conjugated microporous 
 polymers ( cmps ) and related materials , such as 
 covalent triazine - based frameworks ( ctfs ) and 
 porous aromatic frameworks ( pafs ) , are a fascinating 
 class of materials that can , in some cases , combine microporosity 
 with useful physical properties that arise from their electronic conjugation .
cmps typically comprise aromatic organic units covalently bonded 
 to three or four neighboring units .
insoluble cmps and related polymers , 
 such as pafs , hence have commonly been envisaged as extended 3- or 
 4-connected networks that contain rings but only a small number of 
 end - groups .
if this picture is true , then cmps , ctfs , pafs , and their like , 
 would be the closest organic polymer analogues of well - known inorganic 
 materials such as boron nitride , zinc oxide , and ( alumino)silicates , 
 materials whose structures are also based on 3- or 4-connected networks .
alternatively , however , these materials could also resemble highly 
 branched polymers or dendrimers , which are tree - like molecules with 
 a relatively large number of end - groups but , typically , no closed 
 rings .
structural hypotheses and models aside , it is essentially unknown if cmps , ctfs , and pafs can be 
 best described as highly branched polymers or as extended networks , 
 or even as small but insoluble oligomers , which might nonetheless 
 exhibit permanent microporosity .
this 
 uncertainty over structure mainly stems from the amorphous or poorly 
 crystalline nature of the experimental samples , coupled with their 
 total lack of solubility .
however , the need to understand structure  property 
 relationships is highlighted by the recent discovery of new porous 
 materials , such as soluble , hyperbranched cmps and porous dendrimers , organic molecules of intrinsic microporosity , and other discrete organic molecules that can show high levels of microporosity in the amorphous , solid 
 state .
hence , extended networks are not a prerequisite for microporosity , 
 and network structures can not simply be assumed because a given material 
 is microporous , particularly since typical cmp structures , which lack 
 any deliberate solubilizing functionality , would be expected to become 
 insoluble at quite modest molecular weights .
it is important , therefore , 
 that methods are developed to elucidate the molecular structures of 
 cmps , rather than simply inferring them based upon observations of 
 permanent microporosity .
the experimental determination of the 
 number of end - groups , for 
 example by solid - state nmr , should in principle allow us to differentiate 
 between , e.g. , dendrimers and highly extended condensed networks . 
 in practice , however , this is difficult because side - reactions might 
 cleave such end - groups , making them invisible to nmr and artificially 
 lowering the end - group - to - molecular unit ratio .
for example , in nickel - coupled 
 yamamoto polymerizations of tetrahedral aryl halides , it is possible that halogen end groups might be removed by metal - catalyzed 
 dehalogenation .
additionally , entrained gases , physisorbed water vapor , 
 or residual catalyst can all lead to difficulties in the precise determination 
 of end - group concentration by elemental analysis . in this study , 
 we take an alternative approach and focus on optical 
 absorption and fluorescence spectra for cmps as a handle on their 
 molecular structure .
we believe that this is a useful strategy , because 
 the electronic properties of cmps are in any case central to many 
 of their most interesting applications .
we demonstrate that the optical properties of cmps give a unique 
 insight into the structural elements present in the cmp .
furthermore , 
 we show that this structural knowledge can also be exploited in terms 
 of engineering the optical properties of cmps and related compounds . 

we concentrate here on cmps based on the polymerization of pyrene 
 monomers , but the general approach should be broadly 
 applicable to other cmps and compounds such as ctfs and pafs .
thin layer chromatography ( tlc ) was performed 
 using precoated aluminum sheets with silica gel 60 f254 ( merck ) and 
 visualized by uv light (  = 254 or 280 nm ) .
solution h nmr 
 spectra were collected on a bruker uxnmr / xwin - nmr 400 mhz spectrometer 
 while the solid - state c nmr spectrum for the ecmp was 
 collected a 9.4 t bruker dsx nmr 400 mhz spectrometer equipped with 
 a 4 mm hxy triple - resonance mas probe ( in double - resonance mode ) . 

