Constructing a self-assembling C3-symmetric covalently linked (fused) donor-acceptor-type molecule containing a hexaazatriphenylene core

Article abstract of DOI:10.1039/c8nj03793k

A C₃-symmetric covalently linked (fused) molecule, HAT-IPN, comprising a 13 fused ring system was synthesized by a condensation-cyclisation reaction between hexaazatriphenylenehexacarboxylic acid trisanhydride and 4,5-diamino-1,2-dioctyloxybenzene. This fused system exhibits ambipolar behaviour due to an n-type HAT-core at the center and three outer p-type fused dialkoxybenzimidazole moieties, which is confirmed by cyclic voltammetry. The self-assembling nature of this disc-shaped donor-acceptor type system is evident in¹³C NMR spectroscopy, dynamic light scattering studies, differential scanning calorimetry and atomic force microscopy (AFM) images, which illustrate the formation of the one-dimensional aggregates both in solution and in the film state; indicating its potential as an active component in organic electronic devices.

Full text of DOI:10.1039/c8nj03793k

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The role of atropisomers on the photo-reactivity and fatigue of
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Constructing a selfꢀassembling C3ꢀsymmetric covalently linked
(fused) donorꢀacceptorꢀtype molecule containing
hexaazatriphenylene core
Viraj J. Bhanvadia†, Arun L. Patel†* and Sanjio S. Zade‡*
†Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda,
Vadodara − 390002, India.
‡Department of Chemical Sciences, Indian Institute of Science Education and Research
(IISER) Kolkata, Mohanpur – 741246, India.
arunpatel_5376@yahoo.co.in; sszade@yahoo.co.in
Abstract:
A C3ꢀsymmetric covalently linked (fused) molecule HATꢀIPN, comprising of 13 fused ring
system was synthesized by the condensationꢀcyclisation reaction between
hexaazatriphenylenehexacarboxylic
acid
tris(anhydride)
and
4,5ꢀdiaminoꢀ1,2ꢀ
dioctyloxybenzene. This fused system exhibits an ambipolar behaviour in consequence of the
nꢀtype HATꢀcore at center and three outer pꢀtype fused dialkoxy benzene moieties, which is
confirmed by cyclic voltammetry. The selfꢀassembling nature of this discꢀshaped donorꢀ
acceptor type system is evident by the 13C NMR spectroscopy, dynamic light scattering
studies, differential scanning calorimetry and the atomic force microscopy (AFM) images,
which illustrate the formation of the oneꢀdimensional aggregates both in solution and film
state; indicating its potential as an active component in organic electronic devices.
1

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1. Introduction
Fused polycyclic aromatic cores, demonstrating the extended πꢀconjugation have
gained significant attention in the field of organic electronics.1 Many linear and starꢀshaped
molecules comprised of the polycyclic aromatic core and extended πꢀconjugation have been
identified as an active component in optoelectronic devices2–4 in view of their adequate
HOMO and LUMO energy levels,5,6 isotropic absorption properties and strong tendency to
assemble into quasiꢀ1D columns which in turn helps to generate a preferred charge
transporting pathway.7 The nꢀtype and pꢀtype semiconducting aromatic molecules having
tendency to form the columnarꢀtype πꢀstacked structures and selfꢀassembled oneꢀdimensional
aggregates exhibit prominent candidature for electron and hole transporting materials8–13 than
glassyꢀtype amorphous aromatics which show unfavourable positional and energy disorder.14
Recently, 1,4,5,8,9,12ꢀhexaazatriphenylene (HAT) and its extended derivatives have
been demonstrated as good nꢀtype semiconducting materials.3,10,15 These HATꢀbased
materials, when properly substituted, show selfꢀassembling behaviour both in solution and
bulk state to form oneꢀdimensional columnarꢀtype aggregates.7,10,15,16–18 Though having selfꢀ
assembling nature, most of these reported large polycyclic aromatic molecules are either
showing nꢀtype or pꢀtype semiconducting characteristics. The construction of electron donorꢀ
acceptor ensembles, capable of transporting electrons and holes at the same time is important
for their application in organic field effect transistors (OFETs) and organic light emitting
field effect transistors (OLETs).19–21 The charge transfer complexes containing alternating
donorꢀacceptor stacks exhibit high order geometry.22 The prime requirement for building
highly ordered donorꢀacceptor assemblies is strong interaction between electron donorꢀ
acceptor pairs as a result of complementary electronic characters. Such wellꢀknown pairs are
bipyridinium22 or aromatic diimide23–27 as acceptors and dioxyarenes23–30 or
tetrathiafulvalene28,30–33 (TTF) as donors. Klivansky et al. have reported one of such highly
ordered donorꢀacceptor (DꢀA) assembly using HATꢀtriimide as an electron acceptor and
hexaalkoxytriphenylene as electron donor utilizing the π−π interaction between DꢀA pairs.
