A Covalent Organic Framework with 4 nm open poresw

Porous crystalline materials with a well-defined pore size distribution are essential for applications in gas storage, catalysis, separation, optics and chemical sensing. Specifically, large pores reaching several nm diameter permit the design of a multitude of nano-devices interacting with large guest molecules. To gain maximum control over local structure, chemistry and physics in the pore system, it would be highly desirable to have access to multi-nm pores with molecularly defined walls. For example, Sonnauer et al. reported a mesoporous Metal–Organic Framework (MOF) exhibiting giant cages with diameters of up to 4.6 nm, although the windows connecting these cages are much smaller with dimensions of about 2.0 nm. Other mesoporous MOFs with similar pore dimensions have been described. A new class of materials called Covalent Organic Frameworks (COFs) with organic open pore structures exhibit significant stability due to covalent bonding. They feature great structural diversity and also allow precise geometric control through the design of the molecular building blocks (reticular chemistry). Until now mesoporous COFs with pore sizes larger than 3.4 nm have not been reported. Here we describe the synthesis and characterization of a new mesoporous Covalent Organic Framework BTP-COF, with fully accessible pores having an open diameter of 4.0 nm. To the best of our knowledge, this is the largest open pore diameter of any crystalline material ever reported. The new BTP-COF was synthesized under solvothermal conditions by co-condensation of 1,3,5-benzenetris(4-phenylboronic acid) (BTPA) and the polyol 2,3,6,7-tetrahydroxy-9,10-dimethyl-anthracene (THDMA) (Fig. 1). The synthesis parameters for the new COF structure were found and optimized in a high throughput screening approach using a robotic dosing system. The investigated reaction parameters were solvents, solvent ratio, concentration, ratio of the starting materials, time and temperature. Thereby it was observed that the solvent mixture is the most important parameter for successful network formation, whereas the latter ones influence the purity (crystallinity and yield) of the system. We can conclude that the reversibility of the ester condensation depends strongly on the polarity of the reaction solution. Hence the crystallization is only possible if the appropriate reaction conditions are adapted for every linker system. The COF materials were obtained under microwave synthesis conditions. The different reaction parameters are described in the ESI.w Powder X-ray diffraction (PXRD) confirms the formation of a highly crystalline Covalent Organic Framework. In order Fig. 1 Co-condensation reaction for the BTP-COF with 4.0 nm pores.