ftir spectra for the ecmp and its precursors were collected on a bruker 
 tensor 27 spectrometer equipped with a specac attenuated total reflectance 
 module .
gel permeation chromatography ( gpc ) utilized a lc 1120 hplc 
 pump , a pl - els 1000 evaporative light scattering detector , a pl gel 
 5 mm mixed - c gpc column and midas autosampler ( polymer laboratories 
 ltd .
thf was used as the eluent with flow rate of 1.00 ml / min 
 at 40 c and polystyrene as the standard .
full synthetic details 
 and nonoptical characterization results are given in section esi-1 
 of the supporting information .
solution measurements for 
 the soluble materials were obtained at 0.04 and 0.004 mg / ml ( 1,3-linear 
 pyrene polymer and scmp ) and 1.5 mg / ml ( py(5 ) dendrimer ) respectively 
 in dcm .
for the same soluble materials thin film samples were prepared 
 by dissolving 20 mg ( 1,3-linear pyrene polymer and scmp ) or 6 mg ( py(5 ) 
 dendrimer ) of the compounds in 1 ml of dcm , after which the resulting 
 solution was added to a quartz cuvette held at an angle and left overnight 
 to evaporate .
the resulting film on the inside of the cuvette provided 
 a range of film thicknesses for analysis ( thickest at the bottom of 
 the cuvette ) .
measurements for solid - state powders were obtained by 
 grinding the sample with kbr .
vis 
 spectra were 
 obtained using a shimadzu uv-2550 uv  vis spectrophotometer 
 running the uvprobe software , version 2.34 .
all spectra were obtained 
 as absorbance measurements from 200800 nm , with scan speed 
 set to fast and using a slit width of 5 nm .
film and solution samples 
 were measured in a quartz cuvette as a transmission measurement .
solid 
 powdered samples were analyzed using the isr-2200 integrating sphere 
 attachment with a quartz solid sample holder as diffuse reflection 
 measurement .
emission and excitation 
 spectra were obtained 
 on a shimadzu rf-5301pc spectrofluorophotometer running rfpc software , 
 version 2.04 .
spectra were obtained using a fast scan speed and with 
 sensitivity set to high .
slit widths were adjusted so as to maximize 
 the signal - to - noise for each sample .
solution samples were analyzed 
 in a quartz cuvette with the standard cell holder attachment .
film 
 samples were analyzed adhered to the wall of a quartz cuvette placed 
 in the solid ( powder ) holder attachment .
powder samples were analyzed 
 in a quartz solid sample holder held in the solid ( powder ) sample 
 holder attachment .
data was exported to excel for further processing , 
 and second order diffraction peaks of the excitation wavelength were 
 manually removed from the spectra and the long wavelength baseline 
 set to zero .
the excited 
 state properties 
 of pyrene oligomer model clusters were calculated using a six - step 
 approach .
first , for each cluster model we performed a conformer search 
 using the opls - aa forcefield and the 
 low - mode sampling algorithm as implemented 
 in macromodel 9.3 ( for more details about the conformer search see 
 section esi-2 of the supporting information ) second , the ground state singlet ( s0 ) geometries of 
 the low - energy conformers found in the conformer search were optimized 
 using ground state density functional theory ( dft ) calculations .
third , 
 where possible , the harmonic frequencies at these optimized s0 geometries were calculated using the same dft setup to verify 
 that the optimized structures correspond to proper minima on the s0 energy surface .
fourth , the excitations at the optimized 
 s0 geometry were calculated using time - dependent density 
 functional theory ( td - dft ) .
fifth , the lowest singlet excited state 
 of each oligomer was relaxed using td - dft , to obtain its minimum energy geometry . finally , frequency calculations 
 on the excited state minima were performed for selected oligomers 
 to verify that they correspond to proper minima on the td - dft excited 
 state energy surface .
for the dft / td - dft calculations mainly the range - separated 
 hybrid xc - functional cam - b3lyp was used 
 and in some selected cases also the plain hybrid xc - functional b3lyp ( mainly to study the ground state energetics ) , 
 where for all excitation calculations the tamm 
all the dft / td - dft calculations employing 
 the cam - b3lyp xc - functional used a combination of nwchem 6.0 ( vertical excitation energies ) and gamess - us version 1 october 2010 r1 ( vertical excitation 
 energies , excited state optimizations ) and the 6 - 31 g * * split - valence 
 basis - set .
all the dft calculations employing 
 the b3lyp xc - functional were performed using the turbomole 6.3.1 code , and employed the double- dzp basis - set .
the pyrene - based 
 cmp network ( scheme 1b ) 
 obtained by homopolymerization of 1,3,6,8-tetrabromopyrene ( see scheme 1a for atom labeling of pyrene ) has a reported brunauer 
emmett  teller 
 ( bet ) surface area of 1508 m / g and is highly fluorescent .
note , solubilizing tert - butyl groups on the 7-position 
 of structures c and 
 e have been removed for clarity .
the pyrene - based 
 cmp network is a member of a broad family of related 
 pyrene - based polymers and oligomers reported previously in the literature . 

