Synthesis and luminescent properties of perylene bisimide-cored dendrimers with carbazole surface groups

Article abstract of DOI:10.1016/j.polymer.2011.06.019

A group of perylene bisimide (PBI) cored dendrimers were designed and synthesized. The polyphenylene dendrons containing carbazole functional groups at periphery were attached to the PBI core with expectation to control the intermolecular interaction and to tune the charge transporting ability of dendrimers. Their photophysical, electrochemical and thermal properties were investigated. The spectral data showed that energy transfer and photoinduced charge transfer coexist between the carbazole peripheries and the PBI core, competing to influence the luminescent properties of these dendrimers. The red OLEDs were fabricated with these dendrimers as non-doped emitting layer by solution method. The dendrimers bearing carbazoles exhibited improved EL performance than those model compounds. The improved charge balance state caused by these carbazole units is suggested to be responsible for the EL performance.

Full text of DOI:10.1016/j.polymer.2011.06.019

Polymer 52 (2011) 3639e3646
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and luminescent properties of perylene bisimide-cored
dendrimers with carbazole surface groups
,
Huicai Ren a, Jiuyan Li b, Renjie Wang b, Ting Zhang b, Zhanxian Gao a, Di Liu a
*
a School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
b State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A group of perylene bisimide (PBI) cored dendrimers were designed and synthesized. The polyphenylene
dendrons containing carbazole functional groups at periphery were attached to the PBI core with
expectation to control the intermolecular interaction and to tune the charge transporting ability of
dendrimers. Their photophysical, electrochemical and thermal properties were investigated. The spectral
data showed that energy transfer and photoinduced charge transfer coexist between the carbazole
peripheries and the PBI core, competing to influence the luminescent properties of these dendrimers.
The red OLEDs were fabricated with these dendrimers as non-doped emitting layer by solution method.
The dendrimers bearing carbazoles exhibited improved EL performance than those model compounds.
The improved charge balance state caused by these carbazole units is suggested to be responsible for the
EL performance.
Received 2 December 2010
Received in revised form
9 June 2011
Accepted 9 June 2011
Available online 16 June 2011
Keywords:
Perylene bisimide
Dendrimer
Carbazole
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
emissive core [5]. Dendrimers built in this way can offer good
solubility and relatively high viscosity in common organic solvents
Perylene-3,4,9,10-tetracarboxylic acid bisimides (PBIs) as
a typical n-type semiconductors have found use in a wide range of
applications including organic-lighting emitting diodes (OLEDs)
[1], photovoltaic devices [2], field effect transistors [3] and other
organic electronic devices owing to their outstanding optical,
thermal, chemical properties and high fluorescence quantum yields
[4]. In OLEDs, however, the inherent planar conformation of PBIs
always makes them suffer from two problems which reduce their
emission efficiency and color purity. One is the poor solubility
which makes the synthesis and purification difficult. In addition,
the preparation of thin film layers has to be carried out by vapor
deposition which goes against large-scale production of large area
products due to the limited dimensions of vacuum chamber and its
which make them suitable for wet methods such as spin-coating
and ink jet printing to prepare films and devices. In addition, the
chromophores can be separated from the surrounding medium,
thus preventing
pep stacking and fluorescence quenching effi-
ciently [6]. The extent of the site-isolation effect can be tuned by
different generation of the dendrons. Furthermore, various func-
tional surface groups can be selected to generate the interesting
properties such as carrier-transporting, and energy antenna effects
[7]. And PBIs can be a favorite candidate as a dendrimer building
block since functional groups can be flexibly introduced to either
the terminal imide moiety or the bay positions of perylene ring to
construct multifunctional molecules [8]. Müllen’s group has
developed a series of PBI derivatives including PBI-cored light-
emitting dendrimers containing polyphenylene dendrons [9]. In
our previous report [10] we have prepared a PBI-cored first-
generation dendrimer with fluorinated shell (PBI-F) and demon-
strated that it can be used as multifunctional material in opto-
electronic devices.
In this paper we designed and prepared a series of novel
solution processible PBI-cored dendrimers (G1-Cz and G2-Cz in
Scheme 1) bearing the polyphenylene dendritic wedges in the bay
region of perylene ring and the specific functional groups of
carbazole at the molecular surface. PBI with 2,6-diisopropyl phenyl
groups appended at the terminal imide was designed as the
higher cost. The other one is self-quenching caused by
pep
stacking between the chromophores which results in lower device
efficiency and device degradation.
Up to date, an effective way to solve both these problems
simultaneously is to build dendrimers with highly branched
architectures. Light-emitting dendrimers are usually built by
attaching bulky dendrons (with or without surface groups) to the
* Corresponding author. Tel./fax: þ86 411 84986233.
E-mail address: liudi@dlut.edu.cn (D. Liu).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.06.019

3640
H. Ren et al. / Polymer 52 (2011) 3639e3646
R
R
R
R
N
N
O
O
O
O
R
R
R
O
O
O
O
R
N
N
O
O
O
O
O
O
O
O
G0
R
R
R
R
R
R
N
N
O
O
R
R
O
O
O
O
R
R
R
R
O
O
R
R
G2: R=H
G2-Cz: R=Cz
R
R
G1
: R=H
G1-Cz
Cz =
: R=Cz
N
Scheme 1. Chemical structures of the dendrimers in present study.
luminescent core due to its high fluorescent quantum yield of red
emission and excellent chemical and thermal stability [4]. Bulky
polyphenylene dendrons were introduced into the bay positions of
PBI in order to construct a non-planar molecule and to provide
a perfect site-isolation effect on the emissive core [9]. 3,6-Di-tert-
butylcarbazole units were grafted to the dendrimer periphery with
the aim to function as hole-transporting (HT) moieties to tune the
carrier-transporting balance property when the target dendrimers
are used as emitting layer in OLEDs. Two generations of dendrimers
(G1-Cz and G2-Cz) were synthesized via continuously cycloaddi-
tion by a divergent way. For comparison, two reference dendrimers
without the carbazolyl substituents (G1 and G2) and a corre-
sponding small molecular parent compound PBI (also called the
zeroth-generation, G0) were also prepared (shown in Scheme 1).
