The effects of concentration on the fluorescence decay kinetics of disulfonated aluminium phthalocyanine (AlPcS2) were studied in several solvents. The degree of aggregation, which increased with total dye concentration, was estimated from the absorption spectra. The measured fluorescence decays were shorter and increasingly non-monoexponential with increasing dye concentration. However, stronger quenching was not correlated with higher aggregation. The fluorescence decays were analyzed using a model that assumes excitation energy migration between diffusing monomeric AlPcS2 and quenching by diffusing dimers, both governed by the Förster energy transfer mechanism. The model can explain the observations in three of the four solvents used (phosphate-buffered saline (PBS) pH = 11.5, ethanol, and 67% glycerol–33% water mixture) on the assumption that different dimer configurations are present and not all of them act as quenchers. In PBS at pH = 7.4 the theory predicts much stronger quenching than observed. Excitation energy migration between monomeric species at high dye concentration was confirmed by the observed decrease of the decay time of fluorescence anisotropy in viscous solutions of 67% glycerol, and appears to be a major factor in fluorescence quenching of AlPcS2 at high concentration.
