Core, shell, and surface-optimized dendrimers for blue light-emitting diodes

Article abstract of DOI:10.1021/ja109734e

We present a novel core-shell-surface multifunctional structure for dendrimers using a blue fluorescent pyrene core with triphenylene dendrons and triphenylamine surface groups. We find efficient excitation energy transfer from the triphenylene shell to t

Full text of DOI:10.1021/ja109734e

COMMUNICATION
pubs.acs.org/JACS
Core, Shell, and Surface-Optimized Dendrimers for Blue
Light-Emitting Diodes
Tianshi Qin,† Wolfgang Wiedemair,‡ Sebastian Nau,‡ Roman Trattnig,‡ Stefan Sax,‡ Stefanie Winkler,§
Antje Vollmer,§ Norbert Koch,§,^ Martin Baumgarten,† Emil J. W. List,*,‡,z and Klaus M€ullen*,†
†Max-Planck-Institut f€ur Polymerforschung, D-55128 Mainz, Germany
‡NanoTecCenter,,Weiz Forschungsgesellschaft mbH, A-8160 Weiz, Austria
§Helmholtz Zentrum Berlin f€ur Materialien und Energie GmbH Elektronenspeicherring, D-12489 Berlin, Germany
^Institut f€ur Physik, Humboldt-Universit€at zu Berlin, D-12489 Berlin, Germany
zInstitut f€ur Festk€orperphysik, Technische Universit€at Graz, A-8010 Graz, Austria
S Supporting Information
b
Scheme 1. Synthesis Strategy toward Dendrimers PYGTPA,
PYG2, and PYGCAP
ABSTRACT: We present a novel core-shell-surface multi-
functional structure for dendrimers using a blue fluorescent
pyrene core with triphenylene dendrons and triphenylamine
surface groups. We find efficient excitation energy transfer
from the triphenylene shell to the pyrene core, substantially
enhancing the quantum yield in solution and the solid state
(4-fold) compared to dendrimers without a core emitter,
while TPA groups facilitate the hole capturing and injection
ability in the device applications. With a luminance of up to
1400 cd/m2, a saturated blue emission CIExy = (0.15, 0.17)
and high operational stability, these dendrimers belong to
the best reported fluorescence-based blue-emitting organic
molecules.
eliable structure-property relationships and exceptional
R
stability are mandatory for the commercialization of organic
semiconductor devices,1 which in particular qualifies dendrimers
with their structural variability, as a promising class of macro-
molecules for such applications.2 Recent progress in organic synthe-
sis provides the buildup of dendritic macromolecules bearing
functional building blocks in the core and the shell, and on the
surface. Different from both small molecules and linear conjugated
polymers, the stepwise generation-by-generation growth of dendri-
mers allows for the controlled synthesis of highly defined three-
dimensional shape-persistent dendritic structures incorporating sub-
units in defined positions.3 Thereby, it is possible to realize design
concepts particularly optimized for “dendrimer light-emitting
diodes” (DLEDs) relevant features, such as photoluminescence
quantum yield (PLQY), exact emission color, overall stability,
solubility, and the ability of charge capture and injection in the
periphery.
a Reagents and conditions: (a) o-xylene, 170 ꢀC, microwave, 91% for 3,
82% for PYGTPA, 84% for PYG2, 81% for PYGCAP; (b) TBAF, THF,
RT, 88% for 4.
for the central energy acceptor unit from which the emission will
occu, thereby allowing the implementation of a wide range of
emitter molecules;5 (iii) suitable redox-active moieties on the surface
enhance selective charge capturing and optimized carrier injection
Hence, we herein implemented all design aspects in the
chosen core-shell-surface dendrimer (PYGTPA, Scheme 1) as
follows: (i) the shape-persistence provides perfect site-definition
of individual chromophore units as required for implementing a
resonance energy transfer donor-acceptor system to boost the
quantum yield and control the emission color at the same time;4
(ii) the spherical nature of the molecule provides a steric shield
Received: October 29, 2010
Published: January 4, 2011
r
2011 American Chemical Society
1301
dx.doi.org/10.1021/ja109734e J. Am. Chem. Soc. 2011, 133, 1301–1303
|

