Expanding the aqueous-based redox-facilitated self-polymerization chemistry of catecholamines to 5,6-dihydroxy-1H-benzimidazole and its 2-substituted derivatives

Article abstract of DOI:10.1039/c5ra25590b

Aqueous-base redox-facilitated self-polymerization can be performed with 5,6-dihydroxy-1H-benzimidazole (DHBI) to generate polymeric material that is analogous to poly(dopamine) (PDA), proving the possibility to expand the catecholamine-exclusive chemistr

Full text of DOI:10.1039/c5ra25590b

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Expanding the aqueous-based redox-facilitated
self-polymerization chemistry of catecholamines
to 5,6-dihydroxy-1H-benzimidazole and its 2-
substituted derivatives†
Cite this: RSC Adv., 2016, 6, 25203
a
b
a
a
*
Ka Wai Fan, Matthew B. Peterson, Peter Ellersdorfer and Anthony M. Granville
Aqueous-base redox-facilitated self-polymerization can be performed with 5,6-dihydroxy-1H-
benzimidazole (DHBI) to generate polymeric material that is analogous to poly(dopamine) (PDA),
proving the possibility to expand the catecholamine-exclusive chemistry to N-heterocyclic catechol
derivatives. DHBI underwent similar reaction pathways as dopamine to self-polymerize into the lightly
cross-linked, p-conjugated poly(5,6-dihydroxy-1H-benzimidazole) (PDHBI). However, it was observed
that the polymerization of DHBI proceeded faster than dopamine, and can be further enhanced under
UV-stimulation, similar to dopamine polymerization. When coated on various substrates, the PDHBI
coatings were capable of promoting surface wettability similar to PDA, but exhibited lower thermal
stability due to a reduced cross-link density. Copolymerization compatibility between DHBI and
dopamine was demonstrated, and it was possible to enhance the thermal stability of PDHBI by
incorporating dopamine as a comonomer/cross-linker. Despite the high level of similarity between the
two polymers, PDHBI possesses the imidazole moieties as unique features. Because of the versatile
chemistry of o-benzenediamine employed for the monomer synthesis, DHBI-based monomers with
specific functionality at the 2-carbon position of the imidazole ring can be prepared by choosing
a desirable carboxylic acid. Two 2-substituted derivatives of DHBI were synthesized to demonstrate the
ability to intrinsically modify the properties of PDHBI-based polymeric materials in terms of solubility,
structure, and thermal stability.
Received 1st December 2015
Accepted 29th February 2016
DOI: 10.1039/c5ra25590b
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species during the polymerization.20,21 As the reaction proceeds,
DHIn molecules are linked through the 4-, 7- and, to a lesser
degree, 2-carbon positions to form a substantially cross-linked
polymeric material.
Introduction
Since the seminal work of Lee et al.1 on the self-polymerization
of dopamine, research in this area has increased dramatically.
Poly(dopamine) (PDA) has been explored in: (i) the development
of biocompatible2–4 and biodegradable5 drug delivery,6–8 (ii) in
vivo near-infrared photothermal cancer therapy,9 (iii) electron-
transfer enhancement for Li-ion batteries10,11 and articial
photosynthesis,12 (iv) facile surface modication,13,14 (v)
membrane technologies for water15 and gas16 separation, and
(vi) microuidic systems.17 Although there is still conjecture as
to the exact mechanism of the self-polymerization,18,19 it has
been shown that the quinone form of the cyclized dopamine
intermediate, 5,6-dihydroxyindole (DHIn), is the reactive
Over the years, the self-polymerization technique has been
applied mainly to catecholamines such as dopamine and its
derivatives,22–25 norepinephrine,26,27 and L-3,4-dihydroxy-
phenylalanine (L-DOPA).28,29 Recently, the range of monomers
have been expanded to polyphenols and pyrogallol 2-amino-
ethane,30 as well as to derivatives of DHIn with different
substituents at the C-3 carbon.31,32 The use of monomers other
than dopamine has the benet of furthering the chemical
versatility of the PDA-based structure. We took the structure of
DHIn as inspiration and realized the potential to broaden the
development of self-polymerization chemistry by substituting
dopamine/DHIn with other heterocyclic catechol derivatives
which are also capable of forming an activated quinone struc-
ture under the same reaction conditions. The concept grew
from previous work incorporating 5-hydroxy-1H-indazole and,
more recently, 5,6-dihydroxy-1H-indazole into PDA,33,34 which
spawned the work reported here on the synthetic catecholic
benzimidazole – 5,6-dihydroxy-1H-benzimidazole (DHBI). The
aCentre for Advanced Macromolecular Design, School of Chemical Engineering, The
University of New South Wales, Kensington, NSW 2052, Australia. E-mail:
agranville999@gmail.com
bSchool of Chemistry, The University of New South Wales, Kensington, NSW 2052,
Australia
† Electronic supplementary information (ESI) available: 1H, 13C, 2D NMR spectra,
13C solid-state NMR spectra, ATR-FTIR spectra, TGA thermograms, data obtained
from DLS and GPC. See DOI: 10.1039/c5ra25590b
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successful self-polymerization of DHBI is of substantial interest
to us, as the resulting polymer, poly(5,6-dihydroxy-1H-benz-
imidazole) (PDHBI) has the advantage of being synthesized
through an aqueous-based process same as PDA while having
imidazole moieties instead of pyrrole moieties within the
structure. The presence of the imidazole moieties, which can
potentially serve as coordination sites for ligating metals and
drugs,35,36 and also binding and release site for nitric oxide,26,37,38
sparks biomedical signicance for PDHBI. In addition, as
indicated by Scheme 1, PDHBI shares some structural similar-
ities with poly[2,20-(m-phenylene)-5,50-bisbenzimidazole], which
is also known as poly(benzimidazole) (PBI).39 The outstanding
mechanical, chemical and thermal stabilities, especially at
elevated temperatures,40–45 of PBI makes it a desired material in
several industry-orientated applications, including membrane
operations,40,41 proton-conductive membranes for fuel cells,42,43
and polymer-based adhesive.44,45 Despite its industrial signi-
cance, the synthesis of PBI and its more recently improved
variants is accompanied with harsh reaction conditions
and oen requires an inert environment.41–44 The
self-polymerization of DHBI provides an alternative route to
prepare materials with structure and, perhaps, performance
comparable to PBIs at ambient, aqueous-based conditions.
Since DHBI was synthesized based on the reaction between
o-benzenediamine and carboxylic acids,46–49 this particular
branch of chemistry can be used as a versatile synthesis plat-
form to formulate a series of DHBI-based monomers with
tailored functionality at the 2-carbon position without impair-
ing the self-polymerizing ability (Scheme 2).
