| Abstract
| - Photoinduced charge separation (CS) and charge recombination (CR) processes have been examinedin various porphyrin−fullerene linked systems (i.e., dyads and triads) by means of time-resolved transientabsorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise ahomologous series of rigidly linked, linear donor−acceptor arrays with different donor−acceptor separationsand diversified donor strength: freebase porphyrin−C60 dyad (H2P-C60), zincporphyrin−C60 dyad (ZnP-C60),ferrocene−zincporphyrin−C60 triad (Fc-ZnP-C60), ferrocene−freebase porphyrin−C60 triad (Fc-H2P-C60), andzincporphyrin−freebase porphyrin−C60 triad (ZnP-H2P-C60). 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-C60 triad, has been generated with the highest quantum yields (close to unity) andreveals a lifetime as long as 16 μs. Determination of CS and CR rate constants, together with the one-electronredox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the drivingforce 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 couplingmatrix element (V) in light of the Marcus theory of electron-transfer reactions: λ = 0.66 eV and V = 3.9cm-1 for ZnP-C60 dyadand λ = 1.09 eV and V = 0.019 cm-1 for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clearevidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. Thecoefficient for the distance dependence of V (damping factor: βCR = 0.58 Å-1) is deduced which dependsprimarily on the nature of the bridging molecule.
|