this family of structures includes a 1,3-linear polymer based on the 
 yamamoto coupling of 1,3-dibromo-7-tert - butylpyrene 
 ( scheme 1c ) , first - and 
 second - generation dendrimers ( py(5 ) and py(17 ) , respectively ) based 
 on pyrene coupled via the 1,3,6 and 8 positions ( scheme 1e ) , and a branched statistical 
 copolymer of 1,3,6,8-tetrabromopyrene and 1,3-dibromo-7-tert - butylpyrene .
the latter copolymer combines 
 porosity with solubility in selected solvents , and we hence refer 
 to this as a soluble cmp or  scmp  .
finally , we note 
 that the synthesis of a linear polymer , obtained through coupling 
 of pyrene via its 2 and 7 positions , has also been reported .
however , this material is less closely related 
 to our pyrene cmp than other pyrene polymers 
 and oligomers because the pyrene monomer is linked through positions 
 other than 1 , 3 , 6 , and/or 8 , and it has very bulky aryl substituents 
 on the 4,5,9 and 10 positions . as such
hence , we have considered four possible basic architectures : 
 a linear soluble pyrene polymer , perfectly branched , soluble pyrene 
 dendrimers , a branched soluble pyrene polymer ( the scmp ) , and a branched 
 pyrene cmp  network  . as a first step in understanding 
 the structure of the pyrene - based 
 cmp network , we prepared samples of the insoluble cmp network , and the soluble 1,3-linear polymer , py(5 ) first - generation dendrimer , and scmp .
we also 
 synthesized a novel copolymer ( ecmp , for  expanded - cmp ) 
 via the cross - coupling of 1,3,6,8-tetrabromopyrene and 1,3-diboronic 
 ester-7-tert - butylpyrene ( scheme 2 ) .
unlike the scmp , which is a statistical copolymer based 
 on similar monomers and where , for example , two 1,3-substituted pyrene 
 monomers ( bifunctional units ) might link directly to each other , the 
 ecmp will have an alternating copolymer structure , where 1,3,6,8- 
 substituted pyrene monomers ( tetrafunctional units ) are only linked 
 to 1,3-substituted pyrene monomers and vice versa . a further immediate 
 difference between ecmp and scmp
is the lack of solubility for ecmp : 
 by contrast , the analogous scmp statistical copolymer is soluble in 
 a range of common solvents .
full synthetic 
 details and characterization data are available in the supporting information for all these materials 
 ( section esi-1 ) .
we next measured the absorption 
 and fluorescence spectra for these 
 samples in solution , where possible , as a thin film cast from solution 
 ( again , where possible ) , and as a powder mixed with kbr ( the only 
 option for the insoluble cmp and ecmp ) .
these measurements were compared 
 with those reported previously in the literature for the 1,3-linear 
 polymer material ( solutions and thin films ) , for the py(5 ) material ( solution ) , for 
 the scmp material ( in solution ) , and 
 for the insoluble cmp ( powder ) .
this is 
 the first systematic comparison of the solid - state and solution phase 
 spectra of these materials .
we concentrate here mostly on the fluorescence 
 spectra , partly because the powder absorption spectra appeared significantly 
 broadened relative to thin films , possibly due to scattering effects 
 related to the polymer particle size distribution ( there is also a 
 small red shift in the fluorescence peak position between the thin 
 film and powder spectra , e.g. , 10 nm in the case of 1,3-linear polymer 
 material , possibly also due to scattering , but with no real effect 
 on the spectrum shape , for more details see section esi-3 of the supporting information ) .
moreover , in the case 
 of a structurally heterogeneous material , such as the insoluble pyrene 
 cmp / ecmp and scmp are likely to be , the absorption spectrum will be 
 a convolution of the absorption spectra of a range of different structural 
 elements ( chromophores ) , all of which displaying vibrational broadening . 
 as a result ,
figure 1 shows the normalized 
 experimental 
 fluorescence spectra for the various powder samples . the insoluble 
 cmp 
network fluorescence spectrum is significantly 
 red - shifted compared to the other materials , with the wavelength of 
 the fluorescence maximum increasing as follows : 1,3-linear polymer 
 ( 479 nm , 2.6 ev ) <
py(5 ) dendrimer ( 501 nm , 2.5 ev ) < scmp ( 526 
 nm , 2.4 ev ) < ecmp ( 530 nm , 2.3 ev ) < insoluble cmp ( 618 nm , 
 2.0 ev ) .
it is apparent that the chromophores responsible for fluorescence 
 in the cmp network and ecmp are quite distinct from those from the 
 fluorescent chromophores in the 1,3-linear polymer and the py(5 ) dendrimer . 

