Pi-Radicals as Building Blocks for Electron-Responsive Molecular materials and Organized Assemblies – PRIMO

Two strategies will be used to exert an electronic control over the assembled and disassembled states of molecular systems or to construct oriented molecular assemblies:
(i) the formation of soluble coordination polymers and supramolecular objects that will include redox-responsive viologen-based hinges in their skeleton
(ii) taking advantage of the interactions between cation-radicals of viologens (p-dimer) to generate linear stacks of aromatic building blocks. The thermal stability of self-assembled structures is a usual drawback of supramolecular scaffolds, thus, in the latter case, a posteriori locking of the assemblies will be used.

Since this project began in November 2012, work has focussed on the synthesis of 4,4'-bipyridium fragments capable of pi-dimerization. Using derivatives built upon a flexible calix[4]arene hinge, we showed that the orbital overlap between the two electrogenerated radical cations of the pi-dimer depend on the structure (length, steric bulk) of the organic linker between the calixarene and the 4,4'-bipyridinium. In another aspect of the project, a general path was developed for the seven-step synthesis of 4,4'-bipyridine derivatives bearing four alkyl ethers of various lengths.

Numerous intellectual and technical obstacles still remain on the pathway leading to smart molecular material that will find applications in the market place. However, it is now recognized that controlling the organization within supramolecular systems through discrete switchable molecular processes is a key objective essential to many current challenges in nanosciences.
This is a fundamental research project that takes an active part in the highly competitive world-wide race to discover useful and marketable technologies based on switchable molecules. The main objective of this project is to produce knowledge and explore new designs for switchable molecular materials. Fundamental information or discoveries may find useful applications in various fields related to materials science and nanoscience.

An article reporting the redox control of molecular motions in bipyridinium appended calixarenes was recently submitted for publication.
Preliminary results were also presented and discussed in formal lectures delivered by the project's partners in the context of seminars or conferences.

The aim of this proposal is to explore a new concept of redox-responsive molecular materials. Electron-responsive switchable architectures have long been recognized as the most straightforward and viable choice to produce practical devices; however, efficiently designing such systems on the molecular scale still remains a challenge. The approach developed in this project relies on the electrochemically triggered production of pi-radicals from viologen derivatives. The objective is to induce the formation of non-covalent and reversible binding between these pi-radicals to control the conformation of supramolecular scaffolds. The nanomaterials will comprise molecular wires arising from coordination polymers and molecular wires produced by pi-interactions along the wire’s axis implemented by peripheral weak interactions such as lipophilic side chain aggregation or H-bonding. The initial efforts will focus on the optimization of pi-dimer and pimer formation, with or without assistance of secondary interactions. After determining the parameters that yield the most efficient pi-directed self-assembly, suitable viologen based motifs will be introduced in more sophisticated building blocks. During the synthesis of the assemblies, pi-dimerization used as the primary driving force for the self-assembly of nanomaterials may be consolidated by covalent locking of the structures. In the end, the electro-stimulation of pi systems will provide dynamic molecular assemblies in which the morphology will be responsive to redox stimuli.
Two partners from complementary fields and common interests have grouped together to present this proposal: "Département de Chimie Moléculaire", UMR 5250 CNRS/UJF, Grenoble, "Institut de Chimie de Strasbourg", UMR 7177 CNRS/UDS, Strasbourg