Stabilizing Pentacene by Cyclopentannulation

Article abstract of DOI:10.1002/anie.201508650

A new class of stabilized pentacene derivatives with externally fused five-membered rings are prepared by means of a key palladium-catalyzed cyclopentannulation step. The target compounds are synthesized by chemical manipulation of a partially saturated 6,13-dibromopentacene precursor that can be fully aromatized in a final step through a DDQ-mediated dehydrogenation reaction (DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone). The new 1,2,8,9-tetraaryldicyclopenta[fg,qr]pentacene derivatives have narrow energy gaps of circa 1.2 eV and behave as strong electron acceptors with lowest unoccupied molecular orbital energies between -3.81 and -3.90 eV. Photodegradation studies reveal the new compounds are more photostable than 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene). Five in a row: A palladium-catalyzed cyclopentannulation followed by a DDQ-mediated dehydrogenation converts a partially hydrogenated pentacene precursor into stabilized pentacenes. These pentacene derivatives are excellent electron acceptors and have small energy gaps. DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

Full text of DOI:10.1002/anie.201508650

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International Edition: DOI: 10.1002/anie.201508650
German Edition: DOI: 10.1002/ange.201508650
Pentacenes
Stabilizing Pentacene By Cyclopentannulation
Sambasiva R. Bheemireddy, Pamela C. Ubaldo, Peter W. Rose, Aaron D. Finke,
Junpeng Zhuang, Lichang Wang, and Kyle N. Plunkett*
Abstract: A new class of stabilized pentacene derivatives with
externally fused five-membered rings are prepared by means of
a key palladium-catalyzed cyclopentannulation step. The target
compounds are synthesized by chemical manipulation of
a partially saturated 6,13-dibromopentacene precursor that
can be fully aromatized in a final step through a DDQ-
mediated dehydrogenation reaction (DDQ = 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone). The new 1,2,8,9-tetraaryldicyclo-
penta[fg,qr]pentacene derivatives have narrow energy gaps of
circa 1.2 eVand behave as strong electron acceptors with lowest
unoccupied molecular orbital energies between À3.81 and
À3.90 eV. Photodegradation studies reveal the new compounds
are more photostable than 6,13-bis(triisopropylsilylethynyl)-
pentacene (TIPS-pentacene).
Figure 1. Pentacene structures. According to Clar’s sextet rule, 1, 3,
H
igher acenes are known to readily undergo photoinduced
and 4 have a single migrating p sextet while 2 has four p sextets.
oxidation, dimerization, or polymerization largely owing to
a biradical character in the electronic ground state.[1–3] These
decomposition pathways are undesirable because acene
stability has been identified as a critical design parameter
for their incorporation in electronic devices,[4] considering
that acene-based devices have been found to fail as a result of
decomposition of the active material.[5] To address this
problem, useful strategies to stabilize acenes have employed
ethynylation,[6–9] thiolation,[10–12] cyanation,[13,14] or bulky sub-
stituent incorporation[15–18] to access electronically modulated
or sterically blocked structures.[19] Benzannulation, or higher
ordered six-membered ring fusions, of the acene core is an
alternative strategy to obtain stabilized compounds, yet the
resulting electronic properties of these derivatives no longer
possess the desirable acene character. For example, upon
of the polycyclic aromatic hydrocarbon (PAH).[21] Although
benzannulated 2 contains four isolated p sextets, a pentannu-
lated acene core (for example as in 3) alleviates this
conjugation dilemma owing to the isolation of a double
bond in the five-membered ring. From the Clar sextet
representation, the electronic structure of 3 should be similar
to that of a native pentacene with a single migrating p sextet
(Figure 1). Owing to our recent interest in cyclopenta-fused
polycyclic aromatic hydrocarbons (CP-PAHs),[22–26] we set out
to prepare pentacene derivatives working under the hypoth-
esis that the fusion of five-membered rings would minimize
the biradical character at the central carbons leading to
stabilized structures. Towards this new class of compounds, we
demonstrate herein that cyclopentannulation of a partially
unsaturated pentacene precursor followed by an aromatiza-
tion step with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) leads to the formation of new pentacenes that are
more stabilized than the well-known 6,13-bis(triisopropyl-
silylethynyl)pentacene (4; TIPS-pentacene). These molecules
behave as strong electron acceptors and possess significantly
decreased energy gaps.
