Solution-Processable Red-Emission Organic Materials Containing Triphenylamine and Benzothiodiazole Units: Synthesis and Applications in Organic Light-Emitting Diodes

Three solution-processable red-emissive organic materials with a hole-transporting unit triphenylamine (TPA) as the core part and a D-pi-A bipolar structure as the branch part, TPA-BT (single-branched molecule), b-TPA-BT (bibranched molecule), and t-TPA-BT (tribranched molecule), were synthesized by the Heck coupling reaction. Herein, for the D-pi-A push-pull structure, we use TPA as the electron donor, benzothiodiazole (BT) as the electron acceptor, and the vinylene bond as the pi-bridge connecting the TPA and BT units. The compounds exhibit good solubility in common organic solvents, benefited from the three-dimensional spatial configuration of TPA units and the branch structure of the molecules. TPA-BT, b-TPA-BT, and t-TPA-BT show excellent photoluminescent properties with maximum emission peaks at ca. 630 nm. High-performance red-emission organic light-emitting diodes (OLEDs) were fabricated with the active layer spin coated from a solution of these compounds. The OLED based on TPA-BT displayed a low turn-on voltage of 2.0 V, a maximum luminance of 12192 cd/m2, and a maximum current efficiency of 1.66 cd/A, which is among the highest values for the solution-processed red-emission OLEDs. In addition, high-performance white-light-emitting diodes (WLEDs) with maximum luminance around 4400 cd/m2 and maximum current efficiencies above 4.5 cd/A were realized by separately doping the three TPA-BT-containing molecules as red emitter and poly(6,6'-bi-(9,9'-dihexylfluorene)- co-(9,9'-dihexylfluorene-3-thiophene-5'-yl)) as green emitter into blue poly(9,9-dioctylfluorene-2,7-diyl) host material with suitable weight ratios.