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 π-conjugation lengths. The trimethylsilyl substituent has no electrondonating 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.

We have studied thin films of the conjugated polymer poly[2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), prepared from polymer samples whose weight-average molecular weight (M w) was varied in the broad range of 10−1600 kg/mol. Anisotropic refractive index measurements by means of waveguide prism coupling and reflectometry as well as polarized infrared spectroscopy were used to analyze the polymer chain orientation in the films. We found that the film morphology depends significantly on the molecular weight, especially in the range M w < 400 kg/mol. Thin films of high molecular weight MEH-PPV have most polymer chain segments oriented parallel to the film planein contrast to low molecular weight samples which have nearly random orientation of the chain segments. Appropriate choice of molecular weight enables fine-tuning of the refractive index of slab waveguides and reduction of their mode propagation losses to less than 1 dB/cm.

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-2methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (PFMEH-PPV), and poly[N-ethylhexyl-3,6-carbazolevinylene-alt-2-methoxy-5-(2-ethylhexyloxy)-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 °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 PL eff (Φ ) 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 × 10 8 and 1.2 × 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.

Articles you may be interested inMorphology and chain aggregation dependence of optical gain in thermally annealed films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] Neutron reflection study on soluble and insoluble poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene) films J. Appl. Phys. 91, 9066 (2002);

Novel cyclohexylsilyl- or phenylsilyl-substituted poly(1,4-phenylene vinylene) (PPV) derivatives, poly[2,5-bis(dimethylcyclohexylsilyl)-1,4-phenylene vinylene] (BDMCyS−PPV), poly[2,5-bis(dimethylphenylsilyl)-1,4-phenylene vinylene] (BDMPS−PPV), poly[2-dimethylcyclohexylsilyl-1,4-phenylene vinylene] (DMCyS−PPV), and poly[2-dimethylphenylsilyl-1,4-phenylene vinylene] (DMPS−PPV), were synthesized via the bromine precursor route (BPR) and Gilch dehydrohalogenation polyaddition. Thin films of the insoluble BDMCyS−PPV and BDMPS−PPV were fabricated from soluble polymer precursor materials by thermal conversion, and the electronic properties of these films were investigated. Monosilyl-substituted DMCyS−PPV and DMPS−PPV exhibited good solubility in the conjugated state, good film-forming properties, and high molecular weights. Moreover, they showed better thermal stability and higher values of T g (DMCyS−PPV, 125 °C; DMPS−PPV, 127 °C) than other PPV derivatives including alkylsilyl-substituted PPVs; this improved mechanical stability led to good electroluminescence performance. Monocyclohexylsilyl- or phenylsilyl-substituted DMCyS−PPV and DMPS−PPV exhibited sharp PL emissions at about 511 and 513 nm, respectively, along with extremely high photoluminescence (PL) efficiencies in both solution and film (DMCyS−PPV, Φfilm = 0.83; DMCPS−PPV, Φfilm = 0.82). LED devices fabricated from DMCyS−PPV and DMPS−PPV using the configuration ITO/polymer/Al showed electroluminescence (EL) maxima at 510 and 515 nm, respectively, with external EL quantum efficiencies of 0.02% and 0.03%. Incorporation of PVK as a hole-transporting layer between the ITO and polymer with an air-stable aluminum cathode caused a substantial improvement in the EL quantum efficiencies, increasing the values to 0.07% for DMCyS−PPV and 0.08% for DMPS−PPV.