Giant extended π-conjugated dendrimers containing the 10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene chromophore: Synthesis, NMR behaviors, optical properties, and electroluminescence

Article abstract of DOI:10.1021/jo049268p

This paper reports the facile synthetic strategy of a series of novel π-conjugated dendrimers (G0 and G1) based on 10,15-dihydro-5H-diindeno [1,2-a;1′,2′-c]fluorene (truxene) in which the benzene cores are generated "in-situ" from acetyl aromatics by the

Full text of DOI:10.1021/jo049268p

Gia n t Exten d ed π-Con ju ga ted Den d r im er s Con ta in in g th e
10,15-Dih yd r o-5H-d iin d en o[1,2-a ;1′,2′-c]flu or en e Ch r om op h or e:
Syn th esis, NMR Beh a vior s, Op tica l P r op er ties, a n d
Electr olu m in escen ce
Xiao-Yu Cao,† Xue-Hui Liu,‡ Xing-Hua Zhou,† Yong Zhang,§ Yang J iang,‡ Yong Cao,*,§
Yu-Xin Cui,*,‡ and J ian Pei*,†
The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University Healthy Science Centre,
Peking University, Beijing 100871, China, and Institute of Polymer Optoelectronic Materials and Devices,
South China University of Technology, Guangzhou 510640, China
jianpei@chem.pku.edu.cn
Received April 30, 2004
This paper reports the facile synthetic strategy of a series of novel π-conjugated dendrimers (G0
and G1) based on 10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene (truxene) in which the benzene
cores are generated “in-situ” from acetyl aromatics by the acid-promoted cyclotrimerizations. The
unique NMR behaviors, physical properties, and electroluminescence device applications are also
presented. Besides the purity, the structure, and the confirmation of the successful formation and
isolation of desired compounds by clear assignments of every molecule by 1H and 13C and 2D NMR
characterizations, several astonishing NMR behaviors have been observed in various solvents. For
1, chemical shift values belonging to H-2′ of hexyl substituents move to the most upfield; however,
such chemical shift values move from 0.48 to 0.85 ppm when pyridine-d5 or benzene-d6 is used as
solvent. Our dendrimers as the emissive layers in organic light-emitting diodes gave blue-green
light with an external quantum efficiency up to 0.16% both for G0 and for G1 in nitrogen,
respectively, which exhibit unique electroluminescence spectra in comparison with their corre-
sponding photoluminescence ones.
In tr od u ction
electroluminescent properties for applications in elec-
tronic and photoelectronic devices including organic light-
emitting diodes (OLEDs).2 One of the important chal-
lenging goals of dendrimer chemistry is to develop new
families of π-conjugated dendrimers with novel branches
and cores, to investigate their physical and chemical
properties, and to understand the structure-property
relationship within such structures.
The convergent approach to these dendritic molecules
provides greater structural control, a lower number of
reactions at each growth step, and more structural purity
and synthetic versatility than their divergent counter-
part.1h Recently, several synthetic strategies have been
used to construct extended π-conjugated dendrimers.3
However, such methodologies employ the repetition of
activation and coupling steps to access target molecules
iteratively, which not only requires a massive excess of
Triggered by their well-defined, unique macromolecu-
lar structures and fascinating properties, considerable
interest has been devoted recently to the design and
synthesis of novel dendrimers and star-shaped mol-
ecules.1 In particular, extended π-conjugated dendrimers
and star-shaped molecules with definite nanometer sizes
and intrinsic stiff nature have been proven to exhibit
interesting electrical, optical, nonlinear optical, and
† College of Chemistry and Molecular Engineering, Peking Univer-
sity.
‡ Peking University Healthy Science Centre.
§ South China University of Technology.
