Double Diels-Alder strategies to soluble 2,9- and 2,9,6,13- tetraethynylpentacenes, photolytic [4 + 4] cycloadditions, and pentacene crystal packing

Article abstract of DOI:10.1021/jo0709807

(Chemical Equation Presented) Four new classes of organic solvent soluble ethynylpentacene derivatives (2,9-, 2,10-, 2,6,9,13-, 2,6,-10,13-) have been prepared by complementary, versatile, double Diels-Alder strategies. Functional groups on the A, C, and

Full text of DOI:10.1021/jo0709807

Double Diels-Alder Strategies to Soluble 2,9- and
2,9,6,13-Tetraethynylpentacenes, Photolytic [4 + 4] Cycloadditions,
and Pentacene Crystal Packing
Christophe P. Be´nard,† Zhe Geng,§ Matthew A. Heuft,‡ Kelly VanCrey, and
Alex G. Fallis*,|
Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5,
Naeja Pharmaceuticals Inc., 4290-91A Street, Edmonton, Alberta, Canada T6E 5V2, Xerox Research
Centre of Canada, Sheridan Science and Technology Park, 2660 Speakman DriVe, Mississauga, Ontario,
Canada L5K 2L1, Toronto Research Chemicals, 2 Brisbane Road, North York, Ontario, Canada M3J 2J8,
and Torcan Chemical Ltd., 110 Industrial Parkway North, Aurora, Ontario, Canada L4G 3H4
afallis@uottawa.ca
ReceiVed May 23, 2007
Four new classes of organic solvent soluble ethynylpentacene derivatives (2,9-, 2,10-, 2,6,9,13-, 2,6,-
10,13-) have been prepared by complementary, versatile, double Diels-Alder strategies. Functional groups
on the A, C, and E rings can be manipulated to increase the solubility, modulate the electronics, and alter
the solid-state packing. Cycloaddition reactions with diene 2 and 1,4,5,8-anthradiquinone (3) or ortho-
quinodimethane 19 with 1-butyl-3-methylimidazolim iodide (18) as the iodide source (a significant
improvement over NaI) and benzoquinone (20) followed by in situ aromatization afforded the quinones
4, 5, 21, and 22, respectively. For the 2,9- and 2,10- families, a one-pot desilylation/triflation was developed.
Palladium(0) coupling and reductive aromatization afforded 2,9-di(triisopropylsilylethynyl)pentacene (10)
and 2,10-di(triisopropylsilylethynyl)pentacene (11), respectively. Photodimerization of these pentacenes
afforded the air-stable [4 + 4] cycloaddition pentacene precursors (tetrakisnaphthotricyclo[4.2.2.22,5]-
dodecanes, 12-15). Thermal cycloreversion of the dimers (∼13 J/g, ∼4 kcal/mol) produces the parent
pentacenes (10 or 11). The tetrasubstituted family utilized a parallel route with extra versatility as the
timing of the Grignard and palladium(0) coupling step may be varied depending upon the functional
group combinations desired. The subsequent reactions provided the tetraethynylpentacenes 28-30, 33-
35 (para-isomers), and 38 (meta-isomer). X-ray crystallography analysis of 28, 29, and 33 revealed various
π-π stacked packing motifs that differ from the unfavorable herringbone pattern of pentacene.
Introduction
transistors1 because of high hole mobilities in excess of 1 cm2/V
per s compared to that of other organic compounds.2 Pentacene
is insoluble in common organic solvents and sensitive to light
and atmospheric oxygen. Consequently, the development of
solution-based processing of pentacene for organic electronic
Pentacene (1) is among the most widely employed small
molecule p-type organic semiconductor for organic thin film
† Naeja Pharmaceuticals Inc.
‡ Xerox Research Centre of Canada.
§ Toronto Research Chemicals.
Torcan Chemical Ltd.
(1) Lin, Y.-Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Trans.
Electron DeVices 1997, 44, 1325.
| University of Ottawa.
10.1021/jo0709807 CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/23/2007
J. Org. Chem. 2007, 72, 7229-7236
7229

Be´nard et al.
