FHBC, a Hexa-peri-hexabenzocoronene–Fluorene Hybrid: A Platform for Highly Soluble, Easily Functionalizable HBCs with an Expanded Graphitic Core

Article abstract of DOI:10.1002/anie.201711739

Materials based upon hexa-peri-hexabenzocoronenes (HBCs) show significant promise in a variety of photovoltaic applications. There remains the need, however, for a soluble, versatile, HBC-based platform, which can be tailored by incorporation of electroactive groups or groups that can prompt self-assembly. The synthesis of a HBC–fluorene hybrid is presented that contains an expanded graphitic core that is highly soluble, resists aggregation, and can be readily functionalized at its vertices. This new HBC platform can be tailored to incorporate six electroactive groups at its vertices, as exemplified by a facile synthesis of a representative hexaaryl derivative of FHBC. Synthesis of new FHBC derivatives, containing electroactive functional groups that can allow controlled self-assembly, may serve as potential long-range charge-transfer materials for photovoltaic applications.

Full text of DOI:10.1002/anie.201711739

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Hexa-peri-hexabenzocoronene-fluorene hybrid: A platform for
highly soluble, easily functionalizable HBCs with expanded
graphitic core
Authors: Tushar Navale, Maxim V Ivanov, Mohammad Hossain, and
Rajendra Rathore
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201711739
Angew. Chem. 10.1002/ange.201711739
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10.1002/anie.201711739
Angewandte Chemie International Edition
COMMUNICATION
Hexa-peri-hexabenzocoronene–fluorene hybrid: A platform for
highly soluble, easily functionalizable HBCs with expanded
graphitic core
Tushar S. Navale, Maxim V. Ivanov, Mohammad M. Hossain and Rajendra Rathore*[a]
This paper is dedicated to Sir Fraser D. Stoddart
Abstract: Materials based upon hexa-peri-hexabenzocoronenes
(HBCs) show significant promise in variety of photovoltaic
a highly-soluble HBC–fluorene hybrid, hereafter referred to as
FHBC, and show that it can be readily functionalized at all six
vertices (Figure 1). We will also show that expansion of the
graphitic core of HBC affords multiple (reversible) 1-e- oxidation,
producing a stable, non-aggregated cation-radical salt in solution.
The successful synthesis of this new, readily functionalizable
graphitic platform detailed here, offers potential for the design
and syntheses of next-generation materials for applications in
modern photovoltaic devices.
a
applications. There remains the need, however, for a soluble,
versatile, HBC-based platform, which can be tailored via
incorporation of electro-active groups or groups that can prompt self-
assembly. Herein, we report the successful synthesis of a new HBC-
fluorene hybrid with expanded graphitic core that is highly soluble,
resists aggregation, and can be readily functionalized at its vertices.
We also show that this new HBC platform can be tailored to
incorporate six electro-active groups at its vertices, as exemplified by
a facile synthesis of a representative hexaaryl derivative of FHBC.
Synthesis of new FHBC derivatives, containing electro-active
functional groups which can allow controlled self-assembly, may
serve as potential long range charge-transfer materials for
photovoltaic applications.
Hexa-peri-hexabenzocoronenes (HBCs) are promising
materials for application in thin film electronic devices, field
effect transistors, and photovoltaic applications.1,2 Indeed, the
design and synthesis of improved HBCs continues to garner
tremendous attention,3 as expanding the size of the flat π-
conjugated graphitic core is expected to result in high charge
carrier mobility.4,5 While the parent HBC (Figure 1) can be
readily accessed via oxidative cyclodehydrogenation of
hexaphenyl-benzene, it is insoluble in common organic
solvents.5,6 HBCs incorporating alkyl groups at the vertices
(RHBC, Figure 1) display improved solubility, yet often form
aggregates in solution, as evidenced by broadened signals in
their 1H/13C NMR spectra at ambient temperatures.7,8
Unfortunately, the incorporation of solubilizing groups at the
vertices of HBCs restrict their further functionalization with
desired electro-active groups.
Figure 1. A comparison of the relative sizes of the parent HBC, HBC
functionalized at its vertices with solubilizing groups (e.g. R = n-alkyl or tert-
butyl), and newly designed HBC–fluorene hybrid (FHBC, R = n-alkyl) with
provisions for both solubility and sites for on-demand functionalization
(indicated by black dots).
