Click-type reaction of aromatic polyamines for improvement of thermal and optoelectronic properties

Article abstract of DOI:10.1021/ja8061612

A quantitative addition reaction between aromatic amino-substituted alkynes and tetracyanoethylene (TCNE), yielding donor-substituted tetracyanobutadiene chromophores, was for the first time employed as a click-type reaction to improve the thermal and optoelectronic properties of aromatic polyamines. The first reduction potentials or the LUMO levels of the aromatic polyamines were found to linearly decrease by the stepwise TCNE addition. Copyright

Full text of DOI:10.1021/ja8061612

Published on Web 10/04/2008
Click-Type Reaction of Aromatic Polyamines for Improvement of Thermal and
Optoelectronic Properties
Tsuyoshi Michinobu
Global Edge Institute, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Received August 6, 2008; E-mail: michinobu.t.aa@m.titech.ac.jp
Click chemistry can be defined as a highly efficient addition
reaction, essentially yielding no byproducts. The most famous and
well-studied click reaction is the copper(I)-catalyzed azide-alkyne
cycloaddition reaction (CuAAC), forming a triazole ring,1 which
has been widely used for the preparation of functional materials.2
However, the development of other click-type reactions has
simultaneously been pursued,3 because CuAAC has several limita-
tions, such as the need for a metal catalyst to improve the reaction
yield and regioselectivity, the explosive nature of the azide
substances, and the poor optoelectronic properties of the triazole
products. In particular, the last limitation is most significant when
this click chemistry is applied to conjugated systems in optoelec-
tronic devices. Indeed, there are some reports on the synthesis of
conjugated molecules and polymers,4,5 but the recently reported
photovoltaic performance was not remarkable.6
One of the reactants in the click-type reactions usually contains
a carbon-based multiple bond. Recently, it was found that electron-
rich alkynes activated by aromatic amino substituents undergo a
[2+2] cycloaddition reaction with a strong electron acceptor,
Figure 1. (a) Reaction of the aromatic polyamine containing electron-rich
alkynes with TCNE. (b) Changes in the UV-vis spectral shape and (inset)
the peak top value from 0 to 1 equiv TCNE addition in 1,2-dichloroethane.
tetracyanoethylene (TCNE), to form the cyclobutene rings, which
spontaneously open to yield the donor-substituted 1,1,4,4-tetracy-
anobuta-1,3-diene (TCBD) chromophores in quantitative yield at
room temperature.7 The advantages of this quantitative addition
reaction are that metal catalysts are not necessary and the products
feature strong charge-transfer (CT) interactions in the visible
absorption region, potent redox activities, and related properties.8
The controlled introduction of these chromophores into conjugated
systems, especially into conjugated polymers, is useful for opti-
mization of the electronic states, thereby leading to the enhanced
performance of the optoelectronic devices, such as organic light-
emitting diodes (OLEDs), organic field-effect transistors (OFETs),
and photovoltaic cells. We now report for the first time the
application of this reaction as a new click-type reaction to improve
the thermal and optoelectronic properties of conjugated polymers.
The fluorene-based aromatic polyamine bearing electron-rich
alkyne side chains was designed as an active precursor9 and was
prepared by the palladium-catalyzed amination reaction between
the dibromofluorene monomer and a tolane-based aniline mono-
mer.10 The molecular weight (Mn) and the polydispersity (Mw/Mn)
of the polymer, determined by gel permeation chromatography
(GPC) (eluent, THF), were 13 300 and 3.43, respectively. The
addition of a TCNE solution in 1,2-dichloroethane to the precursor
polymer at room temperature results in an immediate color change
from yellow to red, accompanying a gradual increase in a CT band
with the most intense absorption centered at ∼470 nm (Figure 1).
The presence of the isosbestic point (428 nm) in the UV-vis
spectral change indicates the absence of any undesired side
reactions. To reveal the quantitative yield of the polymer reaction,
the full TCNE adduct (x ) 1) was prepared and unambiguously
characterized. In the 13C NMR spectrum, 20 of the 22 desired peaks
in the aromatic region were explicitly observed (Figure 2SI
