Functionality Hyperbranched Conjugated Polymer Synthesis, Characterization And Photophysical Properties

Since the 1990's,after discovering the electroluminescence phenomenon of PPV,π-conjugated polymers have attracted more and more interests owing to their wide-ranging application potentials among the field of polymer science.A variety of conjugated polymers with different light-emitting wavelengths and structures are being developed.Due to the simple preparation process together with the unique highly branched and globular structure which helps to suppress the aggregation and excimer,hyperbranched structure has been introduced into the conjugated polymers. Meanwhile,the copolymerization of lower band-gap monomers with higher band-gap chromophores is one of the most promising methods to tune the light-emitting wavelength and realize white light-emitting from a single polymer chain.Now,the energy transfer process has decisive influence on the light-emitting colours and fluorescent efficency.In this dissertation,based on the previous work mentioned above and the research experience of our group,a series of hyperbranched polymers with different functional groups were designed and synthesized.Their thermal stability and energy transfer process were investigated in the following two parts:Part one:Synthesis,characterization,and thermal stability of hyperbranched polyfluorenes containing sterically hindered groups.To combine the advantages of hyperbranched structure and the sterically hindered monomers, 9,10-dibromoanthracene and 2,7-dibromospiro(fluorene-9,9-xanthene) were introduced into the polyfluorenes containing different branching units such as benzene or Triphenylamine via Suzuki polycondensation.All the resulting hyperbranched copolymers are soluble in common organic solvents,such as tetrahydrofuran, chloroform and toluene.The structures of the hyperbranched polymers were fully characterized by NMR,GPC,Elementary analysis and thermogravimetric analysis. These highly branched polymers possessed good thermal and spectral stability.The polymer films exhibited very stable blue light emission even after being annealed at elevated temperature in air.Apparently,with the introduction of spirofluorene/large aromatic units to the hyperbranched structure,the red-shifted phenomena and the undesirable low-energy long wavelength emission band are reduced remarkably.All results indicated that the tendency to form aggregate/excimers in the solid state is efficiently suppressed and the spectral stability of the polymers was greatly improved. Part two:Research of energy transfer process on hyperbranched polymers. Different contents of branching unit(Triphenylamine as the core) was brought into the copolymers of fluorene and benzothiadiazole,and besides,the feed ratio of fluorene/ benzothiadiazole was changed with constant quantity of branching unit,a series of hyperbranched polymers were developed.The photophysical properties and energy transfer process were explored.The narrow band-gap comonomer benzothiadiazole unit acts as a powerful exciton trap which allows efficient energy transfer of the exciton from fluorene segment to benzothiadiazole unit in/between the polymer chains.The experiment data demonstrated that the intrachain energy transfer played the leading role in solution and the energy transfer efficiency varied along with the content of benzothiadiazole as well as the branching degree on account of the globular structure;the interchain energy transfer prevailed and the blue emission of fluorene was fully quenched in the solid state by reason of the compact stacking of polymer chains.