crystal engineering  the 
 rational design of crystalline molecular 
 solids  remains an important challenge for chemistry .
crystal structure prediction is not yet feasible 
 in all cases , and it is therefore useful 
 to develop motifs which allow families of structures to be generated 
 in a reliable fashion .
there is special 
 interest in motifs which lead to microporous ( or nanoporous ) crystals 
 with voids on the scale of 0.52 nm , sufficient to 
 accommodate molecular guests .
such materials offer various functionalities , 
 such as inclusion and storage of gases , and other guest molecules , the enhancement 
 of optical properties of included guests , the use of pores as reaction vessels to promote the formation of 
 desired products , and the separation of 
 mixtures including enantiomers from racemates . at the same time
crystallizing species will usually 
 attempt to maximize contact with each other , thus minimizing any void 
 space . to counter this trend
is not straightforward and will often 
 require the construction of specially shaped rigid components or units 
 capable of strong and directional interspecies interactions .
successful approaches to nanoporous crystal engineering may be 
 divided into two categories . on the one hand
( pcps ) or  metal  organic 
 frameworks  ( mofs ) , formed by combining 
 metal ions with rigid multivalent ligands . on the other
are purely 
 organic systems , which rely on noncovalent bonding to regulate crystal 
 packing .
the organic systems may be further 
 divided into intrinsically and extrinsically porous molecular crystals .
intrinsically porous crystals are based on 
 molecules with predefined open spaces ( macrocycles , cages etc . ) , whereas extrinsic porosity 
 results simply from crystal packing . of these three approaches ( hybrid , 
 intrinsic / extrinsic organic )
, the latter is probably the most challenging 
 as open frameworks must be maintained without the help of powerful 
 directional coordination bonds or pre - existing cavities .
some solutions 
 have emerged through serendipity , such as the classic urea inclusion 
 compounds .
there are few motifs 
 which generate families of readily accessible nanoporous crystals , allowing tuning of void dimensions and material 
 properties .
the work described in this paper is founded on a 
 serendipitous 
 discovery made a few years ago in the course of our program on anion - binding 
  cholapods  .
these powerful 
 receptors combine a rigid steroidal scaffold , derived from cholic 
 acid 1 ( chart 1 ) with various 
 combinations of h - bond donor groups .
most are reluctant to crystallize 
 but a small subset , represented initially by 24 , were found to form needles from methyl acetate - water or 
 acetone - water mixtures .
all three were subjected to x - ray crystallography , 
 with interesting results . despite the 
 significant differences between 24 ,
the external similarity of the crystals was reflected in the internal 
 structures ; the three were isomorphous , with almost identical unit 
 cell dimensions and packing arrangements for the invariant steroidal 
 cores .
the packing involved the formation of helices with hexagonal 
 symmetry ( space group = p61 ) , surrounding 
 solvent - filled channels .
the arrangement is illustrated in figure 1 , using tris - urea 3 as an example . 

individual steroid molecules bind to a single molecule of co - crystallized 
 water through 5 h - bonds ( figure 1a ) and stack 
 to form columns ( figure 1b ) .
the columns then 
 pack in a hexagonal arrangement to generate the solvent - filled pores . 

the orientation of the steroids in the columns is such that the terminal 
 groups ( methoxy and nhph ) face into the pores and largely determine 
 the nature of the channel surface .
effectively , the terminal groups can 
 expand into the channel interior without affecting the packing of 
 the columns which maintain the structure .
the channels , moreover , 
 are unusually wide . in the case of trifluoroacetamide 2 , the average diameter was found to be 16.4  . the average diameter 
 for 3 was only slightly less at 15.7  , although 
 the surfaces are more irregular ( figure 2 ) . 

