Chapter 4
Crystal Structures of Monomers and Oligomers
Containing Azulene Unit – Model Compounds for the
Corresponding Polymers
Introduction
In chapter 3, we have described the interesting properties of conjugated copolymers
containing azulene units in the main chain, such as stimuli-response upon protonation,
stability of the radicals cations, high conductivity upon doping or protonation. All these
interesting properties come from their unique electronic structures. However, structural
studies of these copolymers have been limited by the defects of these copolymers. For
example, due to statistical chain length distribution and interruption of the conjugated
chain by mislinkages and other defects, the obtained polymers, lack a rigidly defined
structural principle and conjugation is severely disturbed.
1
The structure at the chain
segment level is also hard to characterize.
2
These inaccessibility of the copolymer solids
is one of the reasons why model compounds, well-defined oligomeric analogues of the
copolymer, have met increasing interest in the filed of conjugated organics.
3

Because of the chemical purity and relatively low molecular mass, these model
compounds are usually sufficiently crystalline to allow structural resolution by means of
diffraction methods. Although X-ray data of oligomers are scarce, X-ray data of model
compounds, such as inter-ring bond lengths, torsional angles between the moieties planes,
and positioning of the alkyl chains, can give valuable information on polymer
properties.
4-6
It also provides information to study the solid-state polymer properties.

7,8


119

Furthermore, the physical properties of these monomers and oligomers, may be
extrapolated to those of their corresponding polymers. Additionally, it has been well
demonstrated that oligomers themselves represent excellent candidates for electronic
materials.
9-12
Thus, to have a better understanding of the polymer structure and
properties, we prepared the single-crystals of a series of monomers and oligomers
containing azulene unit. To further investigate the side chain effect on solid-state polymer
properties, model compound bearing long side chains was synthesized and its single
crystal structure was studied. These compounds are analog of the copolymers and their
solid state structures, optical and electrical properties were studied as model compounds
for the corresponding polymers.
Results and Discussion
Model compounds design and synthesis
Synthesis and characterization of Monoa and Monob are reported in Chapter 3.
Palladium-catalyzed Stille coupling reaction
13
was employed to prepare the oligomers as
described in Scheme 4-1. The common intermediate for synthesis of Oligoa and Oligob,
4’,4’’-Dibromo(1,3-(3’-methyl-thienyl))-azulene, was obtained by bromination of
Monob with NBS in chloroform and acetic acid (1:1). Because Monob is less reactive
with respect to the corresponding thiophene derivatives, bromination of Monob needed
longer reaction time. 3-Methyl-2-phenyl-thiophene was prepared by Grignard coupling
reaction between 2-bromo-3-methyl-thiophene and bromobenzene, it was then
brominated with NBS in chloroform and acetic acid (1:1) as discussed previously. The

Stille reagents for synthesis of Oligoa and Oligob were obtained in high yield by reacting
the corresponding Grignard reagent with tributylstannyl chloride in ether. The obtained

120

Stille reagents are enough stable to store at room temperature for more than one week
without significant change (detected by
1
HNMR).
Stille coupling reaction of 4’,4’’-Dibromo(1,3-(3’-methyl-thienyl))-azulene with
corresponding Stille reagents catalyzed by Pd(PPh
3
)
4
in refluxing toluene, yielded Oligoa
and Oligob as green and yellowish-green solid, respectively. Their single crystals were
obtained by evaporating of their dilute solution in hexane and CH
2
Cl
2
at room
temperature.
SS
Me
Me
SS
Me
Me
Br
Br

Br
MgBr
SnBu
3
S
Me
Br
S
Me
S
Me
SnBu
3
SS
Me
Me
SS
Me
Me
S
S
Me
Me
Monob
i
iii
i
iv
iii
v

v
ii
Oligoa
Oligob

Scheme 4-1. Synthesis of Oligoa and Oligob. Reagents and conditions: (i), Mg/Et
2
O, (ii),
Bu
3
SnCl/Et
2
O, (iii), NBS/CHCl
3
/AcOH, (iv), Bromobenzene, (v), AsPh, LiCl, Pd(Ph
3
)
4
, 1,4-
dioxane.