Figure 2. MALDI-TOF mass spectrum of the aromatic polyamine after
reaction with an equivalent amount of TCNE (x)1) (matrix, dithranol).
(Supporting Information)). In the IR spectra before and after the
reaction, a weak alkyne vibrational peak at 2207 cm-1 was replaced
by a strong cyano one at 2217 cm-1 (Figure 3SI). GPC showed a
reasonable increase in the molecular weight (Mn ) 14 400) and a
noticeable decrease in the polydispersity (Mw/Mn ) 2.00).11
Furthermore, strong evidence was obtained from the MALDI-TOF
mass spectra. Two sets of polymer peak profiles were observed
for both the precursor (x ) 0) and the TCNE adduct (x ) 1)
(Figure 1SI and Figure 2). The observed peak values and the
intervals substantiated the polymer structures. After the polymer
reaction, the peak interval was expanded in accordance with the
molecular weight of TCNE. Importantly, the addition of 1.5 equiv
of TCNE provided the same MALDI-TOF mass spectrum as Figure
2, indicating that the products are inert to excess TCNE. The blue
set of peaks suggests that the termination process of the polycon-
densation is debromination and hydrogenation.
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14074 J. AM. CHEM. SOC. 2008, 130, 14074–14075
10.1021/ja8061612 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Summary of the Thermal Properties of the Aromatic
Polyamines with Addition of Different Amounts of TCNE
TCNE addition x (equiv)
Tg (°C)a
Td (°C)b
0
125
129
141
146
149
152
291
368
381
387
400
409
0.2
0.4
0.6
0.8
1.0
a Glass transition temperature determined by DSC measurements. b Decom-
position temperature determined by derivative thermogravimetry.
In summary, the quantitative addition reaction between electron-
rich alkynes and TCNE was found to serve as a new click-type
reaction to improve the thermal and optoelectronic properties of
the aromatic polyamine. Advanced studies of device fabrication
using these polymers are currently under way. Since the addition
amount of the acceptor molecule can be regulated, fine-tuning of
the electronic states of the conjugated systems will be possible.
These findings should be applicable to a variety of conjugated
molecules and polymers.
Figure 3. (A) Differential pulse voltammetry (DPV) and (B) the Ered,1 of
the aromatic polyamine with addition of different amounts of TCNE (a,
0.2 equiv; b, 0.4 equiv; c, 0.6 equiv; d, 0.8 equiv; e, 1.0 equiv).
Very interestingly, when the most intense CT absorption is noted,
the peak top values shifted bathochromically with increasing amount
of TCNE addition, and the shift was almost saturated at the
equivalent amount of TCNE (Figure 1b, inset). The possibility of
the intermolecular aggregation was ruled out (Figure 4SI). If the
side-chain chromophores do not interact with each other, the peak
top values should be constant (Figure 5SI). This result suggests
that this click-type reaction can affect the electronic state of the
whole conjugated polymers. To prove this idea, electrochemical
measurements were performed for the polymers with different
TCNE addition amounts. Cyclic voltammetry (CV) of the precursor
polymer (x ) 0) displayed only reversible oxidation steps, whereas
the TCNE adducts showed reversible reduction steps together with
anodically shifted oxidations (Figure 6SI). Since it was difficult to
discuss the exact potentials from the CV measurements, differential
pulse voltammetry (DPV) was also measured. As reported for the
small chromophores,7 two well-resolved reduction steps ascribed
to the TCBD moieties of the polymers appeared (Figure 3A), while
the oxidation potentials were still elusive. The first reduction
potential, Ered,1, which is associated with the LUMO level of the
polymers, shifted anodically in a linear fashion with increasing
amount of the TCNE addition (Figure 3B). This result is consistent
with the UV-vis spectral change and again suggests that this click-
type reaction can be employed for tuning the energy levels of the
conjugated polymers.
Acknowledgment. This work was supported, in part, by a
Grant-in-Aid for Scientific Research and the Special Coordination
Funds for Promoting Science and Technology from MEXT, Japan.
Supporting Information Available: Experimental details on syn-
thesis and measurements. This material is available free of charge via
the Internet at http://pubs.acs.org.
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