Ultrafast Charge Recombination of Photogenerated Ion Pairs to an Electronic Excited State

The dynamics of charge recombination (CR) of ion pairs formed upon electron-transfer quenching of Zn tetraphenylporphine (ZnTPP) in the S 2 state has been investigated by fluorescence upconversion. These ion pairs have two possible CR pathways: (A) a highly exergonic CR to the neutral ground state and (B) a moderately exergonic CR leading to the formation of ZnTPP in the S 1 state. Upon addition of quencher, the S 2 fluorescence decreases considerably, while the S 1 fluorescence is unaffected, indicating unambiguously that CR occurs via path B. A large fraction of the S 2 fluorescence quenching occurs in less than 100 fs. CR to the S 1 state of ZnTPP takes place with time constants around 400 fs.