Fluorescent conjugated dendrimers with fluorinated terminal groups: Nanofiber formation and electroluminescence properties

Article abstract of DOI:10.1021/ol801001h

(Figure Presented) Two conjugated dendrimers with fluorinated terminal groups have been designed and synthesized. Dendrimer TP1 exhibits nanofibrous self-assembly ability accompanied by an unusual blue-shifted emission from nanofibers, and TP2 shows effic

Full text of DOI:10.1021/ol801001h

ORGANIC
LETTERS
2008
Vol. 10, No. 14
3041-3044
Fluorescent Conjugated Dendrimers with
Fluorinated Terminal Groups: Nanofiber
Formation and Electroluminescence
Properties
Zujin Zhao,† Juo-Hao Li,‡ Xiaopeng Chen,† Ping Lu,*,† and Yang Yang*,‡
Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, P. R. China, and
Department of Materials Science and Engineering, UniVersity of California,
Los Angeles, California 90095
pinglu@zju.edu.cn; yangy@ucla.edu
Received May 1, 2008
ABSTRACT
Two conjugated dendrimers with fluorinated terminal groups have been designed and synthesized. Dendrimer TP1 exhibits nanofibrous self-
assembly ability accompanied by an unusual blue-shifted emission from nanofibers, and TP2 shows efficient electroluminescence property in
single-layer organic light-emitting diodes (OLEDs).
Light-emitting dendrimers have attracted intense interest
currently due to their potential use in full color displays.1
Generally, dendrimers are hyperbranched macromolecules
composed of a core, dendrons, and surface groups. It is
envisioned that dendrimers possess many merits such as well-
defined structures, super chemical purity, simple solution-
processable film formation, efficient photon-harvesting,
energy transduction capability, and controllable optical and
electrical characteristics through core, dendron, and surface
group selection. Moreover, the dendritic architecture provides
a way to modulate intermolecular interaction and suppress
excimer formation by controlling dendrimer generations.2
There are several classes of well-studied fluorescent den-
drimers, such as dendrimers extended by “stiff dendrons”
(stilbene)2a,b,3 and “soft dendrons” (poly(benzyl ether)).1g,h,4
† Zhejiang University.
‡ University of California.
(1) (a) Burn, P. L.; Lo, S.-C.; Samuel, I. D. W. AdV. Mater. 2007, 19,
1675. (b) Howard, W. E. Sci. Am. 2004, 290, 64. (c) Holder, E.; Langeveld,
B. M. W.; Schubert, U. S. AdV. Mater. 2005, 17, 1109. (d) Ding, J.; Gao,
J.; Cheng, Y.; Xie, Z.; Wang, L.; Ma, D.; Jing, X.; Wang, F. AdV. Funct.
Mater. 2006, 16, 575. (e) Anthopoulos, T. D.; Frampton, M. J.; Namdas,
E. B.; Burn, P. L.; Samuel, I. D. W. AdV. Mater. 2004, 16, 557. (f) Markham,
J. P. J.; Lo, S.-C.; Magennis, S. W.; Burn, P. L.; Samuel, I. D. W. Appl.
Phys. Lett. 2002, 80, 2645. (g) Furuta, P.; Brooks, J.; Thompson, M. E.;
Fre´chet, J. M. J. J. Am. Chem. Soc. 2003, 125, 13165. (h) Freeman, A. W.;
Koene, S. C.; Malenfant, P. R. L.; Thompson, M. E.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 2000, 122, 12385. (i) Kwon, T. W.; Alam, M. M.; Jenekhe,
S. A. Chem. Mater. 2004, 16, 4657. (j) Wang, P. W.; Liu, Y. J.; Decadoss,
C.; Bharathi, P.; Moore, J. S. AdV. Mater. 1996, 8, 237. (k) Cha, S. W.;
Choi, S.-H.; Kim, K.; Jin, J.-I. J. Mater. Chem. 2003, 13, 1900.
