High-efficiency pyrene-based blue light emitting diodes: Aggregation suppression using a calixarene 3D-scaffold

Article abstract of DOI:10.1039/c2cc30995e

An efficient blue light emitting diode based on solution processable pyrene-1,3-alt-calix[4]arene is demonstrated, providing a record current efficiency of 10.5 cd A^-1 in a simple non-doped OLED configuration. Complete suppression of pyrene aggregation in the solid state is achieved by controlling chromophore dispersion using the 1,3-alt-calix[4]arene scaffold.

Full text of DOI:10.1039/c2cc30995e

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High-efficiency pyrene-based blue light emitting diodes: aggregation
suppression using a calixarene 3D-scaffoldw
Khai Leok Chan,*a Jacelyn Pui Fong Lim,a Xiaohui Yang,b Ananth Dodabalapur,c
Ghassan E. Jabbour*b and Alan Sellinger*d
Received 10th February 2012, Accepted 29th March 2012
DOI: 10.1039/c2cc30995e
An efficient blue light emitting diode based on solution processable
pyrene-1,3-alt-calix[4]arene is demonstrated, providing a record
current efficiency of 10.5 cd AÀ1 in a simple non-doped OLED
configuration. Complete suppression of pyrene aggregation in the
solid state is achieved by controlling chromophore dispersion using
the 1,3-alt-calix[4]arene scaffold.
progress in using the pyrene moiety as a blue emitter with
moderate success.16–21
We have been exploring the use of multi-functionalised 3-D
hybrid cores such as octavinylsilsesquioxane and cyclic triphos-
phazenes as platforms from which organic chromophores, such as
pyrene, can be attached to provide OLED materials.22–24 These
inert rigid platforms serve to electronically isolate the intra-
molecular chromophores and disperse them in radial directions so
as to minimise aggregation. Monodispersed star-shaped materials
that are prepared in this way, such as pyrene-functionalised
octavinylsilsesquioxane, have demonstrated high solution
processability, high purity, and excellent OLED properties.23
Calix[4]arene, a 3-D organic platform, that exists in one of
four possible conformational isomers—cone, partial-cone,
1,2-alternate, and 1,3-alternate—has been extensively used as
molecular scaffolds in the design of supramolecular guest–host
systems and in the construction of novel molecular architectures
because of its tunable and unique three-dimensional structures
together with the ease of chemical functionalisations.25–27 However
the use of calix[4]arene for light emitting applications has been
limited, and only the cone conformer has been deployed, with
chromophores substituted non-conjugatively to the bottom rim to
give low efficiency OLEDs.28–30
Organic light emitting diodes (OLEDs) have gained enormous
attention in the scientific community due to their potential
applications in next-generation flat panel displays and solid-state
lighting.1–5 The development of highly efficient, color-stable and
readily processable organic emitters, especially blue, constitutes a
critical component in the technology roadmap of OLEDs.6,7 Blue
emitting dopants dispersed in host matrices have been demon-
strated to provide efficient fluorescent and phosphorescent blue
OLEDs, but such systems suffer from potential phase segregation
and associated color stability issues when subjected to prolonged
operation.8 The alternative system that consists of a single layer of
non-doped emitter is advantageous for its reliability and simplicity
in fabrication, but organic chromophores tend to take on a planar
construct at the molecular level that display high tendency towards
aggregation/excimer formation in a condensed state, and this can
lead to a red-shift of emission and quenching of fluorescence.
Pyrene is a classical example of a conjugated aromatic
chromophore that emits deep blue with high quantum efficiency
in dilute solution, but aggregates in the condensed state.9–12 Its
long fluorescence lifetime,13 high charge carrier mobility14,15
and chemical stability make it a potentially good candidate for
OLED applications. Despite this, many reports have shown
In this report, we present a novel pyrene-functionalised
calix[4]arene 1 in Scheme 1 in which pyrene chromophores
are attached to a 1,3-alt-calix[4]arene scaffold. The scaffold
forces intramolecular pyrene units to be arranged radially
orthogonal to each other, thus ensuring complete suppression
of chromophore aggregation in the solid state. The solution-
processable 1 provides a blue OLED with a current efficiency
of 10.5 cd AÀ1 and a maximum external quantum efficiency
(EQE) of 6.4% that exceeds the classical 5% upper limit for
fluorescent-based OLEDs. We demonstrate the importance of
the 1,3-alt-calix[4]arene scaffold in suppressing aggregation by
comparing with the cone-calix[4]arene conformer 2 and model
compound 3. In the latter cases, significantly lower efficiencies
were observed in electroluminescent devices. These observa-
tions highlight the effectiveness of 1,3-alt-calix[4]arene as a
scaffold in dispersing and suppressing aggregation between
chromophores, and in enhancing the performance of OLEDs.
The synthesis of pyrene-substituted calix[4]arene 1 follows
3 simple synthetic steps depicted in Scheme 1.31,32 A detailed
description of the synthesis can be found in the ESI.w
a Institute of Materials Research and Engineering (IMRE),
Agency for Science, Technology and Research (A*STAR),
3 Research Link, Singapore 117602, Singapore.
E-mail: chankl@imre.a-star.edu.sg
b Solar and Alternative Energy Engineering Research Center,
King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Kingdom of Saudi Arabia.
E-mail: ghassan.jabbour@kaust.edu.sa
c Microelectronics Research Center, University of Texas at Austin,
Austin, TX 78758, USA
d Center for Advanced Molecular Photovoltaics (CAMP),
476 Lomita Mall, Stanford University, Stanford, CA 94305-4045,
USA. E-mail: aselli@stanford.edu
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterisation of molecules, and OLED device data.
See DOI: 10.1039/c2cc30995e
c
5106 Chem. Commun., 2012, 48, 5106–5108
This journal is The Royal Society of Chemistry 2012