it is also clear that the insoluble cmp fluorescence chromophore is 
 absent in the analogous soluble polymeric materials 
most obviously 
 in the 1,3-linear polymer , which does not fluoresce at all in this 
 wavelength range .
these fluorescence spectra demonstrate that at least 
 part of the insoluble pyrene cmp network and the ecmp structures are 
 fundamentally different in character from those of the 1,3-linear 
 pyrene polymer and the py(5 ) dendrimer .
experimental fluorescence 
 spectra for the 1,3-linear pyrene polymer 
 ( purple ) , py(5 ) pyrene dendrimer ( blue ) , scmp ( cast film green , precipitated 
 film orange ) , ecmp ( light red ) and cmp ( dark red ) , clearly showing 
 the red shift in the spectra . in all cases
we suggest that it is unlikely that these profound differences 
 in the fluorescence spectra can be ascribed simply to the size or 
 effective molecular weight of the conjugated pyrene system .
for example , 
 from the solution data of figueira - duarte et al . , we know that the difference in fluorescence energy of the 
 two dendrimers py(5 ) and py(17 ) is less than 0.1 ev while the system 
 more than triples in size .
similarly , we know , based on solution data 
 for the 1,3-linear pyrene dimer and trimer oligomers and the 1,3-linear 
 pyrene polymer , that the fluorescence energy is effectively converged 
 with the pyrene dimer .
it is thus unlikely that simply 
 increasing the number of conjugated units can explain the large observed 
 red shift in fluorescence for the insoluble pyrene cmp , or indeed 
 even the much smaller red shift observed for ecmp .
we also believe 
 that these differences do not arise from agglomeration related effects . 

we studied two different scmp samples ; one sample obtained through 
 precipitation with an antisolvent ( orange line in figure 1 ) and one obtained through slow evaporation of the 
 solvent ( green line in figure 1 ) .
both samples 
 differ strongly in density and porosity to gases ( precipitated materials 
 have much higher porosity than cast materials , where the precipitated 
 material absorbs nitrogen in micropores while the cast material is 
 impervious to nitrogen ) and thus probably 
 in degree of agglomeration .
the difference in the position of the 
 peak maximum for cast and precipitated scmp ( 8 nm , 0.03 ev ) is , however , 
 small , especially in comparison with the red shift between the 1,3-linear 
 polymer and the pyrene cmp ( see section esi-3 , supporting information ) .
a comparison of the experimental 
 solid - state ( powder ) absorption 
 spectra ( see figure s3 , section esi-3 of the supporting 
 information ) , while complicated somewhat by the peak - broadening 
 problems discussed above , also suggests that the bulk of the cmp and 
 ecmp structure are different from that of the 1,3-linear pyrene polymer 
 and the pyrene py(5 ) dendrimer . not only are the first absorption 
 maxima of the cmp ( 417 nm , 3.0 ev ) and the ecmp ( 430 nm , 2.9 ev ) red - shifted 
 compared to both the 1,3-linear polymer ( 370 nm , 3.4 ev ) and py(5 ) 
 dendrimer ( 397 nm , 3.1 ev ) , but the cmp and ecmp peak are also much 
 broader .
the dendrimer absorption spectrum reaches 25% of the maximum 
 intensity of the first peak at 477 nm ( 2.6 ev , i.e. 0.5 ev below the 
 peak maximum ) while the absorption spectrum of the cmp reaches the 
 same 25% already at 605 nm ( 2.0 ev , i.e. 1.0 ev below the peak maximum ) . 

part of this 0.5 ev of extra broadening of the cmp absorption 
 spectrum might be due to a difference in the inherent vibrational 
 broadening of the cmp and the dendrimer and/or a difference in the 
 scattering - related broadening highlighted above .
however , the extent 
 of the broadening also suggest , like for the fluorescence data , the 
 presence of chromophores in the insoluble pyrene cmp and ecmp that 
 absorb at wavelengths longer than the peak maximum ( i.e. , abs > 417 nm and > 430 nm respectively)that is , 
 a much 
 greater red shift than suggested by the peak maximum alone . again , 
 solution data appear to rule out an explanation for this red shift 
 based on an increased conjugation length . in summary ,
both solid - state 
 absorption and fluorescence spectra suggest that a significant component 
 of the insoluble pyrene cmp and ecmp are structurally different to 
 both the 1,3-linear polymer and the py(5)/py(17 ) dendrimer .
we therefore decided to use computational chemistry calculations 
 on cluster models of the polymer as a means to rationalize these qualitative 
 observations and to elucidate the type of chromophore that might explain 
 the large observed spectral red shift in the pyrene cmp and ecmp networks . 