(2) (a) Halim, M.; Pillow, J. N. G.; Burn, P. L.; Samuel, I. D. W. AdV.
Mater. 1999, 11, 371. (b) Lupton, J. M.; Samuel, I. D. W.; Beavington, R.;
Burn, P. L.; Ba¨ssler, H. AdV. Mater. 2001, 13, 258. (c) Lupton, J. M.;
Samuel, I. D. W.; Beavington, R.; Frampton, M. J.; Burn, P. L.; Ba¨ssler,
H. Phys. ReV. B 2001, 63, 155206.
(3) (a) Frampton, M. J.; Beavington, R.; Samuel, I. D. W.; Burn, P. L.
Synth. Met. 2001, 121, 1671. (b) Ma, D.; Hu, Y.; Zhang, Y.; Wang, L.;
Jing, X.; Wang, F.; Lupton, J. M.; Samuel, I. D. W.; Lo, S.-C.; Burn, P. L.
Synth. Met. 2003, 137, 1125. (c) Pillow, J. N. G.; Halim, M.; Lupton, J. M.;
Burn, P. L.; Samuel, I. D. W. Macromolecules 1999, 32, 5985. (d) Kwok,
C. C.; Wong, M. S. Macromolecules 2001, 34, 6821
.
10.1021/ol801001h CCC: $40.75
Published on Web 06/21/2008
2008 American Chemical Society

However, the performance of the electroluminescence (EL)
devices utilizing these dendrimers is far from satisfactory.
There are relatively few reports on the synthesis of den-
drimers with acetylene linkages because they are considered
as unpromising light-emitting materials for organic light-
emitting diodes (OLEDs). The facile formation of excimer
quenches the photoluminescence (PL) and EL, which has
significantly lowered their performances.1a,j,k,5
Scheme 1
On the other hand, one-dimensional self-assembly of
functional materials has attracted increasing interest in the
fabrication of nanoscale optoelectronic devices.6 Recently,
some reports suggest that the aromatic organic molecules
and large macrocyclic aromatic molecules are prone to one-
dimensional self-assembly through strong π-π interaction.7
There are also reports on the self-assembly of stiff polyphen-
ylene dendrimers with pentafluorophenyl units.8 The driving
force for nanofiber formation is attributed to the increase in
intermolecular π-π stacking and van der Waals interactions
among dendrons by pentafluorophenyl units. In acetylene-
linked dendrimers, their stretched and planar structures may
enable facile π-π stacking, resulting in efficient intermo-
lecular electronic coupling. Thus, it is expected that acetylene-
linked dendrimers with the proper modification of dendrons
can self-assemble into high-order structures under suitable
conditions.
In this communication, we wish to report two novel
solution-processable, acetylene-linked dendrimers composed
of a pyrene core and carbazole/fluorene dendrons based on
our previous work.9 The fluorine atoms are introduced at
the peripheries of the dendrimers. Their strong electron-
withdrawing property may enhance electron transportation,10
thus balancing the number of holes and electrons in LEDs.
The structures of the dendrimers, TP1 and TP2, and their
synthetic routes are illustrated in Scheme 1. The syntheses
of intermediates 111 and 59 have been submitted. Compounds
2-4, TP1, and TP2 are prepared following the reference
methods12 involving a Sonogashira coupling reaction. The
1
structures of intermediates and products are verified by H
and 13C NMR spectroscopy, MALDI-TOF MS measurement,
and elemental analysis. Both dendrimers are highly soluble
in common organic solvents, such as CH2Cl2, CHCl3, THF,
and toluene. Their thermal stability is investigated by
differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) in N2 at a heating rate of 20 °C/min.
Dendrimers TP1 and TP2 exhibit high glass-transition
temperatures (Tg’s) at 142 and 130 °C, respectively, and
decomposition temperatures (Td’s, corresponding to a 5%
weight loss) at 456 and 444 °C, respectively.
