Polybenzobisimidazole-derived two-dimensional supramolecular polymer

Article abstract of DOI:10.1002/pola.28470

We report a novel crystalline supramolecular polybenzobisimidazole (SP-PBBI) capable of providing a two-dimensional polymer (2DSP-PBBI) by liquid-phase exfoliation. A regular arrangement of rigid rod-like polybenzobisimidazole (PBBI) chains is achieved by interchain hydrogen bonding. Titration of 2DSP-PBBI with cobalt chloride (CoCl₂) using UV-Vis spectroscopy demonstrates the presence of bidentate NO ligands on the PBBI backbone and NO–Co(II) complexation. Imaging analysis using atomic force microscopy (AFM) reveals the planar surface morphology of exfoliated 2DSP-PBBI sheets with lateral dimensions of <1 μm and thickness of <30 nm. The size of the polymer crystal growth is tuned by employing condensation/precipitation polymerization under nonisothermal conditions.

Full text of DOI:10.1002/pola.28470

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Polybenzobisimidazole-Derived Two-Dimensional
Supramolecular Polymer
Wanji Seo, Keith L. Carpenter, James A. Gaugler, Wenting Shao, Keith A. Werling,
Philip M. Fournier, Daniel S. Lambrecht, Alexander Star
Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260
Correspondence to: A. Star (E-mail: astar@pitt.edu)
Received 19 October 2016; accepted 24 November 2016; published online in Wiley Online Library
DOI: 10.1002/pola.28470
ABSTRACT: We report a novel crystalline supramolecular poly-
benzobisimidazole (SP-PBBI) capable of providing two-
dimensions of <1 lm and thickness of <30 nm. The size of the
polymer crystal growth is tuned by employing condensation/
a
precipitation polymerization under nonisothermal conditions.
dimensional polymer (2DSP-PBBI) by liquid-phase exfoliation. A
regular arrangement of rigid rod-like polybenzobisimidazole
(PBBI) chains is achieved by interchain hydrogen bonding. Titra-
tion of 2DSP-PBBI with cobalt chloride (CoCl2) using UV-Vis spec-
troscopy demonstrates the presence of bidentate NO ligands on
the PBBI backbone and NO–Co(II) complexation. Imaging analy-
sis using atomic force microscopy (AFM) reveals the planar sur-
face morphology of exfoliated 2DSP-PBBI sheets with lateral
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2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
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KEYWORDS: conjugated polymer; polybenzimidazole; polycon-
densation; precipitation polymerization; supramolecular polymeri-
zation; supramolecular structure; two-dimensional polymer
failed to provide COF-Salophen. We also attempted to neutralize
1,2,4,5-benzenetetramine tetrahydrochloride before polymeriza-
tion, but found that spontaneous air-oxidation of an unstable 2
provided 3 (3,6-diimino-1,4-cyclohexadiene-1,4-diamine) under
atmospheric conditions. Despite the conversion into 3, we have
utilized it as a monomer for polycondensation with 1 to investi-
gate whether the product forms a conjugated linear polymer that
may possess distinct electrical properties. Interestingly, we have
found that a novel supramolecular polymer (SP-PBBI) is formed
by self-assembly of linear polybenzobisimidazole (PBBI) chains
in a one-step reaction. SP-PBBI features a regular arrangement of
rigid linear PBBI chains stabilized by intramolecular hydrogen
bonding within a planar sheet. The size of SP-PBBI crystal
growth is controlled by gradually raising the temperature during
the precipitation polymerization process.
