Synthesis, solid structure, and optical properties of new thiophene-based alternating π-conjugated copolymers containing 4-alkyl-1,2,4-triazole or 1,3,4-thiadiazole unit as the partner unit

Article abstract of DOI:10.1021/ma0488820

The synthesis, characterization, and solid-state structure of new thiophene-based alternating π-conjugated polymers containing the 4-alkyl-1,2,4-triazole or 1,2,4-thiadiazole unit as per partner unit were reported. A new thiophene-based CT type alternating copolymers was synthesized to obtain further scope about the packing structure and chemical properties of such CT type alternating π-conjugated copolymers. It was observed that the difference between the syn and anti conformations was small. It was also observed that the calculated density (1.79 g cm^-3) according to the packing model agreed with the observed density (1.73 g cm^-3).

Full text of DOI:10.1021/ma0488820

1500
Macromolecules 2005, 38, 1500-1503
Notes
thiophene-based CT type alternating copolymers by
choosing 4-alkyl-1,2,4-triazole and 1,3,4-thiadiazole as
the electron-accepting five-membered ring unit. 1,2,4-
Triazole, 1,3,4-thiadiazole, and 1,3,4-oxadiazole are
typical electron-accepting compounds.5-7
Synthesis, Solid Structure, and Optical
Properties of New Thiophene-Based
Alternating π-Conjugated Copolymers
Containing 4-Alkyl-1,2,4-triazole or
1,3,4-Thiadiazole Unit as the Partner Unit
Takuma Yasuda,† Tatsuya Imase,‡
Shintaro Sasaki,§ and Takakazu Yamamoto*,†
Chemical Resources Laboratory, Tokyo Institute of
Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan, Department of Electronic
Chemistry, Interdisciplinary Graduate School of Science
and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan,
and Japan Advanced Institute of Science and Technology,
1-1 Asahidai, Tatsunokuchi, Ishikawa, 923-1292, Japan
It is known that 1,3,4-oxadiazole-containing com-
pounds are useful as an electron-transporting material
to increase the quantum efficiency in OLEDs.8 π-Con-
jugated copolymers bearing thiophene and 1,3,4-oxa-
diazole alternating units were previously prepared by
a cyclodehydration reaction of a precursor polymer.9
However, π-conjugated polymers consisting of the 4-alkyl-
1,2,4-triazole and 1,3,4-thiadiazole units have received
less attention.
We now report the synthesis, characterization, and
solid-state structure of new thiophene-based alternating
π-conjugated copolymers containing the 4-alkyl-1,2,4-
triazole or 1,3,4-thiadiazole unit as the partner unit.
Received June 7, 2004
Revised Manuscript Received November 11, 2004
Introduction
There is currently much attention focused on π-con-
jugated polymers due to their interesting electronic and
optical properties.1
It is reported that five-membered ring heteroaromatic
π-conjugated polymers such as regioregular head-to-tail
poly(3-alkylthiophene-2,5-diyl)s (HT-P3RTh),2 poly(4,4′-
dialkyl-2,2′-bithiazole-5,5′-diyl)s (HH-P4RBTz),3 and al-
ternating copolymers of 4-alkylthiazole and thiophene
(PTz(R)Th)4 form a π-stacked solid structure assisted
by side chain aggregation. Because of the formation of
the π-stacked structure, they exhibit interesting optical
and electronic properties in the solid.
Among the above shown polymers, PTz(R)Th (cf.
Chart 1) is considered to have a charge-transfer (CT)
electronic structure, because the alternating copolymer
is constituted of electron-donating thiophene unit and
electron-accepting thiazole unit.4 PTz(R)Th assumes a
unique π-stacked structure in the solid assisted by the
CT electronic structure.
Results and Discussion
Synthesis. As shown in Scheme 1, new copolymers
were prepared via palladium-catalyzed Stille cross-
coupling reaction in good yields (76-92%). Synthetic
procedures and spectroscopic and analytical data of the
monomers and polymers are described in the Supporting
Information. The polymers 2b-2e with long alkyl
chains were partially (ca. 60-90%) soluble in CHCl3,
THF, and NMP; whereas 2a with the propyl group was
almost insoluble in these solvents. 2a-2c were com-
pletely soluble in hexafluoro-2-propanol and CF3COOH.
2d and 2e were partly soluble in hexafluoro-2-propanol
and completely soluble in CF3COOH. The nonsubsti-
tuted polymer 4 was only partly (about 30%) soluble in
CF3COOH and was not soluble in other solvents tested.
GPC analysis (eluent ) hexafluoro-2-propanol (for
2a-2c) or CHCl3 (for 2d and 2e; soluble part)) indicated
that the polymers had number-average molecular weights
(Mn’s) of 4100-8800, as exhibited in Table 1. The
1H NMR spectra of 2b-2e in CDCl3 gave a broad
aromatic peak at about δ 7.6 due to the protons in the
thiophene rings (Figure S1). Signals of alkyl protons
appeared at normal positions and the peak area ratios
agreed with the structure. Thermogravimetric analysis
(TGA) indicated that the 5% weight-loss temperature
(Td) of all the polymers was higher than 310 °C.
Chart 1. Five-Membered Ring π-Conjugated Polymers
Which Show π-Stacking in the Solid
To obtain further scope about the packing structure
and chemical properties of such CT type alternating
π-conjugated copolymers, we have synthesized new
* Corresponding author. E-mail: tyamamot@res.titech.ac.jp.
† Chemical Resources Laboratory, Tokyo Institute of Technology.
‡ Department of Electronic Chemistry, Interdisciplinary Gradu-
ate School of Science and Engineering, Tokyo Institute of Technol-
ogy.
Optical Properties. As shown in Figure 1, the UV-
vis spectrum of 2d in CHCl3 gives the lowest energy
π-π* absorption maximum (λmax) at 353 nm. The film
of 2d showed λmax at a longer wavelength (375 nm) than
that for the CHCl3 solution, suggesting the presence of
§ Japan Advanced Institute of Science and Technology.
10.1021/ma0488820 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/25/2005

