porous organic cages ( pocs ) are a unique 
 class of microporous material 
 composed of discrete molecules with intrinsic , guest accessible cavities 
 ( figure 1a ) . to be porous in the solid state ,
these cavities must be connected 
 by a 1- , 2- , or 3-dimensional pore network ( figure 1b ) . without this connectivity , the intrinsic 
 cavities are isolated and inaccessible to guest molecules .
the cages must also remain shape - persistent upon 
 addition and removal of guests , such as solvent , because collapse 
 of the intrinsic cavity would disrupt the pore network .
the intrinsic porosity inside the cages may also 
 be augmented by extrinsic voids between cages .
the combination of these two requirements ( porous crystal packing 
 and shape - persistence ) , coupled with the synthetic challenge of forming 
 a cage in the first place , makes pocs easy to design  on paper  

( a ) space filling representation 
 of the porous organic imine cage cc3-r , taken from its single crystal structure 
 ( gray , carbon ; white , hydrogen ; blue , nitrogen ) .
the cage is shape - persistent 
 and has an intrinsic cavity that is accessible via four windows .
( b ) 
 schematic representation of crystalline cc3 : each 
 cage packs window - to - window with four adjacent cages to form a 3-d 
 pore network .
the intrinsic cage cavities are connected by extrinsic 
 voids between the cage windows ( gray , core cage structure ; red , cyclohexane 
 groups located on the cage vertices ; yellow , 3-d pore network ) .
the cage packing has 
 a dramatic effect on porosity , and different 
 crystalline polymorphs of the same molecule can have quite different 
 physical properties .
pocs share some 
 similarities with metal  organic frameworks ( mofs ) , covalent organic frameworks ( cofs ) , and porous polymer networks , but because of their discrete molecular nature , they are 
 usually solution processable .
this processability 
 allows pocs to be used in applications that would be more challenging 
 with insoluble porous solids . to give one example ,
the molecular 
 nature of pocs also gives options for characterization ( e.g. , solution 
 nmr , hplc ) and purification ( e.g. , recrystallization , preparative 
 chromatography ) that are unavailable for insoluble , extended frameworks . in the cooper group ,
we have primarily focused on the synthesis 
 of imine pocs , but the following discussion is also relevant to cages 
 formed using other bond forming reactions ( e.g. , boronate esters ) . our aim is to highlight more general experimental 
 strategies by using cc1 and cc3 as detailed 
 worked examples ( scheme 1 ) .
there are a number of challenges involved in the 
 design and synthesis 
 of pocs ; this is especially true for completely new molecules , but 
 even the preparation of structural variants of known pocs can present 
 unforeseen difficulties .
we have found that even subtle changes to bond angles 
 in the precursors can have a dramatic effect on the outcome of the 
 reaction by changing the size and stoichiometry of the cage product , as also found for metal  organic 
 cages .
moreover , essentially any cage 
 precursor combination can , in principle , form an amorphous polymer 
 network instead of a cage .
sometimes , a small change in one of the 
 precursors or the use of unsuitable reaction conditions can  flip  
 the system from being cage - forming to being polymer - forming .
high - dilution 
 synthesis coupled with dynamic covalent chemistry is a common strategy 
 to avoid this , but this will only succeed if the target cage is the 
 thermodynamic product .
the synthesis of imine cages 
 from amine and aldehyde precursors is an example of this .
dynamic covalent routes allow the thermodynamic 
 cage product to emerge from the various kinetic products that are 
 initially formed in the reaction , and this can lead in many cases 
 to high synthetic yields ; for example , the yield of cc3 in batch syntheses usually exceeds 80% .
we have synthesized up to 30 g of cc3 in a single batch 
 ( figure 2a ) , and there 
 is no reason to think that larger scale syntheses are not possible . ( a ) large 
 scale batch synthesis ( > 10 g ) of cc3 yields 
 a pure polycrystalline material .
( b ) large , millimeter - sized single 
 crystals suitable for single crystal x - ray diffraction can also be 
 isolated directly from reaction mixtures .
obviously , it would be a major advantage to have methods 
 to design 
 appropriate precursors for poc synthesis without resorting to trial 
 and error . in collaboration with the jelfs group
, we are exploring 
 methods to compute the size and shape of cages formed from a given 
 set of starting materials , and their likelihood of remaining shape - persistent , before attempting any synthesis in the lab .
for example , if the candidate cage structure is too strained , then 
 it is unlikely to form . in time
, we believe this will be a powerful 
 and generalizable method for in silico poc design . for the moment , 
 though , some intuitive design strategies exist .
for example , if a 
 cage is too flexible , then it will often undergo a structural rearrangement 
 upon desolvation , resulting in collapse of the intrinsic cavity and 
 loss of porosity .
this can be due to the cumulative effect of small 
 degrees of freedom in multiple 
rigid  bonds as well 
 as ( more obviously ) the inclusion of freely rotatable or highly flexible 
 linkers . as a result , the design of large shape - persistent cages ( > 23 
 nm diameter ) is generally more difficult than for smaller pocs , and 
 there are fewer examples of large cages in the literature .
once potential cage precursors have been 
 identified , then suitable 
 synthesis , purification , and isolation conditions must be developed . 

