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

Chapter 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. 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 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, processability, 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 semiconductors 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 electronics.31 The semiconductor properties of the conducting polymers have also been 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. a X X X 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 redshift 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 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. S S S S S S S n PT - -1e Ox - Ox S S S S S S S n "polaron" -1e- Ox - - S S -1e - Ox S S Ox S S Ox S n "bipolaron" S S S S S S S n "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. 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. OMe MeO R S S Me R S OT3a,b R = CH3, CH3S Me S S Me OT3OMe MeO S OMe S S S S OMe MeO S Me 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 OH2H2CCS S S S S O SCH2CH2O C 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- 10 observed from EPR spectrum. When a solution of protonated polymer was extracted with water and methanol to remove the TFA, the EPR signal was no longer observed and the neutral polymer was recovered intact on the basis of NMR and UV-vis spectroscopic analysis. 10G (a) (b) Figure 3-13. EPR spectra of Polymer Polyf (a) in chloroform solution with 5% TFA, (b) exposed to TFA vapor at solid state. EPR is also an ideal tool to investigate the paramagnetic properties of the conducting polymers. In this study, EPR spectra were used to measure the stability of the paramagnetic polycations via TFA protonation of polyazulene and the reversible paramagnetic conversion upon oxygen and nitrogen diffusion to the amorphous polymer film. In a series of EPR studies at room temperature, it was found that in general a chloroform solution of polymers in the presence of TFA showed only about 20% decrease in the population of cation radicals after one week. In the solid state, a protonated polymer film showed no significant change in the EPR signal after weeks. Evidently the azulenium cation radicals formed were stabilized by the extended conjugation in the polymer backbone. And when the TFA-protonated copolymer film was exposed to oxygen flow, the intensity of its EPR signal decreased fast to half value of its original one, with the broadness of the peak-to-peak linewidth (∆Hpp) change from 2.7 G 103 to 3.6 G in about 15 mins. Removal of oxygen by passing the nitrogen gas flow through the TFA protonated film seems to be a reversible process. When the nitrogen gas pass through the protonated film, the EPR signals increase quickly to its high value in about mins. The normalized peak-to-peak amplitude of EPR signals and the peak-to-peak linewidth (∆Hpp) change with oxygen and nitrogen diffusion in the first two cycles were shown in Figure 3-14. It is interesting to note that no matter how many time of the copolymers were exposed to oxygen, the EPR signals were still very strong and very easy recovered to their original intensity when nitrogen pass through. And this process is a reversible one and can be repeat for many cycles. The effect of oxygen on the EPR signal of the protonated copolymers are explained as followings: when the paramagnetic molecules of dioxygen attach to polymer, the exchange interaction between spins of immobile dioxygen molecules and mobile polarons during their collisions results in the Ln of peak-to-peak Amplitude broadening of EPR line of the copolymers.45 10.4 10.2 10.0 9.8 20 40 Tim e, m in 60 80 100 (a) 104 Linewidth, G 3.5 3.0 2.5 20 40 60 80 100 Time, (b) Figure 3-14. The normalized (a), peak-to-peak amplitude and (b), linewidth (∆Hpp) change with oxygen and nitrogen diffusion. XRD analysis. An attempt was made to estimate the crystallinity of polymers by means of X-ray diffractograms. Figure 3-15 shows the wide-angle X-ray diffractions patterns of these