Synthesis of ladder polyaromatics as new molecular device candidates

Article abstract of DOI:10.1016/j.tetlet.2004.02.028

In order to investigate one of the proposed molecular electronics switching mechanisms, we synthesized several molecules whose cores are unable to undergo conformational rotation. Preparation of these molecules, all of which are terminated with the thioac

Full text of DOI:10.1016/j.tetlet.2004.02.028

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2801–2803
Synthesis of ladder polyaromatics as new molecular device
candidatesq
Jacob W. Ciszek and James M. Tour*
Department of Chemistry and Center for Nanoscale Science and Technology, Rice University, MS-222, 6100 Main Street,
Houston, TX 77005, USA
Received 15 December 2003; accepted 6 February 2004
Abstract—In order to investigate one of the proposed molecular electronics switching mechanisms, we synthesized several molecules
whose cores are unable to undergo conformational rotation. Preparation of these molecules, all of which are terminated with the
thioacetyl moiety, takes advantage of an improved synthesis of 2,7-dibromophenanthrene, use of the reduced form of phenan-
threnequinone as a protecting group, and various Sonogashira and halogen to Li to S conversions.
Ó 2004 Elsevier Ltd. All rights reserved.
Ever since the establishment of single molecule con-
ductance,1;2 large on–off ratios, and negative differential
resistance (NDR),3 numerous molecules with these
characteristics have been synthesized and tested. During
this time a variety of hypotheses have been offered
concerning the switching mechanism.3–6 One hypothesis
involves the conformational changes within the central
core of the molecular structures.4 Therefore we have
prepared three molecular systems that have ladder-like
cores that cannot undergo internal rotational motion.
Once assembled and tested in the scanning tunneling
microscopy (STM) testbed, they should provide the
requisite data needed to clarify the conformational
rotation hypothesis.
2,7-dibromophenanthrene (1), 3,8-dibromo-1,10-phen-
anthroline (2), and 2,7-diiodophenanthrenequinone (3).
While 2 could be synthesized in one step using previ-
ously disclosed procedures,7 and 3 could be easily pre-
pared by iodinating the parent quinone with NIS in 85%
yield, previous methods for synthesizing 1 were ineffi-
cient.8–10 The new path to afford 1 (Scheme 1) begins
with diphenic acid, which is brominated with dibro-
moisocyanuric acid (DBI) to afford a mixture of bro-
mination isomers that are subsequently reduced to the
diol. Swern oxidation of the isolated diol gave the di-
aldehyde followed by cyclization11 to afford 1. Hence 1
can now be synthesized in large quantities with an
overall yield of 50% over three steps. The best previous
methodology produced this simple material in only 16%
overall using the more expensive dihydrophenanthrene
and the now difficult to obtain solvent CCl4.
O
O
Br
Br Br
Br
I
I
HO
HO
N
N
O
1
2
3
1) DBI, H2SO4
Br
Br
2) NaBH4, BF3OEt2
54%
O
The potential molecular devices synthesized here origi-
nate from three different halogenated polyaromatics:
OH
OH
4
H
O
Swern
97%
H2NNH2
1
Br
Br
Keywords: Molecular electronics; Ladder polyaromatics; Phenan-
threne.
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2004.02.028
* Corresponding author. Tel.: +1-7133486246; fax: +1-7133486250;
e-mail: tour@rice.edu
AcOH
96%
O
H
5
Scheme 1. Synthesis of 2,7-dibromophenanthrene.
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.02.028

2802
J. W. Ciszek, J. M. Tour / Tetrahedron Letters 45 (2004) 2801–2803
With the halogenated precursors in hand, all three
compounds were converted to a form conducive to or-
dered assembly in a molecular testbed. Since a terminal
thiol group is compatible with all commonly used
molecular electronic testbeds,12 the dihalogenated spe-
cies were converted to five different compounds termi-
nated in the thioacetyl moiety (Scheme 2). The acetyl-
protected form of the thiol is used since it is far easier to
handle (it is not prone to oxidative dimerization as the
thiol is) and can be deprotected in situ.13 As shown in
Scheme 2, in the cases where the polyaromatic species
are terminated with only a single thioacetyl group, the
compound was synthesized by monolithiation of the
dihalide using n-butyllithium followed by quenching
with water to give a mixture primarily containing the
monohalogenated compound. This mixture was then
treated with tert-butyllithium, followed by sulfur and
acetyl chloride to give the monothioacetate as in 6.
Formation of the thioacetates in this manner proved to
be simpler than selective monohalogenation of the
dihalides. In the cases of the dithioacetyl compounds
such as 7 and 8, bis-lithiation followed by treatment
with sulfur and acetyl chloride afforded the desired
products.
The synthesis of the thioacetyl-terminated phenan-
threnequinones required the use of a carbonyl-protect-
ing group. Though the acetal-protecting group was
tried, their later chemoselective removal was not possi-
ble. However, we have found that the quinone could be
transformed to a bis-TBS ether of the dihydroxyquinone
and then reoxidized in air.14 Hence, 3 was reduced with
zinc in the presence of TBSCl to afford 9.15 Thioacetyl
attachment could then be effected as done on 1 and 2.
Reoxidation of the dihydroxyquinone back to the qui-
none was carried outwiht TBAF for silyl removal and
the addition of air. AcOH is added to buffer the TBAF
or else removal of the acetyl moiety ensues.16 Addition
of DDQ in a later test reaction increased the yield
slightly although it had not been routinely used.
Though the shorter molecules, 6, 7, 8, 11, and 13, are
compatible with the STM testbed, most other testbeds12
for molecular devices require a longer distance between
the termini so as to retard metal leakage across the self-
assembled monolayer. Hence, phenylene ethynylene
spacers were added to 1–3 (Scheme 3). In all cases the
basic carbon framework was assembled through single
or multiple Sonogashira couplings. Compound 1 was
1a) n-BuLi (1 equiv)
b) H2O
Br
Br
SAc
2a) t-BuLi (2 equiv)
b) S8
c) AcCl
1
6
40%
1) t-BuLi (4 equiv)
2) S8
Br
Br
Br
Br
AcS
AcS
SAc
SAc
3) AcCl
49%
1
2
7
1) t-BuLi (4 equiv)
2) S8
3) AcCl
12%
N
N
N
N
8
O
O
TBSO
OTBS
Zn, TBSCl
87%
I
I
I
I
3
9
TBSO
OTBS
O
O
1) t-BuLi (4 equiv)
2) S8
TBAF
AcS
SAc
AcS
SAc
9
AcOH
air
57%
3) AcCl
55%
11
10
TBSO
OTBS
O
O
1a) n-BuLi (1 equiv)
b) H2O
TBAF
9
SAc
SAc
AcOH
air
2a) t-BuLi (2 equiv)
b) S8
c) AcCl
12
13
35%
25%
Scheme 2. Synthesis of the planar polyaromatic species.