Macromolecules, Vol. 38, No. 4, 2005
Notes 1501
Scheme 1
Table 1. Synthesis and Characterization of the Copolymers
UV-vis λmax, nm
polymer
yield, %
mol wta
Td,c °C
in solutiond
filmf
PL λem,d nm
Φ,g %
2a
2b
2c
2d
2e
4
84
80
76
83
92
86
8800 (2.0)
7500 (2.0)
4100 (2.9)
8600 (1.9)
7200 (1.9)
b
312
352
381
373
331
322
320e
356
356
353
343
409e
366
368
375
375
366
437
465e
479
459
458
449
535e
b
21
27
29
23
b
a Number-average molecular weight (Mn) estimated from GPC. The values in parentheses are polydispersity index (Mw/Mn). Eluent )
hexafluoro-2-propanol for 2a-2c (vs poly(methyl methacrylate) standards); CHCl3 for 2d and 2e (vs polystyrene standards). b Not
determined. c 5% weight-loss temperature measured by TGA. d Solvent ) CHCl3 unless otherwise noted. e In CF3COOH. f Formed on a
quartz glass plate by casting. g PL quantum yield (Φ) was calculated by comparing with the standard of quinine sulfate (ca. 10-5
solution in 0.5 M H2SO4, having Φ of 54.6%).
M
Figure 1. (a) UV-vis and (b) PL spectra of 2d in CHCl3. (c)
UV-vis spectrum of the film of 2d.
an intermolecular interaction due to stacking of the
polymer molecules in the solid.2,4,10 2a-2e were photo-
luminescent both in solution and in film. The photo-
luminescence (PL) spectrum of 2d in CHCl3 gave the
peak at 458 nm (Figure 1b), which essentially agreed
Figure 2. Powder XRD patterns of 2a-2e and 4.
with the onset position of the absorption band as usually
observed with π-conjugated polymers. The PL quantum
The interchain d1-spacing reasonably increases upon
elongation of the side chain from the N-propyl (2a) to
the N-hexadecyl (2e) group. As shown in Figure 3a, plots
of the d1-spacing against the number of carbons in the
alkyl group give a linear line with a slope of 0.96
Å/carbon, which is smaller than the height of the
-CH2- unit (1.25 Å/carbon). In the case of previously
reported HT-P3RTh, HH-P4RBTz, and PTz(R)Th
(cf. Chart 1), similar plots gave a slope of about 1.8
Å/carbon, and an end-to-end packing in the solid was
proposed.2,4,10 The much smaller slope of 0.96 Å/carbon
observed with 2a-2e suggests that they assume an
interdigitation packing mode, which is depicted in
Figure 3b. The presence of alternating syn-anti confor-
mation (cf. Figure 3b) between the 1,2,4-triazole and
yield of 2d (Φ ) 29%) was comparable to those of 1,3,4-
oxadiazole-containing π-conjugated polymers.9,11 UV-
vis and PL data of the copolymers are included in
Table 1.
Solid Structure. The molecular ordering of 2a-2e
in the solid was investigated by powder X-ray diffraction
(XRD) measurements and density functional theory
(DFT)12 computation approach. In the XRD patterns of
2a-2e shown in Figure 2, the strong peaks observed
in the low-angle region (d1 ) 10.3-22.8 Å) seem to be
assignable to a distance between the conjugated main
chains separated by the alkyl side chains, as previously
proposed for π-conjugated polymers with alkyl side
chains (e.g., HT-P3RTh and HH-P4RBTz).2,10

