on A3, temporal behavior of the EL signal though the energies of LUMO are higher than the experimental
for the external voltage of 20 V was also measured (Figure 11, values. The virtual orbitals are generally more difficult to de-
c). The experiment carried out at a fixed external voltage as a scribe theoretically than the occupied orbitals, and the errors
function of time reveals that the investigated device exhibits in the LUMO eigenvalues are also usually significantly larger,
modest stability, with the signal diminishing after 3 minutes. with the calculated LUMO eigenvalues being much higher in
Clearly, further work towards the protection of such devices is energy than those determined experimentally.
required.
The dichloromethane solutions of A1–A3 reveal photolumi-
It should be noted that the OLEDs presented in this work nescence excited at 488 (A1, A3) and 498 nm (A2). The elec-
have a simple structure because the aim was to investigate the tronic absorption spectra of A1–A3 were calculated by using
ability of the synthesized materials to electroluminesce. Clearly, the CIS(D) method with CH Cl as solvent in the polarizable
2
2
device performance could be improved by the introduction of continuous model (PCM). Selected electronic transitions as-
additional charge transporting layers, such as carrier injection signed to the excitation wavelengths are collected in Table 5.
and charge transport layers.
Taking into account the electronic structures of the compounds,
the luminescence active transitions have mixed π→π*/n→π*
character. Moreover, considering the molecular orbitals in-
volved in electronic transitions, it can be assumed that the exci-
tation processes have intramolecular charge transfer (ICT) char-
acter. In A1, ICT occurs between fluorene-(C H ) and anthra-
DFT Calculations
To gain further insight into the optical and electronic properties
of the anthracene derivatives A1–A3, quantum theoretical cal-
culations at the density functional theory level were performed.
Based on the optimized geometries, the energy and electronic
distribution of molecular frontier orbitals were calculated in di-
chloromethane for comparison with experimental (electro-
chemical) potentials. Three-dimensional plots of the electron
density for the HOMO and the LUMO of A1–A3 are shown in
Figure 12, whereas the orbital energies and compositions are
summarized in Table S1 (see the Supporting Information).
Moreover, the partial density of states diagrams are also in-
cluded (see the Supporting Information, Figure S8). The mol-
ecule of each compound was divided into three fragments such
as: -CC-anthracene-CC-, fluorene-(C H ) , carbazole, Ph N-, re-
4
9 2
cene-ethynyl fragments, in A3 between carbazole-C H and an-
4
9
thracene, and in the case of A2, fluorene-(C H ) and anthra-
4
9 2
cene-ethynyl moieties play the acceptor role. The Stokes shift
of A1 and A3 are similar and about two times smaller than for
A2. The smaller bathochromic shift observed for A1 and A3
may be due to their rigid structure (the nuclear structure of the
molecule does not substantially deviate from its Franck–Con-
don geometry), which enhances the intramolecular energy
transfer process. In the case of A2, the greater nuclear geome-
try distortion with respect to the ground state results in a larger
shift of the emission maximum.
4
9 2
2
Table 5. Selected electronic transitions calculated for CH Cl solutions of A1–
2
2
spectively, and their contribution to each molecular orbital was
calculated. The electronic structures of A1 and A3 are similar
and the HOMO and LUMO in both cases are localized mainly
on the anthracene-ethynyl moiety with some participation of
the fluorene-(C H ) A1 or carbazole-C4H9 A3 orbitals
A3.
Character[a]
λ
ex
λcalc (f)
Transition
A1
A2
488
498
489.7
0.011)
496.1
H-5→LUMO (47 %)
HOMO→L+1 (52 %)
H-3→LUMO (13 %),
H-1→L+2 (15 %)
HOMO→L+1 (71 %)
H-7→LUMO (65 %)
HOMO→L+1 (33 %)
π
flu→ π*anth
(
nanth→ π*flu
π→ π*anth
4
9 2
(
Table S1). The character of the HOMO and LUMO can be de-
(0.0251)
π
Ph2N→ π*flu
π→ π*flu
carb–C4H9→ π*anth
nanth→ π*carb–C4H9
fined as nonbonding (n) and π* antibonding, respectively (Fig-
A3
488
484.7
π
(
0.0175)
[
a] flu, anth, carb denote respectively fluorene, -CC-anthracene-CC- and carb-
azole subunits.
Conclusions
We have presented the synthesis and comprehensive character-
ization of novel symmetrically substituted anthracene deriva-
tives of D-π-A-π-D type: A1, A2, and A3, where anthracene,
acting as acceptor (A), is connected to donating moieties, such
as carbazolyl-fluorene, diphenylamino-fluorene, carbazolyl-
carbazole groups through triple bonds. The novel anthracene
derivatives A1–A3 were prepared by Sonogashira cross-cou-
Figure 12. Contours of HOMOs and LUMOs of A1–A3.