Regardless of the advances in these selfꢀassembly systems, breakthroughs in new materials
development are largely limited by the scarcity of the strongly interacting donorꢀacceptor
pairs.15
An alternate approach to this is to construct DꢀA systems by electronꢀrich groups
conjugated with electron deficient groups, which can selfꢀassemble into columns, thereby
forming a single molecule that is proficient of showing both nꢀtype and pꢀtype behaviour.
2

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One of these types of DꢀA systems has recently been reported by BlasꢀFerrando et al.,7 in
which large starꢀshaped DꢀA system has been synthesized utilizing the nꢀtype
hexaazatriphenylene group fused with three pꢀtype phthalocyanine units and this system
exhibits a strong tendency to aggregate both in solution and in bulk.7 Herein, we report the
construction of C3ꢀsymmetric covalently linked donorꢀacceptorꢀtype molecule in which the nꢀ
type hexaazatriphenylene core has been fused with three pꢀtype diꢀaminoꢀdiꢀalkoxybenzene
moieties by imidazoloꢀpyrrolone type linkage, forming a 13 fused ring system, HATꢀIPN
(Figure 1). Synthesized molecule shows ambipolar behaviour as well as strong selfꢀ
assembling nature, forming oneꢀdimensional aggregates.
Figure 1 Structure of C3ꢀsymmetric donorꢀacceptor type HATꢀIPN
2. Results and discussion
2.1 Synthesis of hexaazatriphenylene core containing molecule HATꢀIPN
HATꢀIPN with six nꢀoctyloxy groups was synthesized by the condensationꢀ
cyclisation reaction of corresponding hexaazatriphenylenehexacarboxylic acid trisanhydride
5 with 4,5ꢀdiaminoꢀ1,2ꢀdioctyloxybenzene 10 (Scheme 1). The compound 5 was synthesized
according to the procedure reported by Kanakarajan et al.34 and compound 10 was
synthesized according to the literature procedure reported by Nagarjuna et al.35 Both the
starting materials required for the synthesis of HATꢀIPN being unstable and moisture
sensitive were used as prepared without any purification attempts. The condensation reaction
between anhydride and diamine was carried out in refluxing dry acetonitrile using
triethylamine as a base. The resulting condensation product was isolated by removing the
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solvent under high vacuum and carried forward to the cyclisation step by dissolving the crude
condensation product in Nꢀmethylpyrrolidineꢀ2ꢀone (NMP) and refluxing the resulting
reaction mixture for 30 h. The crude product HATꢀIPN was extracted using ethyl acetate and
was purified by column chromatography over silica gel.
Scheme 1 (a) Synthesis of HATꢀIPN from HATꢀTrisanhydride 5 (1 eq.) and 4,5ꢀdiaminoꢀ1,2ꢀ
dioctyloxybenzene 10 (4.5 eq.); (b) The effect of stoichiometric ratio of starting materials
over the side product (HATꢀBIM) formation
The stoichiometric ratio of starting materials also plays a key role in the synthesis of
HATꢀIPN. Ideally, the stoichiometric ratio of 1:3 (compound 5: compound 10) is required for
the synthesis of HATꢀIPN, but due to the possibility of aerial oxidation of diamine, a slight
excess of compound 10 (4.5 eq.) was taken. On the other hand, excessive diamine ( > 4.5 eq.)
would lead to the formation of bisbenzimidazole (HATꢀBIM) instead of imidazoloꢀpyrrolone
(HATꢀIPN). Also, HATꢀIPN reacts with 4,5ꢀdiaminoꢀ1,2ꢀdioctyloxybenzene to give
bisbenzimidazole even at lower temperatures. The formation of the side products in the
synthesis of HATꢀIPN can be related to the strain in the imidazoloꢀpyrrolone structure of
HATꢀIPN.29 Similarly, lesser diamine (< 3.0 eq.) would lead to the formation of monoꢀ and
diꢀ imidazoloꢀpyrrolone products. The lower isolated yield of HATꢀIPN (7%) can be justified
by the instability of the starting materials (HATꢀtrisanhydride, 5 and 4,5ꢀdiaminoꢀ1,2ꢀ
dioctyloxybenzene, 10), decomposition products of the starting materials as well as the
possible formation of the side products.
