Synthesis and electronic properties of iso-alkyl substituted hexa-peri-hexabenzocoronenes (HBC's) from a versatile new HBC synthon, hexakis(4-acetylphenyl)benzene

Article abstract of DOI:10.1016/j.tetlet.2008.06.005

Simple syntheses of a variety of soluble hexa-peri-hexabenzocoronenes are accomplished using an easily prepared hexakis(4-acetylphenyl)benzene as a common precursor. The concentration-dependent emissions of various HBC's in solution indicate that they undergo extensive aggregation despite the presence of bulky (peripheral) isoalkyl and tert-butyl groups. The various planar HBC's undergo reversible electrochemical oxidations and form stable monomeric cation-radical salts in solution.

Full text of DOI:10.1016/j.tetlet.2008.06.005

Tetrahedron Letters 49 (2008) 4869–4872
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Tetrahedron Letters
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Synthesis and electronic properties of iso-alkyl substituted
hexa-peri-hexabenzocoronenes (HBC’s) from a versatile new
HBC synthon, hexakis(4-acetylphenyl)benzene
*
*
Vincent J. Chebny, Chengeto Gwengo, James R. Gardinier , Rajendra Rathore
Department of Chemistry, Marquette University, PO Box 1881, Milwaukee, WI 53233, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Simple syntheses of a variety of soluble hexa-peri-hexabenzocoronenes are accomplished using an easily
prepared hexakis(4-acetylphenyl)benzene as a common precursor. The concentration-dependent emis-
sions of various HBC’s in solution indicate that they undergo extensive aggregation despite the presence
of bulky (peripheral) isoalkyl and tert-butyl groups. The various planar HBC’s undergo reversible electro-
chemical oxidations and form stable monomeric cation-radical salts in solution.
Received 6 May 2008
Revised 1 June 2008
Accepted 2 June 2008
Available online 6 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Polycyclic aromatic hydrocarbons (PAH) hold potential as build-
ing blocks for the preparation of electro-active materials for the
practical applications in the emerging areas of molecular electron-
ics and nanotechnology.1 Although Clar2 in his classical papers in
the early 1960s suggested that ‘fully benzenoid’ hexa-peri-hexa-
benzocoronene (HBC) should possess exceptional thermal and
photochemical stability, the study of this novel PAH was not vigor-
ously pursued owing to its insolubility in most organic solvents.
Recently, Mullen and co-workers have shown in a series of publi-
cations3 that the solubility problem is overcome by attaching the
long alkyl chains and/or bulky tert-butyl groups in the para posi-
tions of hexa-peri-hexabenzocoronene and its derivatives. Despite
numerous ingenious designs of novel hexa-peri-hexabenzocoron-
enes using multi-step syntheses,3,4 need still remains for the devel-
opment of simple, rational synthetic approaches which will allow
easy access to a wide variety of functionalized hexa-peri-hexa-
benzocoronenes (HBC) in order to realize their full potential in
materials’ applications.
the myriad of various transformations of the acetyl group. The syn-
thetic utility of 2 is first demonstrated herein by the preparation of
various iso-alkyl substituted HBC’s (isopropyl, isobutyl, and isooc-
tyl were chosen as solubilizing groups). The HBC derivatives
obtained herein were characterized by 1H/13C NMR spectroscopy
and mass spectrometry, and their redox and optical properties
were measured by electrochemistry, UV–vis, and emission
spectroscopy.
The common precursor, that is, hexakis(4-acetylphenyl)-ben-
zene (2), was easily synthesized by a Co2(CO)8-catalyzed trimeriza-
tion of diarylacetylene 1 in refluxing dioxane, which in turn was
prepared from a modified Sonogashira coupling7 of the commer-
cially available 4-iodoacetophenone (see Supplementary data for
the experimental details). The identity of 2 was established by
1H/13C NMR spectroscopy, mass spectrometry, and was further
confirmed by X-ray crystallography (see Scheme 1).
