Synthesis and electro-optical properties of poly(p-phenylenevinylene) derivative with conjugated 1,3,4-oxadiazole pendant and its AC electroluminescence

A luminescent polymer, poly{2-[10′-(2″-(4″′-oxyphenyl)-5″-phenyl-1″,3″,4″-oxadiazole)-decyloxy]-1,4-phenylenevinylene-alt-2,5-didodecyloxy-1,4-phenylenevinylene} (PPOXA), was synthesized. The electron withdrawing pendant, 2′-(4″-phenoxy)-5′-(4″′-phenyl)-1′,3′,4′-oxadiazole (OXD), is separated from the main chain via a linear 1,10-decamethylenedioxy chain. The band gap and photoluminescence (PL) maximum wavelength of PPOXA and HDO (poly(2-hexadecyloxy-1,4-phenylenevinylene-alt-2,5-didodecyloxy-1,4-phenylenevinylene)) exactly match each other (2.19 eV and 557 nm). In addition, the electroluminescence (EL) maximum wavelength of PPOXA is 559 nm, which is very close to that of HDO (557 nm). In the PL and EL spectra of film, the emission from conjugated 1,3,4-oxadiazole (OXD) unit was not observed and did not affect the EL and PL maxima of PPOXA. The HOMO and LUMO energy levels of the main chain figured out from the cyclic voltammogram and the UV-vis spectrum are −5.11 and −2.94 eV, respectively, which are very close to those of HDO (−5.13, −2.94 eV). The estimated HOMO and LUMO energy levels of the pendant are −6.54 and −2.84 eV, respectively. Single layer EL device based on PPOXA has a maximum external quantum efficiency of 0.23 cd/A, which is significantly higher than that of HDO (0.043 cd/A) measured under the same conditions. The energy levels figured out from optical and electrochemical data strongly support that the OXD pendants have hole blocking properties. Single layer structured EL device based on PPOXA (ITO/PPOXA/Al) shows the emission at the reverse bias with an efficiency of 0.043 cd/A, which is very comparable to that of ITO/HDO/Al under the forward bias.