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

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.