Contributions of Nonconjugated Spacers to Properties of Electroluminescent Block Copolymers

Six electro-optically active alternating block copolymers consisting of short substituted phenylene vinylene sequences interspersed with optically inert oligomethylene blocks were synthesized using a Wittig condensation scheme. In this study, the focus was on the effect of the aliphatic spacer length, which was systematically varied from two to twelve methylene units. The copolymers were soluble in standard organic reagents and were characterized by NMR, FTIR, elemental analysis, GPC, UV-vis, and DSC. The vinylene groups in the conjugated blocks were isomerized into the all-trans form by refluxing in toluene with catalytic iodine. The electroluminescent spectra of the six copolymers were similar and displayed a maximum around 465 nm. The EL efficiency of the single-layer LEDs fabricated with the copolymer chromophores was a function of the length of the nonconjugated spacer blocks; copolymers with longer spacers yielded higher-efficiency devices.