Open‐cage silsesquioxane, in which traditional closed‐cage silsesquioxane is partially hydrolyzed, is a promising class of building blocks for constructing organic‐inorganic hybrids. It has advantages of high thermal stability, low crystallinity, transparency, designability, etc. Although the structural effects of closed‐cage silsesquioxanes have been elucidated in detail so far, those of open‐cages have never been studied. Herein, we synthesized corner‐ and side‐opened cage silsesquioxanes having carbazole units at the open moieties. The former was amorphous in the solid state and polymer matrix, while the latter formed crystalline structures. These different packing structures caused significant difference in the performance as an emission layer of organic light emitting device.

Open‐cage silsesquioxanes have lower crystallinity without diminishing the thermal stability than traditional closed‐cage silsesquioxanes. Two of the three siloxane groups of commercially available corner‐opened cage silsesquioxanes were selectively substituted, and the remaining one was functionalized for the mono‐methacrylate monomer. The resultant polymer formed a homogeneous and transparent film with high‐thermal stability.

Bis(hydroxyethoxypropyldimethylsiloxy)-(triphenylsilyloxy)-heptaisobutyl open-cage silsesquioxane (2) was prepared from bis(dimethylsiloxy)-(triphenylsiloxy)-heptaisobutyl open-cage silsesquioxane (1) via our previous report of one-pot procedure from heptaisobutyl trisilanol cage silsesquioxane with triphenylchlorosilane and subsequent reaction with dimethylchlorosilane....

The Cover Feature shows three types of cage silsesquioxanes bearing luminophores: closed, side‐opened, and corner‐opened cage structures. Among them, the corner‐opened cage silsesquioxane has excellent film‐formability to exhibit high OLED performance. The most reduced symmetry of the cage effectively lowers the crystallinity. This is the first comparison of the materials properties of different types of open‐cage silsesquioxanes. More information can be found in the Full Paper by K. Naka et al.