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1
The synthesized HATꢀIPN was characterized by H and 13C NMR and elemental
analysis. 1H NMR spectrum of HATꢀIPN showed a single peak at 7.05 ppm corresponding to
the aromatic proton, proton a, as shown in Figure 2. Rest of the signals correspond to the nꢀ
octyl chain protons, protons b, c, d, e, and f. In 13C NMR spectrum of HATꢀIPN (Figure S10),
carbonyl carbon a of pyrrolone ring appears at 173.2 ppm. The imidazole ring bridgeꢀhead
carbon c is at 152.2 ppm and the inner aromatic ring carbon d of hexaazatriphenylene core
appears at 131.0 ppm. The outer aromatic ring carbon b and f of hexaazatriphenylene core
appear as two different signals at 147.2 and 130.0, respectively due to the fused pyrrolidinone
ring attached to it, while the outer benzimidazole ring carbons e, g, h and i appear at 132.4,
128.2, 126.5 and 99.7 ppm, respectively.
Figure 2 1H NMR spectrum of HATꢀIPN in CDCl3
Rest of the signals correspond to the carbons of the nꢀoctyloxy group. The oneꢀ
dimensional πꢀstacking of HATꢀIPN was evidenced by 13C NMR spectroscopy. Because of
the oneꢀdimensional πꢀπ stacking of the HATꢀIPN molecules in CDCl3, it behaves as a
physical polymer rather than behaving as individual molecules, owing to which the relaxation
time (delay time D1) required for the magnetic vector of 13C nuclei is higher. Thus, the
complete 13C NMR spectrum of HATꢀIPN with all corresponding carbon signals was
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obtained by increasing the number of scans from 1024 to 6144 and given delay time D1 of
5.00 seconds (See Supporting Information; Figure S2 a, b and c).
2.2 Photoꢀphysical properties and dynamic light scattering studies
The UVꢀvisible absorption spectrum of HATꢀIPN exhibits absorption resulting from
the monomers as well as from the aggregates as shown in Figure 3a. The UVꢀvisible
spectrum of HATꢀIPN in methanol exhibited two absorption bands; the first band at 292 nm
and the second broad structured band ranging from 320 nm to 700 nm. The second lower
energy absorption band is broadened and the intensity of the band is much lowered because
of the strong aggregation.7,29 On the other hand, the intensity of the first absorption band at
292 nm, which corresponds to the π−π transitions is greatly enhanced because of the
formation of πꢀstacked aggregates.29 The steadyꢀstate fluorescence spectrum of HATꢀIPN in
methanol (Figure 3b) showed emission at 330 nm when excited at 292 nm by a single
monochromatic beam. The emission spectrum showed combined emission from both the
molecules and aggregates, as suggested by the broadening of the emission peak. The relative
fluorescence emission quantum yield of HATꢀIPN in methanol is measured to be 23.1% with
the reference of quinine sulfate (54%) in 0.01 M H2SO4 solution.
