Synthesis of giant rigid π-conjugated dendrimers

Article abstract of DOI:10.1021/ol702063e

(Chemical Equation Presented) A novel family of giant π-conjugated dendrimers (G0, G1, and G2) solely constructed by 5,5,10,10,15,15- hexahexyltruxene units has been developed in a convergent manner through a Suzuki cross-coupling reaction. The overall yields to such large rigid conjugated dendrimers are quite satisfying. The structures and purity of these nanosize rigid dendrimers are verified by ¹H and ¹³C NMR, MALDI-TOF MS, and elemental analysis.

Full text of DOI:10.1021/ol702063e

ORGANIC
LETTERS
2
007
Vol. 9, No. 22
539-4542
Synthesis of Giant Rigid
Dendrimers
π-Conjugated
4
Yang Jiang, Yi-Xuan Lu, Yu-Xin Cui,* Qi-Feng Zhou, Yuguo Ma,* and Jian Pei*
Key Laboratories of Bioorganic Chemistry and Molecular Engineering and of Polymer
Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular
Engineering, and Health Science Center, Peking UniVersity, Beijing 100871, China
jianpei@pku.edu.cn; ygma@pku.edu.cn
Received August 22, 2007
ABSTRACT
A novel family of giant
π-conjugated dendrimers (G0, G1, and G2) solely constructed by 5,5,10,10,15,15-hexahexyltruxene units has been devel-
oped in a convergent manner through a Suzuki cross-coupling reaction. The overall yields to such large rigid conjugated dendrimers are quite
1
13
satisfying. The structures and purity of these nanosize rigid dendrimers are verified by H and C NMR, MALDI-TOF MS, and elemental analysis.
In the past decades, the design and synthesis of π-conjugated
dendrimers have been explored extensively because of their
unusual molecular structures and their potential applications
as active chemical components in a wide range of electronic
and optoelectronic devices.1 In comparison with linear
In our previous contributions, we reported the synthesis
and photophysical properties of several π-conjugated den-
drimers based on truxene units, which exhibited good
properties as active materials for optoelectronic devices
3
including organic light-emitting diodes (OLEDs). Herein,
2
π-conjugated oligomers and polymers, such rigid dendritic
systems with large branching building blocks exhibit intrinsic
we design and develop a novel family of π-conjugated
dendrimers (G0, G1, and G2) constructed solely with 5,5,-
10,10,15,15-hexahexyltruxene units. A Suzuki cross-coupling
reaction is employed to construct such giant dendrimers with
large steric hindrance. The numbers of the 5,5,10,10,15,15-
two- or three-dimensional architectures, which bring them
1i
new electrical, photophysical, and morphological properties.
Therefore, development of new, large π-conjugated den-
drimers can allow us to further expand their potential
applications in the plastic electronic field.
(2) (a) Salaneck, W. R.; Lundstr o¨ m, I.; R a¨ nby, B. Conjugated Polymers
and Related Materials; Oxford University Press: Oxford, 1993. (b) M u¨ llen,
K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-
VCH: Weinheim, 1998. (c) Thomas, S. W.; Joly, G. D.; Swager, T. M.
Chem. ReV. 2007, 107, 1339-1386. (d) G u¨ nes, S.; Neugebauer, H.;
Sariciftci, N. S. Chem. ReV. 2007, 107, 1324-1338. (e) Murphy, A. R.;
Fr e´ chet, J. M. J. Chem. ReV. 2007, 107, 1066-1096.
(3) (a) Cao, X.-Y.; Zhang, W.-B.; Wang, J.-L.; Zhou, X.-H.; Lu, H.;
Pei, J. J. Am. Chem. Soc. 2003, 125, 12430-12431. (b) Cao, X.-Y.; Liu,
X.-H.; Zhou, X.-H.; Zhang, Y.; Jiang, Y.; Cao, Y.; Cui, Y.-X.; Pei, J. J.
Org. Chem. 2004, 69, 6050-6058. (c) Jiang, Y.; Wang, J.-Y.; Ma, Y.; Cui,
Y.-X.; Zhou, Q.-F.; Pei, J. Org. Lett. 2006, 8, 4287-4290. (d) Wang, J.-
L.; Luo, J.; Liu, L.-H.; Zhou, Q.-F.; Ma, Y.; Pei, J. Org. Lett. 2006, 8,
2281-2284.
(
1) (a) Moore, J. S. Acc. Chem. Res. 1997, 30, 402-413. (b) Fr e´ chet, J.
M. J.; Tomalia, D. A. Dendrimers and Other Dendentic Polymers; Wiley:
Chichester, U.K., 2001. (c) Newkome, G. R.; Moorefield, C. N.; V o¨ gtle,
F. Dendritic Molecules-Concepts, Synthesis, PerspectiVes; Wiley-VCH:
Weinheim, Germany, 2002. (d) Grayson, S. M.; Fr e´ chet, J. M. J. Chem.
ReV. 2001, 101, 3819-3867. (e) Strohriegl, P.; Grazulevicius, J. V. AdV.
Mater. 2002, 14, 1439-1452. (f) Momotake, A.; Arai, T. J. Photochem.
Photobiol. C: Photochem. ReV. 2004, 5, 1-25. (g) Schryver, F. C.; Vosch,
T.; Cotlet, M.; Auweraer, M. V.; M u¨ llen, K.; Hofkens, J. Acc. Chem. Res.
2
005, 38, 514-522. (h) Bauer, R. E.; Grimsdale, A. C.; M u¨ llen, K. Top.
Curr. Chem. 2005, 245, 253-286. (i) Lo, S.-C.; Burn, P. L. Chem. ReV.
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2
1
0.1021/ol702063e CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/04/2007

