Abstract

Efficient photoisomerization between the cis and the trans states of azobenzenes using low-energy light is desirable for a range of applications in, e.g., photobiology yet challenging to accomplish directly with modified azobenzenes. Herein, we utilize molecular iodine as a photocatalyst to induce indirect cis-to-trans isomerization of 4,4′-dimethoxyazobenzene with 770 nm near-infrared light, showing robustness during more than 1000 cycles in ambient conditions. Intriguingly, the catalysis is mediated by molecular oxygen, and we demonstrate that other singlet-oxygen-generating photosensitizers besides iodine, i.e., palladium phthalocyanine, catalyze the isomerization as well. Thus, we envision that the approach can be further improved by employing other catalysts with suitable photoelectrochemical properties. Further studies are needed to explore the applicability of the approach with other azobenzene derivatives.

Highlights

  • Molecular iodine is known to catalyze organic reactions when illuminated with visible light

  • Solvents with nucleophilic electron pairs blue shift the absorption spectrum of iodine, so we studied the isomerization in three inert solvents of varying density and polarity: dichloromethane (DCM), carbon tetrachloride (TCM), and n-hexane

  • We have shown that singlet oxygen generated upon NIR light excitation of molecular iodine or another sensitizer can be used to induce robust azobenzene photoswitching

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Summary

■ INTRODUCTION

Molecular iodine is known to catalyze organic reactions when illuminated with visible light. An important photochemical reaction that iodine is known to photocatalyze is the cis−trans isomerization of azobenzenes,[21,22] a photoswitch family utilized in photoresponsive pharmaceuticals,[23,24] catalysts,[25] and materials.[26−29] These applications benefit from precisely controlled light-driven isomerization, preferably with low-energy (red/near-infrared) light.[30] the lifetime of the metastable cis isomer is typically short for red-light-absorbing azobenzenes, an undesired feature in most applications This deficiency can be addressed by synthetic modifications, especially ortho substitution with certain moieties[31−35] that stabilize the cis isomer and in some cases separate the low-energy n−π* absorption bands of the isomers, allowing selective trans−cis and cis−trans photoisomerization with visible light. We set out to explore the photocatalytic properties of molecular iodine with the aim of controlling the azobenzene isomerization with low-energy light while broadening the utility of molecular iodine in other photocatalytic processes

■ RESULTS AND DISCUSSION
■ CONCLUSIONS
■ ACKNOWLEDGMENTS
■ REFERENCES
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