Solvent effects on molecular aggregation of highly water-soluble phthalocyanines

Molecular aggregation phenomena of a highly water-soluble, metal-free phthalocyanine, H2Pc, and its copper complex, Cu(Pc); where Pc denotes tetrakis{(2 ', 6 '-dimethyl-4 '-sulfonic acid)phenoxy}phthalocyaninate) in water, ethanol, and their mixed solvent systems have been investigated by spectroscopic approach. In both aqueous and ethanolic solutions, H2Pc molecules aggregate in a slipped-cofacial manner, although very weakly in ethanol. On the other hand, Cu(Pc) molecules aggregate in a cofacial manner in water while aligning in a slipped-cofacial manner in ethanol. Magnetic circular dichroism spectroscopy was undertaken to investigate aggregation behaviors of Cu(Pc) in water and ethanol. Presence of 4-phenylpyridine in ethanol effectively inhibited aggregation of Cu(Pc) molecules, although little affected the spectra of the monomers. In a mixed solvent system, where water/ethanol = 10/90 (v/v), Cu(Pc) is free from aggregation up to almost 10(-4) M. Driving forces of this molecular aggregation are discussed on the basis of pi-pi interaction and coordination chemistry involving copper ions.