# Electro-Optical and Electrochemical Properties of a Conjugated Polymer Prepared by the Cyclopolymerization of Diethyl Dipropargylmalonate

Authors: Yeong-Soon Gal; Won-Chul Lee; Won Seok Lyoo; Sung-Ho Jin; Kwon Taek Lim; Young-Il Park; Jong-Wook Park
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The electro-optical and electrochemical properties of poly(diethyl dipropargylmalonate) were measured and discussed. Poly(diethyl dipropargylmalonate) prepared by (NBD)PdCl2 catalyst was used for study. The chemical structure of poly(diethyl dipropargylmalonate) was characterized by such instrumental methods as NMR (1H-, 13C-), IR, and UV-visible spectroscopies to have the conjugated cyclopolymer backbone system. The microstructure analysis of polymer revealed that this polymer have the six-membered ring moieties majorly. The photoluminescence peak of polymer was observed at 543 nm, which is corresponded to the photon energy of 2.51 eV. The cyclovoltamograms of the polymer exhibited the irreversible electrochemical behaviors between the doping and undoping peaks. It was found that the kinetics of the redox process of this conjugated cyclopolymer might be controlled by the diffusion-control process from the experiment of the oxidation current density of polymer versus the scan rate.


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## Body

## 1. INTRODUCTION

Considerable progress has been
made in the synthesis and optical characterization of conjugated organic
polymers [1–3]. Polymers having a conjugated backbone are expected to show
unique properties, such as electrical conductivity, paramagnetism, migration
and transfer of energy, color, chemical reactivity, and complex formation
ability [3–7]. Because of these properties, polyacetylene and its homologues
have been promising materials for photovoltaics, displays, lasers, nonlinear
optical materials, membranes for gas separation, and for liquid-mixture
separation and chemical sensors [4, 5, 8–12]. There are considerable interests
in the development of electronic devices incorporating polymeric semiconductors
owing to their superior properties, simplicity of fabrication, light weight,
and potentially lower cost when compared to conventional inorganic
semiconductors [1–3, 8, 13]. Among the various types of reported polymer
semiconductors, poly(p-phenylenevinylenes),
polythiophenes, poly(9,9-dialkylfluorenes), and their derivatives are widely
used in solar cell applications as p-type
donor materials [14, 15].The synthesis of conjugated polymer via the cyclopolymerization
of dipropargyl monomers has been very interesting method for the introduction of
conjugated system in the polymer main chain via an alternating intramolecular–intermolecular
chain propagation [16, 17]. 1,6-Heptadiyne itself had been polymerized by
various initiator systems to give poly(1,6-heptadiyne) with a polyene cyclic
structure [17]. However, the resulting poly(1,6-heptadiyne) was insoluble in
any organic solvent and unstable to air oxidation as like with that of
polyacetylene [17]. Thus it was difficult
for practical
applications to optoelectronic devices as an active material.  Introduction of substituents at 4-position of
1,6-heptadiyne can enhance the processibility and stability of the polyene
systems, thus a variety of substituted poly(1,6-heptadiyne)s were designed and
synthesized [5, 7, 10, 18–20].Diethyl dipropargylmalonate
(DEDPM) is a well-known cyclopolymerizable dipropargyl monomer since 1990 [21].
The polymerizations of
DEDPM have been performed by MoCl5, Mo(CO)6, and Cp2MoCl2 catalyst, and high oxidation state alkylidene molybdenum complexes such
as Mo(CHCMe2R)(N-2,6-C6H3-i-Pr2)(OCMe2CF3)2 and Mo(CHCMe2R)(N-2,6-C6H3-i-Pr)2(O-t-Bu)2 [21–24]. However, to date, the
electro-optical and electrochemical properties of the resulting polymer from
DEDPM were not
systematically characterized.Here, we prepared the conjugated
cyclopolymer by the cyclopolymerization of DEDPM by (NBD)PdCl2. And
we report the electro-optical and electrochemical properties of poly(DEDPM), a
representative conjugated cyclopolymer (Figure 1).Figure 1
Chemical structure of
poly(DEDPM).