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Fig. 1 The geometrically optimized 3D molecular model of compound
1 from (a) side view and (b) top view clearly showing the orthogonal and
radial arrangements of the pyrene chromophores that are attached to
the 1,3-alt calix[4]arene scaffold.
scaffold, and the 1,3-alternate directionality of the substitution
around the scaffold. The chromophore orientation clearly prevents
pyrene units from p-stacking with each other, and this therefore
reduces the likelihood of intramolecular aggregate or excimer
formation.
Scheme 1 Synthesis of pyrene-functionalised calix[4]arenes in the 1,3-
alternate (1) and cone (2) conformations. Reagents and conditions: (a)
n-butyl iodide, K2CO3, acetonitrile, 80 1C then n-butyl iodide,
Cs2CO3, acetonitrile, 80 1C, 37%; (b) N-bromosuccinimide, aceto-
nitrile, room temperature, 58%; (c) n-butyl iodide, NaH, DMF, 70 1C,
68%; (d) N-bromosuccinimide, 2-butanone, room temperature, 86%;
(e) 4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxaborolane, Pd(PPh3)4,
2 M Na2CO3, toluene, 100 1C, 65% (1), 53% (2) and 90% (3).
The thermal properties of pyrene-1,3-alt-calix[4]arene 1 were
evaluated using thermogravimetric anaylsis (TGA) and differential
scanning calorimetry (DSC) (Fig. S14 and S15, ESIw). The melting
temperatures and decomposition temperatures were measured at
120 1C and 250 1C, respectively, and no glass transition was
observed. Similar values (118 1C and 260 1C, respectively) were
observed for pyrene-cone-calix[4]arene 2. The HOMO energy
levels of 1 and 2 were measured by cyclic voltammetry and found
to be the same at À5.2 eV (Table S1, ESIw).
Both calix[4]arenes 1 and 2 were found to exhibit high
solubility in common organic solvents such as THF, chloro-
form and toluene, and readily purified via column chromato-
graphy. The conformations of the calix[4]arene cores 1 and 2
were confirmed by 1H NMR. Two broad bands were observed
in the spectrum of 2 at 3.50 and 4.15 ppm which corresponds
to the bridging exo- and endo-hydrogen atoms, respectively, of
the cone-calix[4]arene core. On the other hand, a single broad
band at 3.98 ppm is observed in the spectra of 1 corresponding
to the stereoscopically identical bridging hydrogens of the 1,3-
alt-calix[4]arene core (Fig. S6 and S7, ESIw).
The photophysical properties of both calix[4]arene cores 1
and 2 were examined by UV–Vis absorption and emission
(PL) spectroscopy in dilute chloroform solutions (10À7 M) and
in thin films (Table S1, Fig. S9 and S10, ESIw). The UV–vis
and PL spectra of 1 and 2 in solution and thin film state are
very similar indicating that aggregation is minimized in the
solid state. For example, no appreciable change in peak
positions or emission band widths was detected as solution
concentrations were varied over six orders of magnitude from
10À2 to 10À7 M in chloroform. In contrast, model compound 3
displayed a distinct bathochromic shift of emission of about
70 nm on going from solution to solid state (Fig. S11, ESIw). This
bathochromic shift is well documented for pyrene derivatives
transitioning from single molecules to calix[4]arene cores (both
1,3-alt and cone).33
Light emitting diodes (OLEDs) with the architecture ITO/
PEDOT:PSS/compound 1, 2 or 3 (80 nm)/TPBI (30 nm)/CsF/
Al were fabricated by spin-coating solutions of the pyrene-
emitters in chloroform (10 mg mlÀ1) over the PEDOT:PSS-
coated ITO substrate followed by a hole blocking TPBI layer
and CsF/Al cathode evaporation. Blue emission was observed
in both 1 and 2, with Commission Internationale de l’Eclairage
(CIE) coordinates measured at (0.15, 0.24) and (0.17, 0.23),
respectively. The EL profiles of each compound showed
remarkable similarity to its PL profiles in both solution and
solid state, with the emission peaks of all EL and PL spectra
for each compound observed to fall within a 5 nm spectral
range of one another (Fig. S12, ESIw). This indicates that the
chromophore species of 1 (and 2) are similar in the various
environments and are independent of the mode of excitation.
The EL performances of the devices are summarized in
Table 1. The device from pyrene-1,3-alt-calix[4]arene 1 shows
a maximum current efficiency of 10.5 cd AÀ1 at 14 mA cmÀ2
(EQE = 6.4%) and a maximum luminous efficacy of 4 lm WÀ1
at 6.1 mA cmÀ2 (Fig. 2). Moreover, the current efficiency
remained a high 9.5 cd AÀ1 (EQE = 5.8%) when the bright-
ness was increased to ca. 5000 cd mÀ2 (11 V). The turn on
voltage is 6 V. The system shows a five-fold improvement in
efficiency (at 100 cd mÀ2) over the leading solution-processed
pyrene-based OLED,19 and represents among the most effi-
cient non-doped, solution processed fluorescent blue OLED
reported to date. The remarkable OLED performance of
pyrene-1,3-alt-calix[4]arene 1 is attributed to the unique ability
of the 1,3-alt-calix[4]arene scaffold at dispersing pyrene units
that are otherwise prone to aggregation and the luminescent
To better understand how the 1,3-alt calixarene scaffold
suppresses aggregation, molecular modeling was carried out
on pyrene-1,3-alt-calix[4]arene 1 (Fig. 1).34 The modeling
shows a pair of pyrene chromophores on each of the top
and bottom rims of the calix[4]arene scaffold, and all the
intramolecular chromophores directed away from the core
and aligned orthogonal (B901) to one other. This arrange-
ment is the result of the square geometry of the calixarene
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5106–5108 5107