(1) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concepts, Synthesis, Perspectives; Wiley-VCH: Weinheim,
Germany, 2002. (b) Fre´chet, J . M. J .; Tomalia, D. A. Dendrimers and
Other Dendritic Polymers, Wiley: Chichester, U.K., 2001. (d) Den-
drimers V: Functional and Hyperbranched Building Blocks, Photo-
physical Properties, Applications in Materials and Life Sciences; Vo¨gtle,
F., Schalley, C. A., Eds.; Topics in Current Chemistry No. 228;
Springer-Verlag: Berlin, 2003. (d) Dendrimers IV: Metal Coordination,
Self-Assembly, Catalysis; Vo¨gtle, F., Schalley, C. A., Eds.; Topics in
Current Chemistry No. 217; Springer-Verlag: Berlin, 2001. (e) Den-
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drimers II: Architecture, Nanostructure and Supermolecular Chemistry;
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M. Macromol. Mater. Eng. 2002, 287, 442-461.
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Grim, P. C. M.; De Feyter, S.; Wiesler, U.-M.; Berresheim, A. J .; Mu¨llen,
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10.1021/jo049268p CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/13/2004
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J . Org. Chem. 2004, 69, 6050-6058

Study of Giant Extended π-Conjugated Dendrimers
CHART 1
one reactant but also inevitably generates partially
substituted byproducts, making the purification trouble-
some. Moreover, the preparation of giant dendrimers is
easily complicated by steric inhibition, resulting in
sharply decreased yields.1h The cyclotrimerization reac-
tions have provided a powerful strategy for the construc-
tion of a 1,3,5-trisubstitued benzene moiety or hexasub-
stituted (tris-annulated) benzenes,4 which have become
important building blocks for the synthesis of bowl-
shaped polycyclic aromatic hydrocarbons5 and the ratio-
nal synthesis of C60,6 starburst or star-shaped molecules,7
liquid crystals, and C3 tripods materials in asymmetric
catalysis and chiral recognition.8 The reaction involves
the “in-situ” formation of the central benzene moiety by
a triple aldolization and dehydration of 3 equiv of acetyl-
aromatic molecules catalyzed by acids. It exhibits unique
features including merely one reactant and one desired
product (1,3,5-trisubstitued benzene) along with only a
few of â-methylchalcone intermediate.
CHART 2
Due to their rigid planarized oligophenyl unit within
the backbone as well as the possibility of facial function-
alization at the methylene bridge, poly(9,9-dialkyl-
fluroene), oligo(9,9-dialkylfluroene), and ladder-poly(p-
phenylene) (LPPP) derivatives have been exhibited unique
chemical and physical properties and emerged as the
most promising light-emitting materials for organic light-
emitting diodes (OLEDs).9 Recently, we chose 10,15-
dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene (truxene) as
building block for novel π-conjugated star-shaped mol-
ecules and dendrimers (Chart 1).10 Three fluroene moi-
eties are annulated to afford a unique three-dimensional
symmetric truxene moiety by sharing a central benzene
ring (Chart 2). Investigation of their photophysical
properties would give insight into the effect of delocal-
ization through the benzene core in dendrimers and star-
shaped molecules based on polycyclic aromatics. The
synthesis and properties of star-shaped oligothiophenes-
functionalized truxene derivatives was recently reported.10a
These molecules represented oligothiophenes and were
interesting in that they provided a model of interchro-
mophore contacts not only in main-chain polychro-
mophores such as polythiophenes but also in star-shaped
π-conjugated molecules. However, a question of funda-
mental importance for dendrimers constructed by poly-
cyclic aromatics with C3 tripods concerns the extent of
delocalization for the inter-ring state, since it plays a key
role in determining the frequency and efficiency of
emission and because it reflects the ability of large
fragments to delocalize through the small core.
