Dynamic chemical devices: photoinduced electron transfer and its ion-triggered switching in nanomechanical butterfly-type bis(porphyrin)terpyridines

A series of butterfly-type molecular constructs has been prepared in good yield by using a double Stille coupling synthetic protocol. They are composed of a terpyridine (terpy) scaffold and two wings composed of appended porphyrins that are capable of switching from an extended W geometry to a compact U geometry upon cation coordination of the terpy unit. The porphyrin moieties exist in the constructs either as free bases or they can be sequentially metallated, thus giving rise to wings of different "colours". Stationary and time-resolved emission studies of the HZn, ZnAu and Zn2Au constructs show that the electronic properties are strongly dependent on the geometry. In the extended W conformation an energy-transfer process is seen from the free base to the Zn-metallated porphyrin. In the U conformation in Zn2Au the donor luminescence resulting from the singlet excited state of the Zn wing is strongly, quenched not only due to the heavy atom effect but also due to a fast electron-transfer process to the ground state of the Au wing. Furthermore, the binding of (alpha,omega)-diamine substrates to the Zn(II)-porphyrin sites can also influence the conformation of the system. For the Zn2Zn construct, single-crystal diffraction experiments with synchrotron radiation allowed the structure to be solved by direct methods and fully refined; it shows the expected U conformation. The central Zn atom is six-coordinate, whereby the zinc atom is coordinated by the eta3-terpy ligand as well by monodentate and semi-chelating acetate anions. The structure is made rigid by hydrogen bonds involving the aqua ligands on the outer Zn centres and acetate oxygen atoms. The present system thus represents a double-trigger-modulated optomechanical switching device with selective substrate binding for either metal atoms or tailored ligands. Both energy- and electron-transfer processes can be controlled opening a means of improving the on/off ratio in future constructs.     

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, 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.     

Modification of Fluorescent Photoinduced Electron Transfer (PET) Sensors/Switches To Produce Molecular Photo‐Ionic Triode Action

The fluorophore‐spacer₁‐receptor₁‐spacer₂‐receptor₂ system (where receptor₂ alone is photoredox‐inactive) shows ionically tunable proton‐induced fluorescence off‐on switching, which is reminiscent of thermionic triode behavior. This also represents a new extension to modular switch systems based on photoinduced electron transfer (PET) towards the emulation of analogue electronic devices.     

Photoinduced Electron Transfer of Porphyrin Isomers: Impact of Molecular Structures on Electron‐Transfer Dynamics

Porphyrins have been investigated for a long time in various fields of chemistry owing to their excellent redox and optical properties. Structural isomers of porphyrins have been synthesized, namely, porphycene, hemiporphycene, and corrphycene. Although the number of studies on these structural isomers is limited, they exhibit interesting properties suitable for various applications such as photovoltaic devices, photocatalysts, and photodynamic therapy. In the present review, we summarized their photoinduced electron‐transfer processes, which are key steps of various photofunctions. Their electrochemical and photophysical properties are summarized as basic properties for the electron transfer. Furthermore, differences among these isomers in the electron‐transfer processes are clarified, and its origin has been discussed on the basis of their molecular structures.