Abstract

Glasses are generally regarded as highly disordered and the idea of “controlling” molecular packing in glasses is reasonably met with skepticism. Physical vapor deposition (PVD) allows a surprising amount of control over molecular packing in organic glasses and can be used to test the limits of amorphous packing in two ways, both of which are relevant for applications such as organic light emitting diodes (OLEDs). PVD can prepare glasses that approach the limits of the most dense and lowest energy amorphous packings that are possible. The activation barriers for rearrangements in these materials are very high, giving rise to high thermal and chemical stability. In addition, PVD allows control over anisotropic packing in glasses.  For rod-shaped molecules, for example, glasses can be prepared in which the molecules have a substantial tendency to stand-up or lie-down relative to the substrate. The high density and anisotropic packing of PVD glasses can be explained by a mechanism that is “anti-epitaxial” as structure is templated by the top surface rather than by the underlying substrate.