NH 2 -MIL-53(Al): A High-Contrast Reversible Solid-State Nonlinear Optical Switch Pablo Serra-Crespo, †,⊥ Monique A. van der Veen, ‡,§,⊥ Elena Gobechiya, ‡,⊥ Kristof Houthoofd, ‡ Yaroslav Filinchuk, ∥ Christine E. A. Kirschhock, ‡ Johan A. Martens, ‡ Bert F. Sels, ‡ Dirk E. De Vos,* ,‡ Freek Kapteijn, † and Jorge Gascon* ,† † Catalysis Engineering, Chemical Engineering Department, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands ‡ Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, University of Leuven, 3001 Leuven, Belgium § Molecular Electronics and Photonics, Department of Chemistry, University of Leuven, 3001 Leuven, Belgium ∥ Institute of Condensed Matter and Nanosciences, Universite ́ Catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium * S Supporting Information 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 ever- increasing interest due to multiple applications in information processing, electro-optical switching, and tele- communications. 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 their framework structure when guest molecules are introduced. This results in phenomena such as breathing 21,22 or gate opening, 23,24 where pores open or contract upon adsorption. Examples of a breathing material are MIL-53 and its functionalized derivatives. 25−27 MIL-53 is built from MO 4 (OH) 2 octahedra (M = Fe 3+ , Cr 3+ , Al 3+ , Ga 3+ ) and 1,4- benzenedicarboxylate (terephthalate) linkers. In this way, a crystalline material with one-dimensional diamond-shaped pores is formed. During the past few years, we have intensively studied the adsorptive and catalytic properties of the amine- functionalized version of MIL-53(Al), 28−31 hereafter denoted as NH 2 -MIL-53(Al). Its outstanding CO 2 selectivity together with a fair capacity and high thermal stability make this flexible material an excellent candidate for the selective separation of CO 2 from different gas mixtures. Very recently we demon- strated that the adsorptive separation performance of NH 2 - MIL-53(Al) is mostly due to a delicate interplay of weak dispersion forces that control the flexibility of the framework: in contrast to its unfunctionalized counterpart, the unit cell contracts to a very narrow pore (vnp) configuration after solvent removal as a result of −NH 2 ···[AlO 6 ] ∞ hydrogen- Received: January 20, 2012 Published: May 11, 2012 Communication pubs.acs.org/JACS © 2012 American Chemical Society 8314 dx.doi.org/10.1021/ja300655f | J. Am. Chem. Soc. 2012, 134, 8314−8317