To address this issue, we will show that incorporating six
fluorene rings, substituted with solubilizing groups at their C9
methylenes, into the ‘bay areas’ of parent HBC core produces a
hybrid structure that eliminates the issues plaguing parent HBC
(see Figure 1). Specifically, the hybrid structure provides (i) an
expanded graphitic core, (ii) increased solubility, and (iii)
contains unsubstituted vertices for subsequent functionalization
(vide infra). Accordingly, we describe the successful synthesis of
The strategy for the preparation of FHBC involves an
oxidative cyclodehydrogenation of
a
hexaphenylbenzene
derivative 5, in which alternate phenyl groups (i.e. 1,3,5 phenyls)
are functionalized with two fluorenyl rings (Scheme 1). Initially,
we attempted to access 5 via the selective conversion of readily
available hexakis(4-bromophenyl)benzene9 (1) to a symmetrical
t
o
1,3,5-TMS derivative 2 by lithiation with BuLi at -90 C followed
by reaction with TMSCl.10,11 Although 2 was easily prepared in
high yield, a series of functional group transformations onto 2
(Scheme 1) returned the desired hexabromo derivative 4 in
meager yield, largely due to the poor solubility of 4 and its
precursors. The solubility issues forced us to abandon this route
for accessing FHBC (see Scheme 1 and Supporting Information
for additional details).
[a]
Dr. Tushar S. Navale, Dr. Maxim V. Ivanov, Mr. Mohammad H.
Hossain and Prof. Rajendra Rathore
Department of Chemistry
Marquette University
P.O Box 1881, Milwaukee, WI 53201-1881
E-mail: rajendra.rathore@marquette.edu
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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10.1002/anie.201711739
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COMMUNICATION
Br
Br
Br
Br
Br
Br
Br
Br
Me3Si
SiMe3
H2N
NH2
Br
Br
Br
Br
a
b,c,d
e
Br
Br
4
Br
Br
1
2
3A
Br
Br
Br
Br
NH2
Br
SiMe3
R' = n-hexyl
R'
R'
R'
R'
6
R'
R'
FHBC
R'
R'
h
i or j
g
f
I
7
Br
Br
R' R'
Br
Br
8
R'
9
R'
5
R' R'
R'
R'
+
1,2,4 isomer (5')
Scheme 1. Two different synthetic approaches for the preparation of FHBC. a. i) tBuLi (6 equiv)/-90oC; ii) Me3SiCl/-90oC, yield: 76%. b. 70% HNO3/Ac2O/80 oC,
yield: 40%. c. (i) Sn/HCl/DME/90 oC, yield: 74%; (ii) Pd(OAc)2/PPh3/aq K2CO3/n-butanol/100 oC, yield: 68%. d. aq. HBr (48%)/H2O2 (30%)/THF/ H20, yield: ~76%.
e. i) NaNO2/H2SO4; ii) H3PO2, produced a highly insoluble mixture of products which could not be fully characterized. f. PdCl2(PPh3)2/CuI/diisopropylamine/40
oC/4h, yield: 97%. g. 9,9-dihexylfluorene-2-boronic acid pinacol ester/Pd(PPh3)4/aq. K2CO3/ ethanol/toluene/ reflux/24h, yield: 89%. h. Co2(CO)8/p-dioxane/
reflux/14h, Yield: ~96%. i. DDQ (22 equiv)/CH2Cl2-CH3SO3H (9:1) mixture/0 oC/6h, yield: 28% (after chromatographic purification). j. FeCl3 (100 equiv)/
CH2Cl2/CH3NO2/22 oC, yield: 18%.
