Synthesis and properties of novel perylenebisimide-cored dendrimers

Article abstract of DOI:10.1016/j.dyepig.2011.04.008

A group of perylenebisimide-cored dendrimers with polyphenylene dendrons at four bay positions were synthesized and characterized. The electron-deficient pentafluorophenyl or cyano groups were grafted at the dendrimer surface with the aim to further increase the n-type features of the resulted dendrimers. The electrochemical and optical properties were investigated. All the dendrimers show good solubility and film-forming properties, high EA values of 3.8-3.9 eV, and high fluorescent quantum yields. All these merits indicate that they are potential multifunctional materials for application in optoelectronic devices such as solar cells or organic light-emitting devices.

Full text of DOI:10.1016/j.dyepig.2011.04.008

Dyes and Pigments 91 (2011) 298e303
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and properties of novel perylenebisimide-cored dendrimers
a
b
b
b
a
a,
*
Huicai Ren , Jiuyan Li , Ting Zhang , Renjie Wang , Zhanxian Gao , Di Liu
a
School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 February 2011
Received in revised form
A group of perylenebisimide-cored dendrimers with polyphenylene dendrons at four bay positions were
synthesized and characterized. The electron-deficient pentafluorophenyl or cyano groups were grafted at
the dendrimer surface with the aim to further increase the n-type features of the resulted dendrimers.
The electrochemical and optical properties were investigated. All the dendrimers show good solubility
and film-forming properties, high EA values of 3.8e3.9 eV, and high fluorescent quantum yields. All these
merits indicate that they are potential multifunctional materials for application in optoelectronic devices
such as solar cells or organic light-emitting devices.
7
April 2011
Accepted 7 April 2011
Available online 21 April 2011
Keywords:
Perylenebisimide
Dendrimer
Ó 2011 Elsevier Ltd. All rights reserved.
Polyphenylene dendron
Fluorinated shell
Electrochemistry
Optoelectronic device
1. Introduction
functional groups can be flexibly introduced to either the terminal
imide moiety or the bay positions of perylene ring to construct
Organic semiconductors have attracted tremendous attention for
applications in electronic and optoelectronic devices [1e12] such as
photovoltaic cells [2e5], light-emitting diodes [6e9], and field-effect
transistors [10,11]. However, the n-type (electron-transporting)
semiconductors still lag behind p-type (hole-transporting) materials
in overall performance. And it is desired to design and synthesize
new n-type organic semiconductors with high performance and
environmental stability [12]. Correspondingly, flexibly functional-
ized dendrimers, which combine the advantages of small molecules
and polymers, have ascended to be promising materials in this area
recently [13]. The well-defined three-dimensional dendritic struc-
tures can greatly improve the solubility and film-forming ability and
make these materials suitable for solution processing with agreeable
optoelectronic properties.
One typical feature of the dendrimer is that the core, dendron
and surface group can be flexibly and separately functionalized to
generate different functional materials [14]. For example, den-
drimers with different surface groups have been shown to be
promising sensing applications [15], while dendrimers with certain
chromophores can serve as emitters in OLEDs and light-harvesting
systems [16,17]. Perylene-3,4,9,10-tetracarboxylic acid bisimides
multifunctional molecules [18e20]. More importantly, PBIs possess
compact and electron-deficient cores and have great potential as
n-type semiconductors [21e23]. In particular, Müllen’s group and
Tian’s team have developed a series of PBI derivatives including
PBI-cored dendrimers for use in organic light-emitting diodes and
sensing applications [20,24,25]. In our previous report [26] we have
applied these concepts to prepare a PBI-cored first-generation
dendrimer with fluorinated shell (G1-F) and demonstrated that it
can be used as n-type material in optoelectronic devices.
To extend this work, we designed and prepared the second-
generation analog of G1-F (G2-F) and dendrimers with cyano
peripheries up to the second-generation (G1-CN and G2-CN)
(structures shown in Scheme 1). The electrochemical and optical
properties of these dendrimers were investigated and the potential
applications were suggested.
2. Experimental
2.1. Materials and instruments
All the reagents are of analytical grade and used as received from
(
PBIs) is a favorite candidate as dendrimer building block since
1
13
commercial sources without further purification. H NMR and
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)
C
(
*
Corresponding author. Tel./fax: þ86 411 84986233.
