Selective photoinduced charge separation in perylenediimide-pillar[5]arene rotaxanes

Nicholas Pearce,Stephen P Argent,Ferdinando Malagreca,Xue Z Sun,Katherine E A Reynolds,Akihito Yamano,Michael W George,Neil R Champness,Sho Ito,Christian J Schürmann,E Stephen Davies,Surajit Kayal

doi:10.1038/s41467-022-28022-3

Abstract

The ability to control photoinduced charge transfer within molecules represents a major challenge requiring precise control of the relative positioning and orientation of donor and acceptor groups. Here we show that such photoinduced charge transfer processes within homo- and hetero-rotaxanes can be controlled through organisation of the components of the mechanically interlocked molecules, introducing alternative pathways for electron donation. Specifically, studies of two rotaxanes are described: a homo[3]rotaxane, built from a perylenediimide diimidazolium rod that threads two pillar[5]arene macrocycles, and a hetero[4]rotaxane in which an additional bis(1,5-naphtho)-38-crown-10 (BN38C10) macrocycle encircles the central perylenediimide. The two rotaxanes are characterised by a combination of techniques including electron diffraction crystallography in the case of the hetero[4]rotaxane. Cyclic voltammetry, spectroelectrochemistry, and EPR spectroscopy are employed to establish the behaviour of the redox states of both rotaxanes and these data are used to inform photophysical studies using time-resolved infra-red (TRIR) and transient absorption (TA) spectroscopies. The latter studies illustrate the formation of a symmetry-breaking charge-separated state in the case of the homo[3]rotaxane in which charge transfer between the pillar[5]arene and perylenediimide is observed involving only one of the two macrocyclic components. In the case of the hetero[4]rotaxane charge separation is observed involving only the BN38C10 macrocycle and the perylenediimide leaving the pillar[5]arene components unperturbed.

Highlights

Using time-resolved infra-red (TRIR) spectroscopy we show that photoinduced charge transfer in hetero[4] rotaxane [PDI-BN38C10-(P5A)2](PF6)2 proceeds between the PDI and BN38C10 components and does not involve pillar[5]arene groups, as observed in [PDI-(P5A)2](PF6)2, the pathway of energy transfer in these mechanically interlocked rotaxanes is directed by the nature and relative organisation of the macrocycles
The typical strategy for the synthesis of pillararene-based rotaxanes relies upon the strong interaction between the electron-rich cavity of P5A and electrondeficient threading groups, such as imidazoles25,28,29
We synthesised an N,N′-symmetrical PDI equipped with octylimidazole groups to serve as the axle of the two different rotaxanes, providing the necessary threading groups for interactions with the P5A macrocycles

Summary

Introduction

We show that hetero-rotaxanes can be employed to create assemblies of different redox- and photo-active components that enable selective tuning of energy transfer pathways. Using time-resolved infra-red (TRIR) spectroscopy we show that photoinduced charge transfer in hetero[4] rotaxane [PDI-BN38C10-(P5A)2](PF6)2 proceeds between the PDI and BN38C10 components and does not involve pillar[5]arene groups, as observed in [PDI-(P5A)2](PF6)2, the pathway of energy transfer in these mechanically interlocked rotaxanes is directed by the nature and relative organisation of the macrocycles.

Results

Conclusion