INTRODUCTION An increasing number of two-dimensional
polymers (2DPs) have been developed and found applica-
tions in catalysts,1 photovoltaic cells,2 and colorimetric sens-
ing.3 The importance of 2DPs as substrates for hierarchically
ordered heterostructures with novel properties is rapidly
being recognized.4 2DPs are constructed by connecting build-
ing units covalently (e.g., covalent organic frameworks) or
noncovalently (e.g., supramolecular polymers).5 Envisioning
realization of a 2D semiconducting and chemical sensing
material for field-effect transistor (FET) devices, we designed
a novel covalent organic framework COF-Salophen consisting
of multiple salophen macrocycles (Scheme 1). This covalent
organic framework (COF) can be synthesized by condensa-
tion of aromatic monomers bearing aldehyde and amine
functional groups,6 which provides an imine linker connect-
ing salophen units and pores with two different sizes. The
salophen ligands can serve as a sensing moiety for the detec-
tion of Co(II) ions,7 and the extended p-conjugated system
imparts planarity and rigidity, thereby providing semicon-
ducting properties to the polymer.8 The high surface-to-
volume ratio of 2D COFs allows for facile electron transfer
due to the excellent surface contact within FET devices.5(a)
However, an initial effort for direct condensation of 1 and com-
mercially available 1,2,4,5-benzenetetramine tetrahydrochloride,
the precursor of 2, in the presence of N,N-diisopropylethylamine
The rigid rod structure of crystalline polybenzimidazoles (PBI)
and their derivatives were previously reported,9 some of which
possess interchain hydrogen bonds and are able to form spun
fibers.10 PBI can be synthesized with condensation of an amine
and a carbonyl group (e.g., carboxylic acid,11 ester,12 or
aldehyde13) at high temperatures of 100 to 350 8C. However,
neither two-dimensional morphology nor supramolecular
polymerization of PBI has been investigated.14 Herein, we
report the one-step synthesis of supramolecular polymer
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Attempted synthesis of COF-Salophen yields a supramolecular polybenzobisimidazole (SP-PBBI) consisting of multiple
polycrystalline domains, and exfoliation of bulk SP-PBBI affords a two-dimensional supramolecular polymer (2DSP-PBBI). [Color
figure can be viewed at wileyonlinelibrary.com]
polybenzobisimidazole (SP-PBBI) by nonisothermally con-
trolled condensation/precipitation polymerization. The sur-
face morphology of 2DSP-PBBI isolated by liquid-exfoliation of
as-synthesized, bulk SP-PBBI is analyzed with atomic force
microscopy (AFM) and transmission electron microscopy
(TEM). The bidentate NO ligands of PBBI chains are further
investigated by UV-Vis spectroscopic titration with Co(II).
(3.3 mg, 0.02 mmol) in N,N-dimethylformamide (1 mL) was pre-
pared in a 25 mL round bottom flask. The solution of the mono-
mers was degassed by two freeze-pump-thaw cycles and was
sealed with a septum and PTFE tape. After sonication for 5 min,
the reaction mixture was kept at 95 8C (for 2 days) in an oil bath
without stirring and then at 100 8C for 1 day. After gradually
raising the temperature to 145 8C, the reaction was left undis-
turbed for 3 days. After total 6 days, the reaction flask was
cooled to room temperature for 2 h, and the crude polymer
was washed with dichloromethane (4 3 15 mL) and methanol
(4 3 15 mL) over a Millipore filter and a 200 nm fluoropore
filter membrane. After drying in vacuo for 48 h over 100 8C,
2DSP-PBBI was obtained as a dark brown solid (89%). 13C
CP MAS solid-state NMR (15 kHz) d ppm, 198.6, 150.0, 147.0,
138.2, 129.9, 121.3, 119.0, 116.5, 99.0. IR (KBr, ATR) 3327,
EXPERIMENTAL
General
All chemicals were obtained from Sigma Aldrich and used
without further purification. All reaction solvents were anhy-
drous reagent graded. Detailed synthetic procedures, materi-
als characterization, and instrumentation details can be
found in the Supporting Information.
1641, 1580, 1557, 1483, 1440, 1394, 1302, 1152, 839 cm21
.
Synthesis of SP-PBBI
Preparation of 2DSP-PBBI
Monomers 1 and 3 were synthesized as described in the Sup-
porting Information. A solution of 1 (6.0 mg, 0.04 mmol) and 3
Exfoliation was performed in a 100 mL round bottom flask
containing dry 2DSP-PBBI (2.0 mg) and anhydrous DMF
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(12 mL) with gentle stirring for >24 h at 60 8C. Superna-
tants contained relatively well-suspended polymer sheets
and exhibited varying degrees of orange hues, depending on
the exfoliation time.
macromolecular structure.15 Condensation of 1 and 3 allowed
formation of two imine bonds and subsequent cyclization to
a benzobisimidazole moiety. To confirm the formation of
benzobisimidazole from 3, a model compound 4 was pre-
pared with salicylaldehyde and 3 under the same nonisother-
mal conditions [Supporting Information, 1H and 13C NMR
spectra in Fig. S2(c–e)]. The reaction afforded a brown powder
of as-synthesized 2DSP-PBBI in 89% yield, exhibiting insolubili-
ty in water and most common organic solvents at room tem-
perature. The sufficiently long reaction time (total 6 days) was
crucial to achieving high polymer conversions, whereas reac-
tions performed for less than 3 days formed the precipitated
product in much lower conversions.
