Article pubs.acs.org/ac
Point Group Symmetry Determination via Observables Revealed by Polarized Second-Harmonic Generation Microscopy: (2) Applications Monique A. van der Veen,*,†,‡ Frederik Vermoortele,† Dirk E. De Vos,† and Thierry Verbiest‡ †
Centre for Surface Chemistry and Catalysis, KU Leuven, University of Leuven, 3001-Leuven, Belgium Molecular Electronics and Photonics, KU Leuven, University of Leuven, 3001-Leuven
‡
S Supporting Information *
ABSTRACT: In this work, the theory presented in part 1 (van der Veen, M. A.; Vermoortele, F.; De Vos, D. E.; Verbiest, T. Anal. Chem. 2012, DOI: 10.1021/ac300936q) for determination of the point groups symmetry based on easily distinguishable observables present in simple polarization dependent tests in second harmonic generation microscopy is tested. It is shown experimentally that the methodology can be applied for point group symmetry determination for a variety of structures among which molecular crystals and host/ guest systems where the symmetry of the guest molecules cannot be inferred from conventional diffraction methods. Uniquely, this second-harmonic generation based method can discriminate between chiral and achiral structures regardless of their orientation. The method allows for in situ and in vivo studies with spatial resolution. used for a final assignment. We tested the methodology by applying it to two periodic structures of known symmetry: one is a metal−organic framework of D3 symmetry, namely, MIL103;9 the other is an organic molecular crystal of C2v symmetry.10 The metal−organic framework MIL-103 is built up from lanthanide ions and 1,3,5-benzene-tris-benzoate.9 We used the presented methodology to determine the point group symmetry of as-synthesized MIL-103(Sm).11 Small crystallites of ∼1 μm were obtained. As monocrystalline, nonaggregated structures are necessary for a correct point group symmetry assignment, we followed the strategy proposed by Brasselet et al.2 To avoid aggregated particles, which typically are more intense, only particles of low intensity corresponding to the size of the crystallites were selected from the SHG-images for further analysis (see Figure 1i). For a significant number of these single structures, we plotted the average SHG-intensity for the four polarization dependent tests described in part 11 per structure. In these tests, either the plane of polarization of the incident light or the sample is rotated over 360°. Examples for two crystallites are given in Figure 1a−h. The next step is to count the observables present in each polarization plot, namely, the number of times the function goes to zero (number of zeros, abbreviated “z”) and the number of times the function behaves as an even function around a maximum or minimum (number of symmetry axes, abbreviated “sa”). These are also depicted in Figure 1a−h. For a
S
tructural information is important to unravel the structure− activity relationship of functional compounds and materials. The point group or symmetry relations of a material are directly linked to its properties. Second-harmonic generation (SHG) is a technique especially sensitive to the symmetry relations of a system. As such it is ideally suited for noncentrosymmetric structures, for which SHG is especially sensitive, to determine the point group symmetry. In part 1 of this study,1 we developed a methodology for this determination that can be applied on standard SHG-microscopes. In this part of the study, the method is tested on materials with known point group symmetry and tested on a material of which the point group could not be inferred with standard diffraction based methods. Additionally we show that the method can be used to discriminate between chiral and achiral materials. Moreover, as the method is based on imaging and is generally only sensitive to noncentrosymmetric structures, it allows one to deduce the symmetry in situ in complex systems2,3 and in vivo in biological systems.4−6
■
APPLICATIONS
In this study, a wide-field SHG-microscope coupled to a femtosecond Ti-sapphire laser was used. However, the methodology described in part 11 is applicable to both widefield microscopes and scanning microscopes (the latter with N.A.
