Re-entrant supramolecular interactions in inverse-melting α-cyclodextrin·4-methylpyridine·water mixtures: an NMR study.

Inverse melting α-cyclodextrin·4-methylpyridine·H2O ternary mixtures were studied via solution, high-resolution magic-angle-spinning and 2D-NOESY NMR spectroscopy with the aim of unveiling the supramolecular driving force for the unusual inverse-melting phase transition. The role of solvent-solvent interactions as well as solvent-solute interactions in the emergence of the inverse-melting phenomenon is revealed. A surprising re-entrant solvent-solvent interaction is found in the system and in the 4-methylpyridine·water solvent system, supporting the non-coincidental relationship between inverse melting and re-entrant phase transitions, at least in the present system.

Measuring Cumulative Exposure to Oxygen with a Diphenylphosphine-Alkyl Naphthaleneimide Luminescence Turn-On Dyad.

2-(2-Diphenylphosphanylethyl)benzo[de]isoquinoline-1,3-dione is a poorly luminescent, photoinduced-electron-transfer (PET) dyad, NI-(Ph)2 P:, in which the luminescence of its naphthaleneimide (NI) part is quenched by the lone-pair electrons of the phosphorus atom of the (Ph)2 P: group. Photoinduced oxidation of (Ph)2 P: to (Ph)2 P=O by molecular oxygen regenerates the luminescence of the NI group, because the oxidized form (Ph)2 P=O does not serve as a quencher to the NI system. The oxidation of (Ph)2 P: is thermally inaccessible. The NI-(Ph)2 P: system was applied to monitoring the cumulative exposure of oxidation-sensitive goods to molecular oxygen. The major advantage of this new PET system is that it reacts with oxygen only via the photoinduced channel, which offers the flexibility of monitoring the cumulative exposure to oxygen in different time periods, simply by varying the sampling frequency. Electronic-energy calculations and optical spectroscopic data revealed that the luminescence turn-on upon reaction with molecular oxygen relies on a PET mechanism.

Chirality dependent inverse-melting and re-entrant gelation in α-cyclodextrin/1-phenylethylamine mixtures.

Solutions of cyclohexakis-(1→4)-α-d-glucopyranosyl, α-cyclodextrin, αCD, in R-(+)-1-phenylethylamine, αCD/R-PEA, and S-(-)-1-phenylethylamine, αCD/S-PEA, display abnormal phase transitions that strongly depend on supramolecular diastereomeric interactions. While αCD/R-PEA mixtures show one sol-gel inverse-melting phase transition, αCD/S-PEA mixtures show temperature dependent gel-sol-gel re-entrant behavior. NMR, Raman spectroscopy, microscopy and X-ray scattering measurements reveal that hydrogen bond weakening in solution, as well as changes in crystal composition are responsible for entropy increase and gel formation upon heating.