Unusual molecular conformations in fluorinated, contorted hexabenzocoronenes

Article abstract of DOI:10.1021/ol102016m

Fluorinated, contorted hexabenzocoronenes (HBCs) have been synthesized in a facile manner via Suzuki-Miyaura coupling of fluorinated phenyl boronic acids followed by photocyclization and Scholl cyclization. In addition to the molecular conformation observ

Full text of DOI:10.1021/ol102016m

ORGANIC
LETTERS
2010
Vol. 12, No. 21
4840-4843
Unusual Molecular Conformations in
Fluorinated, Contorted
Hexabenzocoronenes
Yueh-Lin Loo,*,† Anna M. Hiszpanski,† Bumjung Kim,‡ Sujun Wei,‡
Chien-Yang Chiu,‡ Michael L. Steigerwald,‡ and Colin Nuckolls*,‡
Department of Chemical & Biological Engineering, Princeton UniVersity,
Princeton, New Jersey 08544, United States, and Department of Chemistry and the
Center for Electronics of Molecular Nanostructures, Columbia UniVersity,
New York, New York 10027, United States
lloo@princeton.edu; cn37@columbia.edu
Received August 25, 2010
ABSTRACT
Fluorinated, contorted hexabenzocoronenes (HBCs) have been synthesized in a facile manner via Suzuki-Miyaura coupling of fluorinated
phenyl boronic acids followed by photocyclization and Scholl cyclization. In addition to the molecular conformation observed in previous HBC
derivatives, close-contact fluorine-fluorine intramolecular interactions result in a metastable conformation not previously observed. Heating
the metastable HBCs above 100 °C irreversibly converts them to the stable conformation, suggesting that the metastable conformation arises
from a kinetically arrested state during cyclization.
Hexabenzocoronenes (HBCs) have attracted significant sci-
entific attention given their interesting electronic and self-
assembly properties. Both planar and contorted HBCs, for
example, have been incorporated as active layers in organic
thin-film transistors; devices comprising unsubstituted planar
HBC1 and dodecyloxy-substituted contorted HBC2 exhibit
hole mobilities of the order of 100 and 10-2 cm2/V·s,
respectively. These materials also have a strong tendency to
self-assemble into supramolecular structures; both planar and
contorted HBCs readily pack in a columnar fashion to
generate nanorods and wires affording field-effect transis-
tors.1-3 Unlike planar HBCs, whose structures resemble
small sections of graphene, the molecular conformation of
contorted HBCs is distorted from planarity by 20° on its
(2) Xiao, S.; Myers, M.; Miao, Q.; Sanaur, S.; Pang, K.; Steigerwald,
† Princeton University.
M. L.; Nuckolls, C. Angew. Chem,. Int. Ed. 2005, 117, 7556–7560.
‡ Columbia University.
(3) Xiao, S.; Tang, J.; Beetz, T.; Guo, X.; Tremblay, N.; Siegrist, T.;
(1) van de Cratts, A. M.; Warman, J. M.; Fechtenko¨tter, A.; Brand, J. D.;
Harbison, M. A.; Mu¨llen, K. AdV. Mater. 1999, 11, 1469–1472.
Zhu, Y.; Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128,
10700–10701
.
10.1021/ol102016m  2010 American Chemical Society
Published on Web 10/05/2010

