Terpy(Pt–salphen)2 Switchable Luminescent Molecular Tweezers

The design and synthesis of switchable molecular tweezers based on a luminescent terpy(Pt‐salphen)₂ ( 1 ; terpy=terpyridine) complex is reported. Upon metal coordination, the tweezers can switch from an open “W”‐shaped conformation to a closed “U”‐shaped form that is adapted for selective recognition of cations. Closing of the tweezers by metal coordination (M=Zn²⁺, Cu²⁺, Pb²⁺, Fe²⁺, Hg²⁺) was monitored by ¹H NMR and/or UV/Vis titrations. During the titration, exclusive formation of the 1:1 complex M( 1 )] was observed, without appearance of an intermediate 1:2 complex M( 1 )₂]. The crystallographic structure of the 1:1 complex was obtained with Pb²⁺ and showed a distorted helical structure. Selective intercalation of Hg²⁺ cations by the closed “U” form was observed. The tweezers were reopened by selective metal decoordination of the terpyridine ligand by using tris(2‐aminoethyl)amine (tren) as a competitive ligand without modification of the Pt–salphen complex. Detailed photophysical studies were performed on the open and closed tweezers. Structured emission was observed in the open form from the Pt–salphen moieties, with a high quantum yield and a long lifetime. The emission is slightly modified upon closing with 1 equivalent of Zn²⁺ or Hg²⁺, whereas a dramatic quenching was obtained upon intercalation of additional Hg²⁺.