Synthesis and Characterization of Naphthyl-Substituted Poly(p-phenylenevinylene)s with Few Structural Defects for Polymer Light-Emitting Diodes

A series of naphthyl-substituted poly(p-phenylenevinylene)s (2N-PPV, 4N-PPV, and NAP-PPV) has been synthesized and characterized by Fourier transform IR, ¹H NMR, and elemental analysis. The polymers possess excellent solubility, high molecular weights, good thermal stability, and high photoluminescence efficiencies. Thermogravimetric analysis reveals the onset of degradation to be 347, 301, and 306°C for 2N-PPV, 4N-PPV, and NAP-PPV, respectively. The differential scanning calorimetry investigation gives the respective glass-transition temperature values of 118, 135, and 141°C. The UV and photoluminescence spectra measurements reveal that the polymers exhibit similar optical properties, indicating that side-chain substitution has little effect on the optical properties of this series of polymers. Proton NMR measurement of the signal due to tolane-bisbenzyl defects at around 2.7 ppm indicates that all the polymers have negligible amounts of tolane-bisbenzyl defects along the polymer main chain as a result of the steric bulk imposed by the naphthalene side chain. The highest occupied and lowest unoccupied molecular orbital energy levels of the polymers are investigated through cyclic voltammetry. Polymer light-emitting diodes utilizing the polymers as the emissive layer with a configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene)/polymer/Ba/Al are fabricated and evaluated. The diodes emit blue-green to yellow-green light with maximum peaks at 518, 542, and 486 nm for 2N-PPV, 4N-PPV, and NAP-PPV, respectively. The respective turn-on electric fields of the diodes are 0.84, 0.69, and 0.83 MV/cm and the respective maximum external quantum efficiencies are 0.08, 0.54, and 0.02%.