Porosity Switching in Polymorphic Porous Organic Cages with Exceptional Chemical Stability

Abstract

Porous solids that can be switched between different forms with distinct physical properties are appealing candidates for separation, catalysis, and host–guest chemistry. In this regard, porous organic cages (POCs) are of profound interest because of their solution‐state accessibility. However, the application of POCs is limited by poor chemical stability. Synthesis of an exceptionally stable imine‐linked (4+6) porous organic cage (TpOMe‐CDA) is reported using 2,4,6‐trimethoxy‐1,3,5‐triformyl benzene (TpOMe) as a precursor aldehyde. Introduction of the ‐OMe functional group to the aldehyde creates significant steric and hydrophobic characteristics in the environment around the imine bonds that protects the cage molecules from hydrolysis in the presence of acids or bases. The electronic effect of the ‐OMe group also plays an important role in enhancing the stability of the reported POCs. As a consequence, TpOMe‐CDA reveals exceptional chemical stability in neutral, acidic and basic conditions, even in 12 m NaOH. Interestingly, TpOMe‐CDA exists in three different porous and non‐porous polymorphic forms (α, β, and γ) with respect to differences in crystallographic packing and the orientation of the flexible methoxy groups. All of the polymorphs retain their crystallinity even after treatment with acids and bases. All the polymorphs of TpOMe‐CDA differ significantly in their properties as well as morphology and could be reversibly switched in the presence of an external stimulus.

The Chemistry of Porous Organic Molecular Materials

“…[23] Such approaches demonstrated that it is the processing of the POC modules that affects pore volumes in these materials, rather than attempting to synthesize larger and often more synthetically challenging POC that may also lack shape persistence. [203,301] POCs [22,23,25,27,28,30,206,313] and extrinsically porous solids [26,31,32,295] have been reported to exhibit polymorphdependent physisorption properties, although extensive screening of crystallization conditions are often required to find functionally different polymorphs in the lab. [295] Indeed, McCrone suggested some time ago that the discovery of different polymorphs is proportional to the amount of effort spent searching.…”

“…Banerjee et al, reported that a structurally related [4+6] imine cage TpOMe-CDA that has more bulky cyclohexyl groups on the imine cage vertices interconverts between three different polymorphs with contrasting sorption properties, using solvent switches initiated with CHCl 3 and dimethylformamide (DMF). [30] Molecules that generate extrinsically porous solids have also been reported to have interesting, polymorph-dependent sorption properties. Credi et al reported that a molecular crystal prepared using shape-persistent azobenzene tetramers, E 4 -1c, formed a porous solid that underwent E→Z photoisomerization reaction, to convert the porous crystals into a nonporous amorphous melt phase.…”

“…[16][17][18][19][20][21] Also, organic molecules can be processed postsynthesis into different structures with contrasting properties. [22][23][24][25][26][27][28][29] Guest responsive materials can also be designed to respond to external stimuli, [22,[30][31][32][33] and organic molecules can be processed into different materials forms, such as membrane supports for separations [34] or for sensing applications. [35,36] While these strategies are not necessarily all unique to porous molecular crystals, we focus in this review on how the functional properties of porous organic molecular materials can be functionalized using traditional solution-based organic synthesis and crystallization approaches, which generally speaking cannot be applied in the same way to extended framework materials.…”

A Polymorphic Azobenzene Cage for Energy‐Efficient and Highly Selective p‐Xylene Separation

“…These stable systems are easily prepared and scaled‐up in addition to being solution processable, [20, 21] which allows them to be combined in a modular way to generate porous co‐crystals [22] and porous liquids [23] . Organic cage crystals can be polymorphic [24, 25] which is a close analogy to zeolites [26] and metal‐organic frameworks (MOFs) [27] where different crystalline polymorphs have different porosity, physical properties, and guest molecules selectivity. Here, a non‐porous polymorphic azobenzene‐based imine cage and its use for selective separation of p ‐xylenes from xylenes mixture and commercial C8 fractions is described.…”

Elucidating heterogeneous photocatalytic superiority of microporous porphyrin organic cage

“…It has been envisioned that the ultrafast photo-induced energy/electron communication between hosts and cavity-confined guests in a close distance 23 may facilitate the rapid conversion of reaction reagents. In addition, the supramolecular frameworks of molecular cages will possess not only the intrinsic intracage cavities but also the intercage porosities 19,27,34 , providing a renewable platform for heterogenizing homogeneous photocatalysts. With these considerations in our mind, we plan to develop visible light photocatalytic molecular cages constructed from porphyrin building units for their photo-induced heterogeneous catalysis.…”