Scheme
2
Reaction of 1,2-diamino-4,5-dimethoxybenzene with
different carboxylic acids to yield DHBI carrying specific functionality
at the 2-carbon position.
polymerization and information about its polymeric nature,
which would not be possible with PDA because of its poor
solubility in most solvents.
Apart from studying the self-polymerization of DHBI, the
present work also reveal the potential to manipulate the prop-
erties of PDHBI. To this end, we successfully copolymerized an
equimolar mixture of DHBI and dopamine which not only
demonstrated the compatibility of the self-polymerization
system, but also showed the ability to reinforce the structure
of PDHBI by increasing the degree of cross-linking with dopa-
mine. Furthermore, we successfully synthesized two
2-substituted derivatives of DHBI – 2-methyl-5,6-dihydroxy-1H-
benzimidazole (2-Me-DHBI) and 2-triuoromethyl-5,6-dihy-
droxy-1H-benzimidazole (2-CF3-DHBI) – as well as preparing the
respective homopolymers. These 2-substituted DHBI-based
polymers were used to exemplify the potential of tailoring, for
example, the structure, solubility, and thermal properties by
selectively choosing the functionality at the 2-carbon position of
DHBI, which in turn emphasized on the versatility of reaction
between o-benzenediamine and carboxylic acids as a versatile
synthesis platform.
Herein we demonstrate the capability of DHBI to
self-polymerize as the sole monomer to prepare the lightly
cross-linked, p-conjugated homopolymer, PDHBI, under
ambient conditions in aqueous environment. It was observed
that the self-polymerization of DHBI shared high level of simi-
larities to that of dopamine in terms of the redox reaction
mechanism and also the possibility to be stimulated by UV
irradiation.50 The reduced cross-link density of the PDHBI
structure allowed us to perform solution-based analyses on
the polymer to gain further understanding of the self-
Experimental
Materials
All reagents and solvents were used as received without further
purication unless specied otherwise. Dopamine hydrochlo-
ride (99%), 1,2-dimethoxybenzene (99%), hydrazine mono-
hydrate (98%), palladium/activated carbon catalyst (Pd/C, 10
wt%), Celite® 535, and tris(hydroxymethyl)aminomethane
(TRIS) buffer ($99.8%) were purchased from Sigma-Aldrich.
Aerosil® OX 50 fumed silica particles were purchased from
Evonik. Nitric acid (70%), hydrochloric acid (32%), dichloro-
methane (99.5%), formic acid (99%), glacial acetic acid, ethanol
(95%), hydrobromic acid (48%), and pH-sticks (0–14) were
purchased from Ajax Finechem Pty Ltd. Decolorizing charcoal
was purchased from Ajax Chemicals. Triuoroacetic acid
($99%) was purchased from Merck KGaA. Deionized water was
obtained from a Millipore water purication unit. The PET lm,
stainless steel plate and Millipore lter sheet (PVDF-based) were
available resources in the laboratory and used in the coating
experiment without any pretreatment.
Scheme 1 Self-polymerization of dopamine (a) and 5,6-dihydroxy-
1H-benzimidazole (b), and the general structure of the industrially
relevant poly(benzimidazole) (c).
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Synthesis of 1,2-dimethoxy-4,5-dinitrobenzene
Synthesis of 2-methyl-5,6-dihydroxy-1H-benzimidazole
The procedure was adopted from Bo et al.51 1,2-Dimethoxy- The procedure was developed from the work of El-Nezhawy et al.
benzene (10 mL, 78.2 mmol) was added to concentrated nitric and Weinberger et al.46,47 Glacial acetic acid (2.00 mL, 35.0
acid (65%, ca. 100 mL) dropwise under vigorous stirring over an mmol) was added dropwise to 1,2-diamino-4,5-dimethoxy-
ice-water bath. The reaction mixture was brought to room benzene (1.0 g, 5.9 mmol) dissolved in hydrochloric acid (4 N,
temperature, which was then heated for 2 h at 80 ꢀC [CAUTION! 8.0 mL) and the resulting mixture was reuxed for 4 h at 100 ꢀC.
nitrogen dioxide is released during the reaction]. Upon The reaction mixture was then chilled over an ice-water bath
completion, the clear yellow reaction mixture was allowed to and further concentrated by reducing the volume with an air jet.
stand at room temperature and then chilled in an ice-water bath. The precipitate formed was ltered out and puried with
The precipitate formed was ltered, washed with deionized decolorizing charcoal to obtain the intermediate. Excess
water until the ltrate was neutral, and then recrystallized from hydrobromic acid (48%, ca. 35 mL) was added to the interme-
hot ethanol with quantitative yield: dH (300 MHz; CDCl3), 4.01 diate and the mixture was reuxed for 8 h at 100 ꢀC. Upon
(6H, s, OMe), 7.33 (2H, s, Ar H) ppm. dC (75 MHz; CDCl3), 57.19 chilling, a precipitate formed in the reaction mixture. The
(C–O), 107.1 (C-3, C-6), 136.8 (C-4, C-5), 152.0 (C-1, C-2) ppm.
precipitate was puried with decolorizing charcoal and the pale
purple product was collected aer solvent removal (39% yields):
dH (300 MHz; DMSO-d6), 7.02 (2H, s, Ar H), 2.68 (3H, s, CH3)
ppm. dC (75 MHz; DMSO-d6), 12.08 (C-10), 98.24 (C-4, C-7), 123.8
(C-8, C-9), 145.6 (C-5, C-6), 147.5 (C-2) ppm.
Synthesis of 1,2-diamino-4,5-dimethoxybenzene
The procedure was adopted from Li et al. with some modica-
tion.52 1,2-Dimethoxy-4,5-dinitrobenzene (1.0 g, 4.38 mmol) and
Pd/C catalyst were dispersed in ethanol (45 mL). Hydrazine
monohydrate (2.13 mL, 43.8 mmol) was added dropwise to the
mixture over an ice-water bath. The mixture was reuxed for
30 min at 80 ꢀC. Upon completion, the mixture was ltered
through Celite® and the residual material was rinse with small
portions of hot ethanol. The ltrate was collected and the
off-white, crystalline product was obtained by removing
the solvent. The product was dried and stored under nitrogen
(96% yields): dH (300 MHz; CDCl3), 3.72 (6H, s, OMe), 6.31
(2H, s, Ar H) ppm. dC (75 MHz; CDCl3), 56.62 (C–O), 103.7
(C-3, C-6), 127.7 (C-1, C-2), 142.9 (C-4, C-5) ppm.
