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.

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.

High pressure studies on structural and secondary relaxation dynamics in silyl derivative of D-glucose.

In this paper, broadband dielectric spectroscopy was applied to investigate molecular dynamics of 1,2,3,4,6-penta-O-(trimethylsilyl)-D-glucopyranose (S-GLU) at ambient and elevated pressures. Our studies showed that apart from the structural relaxation, one well resolved asymmetric secondary process (initially labeled as ß) is observed in the spectra measured at p = 0.1 MPa. Analysis with the use of the coupling model and criterion proposed by Ngai and Capaccioli indicated that the ß-process in S-GLU is probably a Johari-Goldstein relaxation of intermolecular origin. Further high pressure experiments demonstrated that there are in fact two secondary processes contributing to the ß-relaxation. Therefore, one can postulate that the coupling model is a necessary, but not sufficient criterion to identify the true nature of the given secondary relaxation process. The role of pressure experiments in better understanding of the molecular origin of local mobility seems to be much more important. Interestingly, our research also revealed that the structural relaxation in S-GLU is very sensitive to compression. It was reflected in an extremely high pressure coefficient of the glass transition temperature (dTg/dp = 412 K/GPa). According to the literature data, such a high value of dTg/dp has not been obtained so far for any H-bonded, van der Waals, or polymeric glass-formers.

Observation of the nearly constant loss in super rigid saccharides: in search of a hidden crossover in dynamics deep in the glassy state.

The molecular dynamics of three saccharides: D-glucose, 1,6-anhydro-D-glucose (levoglucosan) and 1,6:2,3-dianhydro-ß-D-mannopyranose of various degrees of freedom, number of hydroxyl groups and internal structures was investigated over a wide range of temperatures and frequencies by means of Broadband Dielectric Spectroscopy (BDS). Despite the pronounced variety in the physicochemical properties of the carbohydrates, no change in the shape of the structural relaxation process was observed in the vicinity of the glass transition temperature (ß(KWW) = 0.5). On the other hand further studies of the Debye-Stokes-Einstein relationship between dc conductivity and structural dynamics revealed some significant changes connected with the ability to form strong H-bonded structures. Moreover the presence of nearly constant loss (NCL) at moderate frequencies and just below the T(g) in the glassy state of levoglucosan and 1,6:2,3-dianhydro-ß-D-mannopyranose was noticeable. We followed the temperature evolution of ε'' located at frequencies f = 0.1 kHz and f = 1 kHz, where the NCL is detected. Interestingly, a clear change in the dynamics far below the glass transition was observed in both compounds. This crossover (T(c)), found in different materials, and studied by various experimental techniques, is usually interpreted as being caused by the freezing of the Johari-Goldstein (JG) relaxation process. Alternatively it can also be due to the increasing anharmonicity in the density of vibrational states. Interestingly, it was shown that the slope of ε''(T) measured above the T(c) slightly changes while below the T(c) stays constant after physical aging. This is related to the densification of the sample that might result in steric hindrance and suppression of some kind of motion occurring in the glassy state, involving the larger parts of the molecules.

The kinetics of mutarotation in L-fucose as monitored by dielectric and infrared spectroscopy.

Fourier Transform Infrared Spectroscopy and Broadband Dielectric Spectroscopy are combined to trace kinetics of mutarotation in L-fucose. After quenching molten samples down to temperatures between T = 313 K and 328 K, the concentrations of two anomeric species change according to a simple exponential time dependence, as seen by an increase in absorbance of specific IR-vibrations. In contrast, the dielectric spectra reveal a slowing down of the structural (α-) relaxation process according to a stretched exponential time dependence (stretching exponent of 1.5 ± 0.2). The rates of change in the IR absorption for α- and ß-fucopyranose are (at T = 313 K) nearly one decade faster than that of the intermolecular interactions as measured by the shift of the α-relaxation. This reflects the fact that the α-relaxation monitors the equilibration at a mesoscopic length scale, resulting from fluctuations in the anomeric composition.