benzannulation of pentacene
1
to form dibenzo-
[fg,qr]pentacene 2,[20] significant electronic modulation
occurs and the molecule no longer behaves as an acene but
rather as two electronically separated phenanthrene units
(Figure 1). These properties can be easily rationalized by
invoking Clarꢀs aromatic sextet rule that says the resonance
contributor with the most p sextets (that is, benzene-like
moieties) is the most important contributor to the properties
The palladium-catalyzed cyclopentannulation chemistry
utilized here is an extension of our recent work[26] that was
rooted in the contributions of several other groups.[27–31] For
this work, we needed to adapt the pentannulation synthetic
method from an anthracene to a pentacene core. This new
chemistry required the synthesis of 6,13-dibromopentacene,
or a reactive surrogate, in place of the previously utilized 9,10-
dibromoanthracene. Owing to the unknown nature of 6,13-
dibromopentacene,[32] we turned to a pentacene precursor
that could undergo known transformations to access the
desired CP-PAH substructure and then could later be trans-
formed into the desired pentacene derivatives. To accomplish
[*] S. R. Bheemireddy, Dr. P. C. Ubaldo, P. W. Rose, Prof. J. Zhuang,
Prof. L. Wang, Prof. K. N. Plunkett
Department of Chemistry and Biochemistry and the Materials
Technology Center, Southern Illinois University
Carbondale, IL 62901 (USA)
E-mail: kplunkett@chem.siu.edu
Dr. A. D. Finke
Swiss Light Source, Paul Scherrer Institute
Villigen PSI (Switzerland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508650.
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this goal, we synthesized 6,13-dibromo-1,2,3,4,8,9,10,11-octa-
hydropentacene (6) from the previously known and partially
hydrogenated pentacene 5 (Scheme 1).[33] This stable precur-
sor contains an aromatic core that reacts as a substituted
anthracene and therefore can undergo known chemical
transformations related to that core.[32] We performed our
Figure 2. Representative UV/Vis absorption spectra of 7a and 8a in
chloroform at room temperature. Remaining spectra are given in the
Supporting Information.
ÀꢁÀ
Scheme 1. Synthesis of CP-pentacenes: a) Br2, CCl4, RT; b) Ar
[Pd2(dba)3], P(o-Tol)3, KOAc, LiCl, DMF, 1308C; c) 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ), benzene, 808C.
Ar,
newly optimized palladium-catalyzed cyclopentannulation
between 6 and several diaryl acetylenes utilizing the catalyst
system of [Pd2(dba)3], P(o-Tol)3, and LiCl with KOAc as
a base (Scheme 1; dba = dibenzylideneacetone, o-Tol = ortho-
toluene).[26] Anthracene-based CP-PAHs with electron-rich
(7a), mildly electron-rich (7b), and electron-poor (7c)
substituents on the aryl rings were obtained in consistent
yields of 32–43%.
The resulting compounds have photophysical and electro-
chemical properties similar to previously prepared anthra-
cene CP-PAH derivatives.[26] Each derivative (7a–c) formed
emerald-green solutions in chloroform and their UV/Vis
absorption spectra were almost identical (Figure 2; see also
the Supporting Information), each having with a high-energy
band (labs ꢀ 260 nm), a mid-range band (labs ꢀ 416 nm), and
a broad charge-transfer band (labs ꢀ 590 nm; Table 1). The
cyclic voltammograms show that the compounds are irrever-
sibly oxidized but give two reversible or quasi-reversible
reductions (Figure 3). Both the onset of oxidation as well as
that of the reduction waves vary with respect to the
substituents, with the oxidation occurring at less positive
potentials with the more electron-rich substituents (R =
OCH3) and the reduction waves occurring at less negative
potentials with the more electron-poor (R = F) substituent
(Table 1).