FIGURE 1. Pentacene framework for diverse derivatives.
devices is hindered by these properties,3 although Gelinck et
al. constructed a single sheet transistor display from 1888
pentacene transistors.4 Two strategies have emerged to overcome
these difficulties. The first approach utilizes soluble pentacene
precursors equipped for thermal retro-Diels-Alder reactions to
generate pentacene.5 The second approach involves pentacene
derivatives with solubilizing groups.6 In general, these syntheses
were not designed for further functionalization. We wish to
describe four new classes of pentacene derivatives (Figure 1)
with functional groups on the terminal A and E or A, C, and E
rings designed to increase the solubility, modulate the electron-
ics, and alter the solid-state packing.7
(TBAF/THF) and treated directly with PhNTf2 to afford 6 in
98% yield.11 Sonogashira coupling of ditriflate 6 and TIPS-
acetylene gave diquinone 8. Ditriflate 6 was sparingly soluble
(THF, 1 mg/5 mL) and required complete dissolution before
reagent addition. The most reliable reductive deoxygenation/
aromatization procedure employed a Meerwein-Ponndorf-
Verley reduction (Al, HgCl2, CBr4, cyclohexanol, reflux).12
Reductive/aromatization of 8 afforded the 2,9-disubstituted
pentacene 10. The identical reaction sequence transformed 5
into the 2,10-disubstituted pentacene 11. Pentacenes 10 and 11
were deep purple and soluble in common organic solvents (CH2-
Cl2, THF, Et2O, hexane). This solubility rendered them par-
ticularly sensitive to atmospheric oxygen.13 The initial peroxide
product is unstable and is rapidly oxidized to the C ring quinone.
This could provide an alternative, inefficient route to 24 for
tetrafunctionalized 2,6,9,13-pentacenes! Fritzsche observed 140
years ago that anthracene underwent a photochemically induced
[4 + 4] cycloaddition to generate tetrakisbenzotricyclo[4.2.2.22,5]
dodecane,14 and subsequent research revealed the dimer afforded
anthracene upon thermolysis.15 In the interim, the photochem-
istry of anthracene has been thoroughly investigated.16 Its
photolytic cycloaddition reactivity is due to high singlet energy,
excitation to an encounter exciplex in equilibrium with a more
stable exciplex complex that combines to generate the dimer.17,18
We anticipated that the solubility and enhanced diene character
of pentacenes 10 and 11 would facilitate a photodimerization
capable of thermal reversion. This was the case as independent
photolysis of 10 and 11 afforded a regiochemical mixture (1:1)
composed of the individual isomers (12, 13, and 14, 15), which
were separated by flash chromatography (Scheme 2).19,20 The
pentacene cycloaddition photochemistry likely parallels an-
thracene, in which these pentacenes act as their own singlet
Results and Discussion
Synthesis and Photochemical Dimerization of Pentacenes
10 and 11. Pentacene reacts preferentially at the electron-rich
C ring; thus, direct reactions at the terminal rings are challeng-
ing.8 Therefore, a new double Diels-Alder strategy for penta-
cenes was designed to facilitate early introduction of a substit-
uent onto the terminal A and E rings (Scheme 1). Reaction of
Danishefsky’s diene 29 and anthradiquinone 310 followed by
air oxidation over silica gel afforded diquinones 4 and 5 (1:1,
60%). The isomers were separated by fractional crystallization
from CHCl3. Ditriflate 6 was prepared in a one-pot desilylation/
triflation sequence from the corresponding diquinone to avoid
precipitation of the initial diol. Silyl ether 4 was disilylated
(2) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Matter. 2002,
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D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413.
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Am. Chem. Soc. 2004, 126, 12740. (d) Vets, N.; Smet, M.; Dehaen, W.
Tetrahedron Lett. 2004, 45, 7287.
(6) (a) Takahashi, T.; Kitamura, M.; Shen, B.; Nakajima, K. J. Am. Chem.
Soc. 2000, 122, 12876. (b) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org.