Characterization of SP-PBBI and 2DSP-PBBI
Fourier transform infrared spectroscopy (FTIR) was per-
formed using an IR-Prestige spectrophotometer (Shimadzu
Scientific) outfitted with an EasiDiff accessory (Pike Technol-
ogies). Solid samples were ground with KBr to prepare a
homogenous mixture. 13C cross-polarization magic-angle
spinning (CP MAS) spectroscopy was performed on a Bruker
Avance spectrometer (500 MHz). Powder X-ray diffraction
(PXRD) was recorded on a Bruker X8 Prospector Ultra
equipped with a Bruker Smart Apex CCD diffractometer and
a copper micro-focus X-ray source employing Cu Ka radiation
at 40 kV, 40 mA. A ground sample was loaded in a capillary
tube (D: 1 mm) for analysis. The size and the morphology of
materials were analyzed with transmission electron micro-
scope (FEI-Morgani, 80 keV). TEM samples were prepared
by dropcasting 3.5 mL of a sample onto a lacey carbon films/
400 mesh copper grid and dried under ambient conditions
over 24 h. An Asylum MFP-3D atomic force microscope
(AFM) was utilized with high resolution probes (Hi’Res-C14/
Cr-Au) purchased from MikroMasch. AFM samples were pre-
pared by deposition of 10 mL of an exfoliated solution onto
freshly cleaved mica. The sample was spin-coated and then
allowed to dry under ambient conditions over 24 h. UV-Vis-
NIR spectra were acquired using a Lambda 900 spectropho-
tometer (PerkinElmer). Scanning Electron Microscopy (SEM)
was performed with FEI XL-30 (20 keV).
The FTIR spectrum [Fig. 1(a)] confirms the formation of an
imine (C@N) stretch at a very intense peak of 1641 cm21 in
the polymer sample while peaks characteristic to the mono-
mers such as the aldehyde C–H stretch (2866 and
2769 cm21) of 1 and primary amine N–H stretch (3444 and
3417 cm21) of 3 are clearly absent [Supporting Information
Fig. S1(b,c)]. The relatively high frequency of an imine
stretch can be observed due to the highly rigid conformation
arising from the cyclic imine of benzobisimidazole and a
strong intramolecular hydrogen bond occurring between the
hydroxyl hydrogen and the imino nitrogen (OHÁÁÁN@C)16
although a different hydrogen bonding motif (HOÁÁÁH–N) was
proposed by X-ray crystal structure analysis of PIPD.14(c),17
The distinct O–H stretch at 3321 cm21 can be used as a
marker for predicting the crystallinity of the polymer. Also,
the shift in O–H stretch by about 40 cm21 from the mono-
mer 1 and a higher intensity of the peak suggest that a
strong intramolecular hydrogen bond is present.18 When a
less crystalline polymer sample was obtained, only a broad
O–H stretch was observed in the region between 3100 and
3400 cm21 [Supporting Information Fig. S1(d)]. The struc-
ture of the polymer was further confirmed by 13C CP MAS
solid-state nuclear magnetic resonance (NMR) spectrum [Fig.
1(b)]. The characteristic phenolic (C1) and cyclic imino (C2)
carbons resonate at d 5 150.0 and 147.0 ppm, respectively,
and the resonance at d 5 129.2 ppm was attributed to C3
connecting the benzobisimidazole moiety.19 In the case of a
less crystalline sample, a weak resonance of downfield car-
bon was present at d 5 175.6 ppm, most likely a carbonyl
carbon (C@O) resulting from the keto-enamine tautomeriza-
tion [Supporting Information Fig. S2(a)].
Co(II) Titration of 2DSP-PBBI
A 5.3 mM CoCl2 solution was prepared in DMF, and 5 mL
was delivered to an exfoliated polymer solution (600 mL) for
each titration at ambient conditions. The polymer solution
contained about 32 lg of 2DSP-PBBI. The sample for weight
measurement was prepared by depositing an exfoliated
polymer solution on a glass slide and was dried on a hot
plate for 24 h. The optical change of 2DSP-PBBI solution was
measured by UV-Vis absorption spectroscopy immediately
after each addition of CoCl2.
