Cyclic Zinc(II) Bisporphyrin-Based Molecular Switches: Supramolecular Control of Complexation-Mediated Conformational Switching and Photoinduced Electron Transfer.

A cyclic zinc(II) bisporphyrin with flexible linker was employed as a dynamic molecular switch under the regulation of π-acceptors (tetracyanoquinodimethane, trinitrofluorenone, 9-dicyanomethylenefluorene) and bidentate N-donor ligands (1,4-diazabicyclo[2.2.2]octane, pyrazine, 4,4'-bipyridine). The cyclic bisporphyrin host can efficiently encapsulate the π-acceptor guests through the strong π-π interaction, which can be replaced again by using a bidentate N-donor ligand, which coordinates strongly with the metal centers. The open conformation of the bisporphyrin can be efficiently recovered by removing the bidentate ligands using Cu(+) ion. During the process, two porphyrin rings also reversibly change their relative orientation between perpendicular and parallel. The behavior of the cyclic bisporphyrin was followed by using UV/Vis, (1)H NMR, fluorescence, and electrochemical analyses along with X-ray structure determination of the complexes. Moreover, control of photoinduced electron transfer (PET "ON-OFF") is also achieved by the use of guest exchange. Association constants for the host-guest binding were very high, which further explains the robust nature of such assemblies in solution. The experimental evidence is supported by DFT calculations. Such controllable dynamic features can constitute a new step towards "smart" adaptive molecular devices and the emergence of such systems is of significant interest in supramolecular chemistry.