1502 Notes
Macromolecules, Vol. 38, No. 4, 2005
Figure 4. Profiles of the potential energy vs the inter-ring
dihedral angle (ω) for (a) 5, (b) 6, (c) 7, and (d) 8 on ground-
state calculated at the B3LYP/6-31G* level.
linear correlation exhibited in Figure 3a and the densi-
ties of the polymers suggest that 2a-2e have an
isomorphous molecular geometry. For the copolymers
with longer alkyl chains (2b-2e), the d2 peak becomes
intense while the d3 peak becomes obscure with increase
the side chain length, as exhibited in Figure 2. In these
cases, the d3 peak may be hidden under the overwhelm-
ing d2 scattering. This supports the d2 is attributable
to the side chain aggregation. A polarized optical
microscope (POM) image of 2d observed under cross-
polarization conditions indicated formation of an or-
dered structure.
Usually π-conjugated polymers constituted of recur-
ring five-membered rings (e.g., HT-P3RTh, HH-P4RBTz,
and PTz(R)Th shown in Chart 1) are considered to
assume an all anti conformation in the solid.2-4,10
However, in the proposed solid structure of 2a-2e, the
main chain is considered to include a syn conformation.
The possibility of the inclusion of the syn conformation
in the main chain of 2a-2e is checked by the calculation
at a B3LYP/6-31G* level using a simple model bicycle,
3-(2-thienyl)-4-ethyl-1,2,4-triazole (5). Figure 4 repre-
sents energy profiles of four kinds of dimeric model
compounds 5-8, calculated by rotating the bonding
dihedral angle. Alema´n and co-workers reported similar
results for 7.16 As depicted in Figure 4, the anti
conformation is more stable in 6-8. On the contrary, 5
Figure 3. (a) Plots of the d1 value vs the number of carbons
in the alkyl group of 2a-2e. (b) Schematic diagram of two-
dimension interdigitation packing structure for 2a-2e with
the interchain spacing (d1). The polymer molecules are con-
sidered to form a stacked assembly.
thiophene units in 2a-2e allows the formation of the
interdigitation packing structure. Although the N atom
at the 4-position is formally considered to take an sp3-
like structure around the atom, participation of the lone
pair electrons of N atom in the aromatization of the ring
suggests that the N atom at the 4-position takes an sp2-
like structure. Actually, N atoms in analogous com-
pounds such as N-alkylcarbazole form
a planar
structure around the atom as revealed by X-ray crystal-
lography.13
The d2 and d3 peaks observed with 2a in Figure 2 may
be assigned to the side-to-side distance between the
alkyl chains and the stacking distance between the
polymer backbone chains, respectively. Comblike poly-
mers having long alkyl side chains exhibit a sharp
reflection of Bragg spacing 4.2 Å, indicating a formation
of pseudohexagonal packing of the alkyl chains.14 The
broadness of the XRD profile at 2θ ) about 20° suggests
that the side chain aggregation as well as the backbone
π-stacking are not well controlled, compared with cases
of HT-P3RTh and HH-P4RBTz. If the d3 of 3.7 Å is
taken as the stacking distance between the backbone
chains of 2a-2e in the solid state, the packing model
depicted in Figure 3b gives calculated density15 of 1.13,
1.07, 1.06, 1.02, and 1.00 g cm-3 for 2a through 2e,
respectively. These calculated densities show good
agreement with the observed values of 1.15, 1.08, 1.03,
1.00, and 0.97 g cm-3 within the experimental error,
supporting the proposed packing structure. The alkyl
group has an effective diameter of about 5 Å in the
disordered state, and the aggregation will give a broad
scattering at 2θ ) about 20°. The relatively sharp d2
and d3 peaks of 2a may indicate that 2a forms a packing
structure different from those of 2b-2e. However, the
prefers the syn conformation and this may be taken as
the origin of the inclusion of syn conformation of the
postulated packing structure of 2a-2e. Delicate balance
of charge distribution in the dimeric model compounds
and o-hydrogen repulsion is considered to be the reason
for the difference in the stabler form between 5 and the
other compounds (6-8). However, the calculation is
carried out with ideal isolated molecule in the gas phase
at 0 K, and the calculated energy difference between
the syn and anti conformations is not large. Conse-
quently, the energy difference may not be the determin-
ing factor to control the syn and anti conformations;
however, the results shown in Figure 4 affords ad-
ditional support for the packing structure depicted in
Figure 3.
The polymer 4 gives two XRD peaks at d ) 5.5 Å and
3.5 Å. A packing model, which has an ab lateral packing