The effect of both the dendrons and peripheral units on the lumi-
nescent properties of the dendrimer molecules was monitored by
means of photophysical techniques including UV absorption and
fluorescence spectra. The electrochemical and thermal properties
of these dendrimers were studied. They were also used as non-
doped emitting layer to fabricate OLEDs by solution method and
their electroluminescent properties were investigated.
(Micromass) Mass Spectrometer for MALDIeTOFeMS. Thermog-
ravimetric analyses (TGA) and differential scanning calorimetry
(DSC) measurements were carried out using a PerkineElmer ther-
mogravimeter (Model TGA7) and a PerkineElmer DSC 7 at a heat-
ing rate of 10 ꢀC minꢁ1 under a nitrogen atmosphere, respectively.
The fluorescence and absorption spectra were measured on a Per-
kineElmer LS55 fluorescence spectrometer and a PerkineElmer
Lambda 35 UVeVis spectraphotometer, respectively. For the solu-
tion spectra, dendrimers were dissolved in dichloromethane (ca.
10ꢁ5 M) and then put in a quartz cell for measurement. For the thin
film spectra, dendrimers were first dissolved in CH2Cl2 and spin-
coated onto quartz substrate. Cyclic voltammetry (CV) measure-
ments were performed using a conventional three-electrode cell
with a glassy carbon working electrode, a platinum wire counter
electrode, and a Ag/AgCl reference electrode on a computer-
controlled BAS 100W electrochemical analyzer at room tempera-
ture. The dendrimers were dissolved in dichloromethane solution
containing Bu4NPF6 (0.1 M) as a supporting electrolyte. The redox
potentials were obtained at the scan rate of 100 mV/s, and with
ferrocene/ferrocenium (Fc/Fcþ) as an internal standard of the redox
system. The HOMO and LUMO energy levels of the studied den-
ox
drimers were determined from the onset oxidation (E
) and
onset
red
reduction (E
) potential, based on the reference energy level of
2. Experimental
onset
SCE (4.4 eV below the vacuum level), according to the following
equations [11]:
2.1. Materials
À
Á
All the reagents are of analytical grade and used as received
from commercial sources without further purification. PEDOT:PSS
was purchased from Bayer AG. The solvents used in the reactions
were purchased from Aldrich and were treated to be anhydrous
using the standard methods.
ox
HOMO ðeVÞ ¼ ꢁe E
þ 4:4 V
onset
ꢀ
ꢁ
red
LUMO ðeVÞ ¼ ꢁe E
þ 4:4 V
onset
2.3. OLEDs fabrication and measurements
2.2. Characterization and measurements
The light-emitting devices have a configuration of ITO/PEDOT:PSS
(40 nm)/dendrimer (40 nm)/BCP (15 nm)/Alq3 (20 nm)/LiF (1 nm)/Al
(100 nm). The patterned ITO substrates were cleaned by successive
ultrasonications in detergent, deionized water, ethanol, and chloro-
form, followed by treatment with UV-ozone. PEDOT:PSS (Bayer AG)
was spin-coated onpretreated ITO substrate from aqueous dispersion
1H NMR and 13C NMR spectra were recorded on a Varian INOVA
spectrometer (400 MHz), chemical shifts were referenced to TMS.
Mass spectra were recorded on a Micromass Q-Tof (Micromass,
Wythenshawe, UK) mass spectrometer for ESI-MS and a Micro MX

H. Ren et al. / Polymer 52 (2011) 3639e3646
3641
and baked at 120 ꢀC for 1 h. Subsequently dendrimer layer was spin-
coatedonPEDOT:PSS filmfrom chlorobenzenesolution, thethickness
of which was controlled as 40 nm by adjusting the solution concen-
tration and the spin rate. The substrate was transferred into a vacuum
chamber to deposit the BCP/Alq3 layers. Finally a thin layer of LiF
(1 nm) and 100 nm of Al was vacuum deposited on top of organic
layers as cathode. The emitting area of each pixel is determined by
overlapping of the two electrodes as 9 mm2. The EL spectra, CIE
coordinates, and currentevoltageeluminance characteristics were
measured with computer-controlled Spectrascan PR 705 photometer
and a Keithley 236 source-measure-unit. All the measurements were
carried out at room temperature under ambient conditions.
under reduced pressure, and the crude product was purified by
column chromatography on silica gel to afford 2.2 g (55% in yield) 3
as pale yellow solid.
1H NMR (400 MHz, CDCl3)
d
(ppm): 8.28 (d, J ¼ 8.4 Hz, 4H), 8.14
(s, 4H), 7.80 (d, J ¼ 8.4 Hz, 4H), 7.50 (s, 8H),1.47 (s, 36H). ESIeMS (m/
z): calcd for C54H56N2O2, 764.4; found, 764.5 ([M]þ).
