Structures, properties, and applications of soluble polyazulene and azulene containing copolymers 2

Chapter 1
Introduction
At the beginning of this century, the Royal Swedish Academy of Science awarded the
Nobel Prize in Chemistry for 2000 to three scientists who have revolutionized the
development of electrically conductive polymers. Just as the committee said in the Press
releases, the choice was motivated by the important scientific position that the field had
achieved, consequently practical applications, and of interdisciplinary development
between chemistry and physics.
1

Normally the polymers  that is, plastics  are used in electronic applications as
insulators due to the intrinsic property of covalent bonding present in most commercial
plastics. These polymers with localized electrons are incapable of providing electrons as
charge carriers or a path for other charge carriers to move along the chain. At the end of
the 1970s, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa have changed this
view with their discovery that a polymer (e.g. polyacetylene), can be made conductive
almost like a metal.
2
This electrical properties combining with the polymers’ special
characteristics such as low densities, mechanical strength, ease of fabrication, flexibility
in design, stability and resistance to corrosion has prompted great interest in conducting
polymers over the last 20 years.
1. Conducting Polymers
The conducting polymers was defined as the fourth generation polymeric materials –
“metallic polymers” by Prof. Heeger in his lecture at the Nobel Symposium in 2000.
3

Conducting polymers are characterized by the presence of conjugated double bonds along

1

the backbone of the polymers; and conducting forms are usually classified as the cation
or anion salts of highly conjugated polymers.
4
This means, conjugation is not enough to
make the polymer material conductive. In fact, charge carriers in the form of extra
electrons or “holes” have to be injected in the conducting polymers. A hole is a position
where an electron is missing. When such a hole is filled by an electron jumping in from a
neighbouring position, a new hole is created and so on, allowing charge to migrate a long
distance.
From the first moment it was realized that the applicability of polyacetylene is very
limited because of its processing difficulty and the rapid decrease in conductivity upon
exposure to air. Therefore, other conducting polymers that are more environmentally
stable and that can be electrochemically polymerized have been developed. These
polymers include polypyrrole (PPy),
5-8
polyfuran (PF),
9-12
poly(p-phenylene)s (PPP),
13-15

poly(p-phenylene vinylene)s (PPV),
16-18
polyaniline (PAn),
19-21
polythiophenes (PTs)
and copolymers of poly(3-alkylthiophene)s (PATs).
22-28

Among these polymers, polythiophene (PT), highly processable poly(3-alkylthiophene)s

(P3Ats) and other substituted thiophenes have always been the most likely candidates
because of their high thermal and environmental stability both in neutral and doped
states, variety of molecular designs and wide range of potential applications. The
properties of these materials can be varied over a wide range of conductivity, process-
ability, and stability depending on the type of substituents, rings, and ring fusion. The
wide potential technological use of PTs implies profound modulations of the form,
structure and properties of the polymers in order to meet the specific requirements of
each type of envisioned application.
1.1. The conductivities of conjugated polymers

2

The most important aspect of conjugated polymers from an electrochemical perspective
is their ability to conduct electricity. Value of electrical conductivity is represented in
terms of specific conductivity σ (Ω
-1
cm
-1
, S cm
-1
) or its reciprocal, specific resistivity, ρ
(Ω cm). The specific conductivity represents the electric current that flows across the unit
area (1cm
2
) of electrode under the unit external electric field (1 V cm
-1
) applied to the
sample, σ is expressed by Eq. (1-1).
σ = neµ (1-1)
Generally, materials with metallic properties in electrical conduction generally show

conductivities higher than 10
2
S cm
-1
, while materials with with conductivities of less
than 10
-12
S cm
-1
are often defined as insulators. Materials with electrical conductivities
between 10
-12
and 10
2
S cm
-1
are generally referred to as semiconductors. For conjugated
polymers, because the band gap of them is usually fairly large, n is very small under
ambient conditions, suggesting that conjugated polymers are insulators in their neutral
state. Till now, no intrinsically conducting organic polymer has been reported. However,
a polymer can be made conductive by oxidation (p-doping) and/or, less frequently,
reduction (n-doping) either by chemical or electrochemical means, to generate the mobile
charge carriers. The conductivities of most conducting polymers are in the range of semi-
conductors as shown in Figure 1-1a
1
, and the conductivity of conducting polymers spans
a very wide range (10
-12
to ∼10
5

Scm
-1
) depending on doping (Figure 1-1b). The doping
by both organic and inorganic oxidants changes the oxidation state without alternating the
structure of the polymer.
30

At a lower level of doping, conducting polymers behave as semiconductors. Thus
conjugated polymers have high potential for applications as molecular wires in molecular
electronics.
31
The semiconductor properties of the conducting polymers have also been

3

applied in solid-state electronic devices such as Schottky-type barrier diodes,
32
p-n
junctions, transistors,
33
photovoltaic cells and etc.
34
Interestingly, the conductivity of
some polymers has been found to change on exposure of different gases which led to the
use of conducting polymers as gas sensors, some times marketed as “artificial nose”.
35,36

(a)
(b)

Figure 1-1. Comparison of Conductivities (a), of conducting polymers compared to those of
other materials; and (b), of different conducting polymers.