To study the long side chain effect on the solid structure of the polymers, monomers with
short alkyl-substitutent (less than four carbons) had been prepared but they were found to
have difficulty in crystallization. We then turned to monomers with alkoxy side group,

121

which may crystallize easily because of the electro-negative oxygen. MonoO6 was thus
designed and synthesized by Grignard coupling of 1,3-dibromo-azulene and 2-bromo-3-
hexyloxy-thiophene, catalyzed by Ni(dppp)Cl

2
in anhydrous THF. During the preparation
of MonoO6, two methods were used to prepare 3-alkoxyl-thiophene: The first attempt
was made using Ullmann synthesis with copper (I) halide catalysis.
14
This method
suffered from its low yields, long reaction time, and difficulty in purification. In the
second attempt, 3-alkoxyl-thiophene was prepared from 3-methoxyl-thiophene as shown
in Scheme 4-2.
15,16
In this method, 3-bromo-thiophene was first converted to 3-methoxy-
thiophene by reaction of 3-bromothiophene with MeONa in toluene. 3-Hexoxyl-
thiophene was then afforded by reaction of 3-methoxythiophene with 1-hexanol and
NaHSO
4
in toluene. The second method showed advantages in high yields and simple
purification. Although alkoxyl-thiophene are unstable at r.t., the obtained monomers,
MonoO6, however, is quite stable at r.t. This is may be due to the increasing of oxidation
potentials of the product.
S
Br
S
OMe
S
OC
6
H
13
S
OC

6
H
13
Br
BrBr
S
S
O
O
i
ii
iii
iv
v
MonoO6

Scheme 4-2. Synthesis of MonoO6. Reagents and conditions: (i), MeONa/MeOH, (ii), hexanol,
toluene, (iii), NBS, CHCl
3
/AcOH, (iv), NBS, benzene, (v), Mg/Et
2
O, Ni(dppp)Cl
2
.
Characterization
HNMR. Structural characterization of the synthesized oligomers was performed using
1
HNMR and
13
CNMR spectra as shown in Figure 4-1a and Figure 4-2a with the

assignment of the peaks for Oligoa and Oligob. Comparison of the
1
HNMR of Oligoa

122

and Oligob, we found there is less changes of azulene protons even two more thiophenes
were inserted into Oligob than in Oligoa. To illustrate, H4,8, and H2 of azulene in
Oligob, negatively shifted approximately -0.02 ppm compared to that of in Oligoa. In
case of H6 of azulene in Oligoa, there is about 0.04 ppm shift to the low field in Oligob.
However, extension of the conjugation by inserting two more aromatic units can be seen
from the β-H of thiophene. For instance, two β-H on thiophene in Oligoa was found at
7.30 ppm as a single peak, and shifted to 7.14 ppm in Oligob. Another two β-H on
thiophene was found at 7.05 ppm in Oligob. The high field shift of the β-H on thiophene
ring in Oligob was attributed to the increase of conjugation.

(a)
(b)
Figure 4-1. (a), HNMR spectrum of Oligoa and (b), its HMQC spectrum.

123

For the purpose to assign the carbons in these oligomers, 2-D HMQC experiments were
carried out and the results are displayed in Figure 4-1b and Figure 4-2b. These results
also confirmed the expected oligomers structure and revealed that the extension of
conjugation has little effect on azulene’s carbons. For instance, C4,8 of azulene was
found at 138 and 136 ppm in Oligoa and Oligob, respectively.

(a)
(b)

Figure 4-2. (a), HNMR spectrum of Oligob and (b), its HMQC spectrum.
1
HMR spectrum of MonoO6 is displayed in Figure 4-3. Because of the electron-donating
property of oxygen, a large shift of the protons on azulene in MonoO6 comparing to that
in the alkyl-substituted monomers such as Monob was found in its
1
HMR spectrum. In
Figure 4-3, H4,8 of azulene appeared at 8.58 ppm in MonoO6, while H4,8 in Monob
were found at 8.38 ppm (Chapter 3). They shifted approximately 0.2 ppm to higher filed.

124

Similarly, H2 in MonoO6 positively shifted around 0.37 ppm when compared to that of
in Monob. However, H6 and H5,7 did not show significant shifts as we observed for
H4,8 and H2, which may be due to the bond-alternation properties of azulene.

