Vanillin Phosphorescence as a Probe of Molecular Mobility in Amorphous Sucrose

Abstract

Changes in molecular mobility are important in defining the stability and quality of amorphous solid foods, pharmaceuticals, and other solid biomaterials. Predictions of stability must consider matrix mobility below and above T(g) (the glass transition temperature); measurement of molecular mobility in amorphous solids over time scales ranging from <10(-9) s to >10(8) s requires specialized methods. This research investigated how the steady-state and time-resolved emission and intensity of phosphorescence from vanillin (4-hydroxy-3-methoxy benzaldehyde), a common flavor compound, can be used to probe molecular mobility when dispersed within amorphous pure sucrose films. Phosphorescence emission spectra and time-resolved intensity decays, measured in sucrose as a function of temperature in the absence of oxygen, were strongly modulated by matrix molecular mobility. Temperature had a significant effect on vanillin phosphorescence peak frequency and bandwidth, intensity, and lifetime both in the glass and in the melt. Time-resolved phosphorescence intensity decays from vanillin were multiexponential both below and above the glass transition temperature, indicating that the pure (single component) amorphous matrix was dynamically heterogeneous on the molecular level. These data show that vanillin is a promising intrinsic probe of molecular mobility and dynamic heterogeneity in amorphous solid foods and perhaps pharmaceuticals.