2.4.6. Synthesis of 3,4-bis[4,4’-Di(3,6-di-tert-butylcarbazol-9-yl)
phenyl]2,5-diphenyl-cyclopenta-2,4-di-enone (Cp-Cz)
A 25 ml round-bottom flask was charged with compound 3
(800 mg, 1.05 mmol), 1,3-diphenylacetone (219.6 mg, 1.05 mmol),
anhydrous KOH (58.8 mg, 1.16 mmol) and absolute ethyl alcohol
(10 ml). The mixture was stirring for 30 min under reflux. Remov-
ement of the solvent under reduced pressure gave the crude
product, purification of which by column chromatography on silica
gel yielded pure Cp-Cz (550 mg, 56%) as a brown solid.
2.4. Synthesis of intermediates and dendrimers
2.4.1. Synthesis of compounds G0, G1 and G2
The small molecular parent compound (G0) [10] and the two
reference dendrimers (G1 and G2) [9] were prepared according to
literature methods.
1H NMR (400 MHz, CDCl3)
4H), 7.42 (d, J ¼ 8.8 Hz, 4H), 7.36 (m, 14H), 7.23 (d, J ¼ 8 Hz, 4H), 1.47
(s, 36H). ESIeMS (m/z): calcd for C69H66N2O, 939.2751; found,
939.4039 ([M]þ).
d
(ppm): 8.13 (s, 4H), 7.49 (d, J ¼ 8 Hz,
2.4.2. Synthesis of G1-core and G2-core
The synthesis of these key intermediates G1-core and G2-core
followed the literature methods [9]. They were obtained as red
powders.
2.4.7. Synthesis of dendrimer G1-Cz
A mixture of G1-core (150 mg, 0.13 mmol) and Cp-Cz (714 mg,
0.76 mmol) in o-xylene (15 ml) was stirred at 140 ꢀC for 12 h under
nitrogen. The solvent was removed under reduced pressure. The
crude product was purified by column chromatography on silica gel
and then recrystallization to afford dendrimer G1-Cz (400 mg, 65%)
as a deep red powder.
G1-core: 1H NMR (400 MHz, CD2Cl2)
d (ppm): 8.13 (s, 4H), 7.54
(d, 8H), 7.39 (t, 2H), 7.24 (d, 8H), 3.54 (s, 4H), 2.59 (m, 4H), 1.02 (d,
24H). ESIeMS (m/z): calcd for C80H58N2O8, 1175.33; found, 1176
(100%) [M þ H]þ.
G2-core: 1H NMR (400 MHz, CD2Cl2)
d
(ppm): 8.13 (s, 4H), 7.58
1H NMR (400 MHz, CD2Cl2)
(s, 8H), 7.69 (s, 4H), 7.45 (2H, t), 7.30e6.95 (m,120H), 2.7 (m, 4H),1.3
(s, 144H), 1.1 (d, 24H). 13C NMR (400 MHz, CDCl3)
(ppm): 163.2,
d (ppm): 8.17 (s, 4H), 8.0 (s, 8H), 7.98
(s, 4H), 7.93 (t, J ¼ 7.8 Hz, 2H), 7.32 (d, J ¼ 8.5 Hz, 4H), 7.14e6.65 (m,
88H), 3.03 (s, 4H), 3.01 (s, 4H), 2.68 (m, 4H), 1.09 (d, 24H);
MALDIeTOFeMS (m/z): calcd for C208H138N2O8, 2791.0453; found,
2792.2114 (100%) [M þ H]þ.
d
155.8, 154.1, 145.7, 142.6, 141.3, 141.2, 141.1, 140.2, 139.7, 139.4, 139.3,
139.1,138.9,138.8,137.9, 135.8,135.5,132.9, 132.6, 131.7,130.7,130.5,
130.1, 127.9, 127.7, 127.5, 126.8, 126.2, 125.6, 125.4, 124.0, 123.5,
123.2, 123.1, 120.8, 119.3, 116.1, 108.9, 34.6, 32.0, 24.1.
MALDIeTOFeMS: (m/z): calcd for C352H322N10O8, 4820.3853;
found, 4821.3022 ([M þ H]þ).
2.4.3. Synthesis of 4, 40-dibromobenzoin (1)
A 250 ml round-bottom flask was charged with VB1 (5 g,
20 mmol) dissolved in distilled water (5 ml) and ethanol (100 ml).
The mixture was cooled by an ice-salt bath. When the temperature
was below zero, a 10% NaOH solution in distilled water was added
dropwisely until the pH reached to 9. Then 4-bromobenzaldehyde
(20 g, 108 mmol) was added, and the mixture was kept at 65 ꢀC and
stirred for 12 h. The solution was cooled and the precipitate was
filtered off, washed with water and ethanol. The crude product was
a mixture of benzoin and benzil. Crystallization from ethanol gave
pure compound 1 (11 g, 55%) as white crystals. mp: 92e93 ꢀC (lit.:
94e96.5 ꢀC) [12].
2.4.8. Synthesis of dendrimer G2-Cz
A mixture of G2-core (300 mg, 0.1 mmol) and Cp-Cz (966 mg,
1 mmol) in o-xylene (30 ml) was stirred at 140 ꢀC for 48 h under
nitrogen. The solvent was removed under reduced pressure. The
crude product was purified by column chromatography on silica gel
and then recrystallization to afford pure dendrimer G2-Cz (756 mg,
70%) as a red powder.