RESULTS AND DISCUSSION
Synthesis of SP-PBBI
In an attempt to perform imine condensation under neutral
conditions, 1,2,4,5-benzenetetramine tetrahydrochloride was
treated with cesium carbonate (Cs2CO3) in air prior to poly-
merization. Upon exposure to air, 2 was rapidly oxidized to
3, and polycondensation of the isolated 3,6-diimino-1,4-
cyclohexadiene-1,4-diamine 3 and 2,3-dihydroxybenzene-1,4-
dicarbaldehyde 1 in N,N-dimethylformamide (DMF) pro-
ceeded at 95 to 145 8C. An aprotic polar solvent DMF was
employed to completely solubilize both compounds 1 and 3,
thereby initiating precipitate polymerization in a homoge-
nous solution. DMF may also have facilitated p–p stacking
due to the enhanced solvophobic effect and the self-assembled
To support the evidence of a periodic structure with well-
defined crystalline parameters, powder X-ray diffraction
(PXRD) was implemented. As shown in the experimental
PXRD spectrum [Fig. 1(c)], d spacing values corresponding
to the well-defined peaks were found (16.67, 8.43, 6.33,
5.86, and 3.31 Å), based on which a two-dimensional unit
cell model (a and b axes) was proposed [Fig. 1(d)]. To con-
struct a periodic alignment of PBBI, we referred to the struc-
tural analyses of poly(pyridobisimidazole) that is very
similar to PBBI except for the position of one hydroxyl group
and the pyridine moiety.14(c),17,20 Based on the information
of bond lengths and unit cell (either monoclinic or triclinic)
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of poly(pyridobisimidazole),14(c) we indexed a 5 16.67 Å and
b 5 6.63 Å for SP-PBBI parameters [Fig. 1(d)]. These lattice
parameters are very different from those of the COF-
Salophen estimated at the B3LYP/6-31g(d) level [Fig. 1(e)].
We reason that the interlayer spacing (c axis) is 3.31 Å,
which is within van der Waals contact distances arising from
the p–p stacking of aromatic rings and comparable to previ-
ously synthesized arene-based polymers.21 Due to limited
structural information gleaned from the powder diffraction
data, the analysis of angles between unit cell parameters
was not proposed.14(c) Thermogravimetric analysis (TGA)
showed a weight loss of 10% around 330 8C and a
slow decomposition of 20% up to approximately 400 8C
(Supporting Information Fig. S4).
Morphology of 2DSP-PBBI
The morphology and the size of 2DSP-PBBI were investigat-
ed using TEM and noncontact-mode AFM shown in Figure
2(a–d). Both bulk SP-PBBI and exfoliated 2DSP-PBBI samples
exhibit a planar morphology consisting of largely stacked
platelets [Fig. 2(a)]. We confirmed the supramolecular poly-
merization process with 2DSP-PBBI samples that were soni-
cated or manually ground from 5 to 30 min. It appears that
large planar sheets were disintegrated into small rods under
the mechanical forces, probably due to weakened intermolec-
ular bonds [Supporting Information Fig. S5(a)]. The planar
configuration and rigidity arise from the individual PBBI
chain capable of arranging in a quasi-aromatic six-membered
chelate ring induced by intramolecular hydrogen bonding
between the imino nitrogen and the o-hydroxyhydrogen
(OHÁÁÁN@C).14(a),16(a),17 Once linear PBBI backbones are
formed and ready for self-preorganization, secondary amines
(NH), imines (N@C), and 1,2-dihydroxyphenyl (OH) groups
of PBBI provide intermolecular hydrogen bonding motifs for
promoting self-assembly and the resulting formation of 2DSP.
Each layer of 2DSP-PBBI consists of multiple crystallites
aligned in various orientations, as indicated in the presence
ꢀ
of moire fringes shown in two different directions [inset, Fig.
