Reversible energy transfer between monomers and fluorescent dimers of rhodamine S in polyvinyl alcohol films

Research highlights: → Contrary to liquid solutions rhodamine S aggregates in PVA are fluorescent. → Overlap between aggregate fluorescence and monomer absorption leads to reverse energy transfer. → This effect influences strongly quantum yield and emission anisotropy concentration courses. → Monte-Carlo simulation enables estimation of dimer quantum yield and describes well experiments. - Abstract: Nonradiative energy transfer and excitation trapping are studied for rhodamine S in polyvinyl alcohol (PVA) films. It occurs that fluorescent dimers of rhodamine S can play a role of imperfect traps for the excitation energy. At highest dye concentrations experimental data of fluorescence quantum yield and emission anisotropy cannot be described by the model of energy transfer neglecting the possibility of excitation return to the monomers. However, the agreement between experimental data and the results of computations can be much improved, if the reverse energy transfer is taken into account. Based on the quantitative analysis it is possible to estimate selected characteristics of rhodamine S dimer in polyvinyl alcohol matrix. The experiments and corresponding analysis are made both at room and elevated temperature.