Improving the Separation of Organic Small Molecules Purified Via Sublimation

Conjugated small molecule materials are currently being investigated as organic semiconductors for use in next generation display and solid-state lighting applications.  Widespread commercial adoption of these technologies is limited by significant materials costs, partially due to an expensive purification step.  This work investigates the current industrial purification technique, thermal gradient sublimation, in order to better understand the underlying mechanisms and guide process development.  The purification of two common charge transport layer materials, N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPD) and 4,4',4''-tris(carbazol-9-yl) triphenylamine (TCTA), is found to be limited by vapor phase transport and deposition.  Interestingly, mechanisms in the solid phase such as diffusion within the solid feed, reaction at the feed particle surface, and mass transfer within the bed of feed particles are not rate limiting.  The vapor phase transport and deposition process is consistent with diffusion.  These results provide a basis for improving existing thermal gradient sublimation processes and suggest guidelines for the design and operation of future large scale purifications of high value, low volume specialty chemicals.