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

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 p

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 2000.3Conducting polymers are characterized by the presence of conjugated double bonds along

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),13-15poly(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

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 (1cm2) 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 102 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 102 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 1-1a1, and the conductivity of conducting polymers spans

a very wide range (10-12 to ∼105 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

31

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 108 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

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 sp2px 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

En = n2h2/8m(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)2h2/8m(Nd)2 (1-3)

and the lowest unoccupied molecular orbital (LUMO) has the energy:

E(LUMO) = (N/2 + 1)2h2/8m(Nd)2 (1-4)

The energy required to excite an electron from HOMO to LUMO is thus:

∆E = E(LUMO) – E (HOMO) = (N+1)h2/8m(Nd)2 [h2/8md2 ]/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

X X

X

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 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

red-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

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 (I2) will abstract one electron from polythiophene under formation of an I3– 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 (I3–), 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 (I3–) 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 (I3-) 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

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

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 al37-39 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 RS

S R

OT3a,b R = CH3 , CH 3 S

S MeS

S Me OMe MeO

OT3OMe

S

S Me

S

S

S MeOMe MeO

MeO OMe

OT5OMe

Using methyl- and thiomethyl-substituted oligomers such as OT3a,b with blocked

terminal positions, Hill showed that in CH2Cl2 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-

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

OH2H2CCS S

S

S SCH2CH2O C

O

CH2CH2CO

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 CH2Cl2 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-

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 cube45, and the surface resistivity ρs 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,

ρs = ρv/d (1-7)

where d is the layer thickness The intrinsic resistivity of a conducting polymer can be

calculated according to equation 1-8

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

ρv = (∆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

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 Eg (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

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 Eg,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

n n

Q

A

Q A

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

polythiophene and polyisothianaphthene

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

R R

N N S

C6H13 C6H13

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 λmax = 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

S S S

and Cava prepared copolymer poly(benzo[c]thiophene-alt-bithiophene) (PBTBT).67 The

purple copolymer PBTBT showed λmax = 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.68,69Subsequent 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

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 = C12H25); PDA-2 (R1 = CH3 R2 = C12H25); PDA-3 (R1 = CH3 R2 = C12H25); and PDA-4 (R = C12H25) 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

H3C CH 3

H3C CH 3

R

R O O

O S

N S N

O O

) (

n

(

) n )

(

n

)

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

NC R

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

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 λmax of polymers PYHa

and PYHb is centered around 490 nm, while for polymer PYHc it is observed around

440 nm

S

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

N S N

S

N N

N N

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, 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 polythiophenes100,101 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

affinity-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

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)s107 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)s108 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)s110 and from dark blue to pale blue-grey for poly(3,3’-dialkoxy-2,2’-bithiophene)s111 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

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, O2, 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

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)22 + complex of

poly-bpy (PBPy).133,134 The poly-bpy was prepared by the Ni(0) catalysed coupling of dibromo-2,2’- bipyridine, and subsequently metallized by refluxing with Ru(bpy)2Cl2 in water The resulting water-insoluble product consisted of a methanol-soluble fraction,

5,5’-with a UV-visible absorption at ca 450 nm characteristic of the Ru(bpy)32+ chromophore

and a Ru : poly-bpy ratio of ca 0.2

Subsequently, a series of conjugated polymers containing Re(CO)3Cl 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)32+ forms the core of their system owing to its efficient MLCT138,139 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

OR

RO OR

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

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(CH3CN)3 in boiling hexane The polymer complexes obtained are insoluble Characterization by FT-IR spectroscopy and elemental analysis gave the formula (C6H4[M(CO)3]0.25)x in which phenylene rings act as η6-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 conjugated backbone chain

Figure 1-8 UV-visible spectra of undoped polyaniline in 1-methyl-2- pyrrolidinone on

successive additions of CuCl 2 to ca 0.3 per aniline ring

4 Azulene and Polyazulenes

Azulene and its derivatives are a well-known class of polycyclic nonbenzenoid aromatic compounds, and has been a fascinating target due to their unusual electronic structure and unusual photophysical properties The generic name “azulene” was first applied to these blue oils by Piesse in 1864.151 Later, when the structure had been elucidated, the name of