the various cluster models that we investigated included fragments 
 of linear polymers , fragments of dendrimers , and closed pyrene rings 
 comprising between three and six pyrene units .
we predicted the absorption 
 and fluorescence spectra for the various cluster models and , where 
 relevant , considered a range of conformers . in keeping with kasha s 
 rule
, we assumed in the fluorescence 
 calculations that transitions from higher singlet excited states ( e.g. , 
 s2 ) to s1 are so fast that fluorescence occurs in appreciable yield 
 only from the latter s1 state .  
we first focused on the materials 
 for which we have good knowledge 
 of the molecular topology : that is , the 1,3-linear pyrene polymer 
 and the py(5 ) dendrimer . in previous work
, we demonstrated that td - cam - b3lyp gives a good match to experimental 
 absorption and fluorescence spectra of linear 1,3-oligomers and 1,3-linear 
 polymer in solution after applying a rigid downward shift to account 
 for the effect of environment ( i.e. , in this case , solvent ) and an 
 inherent bias of the cam - b3lyp density functional that results in 
 the general overestimation of excitation energies .
moreover , we showed 
 that the spectral features converge rapidly with oligomer length , 
 and that chains of six pyrene units ( in the case of absorption ) and 
 just two pyrene units ( in the case of fluorescence ) give peak maxima 
 predictions that are close to those of an infinite polymer .
also , 
 it was found that the tert - butyl groups could be 
 omitted without significantly changing the results .
since we are interested here in solid - state versions of 
 the various polymers , we repeated this earlier analysis for both the 
 1,3-linear pyrene polymer and the py(5 ) dendrimer ( scheme 1c and 1e , respectively ) , 
 and compared td - cam - b3lyp predictions with experimental data ( figures 
 s4 and s7 in section esi-3 of the supporting information ) for thin films .
we focus on films for these solution - processable 
 materials , rather than powders , because the thin film data have , as 
 discussed above , a much better resolution for the absorption spectra . 

we found that td - cam - b3lyp correctly predicts the relative ordering 
 of the first absorption peak maximum of the 1,3-linear polymer versus 
 the first absorption peak maximum and shoulder of the py(5 ) dendrimer , 
 and also the relative positions of the fluorescence peak maxima for 
 both materials
. for thin film data , application of a rigid downward 
 shift of 0.5 ev to the td - cam - b3lyp predictions resulted in a good 
 absolute match between the predicted and observed positions of spectral 
 features ( see figure 2 ) , and hence , all computational 
 spectroscopic results hereafter include the same absolute shift .
as 
 the absorption and fluorescence maxima of the powder samples are further 
 red - shifted by 0.1 ev relative to those of thin - film samples 
 ( see discussion above and section esi-3 of the supporting information ) , this may mean that our calculated 
 corrected spectra need to be further red - shifted by a similar amount 
 when comparing to powder spectra .
comparison of the experimental ( solid 
 lines ) and td - cam - b3lyp predicted 
 absorption ( dashed lines ) and fluorescence ( insert ) spectra of 1,3-linear 
 pyrene polymer ( purple ) and the py(5 ) pyrene dendrimer ( blue ) . for systems such as the 1,3-linear 
 pyrene polymer and the py(5 ) 
 dendrimer
there are only a limited number of possible conformers that 
 all lie relatively close in energy .
for example , 
 the lowest excitation energy of the py(5 ) dendrimer varies by less 
 than 0.05 ev between the five different conformers ( see also figure 
 s9 in section esi-4 of the supporting information for an example of the effect of oligomers in the case of 1,3-linear 
 pyrene oligomers ) .
next , we considered an alternative chromophore 
 structure that is 
 strongly related to the 1,3-linear pyrene polymer but which has not , 
 to our knowledge , been prepared experimentally : the 1,8-linear polymer 
 ( see scheme 1d ) . nonetheless , this substitution 
 pattern is ( theoretically ) possible , and would result from the  linear  
 yamamoto coupling of 1,3,6,8-tetrabromopyrene , followed by debromination 
 from the remaining two brominated positions per pyrene unit .
1,3,6,8-tetrabromopyrene 
 does not have a c4 axis , but only a c2 axis perpendicular to the plain 
 of the ring ; that is , it is 
hence , the 
 relative orientation of the pyrene units in the chain results in the 
 1,3- and 1,8-linear pyrene polymers being structurally different and 
 having different optical properties .
the 1,8-oligomers , and by extrapolation 
 the 1,8-linear polymer , is predicted to have spectral features that 
 are slightly red - shifted compared to the 1,3-linear polymer , but rather 
 similar to the dendrimer ; fluorescence at 500 nm ( 2.5 ev ) and two 
 strong absorption features at 400 nm ( 3.1 ev ) and 375 nm ( 3.3 ev ) , 
 respectively ( see also table 1 and figure s9 
 in section esi-4 , supporting information ) .
1,8-linear polymers and oligomers thus do not appear to make good 
 candidates for the red - shifted spectral features in the cmp and ecmp . having considered linear polymers and dendrimers , we next considered 
 possible cluster fragments of extended networks containing rings . 