Lett. 2002, 4, 15. (c) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.;
Wudl, F.; Bao, Z.; Siegrist, T.; Kloc, C.; Chen, C.-H. AdV. Mater. 2003,
15, 1090. (d) Payne, M. M.; Delcamp, J. H.; Parkin, S. R.; Anthony, J. E.
Org. Lett. 2004, 6, 1609. (e) Swartz, C. R.; Parkin, S. R.; Bullock, J. E.;
Anthony, J. E.; Mayer, A. C.; Malliaras, G. G. Org. Lett. 2005, 7, 3163. (f)
In parallel with our research, new soluble hexasubstituted ethynalated A-E
pentacenes have been synthesized recently and their properties measured:
Jiang, J.; Kaafarani, B. R.; Neckers, D. C. J. Org. Chem. 2006, 71, 2155.
(7) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem.
Soc. 2001, 123, 9482.
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M.; Duong, H. M.; Starkey, K.; Houk, K. N.; Carter, E. A.; Wudl, F. J.
Am. Chem. Soc. 2004, 126, 7416.
(9) (a) Danishefsky, S. J.; Kitahara, T.; Yan, C. F.; Morris, J. J. Am.
Chem. Soc. 1979, 101, 6996. (b) Danishefsky, S. J.; Yan, C. F.; Singh, R.
K.; Gammill, R. B.; McCurry, P.; Fritsch, N.; Clardy, J. C. J. Am. Chem.
Soc. 1979, 101, 7001.
(11) Hendrickson, J. B.; Bergeron, R. Tetrahedron Lett. 1973, 14, 4607.
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A. J. J. Am. Chem. Soc. 1998, 120, 664. (c) Chan, A. H.; Lee, H. K.; Wang,
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(18) For recent photolytic synthetic studies, see: (a) Wu, D.-Y.; Chen,
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1075. (b) Takaguchi, Y.; Tajima, T.; Yangimoto, Y.; Tsuboi, S.; Ohta, K.;
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J. H. R. Angew. Chem., Int. Ed. 2005, 44, 1072. (d) For the estimated
enthalpy of dissociation for the anthracene dimer (∼12 kcal/mol), see:
Grimme, S.; Diedrich, C.; Korth, M. Angew. Chem., Int. Ed. 2006, 45, 625.
(10) Cory, R. M.; McPhail, C. L.; Dikmans, A. J. Tetrahedron Lett. 1993,
34, 7533.
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Double Diels-Alder Strategies
SCHEME 1. Double Cycloaddition Route to Pentacenes 10 and 11
sensitizer to form an exciplex complex that collapses to the [4
+ 4] cycloaddition products.
rings is reflected in the 2.81 Å distance between C2 and C24,
compared to the increased distance of 7.33 Å between C6 and
C28. The X-ray crystal structure of 14 (from acetone) confirmed
the structural assignment and its interesting “biplanar” motif
(Figure A in the Supporting Information).21,22
Differential scanning calorimetry (DSC) measurements es-
tablished that the thermal reversion onset temperature was
167 °C with a low value of the enthalpy of dissociation (∼13
J/g, ∼4 kcal/mol), a consequence of the elongated C1-C23
bond (1.60 Å) and the stabilized bisbenzylic radical intermedi-
ates from bond cleavage. The tilted nature of the naphthalene
These photochemical adducts may have other attributes by
analogy with tetrahydroanthracene if they can be converted to
tetrahydropentacenes. Tetrahydroanthracene is a precursor for
the synthesis of both tubelike structures23 and a Mo¨bius aromatic
hydrocarbon.24 This implies that tetrahydropentacenes have the
(19) A SciFinder structure search for tetrakisnaphthotricyclo[4.2.2.22,5]-
dodecane failed. It is in the literature named s-dipentacene. Photolysis of
pentacene (1) in 1-chloronaphthalene afforded s-dipentacene, although the
thermal retro-reaction was not investigated. Berg, O.; Chronister, E. L.;
Yamashita, T.; Scott, G. W.; Sweet, R. M.; Calabrese, J. J. Phys. Chem. A
1999, 103, 2451.