1.2. Mechanism of Polymer Conductivity
As we know in a metal, free electrons move easily from atom to atom under an applied
electric field and a value for metallic copper around 10
8
S cm
-1
has been measured. In a
metal, the orbital of the atoms overlap with the equivalent orbital of their neighbouring
atoms in all directions to form molecular orbitals similar to those of isolated molecules.

4

For conducting polymers, we can use a simple free-electron molecular orbital model to
describe quantitatively the difference between a conductor, semiconductor or insulator.
Polyacetylene is the simplest model of this class of materials as shown in Figure 1-2.
Other type conducting polymers such as poly(heterocycles) can be viewed as an sp
2
p
x

carbon chain in which the structure analogous to that of cis-polyacetylene (Figure 1-2).
Assume a row of N atoms separated by a distance d, so the total length of the chain is (N–
1)d or, for large N, approximately Nd. According to the quantum-mechanical model for a
free particle in a one-dimensional box (potential zero inside the box and infinity outside)

the wave functions correspond to a ladder of eigenvalues
E
n
= n
2
h
2
/8m(Nd)
2
, with n = 1,2, 3… , (1-2)
where h is Planck’s constant, m the electron mass and n a quantum number. If we assume
that the π electrons from the N p-orbitals are filled into this ladder, with two electrons per
molecular orbital (according to the Pauli principle), the highest occupied molecular
orbital (HOMO) has the energy:
E(HOMO) = (N/2)
2
h
2
/8m(Nd)
2
(1-3)
and the lowest unoccupied molecular orbital (LUMO) has the energy:
E(LUMO) = (N/2 + 1)
2
h
2
/8m(Nd)
2
(1-4)
The energy required to excite an electron from HOMO to LUMO is thus:

∆E = E(LUMO) – E (HOMO) = (N+1)h
2
/8m(Nd)
2
.[h
2
/8md
2
]/N for large N (1-5)
Obviously the band gap is predicted to decrease as 1/N with increasing polymer length,
and will thus practically vanish for macroscopic dimensions.

5

XXX
a
b

Figure 1-2. The structure of (a), cis-polyacetylene; (b), poly(heterocycles).
When one electron moved from one of the filled molecular orbitals up into one of the
empty molecular orbitals, there is an excited electron configuration and a corresponding
excited state (conducting band) with energy higher than that of the ground state (covalent
band). The minimum energy difference between covalent band and conducting band –
the band gap – that is, the energy needed to create a charge pair with one electron in the
upper (empty) manifold of orbitals and one positive charge or ”hole” in lower (filled)
manifold.
From equation (1-5), we can see that the band gap would vanish for a sufficiently long
chain, thus polyacetylene would be expected to behave as a conductor. However, in
practice, the band gap is related to the wavelength of the first absorption band in the
electronic spectrum of the substance. A photon with wavelength λ can excite an electron

from HOMO level to LUMO if the energy condition is fulfilled:
∆E = E(LUMO) – E (HOMO) = h ν =h c / λ (1-6)
where h is Planck’s constant and ν the frequency of light (the third equality comes from c
= νλ, with c the velocity of light). For polyacetylenes, the optical absorption will be red-
shift with increasing length of the polyacetylene. That is, the band gap ∆E decreases
when more double bonds are added to form molecules with lengthening conjugations, for
example in the progression from ethene to butadiene to hexatriene, etc. However there
seems to be an upper limit beyond which no change will result from further conjugation
into an infinite linear polyacetylene. Thus, polyacetylene was found to be a

6

semiconductor with an intrinsic conductivity of about 10
–5
to 10
–7
S m
–1
. The reason why
polyacetylene is a semiconductor but not a conductor is due to that the chemical bonds in
polyacetylene are not equal: there is an obvious difference between these bonds, with
alternating sigle and double bonds.
However, a polymer can be rendered conductive by doping. A polymer can be made
conductive by oxidation (p-doping) and/or less frequently, reduction (n-doping) of the
polymer either by chemical or electrochemical means, generating the mobile charge
carriers. Here doping of polythiophenes (PT) is used as an example to illustrate the
doping process. As shown in Scheme 1-1, iodine (I
2
) will abstract one electron from
polythiophene under formation of an I

