Applications of charge-density analysis to the rational design of molecular materials: a mini review on how to engineer optical or magnetic crystals

Abstract

Low-temperature, high-resolution X-ray diffraction has become a rather established technique to unveil fine details of the electronic structure of molecules and even polymeric units in crystals. The most recent advances in methods, hardware and software have enabled the reconstruction of crystalline charge densities with unprecedented high accuracy. A natural step in the evolution of the field would be to apply these results to the rationalization and prediction of macroscopic response properties of crystals, such as thermal expansion and conductivity, electrical polarization, magnetization, mechanical elasticity, etc. Consequently, charge-density analysis is nowadays not limited to find ever more accurate refinement models, but it has also accepted the challenge of helping to engineer application-oriented molecules and materials. Therefore, in the last years, experimentally estimated charge densities have been of interest not only to the crystallographic community, but also to the broader fields of chemistry and materials science. In particular, detailed insight can be gained on the optics or magnetism of molecular materials when their crystalline macroscopic behavior is correlated with structural and electronic features (as obtained from X-ray diffraction or first-principle quantum-mechanical calculations) of their molecular and sub-molecular building blocks. In this mini review, such relationships are outlined. The emphasis is put on amino acids and coordination polymers as prototypical systems, but the conclusions are general in the sense that they may apply to different classes of molecular systems. This work can also be read as a tutorial review by the newcomers interested in the entanglement between charge-density analysis and crystal engineering.