Tunable electron acceptors based on cyclopenta[hi]aceanthrylenes

Article abstract of DOI:10.1016/j.tetlet.2015.11.022

A series of substituted cyclopenta[hi]aceanthrylene derivatives with electron donating (NH₂, OCH₃), neutral (H), and electron withdrawing (COOH, CF₃, CN, NO₂) substituents were prepared. A room-temperature Sonogashira cross-coupling reaction between 2,7-dibromocyclopenta[hi]aceanthrylene and an appropriately functionalized phenylene ethynylene precursor was utilized to access the materials that were characterized by Nuclear Magnetic Resonance Spectroscopy (NMR), cyclic voltammetry (CV), and UV-Vis spectroscopy. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were systematically varied when proceeding from electron donating to electron withdrawing substituents. The optical band gap was significantly altered for the most electron donating species, while little change was observed between different electron withdrawing substituents. This study demonstrates the ability to control the frontier orbital energies of this class of cyclopenta-fused polycyclic aromatic hydrocarbon materials through selective substitution.

Full text of DOI:10.1016/j.tetlet.2015.11.022

Tetrahedron Letters 56 (2015) 7105–7107
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Tunable electron acceptors based on cyclopenta[hi]aceanthrylenes
y
⇑
Xinju Zhu , Bingxin Yuan, Kyle N. Plunkett
Department of Chemistry and Biochemistry and The Materials Technology Center, Southern Illinois University, Carbondale, IL 62901, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2015
Revised 3 November 2015
Accepted 8 November 2015
Available online 10 November 2015
A series of substituted cyclopenta[hi]aceanthrylene derivatives with electron donating (NH2, OCH3),
neutral (H), and electron withdrawing (COOH, CF3, CN, NO2) substituents were prepared. A room-temper-
ature Sonogashira cross-coupling reaction between 2,7-dibromocyclopenta[hi]aceanthrylene and an
appropriately functionalized phenylene ethynylene precursor was utilized to access the materials that
were characterized by Nuclear Magnetic Resonance Spectroscopy (NMR), cyclic voltammetry (CV), and
UV–Vis spectroscopy. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) were systematically varied when proceeding from electron donating to electron
withdrawing substituents. The optical band gap was significantly altered for the most electron donating
species, while little change was observed between different electron withdrawing substituents. This
study demonstrates the ability to control the frontier orbital energies of this class of cyclopenta-fused
polycyclic aromatic hydrocarbon materials through selective substitution.
Keywords:
Cyclopenta-fused
Polycyclic aromatic hydrocarbons
Electron acceptors
Organic electronics
Ó 2015 Elsevier Ltd. All rights reserved.
Introduction
clopenta[hi]aceanthrylene16 and phenylene ethynylenes21 with
varying electron withdrawing or donating substituents in the para
Cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs)
are effective electron acceptors owing to their ability to form aro-
matic cyclopentadienyl anion-like structures in their reduced
state.1 Over the past several years, new CP-PAH scaffolds that allow
systematic variation in substituent structure have been reported.2–7
These substitution strategies provide opportunities to tune frontier
orbitals, such as the Lowest Unoccupied Molecular Level (LUMO),
to beneficially interface with donor materials in organic photo-
voltaics8–10 or to access stable ambipolar or n-type organic field-
effect transistors (OFETs).11–15 Here we demonstrate that the
electronic properties of cyclopenta[hi]aceanthrylenes (CPAA)
based materials can be systematically varied depending on the
substituent constant of an attached arylene ethynylene.
We have previously utilized the CPAA scaffold to make both
small molecule16–18 and polymeric materials19 with relatively sta-
bilized LUMOs (ꢀꢁ3.6 eV). In this contribution, we show that by
varying the electronic structure on the substituent rings, we can
tune the HOMO and LUMO as well as the optical band gap of the
material in a controlled fashion. We have employed phenylene
ethynylene substitution of the CPAA core to facilitate electronic
coupling via the alkyne linker. The synthetic strategy employs a
Sonogashira cross-coupling reaction20 between 2,7-dibromocy-
position (Scheme 1). We have utilized the catalyst system of
Pd(PhCN)2Cl2 and P(tBu)3 to perform these cross-couplings
reactions and the purification of the resulting materials is straight
forward via a simple precipitation and filtration to give high yields
of the substituted compounds.
All substituted CPAA compounds are dark green in the solid
state, produce emerald green solutions, and share similar absorp-
tion profiles (Fig. 1). Each UV–Vis trace shows a characteristic high
energy absorption (280–380 nm,
energy absorption (530–880 nm,
e
ꢀ30,000 Mꢁ1 cmꢁ1) and low
e
ꢀ10,000 Mꢁ1 cmꢁ1) with some
variation depending on substituent composition. The onset of the
longest wavelength absorption, which is related to the electronic
band gap, can be influenced by the nature of the phenyl sub-
stituent. While, strongly withdrawing para-substituents (NO2, CN,
CF3, COOH) do not appreciably change the onset of this band, the
more electron rich unsubstituted (H) and electron donating sub-
stituents (OCH3 and NH2) do significantly red-shift the absorption
band in accordance with the strength of the donor substituent.
These observations are in agreement with known donor–acceptor
diads and triads that show reduced band gaps with stronger donor
and/or acceptor strengths.22–25 The optical band gap ranges from
1.43 eV to 1.61 eV for the most electron rich to the most electron
poor substituent, respectively (Table 1). As found with previous
2,7-difunctionalized CPAAs, as well as other CP-PAHs,4,26,27 1–7
are not fluorescent. The crystal structure of 4 shows the aryl groups
are orthogonal to the plane of the CPAA core (Fig. 3). This arrange-
ment could help explain the lack of optical gap variation for
⇑
Corresponding author. Tel.: +1 618 453 2758; fax: +1 618 453 6408.
E-mail address: kplunkett@chem.siu.edu (K.N. Plunkett).
Current Address: Department of Chemistry, Zhengzhou University, Zhengzhou,
y
China.
http://dx.doi.org/10.1016/j.tetlet.2015.11.022
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

