Orientation of an organic anion and second-staging structure in layered double-hydroxide intercalates

Mg-Al layered double hydroxides (LDHs) intercalated with an anionic azobenzene derivative (AzAA) were prepared, and the arrangement of the incorporated AzAA was investigated by varying the starting AzAA/LDH ratios for two types of LDHs with different Mg/Al mole ratios, Mg/Al = 2.0 (LDH2) and 3.0 (LDH3). The observed basal spacings of the complexes, 8 and 23-25 Angstrom, were ascribed to horizontal (parallel to the layers) and vertical (perpendicular to the layers) orientations of the incorporated AzAA, respectively. In general, the horizontal orientation was observed with low AzAA/LDH ratios and the vertical orientation was observed with higher ratios. For the LDH2 complexes, a large basal spacing of 33 Angstrom was observed with intermediate AzAA/LDH ratios, that is, 0.5 and 0.75, which was interpreted as an alternate stacking of the two types of interlayers ("1:1 ordered interstratification" or "second staging"). In the case of the LDH3 complexes having a smaller layer-charge density, second staging was not found with the intermediate AzAA/LDH ratios; instead, two separated phases, 8 and 23 Angstrom, were obtained. TEM images for the complexes with basal spacings of 33 and 25 Angstrom were consistent with the proposed structure model.