(b)300
(a)
1.0
0.8
0.6
0.4
0.2
0.0
250
200
150
100
50
0
300
400
600
700
800
300
350
400
Wav5e0l0ength (nm)
Wavelength (nm)
Figure 3 (a) UVꢀvisible spectrum of HATꢀIPN in methanol (1.2 ppm); (b) Fluorescence
spectrum of HATꢀIPN in methanol excited at 292 nm
The dynamic light scattering (DLS) study revealed selfꢀaggregating nature of HATꢀ
IPN in the solution phase. HATꢀIPN, having a central hexaazatriphenylene moiety, is a discꢀ
shaped flat molecule with an extended πꢀconjugation. These discꢀshaped molecules having a
large aromatic core exhibit selfꢀassembling tendency both in solution and in film state, owing
to the strong π−π interaction between the large aromatic core.7,10,15,29 The dynamic light
scattering (DLS) study of HATꢀIPN solution in methanol (250 ppm) showed the effective
6

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diameter of 364 nm, which indicates that small sized πꢀstacked aggregates are already been
formed because of the strong π−π interactions between the HATꢀIPN molecules. Upon sevenꢀ
fold dilution of this solution, almost fiveꢀfold increase in the effective diameter of the
aggregates is observed, ranging from 364 nm to 1870 nm, which can be ascribed to the
formation of larger πꢀextended aggregates from the smaller aggregates (see supporting
information; Figure S11). These aggregates having wide πꢀface overlap exhibits promising
electronꢀhole transport along the πꢀstacked aggregate structure.10
2.3 Electrochemistry of HATꢀIPN
The ambipolar (DꢀA) character and electrochemical stability of HATꢀIPN were
confirmed by cyclicꢀvoltammetry. Cyclicꢀvoltammogram of HATꢀIPN was recorded using
three electrode system: Ptꢀdisc electrode as working electrode, Pt wire as a counter electrode
and Ag/Ag+ as a reference electrode. As shown in Figure 4a, three separate reduction peaks
were observed at −0.77 V, −1.17 V and −2.05 V versus Fc/Fc+, corresponding to the nꢀtype
HAT core in tetrahydrofuran using tetraꢀnꢀbutylammonium hexafluorophosphate (TBAPF6)
as a supporting electrolyte when scanned at a scan rate of 100 mV/s. On the other hand, one
oxidation peak was observed at +0.64 V corresponding to the three outer pꢀtype
dialkoxybenzoimidazole moieties (Figure 4b).
20.0µ
(b)
(a)
20.0µ
10.0µ
0.0
15.0µ
10.0µ
5.0µ
0.0
ꢀ10.0µ
ꢀ20.0µ
ꢀ30.0µ
ꢀ40.0µ
ꢀ50.0µ
ꢀ60.0µ
ꢀ5.0µ
ꢀ10.0µ
ꢀ15.0µ
0.0
0.5
1.0
ꢀ3.0
ꢀ2.5
ꢀ2.0
ꢀ1.5
ꢀ1.0
ꢀ0.5
+
+
E (V) vs Fc/Fc
E (V) vs Fc/Fc
Figure 4 (a) Reduction wave and (b) oxidation wave of HATꢀIPN in THF using TBAPF6 as
supporting electrolyte at scan rate of 100 mV/s, using cyclicꢀvoltammetry
The frontier orbital energy levels of HATꢀIPN were calculated using the first
oxidation peak (Eoxi) and first reduction peak (Ered). The values of EHOMO and ELUMO were
calculated using the equations: EHOMO = − (Eoxi + 4.8) and ELUMO = − (Ered + 4.8). The LUMO
energy level of HATꢀIPN was found to be −4.03 eV. The low lying LUMO level of HATꢀ
IPN can be attributed to the presence of acceptor type HATꢀcore and the fused imidazoloꢀ
pyrrolone type structure. The LUMO and HOMO energy levels of HATꢀIPN were compared
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with the HOMOꢀLUMO levels of other HATꢀderivatives reported in the literature (Figure
S12). Donorꢀacceptor nature of HATꢀIPN is also evident from DFT calculated frontier
orbitals. LUMOs are mainly located in the central HAT core, whereas the HOMOs have
contributions largely coming from the peripheral dialkoxybenzoimidazole part. HOMO and
HOMO−1, as well as LUMO and LUMO+1 of HATꢀIPN, are nearly degenerate having a
difference of only of 0.3 meV (Figure S1).
