Visible‐Light‐Driven Photoisomerization and Increased Rotation Speed of a Molecular Motor Acting as a Ligand in a Ruthenium(II) Complex

Abstract Toward the development of visible‐light‐driven molecular rotary motors, an overcrowded alkene‐based ligand and the corresponding ruthenium(II) complex is presented. In our design, a 4,5‐diazafluorenyl coordination motif is directly integrated into the motor function. The photochemical and thermal isomerization behavior has been studied by UV/Vis and NMR spectroscopy. Upon coordination to a Ru II bipyridine complex, the photoisomerization process can be driven by visible ( λ max =450 nm) instead of UV light and furthermore, a large increase of the speed of rotation is noted. DFT calculations point to a contraction of the diazafluorenyl lower half upon metal‐coordination resulting in reduced steric hindrance in the “fjord region” of the molecule. Consequently, it is shown that metal‐ligand interactions can play an important role in the adjustment of both photophysical and thermodynamic properties of molecular motors.