Synthesis and characterization of a main-chain donor-acceptor type low-bandgap polymer by post-functionalization of a poly(arylene ethynylene)

Article abstract of DOI:10.1016/j.reactfunctpolym.2014.07.012

I report a facile synthetic route for synthesizing a main-chain donor-acceptor type polymer containing strong electron donating dialkylamino groups and strong electron accepting 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) units. To accomplish this, an electron donating monomer 4,6-diethynyl-N,N,N,N- tetrahexylbenzene-1,3-diamine was successfully synthesized by converting two carbaldehyde groups in the corresponding monomer into acetylene groups using lithium trimethylsilyldiazomethane via Colvin rearrangement. This electron donating monomer was then polymerized with a carbonyl-activated diiodide monomer to afford an electron-donating π-conjugated precursor polymer with a reasonably high molecular weight, which was further reacted with TCNE via cycloaddition/retroelectrocyclization reaction under mild conditions to afford the target polymer with a low bandgap energy (Egopt=1.40eV and EgCV=1.10eV) arising from strong intramolecular charge-transfer interactions between electron donors and acceptors in the polymer.

Full text of DOI:10.1016/j.reactfunctpolym.2014.07.012

Accepted Manuscript
Synthesis and characterization of a main-chain donor-acceptor type low-bandg-
ap polymer by post-functionalization of a poly(arylene ethynylene)
Wenyi Huang
PII:
S1381-5148(14)00139-4
DOI:
http://dx.doi.org/10.1016/j.reactfunctpolym.2014.07.012
REACT 3405
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Reactive & Functional Polymers
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14 June 2014
5 July 2014
Please cite this article as: W. Huang, Synthesis and characterization of a main-chain donor-acceptor type low-
bandgap polymer by post-functionalization of a poly(arylene ethynylene), Reactive & Functional Polymers (2014),
doi: http://dx.doi.org/10.1016/j.reactfunctpolym.2014.07.012
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Synthesis and characterization of a main-chain donor-acceptor type low-
bandgap polymer by post-functionalization of a poly(arylene
ethynylene)**
Wenyi Huang*
Department of Materials Science and Engineering, The Dow Chemical Company, Midland,
Michigan 48667, United States
*Corresponding author: E-mail: whuang1@dow.com (W. Huang)
** This work was done in the Department of Polymer Engineering, The University of Akron.
ABSTRACT
I report a facile synthetic route for synthesizing a main-chain donor-acceptor type
polymercontaining strong electron donating dialkylamino groups and strong electron accepting
1,1,4,4-tetracyanobuta-1,3-diene (TCBD) units. To accomplish this, an electron donating
monomer 4,6-diethynyl-N,N,N,N-tetrahexylbenzene-1,3-diamine was successfully synthesized
by converting two carbaldehyde groups in the corresponding monomer into acetylene groups
using lithium trimethylsilyldiazomethane via Colvin rearrangement. This electron donating
monomer was then polymerized with a carbonyl-activated diiodide monomer to afford an
electron-donating π-conjugated precursor polymer with a reasonably high molecular weight,
which was further reacted with TCNE via cycloaddition/retroelectrocyclization reaction under
CV
mild conditions to afford the target polymer with a low bandgap energy (Egopt = 1.40 eV and Eg
= 1.10 eV) arising from strong intramolecular charge-transfer interactions between electron
donors and acceptors in the polymer.
Keywords: Low bandgap polymers; Intramolecular charge-transfer interactions; π−Conjugated
polymers; Poly(arenylene ethynylene); Cycloaddition/retroelectrocyclization reaction.
1

1. Introduction
The low bandgap energy of π-conjugated polymers is critical for their applications in many
emerging areas such as organic photovoltaic devices, light emitting diodes, and nonlinear optical
devices [1-3]. The intramolecular charge-transfer interactions between electron donor and
electron acceptor represent one of the most effective pathways toward lowering the bandgap
energy of π-conjugated polymers. The enhancement in the strength of electron donors and/or
acceptors can further reduce the bandgap energy of the polymer [1, 2, 4-9]. This prompted me to
designing and synthesizing π-conjugated polymers with a lower bandgap energy by integrating
stronger electron donor and acceptor moieties into the macromolecules.
It is fundamentally difficult to directly incorporate strong electron acceptors like
tetracyanoethylene (TCNE) into the π-conjugated macromolecular chains. Instead, post-
functionalization via the “click” type cycloaddition/retroelectrocyclization reaction of TCNE
with activated alkynes has proven to be an effective approach to attain novel donor-acceptor type
polymers. This was first confirmed by Michinobu [10] for aromatic polyamines with electron-
rich alkyne side chains. Very recently, Huang et al. have demonstrated the feasibility of
achieving a low-bandgap poly(arylene ethynylene)
through the selective reaction of
tetracyanoethylene (TCNE) with dialkylanilino–activated alkynes in the side chains of the
polymer via the same reaction mechanism to afford donor-acceptor type chromophores while
maintaining the important features of alternating aromatic ring and ethynylene bonds in the main
chain of the polymer [11].
However, it is much more difficult to successfully implement the post-functionalization of
main-chain ethynyl groups along the chain in a complete manner if the chemical structure of the
polymer is not properly designed, because the alkynes must be activated by strong electron
donating groups for the cycloaddition/retroelectrocyclization reaction with TCNE [12].
Dialkylanilino group has been recognized as a strong electron donor for many systems,
especially for the activation of alkynes [13-15]. In particular, dialkylanilino group in the para-
position gives the strongest electron donating effect on alkynes, and thus it is desirable to
integrate this structure into the precursor polymer in order to obtain the highest reactivity
between TCNE and main-chain alkynes [16, 17]. However, to the best of our knowledge, there is
not such a report in the literature regarding the synthesis of one benzene molecule bearing a pair
2

