Synthesis of tetracene sulfoxide and tetracene sulfone via a cascade cyclization reaction

Article abstract of DOI:10.1021/ol070467f

A facile synthesis of tetracene sulfoxides and sulfones based on odiallene annulation initiated cascade cyclization was developed. The photophysical behavior of these tetracene derivatives can be generally rationalized by the presence of electron-withdraw

Full text of DOI:10.1021/ol070467f

ORGANIC
LETTERS
2007
Vol. 9, No. 11
2075-2078
Synthesis of Tetracene Sulfoxide and
Tetracene Sulfone via a Cascade
Cyclization Reaction
Yi-Chun Lin and Chih-Hsiu Lin*
Institute of Chemistry, Academia Sinica, 128 Academia Road, Nankang,
Taipei, Taiwan
chl@chem.sinica.edu.tw
Received February 23, 2007
ABSTRACT
A facile synthesis of tetracene sulfoxides and sulfones based on o-diallene annulation initiated cascade cyclization was developed. The
photophysical behavior of these tetracene derivatives can be generally rationalized by the presence of electron-withdrawing sulfoxides and
sulfones at the 5-position.
Nearly 25 years ago, Hoffmann, Chapman, and Kivelson
foretold that the small HOMO-LUMO gap in acenes may
enable these linearly fused aromatic hydrocarbons to exhibit
various unusual material properties than typically associated
with metals and semiconductors.1 The prominent status that
acene compounds hold in the current pursuit for organic
electronics2 is the fulfillment of that prophecy. Most notably,
pentacene and tetracene derivatives are now regularly
employed as the semiconducting layers in field effect
transistors.3 Although the extraordinary hole mobility of
pentacene has become the benchmark to which all similar
studies are measured, acene-based devices still leave plenty
of room for improvement especially concerning the stability
and solubility problems in higher acenes. One common
approach to optimizing the performance of organic electronic
devices is to substitute the parent compounds with various
synthetic derivatives. However, in acene-based devices, the
systematic change of substitutions on acenes poses a great
challenge since there are very few synthetic methodologies
that can serve this purpose.4
Most acene syntheses that appear in the literature are based
on Diels-Alder or aldol condensation.5 In such strategies,
(2) In field effect transistors: (a) Ito, K.; Suzuki, T.; Sakamoto, Y.;
Kubota, D.; Inoue, Y.; Sato, F.; Tokito, S. Angew. Chem., Int. Ed. 2003,
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Tokito, S.; Yamashita, Y. Chem. Mater. 2005, 17, 1261. In light emitting
diodes: (c) Shi, J. M.; Tang, C. W. Appl. Phys. Lett. 2002, 80, 3201. (d)
Danel, K.; Huang, T.-H.; Lin, J. T.; Tao, Y.-T.; Chuen, C.-H. Chem. Mater.
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H.; Weise, W.; Ahles, M.; Schmechel, R.; von Seggern, H. Phys. ReV. Lett.
2003, 91, Art. No. 157406. (i) Santato, C.; Capelli, R.; Loi, M. A.; Murgia,
M.; Cicoira, F.; Roy, V. A. L.; Stallinga, P.; Zamboni, R.; Rost, C.; Karg,
S. E.; Muccini, M. Synth. Met. 2004, 146, 329. In solar cell: (j) Yoo, S.;
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10.1021/ol070467f CCC: $37.00
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the Diels-Alder or aldol adducts invariantly need to undergo
further redox manipulations to furnish the acene structure.
However, the redox steps can cause problems with more
sensitive acene products. We hereby propose a one-step
synthesis of tetracene via a bisallenyl annulation cascade
cyclization reaction without employing such redox opera-
tions.
Allene is a very useful structure motif to trigger annulation
reactions. Such reactions are often employed in the synthesis
of exotic aromatic systems.6 The application of o-diallene
to construct various linearly fused aromatic systems was also
well-documented in the literature.7 In such transformations,
the starting o-diallenyl compounds first undergo annulation
to form transient o-quinonedimethane intermediates. To reach
a stable product, the diradicaloid intermediates can be trapped
by internal hydrogen abstraction or inter- or intramolecular
cyclization.8 When the diradicaloids derived from o-diallenyl
phenylene undergo intramolecular electrocyclic reaction with
a phenyl group, as reported by Saa´ et al.,8a the intermediate
will lead to a dihydrotetracene-type product. We further
reasoned that such a cyclization can be made irreversible if
a leaving group is incorporated into the system to trigger a
post-annulation elimination reaction and furnish the tetracene
structure. We shall use such a proposed mechanism as our
working model in the subsequent investigation.
Scheme 1. Cascade Cyclization of o-Bisallenyl Compounds to
Tetracene Derivatives
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Among the numerous choices to generate the transient
o-bisallenyl intermediates,9 the 2,3-sigmatropic rearrange-
ment of propargylic sulfenate or sulfinate seems best suited
to our purpose. Such reactions are generally carried out under
very mild conditions and the yields of allenic products are
generally very high. More importantly, the sulfoxide or
sulfone functional groups produced after such rearrangements
can serve as the leaving groups to facilitate the post-
annulation elimination reaction. Such a strategy opened an
access to the tetracene derivatives with a facile one-pot four-
step reaction (rearrangement, two annulations, and elimina-
tion).
The strategy was first carried out with the simplest
substrate-R,R′-bis(phenylethynyl)-1,2-benzenedimethanol at
ambient temperature. With three different sulfenyl and
sulfinyl reagents (sulfenyl chloride, p-tolylsulfinyl chloride,
and p-nitrobenzenelsulfenyl chloride) of a wide range of
reactivity,10 we discovered that the reaction rates or the yields
of desired products are quite similar (Scheme 2). These
results indicated that the formation of the sulfenyl (or
sulfinyl) esters is not likely to be the rate-determining step.
Since the two annulation steps are likely to be reversible
before elimination occurs, the success of these reactions
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S. Rearrangements involving sulfones. In The Chemistry of Sulphones and
Sulphoxides; Patai, S., Rappoport, Z., Stirling, C. J. M., Eds.; John Wiley
& Sons: New York, 1988, Chapter 13. (d) Braverman, S. Rearrangements
involving sulfoxides. In The Chemistry of Sulphones and Sulphoxides; Patai,
S., Rappoport, Z., Stirling, C. J. M., Eds.; John Wiley & Sons: New York,
1988, Chapter 14.
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Chem. 2002, 3198.
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Org. Lett., Vol. 9, No. 11, 2007

