Abstract
Abstract Owing to their novel electrooptical properties, conducting polymers like polyanilines (Pans) have been demonstrated to be particularly useful in many applications. Regarding to the applications in light-emitting diodes and solar cells, both HOMO and LUMO energy levels of the employed materials are the critical parameters for the performances of the devices. In literature, the HOMO level of polyaniline（Pan）has only been estimated once by Heeger et. al. as 4.8 eV via a rather complicated approach, i.e., based on a specially prepared hole-only LED device and in combination with the Fowler-Nordheim tunneling theory. Due to the enormous number of fabrication parameter, it is not easy to prepare the LED devices with reproducible quality. Hence, the HOMO level measured based on the LED device may not be sufficiently reliable. The HOMO and LUMO levels of conjugated molecule are often estimated from the on-set potential of the oxidation or reduction peak of its CV curve. However, the previous literature reports indicated that the redox behaviors of Pan in nonaqueous electrolytic solutions were very complicated and actually somewhat confusing. Different reports often showed different CV curve shapes having different number of oxidation peaks and values of oxidation potential. Such non-reproducible and irreversible electrochemical behaviors of Pan in nonaqueous solution were not yet fully understood up to now. Herein, we re-investigated the CV behaviors of Pan in nonaqueous electrolytic solutions in conjunction with ATR-IR, in-situ UV-Vis, and XPS studies. Interestingly, we found that the CV behaviors of Pan in nonaqueous electrolytic solutions are highly depending on its initial oxidation status of Pan. Based on such findings, we have developed a reliable method to estimate the HOMO/LUMO values of Pan. The study suggests that Pans of different initial oxidation states (from LB to PB form) should have different HOMO/LUMO values. Moreover, we have also developed a method to obtain a stable film of fully oxidized PB form of Pan, by which highly substituted Pan（~ 50 mol%）can be prepared via the CRS route. Such results were also useful in confirming the structure and properties of the PB form. In literatures, substituted Pans have always been found to exhibit much lower conductivities than that of the unsubstituted Pan and the results had been attributed to the substituent’s steric hindrance effect (for R and OR) or electron withdrawing effect (for halogen). However, our previous study indicated that, based on the result of butylthio-substituted polyaniline obtained via the CRS route, the unusually low conductivity of alkyl- or alkoxy-substituted Pans is arisen from their backbone conjugation defects (e.g., 1,3-ring linkage structures) as induced by the substitutent group during the growth of the polymer chains. Because of the unusual properties associated with the fluorine atom, such as high electronegativity, small atomic radius, high lipophilicity, and unique chemical reactivity, the fluorinated-polyaniline should be highly interested. But all the previously reported poly(fluoroaniline)s prepared by the OCP method were found to have greatly increased oxidation potential, significantly blue-shifted UV-vis absorption, lack of redox activity, and much lower conductivity. Therefore, we decided to develop a feasible method for introducing the F-substituent to the preformed polyaniline backbones via the CRS method. Via this synthetic approach, the backbone structure of the fluorinated-polyanilines (F-Pans) could be maintained the same as the parent unsubstituted Pan, so that the electron effect of the substituent group could be fairly and reliably evaluated. We then have successfully synthesized a series of fluorinated-polyaniline（F-Pan）with controlled amounts of F-substituent. Surprisingly, the F-Pan samples prepared by the CRS method were found to be much highly conductive and electroactive than the Pan2F samples prepared from the OCP method. Even with a substitution degree of greater than 100 mol%, the obtained F-Pan showed only a slight decrease in its conductivity (~0.1 S/cm). Furthermore, the UV-vis, IR, and CV studies indicated that F-Pan have retained similar good redox activities as that of the unsubstituted Pan. On the other hand, the Pan2F samples prepared from conventional oxidative copolymerization（OCP）method were found to be pooly conductive (10-1~10-5 S/cm), with much higher oxidation potential Eox 1, significantly blue-shifted UV-vis absorptions, and extremely deactivated redox activity. The results of our thorough comparison studies indicated that the F-Pan samples prepared from the CRS method retained the same highly conjugated structure of the parent unsubstituted polyaniline, while Pan-2F samples prepared from the OCP method contained significant amounts of 1,3-linkage conjugation defects, which greatly shortened the effective conjugation extent of the backbone and dramatically reduced their conductivities. Furthermore, the CV results for the CRS-prepared F-Pan samples showed that, as the fluorination degree increased, its oxidation potential actually decreased and eventually became lower than its parent unsubstituted Pan. The results imply that the fluorine atoms on the highly conjugated polyaniline backbone may actually help to increase the electron density of the backbone through the resonance donation of their lone-pair electrons. To better understand the electronic behaviour of the F in the higher conjugated aniline systems, we have systhesized a series of conjugated aniline model compounds (PDA-F、PDA-3F、pentamer-F) and studied the compounds with various 1D and 2D NMR experiments (1H NMR、13C NMR、1H-homodecouple、NOE、HSQC、HMBC) to fully and unambiguously assign the chemical shifts for all the H and C of the compounds. The results clearly confirmed the resonance electron-donating effect of the fluorine atoms in these higher conjugated aniline systems. In addition, we found that the fluorine group in F-Pan can be replaced by other types of nucleophiles under some appropriate conditions to convert into the alkoxy- or alkyl-substituted Pans, which are otherwise very difficult to accomplish directly via the CRS method. Therefore, the results and findings of this dissertation can greatly expand the application scope of the CRS method for the preparation of various types of substituted-polyanilines.