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 ⊂ 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 ⊂ 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.