E-mail address: liudi@dlut.edu.cn (D. Liu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.04.008

H. Ren et al. / Dyes and Pigments 91 (2011) 298e303
R
299
R
2.3.1.2. G2-core. 1H NMR (400 MHz, CD
2
Cl
2
)
d
(ppm): 8.13 (s, 4H,
ArH), 7.58 (s, 4H, ArH), 7.93 (t, J ¼ 7.8 Hz, 2H, ArH), 7.32 (d, J ¼ 8.5 Hz,
H, ArH), 7.14e6.65 (m, 88H, ArH), 3.03 (s, 4H, hCH), 3.01 (s, 4H,
hCH), 2.68 (m, 4H, CH isopropyl), 1.09 (d, 24H, CH isopropyl).
2791.0453; found
R
R
4
O
O
N
O
O
O
O
N
O
O
3
R
R
R
R
MALDIeTOF-MS (m/z): calcd. for C208
H
138
N
2
O
8
O
O
O
O
O
O
O
O
þ
2792.2114 (100%) [M þ H] .
2.3.2. Synthesis of tetrakis(pentafluorophenyl)cyclopentadienone
N
N
(
Cp-F)
To a 100 mL round-bottom flask charged with Co
.5 mmol) and decafluoroetolane 1 (1.5 g, 4.4 mmol), o-xylene
25 ml) was injected by syringe under N atmosphere [29]. After
R
F
R
2 8
(Co) (1.5 g,
4
(
R
R
G1
G2
2
F
rigorously degassing by freeze-pump-thaw, the reaction solutionwas
F
F
F
stirred at room temperature overnight and then heated to reflux
F
F
F
under N
crude product was dissolved into 200 mL dichloromethane and
passed through a 1 cm high neutral Al pad. Recrystallization
red powder (0.87 g, 54%).
20O 744.3; found
ꢀ136.91(d),
2
for 48 h. O-xylene was removed under vacuum and the
G1-F, G2-F: R =
G1-CN, G2-CN: R =
CN
F
F
F
F
F
2 3
O
F
F
F
from dichloromethane gave
a
CN
F
F F
F
ꢁ
M.p. ¼ 231.5e232 C. MS (FD) m/z: calcd. for C29
F
þ
19
Scheme 1. Chemical structures of the studied dendrimers.
744.8 (M ).
F-NMR (CD
2
Cl
2
):
d
¼
ꢀ136.88
w
ꢀ
137.09 w ꢀ137.13(d), ꢀ147.50 w ꢀ147.59 (t), ꢀ149.68 w ꢀ149.77(t),
mass spectrometer for ESIeMS and a Micro MX (Micromass) Mass
Spectrometer for MALDIeTOF-MS. The fluorescence and absorption
spectra were measured on a PerkineElmer LS55 fluorescence spec-
trometer and a PerkineElmer Lambda 35 UVeVis spectrapho-
tometer, respectively. For the solution spectra, dendrimers were
ꢀ158.34 w ꢀ158.42(m), ꢀ160.01 w ꢀ160.08(m).
0
2.3.3. Synthesis of 4, 4 -Dibromobenzoin (2)
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 iceesalt bath. When the temperature
was below zero, a 10% NaOH solution in distilled water was added
ꢀ
5
dissolved in chloroform (ca. 10 M) and then put in a quartz cell for
measurement. For the thin film spectra, dendrimers were first dis-
solved in CH
.2. Electrochemistry
Cyclic voltammetry (CV) measurements were performed using
a conventional three-electrode cell with a glassy carbon working
electrode, a platinum wire counter electrode, and an Ag/AgCl
reference electrode on a computer-controlled BAS 100W electro-
chemical analyzer at room temperature. The dendrimers were
2
Cl
2
and then spin-coated onto quartz substrate.
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
2
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 2 (11 g, 55%) as white crystals. M.p.: 92e93 C (lit.
ꢁ
[30]: 94e96.5 C).
