Synthesis and Characterization of Hemiindigo-Based Glycoside and Glycomacrocycle

As visible light responsive molecular tools, hemiindigo photoswitches are attracting increasing research interest. The development of chiral hemiindigo derivatives could expand their application fields. We synthesized the first hemiindigo-based glycoside and glycomacrocycle in a few steps through Click reaction. These compounds can be readily photoisomerized under visible light irradiation, with a high photoconversion yield and good fatigue resistance under anaerobic conditions. Compared to the acyclic derivatives, the glycomacrocyclic hemiindigo linked through N-substitution on the indoxyl moiety displayed a constrained molecular geometry, significantly modifying the shape of the absorption band in the visible. The hemiindigo glycomacrocyle also exhibited more efficient Z⇄E photoisomerization compared to its acyclic counterpart, with a two-fold increase of the photochromic quantum yield (0.34 for Φ_{Z → E} and 0.12 for Φ_{E → Z}). These observed specific photophysical behaviors were rationalized using DFT and TD-DFT calculations. The most twisted E-macrocycle displayed chiroptical activity, allowing an on-off chiroptical switching upon Z→E→Z photoisomerization. All of the investigated hemiindigo derivatives exhibited fluorescence in their Z-form.