Modulating charge separation and charge recombination dynamics in porphyrin fullerene linked dyads and triads: Marcus-normal versus inverted region

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-C-60 dyad (H2P-C-60) zincporphyrin-C-60 dyad (ZnP-C-60), ferrocene-zincporphyrin-C-60 triad (Fc-ZnP-C-60), ferrocene-freebase porphyrin-C-60 triad (Fc-H2P-C-60), and zincporphyrin-freebase porphyrin-C-60 triad (ZnP-H2P-C-60). Most importantly, the lowest lying charge-separated state of all the investigated systems; namely, that of ferrocenium ion (Fc(+)) and the C-60 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 mus. Determination of CS and CR rate constants, together with the one-electron different solvents, has allowed us to examine the driving redox potentials of the donor and acceptor moieties in force dependence (-DeltaG(ET)(0)) of the electron-transfer rate constants (k(ET)): Hereby, the semilogarithmic plots (i.e., log k(ET) versus -DeltaG(ET)(0)) lead to the evaluation of the reorganization energy (lambda) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: lambda = 0.66 eV and V = 3.9 (-1) for ZnP-C-60 dyad and lambda = 1.09 eV and V = 0.019 cm(-1) for Fc-ZnP-C-60, Fc-H2P-C-60, and ZnP-H2P-C-60 triads. Interestingly, the Marcus plot in Fc-ZnP-C-60, FC-H2P-C-60, and ZnP-H2P-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: beta (CR) = 0.58 Angstrom (-1)) is deduced which depends primarily on the nature of the bridging molecule.