hexahexyltruxene units in the structures of G0, G1, and G2
are 4, 10, and 22, respectively. According to molecular
modeling, the diameter of G2 is about 7.9 nm with molecular
weights up to 18600 Da.
Scheme 2
Scheme 1
Scheme 1 illustrates the synthetic route to the zeroth
of its reduction and a Sandmeyer reaction. Compound 1 was
nitrated in 1,2-dichloroethane (DCE) under mild and efficient
conditions to afford 4 after about 5 min. Further bromination
of 4 formed key intermediate 5 in 97% yield. A Suzuki cross-
coupling reaction of 5 with boronic acid 2 afforded 6 in an
excellent yield (99%). Such a high yield was attributed to
the electron-withdrawing effect of the nitro group. Reduction
generation dendrimer G0. Following our previous reaction
4
sequences, a brominated derivative of 5,5,10,10,15,15-
hexahexyltruxene 1 was lithiated using n-BuLi followed by
quenching with trimethyl borate and then acidic hydrolysis
to afford boronic acid 2. Full bromination of 1 afforded
4
tribromide 3 in 98% yield. Originally, Na
2
CO
3
or K
3
PO
4
was used as base to promote a Suzuki cross-coupling of 2
and 3 with large steric hindrance. However, since bulky
coupling dendrons became less reactive, the Suzuki reaction
of 6 with SnCl produced amine 7 in 90% yield. Since amine
2
7 was insoluble in aqueous media, which was commonly
5
7
used in a Sandmeyer procedure, very low conversion was
gave very low yield (<20%). Then, a strong base, KOH,
was used instead to promote the Suzuki coupling reaction
to afford G0 in 75% yield. From this observation, KOH was
employed to conduct the subsequent Suzuki cross-coupling
reactions in this contribution.
afforded. Finally, we overcame this problem by dispersing
amine 7 in silica, and then it reacted with HNO in aqueous
2
media followed by treatment with KI to form iodide 8 in
60% yield. Triboronate 9 (Scheme 1) was obtained starting
with 3 through procedures similar to those for the preparation
4
A convergent strategy to synthesize G1 is outlined in
of 2. It is well-known that triboronic acids are readily
Scheme 2. One of the important steps is to synthesize AB
2
-
subjected to dimerization and cyclic trimerization by loss of
water to form anhydrides and boroxines. These side reac-
8
type intermediates, such as 8 and 13 (Scheme 4) in this
contribution. Trimethylsilyl (TMS-) was usually used as a
protection group during the preparation of conjugated den-
(6) (a) Miller, T. M.; Neenan, T. X. Chem. Mater. 1990, 2, 346-349.
(b) Xu, Z.; Kahr, M.; Walker, K. L.; Wilkins, C. L.; Moore, J. S. J. Am.
Chem. Soc. 1994, 116, 4537-4550.
(7) (a) Lincker, F.; Bourgun, P.; Masson, P.; Didier, P.; Guidoni, L.;
Bigot, J.-Y.; Nicoud, J.-F.; Donnio, B.; Guillon, D. Org. Lett. 2005, 7,
6
drimers based on phenyl units. However, in our case, the
nitro group was chosen as a substitution group, which could
be easily converted to halogen groups via a reaction sequence
1
505-1508. (b) Blaszczyk, A.; Chadim, M.; H a¨ nisch, C.; Mayor, M. Eur.
J. Org. Chem. 2006, 3809-3825.
(8) Snyder, H. R.; Konecky, M. S.; Lennarz, W. J. J. Am. Chem. Soc.
1958, 80, 3611-3615.
(
(
4) Cao, X.-Y.; Zhang, W.; Zi, H.; Pei, J. Org. Lett. 2004, 6, 4845-4848.
5) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
4540
Org. Lett., Vol. 9, No. 22, 2007

