Polymeric siloxanes with a substituent and the spirobenzopyran moiety: Effect of polar substituent on the photochromic properties

Oligomeric polysiloxanes having the spirobenzopyran moiety and various substituents were prepared. The polymers were photochromic; the color changed from colorless to red on UV irradiation and the color faded on visible irradiation or on standing in the dark. The thermal decoloration was depressed by the polar substituents. However, the effect of the bulky substituent, which had been observed in the rigid siloxane resins, was not detected. In the inorganic/organic hybrid siloxane gels, the silica/(spirobenzopyran-bonding polysiloxane) system and the (spirobenzopyran-bonding silica)/polysiloxane system, the thermal decoloration was also affected by the polar substituent introduced into the polysiloxane portion. The effect was larger in the first case than in the second. The result can be interpreted by the term of the preferential diffusion of polar spirobenzopyran to the vicinity of the polar substituents during the gelation.     

Rotaxane Synthesis by an End-Capping Strategy via Swelling Axle-Phenols

A method for rotaxane synthesis by enlargement of the size of the terminal phenol group of the axle component by aromatic bromination has been developed. This method may be regarded as an end-capping strategy involving the swelling of the phenol group at the axle terminal. The advantages of the present strategy include: ready availability of axle components with a variety of swelling precursors, wide product scope (19 examples given including a [3]rotaxane), mild conditions for the swelling process, rich potential for the derivatization of the brominated rotaxanes, and possible release of the axle component by degradative dethreading of the thermally stable brominated rotaxanes under the basic conditions.     

Akaike's Information Criterion for Stoichiometry Inference of Supramolecular Complexes

“How do we decide the stoichiometry of host–guest complexes?” This question has long been answered by the Job plot since its first report in 1928. However, as the Job plot was claimed to be misleading in 2016, the question became an open question again and called for renewed investigations. An information-theoretic approach, called Akaike's information criterion, is introduced in this study to select the best model of host–guest complexes, which can rank the models with weight of evidence. A few test cases with unique cylindrical hosts were examined to demonstrate the applicability of the information-theoretic method. Consequently, reasonable views over the thermodynamic behaviors of dumbbell-and-cylinder complexes were obtained. Akaike's information criterion can be a useful and superior alternative to statistical null hypothesis testing, which was proposed as a remedy in place of the Job plot.     

Does Chiral Sensitivity of a Structure Depend on the Metal Core? Alkali Ion Complexes of Cyclo(Tyr-Tyr)

Alkali metal complexes of cyclic dipeptide cyclo Tyr-Tyr have been studied under cryogenic ion trap conditions. Their structure was obtained by combining Infra-Red Photo-Dissociation (IRPD) and quantum chemical calculations. The structural motif strongly depends on the relative chirality of the tyrosine residues. For residues of identical chirality, the cation interacts with one amide oxygen and one of the aromatic rings only; the distance between the aromatic rings does not change with the nature of the metal. In contrast, for residues of opposite chirality, the metal cation is located in between the two aromatic rings and interacts with both of them. The distance between the two aromatic rings strongly depends on the metal. Electronic spectra obtained by Ultra Violet Photodissociation (UVPD) spectroscopy and analysis of the UV photo-fragments shed light on the excited state deactivation processes, which depend on both the chirality of the residue and that of the metal ion core. Na⁺ stands out by the presence of low-lying charge transfer states resulting in the broadening of the electronic spectrum.     

Porous Mn2+ Magnet with a Pt−Cl Framework: Correlation between Water Vapor Adsorption/Desorption and Slow Magnetic Relaxation

We report the water adsorption/desorption behavior and dynamic magnetic properties of the Pt−Cl chain complex [{[Pt(en)₂][PtCl₂(en)₂]}₃][{(MnCl₅)Cl₃}₂] ⋅ 12H₂O ( 1 ). Upon heating 1 in a vacuum, we obtained the dehydrated form [{[Pt(en)₂][PtCl₂(en)₂]}₃][{(MnCl₅)Cl₃}₂] ( 1DH ). The framework structures of 1 and 1DH are identical, and both complexes underwent slow magnetic relaxation. However, the magnetic relaxation times for 1DH were shorter than those for 1 , meaning that the dynamic magnetic properties were controlled upon water vapor adsorption/desorption. From detailed analyses of the dynamic magnetic behavior, a phonon-bottleneck effect contributes to the magnetic relaxation processes. We discuss the mechanism for the changes in the magnetic relaxation processes upon dehydration in terms of the heat capacity and thermal conductivity.