Surface induced phases of molecular crystals: origin and stability

Polymorphism is a widely observed phenomenon in molecular crystals, it describes the appearance of different crystal structures from one type of molecule. The project focuses on one hardly considered origin of polymorphism: the influence of a solid surface to the crystallisation. The formation of surface induced crystal structures will be investigated on a specific series of rod-like conjugated molecules based on benzothieno- benzothiophene. Thin films will be prepared from the monolayer regime up to thick films by different methods ranging from physical vapour deposition under ultra-high vacuum conditions up to solution processing by drop casting and spin coating. Several crystallisation parameters will be varied ranging from the temperature of deposition, the supersaturation during the crystallisation process and the total deposited amount of material. Disordered surfaces will be used for the crystallisation, silicon oxide and polymer surfaces with different roughnesses, surface energies and polarities will be used. The prepared films will be characterised in terms of their structural properties by a variety of methods including surface science techniques but also atomic force microscopy, different methods of x-ray diffraction and infrared absorption spectroscopy. A part of the project is associated with modelling to consider molecular packing including force field calculations and molecular dynamics. The project will answer questions related to the origin and stability of surface induced crystal structures. The variation of the growth conditions will reveal important preparation parameters which cause the formation of a surface induced crystal structure. Studies of monolayer formation will give answers, if a first initial wetting layer is an important prerequisite for the formation of surface induced phases. The experimental investigations together with the modelling will reveal characteristic structural features of surface induced phases. In-situ temperature measurements will give answer if the surface induced phases are thermodynamically stable or if they represent a metastable state originated by growth kinetics.

The aim of the project is the search for unknown crystal structures which are formed by crystallization at defined surfaces. This specific class of polymorph crystal structures of molecules can be found within thin films, therefore, they are often denoted as "thin film phases" or also as "substrate-induced phases". A number of substrate-induced phases of different types of molecules were identified, the molecular packing within the crystal structure was determined, general features for their appearance was discussed and their stability was investigated. We use a number of different experimental method for thin film preparation, the structural investigations were performed by X-ray based techniques but also by optical spectroscopy. The search for new polymorphs were quite successful for a number of conjugated molecules, the crystal structure could be solved for several cases. It is found that the substrate-induced phases are preliminary of metastable character, which is of potential interest for applications e-g- for pharmaceutical molecules. However, there is still a long way to go for understanding the underlying mechanisms for the formation of these specific class of polymorph crystal structures.