Synthesis, optical and electrochemical properties of in-situ thermally cross-linkable oligo(10H-alkylphenothiazine)

A series of 10-alkylphenothiazine trimer (3-PTMA) and pentamer (5-PTMA) with in-situ thermally cross-linkable methyl methacrylate were successfully synthesized. Thermogravimetric analysis (TGA) thermograms revealed that 3-PTMA and 5-PTMA are stable up to 270 and 336 °C, respectively. In the first heating scan of differential scanning calorimetry (DSC) thermogram, 3-PTMA and 5-PTMA showed Tg at 91.2 and 99.7 °C. Both 3-PTMA and 5-PTMA showed broad endothermic process in the region of 144–179 °C, which was thermally cross-linking temperature. In the second heating process, Tg of 3-PTMA and 5-PTMA were 113.4 and 149.8 °C and endothermic process was not observed. UV–vis absorption maximum of thermally cured 3-PTMA is 340 nm, which was same as the UV–vis maximum of 5-PTMA. The absorbance at 340 nm of thermally cured 3-PTMA and 5-PTMA film washed with organic solvent such as methylene chloride (MC), chloroform, and toluene was almost same as the cured 3-PTMA and 5-PTMA, indicating that thermally cured films were very good solvent resistance. Thermally cured 3-PTMA and 5-PTMA were electrochemically stable and the HOMO energy level of 3-PTMA and 5-PTMA were −5.58 and −5.54 eV, respectively. Double layer structured polymer light-emitting diodes based on in-situ thermally cured 3-PTMA and 5-PTMA were fabricated. The maximum luminance efficiency of devices based on 3-PTMA and 5-PTMA were 0.685 cd/A at 16.0 V and 0.760 cd/A at 14.5 V, respectively, which were higher than that of the device without thermally cured 3-PTMA or 5-PTMA (0.348 cd/A at 15.0 V).