Norbornadiene photoswitches anchored to well-defined oxide surfaces: From ultrahigh vacuum into the liquid and the electrochemical environment

Abstract

Employing molecular photoswitches, we can combine solar energy conversion, storage, and release in an extremely simple single molecule system. In order to release the stored energy as electricity, the photoswitch has to interact with a semiconducting electrode surface. In this work, we explore a solar-energy-storing model system, consisting of a molecular photoswitch anchored to an atomically defined oxide surface in a liquid electrolyte and under potential control. Previously, this model system has been proven to be operational under ultrahigh vacuum (UHV) conditions. We used the tailor-made norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) and characterized its photochemical and electrochemical properties in an organic electrolyte. Next, we assembled a monolayer of CNBD on a well-ordered Co3O4(111) surface by physical vapor deposition in UHV. This model interface was then transferred into the liquid electrolyte and investigated by photoelectrochemical infrared reflection absorption spectroscopy experiments. We demonstrate that the anchored monolayer of CNBD can be converted photochemically to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC) under potential control. However, the reconversion potential of anchored CQC overlaps with the oxidation and decomposition potential of CNBD, which limits the electrochemically triggered reconversion.

Molecular photoswitches in aqueous environments

“…108 The formed quadricyclane absorption is strongly hypsochromically shifted compared to the starting material. [387][388][389] The isomerization can be reverted photochemically, 390 thermally, or (electro)chemically, [391][392][393] for instance by adding a metal catalyst 394,395 or acid, 396 and (photo)switching is very robust over many cycles. 388,397,398 Norbornadiene and quadricyclane are similar in size and volume and therefore do not offer many possibilities for applications where the photoswitch should induce large geometrical changes in a system.…”

Storing energy with molecular photoisomers

“…Related model studies were performed using a Co 3 O 4 (111) surface with an atomically defined structure. 118,[120][121][122][123] Monolayer films of NBD derivatives with a carboxylic acid functionality were prepared in which the carboxylic acid group serves as an anchor and binds to the surface Co 2+ ions by forming a chelating surface carboxylate. 120 Using the NBD-derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD), the authors prepared a hybrid interface with an anchored QC derivative on Co 3 O 4 (111) in UHV (Figure 6B).…”

“…Although the anchored NBD derivative was photochemically converted to the QC derivative under potential control, the back-conversion could not be triggered electrochemically because decomposition of the compounds competed with the electrochemically triggered energy release. 121 In the future, this limitation may be overcome by using anchor groups that are more resistant to electrochemical oxidation.…”

Optimization of the thermochemical properties of the norbornadiene/quadricyclane photochromic couple for solar energy storage using nanoparticles

“…1, since these have been shown to provide great performance in MOST application where both catalytic and electrochemical activation of the back reaction have been utilized. [22][23][24] The main question we wish to address is if the thermal back reaction can be promoted without compromising the energy storage capabilities of the system using a NP? To answer this question we will address the following:…”