Blue electroluminescent polymers: Control of conjugation length by kink linkages and substituents in the poly(p-phenylenevinylene)-related copolymers

Poly[o(m,p)-phenylenevinylene-alt-2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene], o(m,p)-PMEH-PPV, and poly[o(m,p)-phenylenevinylene-alt-2,5-bis(t rimethylsilyl)-p-phenylenevinylene], o(m,p)-PBTMS-PPV, of varying effective conjugation lengths were synthesized by the well-known Wittig condensation polymerization between the appropriate diphosphonium salts and the dialdeh yde monomers such as terephthaldicarboxaldehyde, isophthalaldehyde, and pht halicdicarboxaldehyde. The conjugation lengths of the polymers were control led by substituents and kink (ortho and meta) linkages. The resulting polym ers were highly soluble in common organic solvents. The synthesized polymer s showed UV-visible absorbance and photoluminescence (PL) in the ranges of 330-430 nm and 440-550 nm, respectively. The maximum emission peak of p-PME H-PPV was blueshifted about 30 nm compared to that of MEH-PPV, which is due to an unsubstituted phenylene unit. In addition, o-PMEH-PPV and m-PMEH-PPV showed PL emission maximum peaks at 500 and 490 nm, respectively, because ortho and meta linkage of the o(m)-PMEH-PPV reduced pi-conjugation lengths. The trimethylsilyl substituent has no electron-donating effect; therefore, the PL maximum of p-PBTMS-PPV was far more blueshifted (to 485 nm). Conseq uently, maximum PL wavelengths for o-PBTMS-PPV and m-PBTMS-PPV appeared aro und 470 and 440 nm, respectively. A single-layer light-emitting diode devic e was fabricated which has a simple ITO (indium-tin oxide)/polymer/Al confi guration. The threshold bias of trimethylsilyl-substituted o(m,p)-PBTMS-PPV was in the range of 8-9 V. As in the photoluminescence spectra, the dramat ic change of emission color was also shown in electroluminescence spectra o f p-PMEH-PPV, p-PBTMS-PPV, and o-PBTMS-PPV when the operating voltage was a bout 8-9 V.