parameters that can affect the outcome of the cage - forming reaction 
 include concentration , temperature , and solvent and catalyst choice . 
 whether or not water ( or other condensate ) is removed during the reaction 

can also be important as can the order and speed of the reactant addition . 

selection of the reaction parameters should be informed by the properties 
 of the reactants , such as their reactivity and solubility .
for instance , 
 the wrong solvent choice can lead to premature precipitation of intermediates 
 from the reaction mixture before the target cage is formed .
once kinetic 
 products precipitate , they may not be able to equilibrate into the 
 desired cage , even if it is the thermodynamic product .
the 
 addition of solubilizing groups ( e.g. , alkyl chains ) is another strategy , 
 although for pocs , this bears the potential disadvantage that these 
 groups may diminish the porosity in the solid state or decrease the 
 propensity of the cage to crystallize , if that is the goal .
a different strategy to improve cage solubility is to decrease the 
 symmetry in the cage , for example , by using mixed linkers , but this may strongly inhibit crystallization .
it may also be necessary to add a catalyst , 
 such as an acid , to enhance the reversibility of the dynamic covalent 
 bond forming chemistry , although in some cases this can also direct 
 the reaction toward other products , such as interlocked catenanes . with sensible precursor selection and 
 optimization of the reaction 
 conditions ,
also , if there is sufficient preorganization in the 
 cage precursors , dynamic covalent chemistry may not always be required . with luck
, the cage may crystallize directly from the reaction mixture 
 in a porous phase that remains stable to direct desolvation ( e.g. , cc3 ) , but this is relatively uncommon .
even if a cage does 
 crystallize directly from the reaction mixture , it may not be easy 
 to determine whether insoluble oligomers are also present as side 
 products , especially if the cage itself is poorly soluble .
in addition , 
 amorphous cage or side products might not be revealed by powder x - ray 
 diffraction ( pxrd ) ; we have found that a significant proportion of 
 amorphous material can be present before there is a noticeable change 
 in the baseline of the pxrd .
hence , even cages that appear to be chemically 
 pure and phase pure ( e.g. , by solution nmr and by pxrd ) might still 
 be contaminated , for example , with a small quantity of insoluble amorphous 
 polymer that is invisible to these techniques .
moreover , crystals 
 that grow directly and in some cases rapidly from a reaction mixture 
 might be of lower quality and possess a greater number of defects 
 than crystals grown in a more controlled process .
this is an important 
 consideration because crystal quality can strongly affect porosity . quite often
, the desired cage product can remain in solution , 
 perhaps 
 with some insoluble polymeric byproduct . in such cases
, one should 
 take care not to mistake this precipitate for the cage product and 
 discard the supernatant ! indeed , even if the cage product crystallizes 
 from the reaction solvent , the supernatant may contain a significant 
 quantity of cage , which can be recovered . a further complication , often overlooked ,
is that even a 100% yield 
 of the cage does not automatically mean that it is the thermodynamic 
 product .
provided that there are no steric clashes , the global thermodynamic 
 product may instead be a catenated cage where two ( or conceivably 
 more ) independent cages become mechanically interlocked . in such cases ,
however , 
 because catenation often requires extended reaction times and conditions 
 that promote reversibility , such as higher temperatures or the use 
 of a catalyst , then the noncatenated cage can often be isolated even 
 if it is not the overall thermodynamic product .
more generally , minor side - products , whether 
 catenanes or other species , need not be fatal to success : as discussed 
 in the methods section , various chromatographic , 
 precipitation , and crystallization methods can be used to isolate 
 cages as chemically pure single components .
finally , solvent choice 
 can play a part in determining the size and shape of the cage . for 
 example , warmuth et al . demonstrated that three different cages could 
 be formed from the same starting materials simply by changing the 
 reaction solvent .
recently , we also found 
 that other imine cages can equilibrate to form new cage products on 
 prolonged standing in certain crystallization solvents , suggesting 
 that a kinetic cage product is formed initially .
once pure cage material has been isolated , we would typically screen 
 a range of conditions to afford either crystalline or amorphous phases 
 of the cage , as required . usually , crystalline phases will be isolated 
 as solvates , although in rare instances the crystallization solvent 
 may be excluded from the crystal , particularly if the cage cavity 
 is small and the solvent is large .
crystals may also spontaneously 
 and rapidly lose solvent at ambient temperature in air while remaining 
 crystalline , particularly if the solvent is volatile and weakly interacting 
 ( see figure s19 in ref ( 1 ) ) : in such cases , it might appear that the cage is crystallizing 
  solvent free  unless the material is characterized 
 immediately upon isolation from the crystallization solvent .
more 
 commonly , the crystalline solvated phase must be carefully desolvated , 
 or  activated  , to isolate a porous solid .
as observed 
 for mofs and cofs , activation is generally more challenging for pocs 
 with low density ( high pore volumes ) and for solvates where the solvent 
 has a high boiling point , particularly if the solvent strongly interacts 
 with the poc framework , for example , by hydrogen bonding .
in such 
 cases , solvent exchange for a less polar , more volatile solvent prior 
 to solvent removal may be necessary .
pxrd 
 in combination with electron microscopy can be used to establish whether 
 the cage has changed phase or become amorphous after desolvation . 