Figure 3. Cyclic voltammograms of 0.2 mm 7a–c and 8a–c (0.2 mm in
THF) with 0.1m tetrabutylammonium hexafluorophosphate as the
supporting electrode, a glassy carbon working electrode, a platinum
counter electrode, and an Ag/AgCl reference electrode. Scan
rate=50 mVsÀ1. Ferrocene (Fc) was added as the internal standard
and referenced to 0 V.
8a–c is fascinating because we were unable to extend the
method to access traditional 6,13-diarylpentacenes by means
of DDQ aromatization of precursors formed following
a Suzuki cross-coupling to 6. The synthetic method described
here is an alternative method to a recent report accessing
internally fused, cyclopenta-fused pentacenes using a Scholl
cyclodehydrogenation.[34] The method reported herein is
more broadly applicable as a result of the inherent stability
of the anthracene precursors and the broadened scope of
substituents to include electron-withdrawing groups that were
known to inhibit the Scholl reaction.
The conversion from the anthracene (7a–c) into penta-
cene cores (8a–c) resulted in significant modulation of the
optoelectronic properties, leading to significant shifts in the
absorption bands and the occurrence of the reduction waves
at less negative potentials. For example, the transition from 7a
Conversion to the fully unsaturated 1,2,8,9-
tetraaryldicyclopenta[fg,qr]pentacenes (8a–c) was conven-
iently accomplished by aromatization with four equivalents of
DDQ. The effectiveness of this aromatization step to form
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Table 1: Summary of the optoelectronic properties of 7a–c and 8a–c.[a–f]
[a]
[a]
[d]
[f]
Cmpd
Eox/onset
[V]
Ered/onset
HOMO[b]
[eV]
LUMO[b]
[eV]
E-chem gap[c]
[eV]
labs
[nm]
Energy gap[e]
[eV]
thalf-life
[min]
[V]
7a
7b
7c
8a
8b
8c
0.48
0.56
0.63
0.51
0.55
0.65
À1.52
À1.50
À1.46
À0.99
À0.98
À0.91
À5.28 (À5.31)
À5.36 (À5.41)
À5.43 (À5.58)
À5.31 (À5.33)
À5.35 (À5.41)
À5.45 (À5.54)
À3.28 (À3.07)
À3.30 (À3.10)
À3.34 (À3.24)
À3.81 (À3.66)
À3.82 (À3.69)
À3.90 (À3.80)
2.00 (2.24)
2.06 (2.31)
2.09 (2.34)
1.50 (1.67)
1.53 (1.72)
1.57 (1.74)
262, 417, 592
261, 415, 587
263, 416, 597
297, 458, 706
308, 455, 706
309, 456, 706
1.59
1.57
1.57
1.22
1.20
1.21
–
–
–
1614Æ172
1063Æ70
1106Æ79
[a] Measurements taken at sample concentration of 0.2 mm and potentials measured relative to a ferrocenium/ferrocene redox couple used as an
internal standard (Figure 3). Eox/onset is the onset of oxidation potential, Ered/onset is the onset of reduction potential. [b] HOMO and LUMO values
calculated on the basis of the oxidation of the ferrocene reference in vacuum (4.8 eV). B3LYP/6-311g(d,p) predicted values are in parenthesis.
[c] Difference between the electrochemically calculated HOMO and LUMO values. [d] Wavelengths at which maximum absorption bands are detected.
