Luminescent poly(p-phenylenevinylene) with 4-methylcoumarin side groups: Synthesis, optical properties and photo-crosslinking behaviors

Poly(p-phenylenevinylene) (PPV) derivatives HOPPV and COUPPV, containing hexyloxy side groups and 7-oxy-4-methylcoumarin groups via hexyloxy spacer, respectively, have been synthesized and characterized. Their optical properties were investigated with absorption and photoluminescence (PL) spectral methods. The emissions of COUPPV and model HOPPV films are similar and show peaks around 543-550 nm which is attributed to PPV backbone. Accordingly, efficient energy transfer from the 4-methylcoumarin chromophores to PPV backbone occurred readily in COUPPV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels have been estimated from their cyclic voltammograms. As compared with HOPPV (LUMO: −2.86 eV), the electron affinity of COUPPV (LUMO: −3.65 eV) was enhanced greatly by introducing 4-methylcoumarin groups. Double-layer EL devices (Al/PEDOT/COUPPV or HOPPV/ITO) revealed green electroluminescence. The photo-crosslinking behaviors of COUPPV film under ultraviolet irradiation were investigated by FT-IR and PL methods. Both CC of PPV and 4-methylcoumarin dimerized to cyclobutane derivatives and lead to crosslinking of the polymers. However, COUPPV exhibited higher photo-crosslinking rate due to pendant 4-methylcoumarin chromophores. Patterned emission from PLED utilizing this effect was also demonstrated.