Synthesis and stability of soluble hexacenes

Article abstract of DOI:10.1021/ol100178s

Figure presented The synthesis of new silylethyne-substituted hexacene derivatives to investigate their solubility, stability, and π-stacking is reported. It was found that "butterfly" dimerization, rather than photooxidation, is the dominant decompositio

Full text of DOI:10.1021/ol100178s

ORGANIC
LETTERS
2010
Vol. 12, No. 9
2060-2063
Synthesis and Stability of Soluble
Hexacenes
Balaji Purushothaman, Sean R. Parkin, and John E. Anthony*
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055
anthony@uky.edu
Received March 7, 2010
ABSTRACT
The synthesis of new silylethyne-substituted hexacene derivatives to investigate their solubility, stability, and π-stacking is reported. It was
found that “butterfly” dimerization, rather than photooxidation, is the dominant decomposition pathway for these molecules and that stability
can be enhanced by functionalization to prevent close contact between specific regions of the aromatic core. Dimerization regioselectivity can
be altered by suitable engineering of the solid-state arrangement of the chromophores.
Electronic devices based on organic materials have been an
area of intense research for the past few decades.1 Among
the organic materials used in these devices, acene-based
semiconductors have been widely studied.2 Pentacene, the
largest sufficiently stable acene for device studies, has shown
charge carrier mobility greater than 3 cm2 V-1 s-1 and is
regarded as a benchmark among organic semiconductors.3
Acenes larger than pentacene have been of great interest due
to their predicted lower reorganization energy,4 potential
higher charge carrier mobility,5 and smaller band gap.6
However, such an improvement in electronic properties is
concomitant with decreasing stability and solubility in
organic solvents, which in turn hampers the synthesis and
application of these interesting molecules.7 A thorough
understanding of the reactivity of these materials will be
required to design new functional compounds with stability
and solubility sufficient for detailed device studies.
The decomposition pathways for acenes are typically
assumed to proceed through photoinduced endoperoxide
formation, with subsequent oxidation to the corresponding
quinone,8 or through a “butterfly” dimerization of the
aromatic rings.9 In pentacene, endoproxide formation is
assumed to dominate in the solid state, whereas dimerization
is the main decomposition pathway in solution.9 Unsubsti-
tuted higher acenes such as hexacene and heptacene have
only been synthesized successfully by photodecarbonylation
of soluble diketone precursors in a polymer matrix, where
such decomposition pathways can be retarded.10
(1) (a) Cheng, Y. -J.; Yang, S. -H.; Hsu, C. -S. Chem. ReV. 2009, 109,
5868. (b) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953.
(2) (a) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452. (b) Anthony,
J. E. Chem. ReV. 2006, 106, 5028. (c) Bendikov, M.; Wudl, F.; Perepichka,
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(3) Klauk, H.; Hallik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.;
Werner, W. J. Appl. Phys. 2002, 92, 5259.
Among the recent functionalization strategies employed
for the synthesis of soluble and stable acene derivatives, the
(7) (a) Clar, E. Chem. Ber. 1939, 72B, 2137. (b) Marschalk, C. Bull.
Soc. Chim. 1939, 6, 1112.
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8614.
(8) Reddy, A. R.; Bendikov, M. Chem. Commun. 2006, 1179.
(9) Berg, O.; Chronister, E. L.; Yamashita, T.; Scott, G. W.; Sweet,
R. M.; Calabrese, J. J. Phys. Chem. A. 1999, 103, 2451.
(10) (a) Mondal, R.; Adhikari, R. M.; Shah, B. K.; Neckers, D. C. Org.
Lett. 2007, 9, 2505. (b) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Am.
Chem. Soc. 2006, 128, 9612.
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Cornil, J.; Bre´das, J. L. J. Chem. Phys. 2003, 118, 3764.
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93, 146405.
10.1021/ol100178s  2010 American Chemical Society
Published on Web 04/07/2010