[e] Energy gap calculated by E=hc/l, where h is Planck’s constant and c is the speed of light. The labs,onset value is taken from the onset of absorption
from the lowest energy band. [f] Average and standard deviation from three runs. TIPS-Pentacene (4)=317Æ18 min (see Figure 6).
to 8a resulted in bathochromically shifted absorption bands
with the most dramatic being in the charge-transfer band
onset that shifted from about l = 750 nm (at the lower energy
side of the onset) to over 1000 nm (Figure 2). Each of the
pentacene derivatives had nearly identical UV/Vis absorption
spectra with solution-based energy gaps of about 1.2 eV and
showed variation only in the molar extinction coefficients (see
the Supporting Information). All derivatives studied here
were nonfluorescent, a property common to many CP-PAH
compounds (see the Supporting Information).[35–37] The cyclic
voltammograms (Figure 3) demonstrate how 8a–c are sig-
nificantly easier to reduce (by circa 0.5 eV) compared to 7a–c
giving lowest unoccupied molecular orbital (LUMO) energies
of À3.81 to À3.90 eV (Table 1). In contrast, the highest
occupied molecular orbital (HOMO) energies were very
similar between the anthracene and pentacene derivatives.
This result was confirmed by DFT calculations that show
HOMO contours of 7a and 8a similar in energy with the
majority of the electron density migrating between the five-
membered rings in the core and two aryl substituents that are
diagonal to each other (Figure 4). Conversely, the LUMO
contour of 8a is significantly different in relation to 7a as it
has electron density spread onto two additional six-mem-
bered rings. The HOMO and LUMO distributions of 8a–c are
therefore significantly different than traditional pentacene
derivatives which leads to the absence of the known “finger-
print” vibronic fine structure in the UV/Vis absorption
spectra.
Figure 4. B3LYP/6-311 g(d,p) DFT-calculated HOMO and LUMO
contours of 7a and 8a.
Single crystals of 8a suitable for X-ray diffraction were
grown by slow evaporation of a chloroform solution.[39] The
molecular structure (Figure 5) reveals the central carbon
framework is very similar to pentacene with a completely
planar arrangement of fused benzene rings. The five-mem-
bered rings are coplanar with the pentacene core; however,
the four appended aryl groups distort from planarity owing to
their steric congestion in agreement with the DFT-calculated
structures. The specific crystallographically determined and
DFT-calculated bond lengths of 8a were summarized and
compared to pentacene (see the Supporting Information).
Overall, the bond lengths show similar trends to pentacene
with alternating short and long bond-length character near
and a more significant absolute value of alternation between
the shorter and longer bonds on the outer ring. Nevertheless,
this crystal structure demonstrates that the general bond
orders are consistent between the cyclopenta-fused pentacene
and traditional pentacene derivatives. With regards to the
crystal packing, the perpendicular arrangement of the fused
benzene rings hinders p stacking with the molecules ordering
into a pseudo-herringbone-like arrangement.
The stabilities of 8a–c were tested in comparison to the
known TIPS-pentacene derivative 4 by photodegradation in
an oxygenated THF solution under ambient light irradiation
(Figure 6).[5] The resulting decomposition half-lives (Table 1)
were obtained by monitoring the disappearance of the
À
the terminal rings. Notable differences include a shorter C6
C8 bond, which is consistent with an indacene fragment,[38]
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fusion on the acene backbone and are consistent with the
previous report of internally fused CP-PAH pentacenes.[34]
The additional stability could be attributed to electronic
factors or added steric congestion at the central rings which is
known to minimize the rate of decomposition of penta-
cenes.[15–18]
In conclusion, we have presented a unique synthetic
pathway that utilizes easily accessible and stable intermedi-
ates to access externally fused, cyclopenta-fused pentacene
derivatives. The key to the described chemistry is chemical
manipulation of a hydrogenated pentacene precursor fol-
lowed by aromatization to the final acene by reaction with
DDQ. The described cyclopentannulation strategy offers
opportunities to access even higher ordered acenes, a topic
we are currently pursuing, and contributes to the growing
number of planar CP-PAH-based materials.[37]
Acknowledgements
This work was supported by a National Science Foundation
CAREER grant (CHE-1352431).