3
–
ion. The removal of one electron from the
polythiophene chain produces a mobile charge in the form of a radical cation, also called
a “polaron”. The “polaron” is localized, partly because of Coulomb attraction to its
counterion (I
3
–
), which has normally a very low mobility; partly because of a local
change in the equilibrium geometry of the radical cation relative to the neutral molecule.
Since the counterion (I
3
–
) to the positive charge is not very mobile, a high concentration
of counterions is required so that the polaron can move in the field of close counterions.
This explains why so much doping is necessary. If a second electron is removed from an
already-oxidised section of the polymer, either a second independent polaron may be
created (“double polaron”) or, the unpaired electron of the first polaron is removed, a
“bipolaron” is formed. In either case, introduction of each positive charge also means
introduction of a negatively charged counter-ion (I
3
-
). The two positive charges of the
bipolaron are not independent, but move as a pair, like the Cooper pair in the theory of
superconductivity. While a polaron, being a radical cation, has a spin of 1/2, the spins of
the bipolarons sum to S=0.

7

S

S
S
S
S
S
S
n
PT
-1e
-
Ox
S
S
S
S
S
S
S
n
S
S
S
S
S
S
S
n
Ox
-
"polaron"

Ox
-
Ox
-
"bipolaron"
S
S
S
S
S
S
S
n
-1e
-
Ox
-1e
-
Ox
"double polaron"

Scheme 1-1. Structure change in polythiophene upon doping with a suitable oxidant.
Furthermore, polymer chain defects are common in conjugated polymers. And the
conductivity in polyacetylene is solitary wave defects, “solitons”. Positive, negative, and
neutral solitons have been developed to explain the conductivity of polymers. Figure 1-3
shows how a cis polyacetylene chain by undergoing “thermal” isomerisation to trans
structure may create a defect, a stable free radical: this is a neutral soliton which can
propagate along the polymer chain but may not carry any charge itself. In a conducting
polymer, a polaron, bipolaron, or soliton can travel along a chain as an entity, the atoms
in its path changing their positions so that the deformation travels with the electron or

hole. Except for the metallic state, these are the entities through which change transport is
accomplished in conducting polymers.

8


Figure 1-3. A soliton is created by summarization of cis polyacetylene (a to b) and
moves by pairing to an adjacentelectron (b-e).
In 1992, Miller et al
37-39
suggested there were two likely conduction methods in oxidized
polythiophene: conduction along a thiophene ring chain via polaron/bipolarons and
conduction between thiophene ring chains mediated through π-dimer and π-stacks. In
their studies, oligothiophenes listed below were used as models for the structural entities
in polythiophenes and provided the evidence for π-aggregation of oxidized chains.
S
R
SS
R
OT3a,b
R = CH
3
, CH
3
S
S
Me
SS
Me
OMe

MeO
OT3OMe
S
S
Me
S
S
S
Me
OMe
MeO
MeO
OMe
OT5OMe

Using methyl- and thiomethyl-substituted oligomers such as OT3a,b with blocked
terminal positions, Hill showed that in CH
2
Cl
2
solution the ESR active cation radicals and
ESR silent dications were sufficiently stable. The cation radicals showed two π-π* bands
at wavelengths much longer than those of the neutral compounds. The dications showed
one π-π* band, located in between the two bands of the cation radical. When the
oligothiophene cation radicals are formed in the more polar solvent such as acetonitrile,
new absorption bands appear which was assigned to intermolecular π-dimers.
37,38
The π-

9

dimers showed three bands, two π-π* bands shifted to shorter wavelength compared to
the undimerized species and a charge transfer (CT) band at longer wavelength in the
near-IR region. As expected for the diamagnetic dimmers, the ESR signal intensity was
small in acetonitrile. Further investigation showed that π-dimer formation was enhanced
for longer oligomers. The dimerization equilibrium constant of OT5OMe was much
larger than that of OT3OMe.
40
The π-stacks were confirmed by the investigation of the
carboxylate-terminated oligothiophenes in aqueous solution and the studies of the crystal
structure of the oligothiophene cation. In aqueous solution solutions, the cations of these
oligothiophenes showed optical conduction bands that were indicative of stack
formation.
41-43

S
OH
2
H
2
CCS
S
S
S
SCH
2
CH
2
OC
O

CH
2
CH
2
CO
n
PE-OTh

To directly test the hypothesis that π-stacks can be important in polymer conductivity,
Hong et al prepared the polyester PE-OTh which has oligothiophene units isolated in the
main chain.
44
Because it does not have continuously conjugated chains, this polymer
cannot conduct via polarons or bipolarons. However, it can form π-dimers and π-stacks.
The synthesized polymer was oxidized with iodine or ferric chloride in CH
2
Cl
2
. UV-vis
and ESR spectra demonstrated that cation radicals were formed in solution and suggested
that stacks were formed in solution. At solid state, the thin film of polymer PE-OTh
showed strong optical conduction band and weak ESR signal. This polymer, which
cannot form bipolarons, exhibits good conductivity, and its ESR and optical spectra are
quite similar to those of oxidized polythiophenes. Thus, the formation of discrete inter-