Synthesis of 2-triuoromethyl-5,6-dihydroxy-1H-
benzimidazole
´
The procedure was developed from the work of Rene et al. and
Weinberger et al.46,48 A 0.5 mM reaction mixture of 1,2-diamino-
4,5-dimethoxybenzene (1.0 g, 5.9 mmol) in triuoroacetic acid
(12 mL) was prepared over an ice-water bath, which was then
reuxed at 70 ꢀC for 16 h. Upon completion, the acid was
removed under reduced pressure. The residue was puried with
decolorizing charcoal. Excess hydrobromic acid (48%, ca. 35
mL) was added to the intermediate obtained and the mixture
ꢀ
was reuxed for 8 h at 100 C. Upon chilling, an off-white/grey
Purication using decolorizing charcoal
precipitate formed in the reaction mixture, which was
collected as the product (25% yields): dH (300 MHz; DMSO-d6),
6.99 (2H, s, Ar H) ppm. dC (75 MHz; DMSO-d6), 100.3 (C-4, C-7),
131.0 (C-8, C-9), 142.4 (C-10), 145.3 (C-5, C-6), 145.9 (C-2) ppm.
The reaction between the synthesized o-benzenediamine and
the respective carboxylic acid results in the formation of
intensely colored side products, which can be removed by using
decolorizing charcoal. The crude product was rst dissolved in
hot ethanol and the charcoal was added to the resulting solu-
Self-polymerization of the synthesized monomers and
preparation of PDHBI-coated substrates
ꢀ
tion. The mixture was stirred for 30 min at 85 C, and ltered
through Celite® while still hot. The desired compound was
collected by removing the solvent from the ltrate. This proce-
dure was used to purify the intermediates as well as the prod-
ucts from monomer syntheses reported below.
DHBI (300.28 mg, 2.00 mmol) was dissolved in a buffer solution
of pH 10 prepared from TRIS (242.4 mg, 2.00 mmol) dissolved in
deionized water (200 mL).34 The resulting solution would have
a nal pH of 8.5, which was stirred for 24 h at ambient condi-
tions. The grey-green polymer precipitated out from solution
and was collected by Millipore ltration. It was then rinsed with
Synthesis of 5,6-dihydroxy-1H-benzimidazole
The procedure was modied from that of Weinberger et al.46 small portions of water to remove residual TRIS. Similar
Formic acid (99.9%, ca. 30 mL) was added dropwise to 1,2- procedure was performed with the other two monomers based
diamino-4,5-dimethoxy-benzene (1.0 g, 5.95 mmol) and the on the same molar content. It is notable that the polymer of
ꢀ
resulting mixture was reuxed for 2 h at 100 C. Hydrobromic 2-CF3-DHBI [P(2-CF3-DHBI)] is water soluble. Thus the TRIS
acid (48%, ca. 30 mL) was then added to the reaction mixture buffer and unreacted material were removed by dialysis against
and reuxed again for 8 h at 100 ꢀC. Upon completion, the deionized water, and the polymer was obtained by freeze-drying
reaction mixture was chilled over an ice-water bath. The the dialyzed solution.
precipitate formed was collected and puried with decolorizing
A similar procedure was adopted to prepare PDHBI-coated
charcoal as described to obtain the beige product (58%): silica particles (PDHBI@SiO2) in situ during the
dH (300 MHz; DMSO-d6), 7.12 (2H, s, Ar H), 9.25 (1H, s, CH) self-polymerization by dispersing the particle templates
ppm. dC (75 MHz; DMSO-d6), 98.37 (C-4, C-7), 123.6 (C-8, C-9), (500 mg) in the reaction mixture.34 Aer 24 h, the surface-coated
136.9 (C-2), 146.4 (C-5, C-6) ppm.
particles were separated from non-surface-bound materials by
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centrifugation (3000 rpm, 5 min) and washing with deionized Spectrophotometer. Attenuated total reectance Fourier trans-
water for several cycles until the supernatant became clear and form infrared (ATR-FTIR) spectroscopy was performed on
colorless. The particles were then ltered and rinsed with a Bruker IFS 66/S single-beam spectrometer.
deionized water. An equimolar mixture of dopamine and DHBI
(i.e. 1 mmol each) was used for the preparation of copolymer-
coated silica particles [P(DHBI-DA)@SiO2]. PDHBI was coated
Results and discussion
on PET lm, stainless steel plate and Millipore lter sheet in A three-step procedure has been employed to synthesize
a similar manner.
5,6-dihydroxy-1H-benzimidazole as the initial monomer for
testing (Scheme 3). It formed the basis of the present work for
its simple structure, when compared to the other DHBI-based
monomers prepared, and the highest resemblance to DHIn.
2D NMR was performed on DHBI to identify the potential
reactive sites during the self-polymerization process (Fig. S1†).
The monomer is soluble in TRIS buffer and compatible with
dopamine as a comonomer in the reaction mixture. Hence, we
were able to prepare materials based on PDHBI and the DHBI-
dopamine copolymer [P(DHBI-DA)] in a 1 : 1 molar ratio for
further study.
We initially suspected that DHBI-based materials would be
cross-linked like PDA since the hydrogen at the C-2 carbon of
DHBI is potentially labile, which is similar to that of DHIn,19,20
and not possible to be analyzed by solution-state analysis.
However, to our surprise, we were able to dissolve a small
fraction of PDHBI in dimethyl sulfoxide (DMSO), which
immediately suggested that the PDHBI possesses a lower degree
of cross-linking than PDA. The difference in cross-linking
density between the two polymers can be justied through
comparing the respective monomers with regard to molecular
structure and reactivity of the ve-member ring. Firstly, it has
been reported in the literature that C-3 of DHIn also plays
a minor role in forming linkages with another monomer unit
during the polymerization although C-2 is the preferential
active site for cross-linking.21,53,54 In contrast, DHBI lacks such
structural advantage since its C-3 has been occupied by the sp2
hybridized nitrogen, which in turn leads to fewer linkage
formations. In terms of reactivity, both the pyrrole and imid-
Kinetics study of the self-polymerization of 5,6-dihydroxy-1H-
benzimidazole by dynamic light scattering
The procedure was the same as for preparing PDHBI@SiO2. The
reaction mixture was divided into eight fractions of equal
volume prior to the 28 h reaction period. The fractions were
taken off accordingly aer 1, 3, 6, 10, 14.5, 17.5, 24 and 28 h, and
puried accordingly to retrieve the kinetics samples. The same
procedure was performed with dopamine to prepare kinetics
samples of PDA-coated silica particles (PDA@SiO2) for
comparison. Measurements were performed to determine the
change in particle size over time.