LUMO
LUMO+1
LUMO+2
HOMO
HOMOꢀ1
HOMOꢀ2
Figure 5 Molecular orbital topologies of HATꢀIPN calculated at DFT//B3LYP/6ꢀ31G(d)
2.4 Thermogravimetric analysis, differential scanning calorimetry and atomic force
microscopy of HATꢀIPN
The HATꢀderivative, having alternate three cyano groups and three hexyloxy groups,
attached on the periphery of HATꢀcore as well as the donorꢀacceptor charge transfer complex
made from the HATꢀtriimide and hexaalkoxytriphenylene exhibit columnar discotic liquid
crystalline phase15,36 which shows that HATꢀcore having flatꢀdisc like structure, with flexible
alkoxy/alkyl side chains may display the presence of columnar discotic mesophase while
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heating and/or cooling. The thermal behaviour of HATꢀIPN was characterized by thermoꢀ
gravimetric analysis (TGA) as well as differential scanning calorimetry (DSC) (Figure 6a and
b). Thermogravimetric analysis of HATꢀIPN (Figure 6a) revealed that the molecule is stable
up to 225 oC.
(a)
(b)
4
100
3
2
80
1
60
0
40
ꢀ1
ꢀ2
20
ꢀ3
0
50
100
150
200
250
300
350
60
80
100
120
140
Temperature (oC)
(o
Temperature
C)
Figure 6 Thermal behaviour of HATꢀIPN; (a) thermogravimetric analysis (TGA) and (b)
differential scanning calorimetry (DSC): heating cycle is showed by red line and cooling
o
cycle is showed by blue line. Scan rate is 10 C/min for both TGA and DSC. N2 is used as
inert gas; (c) POM image of HATꢀIPN upon cooling from isotropic liquid at 101.7 oC
The DSC analysis of HATꢀIPN (Figure 6b) showed the presence of the mesophase
with the endothermic peak at 91.3 oC in the heating cycle, followed by the endothermic peak
at 104.4 oC which can be correlated to the phase transition of HATꢀIPN from crystalline solid
to isotropic liquid. Upon cooling, the exothermic peak corresponding to the phase transition
o
from isotropic liquid to the crystalline solid appears at 75.3 C. The corresponding polarizing
optical microscope (POM) images indicating the phase transition during heating couldn’t be
captured, however, POM images showed that upon cooling from the isotropic liquid, HATꢀ
IPN crystallized out in focal conic texture, showing cogꢀwheel type arrangement (Figure 6c).
The formation of the πꢀstacked aggregate structure of HATꢀIPN was also evidenced by
atomic force microscopy (AFM) observation. AFM image of CHCl3 solution cast film after
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annealing with CHCl3 vapours (Figure 7a) indicates the πꢀstacked selfꢀassembled aggregates
in form of the coneꢀshaped structures37 that average 70ꢀ80 nm in height and 100ꢀ150 nm in
base width. Moreover, the AFM image of the nonꢀannealed film of HATꢀIPN (Figure 7b) did
not exhibit any coneꢀshaped structures, instead of that layered structure was observed which
indicated that the formation of πꢀstacked aggregates takes place only after annealing with
solvent vapours.
Figure 7 AFM images of HATꢀIPN film; (a) after annealing with CHCl3 vapours and (b)
nonꢀannealed film.
3. Experimental section
All the chemicals were reagent grade and used as purchased. Moistureꢀsensitive reactions
were performed under an inert atmosphere of dry nitrogen with dried solvents. Reactions
were monitored by thinꢀlayer chromatography (TLC) using Merck 60 F254 aluminiumꢀcoated
plates and the spots were visualized under ultraviolet (UV) light. Column chromatography
was carried out on silica gel (60–120 mesh). Mass spectra were recorded on ThermoꢀFischer
DSQ II GCMS instrument. NMR spectra were recorded on a Bruker AvanceꢀIII 400
spectrometer in CDCl3 and DMSOꢀD6. Elemental analysis was performed on Perkin Elmer
C/H/N Analyser, 2400 series II. Compound 5 and 10 were synthesized according to the
procedures reported in the literature34,35 (see supporting information).
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3.1 Synthesis of HATꢀIPN.
