Photophysics on surfaces: Absorption and luminescence properties of Pheophorbide-a on cellulose

Solid samples of Pheophorbide-a adsorbed on microcrystalline cellulose were prepared with a wide range of dye concentrations (7.1 × 10−9 to 8.9 × 10−6 mol (g cellulose−1) and studied by diffuse reflectance and steady-state and time-resolved fluorescence spectroscopy in the solid state. Absorption spectra obtained by means of reflectance spectroscopy were compared with those found in different environments and carrier systems. No aggregation may be inferred from the spectroscopic data. Changes in the fluorescence spectra with increasing dye concentration are attributed to: (a) reabsorption of luminescence and re-emission and (b) interaction between the excited dye molecule and ground state neighboring molecules. The first effect accounts for the observed differences in emission spectra for thin and thick layers of a given sample. Emission decays are non-exponential even at low dye concentrations. A Förster energy transfer mechanism leading to energy trapping by molecules of the same type as the excited dye is proposed.