of alkylanilino group and alkynyl group in the para-position (chemical structure of 3 shown in
Scheme 1).
Herein I reported a facile method to achieve 4,6-N,N,N',N'-tetrahexyl-benzene-1,3-diamine
monomer, which was then polymerized with activated p-benzenediiodide monomer to give an
electron donating π-conjugated polymer. The activation of alkynes in this precursor polymer
facilitated the cycloaddition/retroelectrocyclization reaction of main-chain alkynes with TCNE,
thus affording a main-chain donor-acceptor type polymer with a low bandgap energy.
2. Experimental part
2.1. Materials
All of the chemicals, otherwise stated, were purchased from Aldrich and used as received.
N,N-dimethylformamide (DMF) and diisopropylamine were distilled from sodium hydride under
an argon atmosphere. Tetrahydrofuran (THF) was distilled from sodium/benzophenone under an
argon atmosphere. All of the chemical reactions were performed under an inert atmosphere by
applying a positive pressure under an argon atmosphere. Synthesis of 4,6-diethynyl-N,N,N',N'-
tetrahexyl-benzene-1,3-diamine (3) was schematically represented in Scheme 1. 2,5-Diiodo-1,4-
dibutyl terephthalate (4) was synthesized according to the procedure described in a previous
paper [11].
2.2. Characterizations
1H and 13C NMR spectra were obtained on a Varian Mercury 300 spectrometer at 298 K.
Chemical shifts (δ) are reported in ppm relative to the signal of tetramethylsilane (TMS).
1
Residual solvent signals in the H and 13C NMR spectra were used as an internal reference.
Coupling constants (J) are given in Hz. The apparent resonance multiplicity is described as s
(singlet), br s (broad singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). FTIR spectra
were recorded on a Thermo Nicolet 380 FTIR spectrometer (Thermo Fisher Scientific) with a
diamond attenuated total reflectance (ATR) accessory. Thin-layer chromatography was
conducted on aluminum sheets coated with silica gel having fluorescent indicator 254 nm, which
was visualized with a UV lamp (254 or 366 nm). GPC analyses were carried out using a Waters
510 HPLC pump and a Waters 410 differential refractometer with THF as an eluent at a flow
rate of 1.0 mL/min at 30°C, using polystyrene standards. The thermal degradation temperature of
3

the polymers synthesized was determined using a thermogravimetric analyzer (TGA 2050, TA
Instrument) at a heating rate of 10oC/min under a nitrogen atmosphere. UV−vis spectra were
obtained at Hewlett-Packard 8435 UV–visible spectrophotometer. The spectra were measured in
CH2Cl2 in a quartz cuvette (1 cm) at 298 K. Cyclic voltammetry (CV) experiments were
conducted on an EpsilonTM BAS potentiostat-galvanostat coupled with a typical three-electrode
cell. The electrochemical cell consisted of a conical vial fitted with a glassy carbon working
electrode, a reference electrode of Ag/Ag+/CH3CN/(n-C4H9)4NPF6, and a platinum wire as the
counter electrode. All CV measurements were conducted at 20oC in CH2Cl2 containing 0.1 M (n-
C4H9)4NPF6 as the supporting electrolyte at a scanning rate of 0.1 V/s. Dry argon gas was
bubbled carefully through the electroactive solution for at least 10 min before each measurement
to deoxygenate the solution.
2.3. Monomer Synthesis
2.3.1. Synthesis of N,N,N',N'-tetrahexyl-benzene-1,3-diamine (1)
1,3-Phenylenediamine (1.1 g, 10 mmol) was dissolved in 60 mL anhydrous tetrahydrofuran
in a 250 mL round bottom flask with a magnetic stir bar and a reflux condenser. To this solution
was added 1-iodohexane (9 mL, 60 mmol) and 15-crown-5 (0.2 mL, 1 mmol). This was followed
by slow addition of sodium hydride (60% dispersion in mineral oil, 2.4 g, 60 mmol). The
reaction mixture was then allowed to reflux at 65oC for 2 days. After cooling down to room
temperature, the reaction mixture was poured into 100 g ice, and then extracted with ethyl
acetate (3 x 50 mL). The combined organic layers were dried over magnesium sulfate and
filtered. Removal of solvent under the reduced pressure gave the crude product, which was
purified over a silica gel column using hexane as an eluent to give 3.6 g light yellowish liquid.
1
Yield: 81%. TLC: Rf = 0.73 (hexane). H NMR (300 MHz, CDCl3, δ/ppm): 0.90 (t, J = 6.6 Hz,
12H, −CH3), 1.31 (s, 24H, −CH2−), 1.59 (s, 8H, −CH2−), 3.23 (t, J = 8.1 Hz, 8H, −NCH2−), 5.90
(d, J = 2.1 Hz, 1H, Ar−H), 6.00 (dd, J1 = 8.2 Hz, J2 = 2.3 Hz, 2H, Ar−H), 7.03 (t, J = 8.2 Hz, 1H,
Ar−H). 13C NMR (75 MHz, CDCl3, δ/ppm): 14.2 (−CH3), 22.9 (−CH2−), 27.1 (−CH2−), 27.7
(−CH2−), 32.0 (−CH2−), 51.5 (−CH2−), 95.3 (Ar−C), 100.0 (Ar−C), 129.7 (Ar−C), 149.3 (Ar−C).
FTIR (v, cm−1): 2952, 2924, 2854, 1601 (−phenyl), 1570, 1503, 1461, 1365, 1289, 1248, 1219,
4