“azulene” was given to the compound.152 In some instances azulenes found naturally

have been named by prefixing part of the name of the essential oil to the word “azulene” Thus, the azulene from guaiol has been called guaiazulene, that of vetiver oil has been called vetrivazulene, etc The azulene shows a coplanar structure as shown in Figure 1-

10

4.1 Unique structure and interesting properties of azulene

Azulene ring system can be considered as a combination of the aromatic cyclopentadienyl anion and cycloheptatrienyl (tropylium) cation, hence it exhibits a high chemical reactivity towards electrophilic and nucleophilic agents respectively Calculated

by the LCAO-MO according to Hückel, with the resonance integral β9,10 reduced to 0.8β, the π-electron densities were obtained as shown in Figure 1-9

1.142

1.039 1.024

1.024

0.879 0.989

0.893

1.142 0.879

0.989

Figure 1-9 π-electron densities of azulene

As can be seen from the value of the π-electron densities displayed in Figure 1-9, it suggest that in the five-membered ring the atoms 1, 2, and 3 carry excess negative charges, whereas in the seven-membered ring the atom 4, 5, 6, 7, and 8 carry positive charges The charge drift from the seven- to the five-membered ring induces a permanent dipole moment in the molecule, the five-membered ring forming the negative end

For the nonalternant aromatic systems, such as azulene or fulvene, LCAO-MO theory predicts a large dipole moment due to electron drifts away from seven-membered and towards five-memebered rings The moments of azulene predicted from the value of the π-electron densities, with taking into account the reduced resonance integral for the

longer central bond and the interaction of the π-electrons, is about 1.7D All calculation make the five-membered ring the negative end of the dipole

The experimental value found for the dipole moment of azulene is 1.0D, but this value is not entirely free of additional assumptions The electron polarizability of azulene has been assumed to be equal to that of naphthalene because measurements of the refractive index in a region sufficiently from the long wave absorption of azulene could not be carried out

Azulene are chromophores: the deep blue color of the parent compound arise from the electronic transition from the highest occupied molecular orbital to the lowest unoccupied antibonding orbital (the 1Lb band) The azulene ring system is planar and thermodynamically stable, although the 10π electrons display the reactivity expected of

an aromatic system Substitution of the ring can alter the wavelength of this absorption band resulting in different colors for different azulene derivatives There is great difference in UV spectra of azulenes substituted in the five-membered ring, as compared with the seven-membered ring

Table 1-1 gives a survey of the optoelectronic properties of azulene The excited state photophysics is characterized by a weak S2 to S0 fluorescence at 375 nm and an even weaker S1 to S0 emission The energies of the excited states of azulenes and spectral data are also given in Table 1-1 The longest-wavelength transition at λ ≈ 590 nm is related to the small HOMO-LUMO gap and cause the typical azure (blue) color Azulenes hence low ionization potentials and a high electron affinity which is in agreement with the high energetic HOMO and the low energetic LUMO

Table 1-1 The optoelectronic properties of azulene.154

4.2 Recent application of azulene and its derivatives in materials science

Azulene not only has a beautiful deep blue color, but also has a large dipole moment (µ = 0.8-1.08 D) and unusual photophysical properties, the excited state and the electronic spectrum of azulene are of interest for theoretical and practical study Recently, intense interest has been shown towards azulene and azulene derivatives as raw materials for electrically conductive polymers as we will discuss below and for production of various medicals preparation, NLO, catalysts, and sensors.155

