Efficient Electrocatalytic Switching of Azoheteroarenes in the Condensed Phases

Abstract

Azo-based photoswitches have shown promise as molecular solar−thermal (MOST) materials due to their ability to store energy in their metastable Z isomeric form. The energy is then released, in the form of heat, upon photoisomerization to the thermodynamically stable E form. However, obtaining a high energy density and recovering the stored energy with high efficiency requires the materials to be employed in the condensed phase and display a high degree of Z to E switching, both of which are challenging to engineer. Here, we show that arylazopyrazole motifs undergo efficient redoxinduced Z to E switching in both the solution and the condensed phase to a higher completeness of switching than achieved photochemically. This redox-initiated pathway lowers the barrier of Z to E isomerization by 27 kJ/mol, while in the condensed phase, the efficiency of electrochemical switching is improved by over an order of magnitude relative to that in the solution state. The influence of the photoswitch's phase, electrical conductivity, and viscosity on the electrochemical switching in the condensed phase is reported, culminating in a set of design rules to facilitate further investigations. We anticipate the use of an alternative stimulus to light will facilitate the application of MOST materials in situations where phototriggered heat release is unachievable or inefficient, e.g., indoor or at night. Furthermore, exploiting the electrocatalytic mechanism, whereby a catalytic amount of charge triggers Z to E switching via a redox process, bypasses the need for fine tuning of the photoswitching chromophore to achieve complete Z to E switching, thus providing an alternative approach to photoswitch molecular design.

Storing energy with molecular photoisomers

“…116 In addition, arylazopyrazolesbased AZO variants can be electrocatalytically back-converted to the condensed phase, showing an improved efficiency over an order of magnitude compared with the dissolved state. 117 Such electrocatalytic heat generation from MOST can potentially be used for applications such as car windshield de-icing.…”

Electrochemically Triggered Energy Release from an Azothiophene‐Based Molecular Solar Thermal System

“…A particularly intriguing concept is to trigger the energy release of MOST systems electrochemically. [11][12][13]17,[26][27][28][29] While the isomerization reaction itself is not a redox-reaction, charged intermediates may still initiate a chain reaction which drives the isomerisation. [17,26,27,29] In previous work, some of the authors demonstrated for NBD-based MOST systems that this approach enables direct control of the reaction kinetics.…”

“…[11][12][13]17,[26][27][28][29] While the isomerization reaction itself is not a redox-reaction, charged intermediates may still initiate a chain reaction which drives the isomerisation. [17,26,27,29] In previous work, some of the authors demonstrated for NBD-based MOST systems that this approach enables direct control of the reaction kinetics. [26] In stability tests over 1000 storage cycles, a reversibility of up to 99.8 % was achieved.…”

“…[27] Recently, Greenfield et al managed to overcome this limitation by molecular design using azoheteroarene derivatives. [29] In this work, we address the electrochemically triggered energy release from azothiophenes by photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) and density functional theory (DFT). The development of PEC-IRRAS [13] enables us to monitor the storage and release in situ over a large number of cycles.…”

Exhaustive catalytic ortho-alkoxylation of azobenzenes: flexible access to functionally diverse yellow-light-responsive photoswitches

“…Reversible E→Z and Z→E isomerisations of azobenzenes can be effected with UV/visible light 1 , as well as with redox reactions following photocatalysis 2 , electrocatalysis 3 , or X-ray illumination 4 . The high spatiotemporal precision, non-invasiveness, and rapidity with which bulk populations of azobenzenes can be photoisomerised in both E→Z and Z→E directions has made azobenzene photoswitching a powerful tool for manipulating the physical and optical properties of solid materials 5 , the electronic and dynamic properties of soft matter 6 , and for controlling protein functions and downstream cascades in biochemistry, cell biology, and in adult animals.…”