1176, 1138, 1110, 989, 887, 801, 733, 687, 633, 586, 550. Anal. Calcd for C30H56N2 (444.79): C,
81.01; H, 12.69; N, 6.30. Found: C, 81.07; H, 12.71; N, 6.22.
2.3.2. Synthesis of 4,6-dicarbaldehyde-N,N,N', N'-tetrahexyl-benzene-1,3-diamine (2)
Phosphorus(V) oxychloride (2.4 mL, 25 mmol) was slowly added to a solution of N,N,N', N'-
tetrahexyl-benzene-1,3-diamine (2.2 g, 5 mmol) in 15 mL DMF at 0oC and then stirred for 24
hours at room temperature. After the completion of reaction, 1 N sodium hydroxide aqueous
solution was added to neutralize the solution at 0oC and the product was extracted with ethyl
acetate (50 mL x 3). The combined organic layers were washed with saturated sodium
bicarbonate and dried over magnesium sulfate, and the solvent was removed under the reduced
pressure. The residue was purified over silica gel column using hexane/ethyl acetate (10/1, v/v)
as an eluent to give 1.5 g yellow liquid. Yield: 58%. TLC: Rf = 0.54 (hexane/ethyl acetate =
1
10/1, v/v). H NMR (300 MHz, CDCl3, δ/ppm): 0.86 (t, J = 6.6 Hz, 12H, −CH3), 1.25 (s, 24H,
−CH2−), 1.57 (s, 8H, −CH2−), 3.28 (t, J = 7.2 Hz, 8H, −NCH2−), 6.33 (s, 1H, Ar−H), 8.18 (s, 1H,
13
Ar−H), 9.88 (s, 2H, −CHO). C NMR (75 MHz, CDCl3, δ/ppm): 13.9 (−CH3), 22.5 (−CH2−),
26.6 (−CH2−), 27.2 (−CH2−), 31.5 (−CH2−), 53.5 (−CH2−), 107.3 (Ar−C), 120.2 (Ar−C), 139.6
(Ar−C), 157.4 (Ar−C), 188.3 (−CHO). FTIR (v, cm−1): 2952, 2924, 2853, 1673 (−CHO), 1576
(−phenyl), 1517, 1489, 1460, 1374, 1342, 1254, 1150, 1101, 1024, 926, 841, 778, 724, 681, 609,
558. Anal. Calcd for C32H56N2O2 (500.81): C, 76.75; H, 11.27; N, 5.59; O, 6.39. Found: C, 76.73;
H, 11.26; N, 5.60; O, 6.41.
2.3.3. Synthesis of 4,6-diethynyl-N,N,N',N'-tetrahexyl-benzene-1,3-diamine (3)
n-Butyllithium (1.6 M in hexane, 9.4 mL, 15 mmol) was added to a solution of
diisopropylamine (2.1 mL, 15 mmol) in 20 mL dry tetrahydrofuran at –78oC and stirred for 1 hr.
Trimethylsilyldiazomethane (2.0 M in hexane, 7.5 mL, 15 mmol) was added dropwise at –78oC,
and then stirred for 60 min. After that, a solution of 4,6-dicarbaldehyde-N,N,N', N'-tetrahexyl-
benzene-1,3-diamine (2.5 g, 5 mmol) in 10 mL dry THF was added and the mixture was stirred
at –78oC for 2 hours and then heated to reflux at 65oC for 3 hours. Upon the completion of
reaction, the mixture was cooled down to room temperature and stirred overnight. Consequently,
the reaction solution was poured into 50 g crushed ice. The solution was extracted with ethyl
acetate (100 mL x 3). The combined organic layers were washed with water (100 mL x 3), brine
5

