Revisiting the stability of hexacenes

Article abstract of DOI:10.1021/ol0709376

The Strating-Zwanenberg photodecarbonylation was uaed to prepare hexacene (1). Compound 1 waa found to be extremely unatable in solution, undergoing dimerization and oxidation. However, when generated in a polymer matrix, 1 survived for more than 12 h und

Full text of DOI:10.1021/ol0709376

ORGANIC
LETTERS
2007
Vol. 9, No. 13
2505-2508
Revisiting the Stability of Hexacenes
Rajib Mondal, Ravi M. Adhikari, Bipin K. Shah,* and Douglas C. Neckers*
Center for Photochemical Sciences,1 Bowling Green State UniVersity,
Bowling Green, Ohio 43403
neckers@photo.bgsu.edu; bipin@bgsu.edu
Received April 20, 2007
ABSTRACT
The Strating−Zwanenberg photodecarbonylation was used to prepare hexacene (1). Compound 1 was found to be extremely unstable in
solution, undergoing dimerization and oxidation. However, when generated in a polymer matrix, 1 survived for more than 12 h under ambient
conditions. Hexacenes substituted at the 6 and 15 positions with the phenyl, p-tert-butylphenyl, and mesityl groups were synthesized using
the quinone reduction method, but these compounds were also shown to be unstable in solution.
Poly(acene)s are an important class of polyaromatic hydro-
carbons and consist of an aromatic linear array.2 They are
the basic building units of graphite3 and carbon nanotubes4
and are considered to be efficient organic electronic materi-
als.5 Pentacenes, for example, are a current choice in
superconductors, organic thin-film transistors (OTFTs),6 and
organic light-emitting diodes.7 Field effect mobility greater
than 1 cm2 V-1 s-1 has been reported using pentacene as
the active layer in OTFT devices.8 Higher poly(acene)s such
as hexacene and heptacene show lower band-gaps (∆E) than
pentacene and are predicted to exhibit superior electronic
properties.9 However, the insolubility and instability of the
higher poly(acene)s has limited their isolation and proper
study for applications.10
proved to be unsuccessful since the quinones were over-
reduced to hydrogenated acenes.2,11,12 Other methods of
synthesis also failed for higher poly(acene)s, presumably
because of their higher reactivity toward Diels-Alder
additions which ultimately results in formation of either
dimers or oxygen adducts (endoperoxides).13 Recently, we
used the Strating-Zwanenberg photodecarbonylation to
achieve the first unequivocal synthesis of heptacene.14 This
reaction, first discovered in 1969, was employed by the
Strating group in seeking the dimer of carbon monoxide.15
Pentacene was also synthesized using the Strating-Zwanen-
berg photochemical route.16 An impressive attribute of this
reaction is that it is clean, producing a poly(acene) following
the expulsion of carbon monoxide, either in a molecular form
or a dimeric form, from a precursor R-diketone.
Lower molecular weight poly(acene)s (up to pentacene)
are usually synthesized by reduction of the corresponding
quinones. A similar approach used for higher poly(acene)s
(9) (a) Schleyer, P. v. R.; Manoharan, M.; Jiao, H.; Stahl, F. Org. Lett.
2001, 3, 3643. (b) Houk, K. N.; Lee, P. S.; Nendel, M. J. Org. Chem. 2001,
66, 5517 and references therein.
(1) Contribution No. 636 from the Center for Photochemical Sciences.
(2) (a) Clar, E. Polycyclic Hydrocarbons; Academic Press: London and
New York, 1964; Vols. 1, 2. (b) Harvey R. G. Polycyclic Aromatic
Hydrocarbons; Wiley-VCH: New York, 1997.
(3) Chung, D. D. L. J. Mater. Sci. 2002, 37, 1475.
(4) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603.
(5) (a) Anthony, J. E. Chem. ReV. 2006, 106, 5028. (b) Bendikov, M.;
Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104, 4891 and references
therein.
(10) (a) Clar, E. Chem. Ber. 1939, 72B, 2137. (b) Angliker, H.; Rommel,
E.; Wirz, J. Chem. Phys. Lett. 1982, 87, 208.
(11) (a) Bailey, W. J.; Liao, C.-W. J. Am. Chem. Soc. 1955, 77, 992. (b)
Satchell, M. P.; Stacey, B. E. J. Chem. Soc. C 1971, 3, 468.
(12) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2006, 71,
4085.
(13) Fang, T. Heptacene, Octacene, Nonacene, Supercene and Related
Polymers. Ph.D. Thesis, University of California, Los Angeles, CA, 1986.
(14) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Am. Chem. Soc. 2006,
128, 9612.
(15) Strating, J.; Zwanenburg, B.; Wagenaar, A.; Udding, A. C.
Tetrahedron Lett. 1969, 10, 125.
(6) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14,
99.
(7) Wolak, M. A.; Jang, B.; Palilis, L. C.; Kafafi, Z. H. J. Phys. Chem.
B 2004, 108, 5492.
(8) (a) Kagan, C. R.; Afzali, A.; Graham, T. O. Appl. Phys. Lett. 2005,
86, 193505. (b) Tang, M. L.; Okamoto, T.; Bao, Z. J. Am. Chem. Soc.
2006, 128, 16002.
(16) (a) Uno, H.; Yamashita, Y.; Kikuchi, M.; Watanabe, H.; Yamada,
H.; Okujima, T.; Ogawa, T.; Ono, N. Tetrahedron Lett. 2005, 46, 1981.
(b) Yamada, H.; Yamashita, Y.; Kikuchi, M.; Watanabe, H.; Okujima, T.;
Ogawa, T.; Ohara, K.; Ono, N. Chem.sEur. J. 2005, 11, 6212.
10.1021/ol0709376 CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/22/2007

