Archive for the 'Materials' Category

Square Propellers

April 15, 2007

Polyhedral Oligomeric Silsesquioxanes are silicon-oxygen cage or ladder molecules.  The cubic form R8Si8O12 has silicons on the corners, bridged by oxygen atoms (making up the edges).  Each corner can be functionalised by a variety of groups – H, vinyl, allyl, aminopropyl etc. 

 OctaVinyl-POSS 2808 Molecule Diagram

Vinyl Silsesquioxane (these things are hard to draw!)

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JACS ASAP had  a very cool paper this week using octavinylsilsesquioxane and 1-bromopyrene.  Octavinylsilsesquioxane is commercaially available and was coupled to 1-bromopyrene using Heck conditions (palladium catalyst)*.  Varying the ratio of pyrene to cube created 8 to 16 subsituted molecules fo application in organic light emitting diodes.   For monosubsitution at each corner the 8-pyrene form was most abundant, for disubstitution, 14 pyrenes were most abudant, probably due to steric considerations.  Putting so many pyrene derivatives around a small silsesquioxane is hard work.

 When I printed this paper out, I was mesmerized for a few minutes – these molecules are beautiful. They have very high symmetry due to the cube structure and the 14 pyrene molecule is just elegant.  I’m not going to attempt to draw the final products. 

OLEDs were prepared using the 8- and 14- pyrene vinyl silsequioxanes.  The fluorescent efficiency was found to be good, and possibly the highest achieved to date for silsequioxane based emitters.  14-pyrene vinyl silsequioxane gave lower efficiency, probably due to the higher density of pyrene groups and subsequent quenching in the solid state.

Lo et al., JACS 2007; ASAP 


*Heck reactions are palladium -catalyzed carbon carbon couplings betwene aryl or vinyl halides and alkenes, with base.