Charge Separation Mechanisms in Ordered Films of Self-Assembled Donor-Acceptor Dyad Ribbons

Orthogonal attachment of polar and nonpolar side-chains to a zinc porphyrin-perylenediimide dyad (ZnP-PDI, la) is shown to result in self-assembly of ordered supramolecular ribbons in which the ZnP and PDI molecules form segregated pi-stacked columns. Following photoexcitation of the ordered ribbons, ZnP+center dot-PDI+center dot radical ion pairs form in<200 fs and subsequently produce a 30 +/- 3% yield of free charge carriers that live for about 100 mu s. Elongating the side chains on ZnP and PDI in lb enhances the order of the films, but does not result in an increase in free charge carrier yield. In addition, this yield is independent of temperature, free energy of reaction, and the ZnP-PDI distance in the covalent dyad. These results suggest that the free charge carrier yield in this system is not limited by a bound charge transfer (CT) state or promoted by a vibronically hot CT state. Instead, it is likely that pi-stacking of the segregated donors and acceptors within the ribbons results in delocalization of the charges following photoexcitation, allowing them to overcome Coulombic attraction and generate free charge carriers.