Modulating Charge Separation and Charge Recombination Dynamics in Porphyrin−Fullerene Linked Dyads and Triads: Marcus-Normal versus Inverted Region

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Abstract
Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin−fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor−acceptor arrays with different donor−acceptor separations and diversified donor strength: freebase porphyrin−C60 dyad (H 2 P - C 60), zincporphyrin−C60 dyad (ZnP - C 60), ferrocene−zincporphyrin−C60 triad (Fc-ZnP - C 60), ferrocene−freebase porphyrin−C60 triad (Fc-H 2 P - C 60), and zincporphyrin−freebase porphyrin−C60 triad (ZnP - H 2 P - C 60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60•-) pair in the Fc-ZnP - C 60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 μs. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (−ΔG0ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots (i.e., log kET versus −ΔG0ET) lead to the evaluation of the reorganization energy (λ) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: λ = 0.66 eV and V = 3.9 cm-1 for ZnP - C 60 dyadand λ = 1.09 eV and V = 0.019 cm-1 for Fc-ZnP - C 60, Fc-H 2 P - C 60, and ZnP - H 2 P - C 60 triads. Interestingly, the Marcus plot in Fc-ZnP - C 60, Fc-H 2 P - C 60, and ZnP - H 2 P - C 60 has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence of V (damping factor: βCR = 0.58 Å-1) is deduced which depends primarily on the nature of the bridging molecule.