Solar-thermal enters the ring

J. Phys. Chem. Lett. 4, 854–860 (2013)

Credit: © 2013 ACS

Harnessing energy directly from the Sun is key to meeting our increasing energy demands. Most research is focused on photovoltaics or photocatalysis, but another option is to store the energy directly in the bonds of molecules that undergo photoisomerization. In this scenario, light 'charges' the molecule, switching it to a higher energy — but still relatively stable — isomer. An external trigger can then switch the molecule back to its original form, accompanied by the release of heat. The development of practical 'solar-thermal' materials has, however, been hampered by their instability and low energy density. Now E. Durgun and Jeffrey Grossman from MIT have used density functional theory to show that these properties can be improved by incorporating photoswitchable molecules into strained molecular rings.

They exploit the photoisomerization of stable trans-azobenzene to its less stable, higher energy, cis isomer, a reaction that is found to have a ΔH value — a measure of how much energy the molecule can store — of 0.6 eV. The ΔH value is calculated for the all-trans to all-cis reactions of molecular rings formed from multiple (two to six) azobenzene groups connected by CH₂ groups. It was found that the dimer stores less energy than monomeric azobenzene, the trimer stores about the same energy and the tetramer can store around 65% more, with ΔH calculated to be ∼1 eV.

The differences in ΔH arise from steric effects caused by the imposed ring geometry with the cis- and trans-isomers destabilized to different degrees with respect to isolated azobenzene. If the trans form is destabilized less than the cis form, then this leads to an increase in ΔH. Substituting the CH₂ linkers for N₂ groups or adding stabilizing OH groups to each benzene ring were also seen to alter ΔH. The molecules with larger ΔH values for the isomerization reaction, and thus greater energy-storage capacity, also showed an increase in the stability of the high-energy isomer, leading to a longer 'shelf life' of the charged form.