Figure 4-3. HNMR spectrum of MonoO6
FT-IR. FT-IR spectra showed that methyl substituted oligomers resemble those
copolymers. These compounds showed characteristic frequencies at 3015 cm
-1
that was
assigned to the aromatic C-H stretching mode, and 2910-2850 cm
-1
to the aliphatic C-H
stretching. There were no γ
α-C-H
stretching at 3080 cm
-1
in these oligomers as we
observed in MonoO6, which indicated that all the α position of thiophene was linked in

the conjugated oligomers. Absorption at the region 1930-1659 cm
-1
was attributed to
characteristic absorption of the benzene ring. Absorption at 1560, 1430, and 830 cm
-1

characterized the tri-substituted thiophene. Absorption at 1591 (γ
C=C
of azulene) and 734
cm
-1
(δ
=C-H
of azulene) characterized existence of azulene in the conjugated systems.
Peaks at 1170 cm
-1
was attributed to the in-plane C-H deformation, 1070-1000 cm
-1
was
attributed to the in-plane C-H deformation, and 830-756 cm
-1
was attributed to the out-
plane C-H deformation.

125

4000 3000 2000 1000
0.0
0.5
1.0

1.5
MonoO6
Oligoa
Oligob
Wavenumber (cm
-1
)

Figure 4-3. FT-IR spectrum of Oligoa, Oligob, and MonoO6.

Structural Analysis
Structure of Monoa and Monob.
Single crystal of 1,3-dithienyl-azulene (Monoa) and 1,3-di(3-methylthienyl)-azulene
(Monob) were obtained and Figure 4-4 illustrates the atomic numbering scheme
employed for Monoa and Monob respectively. The crystallographic parameters for these
structures are listed in Table 4-1. Both crystals are orthorhombic system but a slight
difference in density is observed with Monoa at 1.406g/cm
3
, and Monob at 1.343g/cm
3
.
The major observation was the large torsion angle between the thiophene ring and
azulene ring. In Monoa, the thiophene ring and the five-membered ring of azulene are all
nearly planar themselves. But one torsion angle between the thiophene and the five-
membered ring of azulene was found to be 43.2
0
(C10-C3-C1’-S2) and the other is 34.4
0

(C9-C1-C1’’-S1). These data are very close to the theory calculation. PM3 calculation

has been carried out on Monoa and the calculated dihedral angle between the thiophene
ring and azulene ring are 48.5
0
and 52
0
at the lowest energy of 619.52 kJ/mol. As

126

expected, larger steric effect of the methyl group increases the torsion angle in between
thiophene and azulene ring. In Monob, one torsion angle between thiophene and the five-
membered ring of azulene ring is 44.7
0
(C10-C3-C1’-S2) and the other is 45.0
0
(C9-C1-
C1’’-S1). The large torsion angles arises from steric hindrance between the seven-
membered ring of azulene and the thiophene ring, that is, the repulsion between H4, H8
of azulene and the H or substituted group at 3-position of the thiophene ring. The large
torsion angles between azulene and thiophene ring, especially the substituted one
(Monob), means that the wave functions of the HOMO for these compounds are at least
somewhat less localized than for the reported coplanar oligothiophenes.
5,17
Monoa
Monob
Figure 4-4. Perspective view and atom labeling of the crystal structure of Monoa and Monob.

Bonds lengths for Monoa and Monob are displayed in Figure 4-5. For example, the
bond-lengths between the azulene and the thiophene ring were found to be 1.459Å (C3-


127

C1’), 1.456 Å (C1-C1’’) in crystal Monoa, and to be 1.465 Å (C3-C1’), 1.461 Å (C1-
C1’’) in Monob. While the corresponding bond-length in the oligothiophene was found
to be 1.451 Å.
18
The large steric hindrance effect on conjugation of Monob than Monoa
can also be seen from bond-length of β-β single bonds in thiophene. The β-β single bond
(C2’-C3’ or C2’’-C3’’) length were found to be 1.415 – 1.416 Å in Monoa and 1.423-
1.428 Å in Monob.
The large torsion angle effect on the physical, optical and electrochemical properties of
corresponding polymers was found in their corresponding polymers studies. To illustrate
it, the conductivity of Polyb was found to decrease one-tenth of the conductivity of
Polya. But to our surprise, although large torsion occurred in the thiophene-azulene
copolymer systems, these polymers still showed high conductivities in the range of 1-
100S/cm.
S
S
1
.
3
8
9
1
.
4
0
2
1.456
1.459