1H NMR (400 MHz, CD2Cl2)
32H), 6.6e7.54 (m, 318H), 2.7 (m, 4H), 1.3 (s, 288H), 1.1 (d, 24H). 13C
NMR (400 MHz, CDCl3) (ppm): 163.2, 155.9, 153.9, 145.7, 142.6,
d (ppm): 8.10 (s, 4H), 7.97e8.0 (s,
2.4.4. Synthesis of 4, 40-dibromobenzil (2)
d
A solution of compound 1 (500 mg, 1.35 mmol), NH4NO3
(135 mg, 1.69 mmol), and Cu(OAc)2ꢂH2O (31.4 mg, 0.17 mmol) in
acetic acid (5.5 ml) was stirred and refluxed for 2 h. Upon cooling to
room temperature, a precipitate was formed. Filtration followed by
washing with water and ethanol gave pure 8 (409 mg, 84.5%) as
yellow crystal. mp: 230e232 ꢀC (lit.: 228e229 ꢀC) [13] IR (KBr):
3090 (]CeH stretch), 1664 (C]O stretch), 1586 (C]C stretch). 1H
141.7, 141.4, 141.3, 141.1, 140.9, 139.8, 139.5, 139.3, 139.2, 139.0, 138.9,
138.6, 138.2, 138.0, 137.8,135.7, 135.4,132.9, 132.6, 131.8, 131.6,131.3,
130.7, 130.6, 130.1, 129.9, 129.7, 129.1, 128.9, 128.6, 128.3, 128.2,
127.8, 127.7, 127.2, 126.7, 126.4, 126.0, 125.6, 125.3, 124.0, 123.5,
123.2, 123.0, 120.8, 120.4, 119.3, 116.1, 109.0, 34.7, 32.0, 24.1.
MALDIeTOFeMS: (m/z): calcd for C752H666N18O8, 10,083.4502;
found, 10,083.0430 ([M]þ).
NMR (400 MHz, CDCl3)
d
(ppm): 7.83 (d, J ¼ 8.4 Hz, 4H), 7.67 (d,
J ¼ 8.4 Hz, 4H). ESIeMS (m/z): calcd for C14H8Br2O2, 368.02; found,
3. Results and discussion
368.0 ([M]þ).
3.1. Synthesis
2.4.5. Synthesis of 4, 40-di(3,6-di-tert-butylcarbazol-9-yl) benzil (3)
A solution of compound 2 (2 g, 5.4 mmol), 3,6-Di-tert-carbazole
(3.34 g, 12 mmol), CuI (207 mg, 1.1 mmol), anhydrous K2CO3
(2.25 g, 16 mmol) and 18-crown-6 (537 mg, 2 mmol) in nitroben-
zene (35 ml) was stirred and refluxed for 5 h under nitrogen. Upon
cooling, the precipitate was filtered. The solvent was removed
The polyphenylene dendrons of the studied dendrimers were
constructed by DielseAlder cycloaddition of tetraphenyl-cyclo-
pentadieneone to the terminal ethynyl groups at the periphery of
the PBI core. The traditional tetraphenyl-cyclopentadieneone
without any substituents (Cp) used for the synthesis of dendrimers

3642
H. Ren et al. / Polymer 52 (2011) 3639e3646
G1 and G2 is commercial available or synthesized according to the
literature method. The tetraphenyl-cyclopentadieneone with
carbazole moieties for the synthesis of novel dendrimers G1-Cz and
G2-Cz, Cp-Cz, was synthesized by a four-step reaction procedure as
shown in Scheme 2. The first intermediate compound 1 was
prepared by benzoin condensation with 4-bromobenzaldehyde as
the starting material in the presence of vitamin B1 and NaOH in
ethanol. It was found that when the pH was higher than 9 or the
temperature was higher than 65 ꢀC, the product yield was
decreased or even no product was observed. This indicates that the
pH and temperature are two key factors for the reaction as they
control the formation of the yield of thiamine that is essential for
the catalysis of the benzoin condensation [14]. Interestingly, two
new spots were observed on TLC analysis corresponding to
compounds 1 and 2, implying that compound 1 was partly oxidized
during the reaction. Without further purification, the mixture of 1
and 2 was directly oxidized with NH4NO3 and Cu(OAc)2 to give
compound 2 completely. Ullmann reaction of compound 2 with
3,6-Di-tert-carbazole in the presence of CuI and 18-crown-6 gave
compound 3 in a moderate yield, which condensed with 1,3-
diphenylacetone to afford the important building block Cp-Cz.
Since Cp-Br can also be easily synthesized from intermediate 2,
another route as marked by the dotted line square in Scheme 2 was
considered to prepare the target Cp-Cz. However, we failed to
obtain Cp-Cz using this alternative route, since the mono-
substituted byproduct (CpeCzeBr) still dominated even after
a long reaction time of several days. Furthermore, the polarity of
the mono-substituted byproduct is so close to Cp-Cz that it is too
difficult to isolate a pure product.
Since the reactive sites in G2-Core are two-fold higher than in G1-
Core, a higher reaction temperature and much longer reaction time
were necessary to guarantee the complete transformation of the
ethynyl groups and thus a monodispersity of the target dendrimers.
Theoretically, the second-generation dendrimer G2-Cz can also be
synthesized through a convergent way, that is, by reaction between
the first-generation core G1-Core and the second-generation
cyclopentadieneone with carbazoles. However, the target mole-
cule was not obtained in this method probably due to the large
steric hindrance of the second-generation cyclopentadieneone
containing surface groups [15]. All these dendrimers are highly
soluble in common solvents such as dichloromethane, chloroform,
or toluene and were purified conveniently by column chromato-
graph and recrystallization to an excellent purity for OLEDs appli-
cation. Characterization by MALDI-TOF mass spectrometry and 1H
NMR spectroscopy unambiguously confirmed the chemical struc-
tures of these dendrimers.