HATꢀIPN was synthesized using the modification of the literature procedure reported
by Xiao et al38. The crude compound 5 (0.44 g; 1 mmol) was dissolved in dry acetonitrile and
kept stirring under nitrogen atmosphere for 5 min at room temperature. Freshly prepared
compound 10 (1.64 g; 4.5 mmol) was suspended in dry acetonitrile along with triethylamine
(1.39 mL; 10 mmol) and was taken in the pressure equalizing addition funnel. The later was
added dropꢀwise to the stirring mixture of compound 5 in acetonitrile at room temperature
under a nitrogen atmosphere. At this point, the color change of the reaction mixture from
orange to red wine was noticed. After complete addition, the reaction mixture was refluxed at
o
90 C under nitrogen atmosphere. After 24 h, heating was discontinued and the reaction
mixture was allowed to cool to room temperature. The crude condensation product was
obtained by removal of the solvent under reduced pressure, which was carry forwarded to the
cyclization reaction without any purification. The crude product was dissolved again in Nꢀ
methylpyrrolidinꢀ2ꢀone (NMP) (50 mL) and subjected to reflux under a nitrogen atmosphere
o
at 190 C. After 30 h, heating was discontinued and the reaction mixture was allowed to
attain room temperature. The reaction mixture was quenched with 500 mL of deꢀionized
water and was extracted using aliquots of ethyl acetate (3X50 mL). The combined organic
layer was washed with brine (2X100 mL) and finally with deꢀionized water (1X100 mL) for
complete removal of NMP. The organic layer was dried over anhydrous sodium sulfate and
was evaporated under vacuum. The crude product obtained was subjected to column
chromatography over silica gel using 80% ethyl acetateꢀpetroleum ether as eluent. The pure
product HATꢀIPN obtained from the column is a yellowish powder which was further
purified by crystallization from methanolꢀwater.
HATꢀIPN: Reddishꢀyellow solid; 1H NMR (CDCl3, 400 MHz): 7.05 (s, 1H), 3.98ꢀ4.01 (t, J =
6.4 Hz, 2H), 1.80ꢀ1.86 (m, 2H), 1.46ꢀ1.50 (m, 2H), 1.30ꢀ1.34 (m, 8H), 0.88ꢀ0.91 (m, 3H). 13C
NMR (CDCl3, 100 MHz; D1 5 sec, NS 6144): 173.2, 152.3, 147.2, 132.4, 131.0, 130.9,
130.0, 128.8, 128.2, 126.6, 99.7, 70.0, 31.8, 29.4, 29.3, 29.3, 26.1, 22.7, 14.1. Elemental
Analysis: % C: 70.59 (70.56); % H: 7.65 (7.61); % N: 11.71 (11.76). Values in parentheses
are calculated values.
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4. Conclusion
In summary, a selfꢀassembling C3ꢀsymmetric covalently linked donor−acceptorꢀtype
molecule, hexaazatriphenyleneꢀtris(dialkoxybenzimidazoloꢀpyrrolone) (HATꢀIPN) has been
constructed for the first time by the triple condensationꢀcyclization reaction between HATꢀ
trisanhydride and 4,5ꢀdiaminoꢀ1,2ꢀdioctyloxybenzene. The donorꢀacceptor behaviour of
HATꢀIPN has been confirmed by cyclicꢀvoltammetry. Moreover, the selfꢀassembling
tendency of the HATꢀIPN was demonstrated by the 13C NMR spectroscopy, dynamic light
scattering, differential scanning calorimetry and atomic force microscopy, indicating the
formation of oneꢀdimensional πꢀstacked aggregates. An efficient electronꢀhole transport
could be achieved within the structure of these extended aggregates.
Conflicts of interest
There are no conflicts to declare.
Acknowledgement
VJB is thankful to University Grants Commission (UGC), Delhi for UGCꢀBSR fellowship.
ALP thanks to CSIR, New Delhi for financial support (02/(0299)/17/EMRꢀII). SSZ thanks
SERB for financial support (EMR/2015/000241).
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DOI: 10.1039/C8NJ03793K
Constructing a selfꢀassembling C3ꢀsymmetric covalently linked
(fused) donor−acceptorꢀtype molecule containing
hexaazatriphenylene core
Viraj J. Bhanvadia†, Arun L. Patel†* and Sanjio S. Zade‡*
†Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda,
Vadodara − 390002, India.
‡Department of Chemical Sciences, Indian Institute of Science Education and Research
(IISER) Kolkata, Mohanpur – 741246, India.
arunpatel_5376@yahoo.co.in; sszade@yahoo.co.in
Graphical Abstract:
A C3ꢀsymmetric fused donorꢀacceptorꢀtype molecule containing hexaazatriphenylene core,
having tendency to form πꢀstaked aggregates has been synthesized and studied.
1