Abstract
We demonstrate the formation of uniform and oriented metal–organic frameworks using a combination of anion effects and surface chemistry. Subtle but significant morphological changes result from the nature of the coordinative counteranion of the following metal salts: NiX2 with X = Br–, Cl–, NO3–, and OAc–. Crystals could be obtained in solution or by template surface growth. The latter results in truncated crystals that resemble a half structure of the solution-grown ones. 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 MOFs are grown on Si/SiOx substrates modified with an organic monolayer or on glass substrates covered with a transparent conductive oxide (TCO). 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. 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. 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. These “colored” crystals displayed polarized emission (red) with a high polarization ratio because of the alignment of these dyes imposed by the crystallographic structure. A second-harmonic generation (SHG) study revealed Kleinman symmetry-forbidden nonlinear optical properties. These surface-bound and oriented SHG-active MOFs have the potential for use as single nonlinear optical (NLO) devices.
Highlights
The morphology and uniformity of crystals are fascinating structural properties that are difficult to predict and design.[1−3] The relationship between the structure of the molecular components and their crystal appearance is often not obvious.[4,5] Crystal design often includes the use of additives or modulators
Wang, Sun, and Yaghi demonstrated that the use of nucleation inhibitors resulted in the formation of covalent organic frameworks suitable for single-crystal X-ray diffraction.[6]
Interface chemistry played a pivotal role in fundamental studies in crystal nucleation by Leiserowitz, Lahav, Weissbuch, and Sagiv using Langmuir−Blodgett films of amino acids.[9]
Summary
The morphology and uniformity of crystals are fascinating structural properties that are difficult to predict and design.[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.[4,5] Crystal design often includes the use of additives or modulators. The crystals are formed under solvothermal reaction conditions Their macroscopic structures were systematically changed by (i) solution versus surface chemistry, (ii) varying the anion, and (iii) changing the concentration of both the metal salt precursor and the organic ligand. The surface-bound crystals are truncated and resemble one-half of the morphology of those grown in solution and can be detached by sonication These crystals, grown on an organic monolayer on silicon and quartz substrates or directly on bare indium−tin-oxide substrates, are orientated with their continuous channels perpendicular to the substrate surface. Both the solution- and the surface-bound crystals showed strong Kleinman symmetry-forbidden second-order nonlinear properties.[69]
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