Orthogonal functionalization of cyclopenta[hi]aceanthrylenes

Article abstract of DOI:10.1021/ol400093g

A synthetic strategy to prepare 2,7- or 4,9-functionalized cyclopenta[hi]aceanthrylenes that are capable of Suzuki cross-coupling reactions is demonstrated. This method has been utilized to create a series of thiophene derivatized compounds that were subsequently used to investigate the role of substitution pattern on the photophysical and electronic properties of cyclopenta[hi]aceanthrylenes. The orthogonal functionalization provides access to unique substitution patterns (e.g., cruciform-like architectures) and materials with small optical band gaps (1.22-1.97 eV).

Full text of DOI:10.1021/ol400093g

ORGANIC
LETTERS
2
013
Vol. 15, No. 6
202–1205
Orthogonal Functionalization of
Cyclopenta[hi]aceanthrylenes
1
Che-Hsiung Lee and Kyle N. Plunkett*
Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale,
Illinois 62901, United States
kplunkett@chem.siu.edu
Received January 11, 2013
ABSTRACT
A synthetic strategy to prepare 2,7- or 4,9-functionalized cyclopenta[hi]aceanthrylenes that are capable of Suzuki cross-coupling reactions is
demonstrated. This method has been utilized to create a series of thiophene derivatized compounds that were subsequently used to investigate
the role of substitution pattern on the photophysical and electronic properties of cyclopenta[hi]aceanthrylenes. The orthogonal functionalization
provides access to unique substitution patterns (e.g., cruciform-like architectures) and materials with small optical band gaps (1.22À1.97 eV).
2
CP-PAH scaffolds based on indenofluorenes, corannulene,
3
Cyclopentafused-polycyclic aromatic hydrocarbons (CP-
PAHs) possess unique properties including high electron
affinities and interesting photophysical properties that have
4
pentalenes, emeraldicene, and cyclopenta[hi]aceanthryl-
5
6
ene scaffolds to name a few. Some of these systems have
1
utility for organic electronic devices. However, successful
recently been incorporated into devices and behave as
f,7
6
postmodification strategies that provide access to small-
molecule CP-PAH conjugates have only recently been devel-
oped. New synthetic advances have led to functional
ambipolar semiconductors in field-effect transistors or
electron acceptors (and fullerene substitutes) in organic
8
photovoltaics. There is still a critical need, however, to
develop chemistry that can facilitate the creation of
(
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1
0.1021/ol400093g r 2013 American Chemical Society
Published on Web 03/05/2013

Scheme 1. Synthesis of Thiophene-Conjugated CP-PAHs
libraries of compounds for the screening of materials
properties. With the apparent utility of the cyclopenta-
[
hi]aceanthrylene scaffold for electronic applications, we
set out to create a series of analogs based on thiophene
conjugates. In this report, we demonstrate an orthogonal
functionalization strategy to selectively modify a cyclopenta-
[
(
hi]aceanthrylene core on either the five-membered rings
2- and 7-positions), six-membered rings (4- and 9-positions),
or both positions in series. We further show that the resulting
optical and electrochemical properties of the CP-PAH are
greatly influenced by the location of substitution.
As demonstrated previously, the formation of the core
,7-bis(trimethylsilyl)cyclopenta[hi]aceanthrylene (1) can
2
be accomplished through a palladium-catalyzed cyclopen-
tenalation reaction between 9,10-dibromoanthracene and
6b,c,9
trimethylsilyl)acetylene (TMS-acetylene).
(
Mild bromi-
nation with N-bromosuccinimide (NBS) at the ipso-position
of the TMS group creates a universal cross-coupling partner (2)
for functionalization on the five-membered rings (Scheme 1).
We have previously shown that a simple Sonogashira cross-
coupling reaction at these locations could create both low
Figure 1. Normalized absorbance of 1 (gray), 3b (green), 5b
(red), and 7 (purple) in THF.
1
2
Using CÀH activation with an iridium catalyst, boronate
esters have been installed at the two least sterically hindered
carbons of 1 to produce a cyclopenta[hi]aceanthrylene that
can be further functionalized at the six-membered rings (4).
We utilized the Suzuki cross-coupling conditions to create
the comparable thiophene-containing analogs (5a,b) in
tolerable yields for a two-step borylation/Suzuki cross-
coupling procedure. The residual TMS groups on these
compounds provided an opportunity to create a cruci-
form-like structure with functionalization at both the six-
and five-membered ring locations. Ipso bromination of the
TMS group with NBS provided the brominated species (6),
which was carried forward with one last Suzuki cross-coupling
9
band gap small molecules and donorÀacceptor copoly-
1
0
mers that behave as electron acceptors. In this work, we
show the bromide can be utilized for a Suzuki cross-coupling
reaction to create thiophene-containing analogs (3a,b), which
are functionalized at the five-membered rings, in moderate
yields. The reduced yields for these compounds, as well as
others in this work, are due to slow decomposition on silica
1
1
gel during purification.
A secondary modification strategy, which leads to ortho-
gonally functionalized CP-PAHs, has also been developed.
(
8) (a) Brunetti, F. G.; Gong, X.; Tong, M.; Heeger, A. J.; Wudl, F.
Angew. Chem., Int. Ed. 2010, 49, 532–536. (b) Gong, X.; Tong, M.;
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Mater. 2011, 23, 2272–2277.
(11) Once purified, all compounds reported in this manuscript pro-
duce air and light stable solids and solutions.
(
9) Wood, J. D.; Jellison, J. L.; Finke, A. D.; Wang, L.; Plunkett,
K. N. J. Am. Chem. Soc. 2012, 134, 15783–15789.
10) Jellison, J. L.; Lee, C.-H.; Zhu, X.; Wood, J. D.; Plunkett, K. N.
(12) (a) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi,
N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390–391. (b) Cho,
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2002, 295, 305–308.
(
Angew. Chem., Int. Ed. 2012, 51, 12321–12324.
Org. Lett., Vol. 15, No. 6, 2013
1203

