Ladder-type conjugated oligomers prepared by the Scholl oxidative cyclodehydrogenation reaction: Synthesis, characterization and application in field effect transistors

Article abstract of DOI:10.1039/c5tc00354g

Two novel well defined ladder-type conjugated oligomers have been successfully designed and synthesized through a solution processing method in an excellent yield. The field effect transistors (FETs) fabricated by these ladder-type oligomers with a nice planar structure exhibit excellent charge carrier mobilities, up to 0.10 cm² V^-1 s^-1 and 0.33 cm² V^-1 s^-1; furthermore, the devices can work well with a low gate voltage. The ladder-type oligomers are both converted from two precursor co-oligomers, poly(2,7-(1,2,-diphenylethene)-9,9-dioctylfluorene) (PDPF), via an anhydrous FeCl₃ oxidative cyclodehydrogenation. The pronounced red shift shown in the preliminary photoluminescence spectra and the changes of band gaps measured by electrochemical analysis both testify that the better electronic transmission capacity in the FET performance is due to the expanded molecular chain planarization after the chemical cyclodehydrogenation. Interestingly, the precursor oligomers having a linear-type chain and a zigzag-type chain (L-PDPF and Z-PDPF, respectively) show many characteristic differences in their thermal, optical and electrochemical properties. The differences caused by the different types of main chains demonstrate that the macromolecular configurations have a tremendous impact on the functioning of the oligomers.

Full text of DOI:10.1039/c5tc00354g

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Ladder-type conjugated oligomers prepared by
the Scholl oxidative cyclodehydrogenation
reaction: synthesis, characterization and
application in field effect transistors†
Cite this:DOI: 10.1039/c5tc00354g
Wei Huang, Hejian Zhang, Ji Ma, Moyun Chen, Haoyun Zhu and Weizhi Wang*
Two novel well defined ladder-type conjugated oligomers have been successfully designed and synthesized
through a solution processing method in an excellent yield. The field effect transistors (FETs) fabricated by
these ladder-type oligomers with a nice planar structure exhibit excellent charge carrier mobilities, up to
0.10 cm2 Vꢀ1 sꢀ1 and 0.33 cm2 Vꢀ1 ꢀ1; furthermore, the devices can work well with a low gate voltage. The
s
ladder-type oligomers are both converted from two precursor co-oligomers, poly(2,7-(1,2,-diphenylethene)-
9,9-dioctylfluorene) (PDPF), via an anhydrous FeCl3 oxidative cyclodehydrogenation. The pronounced red
shift shown in the preliminary photoluminescence spectra and the changes of band gaps measured by
electrochemical analysis both testify that the better electronic transmission capacity in the FET performance
is due to the expanded molecular chain planarization after the chemical cyclodehydrogenation. Interestingly,
the precursor oligomers having a linear-type chain and a zigzag-type chain (L-PDPF and Z-PDPF,
respectively) show many characteristic differences in their thermal, optical and electrochemical properties.
The differences caused by the different types of main chains demonstrate that the macromolecular
configurations have a tremendous impact on the functioning of the oligomers.
Received 5th February 2015,
Accepted 26th April 2015
DOI: 10.1039/c5tc00354g
www.rsc.org/MaterialsC
ladder-type frameworks, not only form graphene-like structures,
but also possess some unique characteristics such as largely
Introduction
Graphene as a promising two dimensional material has attracted defect-free accurate structures and excellent film forming proper-
great interest for a wide range of electronic applications, and the ties.24,25 In addition, unlike the zero-band gap graphene, ladder-
related research has developed rapidly in recent years.1–7 It can type conjugated oligomers (LCOs) with different band gaps show
be obtained by a great number of physical methods, such as the an outstanding semiconducting nature and are fit to be applied
mechanical exfoliation of graphite,8 the unzipping of carbon in manifold electronic devices as the active materials.26–28
nanotubes,9 lithography10 and the reduction of graphite oxide.11
Up to now there exist two principal chemical methods to
Inspired by these developments, many research groups have synthesize ladder-type materials: one is the polymerization of
devoted plenty of effort to the design and synthesis of diverse some multifunctional monomers, generating a ladder type
graphene-like materials in the fields of oligomers and macro- material in just a single reaction; the other is the cyclization
molecules.12–17 Within this research, the conjugated oligomer is of suitably functionalized open-chain precursors in a polymer-
a category of significant materials. The applications of conju- homologous process.29–32 Compared with physical methods,
gated oligomers in photovoltaic cells, electroluminescent diodes, chemical synthesis could precisely control the structure to give
thin film transistors and chemical sensors have been paid the desired shape and dimension of the achieved products.
more and more attention.18–23 Compared to small molecules, Because it is more atom-economical and straightforward, late-
the conjugated oligomers, especially those with ribbon- or stage cyclization by the direct transformation of C–H bonds has
been studied by many chemists.33–35 K. Mu¨llen’s group36,37 and
W. R. Dichtel’s group38 have both employed an efficient way to
State Key Laboratory of Molecular Engineering of Polymers, Department of
Macromolecular Science, Fudan University, Shanghai 200433, P. R. China.