0
2.3.4. Synthesis of 4, 4 -Dibromobenzil (3)
dissolved in dichloromethane solution containing Bu
4
NPF
6
(0.1 M)
A solution of compound 2 (500 mg, 1.35 mmol), NH
4 3
NO
as a supporting electrolyte. The redox potentials were obtained at
the scan rate of 100 mV/s. The Ag/AgCl reference electrode was
(135 mg, 1.69 mmol), and Cu(OAc) O (31.4 mg, 0.17 mmol) in
2
ꢂH
2
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 3 (409 mg, 84.5%) as
þ
calibrated by ferrocene/ferrocenium (Fc/Fc ) as the internal stan-
þ
dard and all potentials are versus Fc/Fc . The HOMO and LUMO
ꢁ
energy levels of the studied dendrimers were determined from the
yellow crystal. M.p.: 230e232 C. IR (KBr): 3090 (¼CeH stretch),
onset
onset
red
1
onset oxidation (Eox ) and reduction (E
) potential, based on
1664 (C]O stretch), 1586 (C]C stretch). H NMR (400 MHz, CDCl
3
)
þ
the reference energy level of Fc/Fc (4.8 eV below the vacuum level)
d
(ppm): 7.83 (d, J ¼ 8.4 Hz, 4H, ArH), 7.67 (d, J ¼ 8.4 Hz, 4H, ArH).
þ
[27], according to the following equations:
8 2 2
ESIeMS (m/z): calcd. for C14H Br O 368.02; found 368 ([M] ).
À
Á
onset
ox
HOMO ðeVÞ ¼ ꢀe E
þ 4:8 V
(1)
(2)
2.3.5. Synthesis of 3,4-Bis-(4-bromophenyl)-2,5-
diphenylcyclopentadienone (4)
À
Á
onset
red
A 25 ml round-bottom flask was charged with compound 3
LUMO ðeVÞ ¼ ꢀe E
þ 4:8 V
(
1.2 g, 3.26 mmol), 1,3-diphenylacetone (600 mg, 3.26 mmol),
anhydrous KOH (200 mg, 3.59 mmol) and absolute ethyl alcohol
15 ml). The mixture was stirring for 30 min under reflux.
(
2
2
.3. Synthesis
Removement of the solvent under reduced pressure gave the crude
product, purification of which by column chromatography on silica
1
.3.1. Synthesis of G1-core and G2-core
The synthesis of these key intermediates G1-core and G2-core
gel yielded pure compound 4 (1.5 g, 85%) as a dark brown solid. H
NMR (400 MHz, CDCl
3
)
d
(ppm): 6.77 (dd, J ¼ 2.1, 8.7 Hz, 4H, ArH),
followed the literature methods [28]. They were obtained as red
powders.
7.1e7.3 (m, 10H, ArH), 7.34 (dd, J ¼ 2.1, 8.7 Hz, 4H, ArH). ESIeMS
þ
(m/z): calcd. for C29
H18Br
2
O 542.26; found 542.0 ([M] ).
2
.3.1.1. G1-core. 1H NMR (400 MHz, CD
ArH), 7.54 (d, 8H, ArH), 7.39 (t, 2H, ArH), 7.24 (d, 8H, ArH), 3.54
s, 4H, hCH), 2.59 (m, 4H, CH isopropyl), 1.02 (d, 24H, CH iso-
1175.33; found 1176
2
Cl
2
)
d
(ppm): 8.13 (s, 4H,
2.3.6. Synthesis of 3,4-Bis-(4-cyanophenyl)-2,5-
diphenylcyclopentadienone (Cp-CN)
(
3
A 25 ml round-bottom flask was charged with compound 4
propyl). ESIeMS (m/z): calcd. for C80
H
58
N
2
O
8
(500 mg, 0.92 mmol), K
(2 mg, 0.01 mmol), P(t-Bu)
4
Fe(CN)
6
(156 mg, 0.37 mmol), Pd(OAc)
CO
2
þ
(
100%) [M þ H] .
3
(0.08 ml, 0.03 mmol) and Na
2
3

300
H. Ren et al. / Dyes and Pigments 91 (2011) 298e303
F
F
F
F
F
F
F
F
O
F
F
MgBr
F
F
F
F
F
F
F
F
A
C
F
F
F
F
F
F
HC CH
I
I
F
F
F
F
B
F
F
1
F
F
F
F
Cp-F
Br
Br
O
O
CHO
Br
D
HO
E
O
F
G
O
O
Br
Br
NC
CN
Br
Br
4
Cp-CN
2
3
(
A) KI, NaOH, NaClO; (B) Mg / Et O; (C) Co (CO) ; (D) VB1, NaOH, H O, EtOH; (E) NH NO
2 2 8 2 4 3,
Cu(OAc) ; (F) KOH, EtOH, 1,3-diphenylacetone, reflux 0.5 h; (G) K Fe(CN) , Pd(OAc) , P(t-Bu) ,
2
4
6
2
3
Na CO , NMP
2
3
Scheme 2. Synthetic routes of the important intermediate Cp-F and Cp-CN.