Ultraviolet-stimulated self-polymerization of 5,6-dihydroxy-
1H-benzimidazole
A reaction mixture of DHBI and TRIS was prepared as described
previously. The volume was divided equally into two round
bottom asks. Both asks were exposed to UV-B source (Sankyo-
Denki G8T5E) irradiated from the CL-1000 ultraviolet cross-
linker (UVP), while one of the asks was covered with
aluminium foil and acted as a control. A small aliquot (16 mL) of
the reaction mixture was sampled each time and diluted by
3 mL of deionized water for absorbance measurement with an
UV-Vis spectrophotometer. Samples were taken every hour for
7 h, while the last sample was taken aer 24 h.
Characterization
1H, 13C, 19F and 2D nuclear magnetic resonance (NMR) spec- azole moieties behave as nucleophiles as the lone electron pair
troscopies were performed on a Bruker Avance III 300 MHz of the sp3 hybridized nitrogen resonates among all other posi-
spectrometer. 13C solid-state (CP-MAS) spectroscopy was per- tions on the ring to induce an overall negative charge.55 They
formed on a Bruker Avance III 300 Solid State spectrometer. would attack the catechol moiety, which behaves as an elec-
Cyclic voltammetry (CV) analyses were performed with an eDAQ trophile during the polymerization, of another monomer unit at
Integrated Potentiostat System (ER466) equipped with an Ag/ C-4 and C-7.56 Such reaction regime is particularly probable for
AgCl reference electrode, and platinum counter and working DHIn since the asymmetrical structure and different acidity of
electrodes. Dynamic light scattering (DLS) and zeta potential its hydroxyl groups direct electron density towards C-2, making
measurements were performed on a Malvern Zetasizer Nano. it a stronger nucleophile,21 whereas the symmetrical structure of
Thermogravimetric analysis (TGA) was performed on a Q5000 DHBI might not be able to achieve the same effect on the
(V3.15 Build 263) instrument purchased from TA Instruments. reactivity at the corresponding C-2. The overall effect of the
Transmission electron microscopy (TEM) images were acquired differences discussed above contributes to the lower
with a JEOL 1400 transmission electron microscope. Electron cross-linking density of PDHBI compared to PDA.
Paramagnetic Resonance (EPR) spectroscopy was performed on
In order to understand the reaction pathway taken by DHBI
a Bruker EMX-Plus X-Band ESR Spectrometer. Gel permeation during the self-polymerization and elucidate the similarities
chromatography (GPC) was performed by a unit purchased from and differences to that of dopamine, PDHBI was partially dis-
Shimadzu Scientic Instruments, and the chromatography solved in deuterated DMSO and analyzed by 1H NMR. The
columns were purchased from Phenomenex. Contact angle soluble fraction was collected through a syringe lter and con-
measurements were performed using a CAM200 Contact Angle tained just enough material to acquire appreciable signal
and Surface Tension Meter. Ultraviolet-visible (UV-Vis) spec- strength. Substantial magnication was needed to illustrate the
trophotometry was performed on a Varian Cary 300 UV-Visible signicant peaks of the PDHBI spectrum since the signals
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extensive peak broadening, indicating an extended polymeric
architecture. The PDHBI homopolymer peaks are sharper than
the other samples polymerized on the silica template due to the
increased polymer concentration and subsequent signal-to-
noise reduction. The DHBI monomer exhibits a sharp peak
for the C-4 and C-7 carbons at 97 ppm. When polymerized,
signicant peak broadening and intensity reduction of the C-4
and C-7 peaks for PDHBI and PDHBI@SiO2, located between
90 and 110 ppm, is observed. This conrms that reactions have
taken place at these carbons, consistent with carbon–carbon
bond formation, generating a p-conjugated system with a large
number of repeating units, as shown in Scheme 1. Such
a structure is analogous to PDA materials previously re-
ported.19,20 Peak broadening at 90–110 ppm is also exhibited by
P(DHBI-DA)@SiO2, whereas when dopamine is polymerized
alone on silica, such peaks are not evident. This result indicates
that the copolymer P(DHBI-DA) successfully incorporates
PDHBI into the PDA structure.
Scheme 3 The synthesis of 5,6-dihydroxy-1H-benzimidazole: (a) (i) –
nitric acid (65%), 0 ꢀC; (ii) – 80 ꢀC, 2 h, (b) Pd/C, hydrazine mono-
hydrate, methanol, reflux at 50 ꢀC, 1 h, (c) (i) – formic acid (99.9%),
reflux at 100 ꢀC, 2 h; (ii) – hydrobromic acid (48%), reflux at 100 ꢀC, 8 h.
1
originated from the residual H at the unreacted positions, if
the polymer is structurally similar to PDA. By comparing the 1H
spectrum of PDHBI against that of the monomer (Fig. 1a), the
peak (9.25 ppm) of the 1H at C-2 of DHBI (Ha) is not observed in
the PDHBI spectrum, but a newcluster of peaks has emerged at
8.00–8.50 ppm. The cluster was attributed to residual unreacted
hydrogens at the C-2 carbon incorporated into the polymer,
subject to a different chemical environment. Furthermore, the
peak (7.12 ppm) of the 1H at C-4 and C-7 of DHBI (Hb) has
exhibited some degree of broadening and reduction in the
integral ratio with the other signicant peaks – the ratio
between Hb and Ha is 2 : 1 for DHBI, whereas that between Hb
and the cluster is 2 : 7 for PDHBI. The lower number of Hb in
the polymer suggests that C-4 and C-7 are reactive during the
self-polymerization. It is notable that the chemical shi for Hb
has not changed signicantly, which is likely due to these
residual hydrogens only existing on the polymer chain ends.
The results above are in agreement with the accepted model
for the self-polymerization of dopamine, and other heterocyclic
catechol derivatives in general.19,20 However, the 1H NMR
spectra was not denitive proof for the lightly cross-linked
nature of PDHBI, i.e. to prove that C-2 of DHBI is a reactive
site. Hence, 13C solid state NMR (CP-MAS) was performed to
elucidate the reactivity of the C-2, C-4 and C-7 carbon in order to
gain further understanding of the polymerization mechanism
Results gathered from 13C solid state NMR conrms that the
C-2 carbon on DHBI, represented by the small peak at 137 ppm,
is indeed reactive enough to facilitate cross-linking reactions
during the polymerization. As indicated by the 13C spectra of the
DHBI-containing polymer samples (Fig. 1b), the C-2 peak has
both broadened and shied upeld. The reactivity of the C-2
carbon does not only suggest that the polymerization of DHBI
is able to yield materials analogous to PDA (Scheme 1),19 it also
explains the poor solubility of PDHBI mentioned in previous
discussion.