E-mail: weizhiwang@fudan.edu.cn; Tel: +86 021-65643836
prepare well defined polycyclic aromatic hydrocarbons by the
Scholl oxidative cyclodehydrogenation reaction, in which a pre-
cursor for a potential graphene-like molecule is oxidized by an
iron(III) chloride (FeCl3)–nitromethane system. Inspired by the
above examples and the properties of LCOs, we propose to
develop a method for synthesizing a novel type of LCO to
† Electronic supplementary information (ESI) available: Experimental section
including materials, measurements and characterizations; mass spectra, NMR
data for new compounds and oligomers, cyclic voltammetry figures and GPC data
for the oligomers. CCDC 1043775 and 1043776. For ESI and crystallographic data
in CIF or other electronic formats see DOI: 10.1039/c5tc00354g
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overcome the drawbacks of small molecules. Moreover, the LCOs (E)-1,2-diphenylethene-1,2-bis(9,9-dioctyl-9H-fluoren-2-yl) (L-DPBF)
will be applied to manufacture high-performance semiconductor and (Z)-1,2-diphenylethene-1,2-bis(9,9-dioctyl-9H-fluoren-2-yl)
devices like FETs.39,40
(Z-DPBF) and their oxidation products L-DPBF-O and Z-DPBF-O
In this work, 9,9-dioctylfluorene and two isomers of 1,2- were synthesized to elucidate the cyclodehydrogenation. The
diphenylethene were chosen as the monomeric units. The efficient planarization of the precursor oligomers into the LCOs
1
reason for choosing fluorene is that the fluorene-based polymers was corroborated by H NMR and FT-IR spectra. Subsequently,
have exhibited favourable chemical and thermal stability and these LCOs were fabricated into FETs as the semiconductor layer
exceptionally high photoluminescence, electroluminescence and to measure their performance. Additionally, the relevant proper-
fluorescence quantum efficiency in thin solid films.41–45 More- ties to prove whether the intramolecular cyclization enhances
over, the long alkyls in the 9-position of the fluorene unit can the electronic transmission capacity by extending the molecular
make the target oligomers exhibit good solubility in common plane are all discussed as well in this work.
organic solvents. Prior to the polymerization, the cis and trans
isomers of the 1,2-diphenylethene monomers were successfully
synthesized, and the isomers were used to obtain two alternating
co-oligomers only differing in their configurations: one is
Results and discussion
(E)-poly(2,7-(1,2,-diphenylethene)-9,9-dioctylfluorene) (L-PDPF) The overall synthetic procedures for the model molecules, mono-
which possesses a linear-type chain, and the other, (Z)-poly- mers and oligomers are clearly outlined in Scheme 1. It is easy to
(2,7-(1,2,-diphenylethene)-9,9-dioctylfluorene) (Z-PDPF), has a see that the linkages of 1,2-diphenylethene in the obtained
zigzag-type chain. Interestingly, the disparate fluorescence L-PDPF and Z-PDPF actually exhibit cis–trans isomerism, so these
emission colours of these two oligomers under the same UV two precursor oligomers possess different main chain configura-
irradiation inspired us to consider the influence of the different tions. To acquire these two configurations, the monomers 1 and
configurations on their performance as well. Therefore, we 2 were directly synthesized according to the methods described
further investigated how the different configurations affect the in the literature.46,47 What is more, the pure single crystals of
properties of these oligomers. Most importantly, L-PDPF and these monomers were both fortunately recrystallized from their
Z-PDPF can be converted into oligomers of a ladder type (L-PDPF-O CH2Cl2–n-hexane solution. The accurate structures of 1 and 2 were
and Z-PDPF-O) after the FeCl3 oxidation. Accordingly, the identified by the X-ray crystallographic analysis in Fig. 1 plus the
corresponding model molecules of the precursor oligomers, data in the ESI† (Table S1). Another fluorene derivative monomer,
Scheme 1 Synthetic approaches for the monomers, model molecules and oligomers. (i) CCl4, ꢀ12 1C/1 h; (ii) Pd(PPh3)4–K2CO3, toluene–H2O, 95 1C/5 days;
(iii) FeCl3, CH2Cl2–CH3NO2, room temperature/overnight; (iv) Pt(PPh3)4, DMF, 90 1C/24 h. R: C8H17
.
