Phosphorescence of Erythrosin B as a Robust Probe of Molecular Mobility in Amorphous Solid Sucrose

Abstract

Luminescence from the triplet probe erythrosin B (tetra-iodo fluorescein, Ery B) provides spectroscopic characteristics such as lifetime and emission energy that are sensitive to molecular mobility of the local environment in amorphous solids. This study investigated how variations in the local concentration of Ery B free acid as well as the presence of the dispersing solvent affect the spectroscopic measurements of solid matrix properties (the free acid of Ery B is poorly soluble in water and thus must be introduced via an organic solvent). The emission energy of Ery B from 5 to 100 °C in thin films of amorphous sucrose at various probe and solvent (N,N-dimethyl formamide, DMF) concentrations was determined using excitation at 500 nm and emission over the range 520–750 nm. The emission lifetime was determined over the same temperature range using a stretched exponential analysis of intensity decays collected using excitation at 530 nm and emission at 680 nm. Variations in the probe/sucrose mole ratio (concentration) over the range from 0.5 to 10 × 10−4 and 10-fold variations in the amount of DMF used to disperse the probe did not affect the emission energy, the shape of the emission spectra, or the measured lifetimes of Ery B in amorphous sucrose. These results thus indicate that erythrosin B introduced into amorphous solids can provide a robust measure of the intrinsic mobility of the solid matrix that is relatively insensitive to final probe concentration or presence of residual solvent.