For instance, a series of poly(6-azulenylethynyl)benzenes substituted with hexyloxycarbonyl chains at 1,3-positions in azulene rings have been prepared by Pd-catalyzed alkynylation of halogenated arenes with substituted 6-ethynylazulene and/or

n-ethynylated arenes with substituted 6-bromoazulene under Sonogashira-Hagihara conditions.156 CV studies of these these novel poly(6-azulenylethynyl)benzene derivatives revealed the presumed multielectron redox properties For instance,

compound hexakis(6-azulenylethynyl)benzene HAETB showed a wide temperature

range of columnar mesophases (Colho and Colro) from 77.3 °C to the decomposition

temperature at ca 270 °C These systems could be utilized to construct advanced

materials for electro-chromic application with liquid crystalline behavior.157

Scheme 1-2

Synthesis and photophysical/photochemical investigations of

bis(2,5-dimethy-3-thienyl)-azulene-1,1-dicarbonitrile (DBThAC) and diphenylazulene-1,1-di-carbonitrile (DDPhAC) also have been reported The photoprocesses and thermal reactions of systems DDPhAC and DBThAC were studied

1,8a-dihydro-2,3-by time-resolved and steady-state techniques under various conditions The

dihydroazulene(DHA)-dithienylethene(DTE) conjugate DBThAC is photochemically

converted into the dihydrothienobenzothio-phene (DHB) isomer and the

vinyl-heptafulvene (VHF) isomer System DDPhAC exhibits exclusively DHA/VHF

photo-chromism For both systems the VHF form thermally reverts back into the DHA form Their rate constant increases with the solvent polarity The photostationary state of the

DBThAC-A to DBThAC-B and DBThAC-A to DBThAC-C photoisomerisation is

sensitive to the irradiation wavelength.160

NC NC

h

∆

NC CN

NC NC

S S

S

S

S S

hυ1 hυ2

it difficult to characterize and process The difficulty in modification of the azulene ring

may be another reason for the lack of interest in polyazulenes Nevertheless, polyazulenes

as likewise azulenes are expected to have interesting electrical and optical properties in materials science.161

Polyazulene is one of the conducting polymers consisting of condensed aromatic rings and can be prepared from azulene monomer by the electrochemical polymerization162,163

or by the chemical polymerization.164 To the best of our knowledge, only one example of polyazulenes was prepared by chemical oxidative polymerization;164 most of the polyazulene were prepared by electrochemical polymerization

Many attempts have been reported to synthesized polyazulene by electropolymerization The general procedure for the electrochemical polymerization of azulenes are as follows: electrochemical polymerization of 10-3 M of azulene in acetonitrile containing 0.1 M of

an approprite electrolyte yields thick, amorphous and electrically conducting films.163 These films can be peeled off from the platinum anode to provide free-standing flexible films with electrical conductivities of 10-2 – 1 S/cm The n value for the film-forming reaction is 2.3, indicating that the polymerization of azulene involves two electrons per monomer The excess charge of 0.3 required for the partial oxidation of the film is balanced by the uptake of counteranions from the electrolyte The polyazulene, like polypyrrole, shows growth of of polyazulene film on Pt electrode in acetonitrile solution

by a.c impedence technique has been studied.165 Analysis of these films is consistent with that of polymer containing bis-coupled azulene units The film contains the anions

of the electrolyte at typical concentration of 1 per 4 units of azulene This situation closely resembles that of polypyrrole

Polyazulene was also prepared by chemical polymerization One attempt is the cationic polymerization in which heating azulene in trifluoroacetic acid (TFA) to form conducting

polyazulene possessing a conductivity of 8.16 × 10-6 S/cm.166 However, NMR analysis showed the obtained polymer was actually 1,2-polyazulene

To have a better understanding of the mechanism of the polymerization, CV studies have been used to illustrate the polymerization process CV data for azulene and some 1-substituted azulene showed that, all of the monomers electrooxidize irreversibly, except for 1,3-di-tert-butylazulene, which exhibits a reversible redox reaction Among these monomers, electropolymerization is observed only for azulene and 4,6,8-trimethylazulene As expected, 1-substituted azulenes do not electropolymerize, but instead form soluble products The reaction of 1-substituted azulenes are characterized by

n value of ≈ 1, indicating the formation of dimeric products.167 The formation of these dimmers can also be observed via cyclic voltammetry utilizing rapid sweep rates For example, successive sweep in the cyclic voltammogram of momeric guaiazulene exhibits

a two-wave reversible redox couple which is in good agreement with the cyclic voltammogram of authentic 3,3’-biguaiazulene These data suggest that the equivalent 1- and 3- position of azulene are important for electropolymerization If the 1-position of azulene is blocked with a substituent, only dimerization can occur upon electro-oxidation This finding is consistent with the electronic structure of the azulene radical cation,168which displays that the position of highest unpaired electron density occur at the equivalent 1- and 3- positions These data identify them as the reactive positions, and also suggest that, in the polymerization of azulene monomer, the monomer linkage in the polymer do occur in 1- and 3-positions Thus the possible of electropolymerization pathway for azulenes can be drawn as below:169

