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A new, cheap, easy-to-synthesize and air-stable photoswitchable nickel(II) complex, QTNiNO2, is reported. The metal centre in QTNiNO2 is coordinated by a nitro group and a [2-methyl-8-aminoquinoline]-1-tetralone ligand. The compound crystallizes in the tetragonal space group I41/a with one complex molecule comprising the asymmetric unit, and the crystals are stable under ambient conditions. Irradiation of the solid-state form of QTNiNO2 with 530–660 nm LED light at 160 K converts the ambidentate nitro moiety fully to the nitrito linkage isomer which is stable up to around 230 K, as indicated by IR spectroscopy measurements. The structures of all species present in the examined crystals and their thermal stability were confirmed via X-ray multi-temperature and photocrystallographic experiments. The impact of temperature on the (photo)isomerization reaction taking place in a single crystal was additionally investigated. The experimental results are supported by computational analyses of crystal packing and intermolecular interactions that influence the isomerization process studied.

We report on the X-ray studies of freely suspended hexatic films of three different liquid crystal compounds. By applying angular X-ray cross-correlation analysis (XCCA) to the measured diffraction patterns the parameters of the bond-orientational (BO) order in the hexatic phase were directly determined. The temperature evolution of the BO order parameters was analyzed on the basis of the multicritical scaling theory (MCST). Our results confirmed the validity of the MCST in the whole temperature range of the existence of the hexatic phase for all three compounds. The temperature dependence of the BO order parameters in the vicinity of the hexatic-smectic transition was fitted by a conventional power law with a critical exponent β ≈ 0.1 of extremely small value. We found that the temperature dependence of higher order harmonics of the BO order scales as the powers of the first harmonic, with an exponent equal to the harmonic number. This indicates a nonlinear coupling of the BO order parameters of different order. We demonstrate that compounds of various compositions, possessing different phase sequences at low temperatures, display the same thermodynamic behavior in the hexatic phase and in the vicinity of the smectic-hexatic phase transition.

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Tetradental cis-enaminoketone Ni (II) complexes with different molecular shape have been synthesized. Intramolecular hydrogen bonds, which stiffen the mesogenic core and restrict rotation of some molecular parts, have been introduced in these compounds. In the case of molecules with two hydrogen bonds and alkoxy terminal chains filling the inner molecular space, the uncommon phase sequence Iso-D(h)-Iso(re)-SmA (series III-3) was detected. For the first time, it was observed that the isotropic re-entrant (Iso(re)) phase (short-range order) is separating the columnar (D) (high-temperature) and the lamellar (SmA) (low-temperature) phases, both revealing long-range ordered structures.

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Two photoswitchable nickel­(II) nitro coordination compounds and their copper­(II) analogues are reported. In all these systems, the metal center is chelated by (N,N,O)-donor ligands containing either 2-picolylamine or 8-aminoquinoline fragments. The studied compounds were thoroughly investigated using crystallographic and spectroscopic techniques supplemented by computational analysis. They are easy to synthesize and stable, and all compounds undergo the nitro group isomerization reaction. Nevertheless, there are significant differences between the copper and nickel systems regarding their structural and switchable properties. According to the solid-state IR spectroscopy results, 400–660 nm light irradiation of the ground-state (η2-O,O′)-κ-nitrito copper­(II) complexes at 10 K induces a rather moderate conversion to a metastable linkage isomer, which is visible only up to approximately 60–80 K. In turn, upon visible light irradiation (ca. 530 nm excitation wavelength), the ground-state nitro isomers of the examined nickel­(II) complexes transform into the endo-nitrito forms. It was possible to achieve about 35% conversion for both nickel­(II) systems and to determine the resulting crystal structures at 160 K in the case of single crystals after 30–45 min of exposure to LED light (crystals decayed with longer irradiation), and roughly 95% conversion was achieved for thin-film samples as indicated by the IR spectroscopy results. Traces of the endo-nitrito linkage isomers remained up to 200–220 K, and the isomerization reaction was proven to be fully reversible.