Photochemical Energy Storage and Electrochemically Triggered Energy Release in the Norbornadiene–Quadricyclane System: UV Photochemistry and IR Spectroelectrochemistry in a Combined Experiment

Abstract

The two valence isomers norbornadiene (NBD) and quadricyclane (QC) enable solar energy storage in a single molecule system. We present a new photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) experiment, which allows monitoring of the complete energy storage and release cycle by in situ vibrational spectroscopy. Both processes were investigated, the photochemical conversion from NBD to QC using the photosensitizer 4,4'-bis(dimethylamino)benzophenone (Michler's ketone, MK) and the electrochemically triggered cycloreversion from QC to NBD. Photochemical conversion was obtained with characteristic conversion times on the order of 500 ms. All experiments were performed under full potential control in a thin-layer configuration with a Pt(111) working electrode. The vibrational spectra of NBD, QC, and MK were analyzed in the fingerprint region, permitting quantitative analysis of the spectroscopic data. We determined selectivities for both the photochemical conversion and the electrochemical cycloreversion and identified the critical steps that limit the reversibility of the storage cycle.

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

“…Upon cycloreversion of QC back to NBD, the system can release up to 100 kJ mol −1 of thermal energy, which defines it as a solar fuel. The release reaction can be initiated by oxidation (via radical cations), thermally, or electrochemically . The introduction of a protonation‐prone substituent (R=H, X=CN, Y=Me 2 N−Ph−C≡C−) resulted in a fully photoreversible switch, constituting at the same time a three‐input (acid, base, and λ =310 nm light) molecular keypad lock; a rare example of a molecular logic system …”

Turn-off mode fluorescent norbornadiene-based photoswitches

“…Once the QC-form is formed, it has several possible mechanistic pathways to revert back to the NBD-form, 30 – 32 where one of the most common routes is by thermal activation. 27 When the QC-form is heated, it relaxes back to the NBD-form.…”

“…Another photochromic molecule that has seen a renewed interest recently due to possible applications in solar energy storage 27 – 30 and data storage 3 is the norbornadiene–quadricyclane (NBD–QC) system ( Scheme 2 ). The NBD–QC pair is a T-type 5 negative photochrome, where the unsaturated diene, i.e.…”

“…the NBD-form, converts to the saturated QC-form upon photoirradiation. When desired, the QC-form can be triggered to relax back to the NBD-form thermally, 27 electrochemically, 30 by catalytic activation 31 or even using light. 32 – 34 This class of photochromic molecules has seen a recent rise in interest, since a relatively low energy photon is used to initiate the photoisomerization of the parent molecule to its isomer compared to positive photochromes.…”

A UV‐LEDs based photomicroreactor for mechanistic insights and kinetic studies in the norbornadiene photoisomerization

“…The features of the well-suited photosensitizer in our case are summarized as follows: high triplet energy (at least larger than the triplet energy of NBD), high intersystem crossing (ISC) rate compared to other deactivation processes of the singlet excited state (S 1 ), long triplet lifetime, high absorbance at 368.3 nm, and low chemical activity 35. In order to find the triplet photosensitizer that suits the NBD/QC photoisomerization system well, four potential triplet PSs, including 4,4 0 -Bis(diethylamino)benzophenone (diethyl Michler's ketone, EMK),36 4,4 0 -bis(dimethylamino) benzophenone (Michler 0 s ketone, MK),37 4,4 0 -diaminobenzophenone (DABZP), and 3,3 0 -4,4 0 -benzophenonetetracarboxylic dianhydride (BTDA)20 were selected(Figure 3a). Absorbance spectra of these photosensitizers dissolved in acetone were analyzed by the UV/VIS spectrophotometer (SHIMADZU UV1800) at room temperature.…”