(50 mL) and dried (MgSO4). The solvent was removed under the reduced pressure, and then the
crude product was dissolved in a mixed solvent of 15 mL anhydrous tetrahydrofuran and 10 mL
methanol, followed by the addition of potassium carbonate (5.6 g, 40 mmol). After stirring at
room temperature for 10 hours, the solid was filtered off and the solvent was removed under the
reduced pressure. The residue was purified over silica gel column using hexane/ethyl acetate
(50/1, v/v) as an eluent to afford 1.0 g yellowish liquid. Yield: 41%. TLC: Rf = 0.57
1
(hexane/ethyl acetate = 50/1, v/v). H NMR (300 MHz, CDCl3, δ/ppm): 0.89 (m, 12H, −CH3),
1.29 (s, 24H, −CH2−), 1.56 (s, 8H, −CH2−), 3.21 (s, 2H, −acetylene), 3.30 (t, J = 7.5 Hz, 8H,
13
−NCH2−), 6.27 (s, 1H, Ar−H), 7.51 (s, 1H, Ar−H). C NMR (75 MHz, CDCl3, δ/ppm): 14.0
(−CH3), 22.7 (−CH2−), 26.8 (−CH2−), 27.5 (−CH2−), 31.8 (−CH2−), 52.3 (−NCH2−), 79.6
(−acetylene), 83.3 (−acetylene), 104.9 (Ar−C), 108.0 (Ar−C), 142.7 (Ar−C), 153.5 (Ar−C).
FTIR (v, cm−1): 3310 (−acetylene), 2954, 2923, 2854, 2096 (−ethynyl), 1589 (−phenyl), 1498,
1460, 1371, 1258, 1167, 1153, 1095, 1016, 901, 799, 748, 722, 640, 568. Anal. Calcd for
C34H56N2 (492.83): C, 82.86; H, 11.45; N, 5.68. Found: C, 82.89; H, 11.41; N, 5.70.
2.4. Polymerization of PBBmN
As shown in Scheme 4, 4,6-diethynyl-N,N,N',N'-tetrahexyl-benzene-1,3-diamine (2.5 g, 5
mmol) and 2,5-diiodo-1,4-dibutyl terephthalate (2.7 g, 5 mmol) were dissolved in a mixed
solvent of 40 mL anhydrous tetrahydrofuran and 20 mL triethylamine, which was thoroughly
degassed through freeze-pump-thaw procedures. Under argon gas atmosphere, copper(I) iodide
(49 mg, 0.25 mmol) and Pd(PPh3)4 (0.29 g, 0.25 mmol) were added to the solution. Then, the
reaction mixture was degassed once again using freeze-pump-thaw procedures. The reaction
solution was stirred at 50oC for 2 days. Upon completion of the reaction, the mixture was passed
through a short silica gel plug using THF as an eluent. The solvent in the collected solution was
removed under the reduced pressure. The crude product was dissolved in chloroform and
precipitated in methanol, and after filtration, the product was redissolved in chloroform and
1
precipitated in hexane to give 2.4 g red powder. Yield: 63 %. H NMR (300 MHz, CDCl3,
δ/ppm): 0.84 (s, 12H, −CH3), 0.91 (s, 6H, −CH3), 1.25 (s, 24H, −CH2−), 1.44 (s, 4H, −CH2−),
1.59 (s, 8H, −CH2−), 1.77 (s, 4H, −CH2−), 3.42 (s, 8H, −NCH2−), 4.38 (s, 4H, −OCH2−), 6.24 (s,
13
1H, Ar−H), 7.64 (s, 1H, Ar−H), 8.09 (s, 2H, Ar−H). C NMR (75 MHz, CDCl3, δ/ppm): 13.7
(−CH3), 14.0 (−CH3), 19.3 (−CH2−), 22.7 (−CH2−), 26.9 (−CH2−), 27.6 (−CH2−), 30.8 (−CH2−),
6

