Synthesis and properties of novel electroluminescent oligomers containing carbazolylene–vinylene–sulfonylene units for a light-emitting diode

As a new class of spin-coatable electroluminescent oligomers, oligo( N-ethylhexyl-3,6-carbazolylenevinylene- alt-4,4′-diphenylvinylene-sulfone) (P1) and oligo( N-ethylhexyl-3,6-carbazoledivinylene- p-phenylenevinylene) (P2) were synthesized through Wittig polycondensation of N-(2-ethylhexyl)-3,6-diformyl carbazole with the diphosphonium salts of bis(bromomethyl- p-phenyl)-sulfone and α,α′-dibromo- p-xylene, respectively. These electroluminescent (EL) oligomers were highly soluble in common organic solvents, forming excellent-quality optical films by spin coating. Films obtained were very transparent, tough, and smooth with initial decomposition temperature of ca. 400°C. Greenish-blue photoluminescence (PL) and electroluminescence (EL) was obtained for both oligomer films. It was found that the relative EL quantum efficiency of single-layer P1 device was five-fold higher than that of the P2 device, which was attributed to the lowered molecular energy levels of the former due to the presence of the sulfonylene group.