synthesized copolymers. Generally, the copolymers with small side chain (Polyb and Polyc) or without side substitution (Polya) displayed semicrystalline nature, while copolymers with long side chain are amorphous and have no side chain alignment. Polyac showed a strong Bragg peak with d spacing at ca. 4.2 Å (2θ = 210) which corresponding to the stacking distance of the aromatic rings between two polymer chains. The larger stacking distance compared to those copolanar polythiophenes (3.2-3.8 Å) was due to the large torsion angle between the azulene ring and the thiophene rings.46,47 It seems that Polyd-g showed nearly identical patterns. They all displayed a strong reflection hump approximately between 2θ = 15 and 210, indicating a higher packing density.48 Not like the reported layered structure for most poly(3-alkylthiophene)s system,49,50 no side chain alignment were found at low angles. This may be due to the large torsion angle which 105 force the resulting polymers to a helical structure, and thus makes the side chain alignment more difficult and leads to the highly amorphous structure. Relative Intensity Poly 20 40 60 Polyf Polye Poly d θ (deg) Figure 3-15. Powder X-ray diffraction pattern of copolymers. Morphology of the neutral and doped copolymers The morphologies of neutral and doped copolymers were investigated by SEM studies as shown in Figure 3-16. Polyf was used as example and the morphologies of both iodine doped (Figure 3-16c,d) and TFA protonated (Figure 3-16b) species are significantly different from that of neutral copolymer. The surface structure of neutral Polyf film has a relatively smooth morphology (Figure 3-16a). Figure 3-16b shows that protonation with TFA results in the formation lumps and channels in the amorphous polymer matrix as observed for polyaniline doped with mineral acids.51 It is in these channels that dopants ions are accommodated between the polymeric chains, resulting in reasonably good conductivity of the samples.52 The great variation in morphology during the iodine doping is the homogenous spots formation on the copolymers surface. A finnier structure is obtained at a magnification of 100,000 as shown in Figure 3-20d, which shows an average size of ca. 20 nm with a concentration of 2.4×1010/cm2 on the copolymer surface. 106 These observed nano-sized globular spheres are believed to be the doping center which are originated from the iodine doping induced aggregation of polymers chains. Upon doping with iodine, the static electrical interactions between the doped copopolymer chains and their conterions brought the neighboring micelle cores together to form a phase separated globular spheric morphology.53,54 (a) (b) (c) (d) Figure 3-16. SEM photograph of copolymer Polyf: (a), neutral state; (b), iodine doped; (c), TFA protonated. Electrochemical Analysis. Conducting polymers can be reversibly switched between anodically conducting (pdoped) state and insulating (neutral) state has enormous potential application, such as in 107 batteries, in sensors, molecular electronic devices, and etc. The cyclic voltammetry (CV) of all the obtained copolymers showed similar CV behavior. The example of the copolymer film of Polyf on Pt foil in 0.1M tetrabutylammonium hexylfluorophosphate (n-Bu4NPF6) solution is shown in Figure 3-17(i). A distinct color change from yellowgreen to dark-green was observed upon oxidative sweeping from to +1.2 V (vs SCE), indicating a quasireversible p-doping (or hole injection) process that is probably related to the formation of a radical cation.55 The anodic oxidation peak appeared at ca. 0.80V vs SCE with the onset at ca. 0.65V, and the reversible reduction peak appears at ca. 0.60V. The reduction sweep (from to –3.2) showed an irreversible n-doing (or electron injection) with color change to red begin at –1.80V. The electrochemically determined HOMO-LUMO gap of 2.45 eV is in good agreement with that measured from the emission spectrum of the film. Figure 3-17(i) also shows the CV behavior change of thin film of Polyf for its first scan and the subsequent