Author:

Abstract:

Second-order nonlinear optical effects have the powerful property that they are only allowed when symmetry is broken, i.e. at interfaces and for non-centrosymmetric materials. The result is 1) that they can selectively study interfaces present between centrosymmetric media, and 2) that they are extremely sensitive to symmetry properties of non-centrosymmetric crystals. The external surface of metal organic frameworks (MOFs) plays a critical role in many applications as it is the first barrier for entering molecules. It would be highly desirable to directly obtain chemical information from the surface. Such information can be obtained using sum frequency generation (SFG), a surface specific vibrational spectroscopy technique. We use SFG to study the external surface of ZIF-8. Furthermore, the extreme sensitivity of nonlinear optics to symmetry is used to determine with second-harmonic generation microscopy the vapour pressure modulated phase transitions of the aluminium metalorganic framework CAU-10 family.