In an alternative strategy, 5 could be prepared as an
isomeric mixture in 3 simple steps (Scheme 1), via a practical if
CHCl3, THF, DMF, etc., and its structure was established by
1H/13C NMR spectroscopy and MALDI-TOF mass spectrometry
(see Supporting Information). A 1H NMR spectrum of FHBC in
CDCl3 showed well-resolved resonances for all unique protons,
indicating minimal or no aggregation at ambient temperatures
(Figure 2). This is distinct from the solution-phase NMR spectra
of other HBCs, which generally show broad signals owing to
extensive aggregation.7,8
non-elegant route. First,
a
Sonogashira coupling of
phenylacetylene (6) with readily-available 1,3-dibromo-5-
iodobenzene12 (7) returned alkyne 8 in excellent yield. A 2-fold
Pd-catalyzed Suzuki coupling of 8 with 9,9-dihexylfluorene-2-
boronic acid afforded alkyne derivative 9 in 86% yield in two
steps. Finally, a Co2(CO)8-catalyzed cyclotrimerization of 9
afforded 5 (1,3,5-isomer) and 5’ (1,2,4-isomer) as a statistical
mixture13 in nearly quantitative yield (Scheme 1). Repeated
attempts to separate highly soluble isomeric mixture of 5/5’,
using flash chromatography and fractional crystallizations, were
unsuccessful.
R'
R'
CDCl3
R'
R'
We subjected the isomeric mixture of 5/5’ to oxidative
cyclodehydrogenation, using FeCl3 as an oxidant, which
afforded a deep-red solid. A chromatographic purification of the
red-solid using hexanes as eluents easily separated FHBC as
an orange-red microcrystalline solid in 18% yield.14 A much
higher yield and purer FHBC (28%) was obtained by oxidative
cyclodehydrogenation of 5/5’, using our recently developed
procedure with [DDQ/acid] as an oxidant system in CH2Cl2.15,16
Note that oxidative cyclodehydrogenation of only 5 (1,3,5
isomer) can produce FHBC, and as the isomeric mixture of 5/5’
contains only ~33% of 5, use of [DDQ/acid]15,16 as oxidant
produces a nearly quantitative yield of FHBC.
g
R'
R'
R'
R'
b
d
R'
R'
R'
a
c
e
e
h
f
R'h
f
g
c ,d
a,b
12.5
11.5
10.5
9.5
8.5
7.5
δ (ppm)
Figure 2. Partial 1H NMR spectrum of 10 mM FHBC in CDCl3 at 22 oC
showing well-resolved resonances for equivalent aromatic protons (labeled).
The HBC-fluorene hybrid (FHBC) was found to be highly
soluble in common organic solvents, such as hexanes, CH2Cl2,
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10.1002/anie.201711739
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COMMUNICATION
The UV-visible absorption spectrum of FHBC is compared
with well-known tBuHBC6,17 in CH2Cl2 in Figure 3A. Each shows
characteristic well-resolved vibronic structure; however, the
significant expansion of the graphitic core in FHBC, leads to a
large red shift (by ~90 nm) of its absorption bands (325, 451,
and 486 nm; ɛ451 = 6.0 x 105 M-1cm-1) and increased molar
The cation radical of FHBC was generated in solution via
quantitative redox titrations using magic blue (i.e., MB+• or tris-4-
bromophenylamminium cation radical, Ered = 0.70 V vs Fc/Fc+,
λmax = 728 nm, εmax = 28,200 cm-1 M-1) as an oxidant (Figure
4B).18,19 The spectrum of FHBC+• remained unchanged at
tenfold higher concentration, as well as in the presence of
excess (up to 10 equivalents) neutral FHBC, suggesting a lack
of aggregation either between the molecules of FHBC+• or
FHBC+•/FHBC in solution. In contrast, tBuHBC+• readily forms a
absorptivity compared to tBuHBC (230, 360, and 390 nm; ε360
=
1.9 x 105 M-1cm-1). Normalized emission spectra of FHBC and
tBuHBC, at the same concentrations, Figure 3B, show a red-shift
of the emission bands of FHBC (582, 612 nm) compared to
tBuHBC (486, 520 nm). At higher concentrations, tBuHBC shows
a broad excimeric emission (at ~560 nm) indicating aggregate
(i.e., dimer, and higher oligomers) formation.17 In contrast, the
emission spectrum of FHBC does not show the appearance of a
new excimeric band (Figure S9 in the Supporting Information).