, has been 
 invoked previously in a structural model for paf-1 based on isomorphous 
 replacement in an amorphous silica model . here
it is known from topological considerations that every 4-connected 
 network will have at least a certain fraction of rings in this size - range .
for the reasons outlined above , 
 the orientation of the pyrene units in the ring lead to structurally 
 different rings .
the long sides ( i.e. , 18 or 36 ) and 
 short sides ( i.e. , 13 or 68 ) of the pyrene units give 
 rise to l(ong ) and s(hort ) edges , respectively .
these l and s edges 
 can combine into rings made of either just one type of edge ( a ) or 
 a combination of both types of edges ( b ) .
henceforth , we label such 
 rings by the number of pyrene units in the ring , followed by the orientation 
 of the edges . for example , the two possible a - type rings formed from 
 four pyrene units are labeled 4llll and 4ssss , respectively , while 
 an example of a possible b - type ring is 4lsls ( see also figure 3 ) . because every pyrene unit has two l and two s 
 sides , a mixture of different ring configurations is always expected 
 to occur in the extended material .
dft - optimized lowest energy configurations 
 of 4llll , 4ssss and 
 4lsls rings ( atoms represented as blue spheres indicate where the 
 rings would connect to the rest of the amorphous pyrene network in 
 the pyrene cmp / ecmp ) .
we also considered a number of distinct conformers for the 
 4- and 
 5-rings ( typically 2 or 3 , distinguished from each other by a superscript 
 number at the end of the ring label : that is , 4llll for 
 the lowest energy , or first , conformer of the 4llll ring , 4llll for the second most stable , etc .
this was done to sample 
 the possible conformations rings could have when they form part of 
 an extended framework .
competing bonding requirements make it unlikely 
 that in an extended material all rings would be in their lowest energy 
 conformation .
figure 4 shows the 4ssss and 4ssss conformers ( 4ssss is shown 
 in figure 3 ) and illustrates that the difference 
 between different conformers lies in the relative orientation of the 
 different pyrene units ( e.g relative to the plane of the ring up  down  up  down 
 for 4ssss , flat - down - flat - up for 4ssss and 
 flat - up  down 
- optimized 
 lowest energy configurations of the 4ssss and 4ssss conformers ( 4ssss shown in figure 3 ) .  
all values 
 are in ev and , as 
 discussed in the text , rigidly shifted downwards by 0.5 ev .
for all 
 4- and 5-ring chromophores , the observed spread in excitation energies 
 for the different conformers is presented with the value for the lowest 
 energy conformer in bold . for the linear polymers and dendrimers , 
 the position of
table 1 summarizes our predictions for the 
 location of the fluorescence and the first absorption peaks for the 
 different rings ( see table s2 in section esi-5 ( supporting information ) for the unshifted energies ) . focusing 
 first on the lowest energy conformers of each ring type , we observe 
 that the lowest absorption and fluorescence peak of the 3sss and 3lll 
 rings lie at significantly lower energy ( and hence longer wavelength ) 
 than those predicted for the different linear polymer and dendrimer 
 cluster models . in the case of the other ring types ,
the position 
 of the lowest absorption and fluorescence peak is , however , generally 
 very similar to those predicted for the structurally related polymers 
 ( i.e. , the 1,3-linear polymer for the pure s rings and the 1,8-linear 
 polymer for the pure l rings ) . that said , the energy of the lowest 
 absorption peak is always slightly lower than that of the corresponding 
 linear oligomer of the same length ( i.e. , 4ssss vs the 1,3-coupled 
 tetramer , see figure s9 in section esi-4 , supporting 
 information ) .
the only exception is the lowest absorption peak 
 ( but not fluorescence peak ) of the 5sssss ring , which is red - shifted 
 by 0.4 ev compared with both the 1,3-coupled linear pentamer and the 
 1,3-linear polymer .
surprisingly , the lowest energy conformers of 
 most rings built from more than three pyrene units thus show generally 
 only minor evidence of ring - strain in their spectra .
the higher energy 
 conformers of the rings ( lying 2060 kj/(mol pyrene ) 
 higher in energy ) , however , show a different picture . for these conformers , 
 the position of the first absorption and fluorescence peaks
more 
 specifically , for each ring - size , we found that the more strained 
 the conformer  that is , the higher its energy with respect to 
 the lowest energy conformer  the lower in energy and higher 
 in wavelength the fluorescence peak and first absorption peak are 
 predicted to lie .
clearly , then , the presence of such strained rings 
 could give rise to a red shift in both the fluorescence and absorption 
 spectra of cmp and ecmp networks .
for the cmp network specifically , 
 the measured fluorescence spectrum 
 fits well with the fluorescence energies predicted for the more strained 
 4- and 5-rings , and suggests that such rings might be the chromophores 
 responsible for the large red shift in the cmp fluorescence .