10

chain aggregates (π-dimers and π-stacks) is responsible for the conductivity in the
polymer.
1.3. Electrical Conductivity Measurement

Two methods are commonly employed for the measurement of d.c conductivity of
conducting polymers. These have been referred to as 2-probe and 4-probe methods. For
semiconductors and insulators where the resistivity of the sample itself is very high, the
contact resistance becomes negligible; 2-probe method is applicable. But for highly
conducting samples, where the sample resistance is of the order of contact resistance, 4-
probe method is preferred.
In literature, a variety of units is used to describe the resistivity or conductivities of a
conducting polymer. The S.I. unit of the intrinsic resistivity is Ω m, but this resistivity is
usually given in Ω cm. The intrinsic resistivity is defined as the resistance between
opposite faces of a unit cube
45
, and the surface resistivity ρ
s
is often used to characterize
the current flow over a materials surface. The relation between the surface resistivity and
the intrinsic resistivity is given in equation 1-7,
ρ
s
= ρ
v
/d (1-7)
where d is the layer thickness. The intrinsic resistivity of a conducting polymer can be
calculated according to equation 1-8.
ρ
v
= (I/A)/(∆V/x) (1-8)
where I is the current (A), A is the cross-sectional area (m
2
), ∆V is the potential drop
across the two inner electrodes, and x is the separation between the two inner electrodes.

For the 4-point-probe method, it is often setup as shown in Figure 1-4.

11


Figure 1-4. Schematic presentation of an intrinsic resistivity measurement with a 4-point-
probe.
The intrinsic conductivity of the material can be calculated using equation 1-9.
46

ρ
v
= (∆V π d)/(I ln2) (1-9)
In this thesis experiemnts, the samples of polymers are pressed into a pellet of diameter
∼10mm having a thickness of ∼0.5mm. The method utilized a special probe head
containing 4 equally spaced pressure contacts made up on the sample surface as shown in
Figure 1-4. The sample is mounted on a copper block approximately of the size
30×20×4mm with appropriate electrical insulation achieving in the process good
electrical insulation between the specimen and the holder. The conductivity is defined as
the reciprocal resistivity. The unit of conductance, the reciprocal Ohm (Ω
-1
), is usually
called Siemens (S).
2. Conjugated polymers band gap engineering
2.1. Band-gap of conjugated polymers
As we have discussed above, electronically conducting polymers are extensively
conjugated molecules, and it is believed to possess a spatially delocalized band-like
electronic structure.
47,48
These bands stem from the splitting of interacting molecular

12

orbitals of the constituent monomer units in a manner reminiscent of the band structure of
solid-state semiconductors. Analogous to semiconductors, the highest occupied band
(which originates from the HOMO of a single aromatic unit) is called the valence band,
while the lowest unoccupied band (originating from the HUMO of a single aromatic unit)
is called the conduction band. The difference in energy Eg between them is called the
energy band gap (Eg) (Figure 1-5). Since band gap Eg depends upon the molecular
structure of the repeat units (monomer), it provides the opportunity and challenge for
chemists to control the polymer energy gap by design at molecular level. Such ‘band gap
engineering” may give the polymers desired electrical and optical properties.
Furthermore, the reduction of the band gap to approximately zero is expected to afford an
intrinsically conducting polymers.
49,50


Figure 1-5. Band structure in an electronically conducting polymer.
2.2 Reduction of band gap conjugated polymers
As we discussed above, the values of E
g
(HOMO-LUMO separation) determines the
electrical and optical properties of the resulting polymers, and is therefore of importance
in applications in electrochromic devices and in non-linear optics.
51
To mimic the metal
conductivities which are due the practically filled band with a semiconductor, the band
gap of the intrinsic conducting polymers should be zero or close to zero. Two major
approaches towards reduction of the band gap in conjugated polymers will be discussed

