ABSORPTION AND FLUORESCENCE SPECTROSCOPIC STUDIES ON DIMERIZATION OF CHLOROALUMINUM (III) PHTHALOCYANINE TETRASULFONATE IN AQUEOUS ALCOHOLIC SOLUTIONS

Dimerization of chloroaluminum (III) phthalocyanine tetrasulfonates (AlPCS) has been observed in different aqueous alcoholic solvents at room temperature by absorption and fluorescence spectroscopic methods. Both absorption and fluorescence spectral bands of the dimer are red shifted by ca 550 cm-1 from the monomer Q bands in the corresponding spectra, suggesting that the interaction energy between the two monomer subunits is very weak. The fluorescence lifetime of the dimer is longer (ca 9.5 ns) than that of the monomer (ca 7-8 ns). These spectral behaviors of AlPCS dimer contrast with those of transition-metallaloid phthalocyanine dimers, which usually have a nonfluorescent face-to-face stacking conformation. The dimer fluorescence is interpreted to be due to the fact that the lowest excited singlet state of the dimer is lower in energy than a charge-resonance state, based on the exciton-coupling theory applied to the face-to-face slipping conformation. The dimerization constant determined spectrometrically decreases with an increase of water content in the aqueous alcoholic solution. Propanol and ethanol have been observed to be more effective than methanol in promoting dimerization. These results indicate that a specific interaction of water with AlPCS plays an important role in the inhibition of dimerization of AlPCS.