Scheme 1. Synthesis of Hexacene Derivatives
Although hexacene (1) is a known compound,10b,11,17 it is
or mesityl groups could not prevent rapid oxidation or
dimerization. Only a very bulky group stabilizes hexacene.18
Bicyclo[2,2,2]oct-2,3,5,6,7-pentaene (5) was coupled either
with 1,2-dibromobenzene or with 2,3-dibromonaphthalene,
and the product was subsequently subjected to aromatization
and oxidation to prepare 6,15-dihydro-6,15-ethanohexacene-
17,18-dione (11) (Scheme 2, see Supporting Information for
synthetic details).
Photodecarbonylation of 11 was carried out both in
solution and in a polymer matrix (Scheme 3). A solution of
11 in toluene showed an n-π* band at 465 nm (ꢀ ) 850),
with additional π-π* transitions at 327 (5000), 345 (5500),
and 367 (4200) nm (Figure 1). This is similar to the
absorption of 11 in the poly(methyl methacrylate), PMMA,
matrix. A weak fluorescence (λem ) 525 nm, ΦF ) 0.003)
was observed when the solution of 11 in toluene was excited
at 460 nm. It also showed phosphorescence (λem ) 570 nm)
at 77 K in a frozen matrix of methanol/ethanol (1:4). An
array of UV-LEDs (395 ( 25 nm) was used to selectively
excite the n-π* transition and to prevent any subsequent
photochemical side reactions.
still elusive in that its reported synthetic schemes are difficult
to repeat. An extensive study of this compound is not
available in the literature because of the synthetic difficulty.
We recently revisited the question of the stability of
hexacenes and ultimately succeeded in synthesizing 1 by the
Strating-Zwanenberg method, employing the photodecar-
bonylation of 6,15-dihydro-6,15-ethanohexacene-17,18-dione
(the precursor R-diketone of 1). Although this method does
not allow one to isolate 1, it does provide a facile pathway
to synthesize and study it. Since the photoprecursor is highly
soluble in common organic solvents this assures flexibility
and 1 can be routinely synthesized and studied using this
method. We also discuss the attempted syntheses of 1 and
its derivatives using the quinone method.
Nucleophilic addition of suitable aryllithiums to 6,15-
hexacenequinone (2) produced diols (3a-c), which were
further reduced by tin(II) chloride dihydrate/HCl to 6,15
disubstituted hexacenes (4a-c, Scheme 1). Matrix assisted
laser desorption ionization (MALDI) mass spectra of the
reaction mixture showed the appropriate molecular peaks,
indicating formation of the hexacenes. A solution of crude
4a showed the anticipated absorption in the visible region
with the maximum at 684 nm. Further characterization was
not possible, because 4a-c could not be purified or isolated.
The failure to isolate 4a-c indicates that not only 1 but
also its derivatives are extremely reactive. Substitution of
the central rings by aryl groups such as the p-tert-butylphenyl
A solution of 11 in toluene (4.0 × 10-4 M) was degassed
using freeze-pump-thaw. Photolysis of this solution pro-
duced initially a new structured absorption band in the 550-
700 nm region with maxima at 574, 612, and 672 nm (Figure
2). The new structured bands can be assigned to the π-π*
transition of 1 and are in good agreement with the absorption
spectrum of 1 recorded in 1,2,4-trichlorobenzene.10b How-
Scheme 2. Synthesis of the R-Diketone of Hexacene
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Org. Lett., Vol. 9, No. 13, 2007