3.2. Optical properties
The influence of both the polyphenylene dendrons and the
carbazole groups to the emissive behaviors of each dendrimer is
investigated by measuring the absorption and the fluorescence
spectra. The spectral data of these dendrimers in dichloromethane
solutions and their solid films are summarized in Table 1. As shown
in Fig. 1, all these dendrimers exhibit three major absorption bands
covering a wide visible range, which are attributed to the absorp-
tion of the perylene bisimide core. One additional absorption peak
at 347 nm was only observed for G1-Cz and G2-Cz, which should be
assigned to the transition absorption of carbazole units. Upon
photoexcitation at 540 nm, all these dendrimers in dilute solutions
transmit strong red fluorescence with a major band at around
610 nm and a shoulder at 650 nm. The absorption spectrum of G1-
Cz film has a 10 nm blue shift compared to its solution (see
Fig. S1(a)). A similar blue shift of 7 nm was also observed in the
fluorescence spectra of G1-Cz film relative to its solution
(Fig. S1(b)). The hypsochromic effect was observed as well for other
first- and second-generation dendrimers studied here. In contrast,
the zeroth-generation analog G0 exhibited the frequently observed
red shift in both absorption and fluorescence spectra (Fig. S2) of the
solid film compared with its solution. It is well established that the
bathochromic effect of most organic molecules in solid sample is
due to strong intermolecular interaction caused by molecular
stacking. The hypsochromic instead of bathochromic effect
observed in these dendritic molecules implies the excellent site-
isolation effect of the bulky dendrons on the PBI emissive core.
For comparison, the absorption spectra of G1 and G2 films on
quartz substrates are also shown in Fig. S3 in the supporting
information.
Since there is a significant overlap between the emission of car-
bazole and the absorption of G0, the intramolecular singletesinglet
Förster energy transfer from the excited carbazole moieties to the
ground state of the PBI core can take place in such dendrimer
molecules [17,18], during which the periphery carbazole groups
exhibit the light-harvesting potential. In order to monitor this
antenna effect of the carbazoles in G1-Cz and G2-Cz, all these five
dendrimers were excited with light of 345 nm, where it is the
characteristic absorption of carbazole units while the G0, G1 and G2
have ignorable absorption and thus are not excited. Both G1-Cz and
G2-Cz exhibited red fluorescence (spectra not shown), although they
are not very strong, while no red emission was detected for other
three analogs without the carbazole groups. It can be detected that
the red emission from the G1-Cz and G2-Cz molecules arises from
the PBI core which is excited by energy transfer from the excited
carbazoles, rather than directly excited by light absorption. This
The target dendrimers were synthesized through the procedure
described in Scheme 3. All the dendrimers were prepared in the
divergent way from the perylenediimide-containing core as the
basic material, the so-called first-generation core (G1-Core). The
first-generation dendrimers, G1 and G1-Cz, were obtained in high
yields by DielseAlder cycloaddition of the corresponding Cp and
Cp-Cz to G1-Core. Cycloaddition with intermediate 4 followed by
deprotection transformed G1-Core to the second-generation core
G2-Core, the cycloaddition of which with the Cp and Cp-Cz once
again gave rise to the second-generation dendrimers G2 and G2-Cz.
O
O
C
Cp-Cz
+
N
Br
Br
Br
Cp-Br
HO
O
O
O
CHO
Br
B
A
Br
Br
Br
Br
N
1
2
C
O
O
O
D
N
N
N
3
Cp-Cz
(A): VB1, NaOH, H2O, EtOH; (B): NH4NO3, Cu(OAc)2; (C): CuI, 18-crown-6,
K2CO3, reflux 5 h, nitrobenzene; (D): KOH, EtOH, 1,3-diphenylacetone, reflux 0.5 h
Scheme 2. Synthetic routes of the important intermediate Cp-Cz.

H. Ren et al. / Polymer 52 (2011) 3639e3646
3643
O
O
N
O
Cp
: R=H
Cp-Cz
: R=Cz:
R
R
A , Cp
G1
O
O
O
O
Cp-Cz
A ,
G1-Cz
O
O
N
G1-Core
O
B , 4
Si
C
Si
4
N
O
O
D , Cp
G2
O
O
O
O
Cp-Cz
D ,
G2-Cz
O
N
O
G2-Core
(A) o-xylene, 20 h, reflux; (B) o-xylene, 12 h, reflux; (C) THF, n-Bu4NF, 30 min., rt.; (D) o-xylene, 48 h,
reflux.
Scheme 3. Synthetic routes of the dendrimers.
result indicates that the antenna effect of peripheral carbazoles to
enhance the emission indeed exists in the photoluminescence
condition of G1-Cz and G2-Cz.
However, the antenna effect of the peripheral carbazole groups is
negligible when their quenching effect via photoinduced electron
transfer (PET) is taken into account. Based on the strong electron-
donating nature of carbazole and the electron-deficient feature of
the PBI core, the photoinduced charge transfer may occur from the
peripheral carbazole units to the PBI core [19], which in principle
results in the quench of the excited states and thus the emission of the
PBI core. In order to verify this effect, these dendrimers are excited at
540 nm for fluorescence measurement, where only the PBI core have
absorptionwhile the carbazole units do not absorb. Asshownin Fig. 2,
upon photoexcitation at 540 nm, the fluorescence intensity of G1 is
hundreds of times higher than that of G1-Cz although they have
identical concentration. Obviously, the presence of carbazole at the
Table 1
Photophysical and electrochemical data of dendrimers.
