Synthesis of electron-poor hexa-peri-hexabenzocoronenes

Article abstract of DOI:10.1039/c2cc33892k

The oxidative cyclodehydrogenation of electron-poor arenes was achieved using DDQ-CF₃SO₃H resulting in hexa-peri- hexabenzocoronenes with electron withdrawing Br, F and CF₃ groups. This method will lead to their expansion into a new class of electron transport materials.

Full text of DOI:10.1039/c2cc33892k

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COMMUNICATION
Synthesis of electron-poor hexa-peri-hexabenzocoroneneswz
David J. Jones,* Balaji Purushothaman, Shaomin Ji, Andrew B. Holmes and
Wallace W. H. Wong*
Received 31st May 2012, Accepted 22nd June 2012
DOI: 10.1039/c2cc33892k
The oxidative cyclodehydrogenation of electron-poor arenes was
achieved using DDQ–CF3SO3H resulting in hexa-peri-hexabenzo-
coronenes with electron withdrawing Br, F and CF3 groups. This
method will lead to their expansion into a new class of electron
transport materials.
mobility of 0.5 cm2 V sꢁ1 has been estimated for this material
by pulse-radiolysis time-resolved microwave conductivity
(PR-TRMC). The primary reason for the rarity of electron-
poor HBC compounds is difficulty in synthesis. In this com-
munication, a clean synthetic procedure for electron-poor
HBC compounds is presented. The photophysical properties
of fluoro- and trifluoromethyl-substituted HBCs are discussed.
The synthesis of HBC was first reported by Clar and Ironside
in 1958.10 Only small quantities of HBC compounds could be
prepared until the development of the oxidative cyclodehydro-
genation (Scholl) route from hexaphenylbenzene starting
Hexa-peri-hexabenzocoronene (HBC) is a six-fold symmetric
polycyclic aromatic hydrocarbon that has been extensively
studied in the literature.1 As a result of strong p–p stacking
interactions, unsubstituted HBC has very poor solubility in
common organic solvents. Functionalisation of the HBC core
improves solution processability and has also been used to
tune structural and photophysical properties. Appropriate
functional groups, such as alkyl or aryl, on the periphery of
the HBC core have led to liquid crystalline systems as well as
assembly into macroscopic nanotube arrays.2 Chromophores
and various electroactive substituents have also been tethered
to the HBC molecule leading to applications as sensor materials3
and light harvesting semiconductors in organic solar cells.4
While the inclusion of substituents results in changes to the
properties of the material, the electronic property of the HBC
core often remains largely the same. The HBC molecule is
moderately electron rich and displays hole transporting
(p-type) semiconducting property in organic electronic devices.
Hole mobility of up to 3 ꢀ 10ꢁ2 cm2 V sꢁ1 has been measured
in organic field effect transistor (OFET) devices with vacuum
deposited unsubstituted HBC films.5
materials by Mullen and coworkers.11 The copper(II) salts and
¨
aluminium trichloride were initially used as reagents for the
oxidative cyclodehydrogenation. The use of iron trichloride
(FeCl3) gave improved functional group tolerance as a result
of milder reaction conditions. However, the iron trichloride
route often led to chlorination of products and left undesirable
iron residues. The development of dichlorodicyano-p-benzo-
quinone (DDQ) as an oxidant in the presence of acids gave
cleaner products without chlorination or iron contamination.12
As the success of the reaction is highly dependent on the
oxidation of the arene starting materials, most Scholl reaction
substrates predominantly contain electron rich arenes that are
relatively easy to oxidise. Electron poor arenes are much less
reactive in oxidative cyclo-dehydrogenations and often led to
incomplete cyclisations.6b,13 The starting point of this study is
to determine the scope of the oxidative cyclodehydrogenation
for electron poor arene substrates.
In order to tune the electronic properties of the HBC core,
electron donating and/or withdrawing functional groups must
be directly attached. Several examples of electron donating
groups, such as alkoxy6 and amines,7 have been reported in the
literature. Perhaps more interesting is functionalization with
electron withdrawing substituents. 2,5,8,11,14,17-Hexafluoro-
hexa-peri-hexabenzocoronene 1 is one of two examples in the
literature (Fig. 1).8 In OFET devices, an electron transport
mobility of 1.6 ꢀ 10ꢁ2 cm2 V sꢁ1 has been reported. The
second example is a dodecafluoro-HBC with alkoxy solubilising
substituents 2 (Fig. 1).9 An intrinsic charge (either hole or electron)
Interestingly, the successful synthesis of 2,11-dibromo-HBC
3 and hexabromo-HBC 4 has not been reported in the
literature (Fig. 1). One study by Mullen and coworkers
¨
reported attempts to synthesise hexabromo-HBC 4 by cyclo-
dehydrogenation of the hexaphenylbenzene precursor with
Bio21 Institute, School of Chemistry, University of Melbourne,
30 Flemington Road, Parkville, Victoria 3010, Australia.
E-mail: djjones@unimelb.edu.au, wwhwong@unimelb.edu.au;
Fax: +61 3 8344 2384; Tel: +61 3 8344 2371
w This article is part of the ChemComm ‘Aromaticity’ web themed issue.
z Electronic supplementary information (ESI) available: Detailed
procedures and characterisation data. See DOI: 10.1039/c2cc33892k
Fig. 1 Hexa-peri-hexabenzocoronenes with electron withdrawing
substituents.
c
8066 Chem. Commun., 2012, 48, 8066–8068
This journal is The Royal Society of Chemistry 2012