generated in solution, it rapidly undergoes dimerization and
oxidation indicating it to be extremely reactive in solution.
Compound 1 undergoes dimerization even when it is
generated at a very low concentration (<10-4 M). These
results are surprising given that 1 is reported to have been
sold commercially in the past!
Scheme 3. Synthesis of Hexacene (1)
When similar experiments were carried out with solutions
saturated with oxygen, immediate formation of endoperox-
ides of 1 was observed (Scheme 4). This was indicated by
ever, the growth of this absorption continued only for a few
minutes, after which its intensity diminished with subsequent
irradiation. MALDI-MS analysis of the over-irradiated (40
1
MALDI-MS as well as by H NMR spectra.
Scheme 4. Oxidation of 1
These results imply that when 1 is formed in solution, it
immediately reacts with oxygen.19 MALDI-MS analysis
indicated the addition of one as well as two molecules of
oxygen. The protons of the bridge C atoms of 11 appear at
1
5.36 ppm in the H NMR spectrum. Two additional peaks
1
appeared between 6 and 6.1 ppm in the H NMR of an
irradiated solution of 11 in CDCl3 purged with oxygen. The
intensity of these peaks increased with the time of irradiation
(Figure 3). These are assigned to the protons at the carbon
Figure 1. Normalized absorption (red), fluorescence (blue), and
phosphorescene (green) spectra of 11 (absorption and fluorescence
speactra recorded in toluene at room temperature and phosphores-
cence spectrum recorded in methanol/ethanol (1:4) matrix at
77 K).
min) solution indicated the formation of dimer of 1 as well
as oxygen adducts. The latter formed through the reaction
of 1 with residual oxygen in solution. Thus, after 1 is
Figure 3. 1H NMR spectra recorded at different times of irradiation
of a CDCl3 solution of 11 purged with oxygen (light source: a
395 nm UV-LED array).
atoms attached to the oxygen bridge in the endoperoxides.
The many peaks in the 6-6.1 region also indicate that
Figure 2. Absorption spectra recorded during and after irradia-
tion of 11 in degassed toluene (inset: enlarged portion from 525
to 725 nm).
(17) Nijegorodov, N.; Ramachandran, V.; Winkoun, D. P. Spectrochim.
Acta, Part A 1997, 53, 1813.
Org. Lett., Vol. 9, No. 13, 2007
2507

oxygen bridges form at different positions of 1 (for example,
the 6-15 and/or 8-13 positions).
The intensity of the 672 nm absorption band of 1 gradually
increased through the first 60 min of irradiation. But further
irradiation resulted in no additional growth. In fact, the
intensity of the 672 nm band started decreasing, irrespective
of irradiation, indicating slow decomposition of 1 even in
PMMA. Compound 1 could be retained in the matrix
overnight under ambient conditions. Thus, in the PMMA
matrix, 1 was found to be more stable than heptacene, which
survives for about 4 h under the similar conditions.14 These
results attest to the unstable nature of 1 even under conditions
manipulated so as to maintain its isolation; that is, in the
solid PMMA matrix. Compound 1 seems to be consumed
by oxidation with a small amount of molecular oxygen that
diffuses into the matrix.
As we have previously shown with heptacene, photode-
carbonylation can be conveniently used to generate 1 in the
PMMA matrix. A semirigid polymer matrix enables retention
of reactive 1 by preventing its dimerization. This also enables
minimal contact of 1 with environmental oxygen reducing
the rate of oxidation. Thus, when a PMMA film (thickness
≈ 0.5 mm) containing 11 (3.5 × 10-3 M) was irradiated
using the UV-LED array, this produced 1 as the film turned
green and showed structured absorption in the region of
550-700 nm (565, 614, and 672 nm) similar to that observed
in solution (Figure 4). Quick dissolution of the irradiated
In summary, hexacene was synthesized using the Strating-
Zwanenberg photodecarbonylation. It rapidly undergoes
dimerization in solution even when generated at very low
concentrations (<10-4 M). Its generation in oxygen saturated
solution exclusively leads to the formation of endoperoxides.
Nevertheless, 1 was observed to be stable for more than 12
h when produced in a PMMA matrix. Substituents such as
the phenyl, p-tert-butylphenyl, and mesityl groups at the 6
and 15 positions did not provide stability to the hexacenes.
Acknowledgment. The authors greatly appreciate the
financial support provided to this work by the Office of Naval
Research N00 14-05-1-0372 and, in part, by the Ohio
Department of Development. R.M. thanks the McMaster
Endowment for a research fellowship.
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 4. Absorption spectra recorded during and after irradiation
of 11 in a PMMA film.
OL0709376
(18) Payne, M. M.; Parkin, S. R.; Anthony, J. E. J. Am. Chem. Soc.
2005, 127, 8028.
(19) Reddy, A. R.; Bendikov, M. Chem. Commun. 2006, 1179.
film in dichloromethane and subsequent analysis by MALDI
mass spectra confirmed formation of 1.
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Org. Lett., Vol. 9, No. 13, 2007