there is thus substantial room , in principle , both for guest molecules 
 and for terminal groups .
preliminary experiments on 2 implied that guest exchange was possible , at least for certain solvents 
 ( meoac , et2o , toluene ) .
evacuation lead to partial degradation 
 ( evidenced by crazing ) , but the powder x - ray diffraction ( xrd ) pattern 
 remained largely unchanged .
the steroid is solvated 
 by a molecule of water which forms hydrogen bonds to all three urea 
 groups .
( b ) molecules of 3 stack to form 
 columns , running along the crystallographic c axis . 
 representation as for ( a ) except that core steroidal atoms are colored 
 blue and green in adjacent molecules , and water molecules are shown 
 with thick bonds .
one column of steroids is highlighted using the 
 coloring from ( b ) , with the och3 and nhph groups now in 
 spacefilling mode .
( d ) a single channel sliced in half along the c axis , viewed in spacefilling mode .
terminal och3 and nhph groups retain their coloring , other atoms near or at the 
 internal surface are shown as light blue .
( e ) 3d schematic representation 
 of a channel , showing helical arrangement of methyl groups and aromatic 
 rings ( spheres and hexagons , respectively ) .
interior surfaces for trifluoroacetamide 2 and tris - n - phenylurea 3 viewed along the c - axis .
the surfaces were calculated using a 1.4  probe . given the space available within 
 the channels , it seemed likely 
 that a wide range of analogues with a common bis-(n - phenylureido)steroidal core ( figure 3 ) would 
 form crystals isostructural with 24 .
variation should be feasible not only at the c3 substituent ( r in figure 3 ) but also at the c24 ester 
 group ( r in figure 3 )
( npsus ) would 
 be isostructural they should be able to form solid solutions ( organic 
 alloys ) , greatly enlarging the range of systems available . since our 
 original publication we have confirmed both of these possibilities . 

