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

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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.

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