dimeric cation radical in solution, i.e. tBuHBC+•
+
tBuHBC à
+•
[
tBuHBC]2 with an equilibrium constant K = 1100 M-1 (see Figure
S8 in Supporting Information). Expansion of the chromophoric
size of FHBC+• (λmax = 460, 528, 664, 1261, and 1418 nm, ε1418
=
36,000 cm-1 M-1) leads to an increased molar absorptivity (by a
factor of ~6) when compared to tBuHBC+• (λmax = 550, 836, 1570,
1740, 2100 nm, ε2100 = 5700 cm-1 M-1), see Figure S7 in the
Supporting Information.6,17
451
486
582
Summarizing, while tBuHBC forms aggregates in neutral,
excited, and cation radical states, as judged by, respectively,
broad NMR spectra, observation of excimeric emission (at ~560
nm),6,17 and observation of intervalence transition (at 1200
nm)6,17 in its cation radical spectrum in the presence of neutral
tBuHBC (see Figures S8/S9 in the Supporting Information), such
spectroscopic signatures of aggregation were completely absent
in the case of FHBC. A cursory examination of the molecular
structures of FHBC and tBuHBC suggests that the narrow bay
areas in FHBC do not afford arrangement of two hexyl chains in
a staggered (sandwich-like) dimer. On the other hand, the
relatively wider bay areas in tBuHBC provide sufficient space for
smaller methyl groups to be accommodated in a sandwich-like
(staggered) dimeric structure (Figure 5A).
As a further probe of the lack of aggregation in FHBC, we
performed (1-ns long) molecular dynamics (MD) simulations at
ambient temperature, which showed that neutral tBuHBC indeed
forms a stable dimer (with the interplanar separations between
the aromatic cores close to van der Waals contact (~3.5 Å),
while in dimeric FHBC the pair of nanographenes lie at a
separation of ~7.8 Å (Figure 5B-C). Indeed, the presence of the
long hexyl chains in FHBC hinders the approach of the graphitic
cores in the dimer in favor of multiple CH-π interactions between
the large π-system and alkyl chains (Figure 5B/C). It is important
to emphasize that access to FHBC platform, which resist self
aggregation, will open new avenues for the preparation of 2-
dimensional extended aggregates via π-π contacts between
the outer phenylenes of fluorene moieties that can be
functionalized with appropriate groups at its vertices.
FHBC
FHBC
tBuHBC
tBuHBC
486
325
612
230
360
390
520
200
400
Wavelength, nm
600 400
550
Wavelength, nm
700
Figure 3. Comparison of the UV-vis absorption (10-6 M) and emission spectra
of FHBC (red) and tBuHBC (blue) in CH2Cl2 at 22 °C.
Electrochemical analysis showed that FHBC displays four
reversible oxidation waves at 0.40, 0.76, 1.01 and 1.19 V (vs.
Fc/Fc+) corresponding to the formation of monocation, dication,
trication and tetracation, respectively (Figure 4A). In contrast,
tBuHBC exhibits a single oxidation wave at 0.64 V vs Fc/Fc+ in
CH2Cl2 (see Figure S4 in the Supporting Information).6,17
Moreover, the expansion of the size of the graphitic core in
FHBC results in a significant lowering of the first oxidation
potential (by ~240 mV) in comparison to tBuHBC.
A
B
.
FHBC+
.
MB+
1.4
0.7
0.0
500
1000
1500
2000
V vs Fc/Fc+
Wavelength (nm)
Figure 4. (A) Cyclic (solid red line) and square-wave (dashed blue line)
voltammograms of FHBC (0.63 mM) in CH2Cl2 containing 0.2 M n-Bu4NPF6 at
a scan rate of 200 mV s-1 and 22 oC. (B) The spectral changes observed upon
-
the reduction of 5.5 x 10-6 M MB+•SbCl6 by an incremental addition of sub-
stoichiometric amounts of FHBC in CH2Cl2 at 22 oC.
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10.1002/anie.201711739
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COMMUNICATION
A
Functionalizaton of FHBC at its vertices would require
access to its hexabromo derivative 7 (Scheme 2). Initial
attempts of a six-fold bromination of FHBC in CH2Cl2 using
bromine resulted in an inseparable mixture of polybrominated
products.20 However, a bromination of the isomeric mixture of
5/5’ (i.e. the precursor to FHBC) in CH2Cl2 with bromine, in the
presence of a catalytic amounts of iodine (in ~4h), afforded
isomers 10/10’, appropriately brominated at the desired
positions of all fluorenes (Scheme 2).