also , if gas adsorption is to be performed , we strongly advocate repeating 
 these characterization techniques after the adsorption measurement 
 to ensure that the cage has not changed phase .
not all applications 
 require crystallinity ; for some purposes , 
 amorphous pocs may be advantageous .
techniques to isolate amorphous 
 phases , or defective crystalline materials , include chemical scrambling , freeze - drying , rapid precipitation , spin - coating , electrospray , and ( conceivably ) mechanical grinding . throughout each stage of synthesis , isolation , and properties evaluation , 

analytical methods should be used to ensure that the purity and structure 
 of the cage material is unchanged .
for example , hplc and solution 
 nmr are simple methods for ensuring chemical stability , and pxrd and 
 electron microscopy , as discussed above , can be used for phase identification 
 at any stage in the process .
all of these methods require only small 
 ( < 20 mg ) quantities of material .
our most studied pocs , cc1 and cc3 , are 
 synthesized by a [ 4 + 6 ] cycloimination reaction 
 in which four molecules of the aldehyde 1,3,5-triformylbenzene ( tfb ) 
 react with six molecules of either ethylenediamine ( eda , cc1 ) or 1r,2r-1,2-diaminocyclohexane 
 ( chda , cc3-r ) ( scheme 1 ) . to date ,
we have published two separate 
 high yielding procedures for the synthesis of cc1 and cc3 : a batch synthesis that affords multigram quantities of 
 poorly crystalline cc1 or crystalline cc3 within a week and a continuous flow synthesis that affords milligram 
 quantities of amorphous cc1 and semicrystalline cc3 within minutes and gram quantities within hours . in the batch synthesis of cc3 ,
a solution of homochiral trans - chda in dcm is 
 layered onto a suspension of tfb in dcm containing a catalytic amount 
 of tfa .
the concentration is relatively high compared with standard 
 cage or macrocycle reactions , which avoids excessive solvent volumes . 

however , the slow dissolution of the poorly soluble tfb over a number 
 of days , along with the crystallization of cc3 from the 
 supersaturated reaction mixture , is essentially equivalent to running 
 the reaction at high dilution with the slow addition of the tfb .
similar 
 approaches that use solubility tuning of the reagents should be possible 
 with other cages .
the rigidity and homochirality of chda ensure that 
 the intermediates are at least partially preconfigured to form the cc3 cage , thus reducing the formation of misaligned kinetic 
 oligomers in comparison with more flexible reactants .
indeed , in this 
 system , no oligomeric side - products are observed to precipitate from 
 the concentrated reaction mixture .
the role of the acid catalyst is 
 to increase the reversibility of the imine reaction and to afford 
 the thermodynamic cage product within a reasonable time frame .
the 
 use of a catalyst is particularly suited to the synthesis of cages 
 that use less reactive starting materials .
heating may also be employed 
 either to increase reversibility or in cases where the reactants or 
 the cage product has poor solubility .
the isostructural poc , cc1 , formed by the batch reaction of tfb with eda ( scheme 1 ) , is synthesized 
 under different conditions .
this reaction is run at low temperature 
 and high dilution with slow addition of tfb to the diamine .
these 
 conditions are required to mitigate the higher reactivity and inherent 
 flexibility of this diamine , which can result in the formation of 
 insoluble oligomeric products at higher concentrations and higher 
 reaction temperatures .
hence , this route is more reminiscent of classical 
 macrocycle syntheses . to synthesize cc1 ,
a solution of 
 tfb in dcm is added dropwise over 48 h to a solution of the eda in 
 dcm at 0 c .
the use of a syringe or peristaltic pump rather 
 than an addition funnel ensures more accurate control of addition 
 rates and better reproducibility . after warming the reaction mixture 
 to room temperature and stirring for a further 24 h ,
the starting 
 materials show complete conversion to cc1 with no soluble 
 byproducts observed .
the high reactivity of the starting materials 
 means that an acid catalyst is not required even at high dilution . 

in fact , when the reaction is run in acetonitrile , the presence of 
 an acid catalyst affords catenated cc1 , which is the 
 true thermodynamic product.cc3 , on the other hand , is unable to catenate due to the presence of 
 the bulky cyclohexane groups on the cage vertices . catenated cages 

are easily spotted because they often display a marked change in their h nmr spectra due to a decrease in symmetry and a pronounced 
 through - space anisotropic effect caused by close contact between the 
 interlocked cages .
they can also be identified , of course , by techniques 
 such as mass spectrometry , hplc , or diffusion - ordered nmr spectroscopy 
 ( dosy ) .
the formation of cc1 and cc3 does not 
 appear to be sensitive to the water liberated during the reaction , 
 either in batch or continuous flow conditions .
however , we have encountered 
 other cages that require the removal of water via a dean  stark 
 apparatus ( a reverse dean 
stark apparatus can be used for chlorinated 
 solvents ) or the addition of a desiccant to ensure good conversion 
 of the reactants to the cage product .
this 
 requirement is often associated with the low reactivity of one of 
 the starting materials due to steric or electronic effects . as a rule of thumb for the synthesis of imine cages under batch 
 reactions ,
reactive starting materials require high dilution ( unless 
 solubility is controlled ) , low temperatures , and slow addition of 
 one of the precursors ( again , unless solubility can act as a control ) . 