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Cao et al.
yieldb (%)
TABLE 2. Com p a r ison w ith Con d ition s of
Cyclotr im er iza tion s to Den d r im er G0
SCHEME 1. Cyclotr im er iza tion of Acetyl
Ar om a tics
entry
conditionsa
1
2
3
4
5
6
7
8
HOAc-HCl, 120 °C
NR
<1
23
SiCl4 (5 equiv), EtOH, reflux
SiCl4 (20 equiv), EtOH-Tol, reflux
TiCl4 (1.5 equiv), o-C6H4Cl2, 180 °C
TiCl4 (1.5 equiv), o-C6H4Cl2, 140 °C
SiCl4 (30 equiv), (HOC2H4)2O-Tol, 160 °C
SiCl4 (20 equiv), EtOH-Tol, Et3N, reflux
SiCl4 (30 equiv), EtOH-Tol, reflux
100c
NR
12
TABLE 1. Acid -P r om oted Cyclotr im er iza tion of Acetyl
Ar om a tics
d
91
a
b
d
24 h. After purification. c Conversion. A â-methylchalcone
dimer was formed.
a strategy to access functional organic materials (espe-
cially like polycyclic aromatic hydrocarbons assembly)
efficiently and conveniently. We deemed it constructive
to extend our studies to a larger moiety with a polycyclic
aromatic hydrocarbon framework. The heptacyclic pol-
yarene truxene moiety, by virtue of its unique three-
dimensional topology, is an attractive building motif for
use as a potential dendrimer building block. However,
the poor solubility and the large steric hindrance ren-
dered the synthesis of these types of dendrimers a
challenge. With the strategy presented above, we intro-
duced six hexyl groups onto the C-5, C-10, and C-15
positions of the truxene moiety, which greatly enhanced
the solubility and made the subsequent modification at
C-2, C-7, C-12 positions by aromatic electrophilic substi-
tution more facile.
However, the cyclotrimerizations of G0 met severe
difficulties due to the steric hindrance of the giant
truxene moiety at first, which naturally forced us to
further study the reaction. Table 2 summarizes the
results under the various conditions. Initially, we at-
tempted to afford G0 by literature procedures (entries 1
and 2).4 Although the addition of toluene could increase
the yields of cyclotrimerizations (entry 3),7a,8a it is far from
enough for some target molecules with giant substituted
aromatic groups since good intrinsic solubility is a
required criteria for both acquiring them and processing
them into organic photoelectronic devices.
However, it reminded us that the solubility was not
the most significant driving force to get across the steric
hindrance barrier. We noticed the TiCl4-dichlorobenzene
system developed by Scott,6 which turned out to be the
most powerful strategy to convert 2 into G0 molecule.
However, we failed to exclude the dark black impurities
from our products by all means of methods (entry 4). We
also found it to be a temperature-dependent reaction.
When the temperature decreased from 180 to 140 °C, the
yield dropped dramatically from quantitative to none,
and even the â-methylchalcone was not formed at this
temperature (entry 5). It seems that such TiCl4-dichlo-
robenzene system is more practical to those molecules
with poor solubility.
a
b
After recrystalization. After flash chromatography. c 5 equiv
of SiCl4.
ride, the solubility of starting materials, and the amount
of tetrachlorosilane (SiCl4) on the acid-promoted cyclo-
trimerizations. Based on these results, we have success-
fully synthesized some extended π-conjugated dendrimers
and star-shaped molecules with good yields, including the
largest one based on truxene containing up to nine
truxene moieties. Besides the standard characterization
by the elemental analysis data, MALDI-TOF/MS mea-
surement, and the absorption and emission spectra,10b,c
we employed H and 13C and 2D NMR (COSY, HMBC,
1
HSQC, and ROESY) techniques to characterize the
structure and the conformation of our dendrimers in
various solvents. The investigation of absorption and
emission properties of the dendrimers in THF solutions
and in the films indicated that the torsion angle between
the benzene ring and truxene skeleton might play a key
role on optical and physical properties. The dendrimers
were also fabricated as the emissive layers in OLEDs.
Resu lts a n d Discu ssion
Rea ction Con d ition s In vestiga tion . Our first strat-
egy was to investigate the effect of the intrinsic solubility
of all molecules on the reactions as shown in Scheme 1.
