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

A novel dendritic C60‐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 C60, H2P, and ZnP units, the outcome depends strongly on the rigidity/viscosity of the environment. In an agar matrix or Triton X‐100, time‐resolved transient absorption spectroscopic analysis and fluorescence‐lifetime measurements confirm the following sequence. Initially, light harvesting is seen by the peripheral C60‐H2P‐ *(ZnP)3 conjugate. Once photoexcited, a unidirectional energy transfer funnels the singlet excited‐state energy to H2P to form C60‐*(H2P)‐(ZnP)3, which powers an intramolecular charge transfer that oxidizes the photoexcited H2P and reduces the adjacent C60 species. In the correspondingly formed (C60).−‐(H2P).+‐(ZnP)3 conjugate, an intramolecular charge‐shift reaction generates (C60).−‐H2P‐(ZnP)3.+, in which the radical cation resides on 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 peripheral ZnP unit reacts directly with C60 to form (C60).−‐H2P‐(ZnP)3.+. Substantial configurational rearrangements— placing ZnP and C60 in proximity to each other—are, however, necessary to ensure the required through space interactions (i.e., close approach). Consequently, the lifetime of (C60).−‐H2P‐(ZnP)3.+ is as short as 100 ps in benzonitrile. Artificial photosynthesis at work: A dendritic porphyrin–fullerene hybrid capable of light harvesting and photoinduced electron transfer was synthesized and photophysically probed (see scheme). Highly viscous environments were applied and a charge‐separated state with lifetimes of up to 460 ns was observed.