periphery due to steric congestion between the proximal
C-H bonds. This doubly concave conformation can thus
provide unique opportunities for complexation with geo-
metrically complementary compounds, such as C60.4
out a double Corey-Fuchs11 reaction on fluorinated penta-
cenequinone to provide a precursor to the fluorinated,
contorted HBCs shown in Figure 1. This reaction yields a
The addition of fluorenyl units5 and oligothiophenes6 at
the peripheral aromatic rings of planar and contorted HBCs,
respectively, has been shown to alter the optical absorbance
and the HOMO and LUMO energy levels of the parent
compound in addition to improving the molecular packing
in the solid state. In the same vein, select fluorination of the
outer aromatic rings of planar HBC is found to enhance the
molecule’s electron-withdrawing nature;7 fluorinated planar
HBC is reported to transport electrons with a mobility of
10-2 cm2/V·s.8 More interestingly, this compound is shown
to adopt a face-to-face type packing motif in the solid state
rather than the herringbone structure that is found in its parent
compound due to the larger van der Waals radius of fluorine.8
Inspired by the work of Mori et al.,8 we wanted to examine
the influence of fluorine-fluorine intramolecular interactions
on the molecular conformation of contorted HBCs given their
already-unusual doubly concave conformation.
Figure 1. Chemical structures of 8F- (1a), 12F- (1b), 16F- (1c),
and 20F-HBC (1d). Close fluorine-fluorine intramolecular contacts
(<2.6 Å) are highlighted in red for 1c and 1d.
In this study, we report the synthesis and characterization
of a series of contorted HBCs with differing amounts of
fluorination on its exterior aromatic rings.9 Functionalized
contorted HBCs have been previously realized via double
Barton-Kellogg reactions of the appropriate pentacene-
quinones followed by photocyclization or Scholl cycliza-
tion.10 The synthetic scheme used here is different than those
employed in the past and is shown in Scheme 1. We carried
tetrabrominated intermediate, 2. Subjecting 2 to a Suzuki-
Miyaura12 reaction with the appropriate fluorinated phenyl
boronic acid (3a-d) yielded the desired bisolefin compound,
4a-d. We then employed the Katz-modified Mallory pho-
tocyclization13,14 on 4a-d; the products from these reactions
consisted of a mixture of half-cyclized and fully cyclized
HBCs (1a-d). Given the solubility differences between the
half- and fully cyclized HBCs, we isolated the half-cyclized
products and imposed upon them Scholl cyclizations per
Plunkett et al.9 to yield the fully cyclized products, 1a′-d′.
Given the wide availability of functionalized phenyl boronic
acids, this route brings about tremendous flexibility and
modularity to the synthesis of contorted HBCs with different
substitution.
Scheme 1
.
General Strategy for Synthesizing Fluorinated,
Contorted HBCs
X-ray crystallography indicates that the fluorinated, con-
torted HBCs that result from photocyclization adopt molec-
ular conformations that are not significantly different from
that of their hydrogen-substituted counterpart. They adopt,
for example, the doubly concave conformation that charac-
terizes contorted hexabenzocoronenes.2-4 At first blush, it
thus appears that substituting hydrogens with fluorines on
the peripheral aromatic rings of contorted HBCs does not
affect their molecular conformations.
(4) Tremblay, N. J.; Gorodetsky, A. A.; Cox, M. P.; Schiros, T.; Kim,
B.; Steiner, R.; Bullard, Z.; Sattler, A.; So, W.-Y.; Itoh, Y.; Toney, M. F.;
Ogasawa, H.; Ramirez, A. P.; Kymissis, I.; Steigerwald, M. L.; Nuckolls,
C. ChemPhysChem 2010, 11, 799–803.
1
The H NMR spectrum of 8F-HBC obtained upon Scholl
cyclization (1a′) is in all respects identical to that of 8F-HBC
obtained after photocyclization (1a), indicating that 1a′ and 1a
are chemically and conformationally indistinguishable. We
(5) Wong, W. W. H.; Singh, T. B.; Vak, D.; Pisula, W.; Yan, C.; Feng,
X.; Williams, E. L.; Chan, K. L.; Mao, Q.; Jones, D. J.; Ma, C.-Q.; Mu¨llen,
K.; Bau¨erle, P.; Holmes, A. B. AdV. Funct. Mater. 2010, 20, 927–938.
(6) Unpublished data.
(7) Etani, S.; Kaji, T.; Ikeda, S.; Mori, T.; Kikuzawa, Y.; Takeuchi, H.;
Saiki, K. J. Phys. Chem. C 2009, 113, 6202–6207.
(11) Neidlein, R.; Winter, M. Synthesis 1998, 9, 1362–1366.
(12) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457–2483. (b)
Bauer, A.; Miller, M. W.; Vice, S. F.; McCombie, S. W. Synlett 2001, 2,
(8) Mori, T.; Kikuzawa, Y.; Takeuchi, H. Org. Electron. 2008, 9, 328–
332.
(9) Gorodetsky, A. A.; Chiu, C. Y.; Schiros, T.; Palma, M.; Sattler, W.;
Kymissis, I.; Steigerwald, M.; Nuckolls, C. Angew. Chem., Int. Ed. published
online Sept. 16, 2010; DOI: 10.1002/anie.201004055.
254–256
(13) Mallory, F. B.; Wood, C. S.; Gordon, J. T.; Lindquist, L. C.; Savitz,
M. L. J. Am. Chem. Soc. 1962, 84, 4361–4362
(14) Liu, L. B.; Yang, B. W.; Katz, T. J.; Poindexter, M. K. J. Org.
Chem. 1991, 56, 3769–3775
.
.
(10) Plunkett, K. N.; Godula, K.; Nuckolls, C.; Tremblay, N.; Whalley,
A. C.; Xiao, S. Org. Lett. 2009, 11, 2225–2228.
.
Org. Lett., Vol. 12, No. 21, 2010
4841

