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

Abstract

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, 1H 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.