Figure 1 shows the UV-vis absorption and PL emission
spectra of TP1 and TP2 in CH2Cl2 solutions (∼10-6 M) and
in thin neat films. The absorption spectra are normalized
using the core absorption bands as references. The absorption
spectra of the dendrimers exhibit two prominent absorption
bands. The first band is attributed to the π-π* transition of
the core (pyrene with a certain extension) with a maximum
absorption peak at ∼501 nm. The π-π* transition of an
unsubstituted pyrene ring is generally located at 337 nm,
while that of a tetrasubstituted conjugated pyrene (for
(4) (a) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74.
(b) Koene, S. C.; Freeman, A. W.; Killeen, K. A.; Fre´chet, J. M. J.;
Thompson, M. E. Polym. Mater. Sci. Eng. 1999, 80, 238. (c) Freeman,
A. W.; Fre´chet, J. M. J.; Koene, S. C.; Thompson, M. E. Polym. Prep.
(Am. Chem. Soc. DiV. Polym. Chem.) 1999, 40, 1246
.
(5) (a) Pugh, V. J.; Hu, Q.-S.; Pu, L. Angew. Chem., Int. Ed. 2000, 39,
3638. (b) Hu, Q.-S.; Pugh, V. J.; Sabat, M.; Pu, L. J. Org. Chem. 1999, 64,
7528
.
(6) (a) Lehn, J.-M. Supramolecular Chemistry-Concepts and Perspec-
tiVes; VCH: Weiheim, Germany, 1995. (b) Law, M.; Goldberger, J.; Yang,
P. Annu. ReV. Mater. Res. 2004, 34, 83.
(7) (a) Song, Y.; Di, C.-A.; Xu, W.; Liu, Y.; Zhang, D.; Zhu, D. J.
Mater. Chem. 2007, 17, 4483. (b) Balakrishnan, K.; Datar, A.; Zhang, W.;
Yang, X.; Naddo, T.; Huang, J.; Zuo, J.; Yen, M.; Moore, J. S.; Zang, L.
J. Am. Chem. Soc. 2006, 128, 6576. (c) Kastler, M.; Pisula, W.; Wasser-
fallen, D.; Pakula, T.; Mullen, K. J. Am. Chem. Soc. 2004, 126, 5234. (d)
Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura,
T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481.
(8) Bauer, R.; Liu, D. V.; Heyen, A.; Schryver, F. D.; Feyter, S. D.;
Mu¨llen, K Macromolecules 2007, 40, 4753.
(9) Zhao, Z.; Li, J.-H.; Chen, X.; Wang, X.; Lu, P.; Yang, Y., Submitted.
(10) Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Tokito, S.; Taga,
Y J. Am. Chem. Soc. 2000, 122, 1832.
(11) Zhao, Z.; Xu, B.; Yang, Z.; Wang. H.; Wang, X.; Lu, P.; Tian, W.
J. Phys. Chem. C 2008, 112, 8511-8515.
(12) Zhao, Z.; Xu, X.; Chen, X.; Wang, X.; Lu, P.; Yu, G.; Liu, Y.
Tetrahedron 2008, 64, 2658.
3042
Org. Lett., Vol. 10, No. 14, 2008

nanofiber is very simple. A certain amount of TP1 is
dissolved in a mixture solvent of CH2Cl2/cyclohexane (1/10
v/v). Since CH2Cl2, a “good” solvent for TP1, evaporates
faster than the “poor” solvent cyclohexane, the solubility of
TP1 in the mixed solvent becomes lower as the solvent
evaporates, which induces the TP1 molecules to aggregate.
