Quenching of zinc tetraphenylporphine by oxygen and by 1,4-benzoquinone in nitrilesolvents: An optoacoustic spectroscopy study

Abstract

The quenching of zinc tetraphenylporphine by oxygen and 1,4-benzoquinone was studied using laser-induced optoacoustic spectroscopy in a homologous series of aprotic nitrile solvents consisting of acetonitrile, propionitrile, butyronitrile, and valeronitrile. An inverse correlation exists between the oxygen quenching efficiency of triplet state zinc tetraphenylporphine and the efficiency of singlet oxygen formation, against solvent polarity, in agreement with the accepted involvement of a charge transfer complex during the quenching process. No structural volume change was observed for the triplet state formation of zinc tetraphenylporphine using the nitrile variation method, whereas electron transfer between zinc tetraphenylporphine and 1,4-benzoquinone resulted in a volume contraction in the four nitrile solvents used. Specific solute–solvent interaction, in addition to electrostriction, is needed to fully understand the relatively large and solvent dependent structural volume change observed in the longer chain nitriles. This is due to the varying interaction strength between the electron-pair donor nitriles and the ZnTPP cation as one moves across the solvent series. The free energy of the electron transfer was, however, very similar within the solvent series. The effect of a specific solute–solvent interaction on the rate of electron transfer is unclear at the moment since the latter is found to be limited by the solvent relaxation time, which is much shorter than the time window of the experiments.