less reactive starting materials tend to require the use of an acid 
 catalyst , higher temperatures , more concentrated reaction mixtures , 
 and the removal of water ( e.g. , with a dean  stark trap ) to 
 improve conversion .
generally 
 reactions proceed more cleanly when the aldehyde is added to an excess 
 of diamine ; the excess amine in the reaction helps to promote reversibility 
 and should minimize oligomer formation .
the synthesis of cc1 and cc3 has also been 
 successfully transferred to a vaportec r - series 
 continuous flow reactor as a proof of concept . by using flow systems
, the reaction temperature can be 
 increased above the boiling point of the solvent .
coupled with the 
 highly efficient reagent mixing characteristic of flow systems , this 
 allows the rate of reaction to be increased and the reaction time 
 to be reduced , in some cases from days to minutes .
the short reaction 
 time allows multiple combinations of starting materials to be assessed 
 rapidly across a range of conditions .
concentration , stoichiometry , 
 reaction time , flow rate , and temperature are all easily varied .
the synthesis of cc3 under flow conditions afforded 
 full conversion of tfb to cage in just 10 min at 100 c , whereas 
 the batch synthesis takes a number of days .
a stoichiometry of 4 tfbs 
 to 6.5 diamines was used in the reaction ( ideal stoichiometry is 4:6 ) 
 to mitigate fluctuations in the pump performance .
the excess diamine 
 was well tolerated and ensured complete conversion of tfb to the cage , 
 whereas the use of excess aldehyde resulted in incomplete cage formation . 

the same effect has been noted before in our group in the batch synthesis 
 of imine cages , where the use of a small excess of amine can result 
 in cleaner and more reproducible reactions .
the effect of temperature 
 and reaction time on the flow synthesis was also studied with lower 
 temperatures or shorter reaction times affording incomplete conversion 
 to the cage product .
higher temperatures resulted in the precipitation 
 of oligomeric material , whereas a longer reaction time had no effect 
 on the outcome of the reaction .
surprisingly , we also found that cc1 could be synthesized using the same reaction conditions 
 despite significant differences between the batch synthesis conditions 
 for each cage ( above ) , and the reduced thermal and hydrolytic stability 
 of cc1 .
unfortunately , due to the poor solubility of 
 tfb in dcm , we were unable to test the effect of more concentrated 
 reaction mixtures on the outcome of either reaction using this flow 
 system .
however , the flow synthesis of cc1 is still three 
 times more concentrated than its batch synthesis , significantly reducing 
 solvent volumes . for both cc1 and cc3 , the 
 reaction stream exiting the reactor afforded a solution of the cage 
 contaminated with only the excess diamines , which are easily removed 
 by antisolvent reprecipitation .
we have found several examples of cages that form in solution 
 in good yields but are sensitive to the isolation procedure and can 
 readily decompose into an insoluble polymeric material upon solvent 
 removal .
one simple monitoring technique 
 is to dilute a sample of the reaction mixture with a suitable deuterated 
 solvent for analysis by h nmr .
although the signals of 
 interest may be weak and partially obscured by the nondeuterated reaction 
 solvent , such spectra often provide sufficient information to determine 
 whether the aldehyde has been consumed and how cleanly the reaction 
 has progressed . if the target cage is 
 symmetrical , then the h nmr spectrum is usually relatively 
 simple .
a more complex h nmr would potentially indicate 
 the formation of oligomeric products , incomplete conversion to the 
 cage , or potentially catenated products .
ms , which can give information on the purity 
 and composition of the reaction mixture .
a final technique that can 
 be used directly on reaction mixtures is dosy . to do this ,
however , 
 the reaction should be performed in deuterated solvents . with careful 
 calibration
, dosy can be used to determine the size and hence stoichiometry 
 of the cage that is formed , if any . for 
 reaction mixtures that form a suspension ( e.g. , batch synthesis 
 of cc3 ) , it is good practice to analyze a sample of both 
 the solid and the supernatant to determine what quantity , 
 if any , of cage has formed and its purity in each phase . depending 
 on the composition of the solid and the supernatant ,
it is often easier 
 to redissolve the solid and combine it with the supernatant to purify 
 the material as a single batch . obviously ,
if the quantity of cage 
 in the supernatant ( or in the precipitated solid ) is insignificant , 
 then that phase can be discarded .
if the cage is mainly present in 
 the solid phase , and even if it gives a pxrd pattern consistent with 
 cage formation , this does not guarantee that it has crystallized 
phase 
 pure  : it is still good practice to dissolve some of this material 
 and to check that it is not contaminated with oligomeric materials 
 or other side products .
pocs are organic molecules , and they can be 
 characterized by the standard range of organic chemistry techniques . 

