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

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

Solar energy storage at an atomically defined organic-oxide hybrid interface

Molecular photoswitches provide an extremely simple solution for solar energy conversion and storage. To convert stored energy to electricity, however, the photoswitch has to be coupled to a semiconducting electrode. In this work, we report on the assembly of an operational solar-energy-storing organic-oxide hybrid interface, which consists of a tailor-made molecular photoswitch and an atomically-defined semiconducting oxide film. The synthesized norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) was anchored to a well-ordered Co3O4(111) surface by physical vapor deposition in ultrahigh vacuum. Using a photochemical infrared reflection absorption spectroscopy experiment, we demonstrate that the anchored CNBD monolayer remains operational, i.e., can be photo-converted to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC). We show that the activation barrier for energy release remains unaffected by the anchoring reaction and the anchored photoswitch can be charged and discharged with high reversibility. Our atomically-defined solar-energy-storing model interface enables detailed studies of energy conversion processes at organic/oxide hybrid interfaces

Electrochemically controlled energy storage in a norbornadiene-based solar fuel with 99% reversibility

Solar fuels based on molecular photoswitches hold the potential to combine solar energy conversion, storage, and release in an extremely simple one-photon one-molecule process. In this work we demonstrate electrochemically controlled solar energy storage and release with high reversibility in a tailor-made norbornadiene photoswitch. We investigated the photochemical conversion of the energy-lean 2-cyano-3-(3,4-dimethoxyphenyl)-norbornadiene (NBD’) to its energy-rich counterpart 2-cyano-3-(3,4-dimethoxyphenyl)-quadricyclane (QC’) and the electrochemically triggered reconversion. Characteristic bands of NBD’ and QC’ were identified by density functional theory (DFT) and monitored in-situ during the energy storage and release process by photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS). We identified the stable regions of both isomers at a Pt(111) electrode and tested the stability of the NBD’/QC’ couple over 1000 storage and release cycles. It is shown that reversibilities of more than 99% per cycle can be achieved in this electrochemically triggered energy storage system