An oxidative
B
cyclodehydrogenation of this isomeric mixture (10/10’) using
DDQ/acid oxidant system followed by column chromatography
returned FHBC-Br6 in 26% yield as a dark-red solid. The
structure of FHBC-Br6 was established by 1H/13C NMR
spectroscopy and further confirmed by MALDI mass
spectrometry (see Supporting Information section).
As a proof of concept experiment, a six-fold Suzuki
coupling of FHBC-Br6 with 2,5-dimethoxy-4-methylphenylboronic
acid, under standard reaction conditions, afforded FHBC-Ar6 as
a deep-red solid in 91% yield. The structure of FHBC-Ar6 was
established by 1H/13C NMR spectroscopy and MALDI mass
spectrometry. A 1H NMR spectrum of FHBC-Ar6 in CDCl3
showed well-resolved resonances (see Supporting Information)
similar to FHBC (Figure 2).
C
In conclusion, we have developed
a highly soluble
versatile HBC platform (i.e. FHBC) that contains an expanded
graphitic core containing 19 Clar sextets and affords the ready
introduction of multiple electro-active groups at its vertices. This
ready tailoring of electro-active groups at vertices of FHBC, as
Figure 5. (A) Structures of tBuHBC and FHBC showing the different sizes of
the bay areas with the aid of circles. (B) Superimposed structures of tBuHBC
and FHBC dimers obtained from molecular dynamics simulations (see
Supporting Information for details). (C) Space-filling representation of tBuHBC
and FHBC dimers.
demonstrated by
a facile synthesis of a hexaarylbenzene
derivative (i.e. FHBC-Ar6), will allow the preparation of
molecules with desirable electro-active functional groups suited
for controlled assembly to form aggregates with tunable
properties such as long-range charge transport for applications
in the modern area of photovoltaics.21,22
5/5'
R' = n-hexyl; Ar = 2,5-dimethoxytolyl
Br
Br
Br
Br
Ar
Ar
k
R'
R'
R'
R'
R'
R'
R'
R'
R'
R'
R'
R'
l
j
R'
R'
R'
R'
R'
R'
R'
R'
R'
R'
R'
R'
Br
Br
Br
Br
Ar
Ar
R' R'
R' R'
R' R'
R'
R'
R'
R'
R'
R'
10
+
Br
Br
Br
Br
Ar
Ar
FHBC-Br6
FHBC-Ar6
1,2,4 isomer (10')
Scheme 2. Synthetic strategy for functionalization of FHBC at its vertices. k. Br2/I2 (catalytic amount)/CH2Cl2/22 oC/4h, yield: ~100%. j. DDQ (22 equiv)/CH2Cl2-
CF3SO3H (9:1) mixture/0oC/1h, yield: 26% (after chromatographic purification). l. 2,5-dimethoxy-4-methylphenylboronic acid/Pd(PPh3)4/ aq.
K2CO3/ethanol/toluene/reflux/24h, yield: 85%.
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10.1002/anie.201711739
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COMMUNICATION
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[12] P. N. W. Baxter, Chemistry. 2003, 9, 5011-5022.
Acknowledgements
We thank the NSF (CHE-1508677) and NIH (R01-HL112639-04)
for financial support and Professor Scott A. Reid for helpful
discussions.
[13] R. Rathore, C. L. Burns, S. A. Abdelwahed, Org. Lett. 2004, 6, 1689-1692.
[14] The oxidative cyclodehydrogenation of the isomeric 5' (i.e., 1,2,4 isomer)
produces a complex mixture of products, which could not be separated
by repeated chromatographic attempts and therefore their
characterization was abandoned.
Keywords: HBC • graphitic nanocarbons • electron transfer
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10.1002/anie.201711739
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COMMUNICATION
COMMUNICATION
Fluorene-HBC hybrid with large
graphitic core is synthesized and
found to be highly soluble and easily
functionalized at its six vertices.
Tushar S. Navale, Maxim V. Ivanov,
Mohamad H. Hossain and Rajendra
Rathore*
Page No. – Page No.
Hexa-peri-hexabenzocoronene–
fluorene hybrid: A platform for
soluble, easily functionalizable HBCs
with expanded graphitic core
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