1
.
4
1
2
1
.
4
0
9
1
.
7
2
6
1
.7
2
7
1
.
7
1
0
1
.
3
4
2
1

.
3
4
2
1.415
1.416
1
.
3
7
3
1
.
3
7
7
1.489
1
.
3
8
4
1
.
3
8
2
1
.
3

9
3
1
.
3
8
7
1
.
7
0
4
1
.
3
7
8
1
.
3
8
0
Monoa
S
S
1
.
4
0
3

1
.
4
0
4
1.461
1.465
1
.
4
1
5
1
.
4
1
5
1
.
7
3
3
1
.
7
3
5
1
.
7

0
9
1
.
3
5
3
1
.
3
4
7
1.423
1.428
1.
3
7
5
1
.
3
7
2
1.484
1
.
3
8
5
1

.
3
8
3
1.39
1
1
.
3
90
1
.
7
0
8
1
.
3
8
8
1
.
3
8
5
Monob
Figure 4-5. Bond distance in (a), Monoa; and (b), Monob.


128


Figure 4-6a is a stereoscopic view of the Monoa crystal, viewed along a axis, which
displays molecular packing. The crystal packing is characterized by the layered structure,
composed of the molecular zigzags and the herringbone structure, which are also
characteristic of organic crystals such as 2,6-dimethylnaphthalene
21
and DMQtT (α,α’-
dimethylquaterthiophene).
20
The smallest distance between the carbon ring is 3.452 Å,
this is roughly twice the carbon van der Waals radius (1.70 Å). The short distance
demonstrates the presence of the close intact of the carbon rings because of the dipolar
moment of azulene. Figure 4-6b presents the overlap of the molecules from a different
view of the molecules packing. Stereo views of Monob are shown in Figure 4-6c,d. To
our surprise, we found that Monob showed a different packing from Monoa because of
the methyl group substitution. The molecule shows the herringbone structure with an
angle of ca. 60
0
between a pair of molecular planes. Figure 4-6d showed the molecular
overlapping. It showed that neighboring molecules are packed in opposite direction, with
a small intermolecular distance ca. 3.58 Å between the C2 of azulene and neighboring
seven-membered ring of azulene. It should belongs to the weak C-H⋅⋅⋅π interaction.
(a)

129

(b)
(c)
(d)
Figure 4-6. Crystal structure of Monoa viewed from (a), a axis; (b), b axis, and Monob viewed

from (c), a axis; (d), c axis .


130

Table 4-1. Crystal data and experimental details.
data
Monoa Monob
empirical formula C
18
H
12
S
2
C
20
H
16
S
2

fw 292.40 320.45
T (K) 223(2) 223(2)
radiation wavelength, Å 0.71073 0.71073
crystal system Orthorhombic Orthorhombic
space group Pbca Pbca
cell dimens
a, Å 17.3267(13) 7.5257(3)
b, Å 7.2617(5) 10.9653(5)
c, Å 21.9512(16) 38.4072(17)

α, deg
90 90
β, deg
90 90
γ, deg
90 90
vol, Å
3
2761.9(3) 3169.4(2)
Z
8 8
D
c
(g cm
-3
) 1.406 1.343
abs coeff, mm
-1
0.370 0.329
F(000) 1216 1344
θ range (deg)
1.86-29.99 2.12-30.00
no. of reflcns measd 21554 24282
final R indices [ I>2σ(I)]

R
1
a
0.0480 0.0534
wR

2
a
0.1103 0.1322
largest difference peak, hole (e Å
-
3
)
0.395, -0.234 0.562, -0.201
a
R
1
= Σ||F
0
| - |F
c
||/Σ|F
0
|; wR
2
= √{Σ[w(F
0
2
– F
c
2
)
2
]/Σ[w(F
0
2

)
2
]}

Structure of Oligoa and Oligob
Single crystal of Oligoa and Oligob were obtained and their molecular structure and
atomic numbering are shown in Figure 4-7a. The crystallographic parameters for these
structures are listed in Table 4-2. Single crystal of Oligoa is monoclinic system with
space group C2/c while crystal Oligob is triclinic system with P-1 space group. The

131

crystal of Oligoa was identified as molecular weight of 472.63 with observed density at
1.329 g/cm
3
, and the molecular weight of Oligob is 664.92 with density at 1.338 g/cm
3
.


