The absorbance and fluorescence spectra of Nile Red (NR) were examined in a series of nonpolar solvents comprising linear alkanes and a range of poly alpha olefins (PAO). These solvents span a 1000-fold range in viscosity and possess very similar dielectric constants and refractive index properties. A high-energy double peak with vibronic structure is observed in both fluorescence and absorbance spectra, possibly indicating that a locally excited (LE) state is accessed in these solvents. In addition, a red shift in peak position is observed with increasing refractive index; however, it is unaccompanied by any changes in Stokes shift. This shift is attributed to changes in the high-frequency polarizability of the solvent, which is a function of the refractive index. Finally, an increase in quantum yield with viscosity is also observed.
Dalia G. Yablon and Alan M. Schilowitz, "Solvatochromism of Nile Red in Nonpolar Solvents," Appl. Spectrosc. 58, 843-847 (2004)