The X-ray structure of 2 comprises of two symmetrically inde-
pendent molecules, which are positioned at their respective crys-
tallographic inversion centers. Both molecules showed a similar
Our continued interest in HBC framework5,6 recently led us to a
practical one-pot synthesis of a soluble HBC, which is substituted
in the para positions with bulky tert-butyl groups, from readily
available hexaphenylbenzene in high yield.5 We observed concen-
tration dependent emission for this derivative, a likely result of self
association. In order to further probe the self-association charac-
teristics of HBC’s framework, we required a series of derivatives
with different solubilizing groups other than tert-butyls. Accord-
ingly, we now describe a synthesis of the hitherto unknown hexa-
kis(4-acetylphenyl)benzene (2), which can serve as a versatile
precursor for the preparation of a variety of HBC’s by exploiting
O
O
a
b
I
1
O
2
* Corresponding authors. Tel.: +1 414 288 2076; fax: +1 414 288 7066 (J.R.G.).
E-mail addresses: james.gardinier@mu.edu (J. R. Gardinier), Rajendra.Rathore@
mu.edu (R. Rathore).
Scheme 1. Reagents and conditions: (a) benzene/DBU/H2O/(PPh3)2PdCl2/CuI/
Me3SiCCH, reflux; (b) Co2(CO)8/dioxane/reflux. X-ray structure of
‘three-up, three-down’ configuration of carbonyl oxygens.
2 showing
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.005

4870
V. J. Chebny et al. / Tetrahedron Letters 49 (2008) 4869–4872
propeller-shaped arrangement of the peripheral aryl groups with
the dihedral angles between the plane of the central and peripheral
aryl rings varying in the range of 66–87°.8 Interestingly, both inde-
pendent molecules adopt a ‘three-up, three-down’ configuration
with carbonyl oxygens of three neighbouring acetyl groups ori-
ented toward the one face of the central phenyl group. The unusual
‘three-up, three-down’ configurational preference of 2 in the crys-
tal lattice most likely arises due to the intermolecular C–Hꢀ ꢀ ꢀO con-
tacts among the molecules in the perpendicular layers (i.e.,
O2ꢀ ꢀ ꢀH66A–C66 with Oꢀ ꢀ ꢀH distance 2.44 Å and O–H–C angle 145°;
and O6ꢀ ꢀ ꢀH26A–C26 with Oꢀ ꢀ ꢀH 2.35 Å and Oꢀ ꢀ ꢀH–C angle of 148°,
see Supplementary data for the numbering schemes of the atoms
in the two independent molecules of 2).
2 ꢁ 10ꢂ3 M solutions in dichloromethane containing 0.1 M tetra-
n-butylammonium hexafluoro-phosphate as the supporting elec-
trolyte. The reversible cyclic voltammograms (CV) obtained at scan
rates between 25 and 400 mV sꢂ1 (see Fig. 1) showed anodic/
cathodic peak current ratios of ia/ic = 1 (theoretical) at ambient
temperatures. The calibration of the CV peaks of 5a–c with ferro-
cene as an internal standard provided the reversible oxidation
potentials Eox1 = 0.97, 0.98 and 0.98 V versus SCE, respectively
(see Fig. S1 and S2 for the cyclic voltammograms of 5b and 5c in
the Supplementary data). Expectedly, the oxidation potentials
(Eox) of 5a–d were rather invariant upon changing the peripheral
alkyl groups from isopropyl to isobutyl to isooctyl to tert-butyl5
(5d, Eox = 0.98 V vs SCE).
Having the multi-gram quantities of hexaacetyl derivative 2 at
hand, we next examined its conversion to the precursors for the
preparation of soluble HBC’s as follows. Thus, a treatment of 2 with
excess methylmagnesium chloride (10 equiv) in tetrahydrofuran
for 12 h followed by a standard aqueous workup afforded the hexa-
alcohol 3a in 96% yield. The crude 3a was reduced by hydrogena-
tion in the presence of catalytic amounts of methanesulfonic acid
and Pd/C (10%) in ethyl acetate to a hydrogen pressure of 50 psi
for 6 h to afford the hexakis(4-isopropylphenyl)benzene 4a in
nearly quantitative yield. Similarly, 4b (from ethylmagnesium bro-
mide) and 4c (from hexylmagnesium bromide) were obtained via a
similar 2-step route as outlined in Scheme 2.
The reversibility of 1-electron oxidation by cyclic voltammetry
of various HBC’s prompted us to evaluate the stability of their cat-
ion-radical salts using stable triarylamine (MB)9 and/or a hydro-
quinone ether (MA)10 cation-radical salt as robust 1-electron
oxidants.