(a)
(b)
Figure 4-7. The molecular structure and atomic numbering of (a), Oligoa, and (b), Oligob.

Although these oligomers showed longer conjugation than the monomers (will discuss in
the following UV-vis part), large torsion angles were observed in these oligomers
between the adjacent moieties. Oligoa is a systemic molecule, the torsion angle between
the thiophene and the five-membered ring of azulene was found to be 48.1
0
(C4-C5-C7-

S1), and the torsion angle between the thiophene and the benzene ring was found to be
9.1
0
(S1-C10-C12-C17). The torsion angle between thiophene and azulene is bigger than
that of in the Monob. However, because of the small torsion angle between the thiophene

132

ring and benzene ring, effective conjugation of Oligoa is longer than that of Monob. For
Oligob, the torsion angle between the thiophene and the five-membered ring of azulene
ring is 48.2
0
(C9-C10-C11-C12) and the torsion angle between the two thiophene ring is
38.9
0
(C13-C14-C16-S3). To our surprise, we found that that the terminal benzene is
nearly vertically positioned to the thiophene ring. We think such a large torsion angle was
due to the steric hindrance between the benzene ring and the methyl group on thiophene.
Compared with the crystal structures of Monob, we found that methyl groups in
oligomers were all arranged differently from that in Monob. In both oligomers, methyl
groups are all pointed to the positions favoring the release of the steric hindrance. Also
we found two adjacent thiophene rings in Oligob adopt all-anti conformation as observed
in most oligothiophenes.
19,20
These arrangement may be better approach of the real
arrange of the side alkyl group in the resulting copolymers. Combined all the observation,
such as the large torsion angle, anti-conformation of the adjacent thiophene rings, and
methyl group positions, we may conclude that the alkyl side group are not arranged in a
same direction. Because of the large torsion angle, the long side alkyl group may arrange
a cylindrical structure with the rigid conjugated backbone in the center. Such “hairy rods”

structure may explain the copolymers’ properties we observed in Chapter 3, such as
similar UV-vis absorption of the polymers in solution and in solid state, amorphous
structures observed in XRD, and etc.

133

(a)
(b)
Figure 4-8. ORTEP drawing of Oligoa and Oligob.
It is of particular interest to compare the crystal structures of the oligomers with the
monomers. Bonds lengths for Oligoa and Oligob are displayed in Figure 4-9. Because
effective conjugation extends in these oligomers, significant bond alternation should be
observed in the oligomers crystals. β-β Single bond in thiophene of Oligoa (C8-C9) was
found to be 1.419 Å, and the α-β double bond was found to 1.361 Å (C7-C8), the
difference is about 0.058 Å. This difference was found to be 0.043 Å and 0.051 Å in
Monoa and Monob, respectively. The bond length difference between β-β single bond
and the α-β double bond in the thiophene ring was found to be 0.048 Å and 0.076 Å,
respectively. Such a difference may be due to the electron-donating properties of the five-
membered ring of azulene and the large torsion angle between the thiophene ring and the
benzene ring.

134


S
1
.
7
2
7

1
.
3
9
8
1
.
4
1
1
1
.
3
7
9
1
.
3
9
0
3
8
5
1.465
1
.
3
6
1
1.419

1
.
3
5
8
1
.
7
2
5
1.467
1
.
(a)
S
S
Ph
1.471
1
.
4
0
1
1
.
3
9
7
1.489
1

.
3
8
6
1
.
3
7
9
9
1
.
7
2
8
1
.
3
6
9
1.417
1
.
3
5
3
1
.
7
3

1
1.457
1
.
7
28
1
.
3
5
4
1.420
1
.
3
5
6
1
.
7
2
4
1
.
3
7
(b)
Figure 4-9. Bond distance in (a), Oligoa; and (b), Oligob.