red
ox
Dendrimers
labs (nm)
lem (nm)
Ffa
E
b (V)
E
b (V)
HOMO (eV)
LUMO (eV)
onset
onset
In CH2Cl2
In film
In CH2Cl2
In film
G1-Cz
G2-Cz
G1
G2
G0
347, 453, 536, 579
347, 453, 537, 580
454, 538, 579
345, 444, 525, 568
346, 446, 526, 568
450, 538, 582
614
624
616
617
610
607
614
619
618
617
0.17
0.63
0.94
0.92
0.96
ꢁ0.63
ꢁ0.63
ꢁ0.62
ꢁ0.62
ꢁ0.6
1.0
1.0
ꢁ5.4
ꢁ3.77
ꢁ3.77
ꢁ3.78
ꢁ3.78
ꢁ3.8
ꢁ5.4
1.26
1.27
ꢁ5.66
ꢁ5.67
ꢁ5.8
454, 538, 580
443, 535, 576
447, 532, 575
447, 537, 583
noc
a
Quantum yields were measured in chloroform relative to G0 as standard in chloroform (96%) (lexc ¼ 540 nm) [16].
b
c
All the potentials are versus SCE.
no: not observed.

3644
H. Ren et al. / Polymer 52 (2011) 3639e3646
polyphenylene dendrons and the carbazole groups, both which are
2.5
2.0
1.5
1.0
0.5
0.0
famous for their thermal and chemical stabilities [20]. The
morphological stability of these novel dendrimers was monitored
by differential scanning calorimetry (DSC). Neither glass transition
nor any other phase transition was observed in DSC thermograms
of G1-Cz and G2-Cz when they were heated even up to a high
temperature just before decomposition (curves not shown). This
implies that these dendrimers may be amorphous when they were
directly obtained from organic solvents. The absence of any phase
transition not even the melting process suggests that it may be
difficult for them to crystallize. In our previous report [9], a similar
dendrimer PBI-F, bearing same PBI core and polyphenylene den-
drons but with fluorinated shell, was found to easily form amor-
phous state as well and have a high glass transition temperature.
However, it can crystallize and melt in addition to glass transition,
which is different from G1-Cz and G2-Cz in present study. This
indicates the peripheral units have great influence on the
morphological behaviors of the dendrimers.
G0
G1
G2
G1-Cz
G2-Cz
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 1. UVeVis absorption spectra of the dendrimers in dichloromethane (normalized
at the lowest energy peak).
3.4. Electrochemical properties
Fig. 3 shows the cyclic voltammograms of these dendrimers. All
the above compounds show two reversible reduction waves arising
from the stepwise reduction (two one-electron transfers) of the
perylene bisimide core. The onset potential of the first reduction
wave shifted gradually to more negative with dendrimer goes from
G0 to G1 and G2 and further to G1-Cz and G2-Cz (Fig. 3 and Table 1).
A difference of 30 mV in onset potential of first reduction wave was
observed between G0 and G2-Cz. This is reasonable since it would
be more difficult for the centered PBI core to capture electrons from
outside when the dendritic shields get thicker. There is one
reversible oxidation peak detected for all the first and second-
generation dendrimers, whereas within the accessible scanning
range in dichloromethane no oxidation wave was detected for G0.
This suggests that the PBI unit is not prone to be oxidized and the
oxidation in other higher generation dendrimers should occur on
the polyphenylene dendrons and carbazole moieties. Compared
with G1 and G2, the onset potential of the oxidation wave for G1-Cz
and G2-Cz shifted to less positive by 260 mV, which should result
from the introduction of carbazole that is a typical electron donor.
The anodic peak current values of G1-Cz and G2-Cz are significantly
higher than G1 and G2 owing to the good electron-donating feature
of carbazole. In addition, the anodic peak current value of G2-Cz is
double than that of G1-Cz due to increased number of carbazole
periphery of the dendrimers quenched the core emission greatly.
Fortunately, this PET process becomes more insignificant when the
dendrimer goes to higher generation. As shown by Fig. 2 and the
fluorescence quantumyields inTable 1, the fluorescence of G2-Cz is 4-
fold stronger than that of G1-Cz under identical concentrations,
despite double amount of carbazole contained in G2-Cz. This is
because the distance between PBI acceptorand the carbazole donor is
much larger in G2-Cz, definitely suppressing the PET process that is
inherently sensitive to distance.
It is evident that the PET effect, rather than the antenna effect, is
the major factor to influence the emission of these dendrimers
bearing carbazole units. Providing that the charge transportation in
OLEDs containing these novel dendrimers is possibly improved, the
expense in emission efficiency via PETcaused by introduction of the
carbazole functional groups would be acceptable.
3.3. Thermal properties
The thermal stability of the studied dendrimers was investi-
gated by means of thermogravimetric analysis (TGA). Both G1-Cz
and G2-Cz start to decompose at a high temperature up to 450 ꢀC
(see Fig. S4), indicating the excellent thermal stability of these
dendrimers which is an essential merit for organic light-emitting
materials especially when they are used under high temperature.
This thermal stability is suggested to benefit from both the
donor. The HOMO and LUMO energy levels were determined from
ox
the onset potential of the first oxidation (E
) and reduction
onset
500
G1
G1-Cz
G2-Cz
400
G2-Cz
300
G1-Cz
G2
200
100
0
G1
G0
500 550 600 650 700 750 800 850
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
Wavelength (nm)
Potential (vs SCE) / V
Fig. 2. Fluorescence spectra of dendrimers G1, G1-Cz and G2-Cz. (1 ꢃ10ꢁ5 mol Lꢁ1 in
CH2Cl2 for all dendrimers, lexc ¼ 540 nm).
Fig. 3. Cyclic voltammograms of studied dendrimers.