scans (from 2nd to 6th). An obvious decrease of oxidation potential and oxidation onset was observed for subsequent scans (from 2nd to 6th). In the first scan of copolymer Polyf, the oxidation peak appeared at +1.0 V with the onset at +0.80 V. In the second scan, the oxidation potential of Polyf appeared at +0.86V with the onset at +0.65 V, the potential decrease up to 0.15 V. This behavior may be induced by the socalled conducting relaxation, which is correlated with the inhomogeneous structure of the polymers film in the first scan.56 Side chain effect was also observed in the cyclic voltammetry measurement. Figure 3-17 (ii) and 3-17(iii) show the repeat scans (from 2nd to 10th) for Polyd and Polyg, respectively. For Polyd, we found there is a little changing after tenth scans except a slight decrease of the anodic current and the potential. But for Polyg, that bearing a long side chain, its oxidation peaks separate into two from the 108 second scan. The first oxidation peak occurred at ca. +0.81V and the second at +0.90 V at scan rate of 20 mV/s for Polyg thin film. If we used thick polymer film or increase the CV scan rate up to 80 mV/s, such change cannot detected in the repeat scans. Based on these observations, we conclude that the oxidation peaks of copolymers bearing long side chain separated in their repeat scans. This is may be due to the longer side chain produces complex modification in the morphology of the polymer during the doping and undoping process.57 2nd-6th scans (ii) 1st scan 2nd scan 10th scan Cathodic Anodic Cathodic Anodic 0.1 mA 0.1 mA (i) 0.4 0.4 0.6 0.8 1.0 0.8 0.6 Potential / V vs. SCE 1.0 Potential / V vs. SCE (iii) 10th scan 0.1 mA 1st scan Cathodic Anodic Anodic Cathodic 0.4 10th scan (iv) 0.1 mA 2nd scan 0.6 0.8 Potential / V vs. SCE 1.0 1.0 0.5 1.5 Potential / V vs. Ag/AgCl 2.0 Figure 3-17. Cyclic voltammogram of (i), Polyf (R = C10H21); (ii), Polyd (R= C4H9); (iii), Polyg (R= C14H29) in CH3CN solution containing 0.1M tetrabutylammonium hexylfluorophosphate (nBu4NPF6) on platinum electrode, and (iv), Polyf film in aqueous HCl solution. 109 To our surprise, the polymers also showed electrical activity in aqueous acid solution as shown in Figure 3-17(iv). In M HCl solution, the copolymers showed anodic oxidation at 1.58 V (vs. Ag/Ag+) with its corresponding cathodic peak at ca. 0.96 V (vs. Ag/Ag+). As the cycling is continued, increase of the current was observed which indicates increase of electroactivity of the copolymers film with further protonation by the acid. The band gap for the copolymer (Polyf) in M HCl solution was found to be about 1.42eV which is in agreement with the result (1.42eV) calculated from the on-set (865nm) of the copolymer protonated with TFA in chloroform. But in TFA aqueous solution, the obtained copolymers film showed only the oxidation peak at ca. 1.0 V (vs. Ag/AgCl) with an onset at ca. 0.72 V (vs. Ag/AgCl). Electrochemical Impedance Spectroscopy study Electrochemical Impedance Spectroscopy (EIS) is a powerful technique for the characterization of electrochemical systems. In recent years, EIS has found widespread application in the field of characterization of materials. It is routinely used in the characterization of coatings, batteries, fuel cells, and corrosion phenomena. It has also been used extensively as a tool for investigating mechanisms in electrodeposition, electrodissolution, passivity, and corrosion studies. It is gaining popularity in the investigation of diffusion of ions across membranes and in the study of semiconductor interfaces. The fundamental approach of all impedance methods is to apply a small amplitude sinusoidal excitation signal to the system under investigation and measure the response (current or voltage or another signal of interest). When the a.c. signal is first impressed, a time-dependent diffusion layer is created. 