Luminescence Properties of Structurally Modified PPVs:  PPV Derivatives Bearing 2-(4-<i>tert</i>-Butylphenyl)-5-phenyl-1,3,4-oxadiazole Pendants

Abstract

Two new poly( p -phenylenevinylene) (PPV) derivatives bearing 2-phenyl-5-(4- tert -butylphenyl)-1,3,4-oxadiazole pendants were prepared, and their photo- and electroluminescence properties were studied. The first polymer ( P-1 ) is poly[2-{4-[5-(4- tert -butylphenyl)-1,3,4-oxadiazolyl]phenyl}-1,4-phenylenevinylene], which is a PPV derivative having diphenyl-substituted 1,3,4-oxadiazole pendant that is known to be an excellent electron-transporting structure. The second polymer ( P-2 ) is poly[2-{4-[5-(4- tert -butylphenyl)-1,3,4-oxadiazolyl]phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]. The only structural difference between P-1 and P-2 is the presence of additional 2-ethylhexyloxy pendant groups in P-2 . Both polymers were prepared by direct polymerization of the α,α‘-dibromo- p -xylene monomers having the pendant group(s) in the presence of excess potassium tert -butoxide. Both polymers reveal much improved electroluminescence (EL) properties when compared with PPV. They emit luminescence light over the wavelength range from about 500 to 600 nm. The external quantum efficiencies of P-1 and P-2 were respectively 16 and 56 times the value for PPV when LED devices were fabricated using an indium−tin oxide (ITO) coated glass anode and the aluminum cathode. In particular, the EL device ITO/poly(3,4-ethylenedioxy-2,4-thienylene)/ P-2 /Al:Li geometry revealed a maximum luminance of 1090 cd/m 2 at the electric field of 2.36 MV/cm with the external quantum efficiency of 0.045%. The maximum brightness of the ITO/ P-2 /Ca/Al was 7570 cd/m 2 at the electric field of 2.80 MV/cm.