Structure–Property Relationships on the Optical Properties of Rhodamine Thin Films

Abstract

The spectroscopic properties of five xanthene dyes with varying structures and charges were studied as dilute solutions in ethanol and as thin films on a glass substrate. Density functional theory calculations were used to assign the spectral peaks in the visible region of the solution absorption spectra. These assignments were used as guides to understand the changes of the absorption and emission spectra of thin films as the surface density of the dye was increased. Dilute solutions and submonolayer thin films exhibited similar spectra but with shifts that were accounted for by the difference of polarity of the ethanol solvent compared to the glass/air interface. As the thin films reach monolayer levels, the absorption and emission spectra exhibit features that are assigned to excitons and excimers. The intensities of the emission spectra are most strongly affected by the surface density of the dyes. The maximum emission intensity is found at about 1 monolayer dye coverage, while the total intensity approximately follows a log-normal distribution as a function of dye surface coverage.