Through the incorporation of a polycyclic heteroaromatic π-linker between their thiophene units, dithienylethene switches are shown computationally to exhibit a photocyclization reaction well exploitable for solar-energy storage, while also occupying a sweet spot for such applications where contrasting requirements on energy-storage densities and thermal cycloreversion barriers can be met.

Abstract

Dithienylethene photoswitches with an aromatic π-linker as the bridge between the two thiophene units are attractive starting materials for developing molecular solar thermal energy (MOST) storage systems, partly because the aromaticity of their ring-open forms is a favorable feature with regard to the energy-storage densities of their ring-closed forms produced by photoinduced electrocyclization (photocyclization) reactions. At the same time, this typically leads to small barriers for their thermal cycloreversion reactions, which are not desirable in this context. Here, we use computational methods to show that this problem can be circumvented with polycyclic heteroaromatic π-linkers. Specifically, through the tuning of the aromatic character of the individual rings of such a π-linker (like indole or isoindole), it is shown to be possible to strike a delicate balance between the seemingly contrasting requirements of simultaneously achieving both a high energy-storage density and a large cycloreversion barrier. Furthermore, this design is also found to provide for a quick and efficient photocyclization reaction, owing to the onset of excited-state antiaromaticity in the π-linker upon light absorption of the ring-open form. Altogether, dithienylethenes with polycyclic heteroaromatic π-linkers appear to have both thermal and photochemical properties suitable for further development into future MOST systems.