peri-functionalization approach utilizing trialkylsilylethynyl
groups has been widely used to develop organic electronic
materials.11 The improved solubility, stability, and induced
π-stacking of pentacene derivatives have paved the way for
the synthesis of stable and soluble higher acenes.12 However,
in order to stabilize these highly reactive materials, signifi-
cantly bulkier substituents had to be used to prevent
Diels-Alder reaction between the alkyne substituent of one
molecule and the reactive acene chromophore of another.13
Hence bulky tri-tert-butylsiliylethynyl (TTBS) groups were
used to stabilize hexacene, whereas for heptacene, a larger
tris(trimethylsilyl)silylethynyl (TTMSS) substituent had to
be employed. In a recent report, Wudl and co-workers have
reported the synthesis of a stable tetraphenyl heptacene
derivative using a tri-isopropylsilylethynyl substituent. The
phenyl groups are effective in preventing the aromatic cores
from reacting with each other or with the alkyne, and as a
result the smaller alkyne substituent could be used.14
Photoxidation of the aromatic core was reported to be the
major decomposition process for this heptacene derivative.
Although TTBS hexacene exhibits reasonable stability in
solution, the poor solubility in solvents such as toluene and
chlorobenzene has prevented the measurement of its transport
properties in organic thin film transistors (OTFTs). A major
objective of the present work is to facilitate study of these
materials by improving the solubility and π-stacking of these
peri-functionalized hexacenes by changing the alkyl substit-
uents on silicon. Concurrently, we planned to study the
decomposition pathways of these molecules, both in solution
and the solid state, to gain a better understanding of reactivity
for the design of effective functionalization strategies.
For our studies, we surveyed a variety of branched alkyl-
or cycloalkyl-substituted silyl acetylene substituents, such
as tri-isobutylsilylethynyl (TIBS), tricyclopentylsilylethynyl
(TCPS), tricyclohexylsilylethynyl (TCHS), and TTMSS.
6,15-Hexacenequinone (1) was converted to a series of
diethynyl diols (2) by treatment with excess acetylide. The
resulting diols were then converted to the desired hexacenes
by treatment with a saturated solution of tin(II) chloride in
10% HCl. Purification of the crude hexacenes by silica gel
chromatography followed by recrystallization gave dark
green hexacene crystals in yields ranging from 8% to 36%,
depending on the substituent. Compared to phenyl-substituted
hexacenes,10a these materials exhibit significant persistence
both in solution and the solid state. These materials showed
higher solubility than the TTBS derivative, and all of them
could be prepared as 1 wt % solutions in toluene.
Scheme 1. Synthesis of 6,15-Bis(trialkylsilylethynyl)hexacene
(3b) exhibited a two-dimensional π-stacking motif with a
close contact of 3.42 Å between the aromatic faces (Figure
1). By changing the substituent to the smaller TIBS group
(3a) the π-stacking motif changed to a one-dimensional
sandwich herringbone packing, and larger TCHS or TTMSS
substituents (3c) or (3d) led to one-dimensional π-stacked
arrangements with close contacts of 3.3 and 3.36 Å respec-
tively (Figure 1). The change in packing motif with the size
of the trialkylsilyl substituent is in accordance with the
functionalization model developed for controlling π-stacking
in pentacene.11a All of these hexacene derivatives exhibit
significant bending of alkyne substituents (CAr-Csp-Csp
≈
176-178° and Csp-Csp-Si ≈ 169-179°) induced by crystal
packing effects. However, the distortion is not as dramatic
as seen with TTBS hexacene (CAr-Csp-Csp ≈ 176° and 173°
and Csp-Csp-Si ) 169° and 174°) and is typical of alkynes
having large substituents.11a,12,13 Another important observa-
tion is the twisting of the acene core (torsion angle 13.4°)
of TCHS hexacene (3c) in comparison to all other derivatives
(<5°), which likely arises to alleviate strain in crystal packing
due to the bulky cyclohexyl substituents.
Our study of the decomposition pathways of hexacenes
began during the recrystallization of TIBS hexacene 3a under
ambient laboratory lighting, where a small amount of yellow
crystals formed along with the green hexacene. Single-crystal
X-ray diffraction analysis of this byproduct showed that this
compound was a symmetrical dimer formed between the
reactive C7 and C14 carbons of two hexacene molecules (4,
Figure 2). Unlike the two dimer products proposed for TIPS
pentacene decomposition,15 only the centrosymmetric dimer
product was observed, as the planosymmetric product is
significantly more sterically hindered.
The dark green crystals of these compounds obtained from
recrystallization were suitable for single-crystal X-ray dif-
fraction analysis. Like TTBS hexacene,12 TCPS hexacene
(11) (a) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4,
15. (b) Miao, S.; Smith, M. D.; Bunz, U. H. F. Org. Lett. 2006, 8, 757. (c)
Tang, M. L.; Reichardt, A. D.; Siegrist, T.; Mannsfeld, S. C. B.; Bao, Z.
Chem. Mater. 2008, 20, 4669.
(12) Payne, M. M.; Parkin, S. R.; Anthony, J. E. J. Am. Chem. Soc.
2005, 127, 8028.
When pure crystals of TIBS hexacene were kept in air in
the dark, they slowly (∼1 month) turned from crystalline
(13) Payne, M. M.; Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org.
Lett. 2004, 6, 3325.
(14) Chun, D.; Cheng, Y.; Wudl, F. Angew. Chem., Int. Ed. 2008, 47,
8380.
(15) Coppo, P.; Yeates, S. G. AdV. Mater. 2005, 17, 3001.
Org. Lett., Vol. 12, No. 9, 2010
2061

