Abstract

Solar thermal fuels (STFs) are an emerging class of materials that store light energy in strained bonding configurations of photoresponsive molecules and release it on demand as heat. They have potential applications as source of heat energy in technology and architecture. A key requirement for STFs to function is the availability of free volume for the photoresponsive molecules to change structure in response to light. For this reason, many STFs have been developed in the solution state, although this can present limitations in terms of storage, containment and energy density. For some applications solid-state STFs would be desirable, although these are challenging to design owing to the lack of steric freedom in dense phases.

In this work, we have been developing solid-state STFs based on molecular photoswitches such as azobenzene confined within metal-organic frameworks (MOFs).[1-3] Using a combination of X-ray diffraction and solid-state NMR we are able to monitor guest-induced breathing upon loading the well-known framework DMOF -1 with azobenzene, with an associated phase transition enthalpy. NMR measurements show both the guest and framework are highly dynamic at ambient temperature. When the composite is exposed to 365 nm light we observe isomerisation to the cisisomer, which is also highly mobile. Upon thermally triggered reconversion to the ground-state trans isomer, we can demonstrate bulk solar energy storage and thermal energy release. By tailoring the guest and framework structure, we are able to optimise the energy density to reach a maximum value of 100 J g–1 which is in the range of other organic phase transition materials and sets a precedent for the further development of light-harvesting materials for thermal energy storage.

1. K. Griffiths, N. R. Halcovitch, J. M. Griffin, Chem. Mater. 2020, 32, 9925-9936.

2. K. Griffiths, N. R. Halcovitch, J. M. Griffin, Inorg. Chem. 2021, 60, 12950-12960.

3. K. Griffiths, N. R. Halcovitch, J. M. Griffin, Chem. Sci. 2022, 13, 3014-3019.