In Table 1, the poor solubility of reactants and products
resulted in relatively low yields, large amounts of starting
materials, and tedious purification procedures (entries
1-6). In contrast, the introduction of two hexyl groups
in the fluorene moiety not only greatly elevated the yield
of the desired product but also significantly facilitated
the isolation procedure (entry 7). Therefore, it provided
Entry 7 revealed that the concentration of hydrogen
chloride played a pivotal role in this reaction. No target
compound was obtained when Et3N was added; instead,
quite a lot of â-methylchalcone was formed under this
condition. Although the synthesis of â-methylchalcone
with small aromatic substituents by using 1 equiv of SiCl4
has been reported before,4d we developed an alternative
method to prepare such compounds with giant aromatic
substituents efficiently by both increasing the amount
6052 J . Org. Chem., Vol. 69, No. 18, 2004

Study of Giant Extended π-Conjugated Dendrimers
SCHEME 2. P r ep a r a tion of Den d r im er s G0 a n d G1
CHART 3. Nu m ber in g System Used for Discu ssion
of Ar om a tic F r a m ew or k a n d of Hexyl Sid e Ch a in sa
a
The carbon atoms and corresponding hydrogen atoms were
numbered the same, R ) n-C6H13
.
in the structure of some dendrimers, which could be used
to help establish the purity of dendrimers as well as to
provide an extremely related structural information.12 In
1
our previous report,10a we reported that H NMR spectra
of 1 and its oligothiophenes-functionalized derivatives in
CDCl3 did not exhibit any change with an increase of the
concentration, indicating that the stretching long sub-
stituents effectively reduced self-association by arene
π-π stacking.10d,e More surprisingly, the chemical shifts
belonging to methylene groups moved more upfield than
those of methyl groups, the values (about 0.5-0.6 ppm!)
were considerably lower than those of methyl and other
methylene groups. What is more, when we turned to a
more extended scope, we found this astonishing behavior
has actually also been existed in the extensively inves-
tigated 9,9-dialkylfluroene moiety.13 By contrast with the
extensive NMR investigation toward annulenes,14 these
perspective building blocks in materials science receive
fairly little attention in this aspect. To shed light on such
astonishing behavior and to better understand the aro-
maticity, the effective conjugation length, the conforma-
tion, and structure of dendrimers, we carried out a more
1
detailed investigation by employing H and 13C and 2D
of SiCl4 and addition of Et3N to eliminate the hydrogen
chloride in the system, which forced the reaction staying
in the â-methylchalcone intermediate. Finally, we varied
the amount of SiCl4 from 20 to 30 equiv (entry 8), and
the yields increased dramatically under this tendency.
Encouraged by such results, we decided to expand the
scope of this strategy to a more ambitious level. With
increasing the amount of SiCl4 to 100 equiv, we even
successfully prepared G1, a larger extended π-conjugated
dendrimer based on truxene, by repetitive Friedel-Crafts
acetylation and cyclotrimerizations (Scheme 2).10b Con-
sidering the rigid nature and severe steric hindrance of
G1, the yield (72%) was quite remarkable after flash
column chromatography. The transformation from acetyl
aromatics to 1,3,5-trisubstituted benzenes was relatively
clean, thus rendering the purification of the desired
products quite simple by column chromatography. All
NMR techniques. For the purpose of discussion, the
carbon atoms and the corresponding hydrogen atoms
were numbered the same. The aromatic framework
(truxene, fluorene, and benzene moieties) and hexyl side
chains were labeled in two numbering systems, starting
from 1 and 1′, respectively (as shown in Chart 3).
The truxene and fluorene moieties (1 and 6) were
served as models to understand the structure of the
dendrimers (Figure 1). It is obvious that chemical shift
values belonging to protons of one methylene group for
both 1 and 6 moved more upfield. Distinct difference in
resonance signals of H-1′ was also observed for these two
compounds: only one group of multiplet peaks appeared
(11) (a) Hawker, C. J . Advances in Polymer Science 147; Springer-
Verlag: Berlin, Heidelberg, Germany, 1999; pp 114-160. (b) Walker,
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Spectrom 1994, 5, 731-739. (c) Leon, J . W.; Fre´chet, J . M. J . Polym.
Bull. 1995, 35, 449-455.
(12) Hawker, C. J .; Fre´chet, J . M. J .J . Am. Chem. Soc. 1990, 112,
7638-7647.
