Energy and electron transfer in β-alkynyl-linked porphyrin-[60] fullerene dyads

Three porphyrin-fullerene dyads, in which a diyne bridge links C ₆₀ with a β-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which π-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H₂TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C₆₀ dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C₆₀ triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP^.+/C₆₀^.- charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP ^.+/C₆₀^.- in PhCN was ∼40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k_CS) to rates for charge recombination (k_CR), k _CS/k_CR, is quite small, ∼7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C₆₀ to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k_CS/k _CR ≈ 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k_CR but not on k_CS. On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the β-linked dyads.