Figure 4-10a,b is a stereoscopic view of the crystal of Oligoa. The overall crystalgraphic

profile is characterized by a layered structure of Oligoa molecules. It was noteworthy that
azulene are pointed to opposite direction at alternative layers as can be seen clearly from
Figure 4-10a. Such molecular packing was attributed to the large dipolar moment of
azulene. The smallest intermolecular distance between the seven-membered ring of
azulene is 3.663 Å which is roughly twice the carbon van der Walls radius (1.70 Å).
21

Stereo views of Oligob are shown in Figure 4-10c,d. The molecules are packed in a
layered structure consisting of slipped stacks. The backbones of adjacent Oligob in the
stack are laterally displaced and only slightly overlap. Similar to that of Oligoa,
neighboring molecules are packed in opposite direction. Interestingly, there is hydrogen
bonding between the proton of seven-membered ring of azulene and the sulfur in the

135

thiophene near to benzene ring. The C-S distance is about 3.666 Å with an angle about
144.6
0
. This interaction may be due to the weak acidity properties of protons on the
seven-membered ring of azulene and the electronegative property of sulfur atom. This
interaction may arrange molecules of Oligob packed in alternative column with opposite
direction.


(
a)


(b)



(c)






136


(d)
Figure 4-10. Stereoscopic view of the oligomers: (a), crystal structure of Oligoa viewed along a
axis, (b), crystal structure of Oligoa viewed along c axis, (c), crystal structure of Oligob viewed
along c axis, and (d), crystal structure of Oligob viewed along a axis.

Table 4-2. Crystal Data and Experimental Details

data
Oligoa Oligob MonoO6
empirical formula C
32
H
24
S
2
C
42
H
32

S
4
C
30
H
36
O
2
S
2

fw 472.63 664.92 492.71
T (K) 223(2) 223(2) 223(2)
radiation wavelength,
Å
0.71073 0.71073 0.71073
crystal system Monoclinic Triclinic Orthorhombic
space group C2/c P-1 Pnaa
cell dimens
a, Å 27.9513(18) 8.5850(5) 8.7590(12)
b, Å 11.0656(8) 9.4484(7) 15.009(2)
c, Å 7.7111(5) 21.2526(16) 42.798(6)
α
, deg
90 85.544(2) 90
β
, deg
97.882(2) 78.446(2) 90
γ
, deg

90 77.887(2) 90
vol, Å
3
2362.5(3) 1650.2(2) 5626.4(13)

137

Z
4 2 8
D
c
(g cm
-3
) 1.329 1.338 1.163
abs coeff, mm
-1
0.245 0.319 0.213
F(000) 992 696 2112
θ
range (deg)
1.98-30.04 1.96-25.00 1.44-25.00
no. of reflcns measd 9753 17761 30337
final R indices [
I>2
σ
(I)]

R
1
a

0.0438 0.0473 0.0836
wR
2
a
0.1181 0.1050 0.1995
largest difference
peak, hole (e Å
-3
)
0.332, -0.211 0.371, -0.244 0.290, -0.266
a
R
1
=
Σ||
F
0
|
-
|
F
c
||
/
Σ|
F
0
|
; wR
2

=
√{Σ
[w(F
0
2
– F
c
2
)
2
]/
Σ
[w(F
0
2
)
2
]}

Structure of MonoO6
Single crystal of MonoO6 was obtained by recrystallization from dichloromethane and
hexane. The obtained crystal is dark green cubic solid with density at 1.163 g/L. Crystal
data for MonoO6 are given in Table 4-3. The space group of MonoO6 was determined to
be Pnaa and the crystal system was found to be a orthorhombic. The numbering scheme
and the molecular structure of MonoO6 are displayed in Figure 4-11
.

Figure 4-11. The molecular structure and atomic numbering of MonoO6.

138



As expected, a larger twist between the thiophene ring and the azulene ring was found.
The torsion angle between the thiophene and the five-membered ring of azulene is 40
0

(C2-C3-C15-S2) and 44
0
(C2-C1-C11-S1). Furthermore, because of the large steric effect
of the long side group, a diminution of the angle between thiophene and azulene was
found. The angle between S2-C15-C3 was found to be 92.5
0
, which is smaller than in
Monob (118
0
). Interestingly, because of the electron-donating property of oxygen, the
bond-alternation in the seven-membered ring decreased compared with other monomers
and the oligomers. From Figure 4-12b, we found the bond-alternation in the seven-
membered ring of azulene is around 0.005Å. While this difference in Monoa, Monob,
Oligoa, and Oligob are found to be 0.015, 0.008, 0.011, 0.007 Å respectively.