(20) In parallel with our atmospheric oxygen experiments and photo-
chemical studies, Wong and co-workers12c reported the rapid reaction with
oxygen to form a peroxide of their soluble 2,3,9,10-tetrakis(trimethylsilyl)-
pentacene. In the absence of air, it was stable for 7 days in chloroform.
They also observed the dimerization of their pentacene in sunlight in
concentrated hexane solution during crystallization.
(21) X-ray data for compounds 14, 28, 29, and 33 have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication numbers CCDC 297309, 652161, 652162, and 652163, respec-
tively. Copies of the data may be obtained free of charge from CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. Fax: (+44) 1223-336-033.
E-mail: deposit@ccdc.cam.ac.uk.
(22) See ref. 19.
(23) Kammermier, S.; Jones, P. G.; Herges, R. Angew. Chem., Int. Ed.
1996, 35, 2669.
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Be´nard et al.
SCHEME 3. Preparation of ortho-Quinodimethane 19:
SCHEME 2. Photochemical [4 + 4] Dimerization of
Pentacenes 10 and 11 and Their Thermal Reversion
Improved Cycloadditions with an Ionic Liquid Iodide Source
ionic halide source for this transformation. The ortho-xylene
TBS ether 17 was prepared from the xylene 16 by free radical
bromination. The initial conditions to generate the desired diene
intermediate 19 involved the exposure of 17 to sodium iodide
and copper triflate in DMF for 8 h (65 °C) in the presence of
benzoquinone (20). These conditions resulted in disappointing
35-37% yields of the Diels-Alder adducts 21 and 22 (1:1). A
significant improvement of this literature method involved the
replacement of sodium iodide with the iodo-ionic liquid 18 (1-
butyl-3-methylimidazolim iodide) as the halogen source. The
yield of the Diels-Alder adducts 21 and 22 (1:1) was doubled
(77%), the reaction time was halved, and the workup was
facilitated. The two quinones were separated by fractional
crystallization from CH2Cl2.
potential to provide access to new members of both these
homologous structural types.
Unfortunately, despite the interesting chemistry above, nature
presented us with several insurmountable complications. Pen-
tacene 10 dimerizes immediately when exposed to ambient
laboratory light (3-5 min). The thermal reversion may be
followed visually because of the appearance of the purple
pentacene, but this method did not afford pentacene 10 with
the purity required for thin film electronics. In addition, the 13
C
NMR spectra could not be recorded nor could the mobility be
measured. Consequently, these results were not publishable in
an independent article.25
The requisite transformations to the new pentacenes followed
a sequence similar to those established above, as summarized
in Scheme 4. The desired 2,9-regioisomer 22 was treated with
fluoride, and the diol product reacted with N-phenyltrifluo-
romethanesulfonimide to afford the ditriflate 23 in 90% overall
yield. Sonogashira coupling with TIPS-acetylene gave sily-
lacetylene 24 in excellent yield (90-93%).
This was followed by double addition of the appropriate
lithium acetylides (TIPS-CtCLi or TMS-CtCLi or Ph-Ct
CLi) to provide the diols 25-27, respectively. Aromatization
was accomplished using tin(II) chloride under acidic conditions
to afford the 2,6,9,13-tetrasubstituted pentacenes 28-30 in 78-
93% yield.
Reordering the experimental steps from the 2,9-disilylquinone
22 afforded a versatile alternate route to the pentacene family
in Scheme 4. This is preferred because of the concomitant silyl
ether deprotection/aromatization step and potential for the
addition of diverse functionality at the 2,9-positions. Double
addition of the appropriate Grignard reagent was conducted
initially with quinone 22 to add the C6 and C13 substituents
and generate the tertiary diol 31 (Scheme 5). Subsequent
exposure to tin chloride generated the 2,9-diols (not illustrated).