tions brought difficulties for purification and characterization
of desired products. To overcome this obstacle, pure tribo-
ronic ester 9 was obtained through the esterification of the
crude boronic acid with pinacol in toluene, and the total yield
over four steps was 41% (as shown in Scheme 1). Subse-
quently, another Suzuki coupling reaction between dendron
2 3
about 20% (entry 8). Finally, catalyst system Pd(OAc) /PCy
was employed to afford arylboronate 10 in 70% yield (entry
9). Moreover, no symmetric biaryl was observed ac-
companied by the formation of arylboronate 10 in the
reaction mixture.
8
and triboronate ester 9 ended the synthesis of dendrimer
G1 in 54% yield.
Toward the synthesis of G2, another important intermedi-
Scheme 4
ate boronate 10 was prepared from 5 as shown in Scheme
9
3. The synthesis of arylboronates developed by Miyaura
Scheme 3
provides a facile access to boronic acid derivatives in the
presence of sensitive functionalities such as nitro groups in
5. Unfortunately, no desired arylboronate 10 was obtained
according to the typical Miyaura procedure (entry 1). After
10
failure to promote this reaction by using other Pd catalysts,
we tried to employ ligand PPh with Pd catalysts (entries
-5, Table 1); no conversion was observed. Prolonged
3
2
Scheme 4 illustrates the synthetic approach to G2. A
Suzuki coupling reaction between iodide 8 and diboronate
Table 1. Arylboronate Formation under Various Conditions
1
0 gave 11 in 77% yield. The reaction sequence converting
entry
catalyst
ligand
isolated yield (%)
the nitro group to iodide substituents was employed again
to produce 13 in 24% overall yield. The final Suzuki cross-
coupling reaction between iodide 13 and triboronate 9 using
KOH afforded giant dendrimer G2 in 37% yield. Considering
the rigidity and steric hindrance within the large molecule
G2, the yield was satisfying. Therefore, we developed a
convergent strategy to construct the desired dendrimers G0,
G1, and G2 via the Suzuki coupling reactions.
1
2
3
4
5
6
7
8
9
PdCl2(dppf)
PdCl2(dppf)
Pd(PPh3)4
Pd2(dba)3
Pd(OAc)2
PdCl2(dppf)
Pd(PPh3)4
Pd2(dba)3
Pd(OAc)2
0
0
0
0
0
5
5
20
70
PPh3
PPh3
PPh3
PPh3
PCy3
PCy3
PCy3
PCy3
Three dendrimers G0, G1, and G2 are readily soluble in
hexane, chloroform, THF, which facilitates the final char-
acterization and confirmation of their structures and purity.
1
13
Almost identical characteristics of their H and C NMR
spectra proved that the protons and carbons of three
generation dendrimers have similar chemical environments,
which is consistent with their structural similarity (see the
Supporting Information). Gel permeation chromatography
(GPC) data exhibited a stepwise decrease of elution time as
the generation grew (Figure 1). A narrow polydispersity
index (PDI ) 1.02-1.05) was observed, indicating their
purity. Figure 2 illustrates MALDI-TOF MS spectra of G0,
G1, and G2. The signal assigned to molecular ions in
MALDI-TOF MS directly verified the presence of the desired
heating resulted in an insignificant change in the reaction
mixture. The replacement of PPh by PCy (Cy ) cyclo-
hexyl) afforded such arylboronate formation. Consequently,
under catalyst system Pd (dba) /PCy , the conversion was
3
3
11
2
3
3
(
9) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60, 7508-
510.
10) (a) Takahashi, K.; Takagi, J.; Ishiyama, T.; Miyaura, N. Chem. Lett.
7
(
2
000, 126-127. (b) Iovine, P. M.; Kellett, M. A.; Redmore, N. P.; Therien,
M. J. J. Am. Chem. Soc. 2000, 122, 8717-8727. (c) Todd, M. H.; Abell,
C. J. Comb. Chem. 2001, 3, 319-327.
(11) Yeh, R. M.; Xu, J.; Seeber, G.; Raymond, K. N. Inorg. Chem. 2005,
4
4, 6228-6239.
Org. Lett., Vol. 9, No. 22, 2007
4541