2(a)].22 A 100 nm lateral resolution AFM image also con-
firms the polycrystalline grain boundaries [Fig. 2(d)]. 2DSP-
PBBI also vertically grows into a 3D bulk, mainly driven by
p–p stacking that exists between the aromatic PBBI back-
bones. Thus as-synthesized samples were multiple stacks
strongly held by each 2DSP-PBBI layer and poorly soluble in
most organic solvents at room temperature. However,
we were successfully able to exfoliate bulk SP-PBBI in DMF
at 60 8C. After 2 h-exfoliation, mostly small molecular weight
and amorphous-like flakes were visible under TEM, but
larger flakes were slowly delaminated into thin layers
(<30 layers) within 4 to 6 days [Fig. 2(b) and Supporting
Information Fig. S5(c)]. The height distribution of exfoliated
layers analyzed by AFM ranges from 0.4 to 8.6 nm, equivalent
to about 1 to 25 layers of 2DSP-PBBI in the sample
[Fig. 2(c,d)].
FIGURE 1 Characterization of as-synthesized SP-PBBI. (a) FTIR, (b)
13C CP MAS solid state NMR, and (c) experimental PXRD spectra of
SP-PBBI. (d) Proposed two-dimensional (ab plane) structure and opti-
mization of the unit cell parameters calculated by B3LYP/6-31g(d) level
˚
˚
(a5 16.13 A) for SP-PBBI, which is close to 16.67 A of an experimental
Large polymer sheets with lateral dimensions of 0.1–10 lm
were observed under TEM [Fig. 2(a) and Supporting Infor-
mation Fig. S5(b)]. Given that cooperative supramolecular
˚
˚
PXRD d spacing value, and (a5 23.55 A, b5 25.69 A) for (e) COF-
Salophen. [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 2 TEM (a, b) and AFM (c, d) micrographs of 2DSP-PBBI. (a) A large flake 2DSP-PBBI showing the stacked edges seen through
ꢀ
multiple layers (1 d-exfoliation). Inset: Moire fringes marked in the red circle is indicative of high crystallinity. (b) 4 d-exfoliation. (c) A
height of 2.5 nm of exfoliated 2DSP-PBBI on mica and its height profile (below). (d) Grain boundaries of a polycrystalline sample show
the height profile range of 6 to 12 nm (below) in a 100 nm lateral dimension. [Color figure can be viewed at wileyonlinelibrary.com]
polymerization is based on the two-step mechanism of (1)
nucleation and (2) elongation,23 the formation of the large
SP-PBBI seems reasonable. The size of as-synthesized SP-
PBBI was dependent on temperature control during poly-
merization. We tested it by initiating the reaction at 95 8C
and completing it at 145 8C. The PBBI crystals grew in larger
sheets if the temperature was gradually raised in multiple
stages. Precipitation polymerization involves monomers that
are initially soluble in the reaction solvent, and the locus of
polymerization remains in a homogeneous solution until the
growing macromolecular network reaches the critical molec-
ular weight for precipitation.24 We reason that slow diffusion
of building blocks and formation of dynamic intermolecular
bonds under the gradual increments of temperature (95–145
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polymerization. Interestingly, the isothermal condition
resulted in the same PXRD pattern as that of the nonisother-
mally treated sample, but yielded much smaller and more
monodisperse polymer flakes [Supporting Information Fig.
S5(d)]. To further investigate the effects of temperature and
solvent in supramolecular polymerization, we attempted a
higher temperature increment (T1 5 95 8C, T2 5 170 8C) in a
mixture of dimethylsulfoxide/mesitylene (10:1) for 6 days.
The reaction yielded a significantly different crystalline poly-
mer PBBI-170 [see FTIR, 13C CP MAS solid-state NMR, and
PXRD in Supporting Information Fig. S1(e), S2(b), and S3,
respectively]. PBBI-170 was hardly exfoliated in most organ-
ic solvents even after 7 days, and only small rod fragments
were isolated, probably delaminated at the early stage of
exfoliation [Supporting Information Fig. S5(e)].