J. W. Ciszek, J. M. Tour / Tetrahedron Letters 45 (2004) 2801–2803
2803
developed and various Sonogashira, and halogen to Li
to S conversion sequences were utilized.
1) t-BuLi (4 equiv)
2) ICH2CH2I
62%
I
I
Br
Br
1
14
Supporting information available: The experimental
details as well as spectroscopic data are available. This
material is available free of charge via the Internet.
SAc
AcS
SAc
Pd/Cu, NPri2Et
15
57%
Acknowledgements
1) TMSA, Pd/Cu,
NPri2Et, 100%
This work was supported by DARPA, ONR, and the
NSF Penn State MRSEC. The NSF, CHEM 0075728,
provided partial funds for the 400 MHz NMR. We
thank Aaron Engel and Dr. Chih-Hsiu Lin for their
assistance and Dr. Ian Chester of FAR Laboratories for
the generous donation of trimethylsilylacetylene.
2
H
H
2) TBAF
67%
N
N
N
16
17
I
SAc
AcS
SAc
N
Pd/Cu, NPri2Et
21%
TBSO
OTBS
SAc
AcS
References and notes
SAc
9
Pd/Cu, NEt3
76%
1. Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.;
Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.;
Weiss, P. S. Science 1996, 271, 1705.
18
2. Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour,
J. M. Science 1997, 278, 252.
O
O
TBAF, AcOH
3. Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science
1999, 286, 1550.
AcS
SAc
air
4. Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm,
L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.;
Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S.
Science 2001, 292, 2303.
19
23%
Scheme 3. Synthesis of elongated polyaromatics.
5. Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am.
Chem. Soc. 2000, 122, 3015.
6. Ramachandran, G. K.; Hopson, T. J.; Rawlett, A. M.;
Nagahara, L. A.; Primak, A.; Lindsay, S. M. Science 2003,
300, 1413.
found to be too unreactive to couple with the 4-ethy-
nyl(thioacetyl)benzene16 in acceptable yields. Thus 1 was
converted to the diiodo species, 14, before undergoing
coupling. Compound
2 would notcouple whit
4-ethynyl(thioacetyl)benzene under moderate condi-
tions. However, coupling first to the more reactive
trimethylsilylacetylene, deprotecting and then coupling
to the aryl thioacetyl group was effective.
7. Saitoh, Y.; Koizumi, T.; Osakada, K.; Tamamoro, T. Can.
J. Chem. 1997, 75, 1336.
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1977, 12, 2355.
The final compound of interest was the phenanthrene-
quinone compound 19. Extensive work with this com-
pound (both by the authors and co-workers17) has
shown that although 3 can undergo Sonogashira cou-
plings, the yields are irreproducible, often giving no
product. A more reproducible route to 19 involves using
9. When the quinone functionality was protected, cou-
pling was reliably achieved. Deprotection and oxidation
afforded the desired compound 19.
10. Brown, W. G.; Bluestein, B. A. J. Am. Chem. Soc. 1943,
65, 1235.
11. Bacon, R. G. R.; Lindsay, W. S. J. Chem. Soc. 1958,
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Chemistry, Devices, Architecture and Programming; World
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13. Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. S.;
Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A.
N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529.
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In summary, we provide detailed syntheses of several
potential molecular devices. All of the target molecules
contain rigid polyaromatic cores that are being used to
discern the mechanism for switching in molecular
devices. In progression toward these devices a new
method for synthesizing 2,7-dibromophenanthrene was
16. Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62, 1376.
17. Zhao, Y.; Shirai, Y.; Chiu, Y.-H.; Yao, Y.; Yang, H.;
Tour, J. M. Unpublished data.