we have described a series of three npsus with aromatic groups in 
 r , and the interesting feature of  water wires  
 in the channels , and also a range of 
 npsu - based organic alloys .
herein we 
 provide a more complete description of our work surveying the scope 
 and properties of npsus , drawing on 25 examples which have been characterized 
 by x - ray crystallography .
we show how the dimensions and shapes of 
 the channels can be tuned , and how their chemical nature can be altered 
 by the introduction of functional groups ( including previously unreported 
 alkene and aldehyde functionality ) .
we also report , for the first 
 time , that npsus can be porous in the strictest sense , stable to evacuation 
 and capable of gas adsorption .
moreover we show that they can adsorb 
 a remarkable range of guests , including organic dyes with molecular 
 weights up to 300 and even the c30 hydrocarbon 
 squalene ( mw = 410 ) .
the core bis-(n - phenylureido)steroidal 
 unit maintains the p61 nanoporous structure , 
 while groups r and r control the size and 
 nature of the pore .
the first are esters of 3,7,12-tris-(n - phenylureido)-5-cholanoic acid 6 and 
 include 3 as well as the 14 variants 720 represented in chart 2 .
ester groups 
 r were chosen for variation in size ( and thus pore diameter ) 
 and surface characteristics ( aliphatic vs aromatic vs fluorocarbon ) 
 and also to showcase the potential for placing chromophores ( e.g. , 
 azobenzenes ) , fluorophores ( e.g. , pyrenes ) , and reactive units ( e.g. , 
 allyl groups ) in the channels .
the second group are derivatives of 
 methyl 3-amino-7,12-bis-(n - phenylureido)-5-cholanoate 5 , including trifluoroacetamide 2 , carbamate 4 , tris - ureas 2126 , and 
 amides 2729 ( chart 3 ) . again the variable group ( r ) was used to change 
 steric and surface properties and to introduce chromophores and functional 
 groups . in this case
aalthough 29 is included 
 here for convenience , it does not adopt the npsu crystal packing . 
 for further details see text .
although 29 is included 
 here for convenience , it does not adopt the npsu crystal packing . 
 for further details
amine 5 is accessible from cholic acid 1 via a multistep but 
 well - established route in 40% overall yield .
tris - urea 3 may be prepared from 5 by 
 treatment with phenyl isocyanate or more directly from 1 via methyl 3,7,12-triaminocholanoate .
esters 720 ( chart 2 ) are available from 3 via equilibration with lithium alkoxide or hydrolysis to acid 6 followed by o - alkylation or carbodiimide - induced esterification
. 
 the derivatives in chart 3 may be prepared 
 from 5 by treatment with an aryl isocyanate ( giving 2126 ) or an acylating agent ( giving 2729 ) .
the preparations of 9 , 10 , 12 , 1419 , 23 , 26 , and 29 have 
 been reported previously .
procedures for the remaining compounds in charts 2 and 3 are given in the supporting information .
the steroids in charts 2 and 3 were crystallized from methyl 
 acetate or acetone , to which small amounts of water had been added , 
 through slow evaporation of the organic solvent . in most cases
other polar solvents , such as methanol 
 or ethanol , or nonpolar mixtures , such as chloroform - hexane , yielded 
 oils or amorphous solids .
all the compounds could be analyzed by single 
 crystal x - ray diffraction ( scxrd ) , and with the single exception of 29 ( see below ) , all formed crystals with the p61 npsu packing .
the structures of 24 , 14 , 16 , and 18 have 
 been reported in communications , the remainder 
 are described for the first time herein .
as expected 
 these show only minor variations , the differences between the steroids 
 being accommodated by changes to the shape , diameter , and surface 
 characteristics of the pores .
unsurprisingly , given the open nature 
 of the pore region , disorder in terminal groups r and 
 ( especially ) r was fairly common , being present in 11 
 of 25 structures .
however , in most cases the groups concerned were 
 divided between just two positions , so that a reasonable model of 
 the crystal ( for estimating pore volume etc . )
this applied to 4 , 8 , 10 , 12 , 13 , and 24 ( disorder in r ) , and 22 ( disorder 
 in r ) . in two cases ,
15 and 19 , deleting one of the two possible positions did not give a viable 
 structure .
however , these crystals could be modeled successfully by 
 assuming equal occupancy of both positions , on an alternating basis . 
 after editing where relevant , the smoothed solvent accessible surfaces 
 and resulting guest - accessible volumes were calculated using materials 
 studio , employing a probe of radius 1.2  .
these values are given 
 in table 1 , while images of the surfaces are 
 available as supporting information .
these were estimated by repeating the calculation using probes of 
 increasing size until the surface was no longer continuous .
the resulting 
 value is , effectively , the diameter of the largest sphere which can 
 pass through the channel . images of selected structures viewed down 
 the pores , with terminal groups shown in spacefilling mode , are shown 
 in figure 4 ( compounds from chart 2 , varying ester group r ) and figure 5 ( compounds from chart 3 , 
 varying c3 terminal group r ) .
, obtained from the materials studio 
 program employing a spherical probe of 1.2  radius .
calculations of total solvent accessible surfaces give higher values 
 but include small voids outside the channel region .
estimated by calculating the  smoothed 
 solvent accessible surface  using differing probe radii ( increments / decrements 
 of 0.05  ) .
the value given is the diameter of the largest probe 
 for which the calculation yields a continuous surface .
these values are slightly smaller 
 than those reported in ref ( 17 ) , due to a change in the method of calculation .
could be built 
 using the assumption that r in neighboring molecules occupied 
 alternating positions , and this model was used for the pore volume 
 and diameter calculations . when the probe diameter is reduced 
 to this value ,
voids are generated outside the channel region while 
 a continuous pore surface has not yet appeared .
mes disordered over two positions , 
 one being removed before pore volume and diameter calculations .
terminal groups 
 r (= 
 nhph ) and or are shown in space - filling mode , with r colored gold .
terminal groups r and 
 or (= ome ) are shown in space - filling mode , with or colored magenta .
table 1 and figures 4 and 5 illustrate the wide variety 
 of structural 
 properties available via the npsu system .
for example , starting at 
 nearly 20% ( for 2 ) , the volume available in the pores 
 can be tuned downward in small increments essentially to zero ( for 15 and 19 ) .
indeed , by taking advantage of alloy 
 formation , continuous variation should 
 be possible with these compounds .
unsurprisingly , pore volumes and 
 diameters are generally determined by the size of the terminal groups , 
 but more subtle effects are also in play .
for example , in the series 
 with varying or ( chart 2 , figure 4 ) , a 2-carbon spacer between the oxygen and an aromatic 
 group tends to allow efficient packing of the aromatic surface against 
 the side of the channels .
thus , for pyrenyl derivative 16 , space remains down the center for hydrogen - bonded chains of water 
 molecules .
in contrast , a 1-carbon methylene 
 spacer directs the aromatic group toward the center of the channel . 
 in the case of pyrenyl derivative 15 ,
this results effectively 
 in full occupation of the channel ; the calculated guest - accessible 
 volume and minimum diameter are both close to zero .
paradoxically , 
 therefore , the larger terminal group ( in 16 ) leaves more 
 space than the smaller group in 15 .
the shape of 
 the channel wall ( smooth vs corrugated ) is another 
 feature which can be altered . as mentioned above , compounds for which 
 r = ch2ch2ar ( e.g. , 14 , 16 , 18 ) tend to adopt structures in which 
 the aromatic groups line the surfaces of the channels .
the resulting pores are relatively smooth and 
 cylindrical , as illustrated for 14 in figure 6 ( top ) . in other systems from chart 2
, the channel surface is presumably corrugated , but with random 
 and/or flexible character due to disorder within the crystal .
an example 
 is provided by 13 , for which the naphthylmethyl group 
 appears in two orientations , one roughly perpendicular and one more 
 nearly parallel to the channel axis .
well - defined corrugated pores 
 may be accessed by placing extended substituents at r ( which 
 is less prone to disorder ) .
thus for both 24 ( r = azobenzene ) and 25 ( r = biphenyl ) the 
 c3-substituent reaches well toward the c axis creating 
 strongly asymmetric helical pores ( figure 6 , middle and bottom ) .
this work also shows that the chemical 
 nature of the pore walls 
 can be subject to wide variation .
the structures collected in figures 4 and 5 feature an alkenyl 
 group c = c ( 8) , a helical strip of fluorocarbon 
 surface ( 10 ) , an aldehyde ( 21 ) , a thioether 
 ( 22 ) , a boc - protected amine ( 23 ) , and an 
 iodobenzene ( 26 ) . as illustrated in figure 7 , all are positioned where they can interact with guest molecules 
 and participate in reactions or noncovalent interactions .
finally , 
 crystallography of acetamide 29 showed that 
 not every molecule defined by figure 3 adopts 
 the p61 npsu structure . in this case
a monoclinic ( p21 ) form obtained from methyl acetate / water 
 was denoted 29 , and a tetragonal ( p432121 ) form which crystallized 
 from acetone / water was denoted 29. the molecular 
 units in the two forms are almost identical , and qualitatively different 
 to those in the npsus ; in particular , the c3 substituent is positioned 
 so that the nh group points inward , creating a binding site which 
 accommodates two water molecules ( see figures 
 s31 and s32 ) . in both crystals
the packing is efficient , leaving 
 no substantial voids ( see figures s57 and s58 ) .
crystal structures of 14 , 24 , and 25 viewed perpendicular to the c axis . for 
 the images on the left ,
the groups which dominate the channel surface 
 ( or for 14 , r for 24 and 25 ) are highlighted in spacefilling mode .
for the 
 right - hand images , the smoothed solvent accessible surfaces have been 
 added using materials studio , and the structures have then been sliced 
 along the c - axis .
space - filling representations of the channel regions in 8 , 10 , 21 , and 22 .
the structures 
 have been sliced along the c - axis to expose the pore 
 interiors and are viewed roughly perpendicular to c and a ( see axes attached to 8) .
conventional 
 coloring is used for the distinctive groups in each structure ( or for 8 and 10 , r for 21 and 22 ) , the remaining atoms being shown as 
 silver - blue .
 