(a)

S
S
O
O
1
.
4

0
1
1
.
4
0
7
1.500
1
.
4
1
0
1
.
3
9
6
1.468
1.461
1
.
7
2
7
1
.
3
6
8

1.414
1
.
3
4
5
1
.
7
1
0
1
.
7
2
3
1
.
3
7
0
1.414
1
.
3
4
4
1
.
6

9
1
1
.
3
7
9
1
.
3
8
0
1
.
3
8
3
1
.
3
8
1
1
.
3
8
1
1
.
3

8
4

(b)
Figure 4-12. (a), ORTEP drawing of MonoO6 and (b), bond distance in MonoO6.

The packing arrangement of MonoO6 is of the “herringbone” type: the molecular axes
are parallel but the molecules in adjacent layers are inclined with respect to each other.
Herringbone is an usual packing of conjugated oligomers, as well as polymers.
22-24
The

139

long axes of the molecule is parallel to the ac-plane. The most noticeable feature in the
packing is the head-to-head arrangement of the molecules. As can be seen in Figure 4-
13a, all these molecules are packed in parallel and the neighboring molecules were
arranged in head-to-head pattern. Figure 4-13b presents a different view of the molecule
along c axis. From this we can see that all the MonoO6 molecules are lined and form
generally zigzags along the c axis.
(a)
(b)
Figure 4-13. Stereoscopic view of the oligomers: (a), crystal structure of MonoO6 viewed along
a axis, (b), crystal structure of MonoO6 viewed along c axis.

UV-vis Spectra and NMR studies
We have discussed the UV-vis spectra of Monoa and Monob in Chapter 3. Compared
with these monomers, oligomers showed distinct bathochromic shift due to the extension

140


of the conjugation (Figure 4-14). Monob with 3 conjugated units showed a λ
max
of 296
nm, while Oligoa, which has 5 conjugated units in its backbone, displayed a λ
max
of 325
nm. Oligob, which has 7 conjugated units in the backbone, was characterized by its
longest wavelength absorption at λ
max
of 377 nm. The bathochromic shift of the longest
wavelength absorption band as the number of aromatic units increase in the backbone is
because addition of every aromatic units will cause hybridization of the energy level and
thus yielding more and more levels. This is in good agreement with the theory
calculation, which showed that the energy level of oligothiophenes was shown as a
function of the oligomers length.
25

400 600
0
1
2

Oligob

Monob

Oligoa
Absorption
Wavelength (nm)


Figure 4-14. Comparison of UV-vis spectra of the monomers and oligomers with different chain
length.

When the visible spectra of these monomers and oligomers were measured under
protonation by TFA in chloroform, two or three new absorptions in the visible region
were gradually developed as shown in Figure 4-15. For comparison, UV-vis spectrum of
Monob at different TFA concentration was also displayed here. As we discussed in

141

Chapter 3, solution color of Monob gradually changed from green to light blue and to
purple with more TFA added. The longest absorption at 545-555 nm was attributed to the
formation of the azulene radical cations as described by the reference.
26

Compared with Monob, Oligoa showed less UV-vis spectrum changes upon protonation,
as shown in Figure 4-15b. When TFA wa added to the solution of Oligoa, two new bands
appeared at 571 and 467 nm, respectively. However, their absorption intensities changed
insignificantly as we observed in Monob.
400 600 800 1000
0
1
2
TFA concentration
0%
2%
5%
10%
20%

30%
40%
Absorbance
Wavelength (nm)
(a)
400 600 800 1000
0
1
2
TFA concentration
0%
2%
5%
10%
20%
30%
40%
Absorption
Wavelength (nm)
(b)

142

400 600 800 1000
0
1
2
TFA concentration
0%
2%

5%
10%
20%
30%
40%
Absorption
Wavelength (nm)
(c)
400 600 800 1000
0
1
2
TFA concentration
0%
2%
5%
10%
20%
Absorption
Wavelength (nm)
(d)
Figure 4-15. UV-vis spectra of (a),Monob (R = Me), and (b), Oligoa, (c), Oligob, and (d),
MonoO6 in different TFA concentration.

Upon protonation, Oligob showed two new peaks appeared at 650 and 532 nm,
respectively (Figure 4-15c). It also showed similar UV-vis spectrum changes to that of
copolymers. Quite different from Oligoa, Oligob showed significant UV-vis spectrum
changes during the protonation process than that of Oligoa. To estimate the response
ability of the oligomers to the TFA concentration, we defined the absorption intensity


143