Triflation provided the ditriflate compound 32, which was
converted to the pentacenes 28 and 33-35 by palladium(0)-
Synthesis of Tetrasubstituted Ethynylpentacenes 28-30,
33-35 (para-, 2,9-), and 38 (meta-, 2,10-). To solve the
problems presented by the reactivity of pentacenes 10 and 11,
we embarked on the synthesis of more highly substituted
pentacenes with 6,13-alkylsilylethynyl functionality to moderate
the reactivity of the central ring and improve crystal packing.
The desirable 2,9-substituents were retained to aid the solubility,
mix and match the silyl groups, explore their effect on the crystal
packing and the potential to extend the chromophore, and attach
heterocyclic groups. This new class of tetrasubstituted pentacene
derivatives possesses functionalized ethynyl groups on the
terminal A, C, and E rings that may be varied as desired. On
the basis of Scheme 1, a related double Diels-Alder tandem
elimination strategy for these new pentacenes was designed to
facilitate introduction of the substituents onto the terminal A
and E rings and allow ring C functionalization (Scheme 3). The
most reliable route to the requisite ortho-quinodimethane, despite
the investigation of alternative synthetic methods,26 was the Cava
procedure in which a tetrabromo-ortho-xylene is treated with
an iodide source.27 This appears to be the first example of an
(24) Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426,
819.
(25) Conference presentations: (a) Heuft, M. A.; Fallis, A. G. Presented
at the 39th IUPAC Congress, Ottawa, ON, Canada, 2003. (b) Fallis, A. G.
Presented at the 88th CSC Conference, Saskatoon, SK, Canada, 2005. (c)
Fallis, A. G. Presented at Pacifichem, Honolulu, HI, 2005.
(26) Chou, T.-s. ReV. Heteroat. Chem. 1993, 8, 65-104.
(27) (a) Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1957, 79, 1701.
(b) Cava, M. P.; VanMeter, J. P. J. Org. Chem. 1969, 34, 538.
7232 J. Org. Chem., Vol. 72, No. 19, 2007

Double Diels-Alder Strategies
SCHEME 4. Synthesis of Tetraethynylpentacenes 28-30
from Quinone 22
SCHEME 5. Alternate Synthesis of Tetrasubstituted
Pentacenes 28 and 33-35 from Quinone 22
because of increased edge-to-face interactions that are present.
In contrast, replacement of TMS groups with TIPS groups,
which are essentially spherical and have an approximate
diameter equal to or less than one-half the pentacene length,
induces a change in the packing arrangement. Consequently,
6,13-TIPS-ethynyl groups prefer a 2-D parallel geometry,
sometimes referred to as a “bricklayer” arrangement, whereas
n-propyl substitution results in a “slipped-stack” parallel
geometry.
The tetra-TIPS pentacene 28 forms stacked parallel sheets
in which the pentacenes are superimposed (Figure 2). The
individual molecules are separated by 10.5 Å. This large
separation is caused by the steric repulsion between the C6 and
C13 TIPS groups, which extend into the edge of the π-system
in rings B and D and cause a ring shift in the overlapping pair.
This feature prevents the close π-stacking of the aromatic rings.
This arrangement can be identified more easily in the ORTEP
diagram in which the packing arrangement is viewed perpen-
dicular to the pentacene plane (Figure B in the Supporting
Information). In principle, the most facile way to remove this
interaction is to replace the silyl group with hydrogen and
compare the X-ray data. Unfortunately, the resulting 6,13-
diethynylpentacene could not be properly characterized because
mediated coupling with the appropriate acetylenic component.
The triflates 23 and 32 are versatile intermediates for the
preparation of various A and E ring 2,9-pentacene derivatives
for the attachment of heterocyclic and electronically useful
groups that have not been available previously.
The 2,10-pentacene isomer 21 was converted in a parallel
manner following the reaction sequence in Scheme 6 to afford
the 2,6,10,13-tetrasilylethynylpentacene 38 in the meta-series.
Crystal Packing Studies: X-ray Analyses of Pentacenes
28, 29, and 33. The excellent recent review by Anthony28
contains a detailed analysis of pentacene packing motifs.
