Hetero-Hydrazone Photoswitches

The fine-tuning of the (photo)physical properties of molecular photoswitches remains an active area of research, and recently, the incorporation of heterocycles into photoswitch scaffolds has emerged as an effective strategy in this vein. To assess the influence that heterocyclic rings have on hydrazone-based systems, we synthesized a series of photoswitches and examined the impact that heterocycles have on the switching efficiency. TD-DFT calculations and structure-property analyses revealed that heterocycles with basic nitrogen and secondary hydrogen-bonding sites (e.g., imidazole) show poor switching efficiency and undergo thermal back-isomerization via an unusual mechanism involving tautomerization followed by rotation, resulting in fast thermal isomerization rates (i.e., half-lives of seconds to minutes). In contrast, less basic heterocycles such as benzoxazole and benzothiazole favor an inversion pathway, leading to improved bistability. Hydrazones lacking secondary hydrogen-bonding sites, on the other hand, exhibit significantly enhanced photostationary states, improved quantum yields, and red-shifted activation wavelengths compared to their 1st-generation phenyl-based analogs. These results showcase the importance of heterocycle basicity and electron-donating ability, in addition to secondary H-binding sites on photoswitching efficiency, features that are not fully elaborated on in other heterocycle-containing switches. These findings, based on the 2nd generation hydrazone switches, establish key design principles for tuning the bistability and efficiency of such photoswitches.