59th Annual Report on Research 2014

Figure 1.  (top) TPM-based HEX and HBC-POPs synthesized by our lab.  (A) N₂ isotherms collected at 77K for each POP and (B) the BET surface areas obtained from the isotherms.  (C) Pore size distributions of each POP.

Results from ACS-PRF Supported Research – To date, our laboratory has synthesized and characterized a series of porous organic polymers (POPs) based on HBC, HEX and tetraphenylmethane (TPM) monomers. One of the main goals of this work was to expand on our previous report on HBC-based POPs by studying the effects that PAH, and rim functional groups exert on the properties of the POPs.  These synthetic modifications have the effects of improving the solubility of the monomers, as well as modulating their ability to pi-stack. Figure 1 describes the synthetic procedure used to obtain these copolymers designated HEX- and HBC-POPs from the corresponding aryl halide monomer and TPM. To determine the accessible surface areas and pore size distributions, these POPs were evaluated through nitrogen adsorption experiments at 77K.  The resulting isotherms and pore size distributions are shown in Figure 1a-c.  The BET surface areas were calculated from these isotherms, ranging from 315 m²/g for HBC-POP-4 to 1141 m²/g for HEX-POP-3.  Carbon dioxide isotherms (Figure 2a) were collected at 273K and 298K, with HEX-POP-3 showing the highest storage capacity at 18 wt% - relatively high for POPs without polar functional groups.  The storage capacities demonstrated here are comparable with other hydrocarbon based porous polymers and much better than some others, despite comparatively low surface areas. Heats of adsorption (HOA) for CO₂ were calculated from these data for all polymers (Figure 2b).

Figure 2.  (A) CO₂ isotherms (273K) and (B) isosteric enthalpies of adsorption for HBC and HEX-POPs. 

Figure 3.  The diffraction patterns of HBC-POP-1 and HBC-POP-2. 

Based on the BET surface areas alone, the HEX-based POPs generally have a higher surface area than the HBC-POPs.  This can likely be attributed to several factors: 1) overall better solubility of the HEX monomers compared with the HBC monomers, resulting in a higher degree of polymerization and 2) poorer self-association between the bulkier, less pi-conjugated HEX monomers.  However, if we consider the HEX and HBC-POPs separately, the trends with regard to peripheral functional group size are opposite.  For example, increasing the size of the functional groups on the edges of the HBC monomers actually serves to decrease the overall surface area and sorption properties of the polymers.  We hypothesize that this is a result of the fact that the HBC-POPs tend to form small micropores (<8A) that become blocked or occupied when methyl or t-butyl groups are added.  HEX-POPs, on the other hand, display the opposite trend – larger, more solubilizing functional groups improve the overall porosity.  HEX-POPs contains a larger fraction of micropores with sizes of 9-15A, indicating that the inclusion of methyl or t-butyl groups somehow favors the formation of larger pores.  Since HEX monomers lack the strong self-association properties of HBCs, the steric effects of the peripheral functional groups may have a larger effect on the microscopic structure of the HEX-POPs whereas the nature of the microscopic structure of HBC-POPs is primarily driven by HBC monomer self-assembly.  To explore this possibility HBC-POPs 1 and 2 were investigated using pXRD methods.  Interestingly, we found that the polymers have little long-range order, however they do display one strong reflection, representing a distance (~3.5A) that corresponds to face-to-face pi-stacking interactions (Figure 3).  It is surprising to see these types of interactions, as they are rarely observed in other POPs despite the near ubiquitous use of aromatic rings in their construction.  This indicates to us that the larger pi-surfaces may be capable of maintaining their interactions at higher temperatures, even in solvents like toluene.  A similar peak was not observed in any of the HEX-POPs, further supporting our hypothesis of functional group influence on micropore size.

Figure 4.  (A) Synthesis of corannulene-based POPs, (B) N₂ isotherm at 77K and (C) CO₂ isotherm at 273K.

One of the specific goals of this proposal was to investigate the effects of curved aromatic monomers on the assembly of porous materials.  To begin to understand the effects of these types of PAH compounds we have also investigated POPs based on the curved PAH corannulene.  The synthesis of these POPs is shown in Figure 4.  Interestingly, the POPs had a very low BET surface area, but were capable of adsorbing a significant amount of CO₂ (273K).  We attribute this effect to the formation of very small pores that are more easily accessed by the smaller CO₂ molecule.  The small pores may be formed in this case as a result of the low symmetry of the tetrabromocorannulene monomer in comparison with the more symmetrical HEX and HBC monomers.  We intend to more fully investigate this phenomenon by synthesizing a pentasubstituted derivative of corannulene that has five-fold symmetry rather than the mirror plane found in the tetrabromo variant.