Cationic alkyl/arylphosphonium meso-tetratolylporphyrins aggregate in an aqueous solution to form H-aggregates, J-aggregates, and long-range assemblies. The ratio between the monomer and various types of aggregates can be controlled by the substitution in the phosphonium units and by the ionic strength. A trimethylphosphonium derivative is predominantly monomeric, dimethylphenylphosphonium forms monomers as well as low-molecular-weight H- and J-aggregates, triphenylphosphonium forms mainly H- and J-aggregates, and tri(n-butyl)phosphonium forms mainly long-range assemblies. Porphyrin monomers associate with calf thymus DNA (binding constant Kb approximate to 10(7) M-1) and oligonucleotides (K-b approximate to 10(5-)10(6) M-1). The large size of the meso-substituents prevents the intercalation between base pairs. All phosphonium porphyrins described in this study were found to bind to the phosphate backbone of a nucleic acid with a significant preference for A-T base pair sequences. Porphyrin aggregates formed in the solution deposit readily on the surface of the DNA and oligonucleotides without changing their structure and size. Porphyrin monomers bound to DNA and nucleotides have photophysical properties (higher quantum yield of triplet states and singlet oxygen) different from those of porphyrin aggregates.