 implies that the crystal is permeable , 
 allowing exchange of small guest molecules , and that this process 
 does not substantially affect the host framework .
ideally , the crystals should also be able to survive the 
 removal of all guest molecules without loss of structure and then 
 show reversible gas adsorption to confirm porosity .
as mentioned earlier , we previously demonstrated that trifluoroacetamide 2 satisfies , at least , the guest - exchange criterion .
the results 
 from evacuation were less clear - cut ; the powder xrd ( pxrd ) pattern 
 remained essentially unchanged , but the crystals crazed and became 
 opaque .
most npsu crystals , especially those with 3-ureido substituents , 
 showed no change in appearance on evacuation .
nonetheless it was clearly 
 desirable to establish the solvation state of a typical npsu , show 
 that the solvent could be removed , and demonstrate that the resulting 
 crystals were unchanged and capable of gas adsorption .
we chose tris - n - phenylurea 3 for this study , as this compound 
 is the most accessible npsu and has proved the most convenient for 
 routine use .
crystals of 3 were obtained as needles from acetone / water ( initial ratio 
 10:1 ) , after washing with acetone and air - drying .
samples were then 
 evacuated at room temperature and 100 c for 24 h. the three 
 samples ( air - dried , rt evacuated , 100 c evacuated ) were then 
 analyzed by h nmr in dmso , using a procedure which allowed 
 the amount of background water to be measured and taken into account .
the composition of the air - dried crystals was found to be 3:water : acetone = 1:3.8:0.2 . allowing for the single water 
 molecule per steroid embedded in the channel wall , this implies that 
 the pores are filled with 3 molecules of h2o per 
 molecule of 3 , with a small amount of organic solvent 
 also present .
after evacuation at rt the composition was 3:water = 1:1 , implying that the channels are empty .
after evacuation at 100 c 
 for 24 h the ratio 3:water reduced slightly to 1:0.9 . 