H. Ren et al. / Polymer 52 (2011) 3639e3646
3645
Table 2
350
EL performance data of dendrimers G0, G1, G2, G1-Cz and G2-Cz.
G0
G1
G1-Cz
Jmax (mA/cm2)a
Lmax (cd/m2)a
hmax (cd/A)
300
Dendrimers
Von (V)
G0
G1
G2
G1-Cz
G2-Cz
9
10
12
8
343 (12)
74.6 (19)
76.8 (21)
251 (14)
90.8 (25)
59.4 (12)
37.95 (19)
31.6 (21)
220.7 (14)
64.58 (25)
0.026
0.15
0.11
250
200
0.22
150
0.33
11
a
100
Values in parentheses are the voltages at which they were obtained.
50
0
red
(E
) wave and the data are listed in Table 1. G1-Cz and G2-Cz
onset
have the highest HOMO level of ꢁ5.4 eV among these den-
drimers, which is near to that for the widely used hole injecting
material PEDOT:PSS. Therefore, a low hole injection barrier from
the PEDOT:PSS layer to G1-Cz and G2-Cz layer is expected when
they are used as emitting layer in OLEDs.
-50
-2
0
2
4
6
8
10 12 14 16 18 20
Voltage (V)
Fig. 5. The current density versus voltage curves for G0, G1 and G1-Cz OLEDs.
3.5. Electroluminescence
G1-Cz OLEDs. The current density at a given driving voltage for
G1-Cz based OLED is distinctly higher than that for G1 device,
suggesting that the peripheral of carbazoles indeed facilitates the
hole injection and transportation in neat film of G1-Cz, and finally
improves the charge balance in G1-Cz device to result in an
increased luminance and efficiency than G1 (Table 2). The
contribution of these carbazole units can be further verified by
comparing the performance of G0 and G1-Cz. As illustrated in
Fig. 5, G0 device gave higher current density than G1-Cz. This is
probably because the G0 molecules are more orderly arranged in
the neat film, finally resulting in higher charge mobility and device
current. However, the luminance and emission efficiency for G0
are much lower than those for G1-Cz. This implies that there is
current leak in G0 device due to non-balanced charge trans-
portation. Strong intermolecular interaction in G0 film is another
possible reason for its poor performance.
The current densityevoltageeluminance (JeVeL) characteris-
tics for G1-Cz as example are illustrated in Fig. 6. The G1-Cz device
exhibited a maximum brightness of 220.7 cd/m2 at 14 V, and G2-Cz
has the highest efficiency of 0.33 cd/A. These data are better or at
least comparable with those for other PBI derivatives reported in
literatures so far [9,19]. As discussed above, G1-Cz and G2-Cz have
lower PL quantum yields than other model compounds since the
carbazole units act as electron donor to quench the emission of the
PBI core via intramolecular PET process. Nevertheless they exhibi-
ted improved EL performance than other analogs. Evidently the
peripheral carbazole groups played different roles in EL to facilitate
To examine the electroluminescent properties, these den-
drimers were used as neat emitting layer to fabricate non-doped
OLEDs. The OLEDs have
a configuration as follows: ITO/
PEDOT:PSS (40 nm)/dendrimer (40 nm)/BCP (15 nm)/Alq3 (20 nm)/
LiF (1 nm)/Al (100 nm), in which PEDOT:PSS (poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate)) acts as the hole
injecting and transporting layer, BCP (2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline) as the hole and exciton blocking layer, Alq3
(tris(8-hydroxyquino) aluminum) as the electron transporting
layer, ITO (indium tin oxide) and LiF/Al as anode and cathode,
respectively. Based on the excellent solubility of these dendrimers
in common organic solvents, high quality neat films without
pinholes can be obtained by spin coating their solutions in chlo-
robenzene to form the emitting layer. As an exception, the G0 film is
deposited by vacuum evaporation since a film of significant thick-
ness could not be obtained via solution process due to the low
molecular weight and limited solution viscosity.
The performance data of these devices are collected in Table 2.
As a whole, G1-Cz and G2-Cz exhibited higher luminance and
efficiencies than other three analogs without the carbazole
substituent, indicating that these peripheral carbazole units
played important role to determine the EL behaviors of these
novel dendrimers. All the devices emit in the red region around
600 and 650 nm. As shown in Fig. 4 for G1-Cz as an example, the
EL spectra is almost identical to the PL spectra of its solid film,
except for a slight blue shift in the EL, confirming that the EL
emission from the devices comes from the dendrimer layer. Fig. 5
shows the current density versus voltage curves for G0, G1 and
250
250
1.0
0.8
200
EL
PL of film
200
150
150
0.6
0.4
0.2
0.0
100
100
50
50
0
0
0
2
4
6
8
10 12 14 16
Voltage (V)
300
400
500
600
700
800
wavelength (nm)
Fig. 6. The current densityevoltageeluminance (JeVeL) characteristics of the G1-Cz
OLED. Insert: the plot of the luminance efficiency versus the current density of the
same device.
Fig. 4. Comparison of the EL spectra and the PL of the solid film for G1-Cz.

3646
H. Ren et al. / Polymer 52 (2011) 3639e3646
the light-emitting performance. First, the presence of these
carbazole groups improved the charge balance state in OLEDs, as
shown by the higher current in G1-Cz device than in G1 device. In
addition, energy transfer from excited carbazole to PBI core was
proved to be possible in above section. In OLEDs the charge
recombination may occur on these carbazole sites to get them
electronically excited in OLEDs, the subsequent energy transfer to
the PBI core definitely contributes to improve the EL emission.