Optical Properties of 2DSP-PBBI
We investigated the optical properties of the exfoliated
2DSP-PBBI using UV-Vis absorption spectroscopy. The
absorption spectrum of 2DSP-PBBI exhibits a broad band
with a maximum at 381 nm and minor peaks at 274 nm and
461 nm (Supporting Information Fig. S7). Each PBBI chain
possesses bidentate NO ligands, capable of complexation
with divalent metal ions such as Co(II), Cu(II), and Zn(II)
ions [Fig. 3(a)].25 To confirm the complexation of NO–Co(II),
titration of exfoliated 2DSP-PBBI in DMF was performed
with cobalt(II) chloride (CoCl2). Upon serial addition of a
5.2 mM CoCl2 aliquot to the polymer solution, the absor-
bance at 381 nm progressively decreases, indicative of the
metal–ligand complexation [Fig. 3(b)]. The titration spectra
also show a successively increasing absorption band at 609
and 674 nm with the addition of Co(II). The inset of Figure
3(b) shows the saturation point of NO–Co(II) complexation,
based on which about a 1.2:1 binding stoichiometry of Co(II)
to the NO ligand was estimated. The CoCl2 solution in DMF
showed absorption bands at 609 and 674 nm, typical of
pseudotetrahedral complexes of Co(II),26 and the wavelength
of these bands did not change during the titration of 2DSP-
PBBI. This result indicates that ligand exchange between the
NO and Cl2/DMF occurred while maintaining the pseudote-
trahedral geometry in the CoCl2 solution.27 After titration of
2DSP-PBBI with Co(II), the polymer sample was analyzed by
SEM imaging, showing that the planar sheet-like shape
remained intact [Fig. 3(c)].
FIGURE 3 Titration of 2DSP-PBBI with Co(II). (a) Complexation
between Co(II) ions and the bidentate ligands (NO) on the PBBI
backbone. M: divalent metal ions. (b) UV-Vis absorption spec-
tra of an exfoliated 2DSP-PBBI solution in DMF upon addition
of a 5.2 mM of CoCl2 prepared in DMF. The arrows indicate the
direction of absorbance change with increasing concentration
of Co(II). Inset: Absorbance at 381 nm. Instrumental artifacts at
319 and 378 nm. (c) A titrated sample of 2DSP-PBBI was depos-
ited on a copper foil and dried at room temperature for SEM
analysis. [Color figure can be viewed at wileyonlinelibrary.
com]
To test the realization of conjugated 2DSP-PBBI as a semi-
conducting material, the energy band gap was estimated
from emission/excitation spectroscopic data (Supporting
Information Fig. S8).28 From the excitation (at 382 nm) and
emission (at 512 nm) spectra, an energy band gap of 1.08
eV was calculated, indicating that exfoliated 2DSP-PBBI can
be potentially semiconducting. We performed dropcasting of
an exfoliated 2DSP-PBBI sample on a glass slide and mea-
sured the polymer film conductivity using manual probe sta-
tion, and no significant electrical current was observed in IV
measurements in the temperature range of 25 to 200 8C.
Additionally, the polymer conductivity was tested by drop-
casting polymer films on Si wafers with gold interdigitated
8C) result in a slow nucleation and growth process of struc-
turally well-defined, large polymer crystallites (ca. >1 lm)
over an extended period of time. Notably, the growth rate of
polymer layer postulated from the morphology of 2DSP-PBBI
suggests that cooperative interchain hydrogen bonding was
more effective in the elongation of polymer sheet than the
interlayer p–p stacking under the given reaction conditions
(i.e., rateab–axis ꢀ ratec–axis). To further verify the tempera-
ture effect, a different reaction batch (PBBI-2) was stored at
a constant temperature of 145 8C throughout the entire
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We were able to develop a strategy for the one-step synthesis of
SP-PBBI by condensation/precipitation polymerization. The
slow crystallite growth and precipitation under nonisothermally
controlled conditions allowed formation of well-defined bulk
SP-PBBI with the lateral dimension of <10 lm. Each PBBI build-
ing block were laterally grown into large self-assembled planar
2DSP sheets primarily by interchain hydrogen bonding bearing
multiple hydrogen bonding motifs, and p–p stacking facilitated
formation of an orderly stacked architecture from individual
2DSP sheets. Liquid-exfoliation of as-synthesized SP-PBBI pro-
vided multiple planar sheets composed of polycrystalline
domains, and distinct optical absorption and emission charac-
teristics of 2DSP-PBBI suspended in DMF were observed.
Despite some concerns regarding the susceptibility to mechani-
cal forces of supramolecular polymers, we anticipate that exfoli-
ated 2DSP-PBBI can provide convenient means for utilization
into various 2D heterostructures and nanodevices.
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