Synthesis of ladder-type graphene ribbon oligomers from pyrene units

Article abstract of DOI:10.1016/j.tetlet.2016.07.106

A series of ladder-type graphene ribbon oligomers have been synthesized through DDQ/acid-mediated oxidative cyclization. The oligomers present a bright blue light emission, and the new oligomers were clearly characterized by NMR and MALDI-TOF.

Full text of DOI:10.1016/j.tetlet.2016.07.106

Accepted Manuscript
Synthesis of Ladder-type Graphene Ribbon Oligomers from Pyrene Units
Feng Liu, Xiaoxiao Shen, Yonggang Wu, Libin Bai, Hongchi Zhao, Xinwu Ba
PII:
S0040-4039(16)30975-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.07.106
TETL 47968
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
12 June 2016
27 July 2016
30 July 2016
Please cite this article as: Liu, F., Shen, X., Wu, Y., Bai, L., Zhao, H., Ba, X., Synthesis of Ladder-type Graphene
Ribbon Oligomers from Pyrene Units, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.
2016.07.106
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Synthesis of Ladder-type Graphene Ribbon Oligomers from Pyrene
Units
Feng Liu, Xiaoxiao Shen, Yonggang Wu*, LibinBai*, Hongchi Zhao, Xinwu Ba
College of Chemistry and Environmental Science, Hebei University, Baoding, 071002, P. R. China
Supporting Information
[
]
ARTICLE INFO
ABSTRACT
A series of ladder-type graphene ribbon oligomers have been synthesized
through DDQ/acid-mediated oxidative cyclization. The oligomers present a
bright blue light emission, and the new oligomers were clearly characterized
by NMR and MALDI-TOF.
Keyword: Graphene; Pyrene; Ladder-type
Introduction
Polycyclic aromatic hydrocarbons (PAHs) as well defined
“piece of graphite” 1-4 have been exploited in various organic
electronic devices including field-effect transistors,5-7
and they exhibited interesting properties, long-range
ribbon-type oligomers are more attractive and seldom.44
In this paper we design a series of π-conjugated graphene
ribbon oligomers (1-4) with increasing length through fusion
at the K-region. The synthesis of the oligomers was in the way
of the typical strategy of PAH production: first, the design and
synthesis of soluble polyphenylene precursor molecules and
second, the formation of aromatic C-C bonds by
cyclodehydrogenation. Thus, appropriate polyphenylene
precursor structures can be converted in a single reaction step
into the desired planarized PAH structures. For a successful
cyclodehydrogenation toward large graphene ribbon, the well
defined precursors (7, 10, 16, 19 and 22) containing pyrene
and biphenyl units as repeated units were synthesized, they
can be converted into planar ribbon-type graphene pieces by
cyclodehydrogenation.
photovoltaic devices,8-10 and light-emitting diodes.11-13
A
representative example of PAHs developed by Müllen et al is
hexa-peri-hexabenzocoronene (HBC),14,15 HBC and its
derivatives were known for their unique self-organization into
columnar superstructures and exhibit one of the highest
charge-carrier mobilities for mesogens.16-20 Various devices
based on nanographene molecules with different structures
and functionalities have been well developed by organic
synthetic or supramolecular protocols, such as Aida’s
nanotubes,21-23 Li’s graphene quantum dots,24,25 and Müllen’s
graphene nanoribbons and large graphenes.1 Ladder or
ribbon-type polymers and oligomers possessing two
dimensionally
planar
conformation,
high
electron
delocalization, and functional optic-electronic properties
attract great scientific interest considering their potential
applications in areas such as electroluminescence, plastic
lasers, and molecular electronics.26-29
Pyrene, which has been widely investigated for its unique
properties, is a promising building block to develop new
extended π-systems for organic optoelectronics.4, 30-36 As the
smallest peri condensed polynuclear benzenoid aromatic
hydrocarbon, pyrene has been used to architecture ladder or
ribbon-type polymers and oligomers.37, 29 By comparison to
polymers, monodisperse conjugated oligomers can be free
from structural defects, possessing high purity from
conventional purification methods, are more suitable for
detailed structure-property relationships studies.38-43 However,
in spite of their unique material properties, the synthesis and
characterization of defect-free, soluble hydrocarbon ribbon
molecule is still one of the ongoing challenges. Although a
few pyrene derivatives fused at the K-region were developed,
3
2
1
4 (Desired Structure)
Figure 1.the structure of ladder-type graphene oligomers.
The synthetic route to oligomers 1, 2 and 14 was shown in
was obtained from
palladium-catalyzed Suzuki coupling reaction between
scheme 1. The precursor
7