(
7
39 mg, 0.37 mmol) and NMP (10 ml). The mixture was stirring for
h at 140 C under nitrogen. Removement of the solvent under
128.9, 128.6, 128.4, 128.2,128.1, 127.9, 127.7,127.3,127.1,126.3, 126.0,
125.9, 125.8, 125.6, 124.0, 123.0, 120.7, 120.6, 120.3, 119.3, 110.2,
ꢁ
reduced pressure gave the crude product, purification of which by
109.9, 29.1, 24.1. MALDIeTOF-MS: (m/z): calcd. for C448
6045.1485; found 6047.3633([M þ H] ).
H
282
N
18
O
8
þ
column chromatography on silica gel yielded pure compound Cp-
1
CN (200 mg, 50%) as a dark solid. H NMR (400 MHz, CDCl
3
)
d
(ppm): 6.95 (d, J ¼ 8.6 Hz, 4H, ArH), 7.12e7.05 (m, 4H, ArH),
3
. Results and discussion
7
.28e7.20 (m, 6H, ArH), 7.45 (d, J ¼ 8.6 Hz, 4H, ArH). ESIeMS (m/z):
þ
18 2
calcd. for C31H N O 434.1419; found 434.1423 ([M] ).
3.1. Design, synthesis and characterization
2.4. General procedure for synthesis of the target dendrimers
PBI was selected as the dendrimer core because of its
outstanding photophysical and electronic properties with relatively
high fluorescent quantum efficiency and high electron affinity and
the corresponding n-type characteristics, whose derivatives have
A mixture of G1-core (0.13 mmol) or G2-core (0.1 mmol) and
the corresponding cyclopentadienone (6 equiv. for G1-core and
ꢁ
1
6 equiv. for G2-core) in o-xylene (15 ml) was stirred at 140 C for
12 h or 48 h under nitrogen. The solvent was removed under
reduced pressure. The crude product was purified by column
O
N
O
chromatography on silica gel and then recrystallization to afford
A, Cp-F
G1-F
G1-CN
1
the target dendrimers. G1-CN: yield 63% as a red powder. H NMR
O
O
O
O
(
400 MHz, C
6
D
6
CO)
d
(ppm): 8.10 (s, 4H, ArH), 7.61 (s, 4H, ArH),
A, Cp-CN
7
CH
.47e6.82 (m, 94H, ArH), 2.78(m, 4H, CH isopropyl), 1.14 (d, 24H,
isopropyl). 13C NMR (400 MHz, CDCl
d
(ppm): 163.1, 155.8,
3
3
)
O
N
O
1
1
1
54.2, 145.6, 144.9, 144.5, 141.2, 140.2, 139.6, 139.1, 138.6, 137.2,
32.9, 132.3, 132.0, 131.5, 131.3, 130.9, 130.5, 129.8, 129.6, 128.6,
28.2, 127.6, 127.2, 126.6, 124.0, 123.1, 120.8, 119.5, 118.6, 110.3, 110.0,
G1-Core
O
2
9.2, 24.1. MALDIeTOF-MS: (m/z): calcd. for
C
200
H
130
N
10
O
8
þ
2799.0073; found 2799.0181 ([M] ).
B
C
2
.4.1. G2-F: yield 55% as a red powder
Si
Si
1
H NMR (400 MHz, CD
2
Cl
2
)
d
(ppm): 8.05 (s, 4H, ArH), 7.54e6.85
iso-
(ppm): 163.1, 155.8, 154.0,
45.6, 145.2,142.7, 141.3,141.1, 140.7, 140.4, 140.2,139.3, 138.8, 138.3,
37.9,137.7, 136.2, 135.8, 135.5,135.4,134.8, 132.9, 131.9, 131.5, 130.6,
29.9, 129.7, 128.6, 128.3, 127.7, 127.2, 126.9, 126.6, 126.0, 124.0,
23.0, 120.7, 120.3, 119.3, 112.4, 111.3, 29.1, 24.0. MALDIeTOF-MS:
5
(
m, 106H, ArH), 2.6 (m, 4H, CH isopropyl), 1.03 (d, 24H, CH
3
13
3
propyl). C NMR (400 MHz, CDCl ) d
1
1
1
O
O
N
N
O
D, Cp-F
G2-F
G2-CN
1
O
O
O
O
D, Cp-CN
(m/z):
calcd.