1
compounds have been fully characterized by H and 13C
NMR, elemental analyses, and MALDI-TOF MS.10b,c
NMR Beh a vior s a n d Molecu la r Mod elin g. The
monodispersity and high symmetry associated with den-
drimers have presented numbers of interesting charac-
terization traits and obtained an unprecedented amount
of the structural information utilizing advanced charac-
terization techniques.11 1H and 13C NMR spectra allowed
the identification of every unique proton or carbon atom
(13) (a) Ranger, M.; Leclerc, M. Macromolecules 1999, 32, 3306-
3313. (b) Destri, S.; Pasini, M.; Botta, C.; Porzio, W.; Bertini, F.;
Marchio`, L. J . Mater. Chem. 2002, 12, 924-933. (c) Alder, R. W.;
Anderson, K. R.; Benjes, P. A.; Burts, C. P.; Koutentis, P. A.; Orpen,
A. G. J . Chem. Soc., Chem. Commun. 1998, 309-310. (d) Ranger, M.;
Be´langer-Garie´py, F.; Leclerc, M. Acta Crystallogr., Part C 1998, C54,
799-805.
(14) Mitchell, R. H. Chem. Rev. 2001, 101, 1301-1315.
J . Org. Chem, Vol. 69, No. 18, 2004 6053

Cao et al.
F IGURE 1. 1H NMR spectra of (A) 6, (B) 1, (C) G0, and (D) G1 at aliphatic regions in CDCl3.
at 1.95 ppm for 6; however, two groups of multiplet peaks
exhibited at about 2.97 and 2.05 ppm for 1, respectively.
All chemical shift values of H2′-6′ in 1 moved to upfield
in comparison with those of 6. Meantime, as in G0 and
G1, the corresponding values continued slightly moving
to downfield and the peaks became broad with the
increase of the generation of dendrimers. 2D 1H-1H
COSY, 1H-13C HSQC, 1H-1H ROESY, and 1H-13C
HMBC results also demonstrated the same results (see
the Supporting Information).
ing regions are inside or outside the anisotropic cones
induced by the intramolecular ring currents in our
aromatic frameworks, either the truxene moiety or the
fluorene moiety. The shielding effect vanishes with
increasing distance from the ring along the molecular
axis. As shown in Figure 2, all hexyl substituents in 1
and 6 are almost perpendicularly attached to the aro-
matic ring plane, and the H-2′ suffered the most severe
upfield shift because they were situated in the shielding
cone of the aromatic ring. Since the shielding effect
decreased with increasing distance from the ring along
the molecular axis, the H-4′ signals shifted downfield in
comparison with those of H-2′. Likewise, due to the
shielding effect difference between their locations, the
H-2′, H-4′, and H-6′ were more shielded than correspond-
ing H-1′, H-3′, and H-5′, respectively. The different H-1′
signals of 1 and 6 could be ascribed to the different
chemical environment surrounding them.
To better elucidate these interesting behaviors, the
energy-minimized structures of 1 and 6 have been
obtained employing the MM2 (MM+) force field optimi-
zation, as implemented in HyperChem Pro. 7.0 (Hyper-
cube Inc.) (as shown in Figure 2). Due to the prohibitively
large number of atoms in the subsequent dendrimers, a
force field method was favored rather than quantum
mechanics methods. The conformation of the resulting
fluorene moiety (6) is in good agreement with that of the
reported crystal structure analysis, in which the all-anti
alkyl side chains in 9,9-dialkylfluorene are orthogonally
stretched to the fluorene ring plane.13b-d As a result, it
would be safely to assume that the calculated structures
of its three-direction analogue truxene moiety (1) and
dendrimers (G0 and G1) are as good an approximation
as the fluorene model is. The substituted truxene moiety
displayed similar side chains conformation as the fluo-
rene segment from the view of the lowest-energy struc-
ture calculation. The optimized structure of 1 was also
employed to build up its dendrimer derivatives G0, and
then the energy-minimized structure of G0 was employed
to build up larger dendrimer G1. After rotation around
the three bonds connecting the truxene moiety (or the
G0 subunit) and central benzene, several conformations
were obtained. The conformations with lowest energy for
the dendrimers G0 and G1 were shown in Figure 3.
