Molecular Weight Engineering in High Performance Ambipolar Emissive Mesopolymers.

Mesopolymers, with structural defects free, high solubility and negligible batch-to-batch variation of conjugated polymers, open a new avenue for organic optoelectronics. For example, organic light emitting transistors (OLETs) that integrate the functions of organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs) together, and are promising devices for next generation display technology and organic electrically pumped laser. However, in most cases, charge transport ability and light emitting strength are contradictory within one conjugated polymer. Therefore, the development of polymeric materials with high carrier mobility and light emitting ability becomes a key challenge. Herein, three low molecular weighted mesopolymers with thienopyrroledione-benzothiadiazole repeating units (meso-TBTF) were obtained by direct arylation polymerization of monomers. Resultant mesopolymers display strong solid state emission and high ambipolar carrier mobility, making them competitive candidates for light-emission devices. The molecular weights of meso-TBTF can be tuned by polymerization temperature, affording products with molecular weights of 3.8 kDa, 6.7 kDa, and 9.9 kDa for device characterization. All mesopolymers exhibit undetectable homo-coupling defects with gradiently-tunable energy levels and better electrical performance over the corresponding high molecular weight polymer. It is found that meso-TBTF with molecular weight of 6.7 kDa has the best electrical performance with an average mobility of 5.65 × 10-3 cm2 V-1 s-1 for electrons and 1.50 × 10-2 cm2 V-1 s-1 for holes, which are among the highest ambibipolar luminescent polymers. In summary, all mesopolymers have photoluminescence quantum yields (PLQY) of about 50% in solution and 10% in solid state. Polymer light emitting diodes of this material are fabricated to explore its potential use in optoelectronic devices.