found this observation to hold true for 12F-HBC also. This
observation, however, does not hold true for 16F-HBC or 20F-
HBC, both of which have close intramolecular fluorine-fluorine
contacts (<2.6 Å). The NMR spectra of 16F-HBC obtained after
photocyclization (1c) and after Scholl cyclization (1c′) are
shown in Figure 2. We observe two sets of resonances
that the outer aromatic rings in 1c alternate into and out of
the plane defined by the coronene core, resulting in a doubly
concave structure that is similar to those exhibited by other
contorted HBCs.2-4 The molecular conformation of 1c′, on
the other hand, has not previously been observed. In
particular, the perfluorinated outer aromatic rings originating
from fluorinated pentacenequinone are contorted out of the
plane defined by the coronene core in the same direction,
resulting in a saddle-like conformation (see the molecular
models in Figure 3 for comparison of the positions of the
outer aromatic rings originating from the fluorinated penta-
cenequinone precursor). The exterior aromatic rings intro-
duced by coupling with 3c are displaced in the opposite
direction, presumably to minimize close fluorine-fluorine
intramolecular contact (highlighted in red for 16F-HBC in
Figure 1). In the solid state, both 1c and 1c′ adopt monoclinic
crystal structures having the P21/C space group with slightly
different unit cell dimensions.15
Minimization of free energy of these molecular conforma-
tions via density functional theory (DFT) calculations
indicates that the armchair structure of 1c depicted in the
left of Figure 3 sits at a lower energy compared to the saddle
structure of 1c′ (right of Figure 3). Indeed, variable-
temperature NMR experiments carried out on 1c′ indicate
that heating above 100 °C induces a transformation in
molecular conformation to that adopted by 1c. This trans-
formation is irreversible; cooling to room temperature does
not allow reversion back to its original conformation.
To quantitatively examine the transformation from the
metastable conformation of 1c′ to that of 1c, we monitored
the kinetics of this process isothermally via NMR. Figure 4
Figure 2.
1H NMR spectra of 1c (top) and 1c′ (bottom). The spectra
have been corrected against the reference peak of tetrachloroethane
at 6.00 ppm.
associated with the two proton environments in 16F-HBC. The
triplet at 7.4 ppm in the NMR spectrum of 1c is assigned to
the protons on the peripheral aromatic rings that are sandwiched
between fluorine substituents on the same rings. The doublet
at 8.7 ppm is attributed to the less shielded protons of 16F-
HBC. Integrating the proton resonances confirms equal con-
tributions to the NMR spectrum of 16F-HBC. The NMR
spectrum of 1c′ appears to be similar to that of 1c, but both the
triplet and the doublet in the spectrum of 1c′ are shifted
downfield relative to those of 1c at the same concentration.
Comparison of the placement of the proton resonance of the
reference solvent (tetrachloroethane; δ ) 6.0 ppm) indicates
that this downfield shift seen in the spectrum of 1c′ is not an
experimental artifact; rather, it reflects real differences in the
molecular conformations of 1c and 1c′. We have also carried
out NMR experiments on mixtures of 1c and 1c′; both sets of
resonances are present, further confirming that the differences
in chemical shifts stem from differences in molecular conforma-
tions.
Figure 3 shows the molecular conformations of 1c and
1c′ observed in the crystal structure of each. We observe
Figure 4.
1H NMR spectra acquired in 5 min intervals during
isothermal transformation of 1c′ at 110 °C.
contains sequential NMR spectra collected of 1c′ at 110 °C
in 5 min intervals. As time progresses, we observe a steady
increase in the intensities of the proton resonances associated
with 1c; this increase occurs concomitantly with a decrease
in intensity of the original proton resonances associated with
Figure 3. Side view of 1c (left) and 1c′ (right) based on X-ray
crystallography. Fluorine is depicted in red. Hydrogens have been
removed for clarity. The stick figures below illustrate the conforma-
tions of the ring planes originating from the fluorinated pentacene-
quinone precursor.
(15) The unit cell dimensions for 1c: a ) 12.966 Å; b ) 8.566 Å; c )
14.310 Å; ꢀ ) 90.27°. The unit cell dimensions for 1c′: a ) 14.740 Å; b
) 13.624 Å; c ) 16.060 Å; ꢀ ) 99.12°.
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Org. Lett., Vol. 12, No. 21, 2010

