"The Sulfur Dance" : Polythioarenes as Universal Templates for Dynamic Covalent Chemistry – SulfurDance

Dynamic Covalent Chemistry (DCC) is fundamental in science. It relies on reversible molecular systems and reactions, which allow self-correction and adaptation to some physical, chemical, and biological environments with peculiar supramolecular and driving forces. Among these reactions, nucleophilic aromatic substitutions are among the most frequently used reactions in chemistry, but were rarely investigated in DCC. Their reversibility was shown in a few cases, which makes them of some significance to DCC, beside a number of reversible reactions already studied within this framework such as: imines, esters, disulfides, thioacyl, dithioacetals, acyl, acetals, dynamic covalent metathesis, SN2, and Diels-Alder reactions. Based on preliminary results, we would like to further explore a most important feature of SNAr reactions, their reversibility and consequently their ability to undergo facile component exchange and constitutional variation leading to adaptive features as is the case in an interesting self-correcting processes occurring in the generation of macrocyclic materials (Swager 2018). Such features open new avenues in dynamic covalent chemistry (DCC). Here, we report the features of a category of reversible nucleophilic aromatic substitution reactions in view of their significance in DCC. We chose as models some per- and poly(thio)arenes and (thio)heteroarenes (hereafter named "asterisks"). Exchange of sulfur ligands surrounding these asterisks cores can occur below 20°C in a process that one may call a "sulfur dance". These reactions present a wide scope, by involving a plethora of nucleophiles and leaving groups on various aromatic or heteroaromatic core components as multifunctional "DCC templates". It creates a manifold of synthetic, structural and physico-chemical features of new systems with their own specific behavior, apart from the aforementioned reversible reactions. The latter combine exalted (turn-on) luminescence and redox properties, as additional sensing, monitoring and imaging parameters during dynamic processes, with a library of asterisk luminophores. Perthioarenes are electron-acceptors in spite of a rich electronic density. They may present unusual photophysical properties with a high density of sulfur atoms, such as being potent all-organic phosphorescent triplet emitters with quantum yields near 100% in the crystalline state and displaying "turn-on" phosphorescence upon aggregation. It makes them "smart" and sensitive to their environment when a molecular rigidification occurs (aggregation-induced emission). Various tasks and methods are clearly identified to master these DCC systems. Persulfuration raises fundamental questions about synthesis and multiple reversible SNAr reactions. For instance, hexakis(phenylthio)benzene may be obtained at 20°C by six substitutions using benzenethiolates and C6X6 (X=F,Cl,Br) in spite of steric hindrance. Persulfuration tackles fundamental questions concerning structural and electronic features (such as aromaticity), reaction mechanisms and kinetics, as well as supramolecular, spectroelectrochemical, and photophysical properties. A plethora of photophysical, structural, analytical, separative, and spectroscopic techniques (NMR, LC-MS, MS) are required, as well as methods for structural and aggregation studies (scXRD, DOSY, DLS, SAXS). Computation at the DFT level will be used as a predictive and assessment tool. In short, the implementation of nucleophilic aromatic substitutions in DCC opens the door to many applications with relatively non-toxic substrates. This is in line with multifunctional, adaptive materials with novel properties, stimuli-responsiveness, and tunable optical character in the fields of nanoscience, imaging, materials and life sciences. In a general context, these findings will contribute to innovations in constitutional dynamic chemistry (CDC) and related chemical and biological covalent and supramolecular processes