Hexaphenylbenzene-based polymers of intrinsic microporosity

Article abstract of DOI:10.1039/c1cc11717c

Microporous polymers derived from the 1,2- and 1,4-regioisomers of di(3′,4′-dihydroxyphenyl)tetraphenylbenzene have very different properties with the former being composed predominantly of cyclic oligomers whereas the latter is of high molar mass suitable for the formation of robust solvent-cast films of high gas permeability.

Full text of DOI:10.1039/c1cc11717c

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COMMUNICATION
Hexaphenylbenzene-based polymers of intrinsic microporosityw
Rhys Short,a Mariolino Carta,a C. Grazia Bezzu,a Detlev Fritsch,b Benson M. Kariukia and
Neil B. McKeown*a
Received 25th March 2011, Accepted 20th April 2011
DOI: 10.1039/c1cc11717c
Microporous polymers derived from the 1,2- and 1,4-regioisomers
of di(30,40-dihydroxyphenyl)tetraphenylbenzene have very different
properties with the former being composed predominantly of
cyclic oligomers whereas the latter is of high molar mass suitable
for the formation of robust solvent-cast films of high gas
permeability.
and highly reactive 2,3,5,6-tetrafluoroterephthalonitrile is
particularly convenient. Hence, to prepare a HPB-PIM, one
of the three possible regioisomers of di-(30,40-dihydroxyphenyl)-
tetraphenylbenzene, which arise from the ortho- (1,2-), meta-
(1,3-) or para- (1,4-) substitution of the catechol units on the
central benzene ring, is required. This communication reports
on the very different properties of the HPB-PIMs derived from
1,2- and 1,4-di(30,40-dihydroxyphenyl)tetraphenylbenzene, 1
and 2, respectively.
Hexaphenylbenzene (HPB) is an interesting structural unit for
the construction of organic materials due to its rigidity and
propeller-like shape, which arise from the mutual steric inter-
actions of the phenyl groups. Derivatives of HPB have been of
considerable interest for a wide variety of applications in
materials chemistry including their use as organic electronic
materials,1,2 as precursors to synthetic graphene fragments,3 as
rigid structural building units for crystal engineering4–9 and
liquid crystals,10 as components for photo-induced electron
transfer systems,11–13 for the site-isolation of reactive species,14
and as proton-transport materials.15,16 They have also been
used extensively for making polymers with high glass-transition
temperatures.17
The syntheses of monomers 1 and 2 and of the polymers
HPB-PIM-1 and HPB-PIM-2 derived from them is outlined
in Scheme 1. The key step involves the well-established
Diels–Alder reaction between appropriately substituted
diphenylacetylenes and tetraphenylcyclopentadienones.17
Attempts to prepare monomer 1 via the reaction between
di(3,4-dimethoxyphenyl)acetylene and tetraphenylcyclopenta-
dienone failed. Monomer 2 was prepared regioselectively with
no apparent contamination of the 1,3-isomer.w
Both HPB-PIM-1 and HPB-PIM-2 proved readily soluble
in a range of organic solvents including THF and CHCl3 but
only the latter could be purified by the removal of oligomeric
material by reprecipitation from the slow addition of the
polymer solution into a non-solvent such as methanol or
acetone. Analysis of crude HPB-PIM-1 by Gel Permeation
Chromatography (GPC) indicates a distinctly bimodal mass
distribution with a minor fraction corresponding to a mass
range of 20 000–40 000 mol gꢀ1 and the major fraction to a
range of only 1000–5000 mol gꢀ1 relative to polystyrene
standards. Matrix Assisted Laser Desorption Ionisation Mass
Spectrometry (MALDI-MS) shows the presence of cyclic
oligomers (n = 2–8; i.e. [2+2], [3+3],. . .[8+8]). On monitoring
the progress of the polymerization by thin layer chromato-
graphy, a distinct yellow spot at relatively low polarity
(Rf 0.8; ethyl acetate/hexane 8 : 2) was observed. Isolation of
this material in up to 15% overall yield was achieved by
column chromatography and MALDI-MS confirmed it to be
the [2+2] cyclic oligomer 3 (Fig. 1). Single crystal X-ray
diffraction (XRD) revealed that the dibenzodioxane rings
within this cyclic oligomer deviate from planarity.26 All of
the crystal structures held within the Cambridge Structural
Database (CSD) of compounds that contain dibenzodioxane
demonstrate the strong tendency of this structural unit to be
planar. For cyclic oligomer 3 this distortion from planarity
must be attributed to significant ring strain originating from
the structure and rigidity of the TPB units. Investigation of the
The shape of HPB with its many shallow concavities leads to
imperfect packing in the solid state as illustrated by the numerous
examples of inclusion crystal structures of its derivatives.4–9 This
feature, often termed ‘internal molecular free volume’,18 is a
prerequisite for the formation of macromolecular structures that
pack inefficiently in the solid state.19 Therefore, it was anticipated
that HPB would be a successful component for making novel
Polymers of Intrinsic Microporosity (HPB-PIMs) with different
properties to those prepared previously from spirobisindane20–23
