Chiral Molecular Rotors Based on Mixed (Phthalocyaninato)(Porphyrinato) Rare-Earth Triple-Decker Complexes

The effective control over the rotary motions of molecular rotors still remains an enormous challenge. Herein, mixed (phthalocyaninato)(porphyrinato) rare-earth triple-decker complexes (R)-/(S)-Eu and (R)-/(S)-Y have been designed and investigated as a new type of molecular rotors with a phthalocyanine ligand as the rotator and a strapped bisporphyrin ligand as the stator. The rotational rates and thermodynamic parameters of the rotors were determined through variable-temperature 1H NMR experiments, revealing a higher rotational rate and a reduced rotational barrier for (R)-Y, in comparison to those of (R)-Eu. Variable-temperature CD experiments demonstrated that these chiral molecular rotors exhibited temperature-dependent chiroptical properties. Upon elevating the temperature, the reduction of CD intensity is closely related to the conformational perturbation involving the increased rotational rate. The DFT calculation results further elucidate that the increased rotational rate of the phthalocyanine rotator in (R)-Y can be attributed to the reduced steric interaction between the phthalocyanine rotator and the binaphthol linkage of the bisporphyrin stator owing to the diminished radius from Eu(III) to Y(III). Thus, effective control over the rotary motions can be realized by tuning the steric interactions involving adjustment of center metal ions in this new type of molecular rotors.