Besides the reactive carbons, the non-reactive quaternary
carbons (C-8 and C-9) are represented by a sharp peak which
arises at 123 ppm. The peak is present in DHBI and all
DHBI-containing polymer samples, i.e. PDHBI, PDHBI@SiO2,
and P(DHBI-DA)@SiO2, while its intensity and peak shape
remain relatively unchanged. On the other hand, the catecholic
carbons (C-5 and C-6) exhibit peak broadening in all PDHBI
polymeric samples, centering around 145 ppm. The broadening
is more pronounced for PDHBI@SiO2 and P(DHBI-DA)@SiO2,
which could arise from the interaction of the catechol groups
with the silica surface. The peak broadening exhibited by C-5
for DHBI and the structure of PDHBI. Fig. 1b compares the 13
C
spectra between the DHBI monomer and its homopolymer, as
well as PDHBI@SiO2 and P(DHBI-DA)@SiO2. All of the poly-
meric samples (including PDA@SiO2 for comparison) exhibit
Fig. 1 Comparison of NMR results acquired for the synthesized materials: 1H spectra of DHBI and PDHBI (a) and 13C solid-state NMR spectra of
DHBI, PDHBI, PDHBI@SiO2, P(DHBI-DA)@SiO2 and PDA@SiO2 (b).
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Fig. 2 Study of the self-polymerization of DHBI and dopamine by cyclic voltammetry (a) and elucidation of the individual polymerization kinetics
by monitoring the growth of the polymer coating on silica particles with respect to changes in thickness (b) and amount of degradable organic
content (c) over the course of the polymerization.
and C-6 carbons is not as pronounced as that displayed by C-4
and C-7, due to their non-linking nature in the overall poly-
mer. It is interesting to note that no appreciable peaks are
present downeld of 160 ppm for the PDHBI samples, indi-
cating that there are low levels of oxidized carbonyl groups
present in the polymer. This is in contrast with PDA, which
exhibits a cluster of weak and broad peaks from 160–180 ppm
attributed to carbonyl groups (Fig. S2†).
The NMR results indicate a high degree of structural simi-
larity shared between PDHBI and PDA, which suggests that
self-polymerization of DHBI proceeds in the same manner as
Fig. 3 TEM images of silica particle templates coated with PDHBI
[scale bar ¼ 100 nm (top) and 0.2 mm (bottom)]. Note that the small
particles in the background were residual nano-size silica.
dopamine, facilitated by the redox chemistry when dissolved in
an aqueous buffer at ambient conditions (Scheme 1). To prove
our hypothesis, the electrochemical properties of the DHBI and
dopamine polymerization were compared using CV. Control
experiments with only TRIS buffer solution without the mono-
mer yielded no electrochemical activity. The polymerization of
DHBI showed strong evidence of oxidation with the reduction
potential peaking at approximately 0.46 V, associated with the
formation of a grey-green precipitate. Similar results were ob-
tained for the dopamine polymerization, with the reduction
potential peaking at approximately 0.48 V, accompanied by the
formation of a ne, black precipitate. Considering the electro-
chemical similarity of the two reactions, we would expect that
DHBI and dopamine share the same redox-facilitated
self-polymerization mechanism. As shown by the voltammo-
gram (Fig. 2a), both DHBI and dopamine polymerizations are
irreversible since no reduction peak was observed as the current
was reversed. While differences in the diffusion coefficients
were believed to be the cause of the different cathodic currents
required to initiate the oxidation of DHBI and dopamine, more
in-depth study would help to elucidate how the current inu-
ences the redox reactions. The working electrode was quickly
coated with a thin layer of polymeric material aer the rst few
CV cycles which impeded further measurements. The polymer
coating formed more rapidly with DHBI than with dopamine,
which suggests faster reaction kinetics for DHBI. To gain
further insight, kinetics study was performed by using dynamic
light scattering (DLS) to determine the thickness growth of the
respective polymer coating on fumed-silica particle templates
over a 28 hour period (Fig. 2b). The growth of PDHBI was found
to be signicantly faster, which plateaued at 50 nm within 10 h
while it took 24–28 h for PDA to reach the same thickness.‡ A
similar trend was observed from the degradable (organic)
content of the kinetics samples when investigated by TGA
(Fig. 2c). Notably, PDHBI-coated samples exhibited a signi-
cantly higher degradable content than PDA-coated samples with
the same coating thickness/polymerization time, meaning that
PDHBI would be a denser material than PDA. The result is also
reected in the TEM images, in which a dense, uniform coating
of PDHBI partially blocked the electron beam from reaching the
silica particle templates (Fig. 3).
Although DHBI is able to self-polymerize in a faster manner
than dopamine, it is possible to shorten the reaction time
further. Sheng et al. have recently demonstrated the stimulation
of the self-polymerization of dopamine by UV irradiation.50
Since DHBI exhibits the same polymerization mechanism as
dopamine, there is a potential to apply UV-stimulation to this
synthetic monomer. As a result, self-polymerization of DHBI
was conduct under UV irradiation and monitored over a 7 h
period. Progressive changes in color and turbidity/translucency
have been observed from both the control (Fig. S3a†) and the
UV-stimulated (Fig. S3b†) samples. The turbidity of the
‡ The coating thickness of 1 and 3 h PDA kinetics samples could not be
determined accurately due to extensive aggregation.