Epa -e -

between polymers structure and properties, synthesis and investigation of corresponding monomers and oligomers are also carried out in this projects

In summary, there are two reasons for studying of the monomers and oligomers:

First, in comparison with polymers, oligomers are well-defined systems of monodisperse (uniform-length) molecules, with greatly reduced occurrence of defects They therefore offer the possibility of better ordering of the molecules and consequently more well-defined electronic and optical properties.173 Thus oligomers can be sued as model systems for theoretical and experimental investigations aimed at extrapolating physical properties of finite oligomers to the corresponding ideal polymer of infinite length This information is not accessible from investigation on the polymeric systems.174 In this project, we studied model compounds with the purpose to get the following useful information:

dependence of the energies and equilibria of neutral and charged excitations as a function of the conjugation length

the crystallography of the monomers and oligomers for elucidating the folding properties of the polymers.175

the charge transfer (CT) properties between the oligomers and electron donor or electron acceptor also provides the model to elucidate the n-doping or p-doping mechanism of the corresponding polymers.176

Secondly, in some cases, oligomers have already been shown to exhibit characteristics superior to those conjugated polymers For this reason, monomers and oligomers have been recently advanced as component for molecular electronics177, 178 and optical

devices179 themselves A variety of spectacular molecular architecture has been designed aimed at the construction of such materials For example, all-α-linked sexithiophene has been successfully employed as an active component in an organic field-effect transistor (EFT)177,178 and in a light-modulating device.180

5.1 Monomers and Oligomers: model compound for understanding of polymer properties

5.1.1 Structure/property relationships

The knowledge of the relationship between the structure and properties of polymers is of great practical and theoretical important However, a general limitation for the structure and properties relationship in π-conjugated polymers is the insolubility of the extended π-electron conjugated polymers Thus, soluble, monodisperse oligomers as finite model systems offer the possibility to attain specific information concerning the electronic, photonic properties of the corresponding polymers.181-183

Poly- and oligo(thiophene)s may belong to one of the most carefully studied types of conjugated materials and have received a great deal of attention for both fundamental and practical reasons.184 Many research work on the monomer and oligomers of thiophene revealed that oligothiophenes were the ideal model compounds for the corresponding polythiophenes Unsubstituted oligothiophenes and substituted derivatives represent the most interesting derivatives

π-The physical properties of the various un-substituted oligothiophenes are summarized in Table 1-2 As expected, with increasing chain length of the oligothiophene, the melting point increases Similiarly, with respect to the electronic properties, the longest wavelength absorption,174 emission,185 as well as the oxidation potentials186 gradually

shift to lower energies with increasing number of thiophene units This reflects the increasing conjugation which is also observed by the colors of the homologous row The color deepens from colorless for bithiophene, to pale and chrome yellow, to orange, to red, and finally to wine red for septithiophene and octithiophene This type of correlation

is also predicted theoretically.187

Table 1-2 Physical properties of unsubstituted oligothiophenes

Oligothiophene

(nT)

Absorption λmax(nm)

Fluorescence λmax(nm)

Oxidation otentials (V)

Figure 1-10 Calculated (frontier) energy levels of oligothiophenes with n= 1- 4 and of

