Cis and trans isomers of a series of double-decker silsesquioxanes (DDSQ) were characterized by two-dimensional NMR techniques. The (1)H NMR spectra of these species have not previously been assigned to a degree that allows for quantification. Thus, (1)H-(29) Si HMBC correlations were applied to facilitate (1)H spectral assignment and also to confirm previous (29)Si assignments for this class of silsesquioxanes. With the ability to identify all the pertinent resonances of the (1)H NMR spectrum, (29)Si NMR is no longer required for quantification and required only for characterization. This not only saves time and material but also provides a more accurate quantification, thus allowing for the ratio of cis and trans isomers present in each compound to be determined. A more accurate measure of the cis/trans ratio enables the investigation of its influence on the physical and chemical properties of DDSQ nanostructured materials.
A fractional crystallization method was used to separate the cis and trans isomers of three double-decker silsesquioxanes (DDSQs) with an aminophenyl moiety in a THF + hexanes solvent mixture. The experimental solubilities were fitted to the Schröder–van Laar equation with activity coefficients determined using the NRTL model by adjusting the binary interaction parameters. The ability to separate these cis and trans isomers was affected by the regioisomer (m- or p-aminophenyl) and the R moiety (cyclohexyl or methyl) coupled via silicon. For a given DDSQ compound, the variances between the solubilities of the cis and trans isomers depend on differences in thermal properties (Schröder–van Laar). Cis isomers were 33 times more soluble than trans isomers for p-aminophenyl (R = methyl) and 22 times more soluble for the analogous m-aminophenyl in a solution of THF and hexanes. For a more sterically hindered m-aminophenyl (R = cyclohexyl), the cis isomers were only 3.5 times more soluble, and the overall solubility was also the lowest. The magnitude of the binary interaction between DDSQ and nonsolvent (hexanes) was used to explain how quickly the solubility decreased as hexanes were added. The solubilities of the two m-aminophenyl structures decreased at similar rates, while the solubility of the p-aminophenyl structure decreased at a much lower rate since the magnitude of the binary interaction between p-aminophenyl and hexanes is smaller.
Closed double-decker shaped silsesquioxanes (DDSQ-(Ph)8-2((Me)(R))) with R as phenyl, para-phenylamine, and para-phenylethynyl phenyl were synthesized. Isolation of nearly pure trans and cis isomers was obtained by fractional recrystallization. Crystallographic and thermal characteristics of these isolated isomers were obtained by X-ray diffraction of a single crystal and differential scanning calorimetry (DSC). It was observed that melting temperature increases as the size of the R group decreases from para-phenylethynyl phenyl to phenyl and the magnitude of the entropy difference at melting increases as the size of the R group increases. Isolated isomers were then mixed to different cis-to-trans ratios, and their thermal characteristics investigated by DSC. The upper portion of the phase diagram was constructed for these DDSQ compounds using results from DSC traces. Interestingly, cis and trans isomers of these DDSQ compounds form a binary eutectic. Experimentally observed eutectic composition and temperature were found to be close to the calculated values based on the ideal eutectic mixing rule. These data allow users to broaden the thermal processability window by reducing the system melting temperature without affecting the reaction onset temperature of the functional groups.
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