Effective Synthesis of Ladder-type Oligo(p-aniline)s and Poly(p-aniline)s via Intramolecular SNAr Reaction

Article abstract of DOI:10.1021/acs.orglett.1c00363

Symmetric ladder-type oligo(p-aniline)s and poly(p-aniline)s were successfully synthesized by an intramolecular ring closure in a highly efficient SNAr reaction from oligo(p-phenylene)s and poly(p-phenylene)s with fluorine (F) and secondary amine (NH) groups. Unlike Cadogan ring closure, the newly designed cyclization reaction will not produce a mixture of symmetric and nonsymmetric structures. Moreover, the introduction of the F atom does not hinder Suzuki polymerization. The result indicates that preparing regular oligomers and polymers with a nitrogen bridge is possible.

Full text of DOI:10.1021/acs.orglett.1c00363

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Letter
Effective Synthesis of Ladder-type Oligo(p‑aniline)s and
Poly(p‑aniline)s via Intramolecular SNAr Reaction
Hui Wang, Hongchi Zhao,* Shuang Chen, Libin Bai, Zhiyi Su, and Yonggang Wu*
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ABSTRACT: Symmetric ladder-type oligo(p-aniline)s and poly(p-
aniline)s were successfully synthesized by an intramolecular ring
closure in a highly efficient SNAr reaction from oligo(p-phenylene)s
and poly(p-phenylene)s with fluorine (F) and secondary amine
(NH) groups. Unlike Cadogan ring closure, the newly designed
cyclization reaction will not produce a mixture of symmetric and
nonsymmetric structures. Moreover, the introduction of the F atom does not hinder Suzuki polymerization. The result indicates that
preparing regular oligomers and polymers with a nitrogen bridge is possible.
ully conjugated ladder-type macromolecules are of great
interest as potential active materials in electronic devices.1
Among the fully conjugated ladder-type materials, carbazole
is one of the archetypical molecules and is adopted as the most
promising building block in optoelectronic applications
because of its good stability and high charge-carrier mobility.
F
The development of synthetic strategies has promoted the
preparation of many unique structures and useful materials for
their physical, optical, and chemical properties.2 The fully
conjugated ladder-type polymers are an important member of
the conjugated polymer family. The unique multiple-stranded
architecture of the conjugated ladder polymer endows material
with a constrained chain conformation that results in good
conjugation, carrier mobility, and luminescence intensity, so it
has been widely investigated in organic light-emitting diodes
and the organic field effect.3 Despite remarkable advances in
the past decade, fully conjugated ladder-type macromolecules
still remain a major synthetic challenge.4
Scherf and Mullen et al. prepared carbazole-based ladder-type
̈
polymers with a methine bridge, ethene bridge, and ethane
bridge, which were all blue-green emitters with emission at
∼470 nm.9 Bo and coworkers synthesized a carbazole-based
ladder-type polymer with an azomethine bridge by a Bischler−
Napieralski reaction.10 Fang et al. prepared carbazole-based
fully ladder-type polymers with an ethene bridge via a RCM
reaction.6b Some other carbazole-based ladder-type polymers
with heteroatom bridges were also reported.11 However,
LPPPs with full nitrogen bridges, usually called ladder-type
poly(p-aniline)s, are attractive,12 and the desire for these
polymers is still present. Inspired by the previous research and
with our continuous interest in exploring the novel methods of
fully ladder-type polymers, we developed a strategy for
preparing LPPPs and oligo(p-phenylene)s with a full nitrogen
bridge that is promising for application in optoelectronic
devices.
The first ladder-type poly(p-phenylene) (LPPP) reported by
Scherf and Mullen in 1991 was synthesized through a popular
̈
two step approach: polymerization followed by ladderization.5
The fused-ring backbones of LPPP and its derivatives were
achieved by transition-metal-mediated polymerization, espe-
cially palladium-catalyzed Suzuki polycondensation. There
have been various intramolecular ring-closure reactions for
the ladder structure of the precursor polymers. In the
preparation of LPPPs, Lewis-acid-mediated Friedel−Crafts
ring annulations were used. Swager and coworkers reported
electrophile-induced cyclization.2c The acetylenic functional
group on the conjugated polymer could be easily cyclized in
In the preparation of ladder-type oligo(p-aniline)s and
poly(p-aniline)s, the intramolecular C−N bond-forming
reaction is the key step. The classical Ullmann reactions are
popular in the C−N bond-forming reaction.13 The palladium-
catalyzed version of C−N bond formation discovered by
Buchwald and Hartwig was a major breakthrough in this
field.14 Transition-metal-catalyzed reactions for C−N bond
the presence of trifluoroacetic acid. Mullen and coworkers used
̈
oxidative dehydrogenation catalyzed by FeCl3 to prepare 2D
graphene nanoribbons.2d Fang et al. developed a strategy for a
fully conjugated ladder-type polymer with an extremely low
level of unreacted defects via the thermodynamically controlled
ring-closing olefin metathesis (RCM) reaction.6 Meanwhile,
various kinds of fully conjugated ladder-type materials with
heteroatom bridges such as N, S, Si, B, and O have also been
reported,7 but fully conjugated ladder polymers with
heteroatom bridges are seldom.2b,8
Received: January 30, 2021
Published: February 26, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00363
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2217−2221
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formation usually proceed in the presence of halogen I, Br, or
Cl. The existence of a halogen may bring mixed cross-coupling
in the preparation of the ladder polymer precursor. Transition-
metal-catalyzed intramolecular oxidation and Cadogan ring-
closure reactions provide an efficient approach for C−N bond
formation,15 but the product of the isomers is an impediment
to the regular ladder structure. Halogen F is usually passive in
the transition-metal-catalyzed coupling reaction but is easily
substituted through an intramolecular SNAr reaction.16 We
combine the advantages of halogen F to design a synthetic
route for ladder-type oligo(p-aniline)s 1, 2, and 3 and poly(p-
aniline)s 4 shown in Figure 1.