## 2. EXPERIMENTAL

(Bicyclo[2.2.1]hepta-2,5-diene)dichloropalladium(II)
(NBD)PdCl2(Aldrich
Chemicals) was used without further purification. Diethyl
dipropargylmalonate (DEDPM) was prepared by the reaction of propargyl bromide
and the sodium ethoxide solution of diethyl malonate according to the
literature procedure [21]. The solvents were analytical grade
materials. They were dried with an appropriate drying agent and fractionally
distilled.Poly(DEDPM) was prepared by the polymerization
of DEDPM by using (NBD)PdCl2 catalyst.
The polymerization procedure of DEDPM by (NBD)PdCl2 is as follows. In a 25 mL reactor equipped with rubber
septum, 2.0 g (8.46 mmol) of DEDPM was added. Then 0.076 g of (NBD)PdCl2 and 8 mL of DMF were added into the polymerization reactor
(monomer to catalyst mole ratio: 30, initial monomer concentration: 0.85 M). After
24 hours at 90°C, the polymer solution diluted with 10 mL DMF was
precipitated into a large excess of methanol. The precipitated polymer was
filtered and dried in vacuum oven at 40°C for 24 hours. The polymer
powder was obtained in 56% yield.NMR (1H- and 13C-) spectra of polymer
were recorded on a Varian 500 MHz FT-NMR spectrometer (model: Unity INOVA) in CDCl3.
FT-IR spectra were obtained with a Bruker EQUINOX 55 spectrometer using a KBr
pellet. The optical absorption spectra were measured by an HP 8453 UV-visible Spectrophotometer. The
photoluminescence spectra were obtained by Perkin Elmer luminescence
spectrometer LS55 (Xenon flash tube) utilizing a lock-in amplifier system with
a chopping frequency of 150 Hz. Electrochemical
measurements were carried out with a Potentionstat/Galvanostat
model 273 A (Princeton
Applied Research). To examine electrochemical properties, the polymer
solution was prepared and the electrochemical measurements were performed under
0.1 M tetrabutylammonium tetrafluoroborate solution; containing acetonitrile.
ITO, Ag/AgNO3, and platinum wires were used as a working, reference, and
counter electrode, respectively.