O
Br
N
3
O
MA+
MB
1.15 (V vs. SCE)
Eored
1.11
The hexakis(4-alkylphenyl)benzenes 4a–c were easily converted
to the corresponding HBC’s (i.e., 5a–c) via oxidative cyclodehydro-
genation using excess FeCl3 (18 equiv) in a 2:1 mixture of dichloro-
methane/nitromethane at 0 °C under a constant flow of argon (see
Scheme 1). Various HBC’s were purified by a simple filtration
through a short pad of silica gel and were characterized by
1H/13C NMR spectroscopy and mass spectrometry. For a detailed
procedure and 1H/13C NMR spectra of 5a–c, see the Supplemen-
tary data.
Figure 2 shows the spectral changes attendant upon oxidation
of 5a to its cation radical 5aÅ+ by incremental addition of MBÅ+
ꢂ
SbCl6 [kmax (log
e
) = 728 nm (4.45)] in dichloromethane at 22 °C.
The well-defined isosbestic points at k = 585 and 768 nm and the
stoichiometry of the spectral changes (disappearance of bands for
MBÅ+ or the appearance of those for 5aÅ+, inset of Fig. 2) established
an uncluttered one-electron transfer.
Furthermore, a plot of the depletion of MBÅ+ and formation of
5aÅ+ against the increments of added neutral 5a (inset Fig. 2) estab-
lished that MBÅ+ was completely consumed after the addition of
1 equiv of 5a; the resulting highly structured absorption spectrum
The electron-donor strengths of various HBC’s were evaluated
by electrochemical oxidation at
a
platinum electrode as
of 5aÅ+ [kmax (log
e) = 551, 753 (sh), 810 (sh), 840 (3.93)] remained
unchanged upon further addition of neutral 5a (i.e., Eq. 1).
R
O
5a þ MBÅþ ! MB þ 5aÅþ
ð1Þ
R
O
Furthermore, the cation-radical absorption spectra of 5aÅ+, 5bÅ+ [kmax
(log e e) = 551,
) = 551, 753 (sh), 810 (sh), 840 (3.92)], 5cÅ+ [kmax (log
R
O
753 (sh), 810 (sh), 840 (3.99)], as well as of the corresponding tert-
butyl derivative 5d5 were similar irrespective the oxidant used, that
is, MBÅ+ or MAÅ+.
R
O
The observed spectra of the cation radicals of 5a–d were readily
assigned to the corresponding monomeric cation radicals by
comparison to the calculated spectrum of hexamethyl-HBC cation
radical.6 The HBC cation radicals did not show any sign of aggrega-
tion in solution, such as the formation of dimeric cation radicals
R
O
2
5a-c
R
O
a
c
OH
R
R
R
2e-6
0
1.5e-6
0.0
R
OH
R
HO
R
A
B
R
b
1.5e-6
3.0e-6
4.5e-6
-2e-6
-4e-6
-6e-6
R
OH
R
HO
R
1.2
1.0
0.8
0.6
0.4
1.2
1.0
0.8
0.6
0.4
R
4a-c
3a-c
R
V vs. SCE
V vs. SCE
OH
Figure 1. (A) Representative cyclic voltammograms of 2 ꢁ 10ꢂ3 M 5a in CH2Cl2
containing 0.1 M (n-Bu)4NPF6 at 22 °C at scan rates between 25 and 400 mV sꢂ1. (B)
A cyclic voltammogram (blue) and the corresponding square-wave voltammogram
a
b
c
( ) R = methyl, ( ) R = ethyl, ( ) R = hexyl
Scheme 2. Reagents and conditions: (a) THF/RMgBr or RMgCl; (b) H+/H2 50 psi/10%
Pd–C; (c) FeCl3/CH2Cl2/CH3NO2.
(red) of 5a at a scan rate of m .
= 200 mV sꢂ1

V. J. Chebny et al. / Tetrahedron Letters 49 (2008) 4869–4872
4871
orange emission [1931 CIE (XYZ) coordinates: 0.405, 0.563,
0.032]. For 5a, this emission occurs as broad band
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
a
(kmax = 544 nm) centered near 558 nm with detectable fine struc-
ture. The solubility limit for these hexa-substituted HBC’s regard-
less of the substitution pattern described in this work is between
10ꢂ4 M and 10ꢂ3 M. Thus, the apparent discrepancy between the
yellow-green color from calculated CIE coordinates of the
‘10ꢂ3 M’ sample and the yellow-orange color of the sample in the
right inset of Figure 3 is due, in part, to the heterogeneous nature
of the mixture, that is, extensive aggregation of HBC’s in solution
at higher concentrations.
