Tunable electroluminescence from silicon-containing poly(p-phenylenevinylene)-related copolymers with well-defined structures

A new class of silicon-containing poly(p-phenylenevinylene)-related copolymers with a uniform pi-conjugated segment regulated by organosilicon units was synthesized by the well-known Wittig reaction between the appropriate diphosphonium salts and the dialdehyde monomer such as terephthaldicarboxaldehyde, 2,5-thiophenedicarboxaldehyde, and N-(2-ethylhexyl)-3,6-diformylcarbazole. The resulting polymers were highly soluble in common organic solvents. They could spin cast onto various substrates to give highly transparent homogeneous thin films without heat treatment. Their glass transition temperatures were in the range of 104-119 degrees C. The UV-visible absorbance of the present polymers shows strong absorption bands at around 347-387 nm, which corresponds to the pi-pi* transition of the conjugated segments. Their maximum photoluminescence wavelengths for SiPPV analogues appeared around 420-480 nm in the blue emission region, except a silicon-containing poly(p-phenylenevinylene)-related copolymer having a thiophene unit showed a strong PL peak at 520 nm in the green emission region. In the case of the polymers containing a carbazole unit, their PL spectra show both the highest peak in the PL emissive band at 450 nm and an additional strong emissive band in the green region. The single-layer light-emitting diode of a Al/SiPhPPV or SiPhPVK/ITO glass is fabricated. The threshold voltage is in the range 6-12 V from the I-V curve. The electroluminescence (EL) spectrum of the SiPhPPV gives the highest peak in the EL emissive band at 450 nm, when the operating voltage of 9 V was applied. For the polymer containing a carbazole unit, the EL spectrum of the polymer shows the highest peak in the EL emissive band at 450 nm as well as an additional strong emissive band in the yellow region, when the operating voltage of 10 V was applied. Interestingly, the SiPhPVK gives a strong white emitting light, when the operating voltage of higher than 12 V was applied.