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UV-stimulated sample increased signicantly faster and the EPR analysis was performed on both PDHBI and P(DHBI-DA)
reaction mixture lost its translucency within 6 h, whereas the supported on silica particles. It was found that both samples
control was still relatively clear aer 7 h, indicating an contained a stable organic free-radical in a similar fashion to
enhanced reaction rate by UV irradiation. Moreover, the UV PDA. A single peak at 3508 G (g-factor ¼ 2.00335) was generated
irradiated sample displayed a darker green color (Fig. S3c,† due to the presence of randomly orientated p-electron free
right) compared to the control (Fig. S3c,† le) aer 24 h. The radical species, which was consistent with results obtained for
darker color of the UV-stimulated sample suggested a higher PDA (Fig. 5a), both in this study and in the literature.58 The EPR
amount of PDHBI was generated. Since PDA and its analogues signal for these materials is suspected to arise from at least two
absorb strongly in the visible light and UV region of the elec- different types of radicals: defects present in the polymer
tromagnetic spectrum,34,50 UV-Vis spectrophotometry was per- backbone, and a second type likely due to an o-benzosemiqui-
formed as
a
means of monitoring changes during the none anion radical.59 Quinone formation in PDHBI has been
UV-stimulated polymerization with comparison made against proven by ATR-FTIR, in which a weak, broad absorption peak
the control sample. The spectra obtained for the control was observed in the 1800–1600 cmꢁ1 region of the spectrum
(Fig. 4a) and UV-stimulated (Fig. 4b) samples showed absorp- (Fig. 5b), though the quinone structure is less prevalent than in
tion peaks appearing at 249 and 294 nm – within the region PDA, as shown by 13C solid state NMR. The EPR results helped
(200–300 nm) that represents the generation of PDA analogue.57 further elucidate the structure of the prepared polymers, of
The change in absorbance at 294 nm, in particular, has been which the presence of free radicals reects an irregular cross-
plotted to further elucidate the inuence of UV irradiation on linked polymer network with
the self-polymerization kinetics of DHBI (Fig. 4c). The plot arrangements.9,61
a
variety of bonding
showed that the UV-stimulated sample increased faster in
PDHBI is also similar to PDA in terms of surface modifying
absorbance than the control by approximately an hour, which capability. Coating of PDHBI can be formed in situ for direct
then began to decrease at 7 h, while that for the control was still surface modication with not only silica particles but also other
at its maximum. The decrease in absorbance was expected as substrates present in the reaction mixture. Coating has been
PDHBI particles/aggregates settledmore rapidly as they grew performed on Millipore lter sheet, stainless steel plate, and
larger, and less material should be detected as a result. Note PET lms. All coated materials exhibited visible lm coating
that the absorbance of UV-stimulated sample remained rela- (Fig. S5†), as well as ATR-FTIR bands similar to those observed
tively unchanged at 7 and 24 h in comparison to the control, for the coated silica particles (Fig. S6†). The change in surface
indicating that the time required for the self-polymerization of wettability of the coated materials was assessed using static
DHBI to approach completion can be shorten under UV stim- contact angle measurements (Table S1†). A marked reduction in
ulation. Therefore, UV irradiation can be used as stimulation contact angle was observed for every coated sample compared
for the preparation of PDHBI. However, the irradiation has no with the naked substrate. While the coated stainless steel plate
inuence on the chemical nature on the degree of cross-linking and Millipore lter sheet exhibited contact angles of less than
of the resulting polymer as indicated by the highly similar 50ꢀ, the PET lm was only reduced to 73ꢀ, attributed to the
ATR-FTIR spectra and thermal degradation traces for the irra- uneven nature of the PET lm. In each case, the ability to coat
diated and non-irradiated samples (Fig. S4†).
various substrates and increase the hydrophilicity of the
Beside the similar polymerization mechanism, DHBI-based substrates is in line with that observed for substrates coated
polymeric materials also possess several intrinsic properties with polymers of dopamine and other catecholamines.1,26,62
which are found in PDA. It has been reported that PDA carries
Although PDHBI is capable of coating on various surfaces
organic free radicals within the structure like its biological like PDA, the resulting coating is expected to be less thermally
analogues melanin and eumelanin.58–60 Under specic condi- stable than that of PDA due to the lower degree of cross-linking
tions, PDA-coated substrates can be used to initiate radical of the DHBI-based polymer as discussed previously. In order to
polymerization.50 Inspired by such interesting phenomenon, gain further insight into the thermal degradation differences of
Fig. 4 UV-Vis spectra acquired for the control (a) and the UV-stimulated (b) samples during the course of the self-polymerization of DHBI, and
the changes in absorbance at 294 nm plotted against time (c).
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Fig. 5 EPR (a) and ATR-FTIR (b) spectra of silica particle templates
coated with PDHBI, P(DHBI-DA), and PDA.
the coatings without substrate inuences, free PDHBI and PDA
were prepared for TGA studies. As illustrated by Fig. S7a,†
PDHBI indeed degraded at a lower on-set temperature (287 ꢀC)
than PDA (322 ꢀC). When the homopolymers were applied as
a coating on silica particles, a similar trend was observed from
the degradation (Fig. S7b†). We further investigated the possi-
bility to enhance the thermal properties of PDHBI-related
materials by introducing dopamine as a comonomer to
promote cross-linking, of which we have demonstrated with
P(DHBI-DA)@SiO2. The incorporation of dopamine increased
the cross-link density for the resulting polymer, and thus
enhanced the thermal stability as indicated by the comparable
on-set degradation temperature to PDA coating and slower
degradation below 450 ꢀC in contrast to PDHBI. It is notable
that PDHBI, as both free polymer and coating, exhibited better
resistance to high temperatures than PDA as it degraded more
slowly from 500 ꢀC onward. The copolymer, on the other hand,
exhibited only slightly better resistance at high temperatures
compared to PDA, but not to the same extent as PDHBI. It was
believed that the thermal resistance of PDHBI towards high
temperatures is attributed to its monomer unit, i.e. the benz-
imidazole origin, which is more abundant in the homopolymer
than in the copolymer.
Fig. 6 ATR-FTIR spectra of DHBI, 2-Me-DHBI, 2-CF3-DHBI (a) and the
corresponding polymers (b).
Copolymerization is readily achievable, allowing structural
modication to manipulate certain properties of PDHBI.
However, the imidazole ring can be easily modied during the
monomer synthesis process to yield a range of DHBI-based
species with tailored functionalities at the 2-carbon position,
which can also result in PDHBI analogues of manipulated
properties. As a proof of concept, we utilized the reaction
between o-benzenediamine and a range of carboxylic acids as
a versatile platform to prepare 2-substituted DHBI monomer
(Scheme 2).49,63–65 There are still technical barriers to be over-
come with di- and tricarboxylic acids, as well as aromatic acids
in terms of reaction conditions and purications. With simple
aliphatic acids such as acetic acid and triuoroacetic acid,
in contrast, we successfully synthesized 2-Me-DHBI and
2-CF3-DHBI, respectively. 2D NMR has been performed to
elucidate the respective molecular structure (Fig. S8 and S9†).
Fig. 6a illustrated a comparison of the ATR-FTIR spectra
between all the DHBI-based monomers, in which the regions
represent the common and unique molecular structures have
been highlighted. These monomers were able to self-polymerize
under the same reaction conditions for the preparation of
PDHBI. The broadening effect of the absorption peaks because
of the increased level of interactions aer the
self-polymerization was observed from all the polymer spectra,66
indicating the successful preparation of P(2-Me-DHBI) and
P(2-CF3-DHBI) (Fig. 6b).