7106
X. Zhu et al. / Tetrahedron Letters 56 (2015) 7105–7107
X
1
2
3
4
5
6
7
NH2
OCH3
H
Br
X
X =
COOHa
CF3
Pd(PhCN)Cl2
P(tBu)3, iPrNH, CuI
Toluene, 50 o
CN
NO2
C
Br
X
Scheme 1. Sonogashira cross-coupling conditions to prepare tunable cyclopenta
[hi]aceanthrylenes. aObtained from ethyl ester via saponification.
Figure 2. Cyclic voltammograms of 1–7 in THF with 0.1 M tetrabutylammonium
hexafluorophosphate, glassy carbon electrode, platinum counter electrode, and an
Ag/AgCl reference electrode. Scan rate = 50 mV/s. Ferrocene added as internal
standard and referenced to 0V.
Figure 1. Absorbance of phenylene-ethynylene substituted cyclopenta[hi]
aceanthrylenes.
Table 1
Summary of electrochemical and optical propertiesa
Cmpd Substituent Eox/onset Ered/onset HOMO
LUMO
(eV)
Optical Egap
(eV)
(V)
(V)
(eV)
1
2
3
4
5
6
7
NH2
OCH3
H
COOH
CF3
0.14
0.39
0.41
0.54
0.57
0.59
0.62
ꢁ1.20
ꢁ1.18
ꢁ1.13
ꢁ1.07
ꢁ1.04
ꢁ1.02
ꢁ1.00
ꢁ4.94
ꢁ5.19
ꢁ5.21
ꢁ5.34
ꢁ5.37
ꢁ5.39
ꢁ5.42
ꢁ3.60
ꢁ3.62
ꢁ3.67
ꢁ3.73
ꢁ3.76
ꢁ3.78
ꢁ3.80
1.43
1.52
1.57
1.59
1.60
1.60
1.61
Figure 3. Crystal structure of 429
.
electron withdrawing to electron donating substituents. All deriva-
tives show irreversible oxidations and at least two reversible
reductions. The onset of the first oxidation wave shows a step-wise
cathodic shift going from electron withdrawing (7, NO2) to electron
donating (1, NH2) substituents (Table 1), which demonstrates that
stronger electron donating groups make the compounds easier to
oxidize. The range in HOMO energies vary from ꢁ4.94 eV to
ꢁ5.42 eV for 1 to 7, respectively, and shows the destabilization of
the HOMO with increasing donating character. The onset of the
first reduction wave shows a step-wise anodic shift going from
electron donating to electron withdrawing substituents. The range
in LUMO energies vary from ꢁ3.60 eV to ꢁ3.80 eV for 1 to 7,
respectively and confirm that stronger withdrawing groups make
the structures easier to reduce. The LUMO values of the 1–7 can
be analyzed via a Hammett Plot (Fig. 4) that shows the variation
of LUMO energy versus the known substituent constants.28 A linear
relationship between the LUMO values and the substituent
CN
NO2
a
Potentials are measured relative to a ferrocenium/ferrocene redox couple used
as an internal standard (Fig. 2). Eox/onset is the onset of oxidation potential, Ered/onset
is the onset of reduction potential. HOMO and LUMO values calculated on the basis
of the oxidation of the ferrocene reference in vacuum (4.8 eV).
electron accepting substituents. Although we were unable to grow
suitable crystals of more electron rich substituents, we hypothe-
size the aryl rings are more coplanar with the CPAA core, which
would facilitate the observable bathochromic shifts.
The cyclic voltammograms (CV) of each substituted CPAA can
be found in Figure 2. While the UV–Vis spectra (Fig. 1) show vari-
ation only with the most electron rich substituents, the CV traces
show that both the highest occupied molecular level (HOMO)
and LUMO are systematically varied across the entire range of

X. Zhu et al. / Tetrahedron Letters 56 (2015) 7105–7107
7107
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was diluted with ethyl ether (150 mL) and extracted twice with brine (50 mL).
The organic layer was collected and concentrated to give a residue that was
purified by silica gel chromatography.
Acknowledgments
This work was supported by a National Science Foundation
CAREER grant (CHE-1352431).
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