2-bpin-4,4'-di- tert-butylbiphenyl 5 and 4,7-dibromo-2,7-di-
tert-butylpyrene 6 in yield 67%, then an intramolecular Scholl
reaction was performed by oxidative cyclization with FeCl3 to
afford the product 1 in yield 70%, the reported synthetic yield
of 1 was only 10%.44 The original synthetic design of product
was transformed into boronic acid pinacol ester group, 19 was
obtained in a total yield 50%. The longest conjugated
precursor 20 could be achieved by coulping 16 and 19 in yield
55%,
unfortuantaly
the
oxidativecyclization
with
DDQ/BF3·EtO2 could not afford the target oligomer 4.
MALDI-TOF spectrum was used to monitor the reaction
process, after 12 hours main product was incomplete oxidation
intermediate, the extension of time led to the chlorinated
intermediate, not target product.
2
was performed by coupling 2,2'-di-bromo-4,4'-di-tert
-butylbiphenyl 8 and 4-bpin-2,7-di-tert-butylpyrene 9 through
the Pd(0)-catalyzed Suzuki reaction, but unfortunately the
Suzuki copuling reaction could not provide the target
precursor 10. The products of coupling 8 and 9 were
complicated.
Scheme 2. Synthetic route to the oligomer 4
Scheme 1. The synthetic route to oligomers 1, 2 and 14
O
Br
B
O
Pd(PPh3)4
Br
O
11
+
B
O
NaHCO3,
THF/H2O
reflux
Br
FeCl3
CH2Cl2
0oC
O
Pd(PPh3)4
B
O
1
16
+
Br
NaHCO3,
THF/H2O
reflux
15
16
Pd(PPh3)4
DDQ/Acid
CH2Cl2
7
6
5
13
3
+
NaHCO3,
THF/H2O
reflux
NaHCO3,
THF/H2O
reflux
O
B
Pd(PPh3)4
Pd(PPh3)4
O
17
Br
x
Br
+
O
O
O
B
B
Br
O
O
O
B
9
8
10
Pb(dppf)2Cl2
DMF,80oC
O
O
O
O
I
B B
Pd(PPh3)4
O
O
Br
B
Br
9
18
19
+
NaHCO3,
THF/H2O
reflux
Pb(dppf)2Cl2
DMF,80oC
11
19
13
Pd(PPh3)4
12
DDQ/Acid
CH2Cl2
+
16
4
NaHCO3,
THF/H2O
reflux
Pd(PPh3)4
FeCl3
Cl
Cl
Cl
Cl
10
13
12
+
NaHCO3,
THF/H2O
reflux
CH2Cl2
0oC
20
DDQ/Acid
CH2Cl2
10
2
Precursor 23 with non-K-region at the end was designed for
insight of the oxidativecyclization, 16 and 4-tert-butylphenyl
boronic ester was coupled to afford 21. Compound 21 was
transformed into the intermediate 22 in a total yield 49%.
Precursor 23 was obtained by coupling 21 and 22 in yield 58%.
The cyclodehydrogenation of 23 was performed by oxidative
cyclization with DDQ/BF3·EtO2, but the target product could
not be obtained. Samely after 12 h MALDI-TOF spectrum
showed that the main product was incompletely oxidative
intermediate. The extension of time led to the chlorinated
intermediate. FeCl3 was also used to achieve the target
product, but the main products were complicated.
14
To achieve precursor 10 in good yield, a key intermediate 12
was obtained by coupling1-bromo-2-ido-4-tert-butylbenzene
11 and 9 at 65 C in yield 81%, then the bromo-group of 10
o
was converted into boronic acid pinacol ester group at the
presence of bis(pinacolato)diboron, CH3COONa, and
Pd(dppf)2Cl2 in solution of DMF, compound 13 was obtained
in a yield 67%. The precursor 10 can be achieved by coupling
compounds 12 and 13 in a good yield 79%. As the general
method for oxidative cyclodehydrogenation, FeCl3 was used
firstly for achieving product 2, but the MALDI-TOF showed
that the main product of cyclodehydrogenation was
tetra-chlorinated side product 14 (see supporting information),
and separated in a yield 85%. The attempt of decreasing the
temperature and controlling the reaction time still could not
afford the product 2. Another attempt for cyclization of 10
with DDQ/BF3·EtO2 was performed and gave the product 2 in
a yield 69%. The product 1 can be also obtained with
treatment of DDQ/BF3·EtO2 from the precursor 7 in a yield
81%. To achieve the enlarged graphene pieces, the
dibromo-intermediate 16 was synthesized from 11 and
4,9-bis-bpin-2,7-di-tert-butylpyrene 15 by selective Suzuki
coupling in yield 44%. Precursor 17 could be obtained by
coulping 13 and 16 in yiled 85%, then the oxidition for
cyclization with DDQ/BF3·EtO2 to afford 3 in a yield 69%.
Intermediate 18 can be obtained by coulping 13 and excess 16,
the product was difficult to separate, after the bromo-group
Scheme 3. Synthetic route to the precursor 23 without the
K-region at the end
O
O
B
O
O
Pd(PPh3)4
Pd(dppf)2Cl2
DMF 80oC
Br
B
16
+
+
NaHCO3
THF/H2O
reflux
22
21
Pd(PPh3)4
22
21
NaHCO3
THF/H2O
reflux
23