Macromolecules, Vol. 38, No. 4, 2005
Notes 1503
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mode similar to those of HT-P3RTh (R ) CH3, cf. Chart
1) and HH-P4RBTz (R ) CH3),10,17 is depicted in the
Supporting Information (Figure S4). The calculated
density (1.79 g cm-3) according to the packing model
agrees with the observed density (1.73 g cm-3) if one
takes account of the presence of amorphous parts in the
polymer solid. As depicted in Figure 4, 6 is considered
to prefer rather the anti conformation, and 4 may
assume all anti conformation similar to polythiophene.18
However, the energy difference between the syn and
anti conformations also seems to be small, and the main
chain of 4 may partly contain a syn joint, instead of the
all anti conformation exhibited in Figure S4.
As described above, the new alternating π-conjugated
copolymers between electron-donating thiophene and
electron-accepting 1,2,4-triazole or 1,3,4-thiadiazole have
been prepared, and the copolymer of 1,2,4-triazole is
considered to assume the unique solid structures.
Acknowledgment. This research was partly sup-
ported by a grant from the 21st Century COE program.
Supporting Information Available: Text giving experi-
mental details, characterization data, and computational
details for the monomers and polymers and figures showing
1H NMR spectra of 2b-2e, FT-IR spectra of 2a-2e and 4, TGA
curves of 2a-2e under N2, and a packing model of 4. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(14) (a) Jordan, E. F., Jr.; Felderisen, D. W.; Wrigley, A. N. J.
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B.; Morawetz, H. J. Polym. Sci.: Polym. Phys. Ed. 1976,
14, 1241. (c) Inomata, K.; Sakamaki, Y.; Nose, T.; Sasaki,
S. Polym. J. 1996, 28, 986.
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MA0488820