The above1H NMR behaviors could be interpreted by
traditional ring current model based on the “Pauling-
Lonsdale-London π model”.15 The shielding and deshield-
With these data in hand, to further understand the
relationship between the NMR and the structure and also
to investigate the effect of the hexyl substituents on the
conformation of the whole molecule, we investigated the
1H and 13C NMR behaviors by varying the solvents. When
the solvent was changed from CDCl3 to pyridine-d5 or
benzene-d6, we observed that chemical shift values of all
protons at hexyl substituents moved to downfield, espe-
cially abnormal chemical shift ones of H-2′ moved to the
downfield from 0.48 to 0.75 ppm, and chemical shift ones
of methyl groups also moved about 0.05 ppm. Figure 7
1
presents the comparison of the H NMR spectra of 1 in
different solvents (as shown in Figure 4). The similar
1
behavior was also observed for the H NMR spectra of 6
(see the Supporting Information).
Although the polar CDCl3 molecule can have electro-
static interactions with a polar substrate, it can only
interact with nonpolar molecules 1 or 6 through van der
Waals forces and had a minimal effect on the chemical
shift value. On the other hand, the disklike benzene or
pyridine molecule possessed large diamagnetic anisotro-
py, hence resulting in an upfield shift (decrease of
chemical shift value). We assumed that the disklike
(15) Gomes, J . A. N. F.; Mallion, R. B. Chem. Rev. 2001, 101, 1349-
1384.
6054 J . Org. Chem., Vol. 69, No. 18, 2004

Study of Giant Extended π-Conjugated Dendrimers
F IGURE 2. Lowest energy calculated structures of the front (a) and side view (b) for 1 and of the front view for 6 (c).
F IGURE 3. Lowest energy calculated structure for G0 (a) and G1 (b).
aromatic solvent molecules interacted with the substrate
by π-π stacking. As a consequence, the H-2′ and some
other protons were exposed to the deshielding regions of
the solvent molecules, which would attenuate the shield-
ing effect from the substrate aromatic frameworks. It is
well-known that the accidentally overlapping resonance
J . Org. Chem, Vol. 69, No. 18, 2004 6055

Cao et al.
F IGURE 4. 1H NMR spectrum of 1 (lower) in (A) CDCl3, (B) pyridine-d5, and (C) benzene-d6 solvent.
F IGURE 5. Absorption (left) and emission (right) spectra for dendrimers in THF solutions.
of chemically different nuclei is usually separated by
using a different solvent.
Op tica l P r op er ties. The absorption spectra for 1, G0,
and G1 in THF solutions are shown in Figure 5.
Normally, π-conjugated dendrimers show a very strong
π-π* electron absorption band in the UV-vis region,
which progressively red-shifted with increase of effective
conjugation length. From absorption spectra in Figure
5, we observed that absorption λmax (309 nm for G0 and
310 nm for G1, respectively) exhibited a small red-shift
with the increase of the truxene moiety and of the
generation of the dendrimer in comparison with that of
1 (307 nm) by virtue of the large molar extinction
coefficients (ꢀ > 105 M-1 cm-1). However, absorption
spectra showed very strong shoulder peaks at 320 nm
for G0 and 330 nm for G1, respectively, while the
intensity of such peak of G1 was higher than that of G0.
We also observed that onsets of absorption spectra were
obviously red-shifted from 1 to G1. The absorption λmax
for these compounds might belong to the π-π* electron
absorption band of the truxene skeleton, and we did not
observe the very obvious π-π* delocalization with the
increase of the truxene moiety, which was owing to the
large torsion angle between the benzene ring and the
truxene system caused by three giant truxene skeletons
at one smaller benzene ring. However, the intensity of
shoulder peaks augmented from G0 to G1, which sug-
gested that the relative delocalization might be promoted
with an increase of the generation of dendrimers.
F IGURE 6. Absorption and photoluminescence spectra of G0
and G1 in solid states.