regarding 
 the absorption spectrum , this again will be a convolution of the absorption 
 spectra of a range of different chromophores . in principle , the red 
 shift and broadening of the cmp absorption spectrum could be explained 
 by a combination of different ring - sizes , configurations , and conformations 
 as present in an amorphous network .
we propose that the bulk of the 
 rings would be moderately strained 4- , 5- , and 6-rings , and that these 
 chromphores are responsible for the shift of the first absorption 
 maximum to 3.0 ev ( 310 nm )
. a smaller fraction of more strained rings 
 could account for the chromophores responsible for broadening the 
 cmp absorption spectrum to even longer wavelengths , the most strained 
 of which would be the strained 4- and 5-ring chromophores responsible 
 for the fluorescence . in line with
this evidence of rings in cmps , 
 energetic estimates ( discussed in the supporting 
 information , section esi-6 ) suggest that the formation of rings , 
 even small or strained ones , is energetically feasible during the 
 kinetically driven cmp synthesis which involves irreversible bond - formation , 
 especially when one takes into account that some of the polycondensation 
 might take place after phase separation within the porous , precipitated 
 network phase .
overall , we thus believe that a structural model of 
 the cmp should contain the above - described rings ( and indeed our previous 
 preliminary model of the pyrene cmp contained rings ) .
the likely microscopic link between strain and 
 the spectroscopic 
 red shift are the pyrene 
smaller torsion 
 angles  that is , flatter structures  are expected to result 
 in increased overlap between the -systems on adjacent pyrene 
 units and hence enhanced conjugation , which would explain , for example , 
 the observed red shift in the absorption energies .
indeed we find 
 that the strained rings that display a large red shift have considerably 
 smaller average torsion angles than those of their chain counterparts ; 
 e.g. , 42 for 3lll and 46 for 4ssss compared with 70 for long 1,3- and 1.8-linear chains . 
 likewise , those rings that have a similar absorption on - set as the 
 linear - polymers also have average torsion angle comparable to that 
 of linear chains ; e.g. , 67 for 4ssss .
see supporting information , section esi-5 , for more 
 information about the torsion angle distributions of the rings .
small 
 torsion angles , in the absence of rings , could also result in a significant 
 red shift ( at least in the case of the absorption spectrum , see for 
 example data for linear chains in figure s10 in section esi-7 of the supporting information ) .
however , it is difficult 
 to envisage where the strain required for the formation of these small 
 torsion angles should originate from , if not from the presence of 
 rings .
a comparison of the data in table 1 and 
 the experimental fluorescence spectrum of the pyrene cmp in figure 1 suggests that fluorescence occurs from only the 
 most strained subset of chromophores and not from the less strained 
 chromophores ( though there might be a shoulder at 540 nm , 2.8 ev ) , 
 despite the fact the less strained chromophores do appear to contribute 
 to the absorption spectrum .
our calculations , discussed in more detail 
 in the supporting information ( section 
 esi-8 ) , suggest that this might be because an excited ring - based system 
 can lower its energy significantly by moving the excited state from 
 larger rings to smaller rings ( e.g. , 5lllll * + 4llll  5lllll 
 + 4llll * , where the asterisk denotes the ring upon which the excited 
 state is localized ) and , for a given ring - size , from a less strained 
 conformer to a more strained conformer ( e.g. , 4llll * + 
 4llll  4llll + 4llll * ) . 
 assuming that excited state transport is generally fast relative to 
 excited state relaxation , then excited state relaxation and fluorescence 
 would indeed only occur at the most strained chromophores in the material , 
 even though all chromophores to some extent contribute to absorption . 
 the possible shoulder at 540 nm ( in the range where fluorescence is 
 predicted to occur from unstrained 4ssss- and 6ssssss / llllll - rings ) 
 might then be indicative of those cases where excited state transport 
 is too slow and/or strained rings too far away from the absorption 
 chromophore for the excited state to reach the most strained ring 
 before fluorescence . in this scenario ,
the appearance of multiple 
 fluorescence peaks or shoulders could potentially be a marker for 
 material heterogeneity ( e.g. , the spatial distribution of small rings 
 or linear fragments , see below , over the bulk of the material ) .
for the ecmp , the measured fluorescence spectrum matches the fluorescence 
 energies predicted for moderately strained 4ssss and 4lsls rings ( configurations 
 by definition including short edges , see figure 3 , in - line with the fact that the bifunctional 1,3-pyrene 
 units , by definition , will be incorporated  short  in any ring that is formed .