31.8 (−CH2−), 52.3 (−NCH2−), 65.4 (−OCH2−), 90.7 (−ethynyl), 97.0 (−ethynyl), 104.7 (Ar−C),
107.0 (Ar−C), 123.0 (Ar−C), 133.5 (Ar−C), 135.2 (Ar−C), 142.1 (Ar−C), 153.2 (Ar−C), 165.4
(−C=O). FTIR (v, cm−1): 2954 (−CH2−), 2924 (−CH2−), 2854 (−CH2−), 2185 (−ethynyl), 1726
(−COO−), 1583 (−phenyl), 1504, 1460, 1427, 1371, 1280, 1221, 1147, 1091, 1022, 956, 916,
806, 786, 727, 694, 605. UV-vis (λmax, nm, ε = 5.7 × 104 mol-1 •L•cm-1 calculated based on the
molecular weight of each repeat unit): 248, 277, 351, 458. Anal. Calcd for (C50H74N2O4)n
(767.15 for each repeat unit): C, 78.28; H, 9.72; N, 3.65; O, 8.34. Found: C, 78.23; H, 9.76; N,
3.61; O, 8.40.
2.5. Synthesis of PBBmN-TCBD
As shown in Scheme 4, to a solution of PBBmN (3.8 g, 5 mmol) in 50 mL dichloromethane
was added a solution of tetracyanoethylene (1.3 g, 10.1 mmol) in 50 mL dichloromethane under
the atmosphere of argon gas at 0oC. The reaction mixture was allowed to stir overnight. The
polymer solution was concentrated under reduced pressure and then precipitated in hexanes. The
obtained product was redissolved in dichloromethane and precipitated in hexane for three times
to give 1.15 g red powder. Yield: 88%. 1H NMR (300 MHz, CDCl3, δ/ppm): 0.89 (s, 18H, −CH3),
1.31 (s, 28H, −CH2−), 1.57 (s, 8H, −CH2−), 1.80 (s, 4H, −CH2−), 3.13 (s, 8H, −NCH2−), 4.45 (s,
4H, −OCH2−), 6.87 (s, 1H, Ar−H), 8.10 (s, 1H, Ar−H), 8.41 (s, 2H, Ar−H). FTIR (v, cm−1): 2959
(−CH2−), 2927 (−CH2−), 2858 (−CH2−), 2217 (−CN), 1716 (−COO−), 1585 (−phenyl), 1493,
1461, 1430, 1373, 1288, 1227, 1153, 1096, 1015, 931, 836, 803,767, 727, 692, 603. UV-vis (λmax
,
nm, ε = 3.9 × 104 mol-1 •L•cm-1 calculated based on the molecular weight of each repeat unit):
230, 308, 578. Anal. Calcd for (C62H74N10O4)n (1023.33 for each repeat unit): C, 72.77; H, 7.29;
N, 13.69; O, 6.25. Found: C, 72.82; H, 7.25; N, 13.74; O,6.19.
3. Results and discussion
To date, the synthetic strategy for the phenylacetylene derivatives has relied heavily on the
Sonogashira cross-coupling protocol [18], which was not applicable for the synthesis of 4,6-
diethynyl-N,N,N,N-tetrahexylbenzene-1,3-diamine (3) in Scheme 1. Initially, attempt has been
made to synthesize 3 by standard method of Sonogashira coupling with acetylene derivatives, but
failed as illustrated in Schemes 2 and 3. In Scheme 2, it was easy to introduce diiodide groups
into 1,3-phenylenediamine by oxidative iodination using potassium iodide and hydrogen
7

peroxide in the presence of methanol and sulfuric acid at ambient temperature to afford 5 in a
high yield [19]. However, the diiodide group in 5 cannot survive in the alkylation reaction
conditions when synthesizing 6. Another attempt was made to couple trimethylsilylacetylene
with 5 using PdCl2(PPh3)2, CuI, and PPh3 as the catalyst in a mixed solvent of THF and
diisopropylamine to give 7 in a good yield, as indicated in Scheme 3. Unfortunately, it was
ofgreat difficulty to obtain 8 in Scheme 3, because trimethylsilyl group was replaced by hexyl
group.
Consequently, a new strategy was developed to successfully synthesize the target monomer
3 by the reaction scheme given in Scheme 1. Specifically, tetraalkylation with 1,3-
phenylenediamine with 1-iodohexane in the presence of sodium hydride and 15-crown-5 gave
the desired tetrahexyldiamine monomer precursor 1 [20]. The use of Na2CO3/N,N-
dimethylformamide reaction system [13] for an extended period only afforded di- or tri-alkylated
monomers. Direct iodization based on 1 was unsuccessful using a couple of methods [21, 22].
Therefore,
this problem was circumvented by reacting 1 with POCl3 and N,N-
dimethylformamide [23] to give 2 having two carbaldehyde groups. Subsequently, 2 was further
reacted with lithium trimethylsilyldiazomethane to afford dialkyne monomer 3 via Colvin
rearrangement [24] in a good yield.
Referring to Scheme 4, the obtained dialkyne monomer 3 was then polymerized with
diiodide monomer 4 by Sonogashira cross-coupling reaction with tetrakis(triphenylphosphine)
palladium and copper iodide as the catalysts in a mixed solvent of tetrahydrofuran and
triethylamine at 50oC. The strong electron donating property of dihexylamino groups in 3 has a
negative effect on the palladium-catalyzed cross-coupling reactions [11, 22], and thus electron
withdrawing (carbonyl) groups are introduced into 4 for improving the reactivity of this coupling
reaction. As a consequence, PBBmN has a reasonably high molecular weight with an Mw of
82,044 and a polydispersity (Mw/Mn) of 3.2 as determined by GPC using polystyrenes as
standards and THF as the eluent, and it shows good solubility in THF, chloroform, and
dichloromethane. In contrast, if diiodide monomer 4 was replaced by 1,4-benzenediiodide, where
the iodide groups were not activated, I only obtained oligomers with Mn = 1,854 and Mw =
3,719.
The alkynes activated by electron-rich groups will react with electron withdrawing TCNE
via thermal [2+2] cycloaddition first to form cyclobutenes and the subsequent spontaneous
8

