Abstract
The dynamics of charge recombination (CR) of ion pairs formed upon electron-transfer quenching of Zn tetraphenylporphine (ZnTPP) in the S2 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 S1 state. Upon addition of quencher, the S2 fluorescence decreases considerably, while the S1 fluorescence is unaffected, indicating unambiguously that CR occurs via path B. A large fraction of the S2 fluorescence quenching occurs in less than 100 fs. CR to the S1 state of ZnTPP takes place with time constants around 400 fs.
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