thus , perhaps counterintuitively , 
 expanding the cmp by inserting bifunctional units between the tetrafunctional 
 units does not seems to lead to an apparent change in the size of 
 the smallest ring in the network , but only the degree of strain that 
 these small rings are under
. the lower strain could be either ( i ) 
 the direct result of having bifunctional units in the ring that are 
 relatively free to move , since they do not form part of any other 
 rings , or ( ii ) an environmental effect due to other rings surrounding 
 the small ring being larger than in the related cmp .
the fact that 
 no shift in the size of the smallest ring is observed fits with the 
 idea that the pyrene ecmp and cmp are kinetic and not thermodynamic 
 products . a similar shift in the fluorescence spectrum , as for 
 the ecmp , 

interpretation of spectra for scmp is considerably more difficult 
 because scmp is a statistical copolymer of tetrafunctional and bifunctional 
 units , and hence the material can contain fragments of tetrafunctional 
 units linked to tetrafunctional units as well as bifunctional units 
 linked to bifunctional units . the presence of the latter
 chains 
 could be an explantion to why the scmp , in contrast , to ecmp is soluble 
 in common organic solvents .
the spectral similarities between both 
 materials suggest that moderately strained 4- and 5-rings also exist 
 in scmp , and that they are the chromophore responsible for the scmp 
 fluorescence .
the excess in fluorescence of scmp relative to ecmp 
 in the 450480 nm range , coincident with the measured 
 and predicted fluorescence maxima of the 1,3-linear pyrene polymer , 
 might be the fingerprint of linear - chain rich , unbranched domains , 
 that are absent , as discussed above , in the strictly alternating ecmp . 

the occurrence of this fluorescence excess also suggests that the 
 chain - rich domains are large enough that at least some of the excitations 
 originally generated there do not diffuse to more strained chromophores 
 before radiative de - excitation to the ground state . finally , a slight 
 fluorescence excess at long wavelengths , at least for the precipitated 
 scmp sample ( flu > 550 nm ) might also suggests 
 the 
 possible presence of scmp-1 domains rich in tetrafunctional units 
 ( i.e. , cmp - like domains ) . on the basis of the above analysis
, 
 the absorption and fluorescence 
 spectra of cmps and related polymers should in principle be able to 
 be engineered by the incorporation of different sizes of rings or 
 rings with different degrees of strain .
the ecmp and scmp are examples 
 of how expanding rings / reducing strain by incorporating bifunctional 
 monomers allows one to blue shift the absorption and fluorescence 
 spectra .
this analysis is also in - line with the blue - shift observed 
 between the pyrene cmp network and the ordered alternating 1:2 copolymer 
 of tetrafunctional 1,3,6,8-tetrabromopyrene and bifunctional 1,4-benzene 
 diboronic acid .
copolymers with other 
 ratios than 1:2 might yield intermediate blue - shifted spectra as long 
 as the bifunctional monomer is homogenously incorporated in the cmp . 
 we have presented examples here for pyrene - based materials , but we 
 believe that these general principles should hold for a much broader 
 class of polymer network materials .
finally , a combination of the 
 approach developed here with the polymatic tool for building structural 
 models of ( amorphous ) polymers of colina 
 and co - workers could in the future lead to improved structural models 
 of cmps and related materials . here the optical spectra of the material 
 would be another constraint for the structural model to fulfill , just 
 as the material s surface area , pore - volume etc .
a combination of theoretical and experimental spectroscopy provides 
 an unrivalled insight into the atomic structure of porous conjugated 
 polymers , where hitherto structures have been largely a matter of 
 speculation and hypothesis .
specifically , we show that , in the case 
 of an insoluble pyrene cmp network , the significant red shift in its 
 absorption and fluorescence spectra relative to related pyrene - based 
 materials can be rationalized by the presence of strained rings in 
 a network structure .
the incorporation of strained rings can , in principle , 
 be exploited to tune the absorption and fluorescence spectra of a 
 polymer , and hence optimize this for potential applications such as 
 photocatalysis and photovoltaics .
certainly , these structural effects 
 will be central in strategies which seek to control photophysical 
 properties in porous organic polymers by  band gap engineering  .
 the photophysical properties of insoluble 
 porous pyrene networks , 
 which are central to their function , differ strongly from those of 
 analogous soluble linear and branched polymers and dendrimers . 
 this 
 can be rationalized by the presence of strained closed rings in the 
 networks . 
 a combined experimental and computational approach was used 
 to obtain atomic scale insight into the structure of amorphous conjugated 
 microporous polymers . 
 the optical absorption and fluorescence spectra 
 of a series of pyrene - based materials were compared with theoretical 
 time - dependent density functional theory predictions for model clusters . 
 
 comparison of computation and experiment sheds light on the probable 
 structural chromophores in the various materials . 
Introduction
Experimental Section
Results and Discussion
Conclusions