Volume changes related to triplet formation of water-soluble porphyrins. A laser-induced optoacoustic spectroscopy (LIOAS) study

The production of triplet states in basic solution of tetrakis(tetrasulfonated-phenyl)porphyrin (TPPS4) is accompanied by a contraction of (10 +/- 3) mL/mol as determined by laser-induced optoacoustic spectroscopy (LIOAS) at different temperatures. The contraction is reverted as the triplet decays. The volume changes should mainly originate in changes in the solvation sphere due to the entire process of ground state to triplet transition (for the contraction) and back to the ground state (for the expansion). The presence of oxygen has no major effect on the contraction and the expansion observed. The present study shows that TPPS4 is not suitable as a calorimetric reference, contrary to our own hypothesis in Heihoff et al. Photochem. Photobiol. 1990, 51, 635-641. It was also observed that triplet state formation of methylene blue and rose bengal is accompanied by a structural volume change of -1.5 and 1 mL/mol, respectively.