"Umpolung" photoinduced charge separation in an anion-bound supramolecular complex

A new supramolecular system, consisting of an expanded porphyrin, cyclo[8]pyrrole (C8) and a pyrene carboxylate (Py) is capable not only of electron transfer upon photoexcitation, but results in "umpolung" or a reversal of the predicted charge-separation behavior in that the higher energy radical ion pair, C8^•+-Py^•- (2.58 eV) is formed as opposed to C8^•--Py^•+ (1.31 eV). This observation is rationalized in terms of the latter radical ion pair, C8^•--Py^•+ being in the Marcus inverted region. Transient absorption spectral studies reveal a charge-separated lifetime of 300 μs, which decays to the triplet state of cyclo[8]pyrrole, also an exceedingly long-lived species. The novel features of this noncovalent dyad highlight the potential utility of anion binding in the construction of supramolecular electron transfer systems.