Yellow coloration phenomena of incorporated indomethacin into folded sheet mesoporous materials

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy that included relaxation time measurement was utilized to evaluate the yellow coloration of evaporated samples (EVPs) of indomethacin (IMC) with commercially available folded sheet mesoporous materials (TMPS). Colorimetric analysis by visible light reflection spectroscopy clarified the color differences in each sample: deep yellow-colored melt-quenched amorphous IMC, a slightly yellow-colored EVP of TMPS-1.5 (pore size: 1.8 nm), and a yellow-colored EVP of TMPS-7 (pore size: 7.3 nm). The color of EVPs changed from yellow to white after washing with ethanol, indicating the reversible coloration without a chemical reaction. Powder X-ray diffractometry and differential scanning calorimetry demonstrated that the EVPs of TMPS-7 entrapped greater amounts of amorphous IMC into the mesopore than TMPS-1.5. The amount of amorphous IMC in the mesopores could affect the strength of yellow coloration. Solid-state 13C NMR spectroscopy that included spin–lattice relaxation time (T1) measurement revealed that the mobility of the aromatic rings of amorphous IMC in TMPS mesopores was higher than that in melt-quenched amorphous IMC. The difference in color between amorphous IMC in TMPS mesopores and melt-quenched amorphous IMC can be explained by their distinct intramolecular π-conjugation systems.

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