To achieve longer ladder-type graphene ribbon oligomers,
another synthetic route was designed as shown in scheme 4. In
this route the precursor 26 could be transformed into
ladder-type graphene oligomer by the cyclodehydrogenation
of only 2H. All the reactions were detected by the
MALDI-TOF. 25 was obtained from 1 through two step
reaction easily. No change was obviously shown in MS
spectrum of 26 from 25, but the UV and PL spectra show blue
shit from 25 to 26. Combining the MS and UV, PL spectra, we
can conclude the structure of precursor 26 may be correct. But
unfortunately the oxidative cyclization with BF3·EtO2 could
still not afford the target oligomer, the MALDI-TOF showed
the chlorinated product only. The effort showed that more than
five repeated units ladder-type graphene oligomers or polymer
with this kind structure could not be obtained.
Scheme 4. Synthetic route to the precursor 26
Sn
Sn
Br2, Fe
Br
24
1
PtCl2, toluene
25
Figure 2. Normalized absorption (a) and emission (b) spectra for
1, 2, 14, and 3in solution CH2Cl2. PL λex=350 nm. Concentrations:
[M] =4×10-6mol/L.
DDQ/Acid
CH2Cl2
chlorination
The film absorption and PL spectra are shown in the
supporting information. The absorption of the oligomers 1, 2,
3 and 14 are similar to those in solution, peaked at 334, 343,
347 and 350 nm, respectively. The film absorption of 1, 2 and
14 show slight red-shift by comparing with in the absorption,
but the film absorption of 3 shows slight 4 nm blue-shift,
which indicates an J-aggregation of 3 in the film. The
photoluminescence spectra of the oligomers 1-3 and 14 in film
show evident differences to those in solution. Two emission
peaks of the oligomers 1 were shown at 447 and 507 nm, the
oligomers 2 shows one main emission peak at 470 with a
shoulder peak at 450 nm, the oligomers 3 shows one main
emission peak at 479 nm with a side peak at 450 nm, 14 shows
approximate one peak at 499 nm. The changes of the
photoluminescence spectra indicate that interchain π-π
stacking in film still occurs in the oligomers 1-3 and 14.
26
The normalized solution UV-vis absorption and
photoluminescent spectra of the oligomers 1, 2, 3 and 14 are
shown in Figure 2, panels a and b, respectively. The oligomers
1-3 and 14 show similar absorption with a maximum at 332,
342, 351 and 347 nm with broad sidebands, respectively. With
the increase of chain length the maximum absorption and
broad sideband of oligomers all red-shifted. The
tetra-chlorinated product 14 shows a slight red-shift compared
to the compound 2. The photoluminescent spectra of the
oligomers 1-3 and 14 exhibit similar broad bands peaked at
427, 455, 462 and 471 nm, respectively. Usually the
ladder-type oligomersand polymers exhibit well-resolved
absorption and emission spectra, which is typical for a rigid,
planar, one-dimensional π-system,32,46 but theoligomers 1-3
and 14 are different in contrast to most stepladder and
ladder-type poly-p-phenylene-based compounds. The PL
quantum yields (ΦF) of all compounds in toluene solution
were measured with 9,10-diphenylanthracene as a reference
standard (toluene, ΦF 0.9), the quantum yield of the oligomers
1, 2, 3 and 14 are 0.37, 0.68, 0.30 and 0.67, respectively.
The thermal properties of the oligomers 1-3 and 14 were
examined by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC). The oligomers1, 2
and 14 show good thermal stability with high decomposition
temperatures of 430, 370 and 380 oC under nitrogen
atmosphere, respectively. The oligomers 3 shows lower
decomposition temperature of 295 oC under nitrogen
atmosphere (see the Supporting Information). DSC analysis of

the oligomers shows neither a melting process (Tg) nor other
14. These oligomers exhibit very intensive fluorescence in
solution, and show good thermal stability.
thermal processes from 30 to 300 oC.
Acknowledgments
1.6x10-4
This material is based on work supported by Natural Science
Foundation of China grants 21274037 and 21474026.Natural
Science Foundation of Hebei Province of China grants
(B2013201174).
1.2x10-4
3
8.0x10-5
Supplementary data
14
Experimental detailsand 1H, 13C NMR and MALDI-TOF
spectra, UV and PL spectra of all compounds in film and TGA
traces. This materialis available free of charge via the Internet at
http://dx.doi.org/.
4.0x10-5
2
1
0.0
-1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
2.0
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Graphical Abstract (for review)
ABSTRACT:

*Highlights
Pyrene-based ribbon oligomers have been synthesized through oxidative
cyclizations. Cyclodehydrogenation with different catalyst was discussed
in the detail. The oligomers present a bright blue light emission and
clearly characterized.