NH₂-MIL-53(Al): A High-Contrast Reversible Solid-State Nonlinear Optical Switch

Abstract

ABSTRACT:The metal−organic framework NH 2 -MIL-53(Al) is the first solid-state material displaying nonlinear optical switching due to a conformational change upon breathing. A switching contrast of at least 38 was observed. This transition originates in the restrained linker mobility in the very narrow pore configuration.T he field of nonlinear optics has experienced an everincreasing interest due to multiple applications in information processing, electro-optical switching, and telecommunications.1,2 While commercial nonlinear optical (NLO) materials are still largely inorganic, organic compounds and metal−organic complexes have attracted much attention.3 As a result, during the past decade, the possibility of changing the quadratic or second-order NLO response by an external stimulus has been increasingly addressed. A molecule or solid able to change its NLO response reversibly is called an "NLO switch". Several families of molecules and metal−organic complexes display this property in the liquid phase.4−9 NLO switches in the solid state, however, are much more scarce. A necessary requirement for a quadratic NLO material is that it be noncentrosymmetric. While it is easy to synthesize individual noncentrosymmetric molecules and metal−organic complexes, these typically dipolar entities often organize in an antiparallel fashion into centrosymmetric crystals. A common strategy to obtain polar order on the macroscopic level is via electric field poling of polymers containing dipolar chromophores. The change of centrosymmetric to noncentrosymmetric order is associated with a large change in quadratic NLO response, but the change is not readily reversible.10 As a consequence, hardly any reversible solid-state second-order NLO switches have been reported to date: only anil crystals (Schiff bases, based on photoswitching)11−14 and thin films of ruthenium complexes (based on redox switching) 15 have been shown to display a certain degree of reversible switching. For these materials, the NLO contrast, defined as the ratio of the second harmonic generation (SHG) intensities (see below) before and after the external stimulus, varies by a factor between 1.3 and 10. This limited contrast is due to the fact that all reported NLO switches essentially retain their noncentrosymmetric order upon switching. Herein we report that the metal−organic framework (MOF) NH 2 -MIL-53(Al), which contains Al 3+ and 2-aminoterephthalate, is a novel solid reversible NLO switch. The switching capacity is due to a reversible conformational change that greatly diminishes the polar ordering of the material.MOFs have also attracted a lot of scientific attention in the field of nonlinear optics, where the design of several noncentrosymmetric frameworks has been reported. 16−19 In a single case, the SHG intensity of a MOF could be modulated by cation exchange, with a contrast of 1.75. 20 However, the effects of organic guest molecules on the SHG intensity have not been reported to date.A special class of MOFs are those that can reversibly alter the...

The oxamate route, a versatile post-functionalization for metal incorporation in MIL-101(Cr): Catalytic applications of Cu, Pd, and Au

“…After that, extensive work was devoted to the less stable IRMOF series bearing different functionalized organic ligands [15,16]. Stability issues were solved by combining late transition metals and functional benzene dicarboxylates (X-BDC; X = -NO 2 , -NH 2 , -Br, -OCH 3 , -CH 3 , -F, -Cl) [17][18][19]. The use of several functional organic sites within a single MOF crystal by approaches like mix-linker or post-synthetic exchange has also attracted a lot of interest during the last few years [12,[20][21][22][23][24][25].…”

An Electric Field Induced Breath for Metal–Organic Frameworks

“…6 Interestingly, for the amine functionalized NH 2 -MIL-53(Al) in the closed pore form an ordering of the −NH 2 substituents is observed, resulting in such a polar space group. 7 In contrast to that, for the centrosymmetric MIL-53(Cr) with its purely induced polarization, on the other hand, a transformation like the pore closing would not change the interaction of the induced dipole moments with the homogeneous field itself (see Figure 2). However, for any dielectric a (usually weak) deformation due to induced polarization is observed in an electric field, which is usually referred to as electrostriction.…”

Synthesis, Structure, and Selected Properties of Aluminum-, Gallium-, and Indium-Based Metal-Organic Frameworks

“…In the presence of a mixture of CO 2 and CH 4 , Al-MIL-53-NH 2 demonstrated excellent selectivity in the separation of these gases [115]. This particular MOF exhibits also a remarkable transition from a non-centrosymmetric crystal structure to a centrosymmetric crystal structure upon adsorption of toluene [116]. The non-centrosymmetric form is SHG active, and hence this MOFs could be potentially useful for the construction of a sensing device [116].…”

“…This particular MOF exhibits also a remarkable transition from a non-centrosymmetric crystal structure to a centrosymmetric crystal structure upon adsorption of toluene [116]. The non-centrosymmetric form is SHG active, and hence this MOFs could be potentially useful for the construction of a sensing device [116]. In order to achieve more control over this breathing effect, mixtures of linker molecules (i.e., terephthalic and 2-aminoterephthalic acid) were employed [87], and solid solutions were obtained.…”