Photophysical Properties of Synthetic Food Dyes

The photophysical properties of generally-recognized-as-safe (GRAS) food dyes, such as Allura Red (AR), Tartrazine (TR), Sunset Yellow (SY) and Fast Green (FG), have rarely been reported. All these dyes exhibit very weak emission in common fluid solvents, which can explain the limited information on their photophysical properties. A significant enhancement of fluorescence emission is observed in highly viscous solvents (glycerol, etc.) and structured systems (e.g., amorphous solids, fat crystal networks, etc.). This observation has supported the hypothesis that these dyes exhibit molecular rotor behavior enabling their use as probes of microviscosity. To enhance the applicability of these potential luminescent probes, detailed information on their photophysical properties was gathered in this study. Their excitation and emission spectra were collected and Stokes shifts analyzed in a series of solvents of different polarity. Additionally, fluorescence quantum yields and lifetimes of all the dyes were determined in water:glycerol mixtures (0-98% glycerol). TR exhibited the largest Stokes shift (Δν = 5000 cm-1), followed by AR and SY (Δν = 2500 cm-1) and finally FG (Δν = 700 cm-1). The effect of environmental polarity on the fluorescence emission peak location was more pronounced for AR than for the rest of the probes. For dyes in water at 296K, the fluorescence quantum yields (Q) and lifetimes (τ) were extremely small (0.0001-0.0005 and 10-20 ps, respectively). However, for all dyes, a significant increase in Q and τ was observed by increasing medium viscosity at fixed temperature (increasing glycerol content in water:glycerol mixtures), by reducing the temperature to 77K, or by a combination of both, reaching values as high as Q = 0.41 and τ = 3.37 ns for FG in glycerol at 77K. This study makes a significant contribution to the currently incomplete photophysical characterization of these edible and safe-to-ingest fluorophores.