this suggests some degradation , although microscopy and pxrd again 
 showed no major changes .
samples of 3 were also heated 
 to 150 c and above , and in these cases clear signs of decomposition 
 were observed both by microscopy ( loss of transparency ) and pxrd ( loss 
 of diffraction peaks ) . having established that the pores could 
 be evacuated without loss 
 of crystallinity , we proceeded to confirm the permanent porosity of 3 using n2 gas adsorption measurements for a sample 
 that had been heated under vacuum at 75 c for 9 h. surprisingly , 
 the n2 adsorption predominately takes place at high relative 
 pressures ( p / p > 
 0.7 ) , 
 and there is significant hysteresis between the adsorption and desorption 
 isotherms giving a type iv isotherm ( see e.g. , figure 8) .
this hysteresis differs from 
 that observed in mesoporous materials ( pore diameter > 20  ) 
 that 
 generally closes at lower relative pressures ( p / po  0.4 ) and which is related to pore 
 evacuation involving capillary action .
furthermore , the desorption 
 isotherm falls below that of the adsorption isotherm at p / po  0.7 .
 desorption cycle is repeated 
 and presumably reflect slow , nonequilibrium , kinetics of n2 adsorption .
such slow kinetics is understandable if access to the 
 pores is restricted to the relatively small number of openings located 
 at the end of the long needle - shaped crystals , which are on average 
 > 2 mm in length .
we have previously shown that the pores in 3 are parallel to the long axis of the crystals .
similar hysteresis was observed by tosi - pellenq 
 et al . from the n2 isotherms of long ( 150 m ) microporous 
 crystals of alpo4 - 5 , which 
 also contain cylindrical channels ( 0.76 nm in diameter ) along the 
 long axis of the crystals . in this case
the fact that the desorption isotherm in figure 8 dips below the adsorption isotherm implies that 
 evaporating nitrogen is lost more rapidly from the channels than gaseous 
 n2 is readsorbed .
this may relate to pressure differences 
 between the interior and exterior of the crystals ; it is reasonable 
 to suppose that when the crystals are compressed , inward gas transfer 
 could be relatively slow , while internal pressure could expand the 
 crystals and assist n2 efflux .
the possibility that the 
 effects are due to collapse of the crystal structure during n2 analysis was discounted by confirming that the structure 
 remained unchanged , as shown by scxrd of a crystal extracted from 
 the sample of 3 used for n2 analysis . the bet surface area calculated from the n2 adsorption 

isotherm is very low ( 29 m / g ) and probably represents 
 only the external surface area of the crystals .
however the pore volume 
 of 0.17 ml / g calculated from the total n2 uptake ( 4.9 mmol / g ) 
 is highly consistent with the guest - accessible volume ( 16.1% ) calculated 
 from the crystal structure ( i.e. , 0.17 ml / g equates to 16% of the 
 total volume given a crystal framework density of 0.941 g / ml ) .
the 
 pore volume obtained from n2 uptake is also consistent 
 with the values calculated from the adsorption of liquid guests , as 
 discussed in the following section .
n2 adsorption (  ) and 
 desorption (  ) isotherms 
 for crystal 3 at 77 k. see text for discussion .
air - dried crystals of 3 were placed 
 in each , left for 1224 h , washed briefly with ether , and subjected 
 to h nmr analysis .
all of the substrates were adsorbed 
 in significant amounts , as summarized in table 2 .
aniline 30 formed a well - defined host  guest 
 1:1 complex which could be characterized by x - ray crystallography . 