Scheme 2. Cascade Synthesis of Tetracene Solfoxide (2a, 4)
and Sulfones (3a) with Various Sulfenyl and Sulfinyl Reagents
Scheme 3. Yields of Cascade Synthesis of Tetracene
Sulfoxides and Sulfones
should largely depend on the efficiency of the rearrangement
and elimination steps. On the basis of this line of reasoning,
we successfully optimized the reaction yield by slightly
raising the reaction temperature since it is well-known that
the elimination of sulfoxide and sulfone is thermally
activated. When the reactions were performed in refluxing
methylene chloride, tetracene sulfoxide 2a and sulfone 3a
were obtained in 79% and 66% yields, respectively. To
confirm our structural assignment, a single crystal of 2a was
obtained and its structure was unambiguously established by
X-ray crystallography (Figure 1). The yields of various
thesized from various phthalaldehyde and lithium phenyl
acetylide) and gave tetracene sulfoxide and sulfone deriva-
tives in good to moderate yields. The dismal yield and poor
purity of 2m in entry 13 must be attributed to the instability
of the pentacene product. In entry 14, the butyl chain in 2n
can provide internal hydrogen sources to divert the desired
cyclization pathway of the quinonedimethane and leads to
the low yield. In reactions where two regioisomeric products
are possible, no apparent selectivity was observed except for
entry 12 (from 1l to 2l). With the exception of 2i and 2i′,
these isomeric mixtures cannot be easily separated and thus
they are characterized as such.
Attempting to further simplify the procedure, we also tried
to perform a multiple-component one-pot reaction by gen-
erating the intermediate bis-sulfenyl ester by in situ trapping
of the bis-lithium alkoxide with sulfenyl chloride (Scheme
4). Unfortunately, this protocol gave rather unsatisfactory
results and it was not pursued further.
The photophysical properties of these tetracene derivatives
are tabulated in Table 1. Compared to the parent tetracene,
Scheme 4. Attempted One-Pot Synthesis of Tetracene
Figure 1. ORTEP picture of cascade cyclization product 2a.
Sulfoxide
tetracene derivatives from this cascade reaction are listed in
Scheme 3.
The strategy was applied to a variety of substituted R,R′-
bis(phenylethynyl)-1,2-benzenedimethanol compounds (syn-
Org. Lett., Vol. 9, No. 11, 2007
2077