electrocyclic ring opening to afford 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) [25, 26]. By this
mechanism, the main-chain akynes in PBBmN were able to react with TCNE due to the
activating effect of dialkylanilino groups in the para-position. This reaction proceeded smoothly
under mild conditions in the absence of any catalyst. Upon the addition of TCNE, I observed the
immediate color change of reaction mixture from light yellow to reddish color.
NMR spectroscopy was investigated to understand the chemical structures of PBBmN and
PBBmN-TCBD, as illustrated in Fig. 1. Upon the completion of reaction with TCNE, the
chemical shift for proton at 1 position of PBBmN has shifted from 6.24 ppm to 6.87 ppm for that
at 1' position of PBBmN-TCBD. Similarly, the chemical shift for proton at 2 position of PBBmN
has also shifted from 7.64 ppm to 8.10 ppm for that at 2' position of PBBmN-TCBD. The large
increase in chemical shifts for these two protons may be attributed to the strong influence of
electron withdrawing effect of TCBD units in PBBmN-TCBD. Further, I observe the same trend
for protons at 3 and 4 positions for PBBmN, which shifts from 8.09 ppm to 8.41 ppm for those at
3' and 4' positions for PBBmN-TCBD. Besides these, there are no additional peaks appearing in
the range of 6 ppm to 9 ppm in the 1H NMR spectrum of PBBmN-TCBD that can be assigned to
other chemical structures. This means that the extent of cycloaddition/retroelectrocyclization
reaction of main-chain alkynes in PBBmN with TCNE should be extremely high, if not
completely. I also observe the variation of chemical shifts of flexible side chains in PBBmN-
TCBD, as compared with those in PBBmN. For example, the chemical shift at 3.42 ppm for
−NCH2− groups in PBBmN has reduced to 3.13 ppm for those in PBBmN-TCBD, arising from
the electron delocalization from electron donating dialkylamino groups to electron accepting
TCBD groups. In contrast, the chemical shift at 4.38 ppm for −OCH2− groups in PBBmN has
increased to 4.45 ppm because of the presence of stronger electron withdrawing TCBD groups in
PBBmN-TCBD. The broader peaks for −NCH2− and −OCH2− groups in PBBmN-TCBD can be
interpreted by the conformation change of side chains due to the intramolecular charge-transfer
interactions between the dialkylamino groups and TCBD units.
The chemical structure of PBBmN-TCBD can be further confirmed by the FTIR spectra
given in Fig. 2. It can be seen from Fig. 2 that the absorption peak appearing at 2185 cm–1 for
main-chain alkynes in PBBmN has shifted to 2217 cm–1 for cyano groups of TCBD units in
PBBmN-TCBD, indicative of the successful cycloaddition/retroelectrocyclization reaction of
alkynes in PBBmN with TCNE [11]. Correspondingly, I oberved the shift of absoprtion peak for
9