Journal of the American Chemical Society
COMMUNICATION
Table 1. Photophysical Properties and Device Performances of Dendrimers
solution
film
b
λem nm
ΦPL
%
λem nm
ΦPL** rel.%c
Lv @8 V cd/m2
Von
V
CIE, x, y
TPG2a
410
456
450
450
27
431
469
463
463
100
324
410
442
300
1132
728
6.0
0.19, 0.18
0.19, 0.27
0.15, 0.18
0.15, 0.17
PYG2
74
88
73
4.4
3.4
3.2
PYGCAP
PYGTPA
1443
a Taken from ref 11. b Relative to quinine sulfate dehydrate. c Relative to TPG2 as 100%.
from the electrode;6 (iv) the strongly twisted benzene rings of the
dendrimer scaffold provide high solubility without the necessity of
further side-chain substitution and thereby yield thermally stable
amorphous thin films with high glass transition temperatures,7
also omitting alkyl chains as solubilizing side groups and thus
enhancing the stability of devices.8
The divergent synthetic route toward a series of blue light-
emitting dendrimers is shown in Scheme 1. The key first-generation
dendrimer 4 was obtained in two steps from 1,3,6,8-tetraethy-
nylpyrene 1 and 1,3-diphenyl-6,9-bis(triisopropylsilyl-ethynyl)cyclo-
pentaphenanthrenone 2 by [4 þ 2] Diels-Alder cycloaddition
and TiPS desilylation in quantitative yields, and allowed the
synthesis of different second-generation dendrimers (PYGTPA,
PYG2, and PYGCAP) with various surface groups in good yields.
All dendrimers are highly soluble in common organic solvents
(∼20 mg/mL in dichloromethane, THF, and toluene), and can
thus be purified by column chromatography. The MALDI-TOF
mass spectrum demonstrated that all dendrimers are mono-
disperse with their molecular weights being identical to the calculated
masses (Figure S1, Supporting Information [SI]).
All substances have been analyzed for their optical properties
in solution and in the solid state (Figures S2 and S3, SI). As
summarized in Table 1 all PYGx dendrimers are characterized by
an unstructured emission spectrum in the blue spectral region
and an absorption band located in the UV. For all PYGx dendrimers
the emission is stemming from the pyrene core as a consequence
of an onging energy transfer.9,10 The efficient dipolar coupling of
the core-shell triphenylene-pyrene system becomes in particular
evident as one observes a drastically enhanced PLQY of all PYGx
systems due to excitation energy transfer. A comparison reveals
that PYGx show a PLQY improvement of up to 88% compared to
27% of previously reported TPG2 in solution.11 As a consequence
of the shape persistent molecular design of PYGx dendrimers the
relative PLQY in thin films was even improved 3-fold for PYG2 and
4-fold for PYGCAP and PYGTPA as compared to TPG2.
To determine the absolute location of the energy levels of the
PYGx dendrimers and the relative energy alignment of the core-
shell and surface groups, ultraviolet photoelectron spectroscopy
(UPS) was performed (Figure S4, SI). The results show that (a)
the energy levels of the pyrene core (HOMO = 5.7 eV; LUMO =
2.8 eV) are located within the band gap of the triphenylene
(HOMO = 6.2 eV; LUMO = 2.8 eV) as required for energy
transfer and (b) that adding the TPA groups on the surface
in PYGTPA drastically reduces the injection barrier from the
PEDOT:PSS electrode by ∼0.65 eV compared to PYG2 and
PYGCAP; thus, PYGTPA represents indeed an optimized
molecular core-shell-surface design for device applications. From
UPS investigations on heterolayer structures PEDOT:PSS/PYGx
1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi) was chosen
as the electron transport layer for devices. TPBi deposition onto
Figure 1. Voltage-current-luminance characteristics of PYGTPA
DLED.
PYGx films lowered the sample work function by 0.2 eV (PYG2
and PYGTPA) and 0.4 eV (PYGCAP) due to interface dipoles at
the organic heterointerfaces.12
On the basis of these results a device configuration ITO/
PEDOT:PSS/PYGx(30 nm)/TPBi(10 nm)/CsF(8 nm)/Al was
used to test the dendrimers in light-emitting devices. The key
results for optimized devices using PYG2, PYGCAP, and PYGTPA
are summarized in Table 1. For PYGTPA the best stability and
highest luminance of 1440 cd/m2 with a deep blue emission
spectrum with CIExy = (0.15, 0.17) and an overall efficiency of
0.26 cd/A was achieved (Figure 1). With these values the presented
dendrimers possess the best brightness and stability values
thus far reported for solution-processed saturated blue-emitting
dendrimer-based devices.13 As compared to fluorescent poly(p-phe-
nylene)-based blue-emitting materials, our results are comparable
with respect to device brightness and stability for single-layer
devices.14,15 This excellent material performance can be attributed
to both the presented dendrimer design as well as the chosen
synthetic concept, allowing for perfect site definition of the dendrimer.
In conclusion, the core-shell-surface design yields a den-
drimer with high quantum yield and optimized charge injection
properties and which, as a consequence of the shape-persistent
molecular design, maintains all properties in the solid state. Further-
more, using a synthetic route based on noncatalytic Diels-Alder
cycloaddition not only allows for a defined generation-by-
generation growth of the dendrimers yielding highly defined
monodisperse molecular weight materials, which are free of
unknown structural defects, but also completely rules out metal
catalyst contamination and polymeric defects.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
b
MALDI-Mass, UV-vis, photoluminescence spectra, UPS, and
1302
dx.doi.org/10.1021/ja109734e |J. Am. Chem. Soc. 2011, 133, 1301–1303

