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

Abstract

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.