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Table 1 Performances of pyrene-calix[4]arenes 1 and 2 and model compound 3 in an OLED device of the structure ITO/PEDOT:PSS/Emitter/
TPBI/CsF/Al
At maximum current efficiency, Zc
At maximum luminous efficacy, Zp
b
Zca/cd AÀ1 Zext (%) Voltage/V Brightness/cd mÀ2 Zpc/lm WÀ1 Jd /mA cmÀ2 Voltage/V
Device Emitter
CIEe [x, y]
1
2
3
Pyrene-1,3-alt-calixarene (1) 10.5
2.4
6.4
1.5
8.8
11.2
1460
984
4.0
0.71
6.1
20.0
7.8
10.0
(0.15, 0.24)
(0.17, 0.23)
Pyrene-cone-calixarene (2)
Model compound (3)
No electroluminescent emission
a
b
Current efficiency. External quantum efficiency (EQE). Luminous efficacy. Current density. Commission Internationale de L’Eclairage
c
d
e
(CIE) coordinates at 9 V.
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quenching of excited states. In comparison, the device based
on pyrene-cone-calix[4]arene 2 was significantly less efficient;
the maximum current efficiency achieved was 2.5 cd AÀ1, that was
less than 25% the efficiency of compound 1. The clustering of all
pyrene units on one face of the cone-calix[4]arene scaffold in
compound 2 likely led to significant aggregation quenching. On
the other hand, the device with model compound 3 as the emitter
showed no electroluminescence. Varying the charge injection/
transport layers in the device and increasing bias voltage also did
not yield any emission. This clearly demonstrates the significant
role played by the calix[4]arene scaffolds in suppressing chromo-
phore aggregation.
In summary, a highly efficient solution processable blue
OLED based on pyrene-1,3-alt-calixarene 1 has been developed,
providing an excellent luminous efficiency of 10.5 cd AÀ1 and an
external quantum efficiency of 6.4% at 1460 cd mÀ2. This, to our
knowledge, represents the most efficient non-doped solution-
processable blue fluorescent OLED reported to date. This simple
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