Scheme 1. Synthetic Routes to Oligo(p-aniline)s 1, 2, and 3
1
terized with H and 13C nuclear magnetic resonance (NMR)
spectroscopy, mass spectrometry (MS), or matrix-assisted laser
desorption ionization−time-of-flight (MALDI-TOF) MS.
The desire for a fully ladder conjugated polymer with
nitrogen bridges kept the synthetic strategy going on, and the
polymerization of monomers 6 and 7 was carried out in a
biphasic mixture of aqueous NaHCO3 and tetrahydrofuran
(THF) with Pd(PPh3)4 as the catalyst (Scheme 2). After the
Figure 1. Structures of ladder-type oligo(p-aniline)s 1, 2, and 3 and
poly(p-aniline)s 4.
As shown in the Scheme 1, we started our synthesis by
preparing compounds 6 and 7. 6 was obtained via the reaction
of 1,4-dibromo-2,5-difluorobenzene with an excess of bis-
(pinacolato)diboron in the presence of the Pd(dppf)Cl2
catalyst in 31% yield. The key intermediate 7 was prepared
by coupling 4-tert-butylbenzeneboronic acid and 3,6-diamino-
2,7-dibomo-9-(2-octyldodecyl)-9H-carbazole catalyzed by cop-
per acetate Cu(CH3COO)2 in 80% yield. Similar to 7,
compound 5 was synthesized from 2-bromoaniline and 4-
tert-butylbenzeneboronic acid in 83% yield. A typical Suzuki−
Miyaura coupling of compounds 5 and 6 afforded precursor 8
of ladder-type oligo(p-aniline)s in 86% yield, which could be
smoothly converted to the desired ladder-type oligomer 1 in
91% yield in a mixture of NaH and dimethylformamide
(DMF) at 60 °C. The coupling of 2-fluorobenzeneboronic acid
and monomer 7 gave precursor 9 in 95% yield, which could be
smoothly converted to ladder oligo(p-aniline)s 2 in 94% yield.
The success of compounds 1 and 2 demonstrated that the
intramolecular SNAr reaction could easily proceed in the
presence of a base. The good yields and unique structures of
compounds 1 and 2 also indicated the possibility for ladder
poly(p-aniline)s. The enlarged conjugated system 3 with six
nitrogen bridges was obtained in 81% yield from the precursor
11, which was achieved by coupling compounds 6 and 10 in
85% yield. Compound 10 could be synthesized by coupling 2-
fluorobenzeneboronic acid and an excess of 7 in 41% yield. All
intermediates and final products were purified and charac-
Scheme 2. Synthetic Route to Ladder-type Poly(p-aniline)s
4
capping of bromo end groups with 2-fluorobenzeneboronic
acid, polymer 12 was obtained in 88% yield. The infrared (IR)
spectrum of polymer 12 showed an obvious absorption peak at
3411 cm−1 due to its secondary amine NH stretching
vibrations. The characteristic signal of the secondary amine
1
NH is shown at 5.41 ppm in the H NMR spectrum (Figure
2). After treatment with NaH in DMF, the ladder-type poly(p-
aniline)s 4 were achieved in 85% yield. The complete
disappearance of the characteristic signals of the secondary
1
amine NH in the H NMR spectrum (ppm 5.41) and the IR
spectrum (3411 cm−1) confirmed that the intramolecular ring-
closure SNAr reaction was efficient. Further evidence of the
formation of the ladder structure was the shift of the resonance
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Figure 2. IR and NMR spectra of the precursor 12 and ladder-type
poly(p-aniline)s 4.
peaks associated with the aromatic protons on the 4-tert-
butylbenzene groups (Hb) from 6.90 to 7.51 ppm and (Ha)
from 7.37 to 8.07 ppm. A series of evidence showed that
ladder-type poly(p-aniline)s were successfully prepared, and
the polymer was easily soluble in common organic solvents
such as dichloromethane and THF.
