Visible light activated energy storage in solid-state Azo-BF switches

We present here a group of Azo-BF 2 photoswitches that store and release energy in response to visible light irradiation. Unmodified Azo-BF 2 switches have a planar structure with a large π-conjugation system, which hinders E-Z isomerization when in a compacted state. To address this challenge, we modified the switches with one or two aliphatic groups, which altered the intermolecular interactions and arrangement of the photochromes in the solid state. The derivative with two substituents exhibited a non-planar configuration that provided particularly large conformational freedom, allowing for efficient isomerization in the solid phase. Our discovery highlights the potential of using double aliphatic functionalization as a promising approach to facilitate solid-state switching of large aromatic photoswitches. This finding opens up new possibilities for exploring various photoswitch candidates for molecular solar thermal energy storage applications. Visible-light-absorbing photoswitches based on the Azo-BF 2 scaffold show reversible isomerization in the solid state, storing photon energy and releasing thermal energy on demand.