also , as discussed above , crystallization of the cage from the reaction 
 solvent may not produce  good  crystals , although in 
 several cases we have obtained excellent quality millimeter - sized 
 crystals directly from reaction mixtures ( figure 2b ) that were suitable for single crystal 
 x - ray diffraction . in many cases , though , 
 it is advisible to grow crystals of purified material in a subsequent 
 step , although this may produce a different polymorph to that which 
 precipitated from the reaction mixture , especially if a different 
 crystallization solvent is used . in this respect ,
soluble pocs differ 
 significantly from insoluble mofs and cofs where the crystals must , 
 by necessity , be used  as synthesized  .
the first priority when 
 developing an isolation procedure to yield a porous crystal ( or an 
 amorphous poc solid ) is to check that you are starting with chemically 
 pure material .
if the chemical purity of the cage is poor , then it 
 can make the subsequent isolation of crystalline phases more difficult . 
 in the batch synthesis of cc3 ,
the cage crystallizes 
 directly from the reaction mixture in good yield and excellent chemical 
 purity , though this is not always the case for all pocs .
pure cc3 is isolated by filtration of the reaction mixture , followed 
 by a displacement wash with 95:5 etoh / dcm to remove any surface impurities .
cc1 , on the other hand , remains in solution when the reaction 
 is complete .
the reaction mixture is first filtered to remove any 
 traces of polymeric material that have formed during the reaction , 
 and this filtered solution is evaporated to dryness at < 20 c ; 
 higher solvent evaporation temperatures can result in polymerization 
 of the cage .
indeed , because of the extended evaporation time as a 
 result of the large reaction volume , some cage polymerization can 
 still occur , even at low temperatures . hence ,
after reaching dryness , 
 the residue is redissolved in a small volume of volatile solvent , 
 usually dcm , refiltered to remove any insoluble polymeric material , 
 and this much smaller volume of solvent evaporated at < 20 c 
 to afford cc1 as a chemically pure solid . more generally , 
 dissolution and filtration is a good method to remove any contamination 
 with insoluble oligomeric side - products . in the flow synthesis 
 of cc1 and cc3 ,
both products exit the reactor 
 as a dcm solution that also contains excess diamine .
cc3 is precipitated directly from the reaction mixture by mixing it 
 with excess hexane .
however , because of the higher solubility of cc1 , the reaction mixture must first be evaporated ; the material 
 is then redissolved in a small amount of solvent , filtered to remove 
 any polymeric material formed during the evaporation , and precipitated 
 by addition to hexane . in both cases , the product is then isolated 
 by filtration to yield the cage in excellent purity and yield
a number of factors affect 
 the choice of antisolvent : it must be miscible with the cage solution 
 and induce precipitation of the cage in good yield and purity .
comparing 
 the hplc of the initial cage solution with the supernatant of the 
 precipitated suspension allows rapid screening of solvent / antisolvent 
 combinations ; in a successful purification , the filtrate will be heavily 
 enriched in impurities as the cage will have precipitated .
a related 
 technique that we often use to isolate cages is to swap the solvent , 
 often dcm or chloroform , to a higher boiling antisolvent .
the cage 
 solution is usually diluted with the antisolvent , which may result 
 in a cloudy solution / suspension , and then the original solvent is 
 slowly evaporated to leave a suspension of the cage .
the cage can 
 then be isolated by filtration while more soluble impurities ( e.g. , 
 unreacted monomers ) remain in solution .
alkanes , such as hexane , and 
 acetone have proven to be particularly effective antisolvents for 
 this technique .
we routinely employ this technique to isolate soluble 
 cages directly from their reaction mixtures , and in some instances , 
 it has led directly to the formation of a crystalline porous phase . 

, common crystallization solvents can be screened on a small 
 scale with heating to establish the solubility of the cage .
on cooling , 
 the recovery and purity of any cage precipitate can again be assessed 
 by hplc analysis of the supernatant . for recovery to be increased , 
 an antisolvent may be added to the hot cage solution .
cages that are 
 thermally unstable may also be purified by adopting the ambient temperature 
 crystallization techniques outlined below .
the correct solvent choice 
 for any of these methods is based on a combination of experience and 
 ( substantial ) trial and error .
if pure cage can not be obtained 
 using any of the methods discussed , 
 then chromatography may be used for purification .
we have had the 
 most success with preparative hplc and have been able to separate 
 desired cage products from soluble impurities , usually cage fragments 
 or oligomers , isostructural cages , and catenated cages . in our hands , 
 purification usually involves injecting a solution of the crude cage 
 onto a c8 reverse phase column then eluting with methanol , the presence 
 of imine bonds in the cages precludes the use of water - containing 
 gradients .
unsurprisingly , we have found huge differences in performance 
 between column manufacturers ; hence , the suitability of a column should 
 first be assessed using an analytical system .
other chromatographic 
 techniques , such as size exclusion chromatography , have also been 
 investigated , but these have so far met with limited success in our 
 experience .
the purification of pocs by vacuum sublimation , the organic 
 chemist s  last resort  , is a further possibility , 
 although the vapor pressure of these relatively large macrocyclic 
 molecules may often be too low for this to be viable . unlike insoluble mofs 
 and cofs
, the porosity in pocs can be modified 
 by postsynthetic crystallization steps . as such , isolation of a cage 
 from a reaction mixture that is initially found to be nonporous
cc3 crystallizes from the batch reaction mixture in a single 
 desolvatable phase , cc3 , where the intrinsic cage 
 cavities are connected via extrinsic cavities to afford a 3-dimensional 
 pore network ( figure 1b ) .
however , as mentioned above , the porosity is influenced by the 
 quality of the crystals that are formed .
we have found that the apparent 
 brunauer  emmett  teller surface area ( sabet ) of highly crystalline cc3 is , reproducibly , 
 409  8 m g , whereas standard 
  as - made  cc3 has been reported 
 with sabet values of 592 and 624 m g. this is thought to be due to defects , such as missing 
 cages , in the crystal packing , which add to the porosity in the material . 