Another device architecture by doping these red emitting den-
drimers in poly(N-vinylcarbazole) (PVK) host was also explored in
our experiment. However, higher efficiency and brightness with
poor color purity were observed due to the emission from the
polymer host. This is consistent with the previously report [9].
Therefore, these dendrimers bearing carbazoles on the outer
surface can be applied as a promising candidate for red-light-
emitting-host materials in OLEDs.
Acknowledgments
We thank the National Natural Science Foundation of China
(20704002,
21072026,
and
U0634003)
the
NKBRSF
(2009CB220009), and the Fundamental Research Funds for the
Central Universities (DUT10LK16) for financial support of this work.
Appendix. Supporting information
Supplementary data associated with this article can be found, in
the on-line version, at doi:10.1016/j.polymer.2011.06.019.
References
[1] (a) Ego C, Marsitzky D, Becker S, Zhang J, Grimsdale AC, Müllen K, et al. J Am
Chem Soc 2003;125:437e43;
(b) Kirstein S, Cohen G, Davidov D, Scherf U, Klapper M, Chmiland K, et al.
Synth Met 1995;64:415e8.
[2] (a) Schmidt-Mende L, Fechtenkötter A, Müllen K, Moons E. Science 2001;293:
1119e22;
4. Conclusion
(b) Dittmer JJ, Marseglia EA, Friend RH. Adv Mater 2000;12:1270e4.
[3] Jones BA, Ahrens MJ, Yoon M, Facchetti A, Marks TJ, Wasielewski MR. Angew
Chem Int Ed 2004;43:6363e6.
[4] (a) Würthner F. Chem Commun; 2004:1564e79;
(b) Langhals H. Heterocycles 1995;40:477e500.
[5] Li JY, Liu D. J Mater Chem 2009;19:7584e91.
[6] (a) Gronheid R, Hofkens J, Kohn F, Weil T, Reuther E, Müllen K, et al. J Am
Chem Soc 2002;124:2418;
(b) Liu DJ, Feyter SD, Cotlet M, Stefan A, Wiesler UM, Herrmann A, et al.
Macromolecules 2003;36:5918.
[7] (a) Du P, Zhu WH, Xie YQ, Zhao F, Ku CF, Cao Y, et al. Macromolecules 2004;37:
4387;
(b) Li SF, Zhong GY, Zhu WH, Li FY, Pan JF, Huang W, et al. J Mater Chem 2005;
15:3221.
[8] (a) Pan JF, Zhu WH, Li SF, Xu J, Tian H. Eur J Org Chem; 2006:986e1001;
(b) Li Y, Tan L, Wang ZH, Qian HL, Shi YB, Hu WP. Org Lett 2008;10:529e32;
(c) Fischer MK, Kaiser TE, Würthner F, Bäuerle P. J Mater Chem 2009;19:
1129e41.
[9] (a) Qu JQ, Zh JY, Müllen K, et al. Macromolecules 2004;37:8297e306;
(b) Qu JQ, Pschirer NG, Liu DJ, Stefan A, Müllen K. Chem Eur J 2004;10:528e37.
[10] Liu D, Ren HC, Li JY, Tao Q, Gao ZX. Chem Phys Lett 2009;482:72e6.
[11] (a) Agrawal AK, Jenekhe SA. Chem Mater 1996;8:579e89;
(b) Zhu Y, Alam MM, Jenekhe SA. Macromolecules 2003;36:8958.
[12] Maria D, Rozwadowska. Tetrahedron 1985;41:3135e40.
[13] Buckingham J. Dictionary of organic compounds. 5th ed. London: Chapman
and Hall; 1982.
[14] Seema B, Vidula VV, Mariam SD. J Chem Res; 2006:267e9.
[15] Grayson SM, Frĕchet JMJ. Chem Rev 2001;101:3819e67.
[16] Seybold G, Wagenblast G. Dyes Pigm 1989;11:303e17.
[17] Lor M, Viaene L, Pilot R, Fron E, Jordens S, Schweitzer G, et al. J Phys Chem B
2004;108:10721.
A series of perylene bisimide-cored dendrimers containing
polyphenylene dendrons and peripheral functional groups of
carbazole were prepared and their luminescent properties were
studied. The hypsochromic effect observed in both absorption
and emission of these dendrimer films proved the perfect site-
isolation effect of the bulky dendrons on the perylene bisimide
core. The carbazole units are unfavorable for a high fluorescent
quantum yield due to intramolecular charge transfer process.
However, their presence definitely facilitated the EL performance.
It is suggested that the hole transporting of the carbazoles
improved the charge balance state in OLEDs and act as antenna
group to transfer energy to the core, consequently resulting in
improved EL emission. It should be noted that most PBI deriva-
tives reported so far, including the present dendrimers, have
excellent fluorescent quantum yields, but their EL performances
are still far from ideal if compared with other kinds of materials
with similar quantum yields. This is probably because this series
of compounds have relatively small Stokes shifts and thus strong
self-absorption. In addition, the dendrimer structures would
generally decrease the charge carriers’ mobility due to the site-
isolation effect, which is another possible reason to account for
the EL performance of these PBI dendrimers. Therefore, it may be
one effective strategy to increase the Stokes shift and thus to
improve performance by ingenious structure modification for PBI
derivatives.
[18] Förster T. Ann Phys 1948;2:55.
[19] Pan JF, Zhu WH, Li SF, Zeng WJ, Cao Y, Tian H. Polymer 2005;46:7658e69.
[20] Albrecht K, Yamamoto K. J Am Chem Soc 2009;131:2244e51.