Synthesis & Spectroscopic Studies of Poly(isothianaphthene)

Abstract

The discovery, research and developments in conductive polymers have drawn extensively scientific and industrial interests worldwide for over two decades. A general review in fundamental theory developed in this field and their applications are presented in chapter 1. In Chapter 2,two routes for the synthesis of the low band gap conductive polymer poly(isothianaphthene), which have significant advantages over previous preparative methods, are examined. The single-step O2-induced polymerization of 1,3- dihydroisothianaphthene, which has been reported previously, is further systematically studied. A modified synthetic method for the O2-induced polymerization of 1,3- dihydroisothianaphthene in the presence of the radical initiator dibenzoyl peroxide leads to the more rapid formation of poly(isothianaphthene) in greater yield, and the product shows better stability as a suspension in organic solvents relative to the products of other preparative methods. In chapter 3, the products obtained were characterized by elemental analysis, and IR and Raman spectroscopy. The variables involved in these two synthetic routes are investigated and possible mechanisms for the reactions involved are discussed. In chapter 4, the iodine-doped PITN were investigated by X-ray photoelectron absorption spectroscopy (XPS). The chemical composition of PITN prepared by O2- induced polymerization and iodine-doped PITN obtained by XPS were consistent with the results of elemental analysis. The doping level of the iodine doped PITN increases up to about 13% as the concentration of the iodine solution used for doping increases. The subband analysis of C ls and S 2p narrow-scan XPS spectra reveals that the polarons are dominant at low doping level and reaches its maximum as the doping level reaches about 5%, and the subband analysis of I 3d narrow-scan XPS spectra suggests that I3- ions are the main dopants in doped PITN. These results are consistent with the spectroscopic studies of Raman and EPR spectroscopy. In addition, it is also found that the O2- and C=O are present in the PITN prepared by O2-induced polymerization. The reason for their existence in PITN is discussed. In chapter 5, far-IR, mid-IR and Raman spectroscopy were used to characterize iodine-doped poly (isothianaphthene) (PITN) films and powders. The far-IR and mid-IR results show changes from absorption mode to reflective mode as the doping level increases, consistent with the iodine-doped PITN becoming more metallic and more conductive at higher doping levels. The far-IR and Raman (514.5 nm laser excitation) results show that I3- is dominant in iodine-doped PITN. The Raman spectral changes observed using 1064 nm excitation is different to those measured using 514.5 nm excitation. The spectra recorded with 514.5 nm excitation show features due to the undoped parts of the polymer, and these indicate that the effective conjugated chain length decreases with increased doping. The Raman spectra obtained by using 1064 nm excitation show features due to polaron and bipolaron states in the doped polymer. In chapter 6, Electron paramagnetic resonance (EPR) and electrical conductivity are studied in undoped, iodine- and Cu(II) complex-doped poly(isothianaphthene) (PITN) powders prepared by O2-induced polymerization. The integrated intensity of the EPR signals of iodine-doped PITN increases as the dopant concentration increases up to 5%, then decreases, while the conductivity shows a sharp increase at this doping level. The EPR linewidth decreases slightly upon dedoping and increases as the doping level increases. These results are discussed in terms of the formation of polarons and bipolarons and a transition from a semiconductor to a metallic regime upon doping. Cu(II) complex-doped PITN prepared by an ion-exchange process shows that the Cu(II) doping level increases 3-5 times compared with that for samples prepared by a direct doping process. Cu(II) complex-doped PITN (σmax=5.2 s cm-1) gives better conductivity than iodine-doped PITN (σmax=0.16 s cm-1) even at lower doping levels. This is possibly due to an increase in the interchain contribution to the conduction mechanism. Chapter 7 describes the new method of the preparations of Cu(II) complexes doped PITN chemically, which were investigated by UV-Vis, XPS and EPR spectroscopy. It found that the doping level of Cu(II) complexes increase 3 - 5 times for the Cu(II) complexes doped PITN prepared by ion-exchange process with pre-iodine-doped PITN. XPS and EPR results both indicate that (l) the ion exchange of Cu(II) complex anions with I3- ions was almost completed and the highest doping level of Cu(II) complex-doped PITN was about 5%; (2) bipolaron states are dominant in these Cu(II) complex-doped PITN through ion-exchange process, which could be related to the effects of the negative charges of dopants. That the analysis of their doping level further confirms that I3- ions are the main dopant in iodine-doped PITN. The conductivity of Cu(II) complexes doped PITN through ion-exchange process increase at least one order in magnitude. Chapter 8 briefly describes some further research directions and projects in the regards of fundamental and applications of PITN.