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to replace THF and the reaction temperature was brought up to
95 1C. This is likely due to the low temperature negatively
influencing the activities of the monomers, which results in a
deviation of their actual concentration ratio in solution; thereby
the monomers could not react in the envisioned proportion in
this alternating copolymerization.
After that, we expected to obtain the LCOs with greater
conjugated planarization via one-step chemical cyclization. An
efficient reaction route, called the Scholl reaction on the basis of
ref. 50, was adopted. Herein, anhydrous FeCl3, which has been
used for the synthesis of polycyclic aromatic hydrocarbons because
of its avoidance of severe disadvantages such as dealkylation,
chlorination or migration of the alkyl substituents, is utilized as
an oxidant for these transformations.51 At the end, methanol
is poured into the mixture to consume the extra anhydrous
FeCl3, and the oxidized products are purified through column
chromatography.
All the prepared monomers and final products were care-
fully purified and further characterized by nuclear magnetic
resonance (NMR) spectra, and the successful intramolecular
cyclization can demonstrated by these 1H NMR spectra. For
instance, the 1H NMR spectra of Z-PDPF and Z-PDPF-O are shown
in Fig. 2a. It is pronounced that the peaks within the 7–8 range
representing the hydrogens on the phenyl rings were weakened
drastically after the oxidative cyclodehydrogenation, which might
Fig. 1 The ORTEP drawings of 1 and 2.
2,7-dibromo-9,9-dioctylfluorene, was prepared in advance as
reported.48 Normally, the long alkyl chains in the 9-position
can enhance the solubility of the oligomer in organic solvents
like THF and toluene, which allows solution-processing and thus
is beneficial to the degree of polymerization.
The elemental approach employed for the synthesis of the
precursor is a Suzuki cross-coupling reaction,49 as described in
Scheme 1. Obviously, the difference between these two oligomers
is that L-PDPF presents a linear-type main chain and Z-PDPF has
a zigzag-type chain, as pictured in the schematic. The much
more flexible zigzag-type chain enables the building blocks in its
twisted chain to rotate more easily. As displayed in Table 1, the
Mw values of L-PDPF and Z-PDPF were determined, using gel
permeation chromatography (GPC, Fig. S1 and S2, ESI†) with
THF as the eluent and polystyrene as the standard, to be 9900
and 6800. Their polydispersity indices (PDIs) calculated by
means of Mw/Mn are 1.32 and 1.36, respectively. THF and toluene
were both tested as the solvent in the polymerization. The
reaction temperature was 60 1C when the solvent is THF, which
led to products with a lower degree of polymerization. The
molecular mass increased obviously when toluene was selected
Table 1 Molecular weights of L-PDPF and Z-PDPF
Oligomer
Yield (%)
Colour
Mw
PDI
1
Fig. 2 (a) H NMR spectra of Z-PDPF and Z-PDPF-O; (b) 1H NMR spectra
L-PDPF
Z-PDPF
44.7
52
Yellow green
Yellow
9900
6800
1.32
1.36 of Z-DPBF and Z-DPBF-O. R: C8H17
.
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indicate the formation of C–C bonds between the phenyl rings
to produce more six-membered rings in the backbones to take
on a ladder-type structure. The detailed 1H NMR spectra of the
model compounds (Z-DPBF and Z-DPBF-O) in the ESI† (Fig. S19
and S21) demonstrate clearly the cyclization between the sub-
stituted phenyl rings. Simultaneously, the corresponding
1H NMR spectra of the products in the L-PDPF system exhibit
the same trend. The complete 1H NMR and 13C NMR spectra of
all the synthesized products, which are included in the ESI†
(Fig. S3–S22), could also be further evidence to illustrate the
success of these syntheses. Moreover, the decline of the aromatic
proton signals after cyclization has been reported in ref. 15, 16
and 37. However, the reason for the decline has lacked a reliable and
reasonable explanation until now. A variable-temperature experi-
ment (25 1C to 50 1C) of Z-PDPF-O in 1,1,2,2-tetrachloroethane-d2
solution was carried out (Fig. S23, ESI†). The unchanged spectra
at different temperatures indicate that the chemical shifts of the
aromatic resonances are independent of temperature, and it is
not attributed to aggregation.