or triptycene-based components.24,25
For PIMs, the microporosity arises from their fused-ring
structure, which prevents conformational rearrangement of
the contorted shape of the macromolecule. An efficient poly-
merisation reaction that provides a fused-ring linking group is
the double aromatic nucleophilic substitution reaction between
monomers that contain catechol (i.e. 1,2-dihydroxyphenyl) and
1,2-difluoroaryl units. For the latter, the commercially available
a School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK.
E-mail: mckeownnb@cardiff.ac.uk; Tel: 44 2920 875850
b Institute of Materials Research, Helmholtz-Zentrum Geesthacht,
Max-Planck-Str. 1, 21502 Geesthacht, Germany
w Electronic supplementary information (ESI) available: Synthetic
procedures and physical methodology. CCDC 819213. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc11717c
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Scheme 1 Reagents and conditions: (i) diphenyl ether, 250 1C; (ii) BBr3, DCM, 20 1C, 1 h; (iii) 2,3,5,6-tetrafluoroterephthalonitrile, DMF, K2CO3,
65 1C, 96 h.
packing within the crystal structure reveals wide, solvent-filled
channels which run along the crystal z-axis. Desolvation of
these crystals occurs very rapidly to give an amorphous
powder, which demonstrates significant nitrogen (N2)
adsorption at 77 K. An apparent BET surface area of
162 m2 gꢀ1 is calculated from the N2 adsorption isotherm
(Fig. 2). Although the observed microporosity for the
amorphous powder of 3 cannot be directly linked to the crystal
structure, the large quantity of included solvent within these
channels demonstrates the inefficient molecular packing that
leads to intrinsic microporosity. Crude HPB-PIM-1 demon-
strates a larger apparent BET surface area of 425 m2 gꢀ1
(Fig. 2).
Table 1 Permeability data for HPB-PIM-2. The corresponding data
for PIM-1 is also given for N2 and CO2
28
Solubility
(cm3(STP)/
Perm.-
Permeability Selectivity
Diffusivity
(cm2 sꢀ1 ꢁ 108) cm3 cmHg ꢁ 103) (Barrer)
PX/PN2
O2
N2
93.5
34.8 (160)
23
19 (37)
217
66.5 (610)
3.3
1 (1)
(PIM-1)
He
H2
CO2
(PIM-1)
CH4
3100
1700
46.2 (160)
1.1
4.3
375 (700)
340
723
5.1
10.9
1730 (11300) 26.1 (18.5)
14.0
87
122 1.8
In contrast, GPC analysis of HPB-PIM-2, purified by
reprecipitation, indicates a relatively high molecular mass
(Mn = 40 000 and Mw = 106 000 g molꢀ1). In addition, no
cyclic oligomers were apparent from MALDI-MS analysis as
might expected from the 1,4-arrangement of the catechol
moieties on the central benzene ring of the TPB unit of
monomer 2, which would inhibit macrocycle formation. The
microporosity of HPB-PIM-2 also proved significantly greater
than that of HPB-PIM-1 with an apparent BET surface area
of 527 m2 gꢀ1 calculated from its N2 adsorption isotherm
(Fig. 2). A particularly attractive property of HPB-PIM-2
arising from its high average molecular mass is its ability to
form robust, optically clear self-standing films by simple
casting from a dilute solution of the polymer in CHCl3. The
availability of robust, defect-free films facilitates the measure-
ment of gas permeability using time-lag methodology. The
permeability, diffusivity and solubility coefficients for O2, N2,
He, H2, CO2 and CH4 are presented in Table 1. In common
with other highly permeable glassy polymers the order of
permeability is CO2 4 H2 4 He 4 O2 4 CH4 4 N2 which
reflects the contribution of both solubility and diffusivity to
the gas permeability.27
PIM-1, derived from the reaction between 5,50,6,60-tetra-
hydroxy-3,3,30,30-tetramethyl-1,10-spirobisindane and 2,3,5,6-
tetrafluoroterephthalonitrile, provides permeability data that
represent an excellent compromise in the trade-off between the
desirable properties of high permeability and good selectivity
required for efficient gas separation membranes.27,28 The data
for HPB-PIM-2 show significantly lower permeabilities
relative to PIM-1 but enhanced selectivities (e.g. selectivity
for CO2 vs. N2 = 26). The reduced permeability relative to
PIM-1 can be attributed both to lower diffusivity due to lower
intrinsic microporosity and lower solubility due to a smaller
concentration of polar groups (i.e. ether and nitrile). Despite its
relatively modest performance relative to PIM-1, HPB-PIM-2
still demonstrates permeabilities that are attractive for gas
separation membranes and may possess other properties that
provide advantages for this application (e.g. reduced physical
ageing effects due to greater chain rigidity). Simple chemical
modifications to introduce greater polarity into the polymer
may also provide enhancement of gas separation properties and
such studies are planned. The highly fluorescent nature of these
polymers also suggest utility as the active material in optical
sensors29 and work related to this is on-going.
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Fig. 2 Nitrogen adsorption isotherms for 3 (’); HPB-PIM-1 (K)
and HPB-PIM-2 (m).
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6824 Chem. Commun., 2011, 47, 6822–6824
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