The observation of concentration dependent emission of
various HBC’s is most likely due to aggregation resulting in exci-
mer-based emissions, similar to those of other polyaromatic
hydrocarbons such as pyrene (Supplementary data).13 The tenta-
tive assignment of two lower energy excimer-based emission
bands (ꢃ520 and 588 nm) likely reflects the different ‘solvation’
environments around dimers when in solution versus the solid
state. The bulkier substituents give rise to powder excimer emis-
sions that are more closely energy-matched to their solution-based
excimer band. We cannot exclude the possibility that in concen-
trated solution simple dimeric species (higher energy excimer
emission) predominate over higher-order aggregates (lower en-
ergy excimer emission) as might likely be found in the solid-state
powder. However, the crystal structure of tert-butyl derivative 5d,
which showed similar emission characteristics to those described
for 5a, has been reported14 which shows a herringbone arrange-
ment of slipped (cofacial) dimers (two molecules related by inver-
sion symmetry). The perpendicular distance between mean planes
of the aromatic ring systems in the dimers is ꢃ3.69 Å, the centroid–
centroid distance is 4.98 Å, and the slip angle, b, is 44.5°; all these
0.0 0.5 1.0 1.5 2.0 2.5
equiv. of 5a
500
1000
1500
2000
Wavelength (nm)
Figure 2. Spectral changes upon the reduction of 2.6 ꢁ 10ꢂ5 M MBÅ+ (red line) by
incremental addition of 1.0 ꢁ 10ꢂ3 M 5a to its radical cation (black line) in CH2Cl2 at
22 °C. Inset: A plot of depletion of absorbance of MBÅ+ (red triangles, at 728 nm) and
an increase of the absorbance of 5aÅ+ (blue circles, at 840 nm) against the equivalent
of added neutral 5a.
resulting from the cofacial stacking of 5+ with neutral 5, as judged
by the singular absence of a new charge-resonance transition11
even in the presence of a large excess of the corresponding neutral
HBC. It is also noted that various HBC cation radicals did not show
any change in their absorption spectra even when the concentra-
tion was increased by 10-fold.
The excited state emission spectra of the hexa-substituted
HBC’s are concentration dependent, exemplified by the hexaiso-
propyl derivative 5a (Fig. 3). In dilute solution (10ꢂ6 M), 5a showed
a structured cyan [1931 CIE (XYZ) coordinates: 0.127, 0.366,
0.507]12 emission with the characteristic emission band at
kmax = 488 nm. At higher concentrations (10ꢂ5 to ꢃ10ꢂ3 M), lower
energy emission bands centered near 520 nm and 558 nm, tenta-
tively assigned as excimer-based, grow in intensity at the expense
of the higher energy (488 nm) band, with an apparent isosbestic
point at 500 nm. Concomitant with the change in band intensities
in this concentration range, the emission changes from cyan
(10ꢂ6 M) to yellow-green [10ꢂ3 M, 1931 CIE (XYZ) coordinates:
0.299, 0.580, 0.121]. In the solid state, HBC’s 5a–d give yellow-
values are outside the accepted ranges for an effective
p
–
p
p
interac-
interac-
tion in the ground state.15 There are two mutual CH ꢀ ꢀ ꢀ
tions involving the hydrogen donor from a tert-butyl substituent
on one molecule and the HBC core of a neighbouring molecule with
a distance and angle of 2.69 Å, 175.7°.16 Owing to the paucity of
structural information on HBC derivatives, it is unclear whether
more extensive slipped stacking arrangements (such as trimers
or higher-order aggregates) can exist in the solid state. Similar
low-energy dimeric arrangements were also found from PM3
geometry optimization for the dimers, each of the hexa-substituted
HBC’s, exemplified for 5a in Figure 4.
In each case, the offset or slipped arrangement with Ci
symmetry was energetically more favorable than the aligned,
cofacially-arranged (b = 0°) D6d symmetric dimer, as expected from
the repulsion of p-electron clouds. In the slipped arrangement, the
methyl groups of peripheral substituents penetrate into the
Figure 3. Concentration-dependent emission spectra of 5a in CH2Cl2 at 22 °C. The
emission spectrum of 5a in solid state is shown as yellow line.