Figure 2. Thermal ellipsoid plots of symmetrical dimer (4), offset
dimer (5), and alkyne dimer-endoperoxide (6).
from reaction on a chromophore where one of the alkyne
substituents had been compromized. Noteworthy for these
dimeric products are their mass spectra: all of these well-
characterized decomposition products showed only signals
for pristine monomer 3a when subjected to laser-desorption
ionization mass spectrometry (LDI-MS, see Figure S6 in
Supporting Information). This result suggests that LDI-MS
may not be a viable method to determine structure or purity
of the larger acenes.
Differential pulse voltammetry was performed on all of
the hexacene derivatives to determine the oxidation and
reduction potentials of these molecules (ferrocene used as
internal standard, Table 1). TTMSS hexacene (3d) has the
lowest oxidation potential compared to the other derivatives
Figure 1. Crystal packing arrangement of hexacenes 3a-d (front-
most trialkylsilyl groups removed for clarity).
green needles into a reddish brown powder. Purification of
this powder by silica gel chromatogrphiy to remove unreacted
hexacene followed by recrystallization and single crystal
X-ray diffraction analysis showed that this material was
another dimerization product formed between the C7 and
C14 carbons of one hexacene molecule with the C8 and C13
carbons of another to give a dimer with tetracene and
anthracene chromophores (5, Figure 2). The change in the
regiochemistry of dimerization arises simply from the
orientation of the chromophores in the solid state.
Another byproduct observed during the synthesis of
hexacene 3a was initially isolated as a deep green material
that turned yellow-orange during chromatographic purifica-
tion on silica gel. X-ray crystallographic analysis of this
purified material showed that initial decomposition arose
from a Diels-Alder reaction of the hexacene core with the
alkyne substituent of another molecule, to yield the initially
observed green species, followed by endoperoxide formation
during chromatography (6, Figure 2), which bleached the
remaining hexacene chromophore. Thus, the only photo-
oxidation product observed in this series of hexacenes arose
Figure 3. Differential pulse voltammetry of TCPS hexacene.
due to the electropositive tris(trimethylsilyl)silylethynyl
group. The lower energy gap is also evident from a ∼13 nm
red shift in toluene compared to other derivatives.
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Org. Lett., Vol. 12, No. 9, 2010

Table 1. Electrochemical Data and Solution Half-Life of 3a-d
Eox
(mV)
Ered
(mV)
HOMO
(eV)
LUMO
(eV)
t1/2
(min)
acene
TIPS pentacene
296
93
38
45
19
3a
3b
3c
3d
181
172
198
123
-1376
-1370
-1370
-1421
-4.98
-4.97
-5.00
-4.92
-3.42
-3.43
-3.43
-3.38
To study their relative stabilities in solution, all of these
hexacene derivatives were subjected to UV-vis stability
studies (Table 1) and compared with TIPS pentacene.
Disappearance of the long-wavelength hexacene absorptions
is coupled with the appearance of a strong absorption at 430
nm, characteristic of an anthracene chromophore, and weak
absorbances at ∼550 nm, characteristic of tetracene (Figure
4). Comparison of the decomposition UV-vis spectrum with
absorption spectra of 4 and 5 (Figure S4 in Supporting
Information) suggests that these are indeed the major
decomposition products in solutions illuminated in air.
Similar decomposition products were observed for hexacene
derivatives 3b-d (Figures S1-S3 in Supporting Informa-
tion). It is noteworthy that TTMSS hexacene 3d, which was
expected to have higher stability due to the effectiveness of
the bulky substituents in preventing dimerization, had a half-
life of only 19 min in solution (Figure S3 in Supporting
Information). This lower stability may be due to the
photolysis of the silane into hexamethyldisilane and a radical
byproduct,16 which effectively reduces the size of the silyl
group and hence its efficacy in stabilizing the hexacene.
Figure 4. UV-vis spectra of 3a recorded at regular interval during
exposure to light.
electronic properties of silylethyne-substituted acenes need
not be tuned to prevent oxidation. Instead, identifying
approaches to prevent dimerization may be the best route to
stabilize these materials.
In conclusion, we have presented a series of soluble
hexacene derivatives using our peri-functionalization ap-
proach to study the modes of decomposition of these
materials. In contrast to other reports, photooxidation was
not found to be a significant decomposition pathway17 and
was only observed in materials where one of the alkyne
substituents had been compromised. Instead, dimerization
was the most common decomposition pathway. We have
developed materials with reasonable solution and solid-state
lifetimes, and with suitable precautions to minimize exposure
to light, studies of the thin film properties of these materials
are currently underway.
1
A H NMR stability study was also performed on TIBS
Acknowledgment. The authors thank the National Science
Foundation (CHE 0749473) for support of this project.
hexacene 3a in deuterated benzene at a concentration of 1
1
wt % (Figure S5 in Supporting Information). The H NMR
Supporting Information Available: Experimental details
and crystallographic CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
spectra of both N2- and O2-purged solutions before and after
exposure to bright light again showed that dimerization,
rather than oxidation, was the dominant degradation pathway
in these silylethyne-functionalized hexacenes. These results
suggest that, unlike most other acene derivatives, the
OL100178S
(17) Kaur, I.; Jia, W.; Kopreski, R.; Selvarasah, S.; Dokmeci, M. R.;
Pramanik, C.; McGruer, N. E.; Miller, G. P. J. Am. Chem. Soc. 2008, 130,
16274.
(16) Ishikawa, M.; Sugisawa, H.; Yamamoto, K.; Kumada, M. J.
Organomet. Chem. 1979, 179, 377.
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