Journal of the American Chemical Society
COMMUNICATION
schematic energy level diagram. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
e.list@tugraz.at; muellen@mpip-mainz.mpg.de
’ ACKNOWLEDGMENT
This work was financially supported by the Deutsche
Forschungsgemeinschaft (DFG) within the frame of the
Sonderforschungsbereich (SFB) 625. S.N. and S.S. acknowledge
the financial support of POLYLED.
’ REFERENCES
(1) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.;
Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.;
Spiess, H. W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384–387.
(2) Lo, S. C.; Burn, P. L. Chem. Rev. 2007, 107, 1097–1116.
(3) Hecht, S.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74–91.
(4) Wang, J. L.; Yan, J.; Tang, Z. M.; Xiao, Q.; Ma, Y. G.; Pei, J. J. Am.
Chem. Soc. 2008, 130, 9952–9962.
(5) Furuta, P.; Brooks, J.; Thompson, M. E.; Frechet, J. M. J. J. Am.
Chem. Soc. 2003, 125, 13165–13172.
(6) Thelakkat, M. Macromol. Mater. Eng. 2002, 287, 442–461.
(7) Abbel, R.; Woffs, M.; Bovee, R. A. A.; van Dongen, J. L. J.; Lou,
X.; Henze, O.; Feast, W. J.; Meijer, E. W.; Schenning, A. Adv. Mater.
2009, 21, 597–602.
(8) Adronov, A.; Frechet, J. M. J. Chem. Commun. 2000, 18, 1701–1710.
(9) Markovitsi, D.; Germain, A.; Millie, P.; Lecuyer, P.; Gallos, L. K.;
Argyrakis, P.; Bengs, H.; Ringsdorf, H. J. Phys. Chem. 1995, 99, 1005–
1017.
(10) Park, Y. H.; Rho, H. H.; Park, N. G.; Kim, Y. S. Curr. Appl. Phys.
2006, 6, 691–694.
(11) Qin, T. S.; Zhou, G.; Scheiber, H.; Bauer, R. E.; Baunigarten,
M.; Anson, C. E.; List, E. J. W.; Mullen, K. Angew. Chem., Int. Ed. 2008,
47, 8292–8296.
(12) Zhu, X. Y.; Kahn, A. MRS Bull. 2010, 35, 443–448.
(13) Burn, P. L.; Lo, S. C.; Samuel, I. D. W. Adv. Mater. 2007, 19,
1675–1688.
(14) Sax, S.; Rugen-Penkalla, N.; Neuhold, A.; Schuh, S.; Zojer, E.;
List, E. J. W.; Mullen, K. Adv. Mater. 2010, 22, 2087–2091.
(15) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471–1507.
1303
dx.doi.org/10.1021/ja109734e |J. Am. Chem. Soc. 2011, 133, 1301–1303