carbonyl groups from 1726 cm–1 for PBBmN to 1716 cm–1 for PBBmN-TCBD, as a consequence
of the electron withdrawing effect of TCBDs in PBBmN-TCBD. TGA data in Fig. 3 show that
the thermal degradation temperatures, which were defined as the heating temperature at 5%
weight loss, for PBBmN and PBBmN-TCBD are 162oC and 198oC, respectively. The
improvement of thermal stability in PBBmN-TCBD may be associated with the intramolecular
charge-transfer interactions along the macromolecular chains [10, 11], which will be elaborated
on by UV-vis spectroscopy and cyclic voltammetry.
Fig. 4 gives plots of UV-vis spectra versus (a) wavelength and (b) energy for PBBmN and
PBBmN-TCBD. Here, the bandgap energy, Eg = 1242/λ (eV), is originated from the equation, Eg
−34
8
J
= hC/λ with h being Planck’s constant (6.626 x 10
J⋅s), C being the speed of light (3.0 x 10
−19
−9
being the onset wavelength (nm x 10 m), and 1 eV being equal to 1.6 x 10
m/s), λ
onset
[11]. It can be observed from Fig. 4a that PBBmN exhibits a broad band with a maximum
absorption peak at 458 nm in the visible absorption region due to the strong electron donating
effect of dihexylamino groups in the polymer. The onset wavelength (λ
) of PBBmN is
onset
about 583 nm, and correspondingly the optical bandgap energy
of PBBmN is about 2.13 eV, as
illustrated in Fig. 4b. In the UV region of Fig. 4a, I also observe strong absorption bands at a
wavelength of 248, 277, and 351 nm, which are equal to 5.0, 4.5, and 3.5 eV, respectively, and
π−π* transitions located on the phenyl and ethynyl subunits. In the case of
thus assigned to the
PBBmN-TCBD (Fig. 4b), I observe a very broad absorption band with a maximum absorption
peak at 578 nm, which is attributed to the intramolecular charge-transfer interactions between
dihexylamino groups and TCBD units in PBBmN-TCBD. The onset wavelength (λ
) of
onset
opt
PBBmN-TCBD is about 890 nm, affording an optical bandgap (Eg ) of
1.4 eV, as confirmed
by Fig. 4b. The strong charge-transfer interactions between dihexylamino groups and TCBD
units in PBBmN-TCBD result in such a low bandgap energy. Similarly, I observe strong
absorption bands at a wavelength of 230 nm and 308 nm, which are equal to 5.4 and 4.0 eV
π−π* transitions located on the phenyl units of PBBmN-
respectively, and thus assigned to the
TCBD.
The electrochemical behaviors of PBBmN and PBBmN-TCBD were also investigated by
cyclic voltammograms with an internal reference to ferricinium/ferrocene (Fc/Fc+) couple. As
displayed in Fig. 5a and Table 1, PBBmN shows a one-electron oxidation step at a half-wave
10

oxidation (E1/2ox) of 0.30 V vs Fc/Fc+ as a consequence of the redox between amine and amine
cation, and the other oxidation step at E1/2 of 0.89 V vs Fc/Fc+, indicative of the subsequent
ox
oxidation from amine cation to bication. As shown in Fig. 5b and Table 1, PBBmN-TCBD
ox
shows an irreversible anondic peak at Epa = 0.99 V and a half-wave oxidation potential of
(E1/2ox) of 1.19 V vs. Fc/Fc+, which are ascribed to the amino groups in PBBmN-TCBD. Also,
two additional quasi-reversible half-wave reduction potentials (E1/2red) of −0.91 V and −1.76 V
vs. Fc/Fc+ are observed in Fig. 5b, which are assigned to the electron accepting TCBD units in
PBBmN-TCBD. Since PBBmN-TCBD exhibits an onset oxidation potential (Φox = 0.48 V) and
an onset reduction potential (Φred = –0.62 V) as given in Fig. 5b, the the highest occupied
molecular orbital energy and the lowest unoccupied molecular orbital energy of PBBmN-TCBD
are determined to be EHOMO = –5.42 eV and ELUMO = –4.32 eV, respectively. These are
calculated based on the assumption that the energy level of Fc/Fc+ is 4.80 eV below the vacuum
level [27]. Thus, PBBmN-TCBD has a low bandgap energy (EgCV) of 1.10 eV, arising from the
strong intramolecular charge-transfer interactions between dihexylamino groups and TCBD units
within the molecule.
4. Conclusions
In summary, a facile synthetic route was developed to achieve 4,6-diethynyl-N,N,N',N'-
tetrahexyl-benzene-1,3-diamine by introducing carbaldehyde intermediates, which was then
converted into acetylene groups via Colvin rearrangement, while the procedures based on
palladium-catalyzed coupling reaction was unsuccessful. This unique strong electron-donating
monomer was then polymerized with a carbonyl-activated diiodide monomer to afford a strong
electron donating π-conjugated polymer having a reasonably high molecular weight. This
precursor polymer was then reacted with TCNE via cycloaddition/retroelectrocyclization
reaction to afford the main-chain donor-acceptor type polymer, which exhibited a very low
bandgap energy due to the strong intramolecular charge-transfer interactions between electron
donors and acceptors along the macromolceular chains.
ACKNOWLEDGEMENTS
11

I acknowledge with gratitude that this study was supported in part by the National Science
Foundation under Grant CBET-0755763.
REFERENCES AND NOTES
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Macromolecules 42 (2009) 6519-6528.
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2476.
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Gross, F. Diederich, Chem. Eur. J. 12 (2006) 1889-1905.
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12

[20] S.W.I. Thomas, T.M. Swager, Macromolecules 38 (2005) 2716-2721.
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13