Despite the common DHBI origin, immediate differences
were observed in the self-polymerization of the 2-CF3-substituted
monomer, namely that a color change was observed whilst no
precipitate was formed. The reaction mixture of 2-Me-DHBI, in
contrast, also exhibited a color change but a dark green precip-
itate was formed, which was similar to the visual changes
observed when DHBI self-polymerized. It can be condently
stated that P(2-Me-DHBI) and P(2-CF3-DHBI) would not be able
to cross-link like PDHBI because the 2-carbon position has been
occupied and averts the formation of linkages, i.e. the
2-substituted polymers are linear in structure. Despite the
structural similarity, the key to the difference in solubility
between P(2-Me-DHBI) and P(2-CF3-DHBI) was believed to be the
C-2-substituent. The CF3 group would be able to induce
a signicantly stronger polarization effect on the overall struc-
ture than the methyl group because of the highly electronegative
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uorine atoms, which also helps to establish hydrogen bonding technique due to the insufficient signal strength available
with the surrounding water molecules. As a result, the more from the dissolve polymer. Fig. 7b illustrates the typical peak
polar P(2-CF3-DHBI) stayed soluble in the TRIS buffer aer broadening/shiing phenomenon to C-4 and C-7 aer the
self-polymerization while P(2-Me-DHBI) precipitated out. This polymerization. The C-8 and C-9 peak was also slightly distorted
preliminary observation effectively demonstrated that the because of the merging of the C-4 and C-7 peak. The peaks of
properties of PDHBI can be manipulated by tailoring the func- C-2, C-5 and C-6, and C-10 only broadened in contrast. The
tionality at the 2-carbon position.
unchanged chemical shi of C-2 further indicates its non-
With the 2-substituted polymers, we have performed NMR reactive nature, which in turn suggests that the occupied
analyses in a similar manner as with PDHBI. Although P(2-Me- 2-carbon position would promote linear chain growth for P(2-
DHBI) is solely linear and more soluble than PDHBI, some Me-DHBI) through p-conjugations.
material could not dissolve into DMSO and was believed to be
In comparison, the higher solubility of P(2-CF3-DHBI) in
due to its higher molecular weight. P(2-CF3-DHBI), in compar- DMSO allowed better 1H and 13C spectra to be obtained
ison, was completely soluble in DMSO because of its higher with solution NMR. Splitting of the 1H peak of C-4 and C-7
solubility than the other two polymers in the chosen solvent. (6.99 ppm) was also observed aer the self-polymerization of
Similar to PDHBI, P(2-Me-DHBI) exhibit changes in its 1H 2CF3-DHBI (Fig. S10†). The resulting cluster gave evidence of
spectrum compared to the monomer (Fig. 7a). Substantial p-conjugations occurred between monomer units, which sub-
magnication was also needed for the signals because of the jected the residual 1H on C-4 or C-7, i.e. the chain ends, to
low concentration of P(2-Me-DHBI) in the sample even though different chemical environments. For the 13C spectrum of
a signicant quantity of the polymer was added. Although being P(2-CF3-DHBI) (Fig. S11†), clusters of peaks have emerged,
non-reactive in nature, the methyl group at C-2 represented by suggesting an increased level of interactions which typically
the singlet at 2.68 ppm was shied up eld to about 2.45 ppm happens aer the self-polymerization.
1
and split into a cluster of peaks which overlapped with the
We were able to associate the residual H to their host and
solvent peak. The split is attributed to the increased interac- neighboring carbons readily by 2D NMR credited to the
tions with neighboring 1H because of the structural conne- similar pattern of proton-to-carbon interactions exhibited by
1
ment resulted from the polymerization. The peak of the H at P(2-CF3-DHBI) and its monomer (Fig. 8). Attributed to the better
C-4 and C-7 of 2-Me-DHBI (7.02 ppm) was also split into two resolution of 13C solution NMR than that of the solid state
singlets of the same integral and shied up eld slightly. The technique, the results allowed us to conrm the presence of at
signicant decrease in the integral ratio between this peak and least three types of chemically different chain ends in the
the methyl peak (from 2 : 3 to almost 2 : 9) aer the polymeri- polymer sample. The 13C peaks that did not show any interac-
zation suggests the displacement of 1H from C-4 and C-7. tion with the residual 1H, except those of C-10, were believed to
Hence, the acquired signals solely originated from the arise from the carbons of the monomer units at the center of
residual 1H present at the chain ends of P(2-Me-DHBI). 13C NMR P(2-CF3-DHBI). With C-10, 19F NMR analysis was performed to
spectrum was also obtained, but it required the solid-state verify changes occurred to the triuoromethyl group aer the
self-polymerization of 2-CF3-DHBI.
As illustrated by Fig. S12,† the singlet of the monomer was
split into a cluster of three peaks, i.e. three new chemical
environments, which is in agreement with the aforementioned
Fig. 8 2D NMR spectrum of P(2-CF3-DHBI) acquired by hetero-
nuclear single-quantum correlation (HSQC, red signals) and hetero-
Fig. 7 Comparison of the 1H NMR (a) and 13C solid-state NMR (b) nuclear multiple-bond correlation (HMBC, blue signals) techniques
spectra of 2-Me-DHBI and P(2-Me-DHBI).
(horizontal ¼ 1H, vertical ¼ 13C).
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results. It should be noted that there is a strong aliphatic signal
1
at 3.44 ppm on the H spectrum of P(2-CF3-DHBI), which has
a corresponding signal at 60.2 ppm on the 13C spectrum
(Fig. S10 and S11†). The origin of these aliphatic signals was
believed to be the residual methoxy groups on some monomer
units resulting from incomplete demethylation. Since the
methoxy 1H would not be displaced during the
self-polymerization, the signal became prominent relative to the
residual 1H on C-4 and C-7 when incorporated into the polymer.
Nonetheless, the presence of residual methoxy groups did not
prevent 2-CF3-DHBI from self-polymerizing.
In brief, the NMR results of P(2-Me-DHBI) and P(2-CF3-DHBI),
again, indicated the successful preparation of these polymeric
materials. These results also strengthened the argument that the
p-conjugation at C-4 and C-7 is the dominant mechanism to
facilitate the self-polymerization of heterocyclic catechol deriva-
tives, which in turn gives exibility on the choice of C-2-substituent
for tailoring the performance of the resulting polymer.
The increased solubility of the newly prepared materials
allowed us to further unravel the less studied polymeric nature
of these analogues of PDA, which would not be possible with
catecholamine-based materials due to their poor solubility in
most solvents. Samples were prepared for GPC in N,N-dime-
thylacetamide (DMAc) with the soluble fraction of PDHBI and
P(2-Me-DHBI), and P(2-CF3-DHBI). The molecular weight (MW)
distribution of the respective polymer was illustrated in Fig. 9a.