13

here: (i) minimization of bond-alternation along the main chain and (ii) alternation of
electronic-donors and –acceptors in the main chain.
2.2.1. Minimization of bond-length alternation
Although there has been some controversy over the relative importance of factors
controlling E
g
,
52-54
the theoretical work concurs that the key factor is the degree of bond
alternation in the polymers. For example, in polythiophene, as shown in Figure 1-7, the
energy difference between the aromatic and quinoid forms is comparatively large (ca. 2
eV) which is due to the small contribution of the energetically unfavourable quinoid
structure to the ground state of the polymer,
55
resulting in a pronounced single bond
character of the thiophene-thiophene linkage and hence a large bond-length alternation.
Increasing the double-bond character of the thiophene-thiophene linkage can be
accomplished by making the quinoidal structure energetically more favourable system as
the case of polyisothianaphthene (PITN).
56
Going from the aromatic to the quinoid state,
the loss of aromaticity in thiophene is coterbalanced by the gain aromaticity in the six-
membered ring. This results in a band gap for polyisothianaphthene of roughly 1 eV, one
full electronvolt (eV) smaller than that of polythiophene.
57,58

S
S

S
S
S
S
S
S
S
S
SS
n
n
n
n
Q
A
Q
A
Polythiophene
polyisothianaphthene

Figure 1-6. Change in the relative stabilities of the aromatic (A) and quinoid (Q) forms of
polythiophene and polyisothianaphthene
.

14

Synthesis of PITN was first reported by Wudl and Heeger group in 1984.
56
Following
this initial work, many papers have appeared on a variety of chemical and

electrochemical syntheses, as well as on other polymers representing structural variations
on the isothianaphthene unit.
59,60
One of the earliest claims of the synthesis of derivatives
of PITN was in 1988, when polymers PITN-OMe, PITN-EXa,b were prepared. The
band gap of PITN-EXa was originally said to be 0.6 eV, based on the band edge, but this
was subsequently modified to ca. 1 eV. A film of PITN-EXa could be recycled
electrochemically between oxidized and reduced form. Two-probe conductivity of
pressed pellets of the film of PITN-EXa was 4 × 10
-4
S/cm, and upon doping with iodine,
the conductivity rose modestly to 6 × 10
-2
S/cm.
61,62
S
S
H
3
CO OCH
3
n
PITN-OMe
OO
RR
n
S
n
NN
S

C
6
H
13
C
6
H
13
n
PITN-EX a, R = H
b, R = Me
PNTH
PTPA

Another polymer related to PITN in which the two CH groups adjacent to the thiophene
rings have been replaced by sterically less demanding nitrogen atoms is polythieno[3,4-
b]pyrazine (PTPA). The polymer was first prepared by Pomerantz and coworkers.
63,64
A
cast film of the polymer showed a band-edge band gap of 0.95 eV, confirming the
quantum-mechanical predictions that the band gap of PTPA would be 0.1 eV lower than
that of the parent PITN.
Further modification of PITN was polydithieno[3,4-d]thiophene (PDiTT), which was
prepared in 1988 by electrochemical polymerization.
65
The neutral polymer was opaque
and have λ
max
= 590 nm, whereas the doped PDiTT was found to be colorless and
transparent. The band edge band gap was reported to be 1.1 eV.

66


15

S
S
S
PDiTT
SSS
n
n
PBTBT

A number of copolymers containing two thiophene rings and one benzo[c]thiophene or a
derivative of benzo[c]thiophene in the repeat unit have been reported.
67
The rationale was
to combine a unit that refers the quinonoid structure in a polymer with thiophene that
prefer the aromatic form in order to form a low band gap polymer. For instance, Lorey
and Cava prepared copolymer poly(benzo[c]thiophene-alt-bithiophene) (PBTBT).
67
The
purple copolymer PBTBT showed λ
max
= 584 nm and a band edge band gap of 1.58 eV.
Another approach somewhat differs from that of PITN is combination of aromatic with
quinoid units in the backbone. It was prepared by oxidative elimination of a precursor
polymer containing all aromatic rings, thus converting some into quinoid rings.
68,69

Subsequent to these reports a number of publications have dealt with the syntheses of
these poly(heteroarylene-methylenes) polymers.
70-73
Another way of canceling the bond-
length alternation is reducing or eliminating the structural deformations that lead to the
localization of alternating double and single bonds along the conjugated main-chain. This
would mean the construction of ladder polymers of which the best-known example is
polyacene.
74,75

2.2.2. Reduction of band gap by donor-acceptor systems
It was shown with PITN that reduction of bond-length alternation by increasing the
double bond character between the repeating units of a conjugated polymer, results in a
decreased band gap. On the other hand, the interaction between a strong electron-donor
(D) and a strong electron-acceptor (A) may also give rise to an increased double bond
character between these units, since they can accommodate the charges that are

16

associated with mesomerism. Hence, a conjugated polymer with an alternating sequence
of the appropriate donor- and acceptor-units in the main-chain may show a decreased
band gap.
The donor-acceptor (D-A) repeat unit strategy was first introduced with polysquaraines
and polycroconaines, and the low band gap arises from the regular alternation of strong
donor and acceptor groups within the conjugated polymers backbone.
76,77
Thus the strong
squaric acid (SQA) and Croconic acid (CRA) were incorporated into the polymers along
with donor moieties containing alkyl groups for solubility. The polymers PDA-1 – PDA-