The optical absorptions of 1, G0, and G1 in the solid
states are shown in Figure 6. The structured absorption
bands observed in solution were also exhibited in the
solid states. However, absorption peaks on the solid state
were slightly red-shifted to their corresponding ones in
solution. The similarity of absorption spectra shape on
the solution and the solid state in these molecules
suggested that there was a slight increase in conjugation
length in the solid state, which indicated that the
π-stacking in solid state was significantly reduced due
to the giant truxene moiety and also because of the large
torsion angle between the truxene group and the phenyl
group. The intensity of shoulder peaks augmented from
6056 J . Org. Chem., Vol. 69, No. 18, 2004

Study of Giant Extended π-Conjugated Dendrimers
F IGURE 7. Current-voltage (a) and current-luminance characteristics of OLEDs with the following configuration: ITO/PEDOT/
dendrimer G0 or G1/Ba/Al.
F IGURE 8. Comparison of external EL quantum efficiency
of dendrimers G0 and G1.
F IGURE 9. EL spectra of dendrimers G0 and G1.
G0 to G1, which suggested that the relative delocaliza-
tion might be promoted with an increase of the genera-
tion of dendrimers.
and 0.16% at 13 V for G1, respectively. The EL spectra
are also illustrated in Figure 9. In comparison with PL
spectra in solid states (in Figure 6), it is obvious that EL
emission is significantly red-shifted compared to that of
The photoluminescence (PL) spectra of 1, G0, and G1
in dilute solution (5 × 10-5 M) are also shown in Figure
PL spectra due to the formation of ketonic defects in the
5. All compounds have structured emission peaks. If the
device processing.16 The further improvement of the LED
0-0 transitions in the emission and corresponding
efficiency of these dendrimers can be achieved by modi-
absorption peaks were considered, the Stoke shift is small
fication of dendrimer/electrode interface in order to
for these three compounds. In dilute solutions, no emis-
enhance and balance the electron and hole injection. The
corresponding studies are in progress.
sion light was observed because the emission light of
these three compounds was in the UV range. The PL
spectra of 1, G0, and G1 in solid states are shown in
Figure 6. In addition of their broad featureless charac-
teristics, the emission spectra of these three compounds
in the solid states slightly red-shifted from the dilute
solution ones, which also indicated the decreased π-stack-
ing in these molecules.
Con clu sion s
In conclusion, we have demonstrated a convenient and
highly efficient convergent approach for the synthesis of
a new family of large-size and precisely well-defined
dendrimers and star-shaped molecules via “in-situ” gen-
eration of benzene core from aryl methyl ketones by acid-
promoted cyclotrimerization reactions. We also have
investigated their photophysical and electroluminescence.
We have developed a platform to synthesize a new family
of dendrimers utilizing giant polycyclic aromatics, further
investigation toward the formation of hexasubstituted
(briannulated) benzenoids using this strategy to create
novel nonalternant polycyclic aromatic hydrocarbons
containing alternating five- and six-membered rings are
in progress. The unique NMR behaviors have exhibited
Or ga n ic Ligh t-Em ittin g P r op er ties. Some conju-
gated dendrimers have been used successively as charge-
transporting layer in OLEDs due to their good charge-
transporting characteristics and the fact that they can
form stable amorphous films. We fabricated organic light-
emitting diodes (OLEDs) from G0 and G1 as the emissive
layer. Preliminary results on intrinsic electrolumines-
cence (EL) of these dendrimers is first presented and
discussed. Device configuration was: ITO/PEDOT/G0 or
G1/Ba/Al. Both the dendrimers emitted blue-green color
at a forward bias (ITO wired positively) above 8 V. A
typical curve of light intensity versus bias (L-V) is given
in Figure 7. The external quantum efficiencies for G1 and
G2 are compared in Figure 8. The external quantum
efficiency (QE) was measure to be 0.16% at 27 V for G0
(16) (a) List, E. J . W.; Guentner, R.; de Freitas, P. S.; Scherf, U.