fluoro and trifluoromethyl. Both literature examples of fluori-
nated HBC compounds 1 and 2 were synthesized using
iron trichloride in 47% and 20% yield respectively.8,9 The
reactivity of a series of hexaphenylbenzene derivatives 8b–d,
with Br, F and CF3, groups towards the DDQ–CF3SO3H
system was examined (Scheme 1). The hexaphenylbenzene
precursors 8b–d were synthesised from the Diels–Alder addi-
tion of acetylenes 6b–d to cyclo-pentadienones 7b–d followed
by CO extrusion and aromatisation (Scheme 1). HBC com-
pounds 9b and 9c were obtained from hexaphenylbenzenes 8b
and 8c in high yields of 82% and 81% respectively. The
solubility of HBC derivatives 9b–d was too low for general
analysis. In MALDI-MS analysis, the parent molecular
cations for compounds 9b and 9c were clearly observed while
a mixture of partially cyclodehydrogenated products was
observed for 9d. The use of additional CF3SO3H, longer
reaction time and elevated reaction temperature did not
improve the product distribution for 9d.
Scheme 1 Synthesis of a series of HBC derivatives containing Br, F
and CF3 substituents. (a) Ph2O, 250 1C; (b) oxidative cyclo-dehydro-
genation with DDQ–CF3SO3H; (c) Pd(PPh3)4, 9,9-dioctyl-fluorene-2-
boronic acid pinacol ester, toluene, Et4NOH (aq.), 90 1C, 14 h.
In order to obtain a more detailed analysis of these novel
electron-poor HBC derivatives, solution processable fluorenyl
HBC compounds 10b and 10c were synthesised by Suzuki–
Miyaura coupling of 9,9-dioctylfluorene-2-boronic acid pinacol
ester with 9b and 9c respectively (Scheme 1). HBC derivative
10a, which has been reported by our group using a different
synthetic route,4a was obtained from dibromo-HBC 3 in 89%
yield. It is also noteworthy that multi-gram scale synthesis of
compound 10a was demonstrated using this improved route
(see ESIz). Fluorenyl HBC compounds 10a and 10b showed
high thermal stability with decomposition temperature
(Td, 5% weight loss) of 409 1C and 404 1C respectively. The
Td of compound 10c was 286 1C which could be a result of the
decomposition of the CF3 groups. All three derivatives 10a–c
showed no significant thermal transitions in differential
scanning calorimetry analysis. The HBC compounds 10a–c
showed strong association behaviour in solution. As reported
FeCl3 which resulted in a mixture of chlorinated products and
starting material.13 In contrast, less electronegative iodo analogue
5 has been used as a precursor to ethynyl substituted HBCs.14
We began our investigations with the oxidative cyclodehydro-
genation of a dibromo derivative of hexaphenylbenzene 8a
(Scheme 1). Precursor 8a was synthesised from the Diels–
Alder addition of diphenylacetylene 6a to cyclopentadienone
7a followed by CO extrusion/aromatisation (Scheme 1).
The reactivity of hexaphenylbenzene 8a was investigated
using a range of oxidative cyclodehydrogenation conditions.
Firstly, treatment of 8a with nitromethane solution of FeCl3
(6 equiv.) in dichloromethane with nitrogen purge gave only
trace amounts of dibromo-HBC 3 (see ESIz for experimental
details). It is of interest to note that there are two proposed
reaction mechanisms for the oxidative cyclodehydrogenation
(Scholl) reaction, an arenium-ion or a radical-cation.15 In both
pathways, acids play a vital role in the progress of the reaction.
With this in mind, trifluoromethanesulfonic acid (CF3SO3H,
6 equiv.) was added with FeCl3 to precursor 8a. Matrix assisted
laser desorption ionisation mass spectrometry (MALDI-MS)
analysis of the reaction mixture revealed the formation of
the desired dibromo-HBC 3 along with some chlorinated
derivatives (Fig. S23, ESIz). In the quest to develop cleaner
Scholl reaction conditions, DDQ was used as oxidant instead
of FeCl3. Interestingly, neither methanesulfonic acid (CH3SO3H)
nor trifluoroacetic acid (CF3COOH) promoted the oxidative
cyclodehydrogenation of 8a with DDQ (see ESIz for experi-
mental details). However, the combination of DDQ and
CF3SO3H gave HBC 3 in 94% yield with the MALDI-MS
analysis revealing a clean product (Fig. S24, ESIz). In addition,
multi-gram scale synthesis of HBC 3 was demonstrated using
this oxidative cyclodehydrogenation condition (see ESIz).
With the reaction conditions optimised for the electron-
poor bromo derivatives, we were interested in the scope of this
strong acid-mediated oxidative cyclodehydrogenation for sub-
strates bearing additional electron withdrawing groups such as
1
previously, the aromatic proton resonances in the H NMR
spectrum of 10a shifted upfield with increasing concentration.4a
This is indicative of strong p–p interactions in solution.
Compounds 10b and 10c showed broadened resonances in
their NMR spectrum (Fig. S19–S22, ESIz). This is a result of
strong aggregation behaviour in chloroform solution. Unlike
compound 10a, no significant concentration dependent aggre-
gation behaviour was observed in NMR experiments for the
fluorinated HBC compounds 10b and 10c.
Fig. 2 Normalised UV-vis and PL spectra of compounds 10a–c in
chloroform solution.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8066–8068 8067