524.5791([M þ H] ).
for
C
H
432 138
160
F N
2
O
8
8523.4712;
found
þ
8
O
2
.4.2. G2-CN: yield 70% as a red powder
1
H NMR (400 MHz, CD
2
Cl
2
)
d
(ppm): 8.04 (s, 4H, ArH), 7.42e6.42
iso-
(ppm): 163.1, 155.8, 153.8,
45.6, 145.0, 144.5, 144.3, 141.6, 141.5, 141.0, 140.9, 140.3, 140.0,
39.7,139.6, 139.3, 139.1,138.8,138.6, 138.3, 138.0, 137.8,137.4, 137.0,
32.9, 132.0, 131.6, 131.3, 131.2, 130.8, 130.6, 130.0, 129.7, 129.6,
(
m, 250H, ArH), 2.6(m, 4H, CH isopropyl), 1.02 (d, 24H, CH
3
G2-Core
A) o-xylene, 20 h, reflux; (B) o-xylene, 12 h, reflux; (C) THF, n-Bu NF, 30 min.,
13
3
propyl). C NMR (400 MHz, CDCl ) d
(
4
1
1
1
rt.; (D) o-xylene, 48 h, reflux.
Scheme 3. Synthetic routes of the dendrimers.

H. Ren et al. / Dyes and Pigments 91 (2011) 298e303
301
Table 1
The photophysical and electrochemical data of the studied dendrimers.
abs (nm)a
l
em (nm)b
F
c
E
onset
red
d
(V)
E
onset
d
(V)
HOMO (eV)
LUMO (eV)
Dendrimer
l
f
ox
e
G1-F
G2-F
G1-CN
G2-CN
441
453
452
452
525
537
536
537
564
579
578
579
595
608
606
606
0.95
0.93
0.75
0.62
ꢀ0.9
ꢀ1.0
e
ꢀ5.96
ꢀ3.9
ꢀ3.8
ꢀ3.83
ꢀ3.8
e
e
ꢀ5.85
ꢀ0.97
ꢀ1.0
0.88
0.84
ꢀ5.68
ꢀ5.64
a
ꢀ5
ꢀ5
Measurements were performed in chloroform solvents (ca. 10 M).
Measurements were performed in chloroform solvents (ca. 10 M, excited at 540 nm).
The fluorescence quantum yield (
b
c
0
F
f
) was measured in chloroform solution relative to 1,6,7,12-tetraphenoxy-N,N -bis(2,6-diisopropyl-phenyl)perylene-3,4,9,10-bis(di-
carboximide) in chloroform (96%) (Excited at 540 nm).
d
e
þ
All potential values are versus Fc/Fc .
Calculated from the optical band gap.
been widely used for electronic and optoelectronics devices
18e26]. The polyphenylene dendrons were selected to build up
generated the important intermediate Cp-CN as a black solid.
[
4 6
Instead of CuCN or KCN, we took K Fe(CN) as the cyanization
the dendrimers due to their famous merits of non-planar steric
conformation and excellent thermal stability [31]. The penta-
fluorophenyl or cyano groups were grafted to the dendrimer
surface to further increase the n-type feature of the target den-
drimers based on their strong electron withdrawing nature. The
dendrimers designed in this way can be expected to have high
luminescent efficiency, good thermal and morphological stability,
high electron affinity and good film-formation ability by solution
processing methods, so that they can be potentially used as multi-
functional materials in optoelectronic devices.
reagent mainly for the sake of safety.
