Intermolecular peripheral 2,5-bipyridyl interactions by cyclization of 1,1 '-silanylene unit of 2,3,4,5-aryl substituted siloles: enhanced thermal stability, high charge carrier mobility, and their application to electron transporting layers for OLEDs

Abstract

Two 2,5-bipyridyl substituted 1,1'-silanylene unit cyclized siloles with 1,1-silacyclopentyl-or 1,1-silacyclohexyl groups (3-Cy5 and 3-Cy6) were prepared from the intramolecular reductive cyclization of silacycloalkyl bis(phenylethynyl) silane with lithium naphthalenide and a a subsequent Pd-catalyzed cross-coupling reaction. Non-cyclized bipyridyl substituted dimethylsilole, 2,5-bis(2,2 ''-dipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), was also synthesized for comparison. All three siloles were characterized by X-ray structural studies. The results showed that the major contribution of three dimensional ordering found in crystal packing originated from distinctive intermolecular C-H center dot center dot center dot pi interactions within the distance range 3.30-3.65 angstrom. Even higher ordering was apparent in the crystal packing due to the additional plane-to-plane packing interactions between the peripheral bipyridyl units in cyclized siloles, 3-Cy5 and 3-Cy6. In accordance with such an increase in the peripheral intermolecular interactions, 3-Cy5 and 3-Cy6 exhibited higher T-g values (95 and 86 degrees C, respectively) than the acyclic analogue, PyPySPyPy (77 degrees C). In particular, 3-Cy5 showed a higher electron mobility of 6.9 x 10(-4) cm(2) V-1 s(-1) at E = 0.581 MV cm(-1) in the solid films. As a result, enhanced OLED performance was observed when 3-Cy5 was used as an electron transporting layer in the multilayered device structure of ITO/PEDOT.PSS/NPB/Alq(3)/3-Cy5/LiF/Al, showing a maximum luminance of 17430 cd m(-2) at 11.5 V and a current efficiency of 4.52 cd A(-1) at 69.4 mA cm(-2) with a turn-on voltage of 2.6 V.