Palladium-Catalyzed Cross-Coupling Reactions of Dithienosilole with Indium Reagents: Synthesis and Characterization of Dithienosilole Derivatives and Their Application to Organic Light-Emitting Diodes

Three dithienosilole (DTS) derivatives bearing naphthyl segments, 5,5'-dinaphthyl-1,1-dimethyldithienosilol (1), 5,5'-dinaphthyl-l-methyl-l-phenyldithienosilole (2), and 5,5'-dinaphthyl-1,1-diphenyldithienosilol (3), are prepared through Pd(II)-catalyzed cross-coupling using an organoindium reagent as a nucleophile. The molecular structure of 3 is confirmed by single-crystal X-ray analysis. In addition, thermal, photophysical, and electrochemical properties for all three compounds are systematically investigated. The introduction of naphthyl segments into the DTS framework leads to an excellent enhancement of thermal stability with relatively high glass transition (T(g): 87 degrees C) and decomposition temperatures (T(d): 320-380 degrees C). A red shift in both absorption and emission is observed in the DTS series as the 1,1-substituents on the ring silicon atom become more electronegative. On the basis of absorption spectra and DFT/TDDFT calculations, intense green photoluminescence observed for all compounds can be attributed to the effective pi-pi* transition of the DTS and naphthyl group with a small contribution of the sigma* orbital of the exocyclic Si-C bond. The electron-transporting properties of 1, 2, and 3 were evaluated by the performance of organic light-emitting diodes (OLEDs), comprising 4,4',4 ''-tris(N-(2-naphthyl)-N-phenylamino)triphenylamine (2-TNA-TA) as hole-injection layer, 4,4'-bis(N-phenyl-1-naphthylamino)biphenyl (NPB) as hole-transport layer, 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano[678-ij]quinolizin]-11-one(C545T)/tris(8-quinolinato)aluminum (Alq(3)) as emitting layer, and the DTS series as electron-transporting layer, respectively. The device shows intense green emission with a high efficiency of 8.0 cd/A at 1000 cd/m(2).