The target dendrimers were synthesized through the procedure
described in Scheme 3. All the dendrimers were prepared in
the divergent way from the perylenebisimide-containing core, the
so-called first-generation core (G1-Core). The first-generation
dendrimers, G1-F and G1-CN, were obtained in high yields by
DielseAlder cycloaddition of the corresponding Cp-F and Cp-CN to
G1-Core. Cycloaddition with intermediate 5 followed by depro-
tection transformed G1-Core to the second-generation core G2-
Core, the cycloaddition of which with the Cp-F and Cp-CN once
again gave rise to the second-generation dendrimers G2-F and G2-
CN. 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 den-
drimers. All these dendrimers are highly soluble in common
solvents such as dichloromethane, chloroform, or toluene and were
purified conveniently by column chromatograph and recrystalli-
zation to an excellent purity. Characterization by MALDI-TOF mass
The traditional divergent approach was employed to synthesize
the second-generation dendrimers in this study. The polyphenylene
dendrons of the studied dendrimers were constructed by
DielseAlder cycloaddition of tetraphenyl-cyclopentedieneone to
the terminal ethynyl groups at the periphery of the PBI core. The
important building blocks of Cp-F and Cp-CN for the synthesis of
the target dendrimers were prepared according to the methods as
described in Scheme 2. Cp-CN was synthesized by a four-step
reaction procedure. The first intermediate compound 2 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
1
13
spectrometry and H and C NMR spectroscopy unambiguously
confirmed the chemical structures of these dendrimers.
ꢁ
temperature was higher than 65 C, the product yield was decreased
3.2. Electrochemical properties
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 [32]. Interestingly, two new spots were
observed on TLC analysis corresponding to compound 2 and 3,
implying that compound 2 was partly oxidized during the reaction.
Without further purification, the mixture of 2 and 3 was directly
oxidized with NH NO and Cu(OAc) to give compound 3
4 3 2
completely, which condensed with 1,3-diphenylacetone to afford
the compound 4 with a high yield. Cyanization of compound 4 by
In order to determine the redox properties of these dendrimers,
cyclic voltammtry (CV) was performed in dichloromethane solu-
tions. From these data, HOMO and LUMO energy levels and the
band gaps were estimated and are summarized in Table 1. The
cyclic voltammograms of these materials are shown in Fig. 1.
In the reductive potential region, two reversiblewaves are observed
for all dendrimers, indicating stepwise single-electron reduction and
radical anion and dianion generation on the perylenebisimide core
[33]. For the first-generation dendrimer G1-F and G1-CN, both the first
and second reduction waves are at less negative potentials than those
K
4
Fe(CN)
6
in the presence of Pd(OAc)
2
, P(t-Bu)
3
and Na
2
CO
3
in NMP
b
a
G1-F
G2-F
G1-CN
G2-CN
-2000
-1500
-1000
-500
-2000
-1000
0
1000
2000
+
+
Potential mV vs. Fc/Fc
Potential mV vs. Fc/Fc
Fig. 1. Cyclic voltammograms of studied dendrimers.

302
H. Ren et al. / Dyes and Pigments 91 (2011) 298e303
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
1.2
G1-F Sol.
G2-F Sol.
G1-F Film
G2-F Film
a
G1-CN Sol.
b
1.0
0.8
0.6
0.4
0.2
0.0
G2-CN Sol.
G1-CN Film
G2-CN Film
.0
3
50
400
450
500
550
600
650
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 2. UVeVis absorption spectra of the dendrimers in chloroform and thin films.
for their corresponding second-generation analogs, indicating that the
first-generation dendrimers capture electrons more easily. This is due
to the “dendrimer effect”, i.e., the thicker the dendron shielding
surrounding the PBI core, the more difficult for the internal PBI core to
accept electrons from outside. In the positive potential region, no
oxidation was observed for the fluorinated dendrimers, whereas one
reversible wave was observed for G1-CN and G2-CN. The absence of
any oxidation in fluorinated dendrimers should result from the
combined effects of the inherent n-type nature of the PBI core and the
additional strong electron withdrawing properties of the peripheral
pentafluorophenyl moieties. Similarly with increasing size and
generation of the polyphenylene dendrons, the oxidation wave of the
G2-CN dendrimer shifted to slightly higher potential than G1-CN. The
electron affinity (EA, corresponding to LUMO energy level) and ioni-
films when spin-coated from solutions onto quartz. The optical
properties of G1-F, G2-F, G1-CN and G2-CN were therefore easily
investigated in both solutions and films. Their absorption spectra in
chloroform solutions and films are shown in Fig. 2. Two main bands, in
the ranges 480e600 nm and 400e460 nm, can be observed from the
spectra. Between 480 and 600 nm the absorption can be assigned to
the S
0
eS
1
electronic transition (along the long PBI axis). The second
eS
absorption band between 400 and 460 nm is ascribed to the S
0
2
electronic transition (along the short PBI axis) [28]. In contrast to the
frequently observed bathochromic effect due to intermolecular
interaction in solid samples, the absorption spectra of thin films of the
present dendrimers were blue-shifted by about 10 nm relative to those
in solutions. The hypsochromic effect in the absorption spectra implies
the tiny intermolecular interaction in the solid films of these den-
drimers, which should benefit from the non-planar molecular
conformation of these bulky dendrimers. The broad light absorption in
visible light range together with high EA value suggests that these
dendrimers are good candidates for use in solar cells.
