Synthesis And Optical Properties Of Fluorescent Polymers Containing Porphyrin And Oxadiazole Groups

PLEDs (polymer light-emitting diode) include emitting materials, electron transport material and hole transport material. Red light-emitting materials, which fully meet the requirements for commercial application, are scarce. The main problem is its relatively low efficiency. Usually red light emitting is achieved by doping red dyes into host materials with a large band gap. Red dyes are prone to aggregate and end up self-quenching their luminescence and their applications are limited. Porphyrin exhibits reasonable fluorescence efficiency and excellent thermal stability, but its film could lead to luminescence quench owing to aggregation, and its fluorescence quantum yield was very low. Atom transfer radical polymerization (ATRP), one of the most powerful CRP techniques, has been successfully used to prepare functional polymers. In this paper, red light-emitting novel star polymers were obtained by the "core-first" strategy.(1) Star polystyrene were prepared using zinc5,10,15,20-tetrakis [4-(2-methyl-2-bromo-propoxy) phenyl] porphyrin(PI) or Zn-PI as initiator, CuBr/PMDETA as a catalyst system. The emission intensity of Zn-PI star polystyrene was much higher than that of PI star polystyrene, it indicated that the coordination of zinc ion and porphyrin initiator could avoid copper ion as catalyst into porphyrin. In solution, the emission maximum of Zn-PI PSt is red-shifted by7nm in comparison with its solid emission, the aggregation of porhyrin of star PSt in solid could decrease obviously. Both Zn-PI and Zn-PI PSt have third-order NLO properties.(2) Carbazole is electron-donor group and attaching a carbazole moiety to the molecular scaffold can significantly enhance the thermal stability and HOMO energy level of light-emitting polymers. Styrene-type monomer9-(4-vinylbenzyl)-9H-carbazole (VBCz) and methacrylate-type monomer2-(9H-carbazole-9-yl)-ethyl methacrylate (CzEMA) were polymerized to star polymers respectively via atom transfer radical polymerization (ATRP) using Zn-PI as an initiator. The emission spectra of two star polymers, star poly(VBCz) and star poly(CzEMA)) in the solid state displayed red light emission, while those of two monomers showed blue light emission. The result demonstrates that effective energy transfer occurs from the carbazole to the Zn porphyrin core. However, two star polymers in DMF solution emit week red light and strong UV light at350-400nm, it points that energy transfer can not occur from the carbazole to the Zn porphyrin core effectively. They exhibit good thermal stability with Td poly(VBCz)=374℃and Td poly(CzEMA)=297℃at5%weight loss. The DSC curves show that the glass transition temperature of styrene-type (Tg Poly(VBCz)=177℃) was better than that of methacrylate-type(Tg poly(CzEMA)=148℃).Besides fluorescent materials, electron-transportation materials are one of PLED materials.2,5-Diphenyl-1,3,4-oxadiazoles derivatives are used for electron accepting materials in polymeric lightemitting diodes. As their analogs,2,5-diphenyl-1,3,4-thiadiazoles derivatives having two electron withdrawing groups C=N, will be promising in luminescence and electron transportation. The synthesis of vinyl monomers and their polymers containing2,5-dipheny-1,3,4-thiadiazole has not been investigated.(3) In this paper,2-(4-bromomethyl phenyl)-5-phenyl-1,3,4-thiadiazole (BMPPT) was synthesized by design and the polystyrene containing2,5-diphenyl-1,3,4-thiadiazole end group was polymerized by BMPPT as initiator. Their emission maximum was390nm, and the emission intensity of lower molecular weight polystyrene was higher.(4) In addition, polymers containing2,5-diphenyl-1,3,4-thiadiazole side chain were further studied by synthesizing three novel monomers which were2-[4-(5-phenyl-1,3,4-thiadiazole-2-yl)-benzyloxy]ethyl methacrylate (PTBEMA),2-[4-(5-phenyl-1,3,4-thiadiazole-2-yl)-phenoxy] ethyl methacrylate (PTPEMA) and2-phenyl-5-(4-vinylphenyl)-1,3,4-thiadiazole (PVPT). Their polymer was obtained by radical polymerization. Due to different π-π interaction affected by substituent and distance between main chain and chromophore, three polymers emit from indigo to green light. Poly(PTBEMA) in the solid emits pure blue light corresponds to coordinates (x=0.15, y=0.13) and poly(PVPT) emits green light corresponds to coordinates (x=0.29, y=0.51) as well as ploy(PTPEMA) emits indigo light corresponds to coordinates (x=0.15, y=0.07). The polymers are red-shifted by ca.40-70nm in comparison with their absorption maxima. It points the formation of static excimer. But poly(PTPEMA) is an exception, its excitation maximum is ca.7nm red-shifted to its absorption maximum. It indicates that poly(PTPEMA) in dilute DMF trend to the formation of dynamic excimer rather than static excimer. The HOMO and LUMO energies of three monomers and their polymers, as estimated from cyclic voltammetry data, were-6.35to-6.14eV and-3.02to-2.84eV, respectively. The optical band gaps (Eg) were similar to those determined from cyclic voltammograms. On the other hand,4-(5-phenyl-1,3,4-thiadiazole-2-yl) phenol (PTP) and4-(5-styreneyl-1,3,4-thiadiazole-2-yl) phenol (STP) were synthesized and reacted with polystyrene containing chloride methyl. The emission maxima of PSt-PTP and PSt-STP were392and420nm.In summary, red light-emitting star polymers with a porphyrin core and the polymers with end group and side chain containing2,5-diphenyl-1,3,4-thiadiazole were prepared, their optical properties were studied in order to provide a theoretical basis for preparation of excellent performance PLED materials.