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

Abstract

Molecular solar thermal (MOST) systems combine solar energy conversion, storage, and release in simple one‐photon one‐molecule processes. Here, we address the electrochemically triggered energy release from an azothiophene‐based MOST system by photoelectrochemical infrared reflection absorption spectroscopy (PEC‐IRRAS) and density functional theory (DFT). Specifically, the electrochemically triggered back‐reaction from the energy rich (Z)‐3‐cyanophenylazothiophene to its energy lean (E)‐isomer using highly oriented pyrolytic graphite (HOPG) as the working electrode was studied. Theory predicts that two reaction channels are accessible, an oxidative one (hole‐catalyzed) and a reductive one (electron‐catalyzed). Experimentally it was found that the photo‐isomer decomposes during hole‐catalyzed energy release. Electrochemically triggered back‐conversion was possible, however, through the electron‐catalyzed reaction channel. The reaction rate could be tuned by the electrode potential within two orders of magnitude. It was shown that the MOST system withstands 100 conversion cycles without detectable decomposition of the photoswitch. After 100 cycles, the photochemical conversion was still quantitative and the electrochemically triggered back‐reaction reached 94 % of the original conversion level.

How Adsorption Affects the Energy Release in an Azothiophene-Based Molecular Solar–Thermal System

“…In addition, we observe negative bands at 1485, 1399, and 1351 cm −1 , which we assign to the ν(NN−CC) azo-thiophene , ν(CC) thiophene , and ν(CH) thiophene of (Z)-3-cyanophenylazothiophene. 30 Note that all spectra are difference spectra, and positive bands indicate consumed species, while negative bands indicate formed species. Therefore, the appearance of the positive and negative bands indicates the conversion from the (E)-to the (Z)isomer.…”

“…Thereby, the dominant conformer is the syn conformer (>99.5%), however, the anti conformer is also present and was therefore considered. 30 The corresponding adsorption energies of the considered (Z)-and (E)-isomers on graphite(0001), Pt(111), and Au(111) are summarized in Table 1, and the adsorption motif of the most preferred syn (Z)-and (E)-isomers is depicted in Figure 4. Note that we provide additionally the relative energies in Table S2.…”

“…17,39 In a recent study, we demonstrated that the energy release of the heteroarylazobenzene 3-cyanophenylazothiophene can be triggered electrochemically with high cyclability. 30 Heterogeneously catalyzed energy release from NBD drivatives was achieved over Ni(111), 35 Pt(111), 38 anchored cobalt(II) porphyrin derivatives, 7 metal oxides, 40 and Cu and Sn salts. 41 For the catalytically triggered energy release in azobenzenes ((Z) to (E) isomerization) different catalysts were found, e.g., cobalt(II) phthalocyanine physisorbed on activated carbon (CoPc@C), [Cu(CH 3 CN) 4 ]PF 6 , 16 and gold in the form of nanoparticles 21,42 and bulk materials.…”

“…To build an efficient MOST storage device, however, it is also essential to control the energy release, an aspect that has received less attention. The most promising approaches to trigger the energy release are the electrochemical ,− or the catalytic pathway. − Both approaches offer a high degree of controllability. In the electrochemical approach, the energy release is controlled directly by the applied potential.…”

“…The energy release in a heterogeneously catalyzed process is controlled using flow reactors. , While the electrochemical energy release requires a catalytically inactive electrode material, heterogeneous catalysts must be catalytically highly active and selective . For E / Z -AZOs and HetABs, electrochemical triggering can be achieved via a reductive reaction pathway. , In a recent study, we demonstrated that the energy release of the heteroaryl­azobenzene 3-cyanophenyl­azothiophene can be triggered electrochemically with high cyclability …”