Abstract
Low band gap polymer is one of the important conjugated conducting polymers that possesses moderate intrinsic conductivity at neutral state and therefore is environmentally stable. It can become transparent upon doping due to a shift of the absorption maximum from the UV-vis region to the near infrared region. Among the various types of low band gap polymers that have been studied, poly(isothianaphthene) (PITN) is of particular interest because it is one of the lowest energy gap polymers known so far (Eg @ 1.0 eV). Its film prepared electrochemically exhibits a high contrast and reversible color change from blue at the neutral state to nearly transparent light greenish-yellow at the doped state. However, practical applications of this polymer have been greatly hindered by lack of solubility in common organic solvents. Its film can only be obtained by deposition on a substrate. Thus, to improve its mechanical property and solubility yet retain high conductivity is an important problem. This research includes synthesis of soluble PITN derivative (poly(5,6-dihexoxyisothianaphthene), PDHOITN) and PITN with ferromagnetic property, and identification of structure and properties by use of various spectroscopies, thermal analyses, and conductivity measurements. The difference of structure and physical properties for PITN obtained by various prepared methods is also explored. A combination of spectroscopic analyses and molecular simulation explores the phenomena of PITN dissolved into solvent, crystalline structure, and transition of electronic and geometric structure of PITN adsorbed on HOPG surface. Moreover, we employ ESR and SQUID to analyze magnetic behavior of PITN and PDHOITN in detail, and MFM to observe the magnetic domain of the PITN thin film surface.The PITN as microgel solution can be prepared via precursor route using SO2Cl2 as dehydrogenation agent to remove the hydrogen from the main chains of its precursor polymer, poly(1,3-dihydroisothianaphthene) (PDHITN). The distribution of PITN obtained from dehydrogenation of PDHITN is narrower. The lmax of PITN as microgel solution in the UV-vis-near-IR spectra is about 858 nm, which can be cast into film with a conductivity of 0.1 S/cm (measured using the four-probe method). Furthermore, the conductivity of PITN remains unchanged at 125℃ for 100 hours in nitrogen atmosphere. At the heating process, the PITN is doped but this reaction is irreversible. The PITN brings about crosslinking after 175 ℃, which causes short in conjugated length. The heavy thermal weight loss is started after 300 ℃. The results of CV and UV-vis-near-IR spectra indicate the band gap, ionization potential, and electron affinity of PITN is about 1.0 eV, 4.4 eV, and 3.4 eV, respectively.Dissolve 1,3-dihydroisothianaphthene (DHITN) into dry chloroform, and then add proper equivalent of ferric chloride in dry chloroform as oxidant dropwisely in order to proceed the oxidative polymerization. After proper reaction time, the PITN as microgel solution is obtained. The distribution of PITN obtained from direct polymerization of DHITN is wider. The lmax of PITN as microgel solution in the UV-vis-near-IR spectra is about 770 nm, which can be cast into film with a conductivity of 10-2 S/cm. Its conductivity retains constant value for 72 hours at 125 ℃. The PITN obtained from this method is more stability without any crosslinking and doping at heating process, which of maximum weight loss appears after 450 ℃.PITN solid thin film exhibits reversible electrochromic properties, changing from deep blue at the neutral state to nearly transparent light yellow at the doped state. There shows two redox peaks on the CV curve, the first oxidative peak is assigned to grow polarons due to loss of electrons at the neutral state; the second is assigned to grow bipolarons due to loss of electrons at the polarons. The appearance of the second redox peaks causes the damage of PITN. The surface analysis of PITN thin film indicates microgel solution obtained form the two prepared methods can be easily cast into smooth thin film with smaller particle size, mending the faults in electrochemical polymerization, such as wasting monomer, monomer being hardly preserved, and rough thin film. Conductivity measurements of PITN thin film indicate better stability at high temperature, having potential of being used as solid electrolyte of capacitor.PITN molecular chains show not only unpaired electron of polarons due to doping, but also unpaired electron on S-atom due to neutral defects. On one hand, PITN obtained from dehydrogenation of precursor indicates to be doped at heating process. The spin concentration is gradually elevated after 80 ℃, yet because of fewer neutral defects it is insufficient to cause intramolecular spins to be coupled; this is paramagnetic property. On the other hand, PITN obtained from direct polymerization of DHITN shows higher spin concentration (1.7×1020 spins/g) because of having more neutral defects, it