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pling with excellent yield in the range of 82–88 %. The novel lized from hexane, to give A1 (169 mg, 87 %) as an orange solid.
1
H NMR (400 MHz, CDCl ): δ = 8.83 (dd, J = 6.6, J = 3.3 Hz, 4 H),
.20 (d, J = 7.8 Hz, 4 H), 7.99–7.97 (m, 2 H), 7.89–7.85 (m, 4 H), 7.79
compounds are yellowish-orange emitters with high thermal
stability and excellent solubility in most organic solvents. Cyclic
voltammetry of all studied compounds reveals that all of them
undergo multistep oxidation and reduction, with fully reversible
first step of these processes. The values of band gaps obtained
electrochemically, optically, and by DFT are very similar. This
result implies that changes to the donor part in anthracene
3
1
2
8
(s, 2 H), 7.74 (dd, J = 6.7, J = 3.2 Hz, 4 H), 7.62–7.60 (m, 4 H), 7.50–
1
2
7
8
1
1
1
.44 (m, 8 H), 7.35–7.31 (m, 4 H), 2.18–2.06 (m, 8 H), 1.28–1.14 (m,
H), 0.90–0.77 (m, 20 H) ppm. 13C NMR (126 MHz, CDCl ): δ = 153.0,
3
51.4, 141.1, 140.9, 139.6, 137.0, 132.2, 131.2, 127.4, 126.9, 126.0,
26.0, 125.9, 123.5, 122.1, 121.8, 121.3, 120.5, 120.1, 120.0, 118.6,
09.8, 103.6, 87.1, 55.6, 40.1, 26.2, 23.1, 13.9 ppm. HRMS (ESI): m/z
+
derivatives A1–A3 might be used as a convenient way to tune calcd. for C H N [MH ] 1109.5774; found 1109.5778. C H N
84
73
2
84 73
2
the band gap, as well as to adjust both HOMO and LUMO en- (1110.51): calcd. C 90.93, H 6.55, N 2.52; found C 89.83, H 6.58, N
ergy levels.
2.10. Purity (DSC): 98.95 mol-%.
It should be stressed that the synthesized anthracene deriva-
tives fulfill the key requirements for organic electrolumines-
cence devices, including (1) suitable ionization potentials and umn chromatography (silica gel; hexane/dichloromethane, 4:1) and
9
,10-Bis{[7-(N,N-diphenylamino)-9,9-dibutylfluoren-2-yl]-
ethynyl}anthracene (A2): The crude product was purified by col-
electron affinity for energy level fitting for the injection of recrystallized from hexane, to give A2 (160.6 mg, 82 %) as a red
1
charge carriers at the interfaces between the electrode/organic solid. H NMR (400 MHz, CDCl
): δ = 8.78 (dd, J
= 6.6, J
= 3.3 Hz,
3
1
2
4
7
2
H), 7.77–7.75 (m, 2 H), 7.70–7.67 (m, 8 H), 7.61 (d, J = 8.2 Hz, 2 H),
.29–7.25 (m, 8 H), 7.15 (d, J = 7.7 Hz, 10 H), 7.05–7.02 (m, 6 H),
.03–1.87 (m, 8 H), 1.21–1.07 (m, 8 H), 0.76–0.66 (m, 20 H) ppm.
NMR (126 MHz, CDCl ): δ = 152.6, 150.9, 147.9, 147.8, 141.8, 135.4,
32.1, 131.0, 129.2, 127.4, 126.8, 125.7, 124.1, 123.3, 122.8, 120.8,
20.8, 119.2, 118.9, 118.6, 104.0, 86.7, 55.2, 40.0, 26.1, 23.0,
3.9 ppm. HRMS (ESI): m/z calcd. for C H N [MH ] 1113.6087;
found 1113.6089. C H N (1114.55): calcd. C 90.61, H 6.88, N 2.52;
found C 89.72, H 6.97, N 2.72. Purity (DSC): 98.36 mol-%.
material and organic material/organic material, (2) the ability to
form uniform films, (3) thermal and electrochemical stability
and (4) high PL quantum yield in CH Cl solution ranging from
1
3
C
2
2
3
7
7 to 98 %. These compounds show electroluminescence re-
1
1
1
sponse and emission of yellowish-orange light at forward ap-
plied voltage. The highest intensities of EL were observed for
OLED devices based on compounds with end-capped diphenyl-
amine units.
+
84 77 2
8
4 77 2
9
,10-Bis{[6-(9H-carbazol-9-yl)-9-butylcarbazol-3-yl]ethynyl}-
anthracene (A3): The crude product was purified by column chro-
matography (silica gel; hexane/dichloromethane, 1:1) and recrystal-
Experimental Section
lized from acetone, to give A3 (154.7 mg, 88 %) as a brown solid.
