Photoinduced Energy and Electron-Transfer Processes in Porphyrin-Perylene Bisimide Symmetric Triads

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• 作者：Marco Ghirotti ; Claudio Chiorboli ; Chang-Cheng You ; Frank W&uuml ; rthner ; Franco Scandola
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：April 17, 2008
• 年：2008
• 卷：112
• 期：15
• 页码：3376 - 3385
• 全文大小：291K
• 年卷期：v.112,no.15(April 17, 2008)
• ISSN：1520-5215

文摘

The photophysics of two symmetric triads, (ZnP)₂PBI and (H₂P)₂PBI, made of two zinc or free-base porphyrinscovalently attached to a central perylene bisimide unit has been investigated in dichloromethane and in toluene.The solvent has been shown to affect not only quantitatively but also qualitatively the photophysical behavior.A variety of intercomponent processes (singlet energy transfer, triplet energy transfer, photoinduced chargeseparation, and recombination) have been time-resolved using a combination of emission spectroscopy andfemtosecond and nanosecond time-resolved absorption techniques yielding a very detailed picture of thephotophysics of these systems. The singlet excited state of the lowest energy chromophore (perylene bisimidein the case of (ZnP)₂PBI, porphyrin in the case of (H₂P)₂PBI) is always quantitatively populated, besides bydirect light absorption, by ultrafast singlet energy transfer (few picosecond time constant) from the higherenergy chromophore. In dichloromethane, the lowest excited singlet state is efficiently quenched by electrontransfer leading to a charge-separated state where the porphyrin is oxidized and the perylene bisimide isreduced. The systems then go back to the ground state by charge recombination. The four charge separationand recombination processes observed for (ZnP)₂PBI and (H₂P)₂PBI in dichloromethane take place in thesub-nanosecond time scale. They obey standard free-energy correlations with charge separation lying in thenormal regime and charge recombination in the Marcus inverted region. In less polar solvents, such as toluene,the energy of the charge-separated states is substantially lifted leading to sharp changes in photophysicalmechanism. With (ZnP)₂PBI, the electron-transfer quenching is still fast, but charge recombination takesplace now in the nanosecond time scale and to triplet state products rather than to the ground state. Triplet-triplet energy transfer from the porphyrin to the perylene bisimide is also involved in the subsequent deactivationof the triplet manifold to the ground state. With (H₂P)₂PBI, on the other hand, the driving force for chargeseparation is too small for electron-transfer quenching, and the deactivation of the porphyrin excited singlettakes place via intersystem crossing to the triplet followed by triplet energy transfer to the perylene bisimideand final decay to the ground state.