Table 1 Photophysical and electrochemical data for HBC compounds 10a–c
UV-vis
PL
Optical energy gapd
(eV)
Eox onsete
(V)
Ered onsete
(V)
EHOMO
(eV)
ELUMO
(eV)
f
g
lmax (nm),
lmax (nm),
lmax (nm),
lmax (nm),
solutiona
filmb
solutionc
filmb
10a
10b
10c
365 (1.8)
365 (0.9)
370 (0.4)
374
365
373
490
502
502
550
532
582
2.79
2.73
2.70
0.33
0.64
0.62
ꢁ2.20
ꢁ2.07
ꢁ2.00
ꢁ5.43
ꢁ5.74
ꢁ5.71
ꢁ2.64
ꢁ3.01
ꢁ3.01
a
b
Absorption measured in CHCl3, 1 ꢀ 10ꢁ5 M, 295 K, corresponding extinction coefficient (ꢀ 105 cm L molꢁ1) in brackets. Film deposited on
glass slides by spin coating from chlorobenzene solution (25 mg mLꢁ1). Photoluminescence (PL) measured in CHCl3, 1 ꢀ 10ꢁ5 M, 295 K,
c
d
excitation wavelength = 365 nm. Determined from the onset of absorption. Cyclic voltammogram measured in chlorobenzene–MeCN 10 : 1,
e
1 ꢀ 10ꢁ3 M, Bu4NPF6 (0.1 M), 295 K, scan rate = 50 mV sꢁ1, versus Fc/Fc+
from ELUMO = EHOMO + optical energy gap.
.
Determined from EHOMO = ꢁ(E
f
onset
ox
g
+ 5.10) (eV). Calculated
The normalised UV-vis and photoluminescence (PL) spectra
of compounds 10a–c in chloroform solution are shown in
Fig. 2. All three compounds have UV-vis absorption bands at
285 nm and 365 nm with vibrational broadening of the 365 nm
band for 10b and 10c (Fig. 2). The absorption onset for 10b
and 10c is slightly red-shifted compared to that for 10a. The
optical energy gap of compounds 10a–c was calculated to be
2.79, 2.73 and 2.70 eV respectively (Table 1). The PL spectrum
of 10b and 10c is also red shifted compared to that of 10a
(Fig. 2). The PL intensity decreased from 10a to 10c. UV-vis
and PL spectrum of thin films of 10a–c showed broadened and
red-shifted signals. This is an expected outcome for HBC
systems which have a tendency to aggregate through p–p
interactions.
This work was supported by the Australian Solar Institute
(fellowship and project grant), the Australian Research Council
(DP0877325), the Victorian Organic Solar Cell Consortium
(www.vicosc.unimelb.edu.au), Victorian State Government
(DBI-VSA and DPI-ETIS) and the University of Melbourne.
Notes and references
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electrode and a platinum counter electrode (see ESIz for
details). The oxidation onset in the CV was used to estimate
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10c (Table 1). Density functional theory calculations on
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performance of these materials in OFET devices is currently
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obtain previously, can be prepared from hexaphenylbenzene
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8068 Chem. Commun., 2012, 48, 8066–8068
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