the only discernible difference between the pxrd patterns of the highly 
 crystalline and  as made 
a polymorph screen with cc3 identified a second crystalline 
 phase , cc3 , which crystallizes from dcm / diethyl 
 ether and has an sabet of 555 m g when desolvated .
hence , different levels 
 of porosity can be obtained for the cc3 molecule depending 
 on the method of crystallization .
deliberate amorphization of cc3 via freeze - drying 
 of cc3 from a solution of dcm affords material with an 
 increased sabet of 859  63 m g. this increase in sabet upon 
 amorphization observed for cc3 is not a general phenomenon ; 
 more commonly , amorphization results in a decrease or a complete loss 
 of porosity , as found for the isostructural cage cc1.cc3 increases its porosity in the 
 amorphous state because the bulky cyclohexane groups on the cage vertices 
 prevent efficient packing ( cc1 lacks these bulky groups ) .
cc1 is isolated from the batch and flow reaction mixtures 
 as a poorly crystalline solid , but a screen of different reaction 
 and crystallization solvents has led to the discovery of a number 
 of crystalline solvates .
cc1 , cc1 , cc1 , and cc1 were 
 isolated from ethyl acetate , dcm , o - xylene , and dioxane , 
 respectively . upon desolvation , cc1 and cc1
undergo subtle structural 
 changes to yield the related desolvated phases , cc1 
 and cc1. each polymorph was observed to 
 possess different porous properties , including different gas selectivities .
cc1 and cc1 are 
 formally nonporous to nitrogen , whereas cc1 , when 
 desolvated , has an sabet of 550 m g. cc1 was found to be unstable to desolvation 
 and afforded a mixture of unidentified phases .
interestingly , it was 
 also found that nonporous and porous phases of cc1 could 
 be interconverted in the solid state simply via exposure to solvent 
 vapor .
the first is to identify new crystalline phases , initially 
 as solvates , and to obtain crystals of sufficient quality to allow 
 them to be solved by scxrd .
once crystalline phases have been identified , 
 the next step is to isolate sufficient phase - pure material to allow 
 characterization of physical properties .
this can be more difficult 
 than isolating the chemically pure cage in the first place .
bulk samples 
 should be analyzed by pxrd to ensure that they are phase pure and 
 that they match the simulated pxrd from the solvated scxrd .
electron 
 microscopy can also be used to ensure sample homogeneity as secondary 
 crystalline or amorphous phases should be visible .
we routinely 
 use a number of techniques to screen for crystalline 
 phases of organic cages .
single or polycrystalline phases can be obtained 
 by slow evaporation of single or mixed solvent cage solutions under 
 a nitrogen flow ; this may be achieved using a desiccator with a gas 
 inlet on the lid .
the solutions should be checked regularly for the 
 appearance of suitable crystals for scxrd during the evaporation . 

after solvent evaporation , the bulk material should also be analyzed 
 by pxrd : a common pitfall here is to select a 
single crystal that is , in fact , not representative of the bulk solid . 

vial - in - vial diffusion of an antisolvent into the cage solution or 
 slow diffusion of an antisolvent layered onto the cage solution can 
 also yield single crystal or polycrystalline materials .
these materials 
 will most likely be isolated as solvates unless the material spontaneously 
 undergoes desolvation upon removal from the solvent .
rapid desolvation 
 of single crystals upon removal from the crystallization solvent may 
 also be accompanied by a loss of singularity , although the material 
 may still be polycrystalline . in practice , it is often better to keep 
 the crystal in the crystallization solvent until it can be analyzed 
 by scxrd .
particularly sensitive crystals may be stabilized by encapsulation 
 in a protective oil before analysis .
one 
 should also be wary of predicting properties from the solvated scxrd 
 structures unless the material can be desolvated without a significant 
 change in crystal packing .
because 
 pocs can be recrystallized after synthesis , it is also 
 possible to produce cocrystals in a modular way containing two or more chemically distinct cages . 