one to four 5,5,10,10,15,15-hexahexyltruxene units, respec-
1
13
tively. Such agreement of H and C NMR, GPC, and
MALDI-TOF results, associated with elemental analysis,
unambiguously proved the structures and purity of three
generation dendrimers, G0, G1, and G2 (see the Supporting
Information).
Figure 1. GPC traces of G0, G1, and G2 in THF.
dendrimers. Furthermore, their MALDI-TOF MS behaviors
also demonstrated that no structural defects were found in
the desired dendrimers G0, G1, and G2. For G0, the clear
molecular ion signal peaked at 3383.4 Da (calcd m/z 3383.5);
moreover, other two signals corresponding to fragments [M
Figure 3. Absorption and emission spectra of G0, G1, and G2 in
toluene solutions (10 M). Emission spectra were recorded upon
excitation at absorption maximum.
+
-6
-
C
H
6
H
13] (calcd m/z 3298.3, found m/z 3298.3) and [M -
+
C
6
12 + Li] (calcd m/z 3306.3, found m/z 3306.0) were
observed. For G1, the predominant MALDI-TOF MS signal
exhibited at 8453.7 Da (calcd m/z 8455.8) for the molecular
ion. MALDI-TOF analysis of G2 exhibited five peaks, which
corresponded to the molecular ion (calcd m/z 18600, found
m/z 18594) and other fragments assigned to the removal of
Figure 3 illustrates the photophysical behaviors of dendri-
-
6
mers G0, G1, and G2 in dilute toluene solutions (10 M).
From the absorption spectra, besides characteristic absor-
bance of the π-π* electron absorption band of the truxene
unit at 310 nm, three dendrimers showed the same absorption
λ
max at around 350 nm. They also exhibited similar emission
spectra with two main peaks at about 386, 405 nm and a
shoulder at 430 nm. Similar photophysical behaviors of three
generation dendrimers implied that the effective conjugation
length did not improve as the generation increased.
In summary, three giant π-conjugated dendrimers (G0, G1,
and G2) built from 5,5,10,10,15,15-hexahexyltruxene units
have been designed and successfully synthesized in a
convergent strategy. The Suzuki cross-coupling provides a
facile approach in constructing such giant dendrimers with
large steric hindrance. The reaction conditions of the Miyaura
arylboronates are also investigated to obtain large arylbor-
onates with sensitive functionalities in a good yield. Ac-
cording to molecular modeling, the diameters are 3.6 nm
for G0, 5.6 nm for G1, and 7.9 nm for G2, respectively.
The three dendrimers show similar UV-vis and photolu-
minescent properties in dilute solution, indicating their similar
effective conjugation length.
Acknowledgment. This work was supported by the
National Basic Research Program (Nos. 2006CB921602 and
2007CB808000) from the Ministry of Science and Technol-
ogy, and National Natural Science Foundation of China
(NSFC 20425207, 50473016, 20521202 and 20604001).
Supporting Information Available: Experimental pro-
1
13
cedures and H and C NMR spectra for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 2. MALDI-TOF MS of G0, G1, and G2.
OL702063E
4542
Org. Lett., Vol. 9, No. 22, 2007