maintains constant value within 220 ℃.The results of molecular simulation for isolated PITN molecular and dichloromethane prove that there is van der Waal attractive force between them, so that PITN molecules can be dissolved into dichloromethane. However, if PITN molecules aggregate, the force among PITN molecules is far greater than that between PITN and dichloromethane, so it can only suspend in solution as microgel form. If many PITN molecules are packed or if they produce crosslinking and mislinkage, then they will be precipitated into powder. Therefore, diluting dehydrogenation agent with great amount solvent and add dropwisely it slowly into reaction system, we can get stable PITN as microgel solution.We use molecular simulation to propose a crystalline structure of PITN and arrangement of PITN molecular chains in unit cell. A combination of STM, molecular dynamics, and UV-vis-near-IR spectroscopy can be employed to describe in great detail the transition of geometric and electronic structure of PITN adsorbed on HOPG, from before adsorption is quinoid to become aromatic after adsorption.Poly(5,6-dihexoxyisothianaphthene) (PDHOITN) is synthesized by introducing hexoxy groups on 5 and 6 positions on the repeat unit of poly(isothianaphthene) (PITN) in order to improve its solubility and processability. The PDHOITN film has a band gap of about 0.95 eV and exhibits a high contrast and reversible color change from bluish black at the neutral state to transparent light yellow (close to colorless) at the doped state. Both the neutral and doped states of this polymer can be dissolved in common organic solvents and cast into freestanding film. After doping with ferric chloride, the conductivity of the film increases from 5×10-5 to 2×10-2 S/cm. The monomer of PDHOITN has dihexoxy groups increasing activity of benzylic hydrogen, and hydrogen of 5, 6- positions on the repeat unit of PDHOITN are replaced, so the crosslinking and mislinkage in the polymerization are not easily produced. The monomers of PDHOITN are not easily to couple due to longer side chain, so the molecular weight is lower and the distribution of molecular weight is narrower than PITN obtained from direct polymerization of DHITN using FeCl3 as oxidant. Both side chain and main chain of neutral PDHOITN are crystalline, which of melting point is 89 ℃ (Ts) and 134 ℃ (Tm), respectively. The glass transition temperature of neutral PDHOITN is 53 ℃. The analyses of IR and TGA show thermal weight loss at 200~300 ℃ due to degradation of side chains, after 300 ℃ appearing quick thermal weight loss due to simultaneous degradation of side and main chains. ESR and UV-vis-near-IR spectroscopies of neutral PDHOITN appear transition stage in the temperature range of Tg and Tm, which is attributed to twist of coplanar subchains.PDHOITN doped with FeCl3 ruin its crystalline structure because the dopants are embedded in spacing of molecular chains packed. Moreover, d-spacing of interlayer for doped-PDHOITN is raised, and the intensity of crystalline peaks is weakened. Analysis of IR spectra for FeCl3-doped PDHOITN heated to 250 ℃ shows thermal undoped and crosslinking between intermolecular chains. The dopants, FeCl4-, are thermal degraded at about 156 ℃, which is changed to FeCl2 from the analysis of ESR. After doping with FeCl3, the spin concentration of PDHOITN enhances to 1 order, and the conductivity increases from 5×10-5 to 2×10-2 S/cm.We have synthesized partially crystalline PITN with ferromagnetic properties, which has polarons/neutral defects from the analytic results of UV-vis-near-IR and ESR. The spin concentration of PITN is higher (1.7×1020 spins/g) and maintains constant value within 220 ℃. The conductive mechanism of PITN is electron transport along the molecular chains and electrons hopping in the interchains. Magnetic measurements by SQUID show that our synthesized PITN has spontaneous magnetization and exhibit field-dependent magnetization and hysteresis at 10~300 K. The Curie temperature can not be estimated, since that obtained by extrapolation (653 K) exceeds the thermal decomposition temperature of the sample (553 K). From the results of theoretical analysis and experiments, we propose the intramolecular spins of PITN to be ferromagnetically coupled and intermolecular spins to be antiferromagnetically coupled. The PITN is ground and blended with PMMA can change its value of coercivity and remanence. The PITN/epoxy blend placed under strong magnetic field can increase various magnetic values. The results of magnetic measurements for FeCl3-doped PDHOITN indicate that spins to be antiferromagnetically coupled between PDHOITN and FeCl4- at lower 250 K.The MFM is employed to identify magnetic domain of PITN thin film surface; we find different features of perpendicular and parallel magnetization. The probes made of different materials will effect the magnetic structure of PITN thin film surface, and induce domain wall movement of PITN thin film surface.