Reversible Photoreduction as a Trigger for Photoresponsive Gels

Emily R Draper,Martijn A Zwijnenburg,Victor Chechik,Philip Groves,Dave J Adams,Ralf Schweins,Riaz Akhtar

doi:10.1021/acs.chemmater.6b02677

Abstract

We present here a new type of photoresponsive, reversible low molecular weight gel. All previous examples rely on a photoisomerisation, ring-closing or dimerization. We show that photoreduction of a perylene bisimide gelator results in the formation of a stable radical anion. The formation of the radical anion results in a change in the packing of the perylene bisimides in the self-assembled aggregates, leading to a change in fibrous network and an increase in the rheological properties of the gels. An increase in the rheological properties is extremely rare for a photoresponsive gel; normally, irradiation results in a gel-to-sol transition, and the gel falling apart. As the radical anion decays, which takes several hours in air, the original gel properties are restored. This photoreduction can be cycled many times. Finally, we show that the mechanical properties are different between irradiated and nonirradiated sections in a patterned gel.

Highlights

Using light as a trigger for changing the properties of materials allows spatial resolution and controlled patterning
The gels are typical low molecular weight gels; rheologically, they break at relatively low strain (3%) and are essentially frequency independent with G′ = 480 Pa and G′′ = 25 Pa (Figure 1c,d)
The choice of wavelength is important. Wavelengths larger than this do not result in formation of the radical anion despite the strong absorption of the Perylene bisimides (PBIs) at 450 nm

Summary

■ INTRODUCTION

Using light as a trigger for changing the properties of materials allows spatial resolution and controlled patterning. The photoinduced ring closing and opening of dithienylethenes can be used to prepare photochromic gels.[10] Ring closing of spiropyrans has been used.[11] For all of these examples where irradiation leads to change in the rheological properties, the end result is generally the destruction of the gel. Van Herpt et al showed that a gel could be maintained as the color was changed via photoinduced ring-closure for a dithienylethene-based gel, the rheological properties were not measured after irradiation.[10] We recently demonstrated a rare exception where the mechanical properties of the gel were improved on irradiation, using the dimerization of a coumarinbased gelator. Samples for UV−vis measurements were placed in a demountable 0.1 mm quartz cuvette and irradiated with a LED for 5 min and the spectra collected.

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES