Coordination-assembly for quantitative construction of bis-branched molecular shuttles

The development and utilization of a new way to build molecular devices is of importance. To build a novel topology of interlocked molecular systems with a controllable mechanical motion, an axle-like compound comprising azobenzene and alkoxy isophthalate moieties was synthesized first. It would form a switchable hemi-rotaxane structure with α-cyclodextrin (α-CD) ring encapsulated in aqueous solution. Next, the hemi-rotaxane was reacted with ethylene diamine palladium nitrate (Pd(en)(NO₃)₂) and ethylene diamine platinum nitrate (Pt(en)(NO₃)₂), respectively, to quantitatively form two bis-branched molecular shuttles in situ. The bis-coordinated Pd(II) complex was formed quickly at room temperature, whereas the bis-coordinated Pt(II) one was effectively treated at 333 K but more stable than the former. In this case, transformation of ring shuttling direction could take place in the stable bis-branched Pt(II) complex. The steric effect of the co-stopper, namely the Pt(II) metal center, made the α-CD ring dynamically shuttle inwards to the alkoxy isophthalate station with the azobenzene's photoisomerization, rather than dethreading from the axle.