Two-way photoswitching norbornadiene derivatives for solar energy storage

Fossil fuels currently dominate global energy consumption and may continue to increase during the next 30 years, exacerbating the already serious environmental impacts.1,2 To address this issue, molecular solar thermal energy storage (MOST) systems are being developed as a possible emission-free energy storage concept.3–7 Unlike combustible fuels, MOST materials can directly convert solar energy into chemical energy through a photoisomerization reaction.8–13 Among the most promising MOST materials are derivatives of norbornadiene–quadricyclane (NBD–QC), known for their high energy storage density and long-term energy storage capabilities.14–18

The stored energy can be released on demand, occurring either spontaneously or through external stimuli, such as electrocatalytic, catalytic, and light.19–26 While both electrocatalytic and catalytic approaches have been developed to trigger energy-releasing conversion from QC to NBD,21,23 it's worth noting that the catalytic system requires physical colocation in fixed bed reactors with the MOST system to operate. To date, very few research activities have been focusing on NBD-based MOST devices in the solid state,27,28 due to the inefficient energy release method. In contrast to other photoswitching systems, such as azobenzenes, which can be reversibly switched between Z and E forms with different wavelengths of light,29–31 there are very few examples of two-way photoswitching NBD–QC systems,32–34 and until now, none of them have been demonstrated to work in the solid state.

To this point, photoisomerization from QC to NBD has been limited due to low photoisomerization yields and the need for short ultraviolet (UV) irradiation wavelengths to activate the QC system. In the case of unsubstituted NBD, it exhibits no absorption beyond 210 nm, necessitating deep UV irradiation for QC to NBD conversion.35 To bathochromically shift absorbing wavelengths, resulting in a better overlap with the solar spectrum, donor–acceptor groups have been introduced into NBD.36–39 Additionally, the photodegradation of NBD, particularly under short UV light, has restricted the development of light-triggered systems.40,41 Guldi et al. reported a photodegradation mechanism for NBD with ester groups, wherein short UV light (258 nm) led to a localized excited state that formed a bicyclo[4.1.0] derivative,42 rendering the formed products unresponsive to photoisomerization. Therefore, addressing these challenges is essential for the development of an efficient optical energy release approach, both in solution and the solid state.

Herein, we introduce a series of NBD-based molecules specifically designed for efficient two-way photoswitching. This was achieved by incorporating acceptor groups such as ester, trifluoroacetyl, and cyano, paired with carefully chosen donors, including benzene substituted with methoxy or amide groups. To mitigate photodegradation, we have employed a tunable UV shielding strategy based on solvents or the polymer matrix to cut-off short UV light, thus enhancing the cyclability of two-way photoswitching. Furthermore, to illustrate the functionality of two-way photoswitching, we have, for the first time, established a continuously operating photo-triggered liquid flow MOST device and demonstrated solid MOST films capable of two-way photoswitching. These findings provide promising pathways for implementing two-way photoswitching NBD-based MOST systems, both in solution and in the solid state.