Highly photoluminescent and blue-green electroluminescent polymers: New silyl- and alkoxy-substituted poly(p-phenylenevinylene) related copolymers containing carbazole or fluorene groups

Abstract

A series of silyl- and alkoxy-substituted poly(p-phenylenevinylene) related copolymers containing carbazole and fluorene groups, poly[9,9-n-dihexyl-2,7-fluorenediylvinylene-alt-2,5-bis(trimethylsilyl)-p-phenylenevinylene] (PFBTS-PPV), poly[N-ethylhexyl-3,6-carbazolevinylene-alt-2, 5-bis(trimethylsilyl)-p-phenylenevinylene] (PCBTS-PPV), poly[9,9-n-dihexyl-2,7-fluorenediyl-vinylene-alt-2-methoxy-5-(2- ethylhexyloxy)-p-phenylenevinylene] (PFMEH-PPV), and poly[N-ethylhexyl-3,6-carbazolevinylene-alt-2-methoxy-5-(2-ethylhexyoxy)-p-phenylenevinylene] (PCMEH-PPV), have been synthesized through the well-known Wittig polycondensation reaction. The synthesized polymers were completely soluble in common organic solvents and exhibited good thermal stability up to 400 degrees C. They showed UV-vis absorbance and photoluminescence (PL) in the ranges of 355-430 and 480-540 nm, respectively. The maximum emission peaks of dialkoxy-substituted PCMEH-PPV and PFMEH-PPV appeared at 520 and 540 nm, respectively, which were both blue-shifted about 55-75 nm compared to that of MEH-PPV. Especially, PCBTS-PPV and PFBTS-PPV showed much more blue-shifted emission maximum peaks at 480 and 495 nm corresponding to the blue and greenish-blue lights, respectively. Although PCBTS-PPV and PFBTS-PPV consist of conjugated segments without any kinked linkages, they showed far more blue-shifted emissions compared with other PPV related copolymers due to little electron-donating effect of the silyl substituent. Moreover, PFBTS-PPV and PCBTS-PPV showed extremely high PL efficiencies. Surprisingly, the PLeff (Phi = 0.81) of PFBTS-PPV is one of the highest values among the currently reported solid film PLeff of PPV derivatives. The single-layer light-emitting diode was fabricated in the configuration of ITO (indium-tin oxide)/polymer/Al. Electroluminescence (EL) maxima of PCBTS-PPV and PFBTS-PPV were shown at 480 and 500 nm, respectively, each corresponding to the blue and greenish-blue emission. The measurement of current vs electric field strength showed the threshold biases of PCBTS-PPV and PFBTS-PPV to be about 1.8 x 10(8) and 1.2 x 10(8) V/m, and PCBTS-PPV and PFBTS-PPV showed 13 and 32 times higher relative EL quantum efficiencies compared to that of MEH-PPV.