Solar thermal fuels make use of molecules that undergo reversible photo-isomerization to store solar energy and convert it into thermal energy. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable alternative energy source. Azobenzene, which undergoes trans- to cis- photo-isomerization, has drawn attention as a candidate material for solar thermal fuels. However, both isomers are photoactive in similar regions of the solar spectrum, and the metastable cis-isomer exhibits a significant absorption coefficient, leading to a photo-stationary state (dynamic equilibrium of the two directions of photoisomerization) with a significant amount of the lower energy trans isomer and a resulting low energy storage capacity. We evaluate possible solutions to this problem, modifying absorption properties and isomerization of the two isomers by (i) functionalizing azobenzene and (ii) designing close-packed semi-crystalline azobenzene/template nanostructures. We aim to maximize the overlap of absorption spectrum of trans-isomer with the solar spectrum while simultaneously minimizing that of cis-isomer with solar spectrum. Using time dependent density functional theory (TDDFT) with Casida's equation, we calculated the absorption properties of trans- and cis-photoisomers and examined how much gain we can get in storage efficiency of solar thermal fuels. We observed that cis-isomer content at the photostationary state varies from 68 to 91 percent depending on the electron-withdrawing character of functional groups and we examined the packing effect induced by templates.

Controlling the photo-stationary state of azobenzene for high efficiency solar thermal fuels: A computational study

247th National Meeting (2014)

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