## 3. RESULTS AND DISCUSSION

We used the
homogeneous (NBD)PdCl2 catalyst
for the preparation of poly(DEDPM), which shows excellent solubility in the
polymerization solvents and shows relatively stable. We had firstly used this
catalyst for the cyclopolymerization of nonconjugated diyne monomers [25]. Unlike the
polymerization behaviors of DEDPM by using Mo-based catalysts, which the
polymerization proceeded rapidly in initial reaction stage [21, 23], the polymerization of DEDPM by (NBD)PdCl2 catalyst proceeded in mild homogeneous manner
to give the moderate yield of polymer (56%). And the number-average molecular
weight and polydispersity of this polymer were 16,300 and 2.59, respectively. The polymers were generally soluble in such organic solvents
as chloroform, 1,4-dioxane, ethyl acetate, chlorobenzene, DMF, DMSO.We
characterized the chemical structure of poly(DEDPM) by using such instrumental methods as NMR (1H-, 13C-),
IR, and UV-visible spectroscopies. The FT-IR spectrum of polymer did not show the acetylenic C≡C bond stretching and acetylenic ≡C–H bond stretching frequencies of DEDPM. The 1H-NMR spectrum of poly(DEDPM) prepared
by (NBD)PdCl2 showed the broad vinyl protons of conjugated polymer
backbone at the region of 5.4–7.2 ppm. The proton peaks at 2.5–3.6 ppm and
3.9–4.1 ppm are originated from the methylene protons adjacent to the
conjugated carbons of polymer backbone and the methylene protons of ethyl side
chains, respectively. The methyl proton peak of ethyl side chains was observed
at 0.9–1.5 ppm.The13C-NMR spectrum of poly(DEDPM) did
not show any acetylenic carbon peaks (72.9, 79.6 ppm) of DEDPM. Instead, the 13C-NMR
spectrum of polymer showed new olefinic carbon peaks of the conjugated polymer
backbone at the region of 118–142 ppm. The carbonyl carbon peak was also
observed at 172 ppm. We obtained more detail information for the
microstructures of poly(1,6-heptadiyne)-based conjugated cyclopolymers from the
studies on the resonance for the quaternary carbon atoms [26]. It has been reported that the two
clusters of resonances for the quaternary carbon atoms in poly(DEDPM) can be
assigned to the quaternary carbons in five-membered rings (57-58 ppm) and
six-membered rings (54-55 ppm), respectively. Figure 2 shows the 13C-NMR
spectrum of poly(DEDPM) in the region of methine carbon peak. The peaks at
56–59 ppm were only observed, which means that the present poly(DEDPM) was
majorly composed of six-membered ring moieties.Figure 2
13C-NMR
spectrum of poly(DEDPM) in the region of methine carbon.The electro-optical properties of poly(DEDPM) were measured
and discussed. Figure3 shows the UV-visible and photoluminescence (PL) spectra
of poly(DEDPM) solution (0.025 wt.%, DMF). In our previous paper [27], we had
reported the UV-visible and PL spectra of poly(2-ethynyl-N-propagylpyridinium
bromide) which has similar polymer backbone, conjugated polyene, and tied
six-membered ring, it showed 460 nm of UV-visible maximum value and 510 nm of PL
maximum value at excitation wavelength of 460 nm. Poly(DEDPM) also exhibited
characteristic UV-visible absorption band at 442 nm and green PL spectrum at 543 nm corresponding to the photon energy of 2.28 eV. The reason why two polymers
showed different maximum values is that poly(DEDPM) does not have an extra
conjugation moiety attached to conjugated polyene which is a polymer backbone.Figure 3
Optical absorption
and photoluminescence spectra of poly(DEDPM)
solution.In order to characterize the electrochemical kinetic
behaviors and an electrochemically stable window, the cyclic voltammograms
(CVs) of poly(DEDPM) including a consecutive scan and various scan rates
(30 mV/s ~150 mV/s) were recorded as shown in Figure4(b). Typical CVs obtained
at the scan rate of 100 mV/s for poly(DEDPM) solution are presented in Figure 4(a).
We have observed that the shape of CVs is almost unchanged, concluding that
poly(DEDPM) is fairly stable without any severe degradation up to 30 cycles of the
consecutive scan. In Figure 4(b), as the speed of scan rate was increased, the
peak potentials were gradually shifted to higher potentials and the current
values were increased. Finally, the oxidation of poly(DEDPM) was occurred at 0.46 V (versus Ag/AgNO3), where the vinylene
group of conjugated polymer backbone might be oxidized in the scan. Poly(DEDPM)
also shows the irreversible reduction at −0.83 V. The redox current value was
gradually increased as the scan rate was increased. This result suggests that
the electrochemical process of poly(DEDPM) is reproducible in the potential
range of −1.5 ~ +1.5 V versus Ag/AgNO3.Cyclic voltammograms
of poly(DEDPM) [0.1M (n-Bu)4NBF4/DMF] (a) consecutive 30
scans under 100 mV/s and (b) 30 mV/sec ~120 mV/sec with various scan rates.
(a)(b)It has been reported that the relationship between the redox
peak current and the scan rate can be expressed as a power-law type as follows [28–30]:(1)ip,a=kvx,(2)Logip,a=logk+xlogv,where ip,a =
oxidation peak current density, v = scan rate, k = proportional
constant, and x = exponent of scan rate.Considering that electrode kinetics satisfies (1), the
electrochemical redox reaction is controlled by either the electron transfer
process, where x = 1, or the reactant diffusion process, where x = 0.5.  The relationship plot of the oxidation current
density (log ip,a) as a function of the scan rate (log v)
is shown in Figure 5. The oxidation current of poly(DEDPM) versus the scan rate
is approximately linear relationship in the range of 30 mV/sec ~150 mV/sec and
the exponent of scan rate x value is found to be 0.262. It is
explained by that the kinetics of the redox process may not reach to the
diffusion-control process and not be so fast electro-active although it has
very stable durability for electrochemical process.Figure 5
Plot of logip,a versus log v for poly(DEDPM).


## 4. CONCLUSIONS

The
electro-optical and electrochemical properties of a typical conjugated
cyclopolymer, poly(DEDPM), were measured and discussed. Poly(DEDPM) was
prepared by (NBD)PdCl2 catalyst in 56% yield. The chemical
structure of poly(DEDPM) was characterized to have the conjugated polymer
system with the designed repeating unit. The present poly(DEDPM) was found to
be composed of six-membered ring moieties majorly from the analysis of methine
carbon peaks in 13C-NMR spectrum. The
photoluminescence (PL) spectra of poly(DEDPM) showed that the photoluminescence peak is located at 543 nm
corresponding to the photon energy of 2.51 eV. The CVs of the polymer exhibited
the irreversible electrochemical behaviors between the doping and undoping
peaks. It was found that the kinetics of the redox process of poly(DEDPM) might be controlled by the diffusion-control process  from the experiment of
the oxidation current density of poly(DEDPM) versus the scan rate.



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*Source: 276027-2008-08-24.xml*