Triptycene

Abstract: Apart from well-established porous materials, such as zeolites, metal–organic frameworks (MOFs), covalent organic frameworks (COFs), or amorphous polymeric organic materials, porous compounds consisting exclusively of discrete organic molecules can be classified as a relatively new type of material. Such materials can be subdivided mainly into two kinds: intrinsic and extrinsic porous materials. Intrinsic porosity is defined as porosity that is already inherent in the molecular structure, such as shape-persistent voids, clefts, or cavities. Typical compounds that form intrinsic pores in the solid state are calixarenes, cucurbiturils, or shape-persistent organic cage compounds. It was shown that shapepersistent cage compounds seem to be superior within this subclass of materials, with Brunauer–Emmett–Teller (BET) surface areas of up to 2071 m g . In contrast to the high surface areas of the intrinsic porous organic materials, the values of extrinsic porous materials are significantly smaller. One of the first examples is tris(phenylenedioxy)cyclophasphazene (TPP), which was introduced in 1964 by Allcock and co-workers. Sozzani et al. demonstrated first that crystals of TPP can be permanently porous, and later, Hulliger et al. reporting a Langmuir surface area of 240 m g . The prediction of the assembling of molecules in the solid state is still very difficult, therefore it seems to be more promising to search for existing compounds that obviously exhibit pores in the crystalline state, but have not yet been proven to be permanently porous. McKeown and coworkers defined some useful criteria for such a search, and revealed that there are already quite a number of potential compounds, which fulfill those criteria. For 3,3’,4,4’-tetrakis(trimethylsilylethynyl)biphenyl (TTEB), they showed that this approach is successful: TTEB is permanently porous (BET surface area 278 m g ) and adsorbs 0.8 wt % H2 at 77 K and 10 bar. Re-investigations by Chen et al. demonstrated that HOF1a, a compound introduced before, is permanently porous, with a BET surface area of 359 m g 1 and selective adsorption of ethylene over ethane. Other extrinsic porous materials from discrete organic molecules with relatively high surface areas are SOF-1a, (BET surface area: 474 m g ), the triptycene-derived trinuclear nickel salphens of the MacLachlan group, with BET surface areas of up to 499 m g , or the macrocyclic bis(urea) CBDU (BET surface area 341 m g ). The highest BET surface area of an extrinsic organic compound were reported for PUNCs (phthalocyanine unsolvated nanoporous crystals), with values between 850 and 1002 m g . Herein, we describe a rational approach of the construction of a readily accessible molecular precursor, which forms a permanent porous crystal with a very high specific surface area of 3020 m g . For the design of the molecular structure, we have taken into account McKeown s criteria and have also systematically searched the Cambridge Structural Database for compounds that form flat ordered sheets by selfassembling by hydrogen bonding. Interestingly, almost all 4,5disubstituted benzimidazolones 22] compounds formed nearly planar ribbon-like structures by directed H-bonds of the imidazolone units (Scheme 1). Therefore, we found benzimidazolones 22] to be a potential subunit for designing our molecular precursor, exploiting the chance to form