Moreover, the mass spectrometric data of the small mole-
cules 1 and 2 and the model molecules are used to demonstrate
the successful synthesis in the ESI† (Fig. S24–S29). The diver-
gences between the mass spectral data of the model molecules
before and after chemical oxidation also testify to the successful
synthesis of the expected oxidized products. Exactly four hydro-
gens are removed after the oxidation. The cyclization proces-
sing could be further confirmed by the measurement of Fourier
transform infrared (FT-IR) spectra, as shown in Fig. 3. From
Fig. 3a and b, the band at about 4050 cmꢀ1 can be taken as a
palpable marker of the existence of the free rotating phenyl
rings in the molecules,36 therefore, it is distinctly observed in
the spectra of L-PDPF and Z-PDPF. The disappearance of this
peak in the spectra of the obtained LCOs indicates the absence
of the hydrogens on the substituted phenyl rings. Altogether,
the changes after the cyclodehydrogenation demonstrate the
Fig. 3 (a) Part of the FT-IR spectra within the near infrared region (4000–
4200 cmꢀ1) of L-PDPF and L-PDPF-O; (b) part of the FT-IR spectra within
the near infrared region (4000–4200 cmꢀ1) of Z-PDPF and Z-PDPF-O.
(c) Part of the FT-IR spectra within the middle infrared regions (550–1120 cmꢀ1
,
2550–3250 cmꢀ1) of L-PDPF and L-PDPF-O; (d) part of the FT-IR spectra
within the middle infrared regions (550–1120 cmꢀ1, 2550–3250 cmꢀ1) of
Z-PDPF and Z-PDPF-O. T means transmissivity.
Table 2 The UV-vis absorption and PL data of the oligomers in THF
solution
a
b
d
Oligomer
labs (nm)
lem (nm)
DSc (nm)
FF (%)
te (ns)
L-PDPF
L-PDPF-O
402
405
420
451
418
447
498
551
16
42
78
81.7
62.9
3.07
39.5
0.501
0.507
1.114
1.988
restrictions to the movement of the substituted phenyl rings. Z-PDPF
Additionally, in Fig. 3c and d, the pronounced disappearance of
Z-PDPF-O
100
a
b
c
the characteristic peaks of these oligomers can be observed by
comparing the spectra from before and after the oxidation.
These reveal the disappearance of the out of plane C–H defor-
mation bands at 698, 728 and 775 cmꢀ1, which are typical for
mono- and disubstituted phenyl rings, and the noticeable triad
Absorption maximum. Emission maximum. Stokes shift calculated
d
from the absorption and emission spectra. Quantum yield estimated
with quinine sulfate (FF = 54% in 0.1 M H2SO4) or fluorescein (FF = 92%
e
in 0.1 M NaOH) as standards. Fluorescence lifetime.
of peaks of the aromatic C–H stretching vibrations at 3023, 3056 498 nm to 551 nm. All of these bathochromic-shifted tendencies
and 3079 cmꢀ1 are attenuated, which verifies the efficient indicate that the broader planarization of the molecular chain
conversion from PDPF into PDPF-O.
and the degree of conjugation after the oxidation are increased
The UV-vis absorption and photoluminescence (PL) data of because of the cyclization between the substituted phenyl rings.
the oligomers in THF are listed in Table 2. L-PDPF-O displays The PL efficiencies (FF) of the oligomers were measured in dilute
an absorption wavelength (labs) at 405 nm, resulting in a 3 nm THF solution using quinine sulfate (FF = 0.54 in 0.1 M sulfuric
red shift compared with that of L-PDPF. The related absorption acid solution) or fluorescein (FF = 92% in 0.1M NaOH solution)
spectra of the obtained oligomers are displayed in the ESI† (Fig. S30), as a reference. The FF of L-PDPF is high, 81.7%, compared to
and their PL spectra have a similar red-shifted behaviour with that of Z-PDPF, merely having 3.1%.
the emission peak moving from 418 nm to 447 nm. For Z-PDPF
It could be observed intuitively by our naked eyes from
and its oxidized product Z-PDPF-O, the corresponding spectral Fig. 4a that L-PDPF emits luminous blue light, while Z-PDPF
shift shows a similar trend: the absorption maximum of Z-PDPF emits faint yellow light under the same 365 nm UV irradiation.
is at 420 nm, compared with 451 nm for Z-PDPF-O. Furthermore, We consider the tremendous intensity difference to be induced
their emission wavelengths exhibit a similar red shift from by the molecular configurations. The zigzag chains of Z-PDPF
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