4 were prepared upon reaction of the acceptors and donors in a higher saturated alcohol
solution with a catalytic amount of mineral acid or a strong base. The band edge band gap
of PDA-1 (R = C
12
H
25
); PDA-2 (R
1
= CH
3
R
2
= C
12
H
25
); PDA-3 (R
1
= CH
3
R
2
= C
12
H
25
);
and PDA-4 (R = C
12
H

25
) were 1.15, 0.5, 0.8 and 1.2 eV, respectively, based on the vis-
NIR absorption maxima of 919, 1378, 992, and 919 nm, respectively. The conductivities
of the pristine polymer films were 10
-7
, 10
-5
, 10
-7
, and 10
-9
S/cm respectively. Doping
with iodine resulted in increased conductivities, up to 1 S/cm, and doping with DDQ also
gave values approximately 1 S/cm.
Calculations have shown that the hybridisation of the energy levels of the donor and the
acceptor in a conjugated polymer, particularly the high-lying HOMO of the donor
fragment and the low-lying LUMO of the acceptor fragment, yield a D-A monomer with
an unusually small HOMO-LUMO separation.
78,79
In these conjugated polymers, the
valence- and conduction-band are curved by space-charge effects, which lead to a
diminished band gap energy. Further hybridisation upon chain extension then converges
to the small band gaps.
80


17

N
N

H
3
C
CH
3
H
3
C
CH
3
R
R
O
O
N
N
H
3
C
CH
3
H
3
C
CH
3
R
R
O
O

O
S
N
S
N
R
2
R
1
O
O
S
N
S
N
R
2
R
1
O
O
O
)
(
n
(
)
n
)
(

n
)
(
n
PDA-1
PDA-2
PDA-3
PDA-4
The initial designs of donor-acceptor conjugated polymers are based on the
copolymerisation of donor molecules with either squaric acid or croconic acid as shown
in PDA1-4. The electron-withdrawing subunit can also be an aryl unit substituted with a
cyano- or a nitro-group, since the latter two are among the most widespread electron
withdrawing groups in organic chemistry. Polymers PTVCNa-c were obtained by
electrochemical polymerization and polymer PTVCNc was claimed to feature a band gap
of 0.6 eV versus 1.5 and 1.4 eV for polymers PTVCNa and PTVCNb, respectively.
81
Polymers PTVHCNa-b were synthesized analogous to polymer PTVCNa-c.
Electrochemical determination of the band gap resulted in values of 1.3 for PTVHCNa
and 1.0 eV for PTVHCNb.
82

S
X
NC
R
PTVCNa-c
a X = S, R = H
n
S
X

NC
n
R
R
OO
b X = S, R = Me
c X = O, R = H
PTVHCNa-b
a R = H
b R = OCH
2
CH
2
O

From the above review of the donor-acceptor conjugated polymers containing a cyano- or
nitro-substituted aryl unit as the acceptor, only polymer PTVCNc is below 1 eV. To find
out the reason why the band gap value of very strong electron-donor and –acceptor units
applied conjugated polymers is still high, a systematic donor-acceptor oligomers were
studied and calculation were carried out.
83-85
These studies revealed that electron-
accepting subunits with large AO coefficients at the coupling positions represents a
crucial issue in designing donor-acceptor conjugated polymers with a small band gap.

18

The most obvious approach is the selection of an aryl unit which bears one or more
electronegative atoms in the ring, close to the coupling positions. The representative
compounds include pyridine, benzothiadiazole, and so on. For instance, via a

polycondensation reaction of monomers using a zerovalent nickel complex, polymers
PYHa-c could be prepared in high yields.
86,87
However, the optical data of the polymers
are not very encouraging in terms of a small band gap since the λ
max
of polymers PYHa
and PYHb is centered around 490 nm, while for polymer PYHc it is observed around
440 nm.
S
N
n
Se
N
n
S
N
N
n
PYHa
PYHb
PYHc

A series of copolymers based on the bithiophene and thiadiazole were prepared and their
band gap were studied.
90
In these copolymers, polymer PTTDA-3 shows remarkable low
band gap, determined from the onset of the p- and n- type doping, of about 0.3 eV. The
dedoped state of this polymer shows the onset of absorption below 0.5 eV. A related
polymer, poly(benzo[1,2-c:4,5-c’]bis(1,2,5-thiadiazole)-4,8-diyl-alt-bithiophene),

(PBTABTh) has also been reported to have a band gap below 0.5 eV.
91
These two
polymers, PTTDA-3 and PBTABTh, are among the lowest band gap polymers reported
to date.
SS
N
S
N
S
NN
SS
N
S
N
N
S
N
n
n
PTTDA-3
PBTABTh