Keywords: acenes · conjugation · organic electronics ·
palladium · polycyclic aromatic hydrocarbons
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15762–15766
Angew. Chem. 2015, 127, 15988–15992
Figure 5. Crystal structure (top) and packing (bottom) of 8a.[39]
[1] M. Bendikov, F. Wudl, D. F. Perepichka, Chem. Rev. 2004, 104,
4891 – 4946.
[2] A. R. Reddy, M. Bendikov, Chem. Commun. 2006, 1179 – 1181.
[3] S. S. Zade, N. Zamoshchik, A. R. Reddy, G. Fridman-Marueli, D.
Sheberla, M. Bendikov, J. Am. Chem. Soc. 2011, 133, 10803 –
10816.
[4] C. D. Sheraw, T. N. Jackson, D. L. Eaton, J. E. Anthony, Adv.
Mater. 2003, 15, 2009 – 2011.
[5] A. Maliakal, K. Raghavachari, H. Katz, E. Chandross, T.
Siegrist, Chem. Mater. 2004, 16, 4980 – 4986.
[6] J. E. Anthony, J. S. Brooks, D. L. Eaton, S. R. Parkin, J. Am.
Chem. Soc. 2001, 123, 9482 – 9483.
[7] M. M. Payne, S. A. Odom, S. R. Parkin, J. E. Anthony, Org. Lett.
2004, 6, 3325 – 3328.
[8] M. M. Payne, S. R. Parkin, J. E. Anthony, J. Am. Chem. Soc.
2005, 127, 8028 – 8029.
[9] B. Purushothaman, S. R. Parkin, J. E. Anthony, Org. Lett. 2010,
12, 2060 – 2063.
[10] I. Kaur, W. Jia, R. P. Kopreski, S. Selvarasah, M. R. Dokmeci, C.
Pramanik, N. E. McGruer, G. P. Miller, J. Am. Chem. Soc. 2008,
130, 16274 – 16286.
[11] K. Kobayashi, R. Shimaoka, M. Kawahata, M. Yamanaka, K.
Yamaguchi, Org. Lett. 2006, 8, 2385 – 2388.
[12] A. L. Briseno, Q. Miao, M.-M. Ling, C. Reese, H. Meng, Z. Bao,
F. Wudl, J. Am. Chem. Soc. 2006, 128, 15576 – 15577.
[13] Y.-F. Lim, Y. Shu, S. R. Parkin, J. E. Anthony, G. G. Malliaras, J.
Mater. Chem. 2009, 19, 3049 – 3056.
[14] S. Katsuta, D. Miyagi, H. Yamada, T. Okujima, S. Mori, K.
Nakayama, H. Uno, Org. Lett. 2011, 13, 1454 – 1457.
[15] I. Kaur, N. N. Stein, R. P. Kopreski, G. P. Miller, J. Am. Chem.
Soc. 2009, 131, 3424 – 3425.
Figure 6. A single photodegradation run of TIPS-pentacene 4 and
CP-pentacenes 8a–c at 15 mm. Experiment ran in triplicate and half-life
average values given in Table 1. O.D.=optical density.
absorption band at l = 455 nm over time (Supporting Infor-
mation). Compounds 8a–c were found to be considerably
more stable than 4 with half-lives on the order of 1063–
1614 minutes, whereas 4 had a half-life of 317 minutes. In
addition, 8a–c showed no decomposition in the solid state
when kept for over a year in the absence of light. These results
clearly demonstrate the stabilizing effect of the cyclopenta
[16] B. Purushothaman, M. Bruzek, S. R. Parkin, A.-F. Miller, J. E.
Anthony, Angew. Chem. Int. Ed. 2011, 50, 7013 – 7017; Angew.
Chem. 2011, 123, 7151 – 7155.
Angew. Chem. Int. Ed. 2015, 54, 15762 –15766
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15765

.
Angewandte
Communications
[17] Q. Miao, X. Chi, S. Xiao, R. Zeis, M. Lefenfeld, T. Siegrist, M. L.
