Light-emitting diodes based on alternating copolymers containing fluorene and oligo(p-phenylenevinylene)

We synthesized poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-cyanostyryl)benzene-1,4-diyl)] [P(FOPV-CN)] and poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-diphenylaminostyryl)benzene-1,4-diyl)] [P(FOPV-Am)]. These polymers have two axes of longitudinal π-conjugation: one in the direction of the polymer backbone and the other in the direction of the oligo(p-phenylenevinylene) chain. From the spectroscopic study, the emissive color of poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(benzene-1,4-diyl)] changed from blue to green by substituting cyanostyryl or diphenylaminostyryl group at the 2,5-positions of the benzene ring. The electronic structures were significantly influenced by the substituents at the ends of oligo(p-phenylenevinylene) moiety. The diphenylamino substituent which has more extended π-conjugation resulted in red-shifted electronic absorption and emission spectra compared to the cyano substituent groups. In the light-emitting diodes (LEDs) based on these two polymers, the turn-on voltage (ca. 9 V) of the device based on P(FOPV-Am) was lower than that of P(FOPV-CN) (ca. 12 V) due to the electron-donating effect of the amino groups. The maximum brightness of the P(FOPV-Am)-based LED was 3100 cd m−2 at 22 V which is much higher than that of the P(FOPV-CN)-based LED (27 cd m−2 at 26 V).