Studies on dynamics and isomerism in supercooled photochromic compound Aberchrome 670 with the use of different experimental techniques.

Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and Broadband Dielectric (BD) spectroscopies were applied to investigate the thermal, structural, photochemical and dynamical properties of a fulgide-type photochromic compound, Aberchrome 670 (Ab670). In the original crystals, characterized by a pale yellow color, molecules take the E conformation. However, upon UV irradiation of either the crystalline or glassy compound, it isomerizes to the closed (C) form, characterized by the intense red tone. Although, we have found that such conversion is not complete (far below 100%). It was shown that due to UV irradiation as well as heating of the studied fulgide to high temperature (above the melting point), the Z isomer is formed. Further FTIR measurements performed on the UV irradiated and molten compound indicated that upon annealing of the sample in the vicinity of the glass transition temperature the Z isomer reverts back to the original E form. The final confirmation of this supposition has come from BDS studies, where the strong shift of the structural relaxation process during time-dependent isothermal measurements was noticed. One can add that a similar pattern of behavior has been observed previously by some of us in the case of tautomerism or mutarotation [Z. Wojnarowska et al., J. Chem. Phys., 2010, 133, 094507; W. Kossack et al., J. Chem. Phys., 2014, 140, 215101; P. Wlodarczyk et al., J. Phys. Chem. B, 2009, 113, 4379-4383; P. Wlodarczyk et al., J. Non-Cryst. Solids, 2010, 356, 738-742]. From the analysis of the time variation of the structural relaxation times, the activation barrier, EA = 18 kJ mol-1, for Z to E isomerization in Ab670 was calculated. Interestingly, it agrees well with the one determined for a similar kind of transformation in stilbenes. Therefore, we found that dielectric spectroscopy can be a very useful technique to track Z to E interconversion in the highly viscous supercooled state. Consequently, a unique opportunity to follow this kind of isomerism at high pressures, high electric fields and under nanometric spatial confinement in pure supercooled compounds appeared.

Towards a better comprehension of interactions in the crystalline N-acetylbenzylamine and its sulphur analogue N-benzyl-ethanethioamide. IR, Raman, DFT studies and Hirshfeld surfaces analysis.

This paper presents the investigation results of the polarized IR spectra of the hydrogen bond in crystals of N-acetylbenzylamine and its sulphur analogue N-benzyl-ethanethioamide. The spectra were measured at 298 and 77 K by a transmission method, with the use of polarized light. The Raman spectroscopy, Hirshfeld surfaces analysis and DFT studies have been also reported. Theoretical calculations of the isolated molecule were performed by using density functional theory (DFT) method at B3LYP/6-311(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311++G(3df,2pd) basis set levels. The geometrical parameters of analyzed compounds are in good agreement with the XRD experiment. The vibrational frequencies were calculated and subsequently values have been compared with the experimental Infrared and Raman spectra. It has been shown that the observed and calculated frequencies are found to be in good agreement, as well as the analysis of the Hirshfeld surface has been well correlated to the spectroscopic studies. Additionally, the highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELUMO), the energy gap between EHOMO and ELUMO (ΔEHOMO-LUMO), molecular electrostatic potential and global reactivity descriptors viz. chemical potential, global hardness and electrophilicity have been calculated. In N-acetylbenzylamine the presence of the N-benzylamide fragment is essential for activity.

Anhydrosaccharides-A new class of the fragile plastic crystals.

In this paper, 1,6-anhydro-ß-D-glucopyranose (anhGLU), 1,6-anhydro-ß-D-mannopyranose (anhMAN), and 1,6-anhydro-ß-D-galactopyranose (anhGAL), three new materials that form the Orientationally Disordered Crystal (ODIC) phase, have been thoroughly investigated using various experimental techniques. All measurements clearly indicated that these compounds possess a series of very interesting physical properties that are considerably different than those reported for ordinary plastic crystals. X-Ray diffraction investigations have revealed enormously long-range static correlations between molecules, reaching even 120 Å. Moreover, dielectric studies showed that besides Freon 113, the investigated anhydrosaccharides are the most fragile systems that form the ODIC phase. Further analysis of Fourier transform infrared spectra indicated that such peculiar behavior of anhydrosaccharides might be closely related to multidirectional H-bonds of various strengths that most likely affect the number of available conformations, density states, and the potential barriers in the energy landscape of these compounds. This is consistent with the results from previous reports [L. C. Pardo, J. Chem. Phys. 124, 124911 (2006) and Th. Bauer et al., J Chem. Phys. 133, 144509 (2010)] showing that the higher fragility of Freon 112 as well as a mixture of 60% succinonitrile and 40% glutaronitrile (60SN-40GN) can be closely related to the enhanced conformational ability and additional disorder introduced by various substituents, which further make energy landscape more complex. Finally, by studying the properties of 2,3,4-tri-O-acetyl-1,6-anhydro-ß-D-glucopyranose (ac-anhGLU) it was found that besides the shape of the molecules, H-bonds or generally strong intermolecular interactions are extremely important parameters contributing to the ability to form the plastic phase. This is in line with current observations that in most cases the ODIC phase is created in highly interacting compounds.

H/D isotopic recognition mechanism in hydrogen-bonded crystals of 3-methylacetanilide and 4-methylacetanilide.

Polarized IR spectra of 3- and 4-methylacetanilide as well as their deuterium derivative crystals were measured at 293K and at 77K by a transmission method. The obtained results were interpreted within the limits of the "strong-coupling" theory. This approach facilitated the understanding of the H/D isotopic, temperature and dichroic effects observed in the hydrogen bond IR spectra. The existence of H/D isotopic "self-organization" phenomenon, depending on the non-random distribution of protons and deuterons in the crystal lattices of isotopically diluted samples of a compound was ascertained. This effect resulted from the dynamical co-operative interactions involving the closely spaced hydrogen bonds, each belonging to a different chain of associated 3- and 4-methylacetanilide molecules. In the case of 4-methylacetanilide crystals weaker but non-negligible exciton coupling also involved adjacent hydrogen bonds in each molecular chain and the H/D isotopic "self-organization" mechanism concerned at least four hydrogen bonds from each unit cell. The source of these phenomena was ascribed to the molecular electronic properties determined by aromatic rings linked to nitrogen atoms of the amide fragments.

H/D isotopic recognition in hydrogen-bonded systems: strong dynamical coupling effects in the polarized IR spectra of 3-methylthioacetanilide and 4-methylthioacetanilide crystals.

This paper presents the investigation results of the polarized IR spectra of the hydrogen bond in crystals of 3- and 4-methylthioacetanilide. The spectra were measured at 293 and 77 K by a transmission method, with the use of polarized light. The main spectral properties of the crystals can be interpreted satisfactorily in terms of the "strong-coupling" theory, on the basis of the hydrogen bond centrosymmetric dimer model. The spectra revealed that the strongest vibrational exciton coupling involved the closely spaced hydrogen bonds, each belonging to a different chain of associated 3- and 4-methylthioacetanilide molecules. A weaker exciton coupling involved the adjacent hydrogen bonds in each individual chain. It was proven that a nonrandom distribution of the protons and deuterons took place in the lattices of isotopically diluted crystalline samples of 3- and 4-methylthioacetanilide. In each case, the H/D isotopic "self-organization" mechanism involved all four hydrogen bonds from each unit cell.