Influence of Distance and Angle Between Pliers and Rotors on Helicity Switching: Exploring the Structure-Property Relationship

Bisporphyrin tweezers can be rendered chiral either by introducing a stereogenic unit or by complexing with a chiral guest via axial coordination. To investigate this mode of chiral induction, four derivatives of the plier were synthesised by varying the distance between the two nitrogen atoms or altering their positional arrangement. In comparison to the meta-pyridine, the para-pyridine pliers exhibit lower photoswitching efficiency and a shorter thermal half-life, which is also observed in the case of the corresponding complexes. Upon coordination of the chiral pliers with achiral rotors, the free rotation of the rotors becomes restricted, leading to the induction of chirality in the rotor and the emergence of a circular dichroism (CD) signal. For the shorter tweezers, a change in the rotor from a meta-substituted to an ortho-substituted derivative results in a shift in chirality preference from M to P. Conversely, when keeping the meta-substituted rotor constant and altering the tweezer structure from meta-pyridyl to para-pyridyl derivative, the chirality preference changes from P to M. In contrast, the longer tweezers, due to their increased distance from the chiral axis, did not exhibit such an opposite chiral preference for ortho-substituted rotors. Moreover, for para-pyridine-substituted pliers, the porphyrin system remained CD-silent. Upon examining the photoswitching behaviour of these complexes, helicity inversion of the rotor was observed in only one case, while all other cases showed no such effect.