Abstract
Although polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesised directly from solution. One-dimensional (1D) covalent polymers that are packed into a framework structure can be viewed as a 1D covalent organic framework (COF), but making a single crystal of this has been elusive. Herein, by combining labile metal coordination and dynamic covalent chemistry, we discover a strategy to synthesise single-crystal metallo-COFs under solvothermal conditions. The single-crystal structure is rigorously solved using single-crystal electron diffraction technique. The non-centrosymmetric metallo-COF allows second harmonic generation. Due to the presence of syntactic pendant amine groups along the polymer chains, the metallopolymer crystal can be further cross-linked into a crystalline woven network.
Highlights
Polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesised directly from solution
Depending on the nature of the strongest bond used in constructing the solids, framework materials are categorised into metal-organic frameworks (MOFs)[3,4,5], covalent organic frameworks (COFs)[6,7,8,9,10], and hydrogen-bonded organic frameworks (HOFs)[11,12]
We first attempt to construct 1D conjugated polymer solely based on dynamic covalent chemistry (DCC) without a metal template
Summary
Polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesised directly from solution. The synthesis of framework materials needs to be carried out under conditions where the bond formation is highly reversible to facilitate the self-correction process necessary for crystal growth In this regard, COFs are among the most difficult to crystallise, owing to the lesser reversibility of their covalent linkages compared to coordination bonds and hydrogen bonds in MOFs and HOFs. The ease of encoding functionalities in COFs and their structural robustness render them potentially useful in wide-ranging applications[13,14,15,16,17,18,19]. Conventionally synthesised COFs are mostly centrosymmetric and do not exhibit second harmonic generation or ferroelectricity[25] In this regard, one-dimensional (1D) COFs, which possess a high degree of freedom in molecular packing, can be a candidate to construct non-centrosymmetric crystals. An advantage of this method is that building blocks can evolve into multiintermediates through reversible reactions or interactions without the need to synthesise each building block individually, which is distinct from the conventional methods of making crystalline organic networks using predetermined building blocks[34,35]
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