In-situ studie van de kristallisatie dynamica van organische materialen met behulp van harmonische lichtverstrooiing

Crystallization is a topic of great interest in many fields, such as pharmaceuticals, catalysis and materials science. An overall goal in the scientific community is to achieve rational crystal engineering, where crystal properties can be tailored for specific applications. However, to achieve this, a deeper understanding of the earliest stages of the crystallization process is needed. Recently, nonlinear optics has emerged as a powerful tool for probing this process at a molecular level. In this research, we use a new, homemade multi-modal optical setup specifically optimized to view the earliest crystallization stages of various molecules in-situ. This setup combines linear and nonlinear optical effects in order to achieve unprecedented insights. It can measure polarized (harmonic) light scattering, (multi-photon) fluorescence, optical rotation and transmission of the sample simultaneously and throughout a dynamic process. This setup was tested for the solvent-induced aggregation of various molecules: a series of poly(3- alkylthiophene)s, a series of chiral polythiophene derivatives and chlorophyll b. The variety of molecules proves the efficiency and broad applicability of our setup. The combination of linear and nonlinear effects proved to be vital during our studies. For example, this combination enabled the determination of pre-aggregation clusters in which the individual molecules are not coupled. Surprisingly, some form of pre-aggregation entity was nearly always observed in the crystallization studies performed. This challenges mainstream theories such as classical nucleation theory, and offers new insights for crystal engineering. For the series of achiral polythiophenes, a general aggregation pathway (with pre-aggregation clusters) was found. Noteworthy is that this pathway proves to be viable across a wide range of polymer lengths. For the chiral polythiophenes, our setup allowed the determination of the mechanistic differences between type I and type II aggregation. These types are linked to polymer properties and this discovery illuminates in greater detail why certain polymer characteristics lead to certain types of formed aggregates. This is of vital importance for crystal optimization in many applications. Studying naturally occurring crystals, like chlorophyll b, enables a greater understanding of the natural world and allows us to use its power in potential applications. In this case, the discovery of pre-aggregation clusters in the aggregation of chlorophyll b may be a key step in improving synthetic light harvesting complexes.