3. Advanced materials based on π-conjugated polymers

19

The past 20 years have witnessed the emergence of conjugated polymers as an important
class of electro-active and photoactive materials.
92,93

Functonalization of conjugated
polymers has led to the development of novel processable polymeric materials with
unusual electrical, electrochemical, and optical properties.
47
Two kinds of interesting
materials come from conjugated polymers are briefly discussed below.
3.1. Chromic effect conjugated polymers
Chromic (thermo-chromic, solvate-chromic, piezo-chromic, ion-chromic, affinity-
chromic, etc.) conjugated polymers have recently attracted a lot of attention, mainly
because of their great potential of application in the areas of sensors, diagnostics and drug
screening.
94-96
Some functionalized polydiacetylenes,
97,98
polysilanes,
99,100
or
polythiophenes
100,101
can show dramatic color changes in the presence of several external
physical or chemical stimuli which can be described as a trans-duction of a physical or
chemical information into an optical signal.
Many different conjugated polymers have been investigated, and on the basis of these
studies, these interesting optical effects have been attributed, to a transition between a
planar (highly conjugated) form and a non-planar (less conjugated) conformational
structure of the backbone. It has been also found that these chromic properties are
strongly dependent upon the nature and the position of the side chains in the repetitive
units of the polymers.
102
Also, it has been suggested that these optical effects are driven

by a delicate balance between steric repulsive interactions and attractive interchain (or
intrachain, due to a chain folding) inter-actions.
103
Thus, addition of flexible not only
enable the conventional conjugated polymers processing, but also create new materials
exhibit enahced electronic and optical properties as compared to the parent polymers. For
instance, polydiacetylene exhibit a phase transition from blue to red with increasing

20

temperature. Furthermore, polydiacetylene shows prominent absorption peaks at around
1.9 eV in the blue phase and 2.3 eV in the red phase. At low temperatures, coplanar
structures of poly(3-alkyl-thiophene)s,
104
poly(3-alkoxy-4-methylthiophene)s,
105
poly(3-
(alkylthio)thiophene)s,
106
and poly(3,3’-bis(alkylthio)- 2,2’-bithiophene)s
107
are disrupted
upon heating due to a disordering of the flexible side chains, resulting in a color change
with temperature. However, in the absence of significant steric interactions, the
conjugated polymer (e.g. poly(3,3’-dialkoxy-2,2’-bithiophene)s
108
can maintain nearly
planar conformations even at high temperatures.
Electrochemical redox processes also result in important changes in the UV-visible range
(electro-chromism), from dark red to blue in the case of poly(3-alkylthiophene)s

110
and
from dark blue to pale blue-grey for poly(3,3’-dialkoxy-2,2’-bithiophene)s
111
and
poly(3,4-ethylene-dioxythiophene).
112

Recently, the chromic behaviour of a soluble ω-hydroxides 3-substituted polythiophene
has been investigated by UV–VIS spectroscopy in different solvent/non-solvent mixtures
over a wide range of low temperatures. The author found that the reversal of the
solvatochromic transformation in substituted polythiopenes and a concentration effect in
dilute pure solvent solutions.
113

ANother interesting thermal- and ion-chromic conjugated polymers are the regioregular
poly(3-alkoxy-4-methylthiophene)s bearing crown ethers of different sizes (12-crown-4),
(PT12C4)and (15-crown-5) (PT15C5). These polythiophene derivatives exhibited a
highly conjugated form in the solid state at room temperature (absorption maximum
around 550 nm) and a less conjugated form upon heating (absorption maximum around
425 nm) as shown in Figure 1-7. In acetone or ethyl acetate ion-chromic responses were
observed upon addition of alkali metal cations. The color change from yellow (without

21

addition of metal ions) to violet (after addition of metal ions). PT12C4 was found more
sensitive to sodium salts while PT15C5 gives more intense ionochromic effects with
potassium salts.
114

Figure 1-7. Temperature-dependent UV-visible absorption spectra of PT15C5 in
acetonitrile.
114
3.2. Conjugated polymers-inorganic hybrids
Incorporation of transitional metal elements into conjugated polymers backbone would
provide another possibilities for super-molecular chemistry and for the properties of the
resulting superstructures.
115
Particular features of interest include a transition metal’s
ability to bind anions and small molecules (CO, O
2
, NO, etc.),
116-119
or effect catalytic
reactions.
120, 121
A large number of transition metal-containing polymers have been
prepared and studied and are of interesting because they allow the electronic, optical, and
catalytic properties of metal complex as discussed below.
Organometallic conductive polymers can be roughly divided into three types of
arrangements of the metals to the π-conjugated polymers.
122


