title: Second harmonic generation microscopy reveals hidden polar organization in fluoride doped MIL-53(Fe).
abstract: Polar metal-organic frameworks have potential applications as functional non-linear optical, piezoelectric, pyroelectric and ferroelectric materials. Using second harmonic generation microscopy we found that fluoride doping of the microporous iron(iii) terephthalate MOF MIL-53(Fe) induces a polar organization in its structure, which was not previously detected with XRD. The polar order is only observed when both fluoride and guest molecules are present, and may be related to a complex interplay between the adsorbates and the framework, leading to a modification of the positioning of fluoride in the inorganic Fe-chains. Combined polarized second harmonic generation microscopy and scanning pyroelectric microscopy show that the polar axis is unidirectional and of the same sense over the whole crystal, extending up to 100 micrometers. This finding shows how MOF materials can be endowed with useful properties by doping MOFs with fluoride.
authors.affiliation: Centre for Surface Chemistry and Catalysis, University of Leuven, Belgium.
authors.forename: Karen
authors.initials: K
authors.lastname: Markey
authors.affiliation: Soft Matter and Biophysics, University of Leuven, Belgium.
authors.forename: Tristan
authors.initials: T
authors.lastname: Putzeys
authors.affiliation: Institut Lavoisier, UMR 8180 CNRS - University of Versailles St-Quentin-en-Yvelines, France.
authors.forename: Patricia
authors.initials: P
authors.lastname: Horcajada
authors.affiliation: Institut Lavoisier, UMR 8180 CNRS - University of Versailles St-Quentin-en-Yvelines, France.
authors.forename: Thomas
authors.initials: T
authors.lastname: Devic
authors.affiliation: Institut Lavoisier, UMR 8180 CNRS - University of Versailles St-Quentin-en-Yvelines, France.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Guillou
authors.affiliation: Soft Matter and Biophysics, University of Leuven, Belgium.
authors.forename: Michael
authors.initials: M
authors.lastname: Wübbenhorst
authors.affiliation: Molecular Electronics and Photonics, University of Leuven, Belgium.
authors.forename: Stijn
authors.initials: S
authors.lastname: Van Cleuvenbergen
authors.affiliation: Molecular Electronics and Photonics, University of Leuven, Belgium.
authors.forename: Thierry
authors.initials: T
authors.lastname: Verbiest
authors.affiliation: Centre for Surface Chemistry and Catalysis, University of Leuven, Belgium.
authors.forename: Dirk E
authors.initials: DE
authors.lastname: De Vos
authors.affiliation: Catalysis engineering, TU Delft, The Netherlands. m.a.vanderveen@tudelft.nl.
authors.forename: Monique A
authors.initials: MA
authors.lastname: van der Veen