Pentacene itself prefers to pack in a herringbone pattern, and
the same pattern is adopted by 6,13-TMS-ethynyl substitution
(28) Anthony, J. E. Chem. ReV. 2006, 106, 5028.
J. Org. Chem, Vol. 72, No. 19, 2007 7233

Be´nard et al.
SCHEME 6. Synthesis of Tetrasubstituted Pentacene 38
from Quinone 21
FIGURE 2. Crystal packing: 2,6,9,13-tetrakis(triisopropylsilylethynyl)-
6,13-pentacene (28).
of its instability, and the corresponding bromoacetylene behaved
similarly. Additional nonbonded interactions between the C9
TIPS group of the top ring and the C2 TIPS substituent of the
lower ring are also restrictive. Despite the magnitude of the
separation of the overlapping pentacenes, the presence of the
TIPS groups has lived up to their reputation of creating a very
ordered mode of crystal packing. Figure 3 shows the crystal
packing of 29.
This beneficial feature is apparent in the X-ray of the 6,13-
TMS compound 33 (Figure 4). The C rings in two pentacenes
are nearly superimposed but are splayed into an X-like orienta-
tion because of rotation about the C ring by the steric
interference between the C13-TMS on the top ring group with
the C9-TIPS group of the nearest neighbor. This inhibits the
alignment for full overlap that might otherwise occur. In
addition, the TIPS groups at the C2 and C9 positions disrupt
rather than aid the alignment. If 33 were a tetra-TMS compound
instead of mixed it is possible the overlap might be improved.
The final crystal structure is for compound 33. The 6,13-TIPS
groups face each other in an interlocking manner like “anthers
about to clash” and dictate the orientation of the other
functionality. Consequently, there is no close π-overlap between
the aromatic rings of the acenes. Instead, they are oriented as a
parallel series of steps along the long axis of the pentacenes.
The 2,9-phenylenynylpentacene samples continually cocrystal-
lized with the solvent, and thus the X-rays were disordered and
further analysis was not informative.
from chlorobenzene or 1,2-dichlorobenzene. The films were
measured before and after annealing at 100 °C.29 The best results
were obtained for films that were drop cast from 1,2-dichlo-
robenzene onto substrates heated at 90 °C. The saturated
mobility was 3 × 10-4 cm2/V per s, whereas the linear mobility
was an order of magnitude lower at 10-5 cm2/V per s for a
10-µm device. The devices were turned on near 0 V and showed
a reasonable ON/OFF ratio of 103 in saturation mode. A
comparison of pentacene and 6,13-TIPS pentacene films
recorded values of 0.02-0.04 and 0.01-0.06 cm2/V per s,
respectively.30 These values are considerably better than those
for 28.
Spectral Data. Electronic absorption and fluorescence spectra
of phenylethynyl-substituted acenes including pentacenes were
reported several years ago.31 More recently, Kafafi and co-
workers32 have examined the photophysical properties of
dioxolane-substituted pentacene derivatives with 6,13-triisopro-
pylsilylethynyl groups dispersed in tris(quinolin-8-olato)alumi-
num(III). Our spectra for 29, 30, and 32 are illustrated in Figures
5 and 6, but the variation in the substitution patterns between
TMS and TIPS had little effect on the absorbance spectra. It is
also interesting that the spectral characters for the ditriflate 32
were very similar to that of the silyl ethynyl compounds. The
normalized fluorescence spectra of these pentacenes in n-hexane
displayed the near absence of Stokes’ shift in the fluorescence
spectra consistent with the rigidity of these molecules. For
On the basis of the simplistic possibility that 2,9-proparyl
alcohol groups might facilitate hydrogen bonding in the crystal,
while retaining the benefit of the 6,13-TIPS substituents, the
compound 35 was prepared, but unfortunately, its crystal
packing could not be determined because of its very poor crystal
quality.
Device Characteristics. The mobility characteristics were
determined for the tetra-TIPS molecule 28. Thin semiconductor
films were then deposited by either spin coating or drop casting
(29) Unfortunately for our collaborators and us, vapor disposition methods
were not available.