as shown in figure 9 , the aniline molecules 
 form a helix within the channel , apparently stabilized by a close 
 interguest chn contact ( dc  hn = 2.68  ) .
the anilines are also held in place by specific 
 favorable interactions with the channel wall , including hydrogen bonds 
 between amino nh and host ester carbonyl ( dn  ho = 2.46  and n  ho = 167.8 ) , nh interactions involving the second 
 amino nh and a host phenyl group ( dn  h = 2.84  ) , and ch interactions to the aniline 
 -system .
guest ratio , although in this case the guest could 
 not be located crystallographically .
a calculation of the volume of liquid absorbed 
 per unit mass of host gave a value of 0.12 ml / g , consistent 
 with the pore volume obtained by gas adsorption ( see above and table 2 ) .
this represents a pore - filling efficiency of 
 70% using the pore volume calculated from the crystal structure , 
 which is consistent with a strong affinity between the crystal and 
 adsorbate .
similar calculations based 
 on the uptake of 3336 suggested 
 that these were absorbed less efficiently . however
the value for squalene 36 , at 65% of the maximum , is remarkable for such a large 
 ( 30-carbon ) guest .
x - ray crystal structure of 3 with adsorbed 
 aniline , 
 viewed along the c - axis ( top ) and a - axis ( bottom ) . the aniline is shown in space - filling mode .
samples of air - dried crystalline 3 were place in aniline and then removed , washed with ether , 
 and analyzed by h nmr after periods ranging from 2 to 
 180 min .
the results showed that the crystals are filled to about 
 half capacity very quickly ( within the first 2 min ) , but that subsequent 
 adsorption is much slower .
we were also interested to discover whether the aniline could 
 be oxidized to polyaniline within the channels .
indeed , treatment 
 of the complex with peroxyammonium sulfate in 0.1 n hcl caused the 
 crystals to turn dark violet ( after 4 h ) then green ( after 12 h ) . 

the diffuse - reflectance uv  vis spectrum of the product showed 
 adsorption maxima at 420 and 795 nm consistent with polyaniline formation 
 ( see figure s75 ) .
pxrd analysis showed that the npsu structure was retained , although 
 the crystals were no longer suitable for single - crystal x - ray structure 
 determination .
another set of experiments involved the adsorption 
 of larger guest 
 molecules from solutions in diethyl ether . in these cases
colored 
 guests were used for ease of analysis and the potential for interesting 
 or useful optical effects .
, solutions of dyes 3743 in ether ( 1020 mm ) were 
 added to crystals of 3 , and the mixtures left to stand 
 for 3 days . after isolation and washing with ether , all crystals were 
 visibly colored . in the case of 3741
the colors were strong enough to show clearly under a microscope 
 ( see figure 10 ) .
as shown in figure 10 , the colors appeared to permeate the crystals 
 and were not localized at ends or edges .
interestingly , the crystals 
 containing nile red 41 were observed to be blue - purple 
 ( figure 10e ) .
this dye is strongly solvatochromic , 
 its optical adsorption moving to longer wavelengths with increasing 
 solvent polarity , and a blue or purple 
 color suggests a highly polar environment . soaking
the crystals in 
 ether for 24 h resulted in loss of color , showing that the dye adsorption 
 was reversible .
a second npsu crystal , trifluoroacetamide 2 , was also investigated as host and was found to absorb azo - dyes 37 and 38 .
the combinations of 3 with disperse red 1 ( 38 ) and azulene ( 43 ) were investigated further , to establish how much dye was included 
 and how fast . in the case of azulene , only 1 mol % was absorbed , 
 while equilibrium was reached within the first hour . in the case of 38 ,
the first 1 mol % was also absorbed quickly , but a further 
 quantity ( nearly 1 mol % ) was taken up in a slower process over 24 
 h. crystals of 3 after exposure to ethereal solutions 
 of ( a ) 37 , ( b ) 38 , ( c ) 39 , 
 ( d ) 40 , and ( e ) 41 . despite the appearance of the crystals there was room for 
 concern 
 that the dyes might not be entering the channels but somehow associated 
 with cracks or defects in the crystals .
to test this possibility , 
 we examined the colored crystals under a microscope using plane polarized 
 light .
if the dyes were occupying the channels , it seemed likely that 
 some ( at least ) would show preferential alignments .
if the transition 
 dipole moments were to lie roughly along the channel axis ( the long 
 axis of the crystal ) the crystals should be dichroic , i.e. , their 
 colors should be dependent on their orientation with respect to the 
 plane of polarization .
figure 11 shows pairs 
 of photomicrographs in which crystals of identical composition , but 
 oriented at roughly 90 to each other , are illuminated with polarized 
 light .
each pair of images shows the same crystals , with the plane 
 of polarization differing by 90. the crystals are clearly dichroic , 
 changing from colored to almost colorless as the plane of polarization 
 is rotated .
the effect was observed for 237 , 238 , 337 , 338 , and 341 but not for 339 or 340 . the guests which lead to 
 dichroism ( 37 , 38 , and 41 ) 

possess extended dipoles due to conjugation of an amino group with 
 an electron acceptor .
this feature should encourage the molecules 
 to adopt a head - to - tail arrangement parallel to the channel axis . 