Fig. 3 illustrates the photoluminescence (PL) spectra of these
dendrimers in chloroform solutions and thin films. Under excita-
tion at 540 nm, all these dendrimers emit strong red fluorescence in
both solutions and thin films. In solutions, the fluorescence maxima
are located at 595, 608, 606 and 606 nm for G1-F, G2-F, G1-CN and
G2-CN, respectively, while the corresponding maxima for the films
are at 594, 614, 630 and 618 nm, respectively. Apparently, the red
emission of these dendrimers is emitted from the PBI core. Similar
to the hypsochromic effect in the absorption spectra, the relatively
small red-shifts observed in PL spectra once again indicate the
weak intermolecular interaction of the emissive core which is
caused by the perfect site-isolation or dendron dilution effect on
the emissive core.
zation potential (IP, corresponding to HOMO level) were determined
onset
from the onset potential of the first reduction (E
)waveandthe first
red
onset
þ
oxidation (Eox ) wave, respectively, by using an Fc/Fc energy level
of ꢀ4.8 eV vs vacuum [27]. The electron affinity was in the range of
3
.8e3.9 eV for these four dendrimers. The IPs for G1-CN and G2-CN are
5.68 and 5.64 eV, respectively. Since no oxidation process was detected
for G1-F and G2-F, their IPs were estimated as ca. 5.96 and 5.85 eV,
respectively, from the optical band gap and EA data according to the
equation of IP ¼ EA þ E
g
.
The high electron affinities of 3.8e3.9 eV for these PBI-cored
dendrimers with strong electron withdrawing peripheral groups
imply that these dendrimers are promising materials to act as
electron acceptors for solar cells or other optoelectronic devices, in
which a high electron affinity of the n-type material is one essential
parameter to influence the device performance.
3.3. Optical properties
The fluorescence quantum yields of these dendrimers
(excited at 540 nm) were measured in chloroform relative to
1,6,7,12-tertraphenoxy-N,N -bis(2,6-disisopropyl-phenyl)perylene-3,
Due to the non-planar molecular conformation aroused from the
bulky polyphenylene dendrons, these PBI-cored dendrimers have
good solubility in common organic solvents and form high-quality
0
4,9,10-bis(dicarboximide) (96% in chloroform) [34]. All dendrimers
1
0
0
0
0
0
.0
.8
.6
.4
.2
a
G1-CN Sol.
1
.0
G1-F Sol.
G1-F Film
G2-F Sol.
G2-F Film
b
G1-CN Film
G2-CN Sol.
G2-CN Film
0.8
0.6
0.4
0.2
0.0
.0
5
50
600
650
Wavelength (nm)
700
750
550
600
650
700
750
Wavelength (nm)
Fig. 3. Fluorescence spectra of dendrimers in chloroform and thin films (
l
exc ¼ 540 nm).

H. Ren et al. / Dyes and Pigments 91 (2011) 298e303
303
exhibit high quantum yields of 0.95, 0.93, 0.75 and 0.62 for G1-
F, G2-F, G1-CN and G2-CN, respectively. The high fluorescent
quantum yields suggest that these dendrimers can be used as red
emitters in organic light-emitting diodes.
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A group of perylenebisimide-cored dendrimers containing pol-
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show good solubility and film-forming properties, and are charac-
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almost the whole visible light range, and high fluorescent quantum
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0
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We thank the National Natural Science Foundation of China
fluorobutyl)-3,4:9,10-perylene diimide. Applied Physics Letters 2007;91:
(
20704002, U0634003 and 21072026), the Ministry of Education
for the New Century Excellent Talents in University (Grant NCET-
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212107.
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