Protonation-Gated Orthogonal Control of Highly Efficient Molecular Motors With Broad Visible-Light Responsiveness

Molecular motors with red-shifted absorption are highly attractive for biomedical applications, yet remain challenging to realize because conventional strategies for extending excitation wavelengths often compromise photochemical efficiency and directional motor function. Here, we report a protonation-gated strategy in which reversible modulation of conjugation topology enables state-dependent molecular motor operation across a broad visible-light window. Modifying coumarin-derived overcrowded alkenes with benzazole substituents, the resulting motors undergo efficient photoisomerization under blue-green light irradiation. Photophysical studies supported by DFT calculations reveal that protonation induces a pronounced conformational inversion with the formation of intramolecular hydrogen bonds, thereby promoting molecular planarization and enhanced electronic delocalization, leading to the efficient photoisomerization under 530-600 nm irradiation while quantitative photoconversion is retained. Overall, these findings establish reversible protonation as a powerful chemical gating strategy to expand the visible-light responsiveness of the motor without permanent structural modification, paving the way for its application in multimodal responsive bioimaging and advanced optical devices.