Synthesis and applications of DPP-based II-conjugated polymers via direct arylation polymerization
Date of Issue2018
School of Chemical and Biomedical Engineering
Diketopyrro lopyrro le (DPP) derivatives are known in industry as efficient and stable pigments for a long time. Nowadays, arising attention is drawn to DPP-based conjugated small molecules and polymers on both bioapplications and electronic devices, owing to their superior photophysical and electrical properties. The synthesis of these materials is typically realized by conventional cross-coupling reactions, which require extra steps to synthesize organometallic reagents and involves toxic tin byproducts (Stille coupling). To overcome the drawbacks of these traditional coupling reactions, direct arylation by activation of C-H bonds has emerged as a new type of cross-coupling reaction as an economically attractive and ecologically benign way. In this thesis, DPP unit is used as the core to construct polymers structures by either direct arylation polymerization or controlled polymerization. A series of DPP-based low-band gap conjugated polymers are synthesized by direct arylation polymerization with detailed optimization. The photophys ical, electrochemical and thermal properties of these polymers are studied and the charge-transport propetties are investigated in organic field effect transistors (OFETs). DPP-Iabelled biocompatible polymers are synthesized by ring opening polymerization and their photophys ical properties are investigated in detail to understand the relationship between the polymer chain length and photophysical properties (especially fluorescent properties). DPP-Iabelled polymer with long polycaprolactone (PCL) chain is found to retain strong fluorescence in both nanoparticles and solid states, which inspires us to explore its applications for cell imaging (nanopaticles) and cell interfacing and longterm tracking (polymer scaffo lds). By combining direct arylation polymerization and control polymerization methods, a grafting through approach is applied to synthesize DPP-based low-band gap conjugated polymer brushes in a single step, which could be used for phtothermal therapy. The detailed contents of the thesis in each chapter are described below: In Chapter 1, a general introduction about the synthesis of conjugated polymers is given first, followed by a review of DPP-based conjugated polymers. The applications of conjugated polymers are then described from electronic devices to biomedical area. After that, a greener synthetic method for conjugated polymers, direct arylation polymerization is discussed. This chapter is ended with the research motivation and objectives of this thesis. From Chapter 2 to 5, DPP-based conjugated polymers are synthesized by direct arylation polymerization with comprehensive characterization and charge-transport study in OFETs. In Chapter 2, detailed optimization of direct arylation polymerization between thiophene-flanked DPP (TDPP) and tetrafluorobenzene (TB) is first carried out to afford target polymer (PTDPP-TB) with molecular weight of over 30 kDa, which exhibits balanced hole- and electron- mobilities in OFETs of over 0.0 I cm2 y-J s-' under ambient conditions. ln Chapter 3, the homo-coupling and branching defects in the synthesis of conjugated polymers via direct arylation polymerization are discussed. Dibrominated TDPP (DBrTDPP), which is used to synthesize TDPP-based conjugated polymers, is found to form hyperbranched conjugated polymers (DBrTDPP as the only monomer) under some certain reaction conditions for direct arylation polymerization. Several different reaction parameters, including catalyst, solvent, reaction temperature, ligand and additive are exclusively examined, which could provide some insights to minimize the defects in the conjugated polymers synthesized by direct arylation polymerization in the future. In Chapter 4, the incorporation of other different building blocks, thienoisoindigo (TIIG) and benzodithiazole (BDTz) with TDPP unit into the conjugated backbones by direct arylation polymerization approach gives a series of conjugated polymers with high molecules weight, well-defined structures and tunable bandgap m high yields. The photophysical, electrochemical and thermal properties of these polymers are studied systematically. The charge transfer properties of these polymers are investigated in OFETs as well and the film morphologies of these polymer films are examined by atomic force microscopy to find out the relationship between film morphologies and charge transfer mobilities. In Chapter 5, a series of phenyl-flanked DPP (PhDPP)-based conjugated polymers were synthesized by DAP and their photophysical, electrochemical and thermal properties were studied as well. Three PhDPP-based conjugated polymers are synthesized with TDPP, tertrafluorobenzene (TB), benzodithiophene (BDT) and 3,4-ethylenedioxythiophene (EDOT) as the other building block. These four polymers show absorption in the visible range and are fluorescent in solutions with Stokes shift from 55 to 74 nm (non-fluorescent for PTDPP-PhDPP). After discussing the application of DPP-based conjugated polymers for organic electronic devices, TDPP-Iabelled biocompatible polymers and TDPPbased conjugated polymer brushes are synthesized and studied for biomedical applications in the next two chapters. In Chapter 6, a series of TDPP-Iabelled biocompatible polymers (PCLDPP- PCL, PCL: polycaprolactone) are synthesized by ring opening polymerization with different chain lengths. lt is found that the fluorescence quenching of DPP in the solid states can be efficiently compressed with extended PCL chain length. PCL-DPP-PCL with long PCL side chain is highly fluorescent and photostable in nanoparticles and solid states, which are used in cell interfacing and long-term tracking (nanofibers scaffolds). In Chapter 7, with previous efforts in the DPP-Jabelled biocompatible polymers and direct arylation polymerization method to the synthesis of conjugated polymers, a grafting through approach was applied to synthesize low-band gap conjugated polymer brushes. with DPP-based conjugated backbone and biocompatible polymers as the side chain in a single step. And these polymers brushes could be served as photothermal therapy agents with strong absorption in the near-infrared range that benefits deep tissue penetration. Finally, in Chapter 8, a summary is given for this thesis, with some outlook for the future efforts in direct arylation polymerization and facile synthetic method for conjugated polymer brushes.