Dendritic Porphyrin-Fullerene Conjugates: Efficient Light-Harvesting and Charge-Transfer Events

A novel dendritic C-60-H2P(ZnP)(3) (P=porphyrin) conjugate gives rise to the successful mimicry of the primary events in photosynthesis, that is, light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions. Owing, however, to the flexibility of the linkers that connect the C-60, H2P, and ZnP units, the outcome depends strongly on the rigidity/viscosity of the environment. In all agar matrix or Triton X-100, time-resolved transient absorption spectroscopic analysis and fluorescence-life-time measurements confirm the following sequence. Initially, light harvesting is seen by the peripheral C-60-H2P-*(Znp)(3) conjugate. Once photoexcited, a unidirectional energy transfer funnels the singlet excited-state energy to H2P to form C-60-*(H2P)-(ZnP)(3), which powers an intramolecular charge transfer that oxidizes the photoexcited H2P and reduces the adjacent C-60 species. In the correspondingly formed (C60)(center dot-)-(H2P)(center dot+)-(ZnP)(3) conjugate, an intramolecular charge-shift reaction generates (C60)(center dot-)-H2P-(ZnP)(3)(center dot+), in which the radical cation resides oil one of the three ZnP moieties, and for which lifetimes of up to 460 ns are found. On the other hand, investigations in organic media (i.e., toluene, THF, and benzonitrile) reveal a short cut, that is, the periplieral ZnP unit reacts directly with C-60 to form (C60)(center dot-)-H2P-(ZnP)(3)(center dot+). Substantial configurational rearrangements-placing ZnP and C-60 in proximity to each other-are, however, necessary to ensure the required through space interactions (i.e., close approach). Consequently, the lifetime of (C60)(center dot-)-H2P(ZnP)(3)(center dot+) is as short as 100 ps in benzonitrile.