Steigerwald, C. Nuckolls, J. Am. Chem. Soc. 2006, 128, 1340 –
1345.
[18] J. Zhang, R. H. Pawle, T. E. Haas, S. W. Thomas, Chem. Eur. J.
2014, 20, 5880 – 5884.
[19] J. E. Anthony, Angew. Chem. Int. Ed. 2008, 47, 452 – 483; Angew.
Chem. 2008, 120, 460 – 492.
[20] W. N. Lipscomb, J. M. Robertson, M. G. Rossmann, J. Chem.
Soc. Resumed 1959, 2601 – 2607.
[30] L. E. Eversloh, Y. Avlasevich, C. Li, K. Müllen, Chem. Eur. J.
2011, 17, 12756 – 12762.
[31] E.-C. Liu, M.-K. Chen, J.-Y. Li, Y.-T. Wu, Chem. Eur. J. 2015, 21,
4755 – 4761.
[32] H. Qu, W. Cui, J. Li, J. Shao, C. Chi, Org. Lett. 2011, 13, 924 – 927.
[33] J. Luo, H. Hart, J. Org. Chem. 1988, 53, 1341 – 1343.
[34] A. Naibi Lakshminarayana, J. Chang, J. Luo, B. Zheng, K.-W.
Huang, C. Chi, Chem. Commun. 2015, 51, 3604 – 3607.
[35] M. J. Bearpark, P. Celani, F. Jolibois, M. Olivucci, M. A. Robb, F.
Bernardi, Mol. Phys. 1999, 96, 645 – 652.
[21] E. Clar, The Aromatic Sextet, Wiley, New York, NY, 1972.
[22] J. D. Wood, J. L. Jellison, A. D. Finke, L. Wang, K. N. Plunkett, J.
Am. Chem. Soc. 2012, 134, 15783 – 15789.
[36] B. D. Rose, L. E. Shoer, M. R. Wasielewski, M. M. Haley, Chem.
Phys. Lett. 2014, 616 – 617, 137 – 141.
[23] J. L. Jellison, C.-H. Lee, X. Zhu, J. D. Wood, K. N. Plunkett,
Angew. Chem. Int. Ed. 2012, 51, 12321 – 12324; Angew. Chem.
2012, 124, 12487 – 12490.
[37] “Planar Cyclopenta-Fused Polycyclic Arenes”: G. E. Rude-
busch, M. M. Haley in Polycyclic Arenes and Heteroarenes
(Ed.: Q. Miao), Wiley-VCH, Weinheim, 2015.
[24] C.-H. Lee, K. N. Plunkett, Org. Lett. 2013, 15, 1202 – 1205.
[25] K. N. Plunkett, Synlett 2013, 898 – 902.
[26] S. R. Bheemireddy, P. C. Ubaldo, A. D. Finke, L. Wang, K. N.
Plunkett, J. Mater. Chem. C 2016, DOI: 10.1039/C5TC02305J.
[27] R. Grigg, P. Kennewell, A. Teasdale, V. Sridharan, Tetrahedron
Lett. 1993, 34, 153 – 156.
[38] K. Hafner, B. Stowasser, H.-P. Krimmer, S. Fischer, M. C. Bçhm,
H. J. Lindner, Angew. Chem. Int. Ed. Engl. 1986, 25, 630 – 632;
Angew. Chem. 1986, 98, 646 – 648.
[39] CCDC 1424587 (8a) contain the supplementary crystallographic
data for this paper. These data are provided free of charge by
The Cambridge Crystallographic Data Centre.
[28] H. Dang, M. A. Garcia-Garibay, J. Am. Chem. Soc. 2001, 123,
355 – 356.
[29] H. Dang, M. Levitus, M. A. Garcia-Garibay, J. Am. Chem. Soc.
2002, 124, 136 – 143.
Received: September 15, 2015
Published online: November 19, 2015
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