Abstract
<jats:p>Aza-diarylethenes (aza-DAEs) are an emerging class of molecular photoswitches that allow reversible C-N bond formation and cleaving upon irradiation. They show not only high potential as T-type photochromes but offer exciting opportunities for the exploration of unique reactivity control. Two types of these photoswitches are currently investigated, which differ substantially in the nature of their metastable states. While classical DAE behavior is found in perfluorinated thiazole derivatives, an unexpected zwitterion formation and concomitant aromatization is observed for non-fluorinated pyrimidine derivatives. The latter offer reversible control over intramolecular charge-separation and intrinsic three-state photoswitching. Their metastable state unleashes thiolate reactivity, which can be harnessed e.g. in photoinitiation applications for smart polymer generation. Despite this highly promising progress, the reason for the divergent photochemical behavior in aza-DAEs is not understood right now. Here we show that substitution of the central five-membered ring is responsible for the different photochemical outcomes and nature of the metastable state. Guided by theory, we deliver a comprehensive mechanistic explanation why the different behavior occurs. We then use this knowledge to reprogram thioazole-derivatives and render them into zwitterion-forming aza-DAEs. With these results, predictable and rational design of aza-DAEs is enabled, which can now be radically changed in their photoresponses and tailored to specific desired properties and applications.</jats:p>