First, the Stokes shifts of tetracene sulfone compounds are
around 100 nm while those of tetracene sulfoxides are only
30-50 nm. Second, the sulfoxide series are on average 5-10
times more efficient emitters than the sulfone series. The
only exception is the nitro-substituted tetracene sulfoxide 4.
When the emission spectra of 2a and 3a were measured in
acetonitrile and ethanol, only 2a exhibits noteworthy solva-
tochromic shifts, indicating that the first excited singlet state
of sulfoxide tetracene has larger charge-separate character
than the sulfone tetracene.
Table 1. Photophysical Data of Tetracene Sulfoxide and
Sulfone Compounds
compd
UV λmax/nm
ꢀ
PL λmax/nm
Φ PL
tetracene
5,12-diphenyl-
474
493
10700
13500
476
tetracene
rubrene
2a
2b
2c
2d
2e
2f
2g
2h
2i
2i′
2j, 2j′
2k, 2k′
2l
2n
3a
3d
3e
528
511
510
512
508
511
512
512
515
495
504
511
511
526
513
503
500
500
499
511
524
499
519
11500
4030
2784
6783
6196
5266
4793
6731
5590
4307
6109
5135
6883
6045
2331
6693
6267
7617
4003
3973
5103
4040
4270
539
554
554
542
556
548
549
542
558
584
579
561
549
579
548
600
598
591
587
612
596
596
551
98
26
34
24
25
32
35
33
38
10
6
29
28
21
24
2.7
2.3
6.6
6.5
0.1
5.2
3.0
0.1
Cyclic voltammetry measurements were performed on the
two simplest compounds 2a and 3a. The oxidation of 3a is
reversible while that of 2a is only quasireversible. The
oxidation potentials of these two compounds by CV experi-
ments (vs Ag/AgCl electrode with ferrocene as the internal
standard) are quite close and surprsingly also very similar
to the parent tetracene (0.70 eV for 2a, 0.77 eV for 3a, and
0.77 eV for tetracene). These findings indicate sulfoxide and
sulfone substitutions both exert influences on the LUMO
levels of both tetracene derivatives and cause the red shitfs
compared to the parent tetracene. The more electron-
withdrawing sulfone group stablizes the LUMO in 3a more
than sulfoxide does 2a and leads to the larger stokes shifts
observed in the tetracene sulfones 3a-k. However, in
systems with such large stokes shifts, the LUMO energy
cannot be accurately deduced from CV and optical absorption
data. TGA analyses indicated the decomposition of 2a and
3a starts at 193 and 174 °C, respectively. As expected, the
weight loss profiles are consistent with the extrusion of SO
and SO2 fragments. Further gradual weight loss can be
observed for both compounds upon pyrolysis up to 500 °C.
In conclusion, we successfully developed a facile synthetic
strategy for tetracene sulfoxide and sulfone via a cascade
cyclization reaction. Various substituents can be incorporated
by using appropriate corresponding starting materials. Due
to the presence of sulfone and sulfoxide groups, these
compounds show marked red shifts in UV and fluorescence
spectra compared to the parent tetracene. The electron-
withdrawing sulfone groups also exert subtle influences on
their photophysical properties. We will attempt to employ
this strategy in higher acene synthesis and explore these
compounds in optical or electronic applications in the near
future.
3f
3i, 3i′
3j, 3j′
3k, 3k′
4
a Spectra are measured in dichloromethane solutions
marked red shifts were observed in both series of compounds
in both absorption and emission spectra. These spectroscopic
data are in the range of those observed in rubrene-type
compounds or 5,12-dialkynyltetracene derivatives reported
by Anthony et al.11 On the contrary, within both series of
compounds, the variations of absorption and emission
wavelengths are much less pronounced. We therefore deduce
that the frontier orbitals are strongly perturbed by the
sulfoxide and sulfone at the 5-position, while other functional
groups at terminal rings only exert minor influences to the
electronic structures of these tetracene derivatives. The more
pronounced bathochromic shifts seen in compounds 2l and
3j are likely due to the cooperative effects of electron-
donating methoxy groups and electron-withdrawing chlorides
and sulfone groups. While most compounds investigated here
have similar absorption spectra around 500-510 nm, two
notable discrepancies in the photophysical behavior between
sulfoxide and sulfone tetracene derivatives are observed.
Acknowledgment. C.-H. Lin thanks generous finnacial
support from Academia Sinica.
Supporting Information Available: Detailed synthetic
procedures, spectroscopic charaterizations (1H, 13C NMR, IR,
HRMS) of all reported compounds, absorption, emission,
CV, and TGA of selected compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) (a) Burgdorff, C.; Kircher, T.; Lohmannsroben, H.-G. Specreochim.
Acta, Part A. 1988, 44, 1137. (b) Odom, S. A.; Parkin, S. R.; Anthony, J.
E. Org. Lett. 2003, 5, 4245.
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