The molecular weights of polymers 12 and 4 were measured
by gel permeation chromatography (GPC) against polystyrene
standards with THF as an eluent. The number-average
molecular weight (Mn) and weight-average molecular weight
(Mw) of polymer 12 were 10.0 and 17.6 kg·mol−1, respectively.
After ring closure, the Mn and Mw of 4 were 10.7 and 18.4 kg·
mol−1, respectively. The molecular weights of the precursor
polymer and the ladder polymer were nearly same. Thermal
stabilities of ladder-type oligo(p-aniline)s 1, 2, and 3 and
poly(p-aniline)s 4 were measured by thermogravimetric
analysis (TGA) thermograms. 1 and 2 exhibited good thermal
stability up to 250 °C. Weight loss (5%) started at nearly the
same temperature, 405 °C, for 1 and 2 and at a lower
temperature, 315 °C, for 3 and 4. When the temperature
increased to 800 °C, the weight loss of 1 was nearly 100%, and
3 and 4 showed nearly the same 50% weight loss. The larger
conjugated structure may cause more residues of 3 and 4.
Differential scanning calorimetry (DSC) studies indicated that
oligomer 2 had a melting point at 157 °C, and no melting
point or phase transition was observed for oligomer 1 or 3 or
polymer 4.
Figure 3. (a) UV and (b) PL spectra of ladder-type oligo(p-aniline)s
1, 2, and 3 and poly(p-aniline)s 4 in CH2Cl2.
type polymer 4 also indicate that the ladder structure was
formed in polymer 4.
The electrochemical behaviors of oligomers 1, 2, and 3 and
polymer 4 in CH2Cl2 solution were investigated by cyclic
voltammetry. As shown in Figure 4, no obvious cathodic wave
was observed for polymer and oligomers. All of the ladder-type
compounds showed reversible processes in the first redox
cycle, and with increasing chain length, the oxidation peaks
moved to more negative values. The band gaps were estimated
The optical properties of all oligo(p-aniline)s 1, 2, and 3 and
poly(p-aniline)s 4 were acquired in diluted dichloromethane
solutions. The normalized solution ultraviolet (UV) and
photoluminescence (PL) spectra are shown in Figure 3. The
UV and PL spectra of the precursors 8, 9, 11, and 12 (see the
Supporting Information) and ladder-type compounds 1, 2, 3,
and 4 present significant differences that could be attributed to
the changes in the conformation. With the increase in the main
chain length, all of the UV absorption and PL emission peaks
red-shifted. The PL quantum yields of compounds 1−4 were
0.93, 0.53, 0.38, and 0.45, respectively. In contrast with LPPPs,
all of the ladder-type compounds did not exhibit well-resolved
absorption or emission spectra, and the results are similar to
some previous literature. The nontypical UV and PL spectra of
ladder-type compounds could possibly be attributed to the
weakness of HOMO−LUMO transitions.6a The differences in
the UV and PL spectra of precursor polymer 12 and ladder-
Figure 4. Cyclic voltammograms of ladder-type oligo(p-aniline)s 1, 2,
and 3 and poly(p-aniline)s 4.
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from the onsets of the absorption spectra. The HOMO,
LUMO, and band gap values are listed Table 1.
ACKNOWLEDGMENTS
■
This material is founded by the Natural Science Foundation of
Hebei Province, China (B2019201337, B2018201281).
Table 1. HOMO and LUMO Energies and Band-Gap Data
of Ladder-type Oligo(p-aniline)s 1, 2, and 3 and Poly(p-
aniline)s 4
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In conclusion, we have described a new approach for the
synthesis of regular ladder-type oligo(p-aniline)s and poly(p-
aniline)s via the intramolecular SNAr reaction as the ring-
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ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge at
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Experimental details, H, 13C, 19F NMR, ESI-TOF, MS,
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compounds, GPC elution curves, TGA and DSC traces,
and DFT calculations (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Hongchi Zhao − College of Chemistry and Environmental
Science, Hebei University, Baoding 071002, P. R. China;
Email: zhc@hbu.edu.cn
Yonggang Wu − College of Chemistry and Environmental
Science, Hebei University, Baoding 071002, P. R. China;
orcid.org/0000-0002-6521-2376; Email: wuyonggang@
hbu.edu.cn
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Authors
Hui Wang − College of Chemistry and Environmental Science,
Hebei University, Baoding 071002, P. R. China
Shuang Chen − College of Chemistry and Environmental
Science, Hebei University, Baoding 071002, P. R. China
Libin Bai − College of Chemistry and Environmental Science,
Hebei University, Baoding 071002, P. R. China;
orcid.org/0000-0002-7202-1746
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Zhiyi Su − College of Chemistry and Environmental Science,
Hebei University, Baoding 071002, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00363
Notes
The authors declare no competing financial interest.
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