Tetraphenylethylene: a versatile supramolecular framework

Supramolecular chemistry is a branch of chemistry that focuses on chemical systems that are made up of a discrete number of assembled molecular components held together by various non-covalent interactions. Supramolecular systems are rarely designed from first principles. Rather, chemists have a range of well-studied structural and functional building blocks that they are able to use to build up larger functional architectures. We have chosen tetraphenylethylene (TPE) as the supramolecular building block in designing various functional materials because of interesting optical and electronic properties of TPE derivatives. We have utilized several intermolecular interactions like hydrogen bonding, coordinate bonding and halogen bonding to obtain materials with remarkable optical and electronic properties in the solid state as well as solution phase that can have potential applications in fields like crystal engineering, material science and organic electronics. TPE functionalized with four carboxylic acid groups was synthesized and crystallized with various bis(pyridines) to yield organic semiconducting materials. These crystals have been characterized by single crystal X-ray diffraction and conducting properties have been studied using conducting probe-atomic force microscopy. Semiconducting properties of these materials can be tuned based on bis(pyridine) component. Two different tetrapyridyl substituted TPEs have been synthesized and their photoluminescent properties have been studied in solution. Fluorescence emission was found to be switchable as a function of solvent mixture as well as pH. Both compounds have been structurally characterized in their free base form as well as in their protonated form as tetraperchlorate salts via X-ray diffraction.

These three compounds have been utilized as supramolecular building blocks in metal organic frameworks (MOFs) as well as organic co-crystals mediated by hydrogen bonding as well as halogen bonding. A fluorescent complex of the tetraacid with zinc has been obtained which crystallized in a non-centrosymmetric space group due to solvent and water ligands on the zinc center. A coordination polymer has been obtained via solvothermal synthesis using tetrapyridyl TPE and zinc chloride. Lewis basicity of tetrapyridyl TPE's has also been utilized in organic co-crystalline assemblies mediated by halogen bonding interactions with iodoperfluoroarenes. Finally, Halobenzoyl esters of TPE based compounds have been synthesized and halogen bonding properties of these compounds have been exploited to achieve solid state porous networks.