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 (ExV2+) and dipyridylthiazolothiazole (TTz2+) electron acceptors doubly-linked by means of two p-xylylene linkers (TTzExVBox4+), which readily incorporates a perylene (Per) guest in its cavity (Per ⊂ TTzExVBox4+) to establish an A–D–A′ system, in which the ExV2+ and TTz2+ units serve as competing electron acceptors with different reduction potentials. Photoexcitation of the Per guest yields both TTz+˙–Per+˙–ExV2+ and TTz2+–Per+˙–ExV+˙ in <1 ps, while back electron transfer in TTz2+–Per+˙–ExV+˙ proceeds via the unusual sequence TTz2+–Per+˙–ExV+˙ → TTz+˙–Per+˙–ExV2+ → TTz2+–Per–ExV2+. In addition, selective chemical reduction of TTz2+ gives Per ⊂ TTzExVBox3+˙, turning the complex into a D–B–A system in which photoexcitation of TTz+˙ results in the reaction sequence 2*TTz+˙–Per–ExV2+ → TTz2+–Per–ExV+˙ → TTz+˙–Per–ExV2+. Both reactions TTz2+–Per+˙–ExV+˙ → TTz+˙–Per+˙–ExV2+ and TTz2+–Per–ExV+˙ → TTz+˙–Per–ExV2+ 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.
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