Materials: All reagents were of analytical reagent grade, and were
used without further purification. Solvents were distilled by using
standard methods and purged with argon before use. Poly(9-vinyl-
carbazole) (PVK) with Mn = 25,000–50,000 was purchased from
Sigma–Aldrich and used without additional purification. Poly[3,4-
1
H NMR (400 MHz, CDCl ): δ = 8.77 (dd, J = 6.6, J = 3.3 Hz, 4 H),
3
1
2
8
4
.50 (d, J = 1.2 Hz, 2 H), 8.34 (d, J = 1.2 Hz, 2 H), 8.20 (d, J = 7.8 Hz,
H), 7.94 (dd, J = 8.5, J = 1.5 Hz, 2 H), 7.68–7.62 (m, 8 H), 7.55 (d,
1
2
J = 8.6 Hz, 2 H), 7.43 (d, J = 3.7 Hz, 8 H), 7.33–7.29 (m, 4 H), 4.45 (t,
J = 7.2 Hz, 4 H), 2.04–1.96 (m, 4 H), 1.56–1.48 (m, 4 H), 1.05 (t, J =
(
(
ethylenedioxy)thiophene]:poly-(styrenesulfonate)
0.1–1.0 S/cm) and substrates with pixilated ITO anodes were sup-
(PEDOT:PSS)
7
1
1
.4 Hz, 6 H) ppm. 13C NMR (126 MHz, CDCl ): δ = 141.9, 141.0, 140.0,
32.0, 130.0, 129.6, 127.4, 126.6, 125.9, 125.8, 124.4, 123.5, 123.2,
22.7, 120.3, 119.8, 119.7, 118.5, 114.1, 110.1, 109.8, 109.4, 103.7,
3
plied by Ossila. Column chromatography was carried out on Merck
silica gel. Thin-layer chromatography (TLC) was performed on silica
[
31]
85.4, 43.4, 31.3, 20.7, 14.0 ppm. HRMS (ESI): m/z calcd. for C H N
gel (Merck TLC Silica Gel 60). 9,10-Dibromoanthracene,
9,10-
2,7-
74 55
4
+
[32]
[33]
[MH ] 999.4427; found 999.4421. C H N (1000.28): calcd. C 88.94,
bis[(trimethylsilyl)ethynyl]anthracene,
2,7-diiodofluorene,
74 55 4
[34]
[35]
H 5.45, N 5.61; found C 87.78, H 5.43, N 5.56.
diiodo-9,9-dibutylofluorene,
3,6-diiodocarbazole,
9-butyl-3,6-
were synthesized
diiodocarbazole,[ and iodoarenes (5, 6, 9)
36]
[37]
Measurements: NMR spectra were recorded in CDCl with Bruker
3
according to reported procedures with minor modifications, and
1
Advance 400 MHz (for H NMR) or Bruker Advance 500 MHz (for
C NMR) instruments. High-resolution mass spectrometry (HRMS)
1
were characterized by H NMR spectroscopic analysis (see the Sup-
13
porting Information for details).
analysis was performed with a Waters Xevo G2 Q-TOF mass spec-
trometer (Waters Corporation) equipped with an ESI source operat-
ing in positive-ion modes. Full-scan MS data were collected from
Synthesis and Characterization
Sonogashira Cross-Coupling for the Synthesis of Anthracene
Derivatives A1–A3. General Procedure: A solution of iodoarene
100 to 1000 Da in positive ion mode with a scan time of 0.1 s. To
ensure accurate mass measurements, data were collected in cen-
troid mode and mass was corrected during acquisition using leu-
(0.350 mmol, 2 equiv.) in THF (15 mL) was bubbled with argon
and stirred for 10 min. After this time, the protected form of 9,10-
TM
cine enkephalin solution as an external reference (Lock-Spray ),
diethynylanthracene 2 (65.2 mg, 0.176 mmol, 1 equiv.), Pd(PPh3)
4
+
which generated reference ion at m/z 556.2771 Da ([M + H] ) in
(
40.5 mg, 0.035 mmol), and CuI (6.6 mg, 0.035 mmol) were added,
and the resulting mixture was bubbled with argon for a further
0 min. TBAF (1 in THF, 0.7 mL, 0.7 mmol) was then injected
positive ESI mode. The accurate mass and composition for the mo-
lecular ion adducts were calculated by using the MassLynx software
1
M
(
Waters) incorporated with the instrument. Elemental analyses were
through the septum, and the mixture was stirred at room tempera-
ture for 24 h. The reaction mixture was filtered and the filtrate was
evaporated under reduced pressure. The residue was purified by
column chromatography.
