Excited State Luminescence of Multi-(5-phenyl-1,3,4-oxadiazo-2-yl)benzenes in an Electron-Donating Matrix: Exciplex or Electroplex?

Multi-(5-phenyl-1,3,4-oxadiazo-2-yl)benzenes show emission in organic solvents from ultraviolet to blue (339−447 nm). The reduction potentials E1/2red cover a large range of −2.11 V for 2,5-diphenyl-1,3,4-oxadiazole to −0.76 V for 1,2,3,4,5,6-hexa(5-phenyl-1,3,4-oxadiazo-2-yl)benzene. An unexpectedly wide spectral range of the oxadiazole (OXD) exciplex emissions in PVK is observed, ranging from 406 to 603 nm. The OXDs also exhibit similar electroluminescence (EL) when blended into polyvinylcarbazole (PVK). A linear correlation between the λmax of the electroluminescence and photoluminescence is observed, implying that the emission mechanisms in both processes are similar. In addition, the linear correlation between the E1/2red versus λmax of EL (eV) reflected that the term of the charge-transfer configuration of the contact electron−hole pair plays a major role in the exciplex emission. The exciplex EL of 1,2,5-tri(5-phenyl-1,3,4-oxadiazo-2-yl)benzene (5) could be as high as 1.0 cd/A. Since the exciplex emission usually has a large Stokes shift, this provides a window for us to generate duo emissions for near white light EL with high efficiency. Among the devices we tried, the device of PVK/2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole/5/2,5,8,11-tetra-tert-butylperylene (100:40:40:4) gave EL with good current efficiency of 1.63 cd/A.

多(5-苯基-1,3,4-噁二唑-2-基)苯（Multi-(5-phenyl-1,3,4-oxadiazo-2-yl)benzenes）在有机溶剂中可发射紫外至蓝光波段（339~447 nm）的荧光。其还原半波电势E1/2red的覆盖范围跨度极大：从2,5-二苯基-1,3,4-噁二唑的-2.11 V，到1,2,3,4,5,6-六(5-苯基-1,3,4-噁二唑-2-基)苯的-0.76 V。我们在聚乙烯基咔唑（polyvinylcarbazole, PVK）中观测到噁二唑（oxadiazole, OXD）类化合物的激基复合物发射光谱范围异常宽泛，覆盖406~603 nm。当将此类噁二唑类化合物掺杂至聚乙烯基咔唑（PVK）中时，其亦表现出相似的电致发光（electroluminescence, EL）特性。研究发现，该类材料的电致发光最大发射波长λmax与光致发光（photoluminescence）最大发射波长呈线性相关，表明两种发光过程的机制具有相似性。此外，还原半波电势E1/2red与电致发光最大发射波长（以eV为单位）之间的线性相关关系，说明接触型电子-空穴对的电荷转移构型在激基复合物发光过程中发挥主导作用。1,2,5-三(5-苯基-1,3,4-噁二唑-2-基)苯（5）的激基复合物电致发光效率可高达1.0 cd/A。由于激基复合物发射通常具有较大的斯托克斯位移（Stokes shift），该类材料为实现高效近白光双波段电致发光提供了可行路径。在我们测试的器件中，采用PVK/2-叔丁基苯基-5-联苯基-1,3,4-噁二唑/5/2,5,8,11-四叔丁基苝（质量比100:40:40:4）的器件，展现出1.63 cd/A的优异电流效率。