Choosing sides: unusual ultrafast charge transfer pathways in an asymmetric electron-accepting cyclophane that binds an electron donor

Constructing functional molecular systems for solar energy conversion and quantum information science requires a fundamental understanding of electron transfer in donor-bridge-acceptor (D-B-A) systems as well as competitive reaction pathways in acceptor-donor-acceptor (A-D-A) and acceptor-donor-acceptor (A-D-A) systems. Herein we present a supramolecular complex comprising a tetracationic cyclophane having both phenyl-extended viologen (ExV(2+)) and dipyridylthiazolothiazole (TTz(2+)) electron acceptors doubly-linked by means of two p-xylylene linkers (TTzExVBox(4+)), which readily incorporates a perylene (Per) guest in its cavity (Per < subset of> TTzExVBox(4+)) to establish an A-D-A system, in which the ExV(2+) and TTz(2+) units serve as competing electron acceptors with different reduction potentials. Photoexcitation of the Per guest yields both TTz(+)-Per(+)-ExV(2+) and TTz(2+)-Per(+)-ExV(+) in <1 ps, while back electron transfer in TTz(2+)-Per(+)-ExV(+) proceeds via the unusual sequence TTz(2+)-Per(+)-ExV(+) TTz(+)-Per(+)-ExV(2+) TTz(2+)-Per-ExV(2+). In addition, selective chemical reduction of TTz(2+) gives Per < subset of> TTzExVBox(3+), turning the complex into a D-B-A system in which photoexcitation of TTz(+) results in the reaction sequence (2)*TTz(+)-Per-ExV(2+) TTz(2+)-Per-ExV(+) TTz(+)-Per-ExV(2+). Both reactions TTz(2+)-Per(+)-ExV(+) TTz(+)-Per(+)-ExV(2+) and TTz(2+)-Per-ExV(+) TTz(+)-Per-ExV(2+) occur with a (16 +/- 1 ps)(-1) rate constant irrespective of whether the bridge molecule is Per(+) or Per. These results are explained using the superexchange mechanism in which the ionic states of the perylene guest serve as virtual states in each case and demonstrate a novel supramolecular platform for studying the effects of bridge energetics within D-B-A systems.