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.