After deposition for several days, green floc aggregates
appear in solution. After shaking, the floc aggregates are
dispersed, resulting in a nanofibrous suspension. No ag-
gregates are formed in TP2 under the same fabrication
condition. Figure 2 displays the TEM micrographs of TP1
Figure 1. UV-vis absorption and PL spectra of dendrimers in dilute
CH2Cl2 solutions (a) and in solid states (b).
instance with phenyl groups) appears in the visible region
(∼395 nm). The much redder core absorption in TP1 and
TP2 reveals that the dendrimers are highly conjugated. The
second band is assigned to the absorption of the dendrons
with a maximum absorption peak at ∼390 nm for TP1 and
∼399 nm for TP2. Dendrimers TP1 and TP2 show
comparable mole absorption coefficients (ε) of the first band
(0.96 × 105 and 1.04 × 105 M-1 cm-1, respectively), while
TP2 shows a larger ε value (11.4 × 105 M-1 cm-1) for the
second band than TP1 (8.59 × 105 M-1 cm-1) due to its
larger dendrons. Similar absorption spectra for both den-
drimers are observed in thin neat films, except for a slight
red shift and a loss of fine structures.
In dilute CH2Cl2 solutions, when excited with a wavelength
within the range of the dendron absorption band, both
dendrimers exhibit narrow PL emission spectra with a full
width at half-emission maximum (fwhm) of about 23 nm
and emission peaks located at 522 nm with a shoulder peak
at about 558 nm, which is attributed to the emission of the
core. There is only a trace emission from dendrons in the
range of 400 ∼ 450 nm, which indicates efficient photon
harvesting and energy transfer from dendrons to the core.
In thin neat films, TP1 and TP2 exhibit strong yellow
emission with peaks at 532 and 530 nm and relatively weak
peaks at 568 nm, respectively. The emission spectra are broad
with fwhm’s of 90 and 85 nm, respectively, which are ∼10
nm red-shifted with respect to those in CH2Cl2 solutions.
These are ascribed to aggregate formation in the solid states.
Both dendrimers are highly fluorescent in solutions and neat
films. The PL quantum yields of TP1 and TP2 are calculated
to be 0.52 and 0.55, respectively, using 9,10-bis(phenyl-
ethynyl)anthrecene (φ ) 1.0 in degassed cyclohexane)13 as
a standard.
Figure 2. TEM micrographs of TP1 nanofiber suspension drop-
casted and dried on a 200-mesh copper grid covered with a carbon
film.
nanofibers drop-casted and dried on 200-mesh copper grids
covered with carbon films. It can be seen that floc aggregates
are composed of entangled nanofibers and bundles. The
diameter of the fiber varies greatly. Some of them show a
diameter of about 6.3 nm or less, which is smaller than the
calculated molecular diameter of TP1 (about 9 nm), while
the diameter of others can be as large as about 200 nm or
more. Interestingly, the nanofiber suspension exhibits a green
emission with a peak at 498 nm, which is blue-shifted by
34 nm with respect to that in thin neat film. This emission
differs from the single molecule emission in dilute CH2Cl2
(521 nm) or in cyclohexane (514 nm) solutions. This blue-
shifted emission is somewhat abnormal because, generally,
aggregation of molecules through intermolecular π-π
interaction should result in a red-shifted emission.7b,14 These
findings suggest that the self-assembly process occurs in a
nonhomocentric way in which the adjacent dendrimer
molecules stack face-to-face because this will lead to a close
packing of the pyrene cores and thereafter a broad, redder
emission from the pyrene aggregates.15 This is further
substantiated by the diameter of some fibers, whose value is
Dendrimer TP1 exhibits a facile one-dimensional self-
assembling property. The procedure of the fabrication of TP1
(14) (a) Balakrishnan, K.; Datar, A.; Oitker, R.; Chen, H.; Zuo, J.; Zang,
L. J. Am. Chem. Soc. 2005, 127, 10496. (b) Hoeben, F. J. M.; Jonkheijim,
P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491
.
(15) (a) Venkataramana, G.; Sankararaman, S. Org. Lett. 2006, 8, 2739.
(b) Nandy, R.; Subramoni, M.; Varghese, B.; Sankararaman, S. J. Org.
