Structural volume changes upon triplet formation of water-soluble porphyrins depend on the resonant effect of the substituents.

Laser-induced optoacoustic spectroscopy was applied to evaluate the photoinduced structural volume changes upon triplet state formation, Δ(T)V, of an aqueous solution of 5,10,15,20-tetrakis-(4-carboxyphenyl)-porphin. Two molecular forms differing in the ionization state of the carboxylic groups at the para-position of the phenyl ring were studied. The contractions, Δ(T)V, for all water-soluble 5,10,15,20-tetra-aryl-porphyrin compounds studied to date show a linear correlation with the Hammett resonant σ(R) constant. This indicates that the resonance electronic communication spreading over the π-orbitals between the meso-aryl rings and the porphyrin macrocycle determines the molecular contraction of the aqueous sphere around the macrocycle in the triplet state.