A water-soluble zinc phthalocyanine (ZnPc) complex with four negatively charged electron-withdrawing sulfonic acid substituents at the nonperipheral positions (alpha-ZnTSPc) is found to have a high singlet oxygen (O-1(2)) quantum yield and exhibits high photostability. The formation of aggregates is hindered and the highest occupied molecular orbital is significantly stabilized, making alpha-ZnTSPc potentially suitable for its use as a photosensitizer for photodynamic therapy and photo-immunotherapy. Atomic force microscopy (AFM) reveals that mixtures of the negatively charged alpha-ZnTSPc complex with a similar positively charged ZnPc were found to result in the self-assembly of one-dimensional accordion-like fibers. Supramolecular fibers can be formed in aqueous solutions through intermolecular electrostatic and donor-acceptor interactions between the two water-soluble ZnPcs.