Chem. 2007, 72, 938.
(13) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules; Academic Press: New York, NY, 1971.
Org. Lett., Vol. 10, No. 14, 2008
3043

smaller than that of a single molecule. In this stage, we have
not been able to confirm the exact assembling pattern.
Although both dendrimers have rigid and hyperbranched
structures, they exhibit good film-forming ability. Their film
morphology is detected by atomic force microscopy (AFM)
at a tapping model, and the images of TP2 are displayed in
Figure 3. An essentially smooth and amorphous film can be
Figure 4. EL spectra (a), and J-V-L characteristics (b) of devices
(ITO/PEDOT/TP1 or TP2/Cs2CO3/Al).
Figure 3. AFM images of TP2 (left, top view; right, side view)
spin-coated on SiO2/Si substrate at a spin speed of 2000 rpm from
2 wt % p-xylene solution.
atoms,9 the efficiency of TP2-based LED is almost double
and the voltages requested to attain a brightness of 100 and
1000 cd/m2 are almost halved. Moreover, the turn-on voltage
decreases obviously. The improvements should be attributed
to the introduction of fluorine atoms at the peripheries of
the dendrimers which facilitate electron transport and thus
balance the number of the hole and electrons. Induction of
different molecular packing patterns may also account for
such enhancement.
In summary, we have synthesized and characterized two
acetylene-linked dendrimers with a pyrene core. Dendrimer
TP1 easily self-assembles into nanofibers under simple
conditions. Dendrimer TP2 exhibits yellowish green (CIE:
0.38, 0.61) EL emission. When compared with dendrimers
with similar structures, TP2-based single-layer LED exhibits
improved performances with a maximum efficiency of 2.7
cd/A at 5.8 V and a maximum brightness of 5300 cd/m2 at
11 V. These results indicate that dendrimers with fluorinated
terminal groups are promising candidates for optoelectronic
materials.
obtained from the dendrimer solution by a simple spin-
coating method, which simplifies the OLED fabrication.
The EL properties of TP1 and TP2 were investigated in
single-layer devices with the configuration of ITO (120 nm)/
PEDOT (25 nm)/TP1 or TP2/Cs2CO3 (1 nm)/Al (100 nm)
fabricated by spin-coating with a speed of 1500 rpm from
their 2% (wt %) p-xylene solutions. The PEDOT (poly(3,4-
ethylenedioxythiophene)) layer was used for hole injection,
and an ultrathin layer of Cs2CO3 was spin coated to
effectively enhance electron injection before the Al cathode
deposited by thermal evaporation. Details of the device
fabrication can be found elsewhere.16 Figure 4 shows the
EL spectra and current density-voltage-luminance
(J-V-L) characteristics of the devices. Dendrimers TP1 and
TP2 exhibit yellowish green (Commission International
d’Eclairage (CIE): 0.38, 0.61; and 0.36, 0.62, respectively)
EL emissions with main peaks at 532 nm and shoulder peaks
at 568 nm, which is almost the same as the emission in films.
The device of TP2 exhibits a maximum efficiency of 2.7
cd/A at 5.8 V and a maximum brightness of 5300 cd/m2 at
11 V, while that of TP1 shows relatively inferior perfor-
mances with a maximum efficiency of 1.2 cd/A at 6.4 V
and a maximum brightness of 2530 cd/m2 at 9 V.
Acknowledgment. P.L. thanks the National Natural Sci-
ence Foundation of China (20674070) and the Natural
Science Foundation of Zhejiang Province (R404109).
Compared with the performances of LEDs utilizing
dendrimers with similar structures but without fluorine
Supporting Information Available: Experimental, NMR,
and mass spectra of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) Huang, J.; Li, G.; Wu, E.; Xu, Q.; Yang, Y. AdV. Mater. 2006, 18,
114.
OL801001H
3044
Org. Lett., Vol. 10, No. 14, 2008