Effects of conjugated substituents on the optical, electrochemical, and electron-transporting properties of dithienosiloles

Dithienosiloles (DTSs) bearing conjugated aryl substituents on the thiophene rings (1 and 2) were prepared and their optical, electrochemical, and electron-transporting properties were investigated in comparison with those of simple DTSs having no conjugated substituents on the thiophene rings (4-6). UV absorption bands of 1 and 2 are red shifted from those of 4-6 by 40-80 nm, reflecting the expanded T-conjugation, whereas the first oxidation peaks in the cyclic voltammograms of 1 and 2 appear at potentials a little lower or almost the same energies relative to those of 4-6, depending on the nature of the substituents. The electron-transporting properties of 1 and 2 were evaluated by the performance of electroluminescent (EL) devices having vapor-deposited DTS, Alq, and TPD layers, as the electron-transport, emitter, and hole-transport, respectively. The results indicated that introduction of aryl substituents to DTSs led to inferior performance of the devices in most cases, while the device with lc bearing trimethylsilylpyridyl substituents exhibited high efficiency of current-luminance energy conversion and emitted a green light with a maximum luminance of 16 000 cd/m(2). A trap-controlled electron transporting model is proposed to explain their performance.