the images in figure 11 provide strong evidence 
 that the dye molecules are indeed in the channels revealed by crystallography .
it should be noted that this phenomenon of dye uptake by organic 
 molecular crystals is rare and may be unprecedented .
it is well - known 
 that dyes may be adsorbed by inorganic crystals , such as zeolites , or by organic  inorganic hybrids ( pcps / mofs ) .
however , the inclusion of dyes in organic molecular 
 crystals is normally achieved by cocrystallization , not by the interaction of substrates with 
 macroscopically sized preformed crystals .
this ability of npsus to 
 adsorb such large guest molecules highlights their unusual combination 
 of robust crystal structures with spacious accessible interiors .
crystals 
 of npsus with included dyes illuminated with polarized 
 light . for each pair of images the plane of polarization is rotated 
 through 90 between left and right .
( a ) 237 , ( b ) 238 , ( c ) 337 , ( d ) 341 .
in 
 principle , the npsu crystal packing represents a powerful tool 
 for the design of functional materials .
first , the structure needs 
 to be generalizable , forming in ( at least ) most of the cases where 
 it might be predicted .
second , the crystals need to be truly porous 
 so that the space within may be exploited .
we have now examined the crystal structures of 26 
 molecules with the general structure represented in figure 3 , and of these only one ( acetamide 29 ) fails to adopt the p61 npsu arrangement . 

the range of npsus now includes examples with vanishingly small pores 
 sizes , strongly corrugated pore surfaces , and several cases with potentially 
 reactive functional groups ( ch2ch = ch2 in 8 , ch = o in 21 , sme in 22 , and nhboc in 23 ) .
it is notable that neither 
 the aldehyde nor nhboc groups , both of which are quite polar , disturbed 
 the npsu packing .
we have also shown that the pores can be evacuated 
 without loss of integrity and that subsequent gas adsorption is possible 
 ( although given the pressures involved and the low pore volume , applications 
 in gas storage are unrealistic ) .
more importantly , organic molecules 
 are also absorbed , including the large rigid nile red 41 ( mw 318 ) , and the even larger but more 
 flexible squalene 36 ( mw 411 ) . 

the ability to orient dye molecules suggests applications in display 
 technology and nonlinear optics .
although not all dyes showed this 
 behavior , the tunability of the pores implies that the phenomenon 
 should be extendable ( e.g. , by tailoring of channel diameter ) .
the 
 fact that small molecules can readily access the pores points to further 
 applications in catalysis , sensing , and separations , especially given 
 the chirality of the crystals and the ability to incorporate effector 
 groups through alloy formation . we hope 
 to explore these and other possibilities in future work .
 previous 
 work has shown that certain steroidal bis-(n - phenyl)ureas , 
 derived from cholic acid , form crystals in the p61 space group with unusually wide unidimensional 
 pores . 
 a key feature of the nanoporous steroidal urea ( npsu ) structure 
 is that groups at either end of the steroid are directed into the 
 channels and may in principle be altered without disturbing the crystal 
 packing . 
 herein we report an expanded study of this system , which 
 increases the structural variety of npsus and also examines their 
 inclusion properties . 
 nineteen new npsu crystal structures are described , 
 to add to the six which were previously reported . 
 the materials show 
 wide variations in channel size , shape , and chemical nature . 
 minimum 
 pore diameters vary from 0 up to 13.1  , while some of 
 the interior surfaces are markedly corrugated . 
 several variants possess 
 functional groups positioned in the channels with potential to interact 
 with guest molecules . 
 inclusion studies were performed using a relatively 
 accessible tris-(n - phenyl)urea . 
 solvent removal was 
 possible without crystal degradation , and gas adsorption could be 
 demonstrated . 
 organic molecules ranging from simple aromatics ( e.g. , 
 aniline and chlorobenzene ) to the much larger squalene ( mw = 411 ) could be adsorbed from the liquid state , while 
 several dyes were taken up from solutions in ether . 
 some dyes gave 
 dichroic complexes , implying alignment of the chromophores in the 
 npsu channels . 
 notably , these complexes were formed by direct adsorption 
 rather than cocrystallization , emphasizing the unusually robust nature 
 of these organic molecular hosts . 
Introduction
Results and Discussion
Conclusion