110 Cdl Rsol Rct EIS is also a good technique to evaluate the electrical conductivities of conducting polymers. Film of copolymer Polyf was prepared by slow evaporation of its toluene solution and the thickness of the obtained films are ca. 0.3mm. Figure 3-18 shows (i) the Nyquist plot for impedance of the copolymers film in HCl/KCl solution and (ii) the equivalent circuit to interpret the impedance diagram. These impedance plots reveal that the reactions are kinetically controlled in the experimental range between 500 kHz and 100 mHz.58,59 At high frequencies, the impedance is determined by the solution resistance Rsol. At very low frequencies, the cell impedance is equal to Rsol + Rct, where Rct is the charge transfer resistance. At intermediate frequencies, the cell impedance is influenced by the value of the double layer capacitance Cdl. The charge transfer resistance can be obtained by the extrapolation of this semicircle on the real impedance axis and the charge transfer conductivity of the film (σct) can be calculated from σct = d/RctA. The copolymers film gives conductivities in the range of 1×10-11 to 10-12 S/cm in 0.1M KCl solution. But when 0.1M HCl was added, the charge transfer resistance decreased greatly and the conductivities increase to 1×10-7 to 10-8 S/cm as shown in Figure 3-18(i). EIS also reveals the anti-corrosion property of the obtained copolymer films. The polymers film showed high charge transfer resistance (500-800MΩ) and low capacity (1.6 × 10-11 to 3.6 × 10-11 F) in the 0.1 M KCl water solution. Combined with the easy processing and high stability of these copolymers, they may be super material than the poor soluble polyaniline (PAn) for the application in the corrosive environment.60 111 Cdl Rsol 200M -Z'' (ohm) Rct 100M (b) a b c d 100M 200M 300M 400M Z' (ohm) (a) Figure 3-22. (i) the Nyquist plot for impedance of the copolymers film in HCl/KCl solution: (a) t = min, (b), t = 10 min, (c), t = hour, (d), t = 12 hours; and (ii) the equivalent circuit to interpret the impedance diagram. Electrical Conductivity. The dc conductivities of polymers Polya-g upon doping with iodine or protonated with TFA were shown in Table 3-1. These copolymers showed high conductivities compared with most of the poly(thiophene-co-arylene)s systems which are in the range of 10-7 to 10-2 S/cm.61-63 We attribute the high conductivities to the specific electronic properties of azulene that can stabilize the polaron or bipolaron formed during the doping process. Further study of the relationship of conductivities with iodine uptake, we found high conductivities were achieved when 30-50% iodine was absorbed by the copolymers. As expected, the conductivities increased linearly with the increase of iodine uptaking and reached a high conductivity at 2.23 S/cm with 45% iodine uptake. But the conductivities decrease greatly when more iodine absorption. High conductivities can also be obtained by protonation with TFA. As shown in Table 3-1, when the neutral copolymer pellets were exposed to TFA vapor in a small chamber for about two days, they generally 112 exhibited conductivities ~ S/cm. And the protonation degree was determined by the integration value of fluoride (F) in TFA using XPS spectroscopy. The results showed that in most case of the protonated copolymers, F continents in the range of 9-15% which indicates the up taking of TFA is in the range of 18-30% for the protonated copolymers. It was established that the copolymers Polya-g behave like insulator-conductor system via a protonation-deprotonation process. Their conductivities in fact were observed to be lower than 10-11 S cm-1 when the TFA was removed from the protonated polymer pellets after washing with water and methanol. Conclusions In conclusion, a series of novel polythiophene (PTs) based conjugated copolymers containing intact azulene units in the main chain were synthesized and their electrical properties upon iodine doping and TFA protonation were studied by using electronic spectroscopy, electrochemistry and surface chemistry techniques. The obtained conducting copolymers display reversible optical and electronic properties based on protonation-deprotonation (P-DP) process. The nature of the reversible P-DP process was