Table 1. Electrochemical properties of PBBmN and PBBmN-TCBD obtained from cyclic
voltammetry measured in CH2Cl2 versus Ag/Ag+ as a reference electrode and calibrated versus
Fc/Fc+
Oxidation Potential (V)
Reduction Potential (V)
Polymers
PBBmN
ox
red
Epa
Epc
∆E
0.18
E1/2
Epa
Epc
∆E
E1/2
0.39
1.02
0.99
1.23
0.21
0.76
0.3
0.26
0.89
Irreversible
0.09
-1.53
-0.73
-1.98
-1.08
0.45
0.35
-1.76
-0.91
PBBmN-
TCBD
1.14
1.19
Note: ∆E = Epa – Epc; E1/2 = (Epa + Epc)/2. Epa and Epc are anodic and cathodic peak potentials,
respectively, with ox denoting oxidation and red denoting reduction.
14

(C6H13)2N
N(C6H13)2
H2N
NH2
C H I,
6
NaH/THF
13
15 Crown 5,65oC
1
(C6H13)2N
OHC
N(C6H13)2
∆
65oC
POCl3
DMF, 0oC
TMSCHN2, LDA
78oC
THF,
CHO
2
(C6H13)2N
N(C6H13)2
C
C
HC
CH
3
Scheme 1. Synthesis of 4,6-diethynyl-N,N,N', N'-tetrahexyl-benzene-1,3-diamine (3).
15

H2N
I
NH2
I
H2N
NH2
KI/H2O2/H2SO4
CH3OH
5
(C6H13)2N
N(C6H13)2
C6H13I, NaH/THF
15 Crown 5,65 oC
I
I
6
Scheme 2. Unsuccessful attempt to synthesize 4,6-diethynyl-N,N,N', N'-tetrahexyl-benzene-1,3-
diamine (3).
16

H N
2
NH
I
2
(H C) Si C CH
3
3
I
5
H N
2
NH
C
2
PdCl (PPh ) , CuI, PPh
3
2
3 2
THF, diisopropylamine
(H C) Si
C
C
C
Si(CH )
3 3
7
3
3
NaH/THF
C H I,
6 13
o
,65 C
15 Crown 5
(C H ) N
13 2
N(C H )
6 13 2
6
C
C
C
C
(H C) Si
3
Si(CH )
3 3
3
8
Scheme 3. Unsuccessful attempt to synthesize 4,6-diethynyl-N,N,N', N'-tetrahexyl-benzene-1,3-
diamine (3).
17

COOC4H9
I
(C6H13)2N
N(C6H13)2
I
C
C
HC
CH
C4H9OOC
3
4
Pd(PPh3)4,CuI THF, Et3N, 50oC
(C6H13)2N
N(C6H13)2
C
C
C
C
COOC4H9
C4H9OOC
n
PBBmN
CH2Cl2
0oC to r.t.
TCNE
(C6H13)2N
NC CN
N(C6H13)2
NC CN
COOC4H9
NC
CN
C4H9OOC
NC
CN
n
PBBmN-TCBD
Scheme 4. Synthesis of the precursor π-conjugated poly(arylene ethynylene) (PBBmN) and
main-chain donor-acceptor type polymer (PBBmN-TCBD).
18

−CH3
1
2
−CH2−
−NCH2−
(C6H13)2N
N(C6H13)2
C
C
C
3
C
COOC4H9
−OCH2−
C4H9OOC
4
3,4
1
2
CDCl3
(a)
(a)
n
−CH3
−CH2−
−NCH2−
(C6H13)2N
N(C6H13)2
1'
NC
CN
NC
CN
−OCH2−
COOC4H9
3'
CDCl3
1'
2'
3',4'
2'
8
((bb))
0
NC
CN
C4H9OOC
NC
CN
4'
9
7
6
5
4
3
2
1
n
Chemical Shift (ppm)
Fig. 1. 1H NMR spectra of (a) PBBmN and (b) PBBmN-TCBD in CDCl3.
19

C
C
(a)
(b)
–CN
3000
2000
Wavenumber (cm-1)
1000
Fig. 2. FTIR spectra of (a) PBBmN and (b) PBBmN-TCBD.
20

100
80
60
40
20
0
0 100 200 300 400 500 600
Temperature (oC)
Fig. 3. TGA data for PBBmN (dashed line) and PBBmN-TCBD (solid line).
21

248
458
351
(b)
(a)
277
230
308
578
1
2
3 4
Energy (eV)
5
6
300 400 500 600 700 800 9001000
Wavelength (nm)
Fig. 4. Plots of UV-vis spectra versus (a) wavelength and (b) energy for PBBmN (dash line) and
(b) PBBmN-TCBD (solid line) in dichloromethane.
22

20
10
0
(a)
-10
-0.5
0.0
0.5
1.0
1.5
Potential/V (vs. Fc/Fc+)
80
60
(b)
40
Φ
ox = 0.48 V
20
0
-20
-40
-60
-80
Φred = –0.62 V
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Potential/V (vs. Fc/Fc+)
Fig. 5. Cyclic voltammograms of (a) PBBmN and (b) PBBmN-TCBD in dichloromethane
solution containing 0.1 M nBu4NPF6. Φox and Φred are the onset oxidation and reduction
potentials, respectively. Red arrows describe the directions of scans.
23