(30) Ostroverkhova, O.; Shcherbyna, S.; Cooke, D. G.; Egerton, R. F.;
Hegmann, F. A.; Tykwinski, R. R.; Parkin, S. R.; Anthony, J. E. J. J. Appl.
Phys. 2005, 98, 33701.
(31) Maulding, D. R.; Roberts, B. G. J. Org. Chem. 1969, 34, 1734.
(32) Wolak, M. A.; Melinger, J. S.; Lane, P. A.; Palilis, L. C.; Landis,
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110, 7928.
7234 J. Org. Chem., Vol. 72, No. 19, 2007

Double Diels-Alder Strategies
FIGURE 3. Crystal packing: 2,9-bis(triisopropylsilylethynyl)-6,13-bis(trimethylsilylethynyl)pentacene (29).
FIGURE 4. Crystal packing: 2,9-bis(2-(trimethylsilyl)ethynyl)-6,13-bis(2-(triisopropylsilyl)ethynyl)pentacene (33).
FIGURE 6. Normalized fluorescence spectra of pentacenes 29, 30,
and 32 in n-hexane (open to air).
FIGURE 5. UV-vis absorption spectra of pentacenes 29, 30, and 32
in n-hexane (normalized at λmax ≈ 650 nm).
example, the absorption maximum for 29 is centered at 647
nm and the emission maximum shifts by less than 1 nm. This
observation indicates that little conformational change occurs
between the ground state and the first singlet excited state. The
fluorescence quantum yield (Φf) was found to be ∼0.1 for 28,
29, 30, and 32 in n-hexane under air when compared to Nile
Red in dioxane (Φf ) 0.7).33 The absorbance and fluorescence
peaks near 650 nm were also very narrow. The full width at
(33) Sackett, D. L.; Wolff, J. Anal. Biochem. 1987, 167, 228.
J. Org. Chem, Vol. 72, No. 19, 2007 7235

Be´nard et al.
mixture of 4 and 5 (1:1). Isomers 4 and 5 could be separated by
preparative HPLC (CH2CL2/hexane, 70:30) or preferably by
fractional crystallization from chloroform.
2,9-Bis(t-butyldimethylsiloxy)-5,7,12,14-pentacenediquinone
(4).
half-maximum (fwhm) for compound 29 is 16.5 and 20 nm for
absorption and fluorescence, respectively. This compares favor-
ably with that of CdSe nanoparticles, noted for their narrow
emission spectra that afford fwhm of ∼30 nm.34
Conclusions
In summary, we have developed versatile four-step syntheses
of soluble silyl-protected 2,9-, 2,10-, 2,6,9,13-, and 2,6,10,13-
ethynylpentacenes by a double Diels-Alder strategy that may
be extended to other molecules. The knowledge gained from
this study should lead to new pentacenes with potential as
organic thin film semiconductors. Pentacene without C ring
substituents underwent light inducted dimerization to tetrakis-
naphthotricyclo[4.2.2.22,5]dodecanes cycloaddition adducts. An
ionic liquid is the preferred halide source for the generation of
ortho-quinodimetanes from bromoxylenes. In addition, the
appropriate members of these families are useful building blocks
for both para-pentacenes 23 and 29-33 and meta-pentacenes
36 and 38 helical cyclophanes with pentacene caps. In particular,
the meta-compounds are key intermediates for shape-persistent,
π-stacked, helical pentacene cyclophanes that parallel our
previous research with benzene in which the superimposed
aromatic rings are separated by ∼3.5 Å, a consequence of the
restricted geometry imposed by the bridges.35
1
Mp: >270 °C; H NMR (500 MHz, CDCl3) δ 9.15 (s, 2H),
8.28 (d, J ) 8.6 Hz, 2H), 7.70 (d, J ) 2.5 Hz, 2H), 7.24 (dd, J )
8.5, 2.6 Hz, 2H), 1.01 (s, 18 H), 0.30 (s, 12H); 13C NMR (125
MHz, CDCl3) δ 181.8 (s), 180.7 (s), 161.9 (s), 136.9 (s), 136.7 (s),
135.6 (s), 130.4 (d), 127.5 (s), 127.0 (d), 126.5 (d), 117.7 (d), 25.5
(q), 18.3 (s), -4.3 (q); MS (EI) m/z 541.2 (M+ - t-Bu, 100), 485.1
(8), 242.0 (19), 162.0 (34); IR (CDCl3) ν 2957.6, 2930.7, 1677.7,
1591.9 cm-1; HRMS calcd for C30H29O6Si2 541.1502 (M+ - t-Bu),
found 541.1510.