monitored by UV/Vis absorption spectroscopy, electron paramagnetic resonance (EPR), and conductivities. The stability of doped and protonated state of the obtained copolymers was evidenced and studied by EPR. Scanning electron microscope (SEM), EPR and conductivities measurement revealed the different conductivity mechanism between iodine doping and TFA protonation process, and the high conductivities are believed to be due to the formation of nano-scale doping center in the iodine doping and formation of conducting channel upon TFA protonation. In addition to the high conductivity and reversible P-DP process, the obtained copolymers also show the 113 corrosion protection effect and can be protonated by HCl aqueous solution with 103 to 104 magnitude conductivities increasing measured by EIS. The stimuli responsibility of the present copolymers combined with their high thermal and environmental stability would allow them to be applied to molecular sensors, switches, corrosion protection coating, and etc. 114 References Nalwa, H. S. Handbook of Organic Conductive Molecules and Polymers, Vol. 4, John Wiley, Chichester 1997. Nomura, R.; Yamada, K.; Masuda, T. Chem. Commun. 2002, 478. Goto, H.; Okamoto, Y.; Yashima, E. Macromolecules 2002, 35, 4590. Sotzing, G. A.; Lee, K. Macromolecules 2002, 35, 7281. Swager, T. M. Acc. Chem. Res. 1998, 31, 201. Salhi, F.; Collard, D. M. Adv. Mater. 2003, 15, 81. Liu, B.; Yu, W-L.; Pei, J.; Liu, S-Y.; Lai, Y-H.; Huang, W. Macromolecules 2001, 34, 7932. Marsella, M. J.; Wang, Z-Q.; Mitchell, R. H. Organic Lett. 2000, 2, 2979. Morisaki, Y.; Ishida, T.; Chujo, Y.; Macromolecules 2002, 35, 7872. 10 Reynolds, J. R.; Ruiz, J. P.; Child, A. D.; Nayak, K., Marynick, D. S. Macromolecules 1991, 24, 678. 11 Redl, F. X.; Köthe, O.; Röckl, K.; Bauer, W.; Daub, J. Macromol. Chem. Phys.2000, 210, 2091. 12 Hendron, W. C. J. Phys. Chem. 1981, 85, 3040. 13 Tobler, H. J.; Bauder, A.; Gunthard, H. H. J. Mol. Spectrosc. 1965, 18, 239. 14 Neoh, K. G.; Kang, E. T.; Tang, T. C. Polym. Bull. 1988, 19, 32. 15 Tourillon, G.; Garnier, F., J. Electroanal. Chem. 1982, 135, 173. 16 Kihara, N.; Nakayama, H.; Fukutomi, T. Macromolecules 1997, 30, 6385. 17 Kumada, M.; Tamao, K.; Sumitani, K. Org. Synth. 1978, 58, 127. 18 Gallazzi, M. C.; Castellani, L.; Marin, R. A.; Zerbi, G. J. Polym. Sci: Part A: Polym. Chem. 1993, 3339. 115 19 Sarker, H.; Ong, I.; Sarker, S.; Searson, P. C.; Poehler, T. O. Synth. Met. 2000, 108, 33. 20 Ng. S. C.; Xu, J. M.; Chan, H. S. O. Macromolecules 2000, 33, 7349. 21 Leclerc, M.; Daoust, G. J. Chem. Soc. Chem. Commun. 1990, 273. 22 Cunningham, D.D.; Laguren-Davidson, L.; Mark, H. B.; Vanpham, Ch.; Zimmer, H. J. Chem. Soc. Chem. Commun. 1987, 1021. 23 Ito, S.; Nomura, A.; Morita, N.; Kabuto, C.; Kobayashi, H.; Maejima, S.; Fujimori, K.; Yasunami, M. J. Org. Chem. 2002, 67, 7295. 24 Farrell, T.; Meyer-Friedrichsen, T.; Malessa, M.; etc. J. Chem. Soc. Dolton Trans. 2001, 29. 25 Giménez, E.; Carilla, J.; Fajarí, L.; Alemán, C.; Juliá, L. J. Org. Chem. 1994, 59, 4630. 26 Inganas, O.; Salanek, W. R.; Osterholm, J. E.; Laakso, J. Synth. Met. 1988, 22, 395. 27 Li, J. Q.; Aoki, K. J. Electroanal. Chem. 1998, 453, 107. 28 Taka, T.; Jylhä, O.; Root, A.; Silvasti, E.; Österholm, H. Synth. Met. 1993, 55-57, 414. 29 Malik, S.; Jana, T.; Nandi, A. K. Macromolecules 2001, 34, 275. 30 McCullough, R. D.; Lowe, R. D. J. Chem. Soc. Chem. Commun. 1992, 70. 31 Wang, W.; Benz, M. E.; LeGoff, E.; Schindler, J. L.; Allbritton-Thomas, J.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1994, 6, 401. 32 Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Macromolecules 1987, 20, 212. 33 Kim, Y. H.; Hotta, S.; Heeger, A. J. Phys. Rev. 1987, B30, 7486. 34 Li, J. Q.; Aoki, K. J. Electroanal. Chem. 1998, 453, 107. 35 Taka, T.; Jylhä, O.; Root, A.; Silvasti, E.; Österholm, H. Synth. Met. 1993, 55-57, 414 36Themans, B.; Salaneck, W. R.; Bredas, J. L. Synth. Met. 1989, 28, C359. 116 37 Yamamoto, T. in “Handbook of Organic Conductive Molecules and Polymers”, ed by H. S. Nalwa, John Wiey, Chichester, 1997, Vol. 2, p. 171. 