(4,5) Crystal design often includes the use of additives or modulators. (1−3) The relationship between the structure of the molecular components and their crystal appearance (e.g., unit cell structure and macroscopic crystalline shape) is often not obvious. The morphology and uniformity of crystals are fascinating structural properties that are difficult to predict and design. These surface-bound and oriented SHG-active MOFs have the potential for use as single nonlinear optical (NLO) devices.
A second-harmonic generation (SHG) study revealed Kleinman symmetry-forbidden nonlinear optical properties.
These “ colored” crystals displayed polarized emission (red) with a high polarization ratio because of the alignment of these dyes imposed by the crystallographic structure. To demonstrate the accessibility and porosity of the macroscopically oriented channels, a chromophore (resorufin sodium salt) was successfully embedded into the channels of the crystals by diffusion from solution, resulting in fluorescent crystals. A propeller-type arrangement of the nonchiral ligands around the metal center affords a chiral structure with two geometrically different helical channels in a 2:1 ratio with the same handedness. Regardless of the different morphologies, the crystallographic packing is nearly identical and is not affected by the type of anion or by solution versus the surface chemistry. The MOFs are grown on Si/SiOx substrates modified with an organic monolayer or on glass substrates covered with a transparent conductive oxide (TCO). The oriented surface-bound metal–organic frameworks (sMOFs) are obtained via a one-step solvothermal approach rather than in a layer-by-layer approach. The latter results in truncated crystals that resemble a half structure of the solution-grown ones. Crystals could be obtained in solution or by template surface growth. Subtle but significant morphological changes result from the nature of the coordinative counteranion of the following metal salts: NiX 2 with X = Br –, Cl –, NO 3 –, and OAc –. We demonstrate the formation of uniform and oriented metal–organic frameworks using a combination of anion effects and surface chemistry.
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