Design, Synthesis, and Photophysical Studies of a Porphyrin-Fullerene Dyad with Parachute Topology; Charge Recombination in the Marcus Inverted Region
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- 作者：David I. Schuster ; Peng Cheng ; Peter D. Jarowski ; Dirk M. Guldi ; Chuping Luo ; Luis Echegoyen ; Soomi Pyo ; Alfred R. Holzwarth ; Silvia E. Braslavsky ; René ; M. Williams ; and Gudrun Klihm
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：June 16, 2004
- 年：2004
- 卷：126
- 期：23
- 页码：7257 - 7270
- 全文大小：295K
- 年卷期：v.126,no.23(June 16, 2004)
- ISSN：1520-5126
文摘
As part of a continuing investigation of the topological control of intramolecular electron transfer(ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad hasbeen synthesized. A symmetrical strap, attached to ortho positions of phenyl groups at opposing mesopositions of the porphyrin, was linked to [60]-fullerene in the final step of the synthesis. The dyad structureswere confirmed by 1H, 13C, and 3He NMR, and MALDI-TOF mass spectra. The free-base and Zn-containingdyads were subjected to extensive spectroscopic, electrochemical and photophysical studies. UV-vis spectraof the dyads are superimposable on the sum of the spectra of appropriate model systems, indicating thatthere is no significant ground-state electronic interaction between the component chromophores. Molecularmodeling studies reveal that the lowest energy conformation of the dyad is not the C2v symmetrical structure,but rather one in which the porphyrin moves over to the side of the fullerene sphere, bringing the two
-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMRdata, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swingingback and forth from one side of the fullerene to the other. The extensive fluorescence quenching in boththe free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process,kET 
1011 s-1, generating porphyrin radical cations and C60 radical anions, detected by transient absorptionspectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitudein these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs inthe Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases.Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formedonly with the free base dyad in toluene, where triplet formation from the charge-separated state is competitivewith decay to the ground state. The photophysical properties of the P-C60 dyads with parachute topologyare very similar to those of structurally related rigid -stacked P-C60 dyads, with the exception that thereis no detectable charge-transfer absorption in the parachute systems, attributed to their conformationalflexibility. It is concluded that charge separation in these hybrid systems occurs through space inunsymmetrical conformations, where the center-to-center distance between the component -systems isminimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systemsbetween 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm-1.
-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMRdata, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swingingback and forth from one side of the fullerene to the other. The extensive fluorescence quenching in boththe free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process,kET 1011 s-1, generating porphyrin radical cations and C60 radical anions, detected by transient absorptionspectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitudein these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs inthe Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases.Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formedonly with the free base dyad in toluene, where triplet formation from the charge-separated state is competitivewith decay to the ground state. The photophysical properties of the P-C60 dyads with parachute topologyare very similar to those of structurally related rigid -stacked P-C60 dyads, with the exception that thereis no detectable charge-transfer absorption in the parachute systems, attributed to their conformationalflexibility. It is concluded that charge separation in these hybrid systems occurs through space inunsymmetrical conformations, where the center-to-center distance between the component -systems isminimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systemsbetween 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm-1.