38 Chen, S-L.; Klein, R.; Hafner, K., Eur. J. Org. Chem. 1998, 423. 39 Farrell, T.; Manning, A. R.; Mitchell, G.; Heck, J.; Meyer-Friedrichsen, T.; Malessa, M.; Wittenburg, C.; Prosenc, M. H.; Cunningham, D.; McArdle, P., Eur. J. Inorg. Chem. 2002, 1677. 40 Xia, Y.; Wiessinger, J. M.; MacDiarmid, A. G. Chem. Mater. 1995, 7, 443. 41 Wang, F.; Lai, Y-H, Han MY, Macromolecules 2003, accepted. 42 Bredas, J. L.; Wudl, F.; Heeger, A. J. Solid State Commun. 1986, 58, 415. 43 Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. Rev. 1988, 88, 183. 44 Chen, X.; Inganäs, O. J. Phys. Chem. 1996, 100, 15202. 45 Houze, E.; Nechtschein, M. Phys. Rev. B 1996, 53, 14309. 46 McCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe, R. D.; Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910. 44 Park, K. C.; Levon, K. Macromolecules 1997, 30, 3175. 48 Hsiao, S. H.; Yang, C. P.; Chu, K. Y. Macromolecules 1997, 30, 165. 49 Chen, T. A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233. 50 Tashiro, K.; Kobayshi, M.; Kawai, T.; Yoshino, K. Polymer 1997, 38, 2867. 51 Singh, R.; Arora, V.; Tandon, R. P.; Chandra, S.; Kumar, N.; Mansingh, A. Polymer 1997, 38, 4897. 52 Mathur, R.; Sharma, D. R.; Vadera, S. R.; Kumar, N. Acta. Mater. 2001, 49, 181. 53 Paschen, S.; Carrard, M.; Senior, B.; Chao, F.; Costa, M.; Zuppiroli, L. Acta Polymerica. 1996, 47, 511. 54 Dai, L.; White, J. W. Polymer 1997, 38, 775. 117 55 Helbig, M.; Hörhold, H-H. Makromol. Chem. 1993, 194, 1607. 56 Andrieux, C. P.; Audebert, P.; Hapiot, P.; Nechtschein, M.; Odin, C. J. Electroanal. Chem. Interfacial Electrochem. 1991, 305, 153. 57 Skompska, M.; Szkurlat, A. Electrochimica Acta 2001, 46, 4007. 58 Osaka, T.; Momma, T.; Nishimura, K.; Kakuda, S.; Ishii, T. J. Electrochem. Soc. 1994, 141, 1994. 59 Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robinson, J. Instrumental Methods in Electrochemistry, Ellis Horwood, Singapore, 1990. 60 Wessling, B. Adv. Mater. 1994, 6, 226 61 Pelter, A.; Maud, J. M.; Jankins, I.; Sadeka, C.; Coles, G. Tetrahedron Lett. 1989, 30, 3461. 62 Ruiz, J. P.; Dharia, J. R.; Reynolds, J. R.; Buckley, L. J. Macromolecules 1992, 25, 849. 63 Yamamoto, T.; Morita, A.; Miyazaki, Y.; Maruyama, T.; Waknyama, H.; Zhou, Z.; Nakamura, Y.; Kanbar, T.; Sasaki, S.; Kubota, K. Macromolecules 1992, 25, 1214. 118 [...]... donor moieties containing alkyl groups for solubility The polymers PDA-1 – PDA4 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,... catalysed coupling of 5,5’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, 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 N N n N Ru(bpy )2 N m PBPy 23 Subsequently, a series of conjugated polymers containing Re(CO)3Cl... Energy of excited states 1B2 2A1 2B2 3A1 4.5 eV 2. 1 eV 3.6 eV 4.3 eV 590 nm 344 nm 28 8 nm 27 6 nm 14500 cm-1 28 000 cm-1 33500 cm-1 35000 cm-1 Fluorescence S2 to S0 S1 to S0 385 nm φF ≈ 0.03 φF ≈ 10-6 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,. .. modification of the azulene ring 31 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 polymerization1 62, 163... shift of the UV/vis spectra of the metal complex was attributed to charge transfer from the benzenoid of polyaniline to the quinoid 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. .. 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 28 Table 1-1 The optoelectronic properties of azulene. 154 Azulene ∆H0f298 50 kcal/mol ERed1 /2 -20 40 mV vs Fc+/Fc 9acetonitrile) EOx1 /2 670 mV vs Fc+/Fc (acetonitrile) 530 mV vs Fc+/Fc (CH2Cl2) Ground state... 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. .. 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 32 polyazulene possessing a conductivity of 8.16 × 10-6 S/cm.166 However, NMR analysis showed the obtained polymer was actually 1 ,2 -polyazulene. .. 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... 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 . 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,. (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. 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