clearly , the main technique for proving such structures is single 
 crystal x - ray diffraction .
however , because it is possible to form 
 cocrystals with significant positional disorder , one may also need to exploit the solubility of pocs , for 
 example , by using hplc and/or nmr to prove that the chemical composition 
 of the cocrystal is the same as that suggested by the x - ray structure 
 refinement .
this is another key difference between pocs and mofs , 
 cofs , and porous organic polymers : their chemical composition can 
 be determined accurately using solution phase analyses without resorting 
 to techniques such as acid digestion or other chemical decomposition .
removal of solvent from poc solvates can 
 have a number of effects . in an ideal case
, the solvent can be removed 
 with no or little change in the crystal packing , such that the pxrd 
 of the activated material matches the pattern calculated from the 
 solvated single crystal structure .
sometimes , desolvation leads to 
 a phase change where a new crystalline phase ( or a mixed phase ) forms . 
 alternatively , and particularly with large and/or flexible cages or 
 highly solvated structures , desolvation can lead to complete amorphization . 

lastly , and less widely recognized , desolvation can sometimes be accompanied 
 by chemical decomposition of some or all of the cage material .
various activation methods for porous 
 materials have been used by our group and by others , including vacuum 
 drying , supercritical drying , and solvent exchange .
thermogravimetric analysis of the solvated material is a useful 
 guide to the desolvation temperature required while also providing 
 information on the thermal stability of the cage .
the best method of activation is dictated by the stability 
 of the crystal packing in the solvate , the boiling point of the crystallization 
 solvent , and the strength of the interaction between the solvent and 
 the cage .
the diamondoid window - to - window packing observed for cc3 is a particularly stable  self - reinforcing  
 packing motif , and cc3 survives rapid desolvation 
 at 80 c in a vacuum oven with no phase change or loss of crystallinity . 

