Blue Electroluminescent Polymers: Control of Conjugation Length by Kink Linkages and Substituents in the Poly(p-phenylenevinylene)-Related Copolymers

Abstract

Poly[o(m,p)-phenylenevinylene-alt-2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene], o(m,p)-PMEH-PPV, and poly[o(m,p)-phenylenevinylene-alt-2,5-bis(trimethylsilyl)-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 dialdehyde monomers such as terephthaldicarboxaldehyde, isophthalaldehyde, and phthalicdicarboxaldehyde. The conjugation lengths of the polymers were controlled by substituents and kink (ortho and meta) linkages. The resulting polymers were highly soluble in common organic solvents. The synthesized polymers 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-PMEH-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). Consequently, maximum PL wavelengths for o-PBTMS-PPV and m-PBTMS-PPV appeared around 470 and 440 nm, respectively. A single-layer light-emitting diode device was fabricated which has a simple ITO (indium-tin oxide)/polymer/Al configuration. 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 dramatic change of emission color was also shown in electroluminescence spectra of p-PMEH-PPV, p-PBTMS-PPV, and o-PBTMS-PPV when the operating voltage was about 8-9 V.