New electron-transporting materials for light emitting diodes: 1,3,4-oxadiazole-pyridine and 1,3,4-oxadiazole-pyrimidine hybrids

We describe the synthesis of three new isomeric 1,3,4-oxadiazole-pyridine hybrids, namely: 2,6-, 3,5- and 2,4-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl] pyridine, (PDPy-2,6, PDPy-3,5 and PDPy-2,4, respectively) and a 1,3,4-oxadiazole-pyrimidine hybrid, namely: 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl] pyrimidine (PDPmDP). The X-ray crystal structures are reported for PDPy-2,4 and the known phenylene analogue 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl] benzene (OXD-7) as a 1 : 1 toluene solvate. The packing motif for molecules of both PDPy-2,4 and OXD-7 is that of discrete layers with the mean planes of all the molecules in the crystals parallel to within 6degrees. We have fabricated light-emitting diodes (LEDs) using poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with rubrene as the emissive material, with and without a thermally evaporated electron conducting/hole-blocking (ECHB) layer of PDPy-2,6, PDPy-3,5 and PDPy-2,4, PDPmDP and OXD-7, in the device configuration ITO/MEH-PPV(Ru)/ECHB layer/Al. Electroluminescence spectra indicate that light is emitted only from the MEH-PPV layer. The bilayer LEDs are considerably more efficient than single layer devices, e. g. the external quantum efficiences of devices incorporating PDPy-2,6, PDPy-3,5 and OXD-7 are 0.14, 0.04 and 0.06% at 40 mA m(-2), respectively, cf. 0.007% for the reference single-layer MEH-PPV( Ru) device. There is no clear correlation between experimental EQE values and the PM3 calculated LUMO levels of the materials.