Adv. Mater. 2002, 14, 374-378. (b) Gong, X.; Iyer, P. K.; Moses, D.;
Bazan, G. C.; Heeger, A. J .; Xiao, S. S. Adv. Funct. Mater. 2003, 325-
330.
J . Org. Chem, Vol. 69, No. 18, 2004 6057

Cao et al.
that the chemical shifts of the H-2′ of hexyl substituents
move to the most upfield (0.48 ppm). The lowest energy
calculations of the structure have proved that this
methylene group is on the top of the shielding cone of
polycyclic aromatic rings. The photoproperties of such
dendrimers indicate that the large torsion angle between
the giant truxene moiety and the benzene ring might
affect full π-delocalization of the whole molecule. We have
developed a platform to synthesize a new family of
dendrimers utilizing giant polycyclic aromatics and to
understand photophysical and electroluminescence prop-
erties of polycyclic dendrimers through the investigation
of such dendrimers. The basic investigations with these
macromolecules would provide novel insights and direc-
tions for the uses of novel extend π-conjugated dendrim-
ers based on truxene in organic materials science. We
expect that further investigation could also be devoted
to the synthesis and characterization of nonalternating
polycyclic aromatics.
to improve the contact angle just before film coating. Onto
the ITO glass was spin-coated a layer of polyethylene-
dioxythiophene-polystyrene sulfonic acid (PEDOT:PSS)
film with thickness of 50 nm from its aqueous dispersion,
aiming to improve the hole injection and to avoid the
possibility of leakage. PEDOT:PSS film was dried at 80
°C for 2 h in the vacuum oven. The solution of dendrimer
in p-xylene was prepared in a nitrogen-filled drybox and
spin-coated on top of the ITO/PEDOT:PSS surface. Typi-
cal thickness of the emitting layer was 50-80 nm. Then
a thin layer of barium as an electron injection cathode
and the subsequent 200 nm thick aluminum protection
layers were thermally deposited by vacuum evaporation
through a mask at a base pressure below 2 × 10-4 Pa.
The deposition speed and the thickness of the barium and
aluminum layers were monitored by a thickness/rate
meter. The cathode area defines the active area of the
device. The typical active area of the devices in this study
is 0.15 cm2. The EL layer spin coating process and the
device performance tests were carried out within a
glovebox with nitrogen circulation. The luminance of the
device was measured with a calibrated photodiode.
External quantum efficiency was verified by the mea-
surement of the integrating sphere, and luminance was
calibrated after the encapsulation of devices with UV-
curing epoxy and thin cover glass.
Exp er im en ta l Section
Ba sic Ch a r a cter iza tion . The NMR spectra were
recorded at 298 K on a spectrometer operating at
frequencies of 499.89 MHz (1H) and 125.71 MHz (13C) in
CDCl3, pyridine-d5, and benzene-d6 with concentrations
of ∼15 mg per 600 µL of solvent. Chemical shifts δ (ppm)
were referenced to internal Me4Si (TMS) for 1H and
solvent peaks for 13C. Two-dimensional gradient-selected
COSY (homonuclear correlated spectroscopy), HSQC
(heteronuclear single quantum coherence), HMBC (het-
eronuclear multiple bond coherence), and ROESY (rota-
tional NOE spectroscopy) experiments were performed
using standard pulse sequences.
Ack n ow led gm en t. This work was supported by the
Major State Basic Research Development Program (Nos.
2002CB613401 and 2002AA324080) from the Minister
of Science and Technology and by the National Natural
Science Foundation of China (NSFC 90201021 and
50103001).
Device F a br ica tion a n d Ch a r a cter iza tion . LED
was fabricated on pre-patterned indium-tin oxide (ITO)
with sheet resistance 10-20Ω/0. The substrate was
ultrasonic cleaned with acetone, detergent, deionized
water, and 2-propanol. Oxygen plasma treatment was
made for 10 min as the final step of substrate cleaning
Su p p or tin g In for m a tion Ava ila ble: Complete experi-
mental procedures; full characterization of all new compounds;
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O049268P
6058 J . Org. Chem., Vol. 69, No. 18, 2004