The MW (Mn and Mw) and dispersity (Đ) of the samples have
been reported on Table S2.† The uncontrolled nature of the
redox-facilitated self-polymerization technique resulted in
a broad distribution for all polymers, of which P(2-Me-DHBI)
exhibited the widest range of MW (Đ ¼ 2.43), followed by
PDHBI and P(2-CF3-DHBI) (Đ ¼ 1.46 and 1.30, respectively). As
discussed previously, PDHBI has a lightly cross-linked struc-
ture. The effect of cross-linking increases with MW, which
reduces the solubility of the high-MW fraction. As a result, only
Fig. 9 Molecular weight distribution (a) and thermal degradation of
PDHBI, P(2-Me-DHBI) and P(2-CF3-DHBI) (b).
performed on the free polymers to elucidate the respective
thermal degradation in order to demonstrate the possibility to
modify, in particular, the thermal properties by the substitution
of different functional groups at the 2-carbon position. As
illustrated by Fig. 9b, P(2-Me-DHBI) exhibited unexpectedly
similar on-set degradation temperature and high-temperature
resistance as PDHBI, though a small change in gradient
ꢀ
around 400 C which was believed to be related to the degra-
the least cross-linked, low-MW fraction (Mw ꢂ 23 000 g molꢁ1
)
dation of the methyl group. The high level of similarity suggests
that the extent of cross-linking of PDHBI may not be enough to
signicantly strengthen the polymer structure, but the benz-
imidazole origin allowed P(2-Me-DHBI) to retain the resistance
to high temperatures. In stark contrast, P(2-CF3-DHBI) exhibi-
ted less stability and the thermal degradation underwent
several stages. As discussed previously, the CF3 group is bulkier
than the methyl group whilst its electron-withdrawing effect can
potentially disrupt the stability of the p-conjugation. As a result,
the onset degradation of P(2-CF3-DHBI) was at a lower
was present in the GPC sample. In contrast, P(2-Me-DHBI) is
linear and its solubility is more likely to be restricted by MW,
i.e. the chain length, in spite of its methyl functionality. Hence,
fraction of higher MW (Mw ꢂ 37 000 g molꢁ1) was able to
dissolve into DMAc. It was expected that P(2-CF3-DHBI) would
exhibit a MW distribution similar to P(2-Me-DHBI) because of
their almost identical monomer units, but the 2-CF3-substituted
polymer has a lower MW and also a narrower dispersity. This
notable contrast could possibly be attributed to the difference
in electron density induced by the respective C-2-substituent.
For instance, the electronegative uorine atoms would have
withdrawn electrons from the aromatic ring and limited its
reactivity. It is also possible that the CF3 group induced steric
hindrance during the self-polymerization because of its bulkier
size than the methyl group and limited the amount of
successful p-conjugations in the same time frame. Although
further investigation will be needed to clarify the matter, the
combination of the above assumptions is in agreement with the
TGA results discussed below.
ꢀ
temperature (197 C). The tunability of thermal properties re-
ported here, in turn, demonstrated the versatility of the reaction
between o-benzenediamine and carboxylic acids as a synthesis
platform for preparing DHBI-based functionalized monomers.
Although the substituent at C-2 impedes the establishment of
linkages between monomer units, dopamine or DHBI can be
employed as a comonomer to aid coating formation with
considerable structural stability in a similar manner to the
copolymerization between dopamine and the non-cross-
linkable 5,6-dihydroxy-1H-indazole which have been previ-
ously demonstrated by our group.34 However, the ability of
applying the 2-substituted monomers for surface coating and
Since the 2-substituted polymers are not cross-linked, no
coated substrate has been prepared. However, TGA was
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property modication was not the focus of this work. Such UNSW Major Research Equipment and Infrastructure Initiative
potential shall be further explored in future research, along with (MREII). We acknowledge the time devoted by Mr Josef Goding
the possibility to employ diacids,63–65 or even triacids, to form for performing CV experiment. We also acknowledge the
linkages at C-2 prior to the polymerization to enforce a highly support of the Electron Microscopy Unit (EMU), Mark Wain-
cross-linked structure for the resulting polymer. It gives the wright Analytical Centre, UNSW. K. W. F. would like to
opportunity to develop polymeric materials with tailored acknowledge the support of the Australian Government
performance, and perhaps comparable to that of PBIs, via the for scholarship funding in the form of an Australian
self-polymerization of DHBI-based monomers under mild Postgraduate Award and a UNSW Engineering Research
aqueous conditions.
Award.
Conclusions
Notes and references
In conclusion, we have demonstrated the potential to perform
aqueous-based redox-facilitated self-polymerization of DHBI
and its 2-subtituted derivatives. This branch of chemistry is no
longer exclusive to catecholamines and other polyphenol
structures, but has been proven expandable to novel synthetic
heterocycles. Although DHBI monomers share similar self-
polymerization mechanism to dopamine, the reaction can
proceed in a signicantly faster manner under the same reac-
tion conditions and can be further accelerated by UV-
stimulation. In order to use PDHBI-based materials for
a particular application, further modication may be required
to enhance or tailor the performance. We showed that it is
possible to manipulate the properties, e.g. thermal stability,
readily by copolymerization with dopamine accredited to the
compatibility of the reaction system. Moreover, property
manipulation can also be achieved through selecting the
desired functionality for the 2-carbon position of DHBI
accredited to the versatile chemistry between o-benzenediamine
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Author contributions
K. W. F., M. B. P., P. E., and A. M. G. performed experimental
work and analyzed experimental results. K. W. F., M. B. P., and 14 S. P. Le-Masurier, G. Gody, S. Perrier and A. M. Granville,
A. M. G. wrote the manuscript. A. M. G. conceived and designed
the project.
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Conflict of interest
The authors declare no competing nancial interests.
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Acknowledgements
We thank Dr James M. Hook and Dr Aditya Rawal for helpful
discussions and aid in performing the solid-state NMR experi- 19 S. Hong, Y. S. Na, S. Choi, I. T. Song, W. Y. Kim and H. Lee,
ments. We also thank Dr Donald S. Thomas for aid in per- Adv. Funct. Mater., 2012, 22, 4711–4717.
forming EPR experiments and the analysis of the results. The 20 J. Liebscher, R. Mrowczynski, H. A. Scheidt, C. Filip,
´
´
˘
equipment in the NMR Facility at the Mark Wainwright
Analytical Centre is supported by the Australian Research
N. D. Hadade, R. Turcu, A. Bende and S. Beck, Langmuir,
2013, 29, 10539–10548.
Council through Linkage Infrastructure, Equipment and Facil- 21 M. d'Ischia, A. Napolitano and A. Pezzella, Eur. J. Org. Chem.,
ities Funding Scheme (LE0989541 and LE120100027) and the
2011, 2011, 5501–5516.
This journal is © The Royal Society of Chemistry 2016
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