to provide the tetrathiophene substituted cyclopenta[hi]-
aceanthrylene cruciform (7).
with a much larger band gap. For 5b, the thin-film ab-
sorbance onset is 632 nm (Supporting Information), which
represents an optical band gap of 1.96 eV, and is ∼370 nm
larger (∼0.7 eV) than that for 3b. Cyclic voltammetry (CV)
ofeachcompound provided some insight into the dramatic
difference in energies. The voltammograms (Supporting
Information) showed little change in the reduction poten-
tial among each of the compounds (Table 1) with all
LUMO energies occurring between À3.53 and À3.57 eV.
The Density Functional Theory (DFT) calculated structures
support this result with the LUMO frontier orbitals of each
compound identical and localized on the cyclopenta[hi]-
aceanthrylene core (Figure 2). The oxidation potentials,
however, were dramatically affected by the substitution loca-
tion. The DFT calculated HOMO contours for 3 and 5 show
the molecular orbitals distributed through each of the cores
and out onto the thiophene appendages. The HOMO for 3b,
however, was À5.03 eV while the six-membered ring sub-
stituted 5b HOMO was much more stabilized at À5.49 eV.
The cruciform-like tetrathiophene 7 possesses a combi-
nation of the electronic properties of the simpler dithio-
phene-substituted analogs (e.g., 3b and 5b) as evident by
the absorption spectra (Figure 1). For 7, the absorption
band centered at 550 nm is similar in energy to the major
long-wave absorption of 5b (530 nm) while the longest
wavelength absorption matches the absorption of 3b (720
nm). The DFT calculated structure is in agreement with
these observations with the LUMO contour remaining
localized on the core much like the LUMOs of 3 and 5. The
HOMO of 7 is visually and energetically similar to 3 while
the HOMOÀ1 is similar to the HOMO of 5. This combi-
nation of absorption transitions leads to a dark purple
material. The fluorescence emission properties of cruci-
form 7 were also intermediate between those of 3a,b and
5a,b. The fluorescence quantum yield was found to be
heavily influenced by substitution location. CP-PAHs
The optical and electrochemical properties of the thio-
phene-conjugated cyclopenta[hi]aceanthrylenesare greatly
influenced by the functionalization location. Substitution
at the five-membered rings (3a,b) creates dark green
materials with relatively small band gaps as demonstrated
by the absorption spectrum in solution (Figure 1) and in
thin film (Supporting Information). The saturated green
color is due to the simultaneous absorption of both blue
(
below 500 nm) and red (above 600 nm) light. For 3b, the
thin film absorbance onset is 1008 nm, which represents an
optical band gap of 1.23 eV (Table 1). Although the
5
a
structures of 3a,b are very similar tothat of emeraldicene,
which is a fused thiophene analog of cyclopenta-
hi]aceanthrylene, the band gaps of the materials presented
[
here are considerably lower. When the thiophene substitu-
tion is switched to the six-membered rings (5a,b), the
materials are visibly different and produce dark red solids
a
Table 1. Summary of Electrochemical and Optical Properties
E
gap (eV)
HOMO
(eV)
LUMO
(eV)
b
c
compd
e-chem
optical
Φ
3a
3b
5a
5b
7
À5.12
À5.03
À5.66
À5.49
À5.08
À3.57
À3.55
À3.53
À3.53
À3.60
1.55
1.48
2.13
1.96
1.48
1.23
1.23
1.97
1.96
1.22
À
À
0.033
0.013
0.0014
a
HOMO and LUMO values calculated from the cyclic voltammo-
grams (Supporting Information) on the basis of the oxidation of the
b
ferrocene reference in vacuum (4.8 eV). Optical band gap calculated
c
from the onset of absorption in thin film (Supporting Information). Quan-
tum yields were calculated relative to anthracene in ethanol (Φ = 0.27).
6
c,9,13
often have reduced or absent fluorescence,
and we
Figure 2. DFT calculated HOMO (bottom) and LUMO (top) contours of 3, 5, and 7 calculated at the B3LYP/6-31G** level.
1
204
Org. Lett., Vol. 15, No. 6, 2013

found that the five-member ring substituted 3a,b showed
no measurable fluorescence. However, when the substitu-
tion was moved to the six-membered ring, we were able to
measure a small, yet noticeable fluorescence quantum yield
demonstrate the importance of functionalization location
on the CP-PAH core. These strategies should provide
access to a variety of well-defined small molecules, as well
as polymeric materials, for utility in organic electronic
applications.
(
Φ) of 0.033 and 0.013 for compounds 5a and 5b, respec-
tively. The cruciform 7, with substitutions on both the five-
and six-membered rings, had a small, intermediate quan-
tum yield of 0.0014.
Acknowledgment. K.N.P. thanks Southern Illinois
University for startup funds and a seed grant.
In conclusion, we have demonstrated an orthogonal
synthetic strategy for the discrete functionalization of
cyclopenta[hi]aceanthrylene. Using this method, we have
created a series of thiophene-substituted analogs that
Supporting Information Available. Experimental de-
tails, NMR spectra, cyclic voltammetry, and UVÀvis of all
compounds in solution and thin film. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(
13) (a) Plummer, B. F.; Hopkinson, M. J.; Zoeller, J. H. J. Am.
Chem. Soc. 1979, 101, 6779–6781. (b) Lin, S.-K.; Shiu, L.-L.; Chien,
K.-M.; Luh, T.-Y.; Lin, T.-I. J. Phys. Chem. 1995, 99, 105–111.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 6, 2013
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