Cyclo[8]pyrrole: An androgynous expanded porphyrin that acts as both an electron donor and acceptor in anion-bound supramolecular electron donor-acceptor complexes

Cyclo[8]pyrrole (C8) is an octapyrrolic expanded porphyrin. In its diprotonated form it stabilizes strong supramolecular complexes (association constants of ca. 10⁵ M^-1) in benzonitrile with both the zinc porphyrin carboxylate (ZnP) and pyrene carboxylate (Py) anions via a combination of hydrogen-bonding and electrostatic interactions. Upon nanosecond laser photoexcitation of the C8-ZnP complex, electron transfer (ET) from the triplet excited state of ZnP to C8 occurs to produce the charge-separated state (C8^̇--ZnP^̇+) in which C8 acts as an electron acceptor as inferred from the characteristic transient absorption spectral features. In this case, the energy of the charge-separated state C8 ^̇--ZnP^̇+ (0.6 eV) is lower than the alternative radical ion pair (C8^̇+-ZnP^̇-) that would be produced if electron transfer were to occur in the opposite direction. In contrast to what is seen for the C8-ZnP complex, photoexcitation of the C8-Py complex results in electron transfer from the singlet excited state of Py to C8; this produces a charge-separated state (C8^̇+-Py ^̇-) wherein C8 acts as an electron donor rather than an electron acceptor. The energy of this charge-separated state (C8^̇+-Py ^̇-; 2.58 eV) is much higher than that of the corresponding alternative charge-separated state, C8^̇--Py^̇+ (1.31 eV). The fact that electron transfer does not occur in the opposite direction to produce this latter alternative charge-separated radical ion pair (C8^̇--Py^̇+) is rationalized in terms of it lying deep in the Marcus inverted region. Because two different directions for photoinduced electron transfer are observed depending on the choice of anion-bound partner, ZnP or Py, we conclude that C8 may act as either an electron donor or acceptor under conditions of photoinduced charge separation. This androgynous character stands in contrast to what is seen for typical porphyrinoids.