General rules for the design of molecular photoswitches based on electrocyclization

Molecular photoswitches undergo a structural transformation upon excitation with light to intraconvert between two or more stable forms. In some cases, the structural rearrangement involves a cyclization reaction, that is, the transition between ring-open and ring-closed molecular forms. In this work we develop simple guidelines for the design of organic molecules able to undergo photochemical electrocyclization, using electronic structure calculations on one of the most widely-used family of molecules in the field, dithienylethene (DTE)-based compounds. We conclude that the ability to photocyclize can be predicted from the localization and symmetry of frontier molecular orbitals. These ideas are based on the Woodward- Hoffmann rules, but go beyond them, allowing the behavior of a more general family of chromophores, chemically substituted DTEs, to be understood. Moreover, our rules have been validated by assessment of a large number of molecular photoswitches that have been previously investigated experimentally. We have also explored the relationship between thermal stability of the closed form and aromaticity. We use the DTE scaffold as the workhorse model in our computational study, however, these simple yet powerful guidelines are expected to be generally valid to guide the design of other diarylethenes.