performed with a Vario EL III (Elementar, Germany). Thermogravi-
metric analysis (TGA) was carried out with a TG Pyris-1, Perkin–
Elmer thermal analyzer with 10 °C/min in a stream of nitrogen
3
(
20 cm /min) in the temperature range of 25–800 °C. Differential
9
,10-Bis{[7-(9H-carbazol-9-yl)-9,9-dibutylfluoren-2-yl]ethynyl}-
scanning calorimetry (DSC) was performed with a TA-DSC 2010 ap-
paratus, under nitrogen atmosphere, using sealed aluminum pans
with a heating rate 20 °C/min. UV/Vis absorption spectra of solu-
anthracene (A1): The crude product was purified by column chro-
matography (silica gel; hexane/dichloromethane, 4:1) and recrystal-
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Full Paper
tions were recorded with a Perkin–Elmer Lambda Bio 40 UV/Vis
uum Model) model. The contribution of a group (anthracene, fluor-
spectrometer. Photoluminescence spectra of solutions were meas-
ured with a Varian Carry Eclipse spectrometer. UV/Vis spectra in the using Mülliken population analysis. GaussSum 3.0 program
ene, carbazole, Ph N-) to a molecular orbital was calculated by
2
[41]
was
solid state as a film deposited on a glass substrate and as blends
with poly(N-vinylcarbazole) on a glass substrate were recorded with
a Jasco V570 UV-V-NIR Spectrometer. Photoluminescence spectra in
the solid state were monitored with a Hitachi F-2500 spectrometer.
used to calculate group contributions to the molecular orbitals and
to prepare the partial density of states (DOS) spectra.
Acknowledgments
The quantum yields (Φ ) in chloroform solution, film, and blend
f
were estimated by using the integrating sphere Avantes AvaSphere- This work was supported by the National Centre for Research
8
0 with an FLS-980 Spectrophotometer (Edinburgh Instruments)
and Development, Poland (grant number PBS2/A5/40/2014).
M. M. acknowledges a scholarship from the DoktoRIS project
co-financed by the European Social. Calculations were carried
out in the Wroclaw Centre for Networking and Supercomputing
under ambient temperature. Quantum yields measurements were
performed by using the absolute method with the excitation wave-
length with the most intense luminescence, in the case of solutions
using chloroform as a blank. The active layer thickness was meas-
ured by atomic force microscopy (AFM) with a Topometrix Explorer
TMX 2000.
(http://www.wcss.wroc.pl).
Keywords: Luminescence · Organic electronics · Thin
films · Cross-coupling · Arenes · Heterogeneous catalysis
Electrochemical measurements were carried out with an Eco Che-
mie Autolab PGSTAT128n potentiostat, glassy carbon electrode
(diam. 2 mm), platinum coil, and silver wire as working, auxiliary,
and reference electrode, respectively. Potentials are referenced with
respect to ferrocene (Fc), which was used as the internal standard.
Cyclic voltammetry experiments were conducted in a standard one-
compartment cell, in CH Cl (Carlo Erba, HPLC grade), under argon.
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Film and Blend Preparation: Films and blends (15 wt.-% concen-
tration of compound in PVK matrix) on glass substrates were pre-
pared by spin coating (800 rpm, 60 s) from chloroform solution
1
[
[
(10 mg/mL).
6
Device Preparation: Devices with two configurations: ITO:PE-
DOT:PSS/compound/Al and ITO:PEDOT:PSS/compound:PVK/Al con-
taining 15 wt.-% of compound dispersed in PVK were fabricated.
They were prepared on OSILLA substrates with pixilated ITO anodes,
cleaned sequentially with detergent, deionized water, 10 % NaOH
solution, water and 2-propanol in an ultrasonic bath. Substrates
were covered with a PEDOT:PSS thin film (40 nm) by spin coating
at 5000 rpm for 60 s and annealed for 15 min at 130 °C. The active
layer was spin-coated on top of the PEDOT:PSS layer from chloro-
form solution (10 mg/mL) at 800 rpm for 60 s and dried at 100 °C
for 10 min. Finally, an aluminum cathode was vacuum-deposited.
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Theoretical Calculations: The calculations were carried out with
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Published Online: ■
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Luminescence
A. Slodek, M. Filapek,
E. Schab-Balcerzak, M. Grucela,
S. Kotowicz, H. Janeczek, K. Smolarek,
S. Mackowski, J. G. Malecki,
A. Jedrzejowska,
G. Szafraniec-Gorol, A. Chrobok,
B. Marcol, S. Krompiec,
M. Matussek* .................................... 1–13
Highly Luminescence Anthracene
Derivatives as Promising Materials
for OLED Applications
Symmetrical anthracene derivatives ized. The compounds were yellowish-
with bulky units connected to the an- orange emitters with high thermal sta-
thracene frame through an ethynyl bility, had excellent solubility in most
bridge were synthesized and their organic solvents, and very high photo-
electrochemical, photophysical, and luminescence quantum yield in solu-
electrical properties were character- tion.
DOI: 10.1002/ejoc.201600532
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