Study On Synthesis Of White Light-emitting Octylfluorene-based Copolymers And The Energy Transfer Therein

Polymer-based White Light-Emitting Devices (PWLEDs) have drawn intense attention in both scientific and industrial communities as a result of their potential applications in full-color flat-panel displays, back-lighting sources for liquid-crystal displays and next-generation solid-state lighting sources. In practice, in order to achieve white emission, mixtures of the three red, green, and blue (RGB) primary colors or two complementary colors are typically required. For those simple blending devices, the effect of light, heat and electricity in process of devices operation often lead to some problems such as poor spectrum stability, increased drive voltage and reduced lifetime. Therefore, copolymerization of different chromophores in a single polymer chain was desigaed to use incomplete energy transfer between these chromophores for white light emission.In this paper, a series of copolymers were synthesized with9,9-dioctylfluorene,2,7-bis(2-thienyl)-9-fluorenone (DTFO) and4,7-bis (2-thienyl)-2,1,3-benzothiadiazole (DBT) by Suzuki reaction. Two white light electroluminescence single-polymers were obtained. Different particle sizes of polymer nano-dispersions were obtained by micro-emulsion ultrasonic method and the relationship between particle size and Forster energy transfer was studied.As a result, adjusting the ratio of DTFO and DBT in the polymer and controlling Forster energy transfer from octylfluorene to DTFO and DBT achieved white light emission. When the content of DTFO group was0.5%(mole fraction), the color coordinate of device was (0.33,0.31) and the maximum brightness was2302cd/m2. When the content of fluorenone and DBT groups was0.025%(mole fraction), the color coordinate of device was (0.37,0.34), the maximum brightness was1632cd/m2and the maximum current efficiency was1.89cd/A.When fluorenone was introduced in the copolymers of octylfluorene and DTFO, emission of the fluorenone was observed. A comparison between the terpolymer and copolymer with the same DTFO content shows that the DTFO emissions of terpolymers were enhanced in film. The results show that the energy transfered from octylfluorene to fluorenone was completely passed to DTFO in nou-radiative Forster energy transfer, leading to increased DTFO emission of terpolymers. Those provide a wider choice of materials, because just selecting the chromophores with appropriate absorption and emission spectra would realize a nou-radiative Forster energy transfer via the bridge group between the wide and narrow band gap groups, which would otherwise not occur.Finally, different particle sizes of polymer nano-dispersions ranging from78to360nm were obtained by micro-emulsion ultrasonic method. As the particle size became larger, the resultant spectra were more similar with those obtained from the film state; and as the particle size became smaller, the resultant spectra were more similar with those obtained from dilute solution. The ordered phase increased as particle size increased, which led more π-π stacking for polymer. As a result, Forster energy transfer from octylfluorene to DBT was increased and the emission of DBT was also enhanced.