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

Jiawang Zhou,Michael R Wasielewski,Indranil Roy,Ryan M Young,Avik Samanta,J Fraser Stoddart,Yilei Wu

doi:10.1039/c8sc05514a

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.

Highlights

We show that an asymmetric cyclophane incorporating two electron acceptor subunits with different reduction potentials is capable of hosting a Per electron donor in its cavity, and can serve as an excellent molecular platform for this purpose
Photoexcited radical anions of polycyclic aromatic molecules can act as strong reductants;[38,39,40,41,42] we show that this is true for the TTz+c radical cation, which makes the Per 3 TTzExVBox3+c complex a useful D–B–A system for studying the role of a non-covalently linked bridge unit in electron transfer reactions initiated from excited doublet states
Since the two reduced electron acceptors ExV+c and TTz+c observed at 1160 and 1340 nm, respectively, decay with the same apparent time constant, sBET 1⁄4 11.4 Æ 0.5 ps, while the back electron transfer (BET) reactions in the symmetric cyclophanes Per 3 ExVBox4+ and Per 3 TTzBox4+ occur in sBET 1⁄4 39.7 Æ 0.3 ps and sBET 1⁄4 5.6 Æ 0.3 ps (Fig. S8†), respectively, the decay of the higher energy TTz2+–Per+c–ExV+c intermediate must involve a more rapid competitive pathway. We propose that this pathway involves the intramolecular charge transfer (ICT) reaction sequence TTz2+–Per+c–ExV+c / TTz+c–Per+c–ExV2+ / TTz2+–Per–ExV2+, in which the ionic states of Per+c are acting as the bridge states in an ICT superexchange mechanism

Summary

Introduction

Cavities,[15,16] and these supramolecular complexes are important for developing an understanding of photoinduced electron transfer reactions in acceptor–donor–acceptor (A–D–A) systems. The strong in uence of the Per bridge on the electron transfer rates is further demonstrated by a control experiment with the TTzExVBox3+c cyclophane itself without the Per guest, where the rate of the TTz2+–ExV+c / TTz+c–ExV2+ BET reaction is about 8 times slower than that of the TTz2+–Per–ExV+c / TTz+c–Per– ExV2+ reaction measured in Per 3 TTzExVBox3+c Despite their unusual nature, these results can be explained in the context of the superexchange mechanism[27] (vide infra), where the electron transfer rates in these systems are controlled by mixing the donor and acceptor states with the closely lying virtual states of the guest molecule. All samples were stirred to avoid localized heating or degradation effects during optical measurements

Results and discussion

16 Æ 1 135 Æ 2

Conclusions