however , not all cages , or indeed polymorphs of the same cage , possess 
 such a strong packing motif , and abrupt desolvation can lead to mixed 
 phases or partial amorphization .
such materials will have different 
 properties to their phase pure constituents , and their gas sorption 
 isotherms may not agree , for example , with isotherms predicted from 
 idealized scxrd structures .
careful solvent exchange , via one 
 or more solvents , to a lower 
 boiling weakly interacting solvent is the most successful method so 
 far for gentler desolvation of pocs , but it is 
 likely that techniques such as supercritical drying , as used successfully 
 for mofs , should also be applicable . ideally , one should also try and obtain a desolvated single crystal 
 structure to provide an accurate structure of the  activated  
 poc . in practice , this can be difficult to achieve , particularly if 
 the crystal structure is heavily solvated .
solvent removal can lead 
 to cracking of the crystals , and single - crystal - to - single - crystal 
 transformations are substantially rarer than crystal - to - crystal transformations . 
 quite often ,
though , the solvent can be removed while the crystal 
 is still mounted in the diffractometer by slowly heating under a flow 
 of nitrogen .
cc3 , for example , can be readily 
 desolvated in situ by heating to 117 c .
pocs should be 
 analyzed postactivation by h nmr to 
 ensure that no chemical decomposition has occurred and also that the 
 material has been fully desolvated : here , again , there is an analytical 
 advantage over mofs and cofs in that the whole sample can be dissolved 
 and any entrained guests released without chemically decomposing the 
 poc . it should be noted that pocs often readily adsorb atmospheric 
 moisture , which could affect any porosity measurements .
this can also affect elemental analyses significantly : 
 a poc can adsorb several weight percent of water , depending on ambient 
 humidity , which is not an issue typically encountered with the elemental 
 analysis of dense , nonporous organic compounds . therefore
pxrd and , ideally , electron microscopy analysis should also be used 
 to ensure that no phase change or amorphization of the sample has 
 occurred when removing the solvent .
these tests should be repeated 
 after gas sorption measurements or exposure to other guests , such 
 as solvent vapors , to ensure that the poc is still chemically and 
 phase pure .
finally , all analysis and characterization should be carried 
 out on a single batch of material to ensure that the measured physical 
 properties match those predicted from the poc structure . throughout the poc discovery process
, 
 a number of analytical tests are required to ensure that chemical 
 and phase purity are maintained .
again , pocs are molecules , and it 
 is possible to analyze more than simple phase purity by pxrd , which 
 should be the minimal analysis requirement . without these various 
 analytical tests , it may be impossible to correlate physical properties 
 with structure : for example , one may end up trying to correlate a 
 gas sorption isotherm calculated from a single crystal x - ray structure 
 with a physical measurement derived from a sample that has become 
 partially amorphous , changed phase , or chemically decomposed .
figure 3 provides an overview 
 of our standard cage discovery workflow and the minimum analytical 
 requirements recommended for each stage of the discovery process .
overview 
 of the cage discovery process and the chemical and physical 
 analysis needed at each stage .
we hope that this perspective raises awareness 
 of the pitfalls 
 that may be encountered along the road to discovering new pocs while 
 at the same time suggesting solutions to these problems .
the field 
 of pocs is wide open for development , but progress requires a combination 
 of techniques developed by organic and supramolecular chemists , coupled 
 with methods borrowed from the world of extended framework solids . 
 in this respect , it is a mistake to think of this area as  organic 
 chemistry  or as  materials chemistry  : it is 
 both .
problems occur when either the synthetic chemistry ( e.g. , insufficient 
 purification ) or the physical chemistry ( e.g. , wrong activation conditions ) 
 are neglected . however , by approaching pocs in a systematic way , it 
 is possible to discover truly remarkable materials .
for example , there 
 are now organic cage molecules in the literature with surface areas 
 exceeding 3,500 m g that begin to 
 rival the most porous mofs and cofs .
dcm ( 100 ml ) was added slowly onto solid 1,3,5-triformylbenzene 
 ( 5.00 g , 30.9 mmol ) without stirring at room temperature .
trifluoroacetic 
 acid ( 100 l ) was added directly to this suspension as a catalyst 
 for imine bond formation .
finally , a solution of 1r,2r-1,2-diaminocyclohexane ( 5.00 g , 44.6 mmol ) in 
 dcm ( 100 ml ) was layered onto the suspension .
the unmixed reaction 
 was covered and left to stand at ambient temperature . over 5 days , 
 all of the solid 1,3,5-triformylbenzene was consumed , and octahedral 
 crystals of cc3-r grew on the sides 
 of the vessel .
the white , crystalline product was removed by filtration 
 and washed with 95:5 etoh / dcm ( 25 ml ) . the solid was dried to constant 
 weight at 80 c in a vacuum oven , resulting in a yield of 6.5 
 g ( 83% ) .
the 
 vaportec reactor was assembled using the r-2+/r-2 pump modules 
 with the r-4 reactor module .
once the system had reached steady state , the suspension was collected 
 for 110 min .
the suspension was isolated by filtration and dried to 
 constant weight in a vacuum oven at 60 c to afford cc3-r as a white crystalline powder ( 0.918 g , 95% ) .
schematic 
 of flow reactor setup showing the optimized parameters 
 for the synthesis of cc3-r .
flow system 
 setup : system solvent , dcm ; reagent a , 0.083 m 1r,2r-1,2-diaminocyclohexane in dcm ( 0.948 g/100 ml 
 of dcm ) ; reagent b , 0.083 m 1,3,5-triformylbenzene in dcm ( 1.34 g/100 
 ml of dcm ) ; reagent c , hexane ; flow rate a , 0.62 ml / min ; flow rate 
 b , 0.38 ml / min ; flow rate c , 4 ml / min ; reactor volume , 10 ml ; reactor 
 temperature , 100 c ; back pressure regulator , 8 bar .
a 
 solution of 1,3,5 triformylbenzene ( 3.75 g , 23.1 mmol ) in dcm ( 1150 
 ml ) was added dropwise over 48 h ( 0.3 ml / min ) via pressure 
 equalizing dropping funnel ( or a syringe or peristaltic pump ) to a 
 solution of ethylenediamine ( 2.08 g , 34.7 mmol ) in dcm ( 850 ml ) at 
 0 c .
after complete addition , the reaction was allowed to stir 
 for another 24 h at room temperature .
the solvent was removed from the filtrate via 
 rotary evaporation ( temperature of the water bath maintained below 
 20 c ) ; the crude product was redissolved in chcl3 ( 100 ml ) , and the solution was refiltered .
the residue was washed 
 with chcl3 ( 50 ml ) , and the combined organic filtrate was 
 concentrated under vacuum on a rotary evaporator ( temperature of the 
 water bath maintained below 20 c ) to give the product as a beige 
 powder with a yield of 4.25 g ( 94% ) .
the vaportec reactor was 
 assembled using the r-2 + pump module with 
 the r-4 reactor module .
once the system had reached steady state , the reaction mixture 
 was collected for 53 min .
the solution was evaporated to dryness at 
 20 c , and the residue was redissolved in the minimum amount 
 of dcm , filtered , and poured into an approximately equal volume of 
 hexane to afford a white suspension .
the solid was collected by filtration , 
 then dried to constant weight in a vacuum oven at 50 c to afford cc1 as a beige powder with a yield of 0.310 g ( 93% ) .
schematic of 
 flow reactor setup showing the parameters for the 
 synthesis of cc1 .
flow system setup : system solvent : 
 dcm ; reagent a , 0.083 m ethylenediamine in 1:3 meoh / dcm ( 0.499 g/(25 
 ml of meoh + 75 ml of dcm ) ) ; reagent b , 0.083 m 1,3,5-triformylbenzene 
 in dcm ( 1.34 g/100 ml of dcm ) ; flow rate a , 0.62 ml / min ; flow rate 
 b , 0.38 ml / min ; reactor volume , 10 ml ; reactor temperature , 100 c ; 
 back pressure regulator , 8 bar .
 porous 
 organic cages present many opportunities in functional materials 
 chemistry , but the synthetic challenges for these molecular solids 
 are somewhat different from those faced in the areas of metal  organic 
 frameworks , covalent  organic frameworks , or porous polymer 
 networks . here , we highlight the practical methods that we have developed 
 for the design , synthesis , and characterization of imine porous organic 
 cages using cc1 and cc3 as examples . the 
 key points are transferable to other cages , and this perspective should 
 serve as a practical guide to researchers who are new to this field . 
Introduction
Design and Synthesis of
New POCs
POC Isolation and Characterization
Methods
Summary
Experimental
Section