Abstract
Experimentally Assisted Molecular Modelling (EAMM) is an original approach for predicting the structure of organic micro-crystals and deducing their habit in the presence of various solvents and additives. It is applied here in the case of n-octylamino-nitrobenzoxadiazole (nOA-NBD), a fluorescent compound. This general approach is first of all described and validated by its blind application to three known and closely-related crystals. Then, the whole process is applied to predict the molecular crystal generated by n-octylamino-NBD, including the molecule conformation, its structure (cell parameters, molecular packing, X-ray powder diagrams) and its theoretical plus actual habits in the presence of a solvent (p-xylene) and two additives (acetic acid and n-dodecane). The conformation of the predicted molecule is validated by comparison with two similar molecules embedded in observable crystals. Then, the choice between two proposed structures (with the same energy and two equiprobable packings) is based on the comparison between experimental and re-computed X-ray powder diffraction diagrams, and also on the interpretation of an actual TEM image in the light of the proposed cell parameters. This choice is confirmed by the comparison between the vapour-grown face development (proposed by the BFDH or the attachment energy models) and the actual face development in a pure solvent. Finally, the actual habit deduced from comparing the crystal attachment energy with the adsorption energy of solvent or additive molecules is validated by SEM images of NBD micro-crystals in presence of the solvent alone or in presence of both additives. In conclusion, the EAMM approach appears to be a decisive tool for quickly simulating structural and habit properties of the molecular crystals, accessible or not by experimentation. Thus, a numerical selection of molecules becomes possible in view of deriving organic materials with predetermined properties, such as the fluorescence properties of the compound considered in the present study.