Figure 4. PM3 geometry optimized structure of 5a dimer.

4872
V. J. Chebny et al. / Tetrahedron Letters 49 (2008) 4869–4872
aromatic ring to allow favorable CH–
p
interactions. It is unclear
Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-336033 or e-
whether CH–
tronic interaction, clearly
investigation.
p
interactions might also be able to mediate this elec-
mail: deposit@ccdc.cam.ac.uk). Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.06.005.
a hypothesis warranting further
In summary, we have developed a versatile synthesis of soluble
hexa-peri-hexabenzocoronenes from an easily synthesized hexa-
kis(4-acetylphenyl)benzene as a common precursor. Various HBC’s
undergo reversible electro-chemical oxidations and form stable
monomeric cation radicals in solution. The emission spectra of
the hexa-substituted HBC’s are concentration-dependent and show
an excimer-based emission both in solution at higher concentra-
tions and in solid state. We are continuing attempts to obtain suit-
able single crystals of various HBC cation radicals and their co-
crystals with various neutral aromatic donors such as substituted
benzenes, naphthalenes, triphenylenes, and pyrenes for X-ray crys-
tallographic studies and evaluation of their solid-state properties.
References and notes
1. (a) Harvey, R. G. Polycyclic Aromatic Hydrocarbons; Wiley: New York, 1996; (b)
Baumgarten, M.; Müllen, K. Top. Curr. Chem. 1994, 169, 1 and references cited
therein.
2. Clar, E. The Aromatic Sextet; Wiley: London, 1972.
3. (a) Berresheim, A. J.; Müller, M.; Müllen, K. Chem. Rev. 1999, 99, 1747; (b)
Watson, M. D.; Fechtenkotter, A.; Müllen, K. Chem. Rev. 2001, 101, 1267 and
references cited therein.
4. Sadhukhan, S. K.; Viala, C.; Gourdon, A. Synthesis 2003, 1521.
5. Rathore, R.; Burns, C. L. J. Org. Chem. 2003, 68, 4071.
6. Nelsen, S. F.; Weaver, M. N.; Yamazaki, D.; Komatsu, K.; Rathore, R.; Bally, T. J.
Phys. Chem. A 2007, 111, 1667–1676.
7. Mio, M. J.; Kopel, L. C.; Braun, J. B.; Gadzikwa, T. L.; Hull, K. L.; Brisbois, R. G.;
Markworth, C. J.; Grieco, P. A. Org. Lett. 2002, 4, 3199.
8. See: CCD database.
Acknowledgments
9. Bell, F. A.; Ledwith, A.; Sherrington, D. C. J. Org. Chem. 1976, 13, 155.
10. Rathore, R.; Burns, C. L.; Deselnicu, M. I.; Denmark, S. E.; Bui, T. Org. Synth. 2005,
82, 1.
11. (a) Kochi, J. K.; Rathore, R.; Magueres, P. L. J. Org. Chem. 2000, 65, 6826; (b)
Hara, M.; Tojo, S.; Kawai, K.; Majima, T. Phys. Chem. Chem. Phys. 2004, 6,
3215.
12. (a) Wyszecki, G.; Stiles, W. S. Color Science: Concepts and Methods. In
Quantitative Data and Formulae, 2nd ed.; Wiley-Interscience: New York,
2000; (b) Commission Internationale de l’Eclairage Proceedings, 1931.
Cambridge: Cambridge University Press.
We thank the National Science Foundation for a Career Award
(R.R.) and the Petroleum Research Fund, administered by the
American Chemical Society, for financial support, and Sergey V.
Lindeman (Marquette University) for X-ray crystallography.
Supplementary data
13. (a) Winnik, F. M. Chem. Rev. 1993, 93, 587; (b) Echeverría, C. G. J. Am. Chem. Soc.
1994, 116, 6031.
14. Herwig, P. T.; Enkelmann, V.; Schmelz, O.; Müllen, K. Chem. Eur. J. 2000, 6, 1834.
15. Rathore, R.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 9393 and
references cited therein.
Synthetic details, spectral data, and NMR spectra of various
compounds in Schemes 1 and 2. Crystallographic data (excluding
structure factors) for 2 have been deposited with the Cambridge
Crystallographic Data Centre CCDC-686846. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
16. (a) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565; (b) Hunter, C. A. Angew. Chem.,
Int. Ed. 2004, 43, 5310.