2,10-Bis(t-butyldimethylsiloxy)-5,7,12,14-pentacenediquino-
ne (5).
Experimental Section
2,9/2,10-Bis(t-butyldimethylsiloxy)-5,7,12,14-pentacenediquino-
ne (4, 5).
1
Mp: >270 °C; H NMR (500 MHz, CDCl3) δ 9.18 (s, 1H),
9.14 (s, 1H), 8.28 (d, J ) 8.6 Hz, 2H), 7.71 (d, J ) 2.4 Hz, 2H),
7.24 (dd, J ) 8.2, 2.6 Hz, 2H), 1.01 (s, 18 H), 0.30 (s, 12H); 13C
NMR (125 MHz, CDCl3) δ 181.8 (s), 180.7 (s), 161.9 (s), 137.0
(s), 136.6 (s), 135.6 (s), 130.4 (d), 127.5 (s), 127.1 (d), 127.0 (d),
126.5 (d), 117.7 (d), 25.5 (q), 18.3 (s), -4.3 (q); IR (CDCl3) ν
2955.8, 2927.2, 1675.0, 1590.9 cm-1; MS (EI) m/z 541.2 (M+
-
t-Bu, 15), 504.9 (3), 162.0 (18), 57.1 (100); HRMS calcd for
C30H29O6Si2 541.1502 (M+ - t-Bu), found 541.1516.
1,4,5,8-Anthradiquinone (3) (2.45 g, 10.3 mmol, 1 equiv) and
trans-3-(t-butyldimethylsiloxy)-1-methoxy-1,3-butadiene (2) (5.14
mL, 21.6 mmol, 2.1 equiv) were combined in CH2Cl2 and stirred
at room temperature (22 °C) for 20 h. The reaction was concentrated
and taken up in THF, silica gel was added, and the suspension was
stirred open to air for 24 h. The reaction was filtered through a
silica gel plug with CH2Cl2 to afford 4 and 5. The plug was flushed
with THF to recover unaromatized material. Silica gel was added
to the THF fraction, and this slurry was stirred open to air for 24
h. Filtration through a silica gel plug with CH2Cl2 afforded
additional 4 and 5. The silica gel oxidation/filtration was repeated
until all of the unaromatized material was converted to 4 and 5.
The diquinones were isolated as a yellow solid (3.72 g, 60%) as
Acknowledgment. We thank NSERC (Discovery Grant) for
financial support, M. K. Fagnou, W. Ogilvie, and D. Fogg for
use of their gloveboxes, D. E. G. Jones and Q. Kwok for DSC
measurements, M. Heeney for device construction and electronic
measurements, M. Frenette for spectral measurements, and I.
Korobkov for X-ray analyses. M.A.H. thanks NSERC, Boer-
ingher Ingelheim, and the University of Ottawa for graduate
scholarships.
Supporting Information Available: Experimental methods and
characterization data for new compounds and complete ref 4 and
CIF data. This material is available free of charge via the Internet
at http://pubs.acs.org.
(34) Maurel, V.; Laferriere, M.; Billone, P.; Godin, R.; Scaiano, J. C. J.
Phys. Chem. B 2006, 110, 16353.
(35) Heuft, M. A.; Collins, S. K.; Fallis, A. G. Org. Lett. 2003, 5, 1911.
JO0709807
7236 J. Org. Chem., Vol. 72, No. 19, 2007