Spectroscopic evidence of monomeric aluminium phthalocyanine tetrasulphonate in aqueous solutions

Aqueous solutions of aluminium phthalocyanine tetrasulphonate (AlPcS4) were investigated by means of absorption and fluorescence spectroscopy. The absorption spectrum of AlPcS4 is independent of concentration in a wide range (from 10^-8 to 10^-4 M). The fluorescence spectrum measured with a standard setup is strongly dependent on AlPcS4 concentration, and the fluorescence maximum is gradually red-shifted with increasing concentration. Calculations that take into account reabsorption of fluorescence (inner-filter effect) fit the experimental observations at low concentrations (up to 10^-6 M). Disagreement between the calculations and spectra recorded at higher concentrations (above 10^-5 M) shows that the reabsorbed light may be reemitted as fluorescence. The influence of inner-filter effects on the spectral shape was demonstrated by the experiments where a fibre-optic front-face fluorescence setup was applied: Under such conditions the shape of the fluorescence spectra for a high concentration (10^-3 M) coincided with that of a low concentration (10^-8 M). In conclusion, the present spectroscopic results show that AlPcS4 does not form aggregates and is a very stable compound in aqueous solutions.