22

 In type I, the metal is tethered to the backbone by a linker such as an alkyl
group.
123-125

 In Type II materials, the metal and backbone are electronically coupled, and can
influence each other's properties. Since p-conjugated backbones and many metal
groups are redox-active, this can lead to systems in which the properties of metal
and backbone may be electrochemically tuned.
126-130

 In type III, the metal group is located directly in the conjugated backbone. In this
arrangement strong electronic interactions between the organic bridge and metal
group are possible.
131-132

In type II, diimine groups, such as bipyridyl (bpy) and 1,10-phenanthroline (phen) have
been used extensively as ligands for transition metals, and their conjugated structure
makes them attractive candidates for incorporation directly into a conjugated polymer
backbone. In this configuration, metal centers coordinated by the diimine are closely
coupled to the polymer, thus allowing strong electronic interactions to occur.
126

The first bipyridine based conjugated metallopolymer was a Ru(bpy)
2
2 +
complex of poly-
bpy (PBPy).
133,134
The poly-bpy was prepared by the Ni(0) catalysed coupling of 5,5’-

dibromo-2,2’- bipyridine, and subsequently metallized by refluxing with Ru(bpy)
2
Cl
2
in
water. The resulting water-insoluble product consisted of a methanol-soluble fraction,
with a UV-visible absorption at ca. 450 nm characteristic of the Ru(bpy)
3
2+
chromophore
and a Ru : poly-bpy ratio of ca. 0.2.
NN N
N
n
m
Ru(bpy)
2
PBPy


23

Subsequently, a series of conjugated polymers containing Re(CO)
3
Cl were prepared in
order to explore interactions between the photophysics of the metal centres and the
polymer backbone.
135,136
The photophysical response of the polymer Re-aryleneethylyne
copolymer (Pbpy-Arylyne) however was characteristic of the individual components; the

chromophores were apparently not strongly coupled. The unusually weak communication
may be due to irregularity in the polymers' composition. A similar strategy of varying the
relative proportions of starting materials was used by Yu et al. in the synthesis of
copolymer of vinylstyrene and bpy (Pbpy-Vinyl) for the purpose of creating new photo-
refractive materials.
137
This approach allows specific control of the metal loading.
Ru(bpy)
3
2+
forms the core of their system owing to its efficient MLCT
138,139
which serves
as a charge generation source, while the conjugated backbone is intended to function as a
charge transport channel and non-linear optical chromophore.
NN
OR
RO
OR
RO
Re(CO)
3
Cl
x
y
Pbpy-Arnyl
NN
OR
RO
OR

RO
Ru(bpy)
2
x
y
RO
OR
Pbpy-Vinyl

2,2’-Bithiazole is a more attractive ligand than bpy for use in p-doped materials because
it is less π deficient which lead to easily oxidize of the polymer, although it bind less
strongly with metals.
140
There are also a variety of Schiff base metal complexes have
been electrochemically polymerized to form films on electrodes.
141,142
In these systems,
the metal can form an integral part of the conducting backbone or can be peripheral.

24

Arenes are also useful π-ligands for transitional metals. The metal-arene bonding
includes both π-donation from ligand to empty d-orbital of metal and π-back-donation
from filled metal d-orbital to π*-orbital of ligand, and thus the frontier orbitals are
frequently of an essentially d-character. It is thus interesting to see how the energy band
structure of linearly π-conjugated poly(arylene)s, such as PPP, are modified by
coordinating to transition metals.
143-145

Yaniger et al. reported the first example of poly(arylene) metal complex in 1984.

144
They
prepared poly(p-phenylene)

(PPP) complex (PPP-Ma-b) of M(CO)
3
by reaction of
insoluble powdery PPP with M(CO)
3
(CH
3
CN)
3
in boiling hexane. The polymer
complexes obtained are insoluble. Characterization by FT-IR spectroscopy and elemental
analysis gave the formula (C
6
H
4
[M(CO)
3
]
0.25
)
x
in which phenylene rings act as η
6
-
ligands. The conductivity of original PPP is σ < 10
-8

S/cm in the undoped state while
increases up to 10
-4
S/cm after coordination to M(CO)
3
moieties, with the color changes
from yellow to dark-brown. This was attributed to the partial doping that increased
planarization of the polymers, giving a longer conjugation length according to IR
spectroscopy. There also have been a number of reports of the complexation of metal
ions with conventional conducting polymers such as polyaniline,
146
polypyrrole,
147
and
polythiophene,
148
although none of these complexes have been well characterized.
In the case of polyanilines, the quinonediimine moieties of the emeraldine base
participate in coordination to transition metals to give conjugated complex PAN-M.
Electronic communication is considered to be permitted between metals through a π-
conjugated backbone chain.

25