Synthesis and Characterization of Siloles, Silole-Containing Polymers and Photoswitchable Polymer Brush

Author

Milind Dhaniram Bisen, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2015

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Scott, Colleen

Abstract

In today's world, there is an increasing demand for new and efficient materials to develop future technologies, address the energy crisis, improve the health care system, and in general for the amelioration of our society. There has been substantial ongoing research effort to develop new organic materials for application in electronic devices and alternative energy such as solar cell, and to develop new biomaterials that will provide fast and efficient diagnosis and treatment of diseases. In this dissertation, we report our progress toward the development of new materials with potential for future application as organic electronics and biomaterials. The first part of this dissertation is focused on the method development and synthesis of silole-based small molecules and conjugated polymers. Silole, a silicon analogue of cyclopentadiene is a very interesting molecule due to its unique photo-physical and electronics properties, and a potential candidate for electroluminescent and photo-luminescent based applications. Although the first example of silole has been reported more than 25 year ago, the difficulty in the synthesis of functional siloles diminished the impact of their potential applications. Herein, we report an efficient and feasible synthetic method to synthesize 2,5-reactive siloles. We utilized this method to synthesize silole monomers, which will be applicable for various palladium catalyzed cross-coupling reactions. In cases where the preparation of reactively functionalized siloles were unsuccessful by our method, we were able to accomplish the synthesis in one additional step; nevertheless, our method is still advantageous compare to the other known methods. We have also synthesized silole molecules that can be further functionalized for application as fluorescent sensors. The synthesis and properties of new silole-containing conjugated polymers are described. The polymers were synthesized by the Suzuki, Sonogashira and Stille coupling polymerization. We have studied structure-property relationship of these polymers by exploring their optoelectrical properties. Co-polymers of silole with diketopyrrolopyrrole showed low band gap and low reduction potentials, which are crucial properties in developing n-type materials. We have also fabricated simple photovoltaic device to study photoconductivity, one of the silole-DPP polymer showed fairly high photoconversion efficiency but low current. Further characterization is required to study the conductivity. Silole-containing polymers were also prepared by the direct arylation reaction. The Direct Arylation reaction via C-H bond activation is emerging as a potential alternative to traditional transition metal-catalyzed cross-coupling reactions. We investigated synthetic methods to synthesize silole containing polymers via direct arylation polymerization by screening catalyst, ligand, solvent, temperature and other parameters. Our preliminary results demonstrated the successful copolymerization of 2,5-diarylhalide functionalized silole monomer with various electron accepting benzodiazole scaffolds to give alternating copolymers with low energy band gaps. However, the molecular weights were not optimal, and therefore future work is needed to optimize the reaction conditions in order to obtain higher molecular weight polymers. In the second part of this dissertation, we prepared photo-switchable polymer brush on silicon surfaces. Photoswitchable polymer brushes consisting of N-isopropylacrylamide and photoactive spiropyran moiety were synthesized by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization method. These brushes reversibly react to external light stimulus by altering their polarity, ionic charge and solubility. In our preliminary study, we have shown that charged species can be adsorbed and release by the brushes with irradiation of UV and visible light, respectively. These polymer brushes can be used for isolation of charged protein/peptides.