Improving Long-Range Intramolecular Proton Transfer—Further Molecular Design Using the Successful Molecular Switch 8-(Benzo[d]thiazol-2-yl)quinolin-7-ol (HQBT) as a Structural Model

Proton cranes are single-molecule photoswitches with rotor and stator parts attached, where the switching event is based on a multi-step, long-range intramolecular proton transfer within the stator. The process of intramolecular motion makes such structures prototypes for machines at a nanomolecular level with broad potential applications as novel materials, necessitating a multifaceted approach to their investigation. Theoretical design of conjugated tautomeric proton cranes, using benzothiazole as a tautomeric rotor and a variety of tautomeric OH-containing heterocycles as possible stators, has been attempted by using density functional theory calculations in various environments. The shape of the ground-state potential energy surface has been used to estimate the suitability of possible proton cranes. A previously developed and studied proton crane, named HQBT, in which the benzothiazole rotor is attached to the eighth position of the quinoline-7-ol stator, has been used as a comparative example. The results indicate that under certain conditions, cinnolin-7-ol-based proton cranes could have practical applicability in non-polar media, where the performance of HQBT is not satisfactory.