1c′. In fact, collapsing the spectra in Figure 4 reveals an
isosbestic point at 7.47 ppm indicating that one species is
converting into another. The conversion from the metastable
to stable conformations is complete after 85 min. We also
tracked the growth of the proton resonances associated with
the more stable conformation; this quantity is plotted in
Figure 5 for the isothermal transformation at 110 °C. The
tion, the bisolefin intermediates 4a-d first undergo half-
cyclization before complete ring closure.9 Given the numer-
ous degrees of rotational freedom of the Suzuki-Miyaura
intermediates, the half-cyclized species are likely to sample
a number of conformations. We speculate that only confor-
mations having minimal steric hindrance between fluorines
on adjacent aromatic rings are capable of complete ring
closure during photocyclization. Ring closure of the remain-
ing half-cyclized speciessdue to additional fluorine-fluorine
steric hindrancesonly occurs during the more energetic
Scholl cyclization. This hypothesis is consistent with our
observation that photocyclization always yields a mixture
of half- and fully cyclized fluorinated, contorted HBCs.
Extending the photocyclization reaction does not further
convert the half-cyclized product into the final product.
In summary, we have demonstrated a facile and modular
method for synthesizing contorted HBC having varying
extents of fluorination. Close fluorine-fluorine intramolecu-
lar contact results in a metastable molecular conformation
not previously observed. Our study highlights the intricacies
of fluorine substitution and hints at the possibility of
accessing unusual and metastable molecular conformations
through fine-tuning of intramolecular steric hindrance.
Figure 5. Rate of transformation from the metastable to the stable
conformations of 16F-HBC at 110 °C. The red line represents the
first-order fit to the data.
Acknowledgment. We thank Rawad Hallani and Prof.
John Anthony (Chemistry, University of Kentucky) for the
fluorinated pentacenequinone and Wesley Sattler (Chemistry,
Columbia) for X-ray crystallography. Y.L.L. acknowledges
financial support from the PIRE Program (DMR-0730243)
and the MRSEC Program through Princeton’s PCCM (DMR-
0819860) at the NSF and the Photovoltaics Program at the
ONR (N000140811175). A.M.H. is supported by an NSF
Nanotechnology for Clean Energy IGERT Traineeship
(0903661). Funding through CHE-0619638 for the acquisi-
tion of the X-ray diffractometer is appreciated. Portions of
this work (CN) were funded by the Center for Re-Defining
Photovoltaic Efficiency Through Molecule Scale Control, an
Energy Frontier Research Center funded by the U.S. DOE,
OfficeofScience,OfficeofBasicEnergySciences(DESC0001085),
and by the Chemical Sciences, Geosciences and Biosciences
Division (DEFG02-01ER15264).
data are well described by first-order kinetics; fitting yields
a rate constant of 0.0004 s-1 or a characteristic half time of
1700 s. Fitting the rate constants obtained at several
temperatures to the Arrhenius equation yielded an energy
barrier of 39 kcal/mol for this transformation. Given that
the gauche-to-trans transformation of C-C bonds is esti-
mated to require 3-6 kcal/mol,16 an energy barrier of 39
kcal/mol seems reasonable for the transformation between
the two conformations of 16F-HBC given the significant
rearrangement of C-C bonds that needs to take place.
We note that the fully cyclized 20F-HBC product upon
photocyclization (1d) adopts a molecular conformation that
resembles that of 1c and those of prior contorted HBCs. The
Scholl cyclized product of 1d′, akin to 1c′, adopts a molecular
conformation that is less stable than that of 1d and undergoes
transformation back to the more stable form on heating.
Examining the chemical structures of 16F- and 20F-HBC,
we notice that both compounds share close fluorine-fluorine
intramolecular contacts (highlighted red in Figure 1). We
believe it is the steric hindrance between adjacent fluorinated
aromatic rings during ring closure that gives rise to the
different molecular conformations.17 During photocycliza-
Supporting Information Available: Procedures and
NMR spectra of compounds. CIF files for 1c and 1c′. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL102016M
(16) Wiberg, K. B.; Murcko, M. A. J. Am. Chem. Soc. 1988, 110, 8029–
8038.
(17) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.;
Grubbs, R. H. Angew. Chem., Int. Ed. 1997, 36, 248–251.
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