In analogy to polythiophenes, solubility of oligothiophenes decreases dramatically with

increasing chain length This is due to the stiffness of the conjugated π-system and the

strong interactions between the chains Generally, the most often used approach to

overcome the low solubility of oligothiophene is the synthesis of corresponding

oligothiophene which bear alkyl group in β-positions For example, the solubility of

dialkylsexithiophene is higher than 400 g/L whereas that of H-T 6 -H 6 is lower than 0.05

g/L.193 Also, the properties of substituted oligo(thiophene) depend critically on the

effective conjugation length (ECL), and the properties of substituted oligo(thiophene) are

effected by structure and conformation because the adjacent alkyl substituents give rise to

steric hindrance and, hence, to a non-planar conformation UV, CV and other techniques

showed clearly the substituents effects as shown in Table 1-3

Table 1-3 Physical properties of β-substituted oligothiophenes

Recently, Bäuerle et al199 used the regioregular head-to-tail coupled

quarter(3-arylthiophene) (Q3ATH) as the model compound for the parent poly(3-quarter(3-arylthiophene)s

and poly(3-alkylthiophene)s.200 Oligothiophene Q3ATH is substituted at the 3-position

of each thiophene subunit with phenyl groups that are in turn substituted at their para

positions with four diversity elements of different electronic nature (R = CF3, H, CH3, OCH3) The phenyl spacer should increase the solubility of the oligomers and ensure the electronic communication between the substituents and the oligomers backbone These

four backbone substituents should influence the electronic structure of Q3ATH without greatly changing the overall geometry of the molecule Thus Q3ATH permits a

systematic investigation of the substituent influence on the energy levels of the molecular orbitals and development of structure-property relationships

S S S S

Oligomers backbone

Variation of the diversity elements:

CF , CH , OCH 3 3 3

Q3ATH

5.1.2 The doping mechanism revealed from the oligomers approach

Oligo(thiophene)s are considered to be ideal model compounds to investigate the polaron and bipolaron formation and charge carries during the charging process.201-204 A series of oligo(a-thiophene)s bearing Me groups in the β positions of some of the monomeric repeat units and extending to the stage of an octamer was reported by Tour and Wu.205From controlled oxidation of these compounds with FeCl3 and subsequent spectroscopic studies, they estimated the delocalization length in PTs for the radical cation (polaron) to

be around n = 12 monomer units and for the dication (bipolaron) around n =10 monomer

units ESR investigation of radical cations and anions of H-T 2 -H2 to H-T 5 -H5 reveal that

the charged oligomers exist not exclusively in an all-trans conformation of the thiophene rings.206 Absorption spectra of oligothiophene radical cations were also obtained by oxidation of the corresponding oligothiophenes in the channels of zeolites where the

subsequent reaction of the reactive cationic species is suppressed by the local environment.207

Similar to the polythiophenes, oligothiophene materials can also be doped by using oxidating agents like iodine, nitrosyl salts NO+X-, electron-acceptors like TCNQ, or using electrochemical oxidation.208 Hotta et al.209 reported the preparation of oligothiophene cations radical consisting of α,α’-dimethylquaterthiophene and five kinds

of anionic species The donor-anion molar ratios were approximately unity for all the salts All of the salts prepared were semiconductive with the electrical conductivities in the range of 10-4 – 10-6 S/cm Electrical conductivity and spectroscopy measurements on the cations radicals salts have suggested that oligothiophene moiety in the present salts is highly oxidized as in oligothiophene doped with nitrosyl salt

Since the oligothiphene are low molecular weight materials, they readily form transfer (CT) complex crystal with an appropriate acceptor such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4).210 For example, a CT complex between dimethylquaterthiophene (DMQtT) and TCNQF4 was prepared by pouring an acetonitrile solution of TNQF4 gently upon the chloroform solution of DMQtT After aging the mixture overnight at r.t., deep-violet needle-shaped crystal were obtained.210 Also, when terthiophene, quaterthiophene, etc were mixed with a stronger acceptor such as NOBF4 and NOPF6 in solution, the solution immediately darkened and soon yielded black precipitates.211 FT-IR spectrum of these materials indicates that they are identical to polythiophene doped with BF4- or PF6- ions Generally, CT complex often obtained as needle-shaped form and the conductivity is often measured along the needle direction The two-probe conductivities (at r t.) of these complex crystals are relatively low, at