Synthesis of self-orienting triptycene adsorbates for STM investigations

Article abstract of DOI:10.1016/j.tetlet.2003.08.056

The syntheses of three C₂ symmetric triptycenes containing two pendant groups are described. The pendant groups are designed to promote oriented adsorption to graphite or gold electrodes such that the unfunctionalized aryl group extends perpendicular to the surface. Initial STM studies are consistent with oriented adsorption on graphite.

Full text of DOI:10.1016/j.tetlet.2003.08.056

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7613–7615
Synthesis of self-orienting triptycene adsorbates for STM
investigations
Adam J. Wolpaw,† Ayal A. Aizer† and Matthew B. Zimmt*
Department of Chemistry, Brown University, Providence, RI 02912, USA
Received 17 June 2003; revised 15 August 2003; accepted 15 August 2003
Abstract—The syntheses of three C2 symmetric triptycenes containing two pendant groups are described. The pendant groups are
designed to promote oriented adsorption to graphite or gold electrodes such that the unfunctionalized aryl group extends
perpendicular to the surface. Initial STM studies are consistent with oriented adsorption on graphite.
© 2003 Elsevier Ltd. All rights reserved.
Self-assembled monolayers (SAMs) are of considerable
interest for tailoring the mechanical,1 biological2 and
electronic properties3 of solid surfaces. SAMs are also
used frequently in molecular electronics investigations.4
SAMs provide some control of molecular orientation at
flat surfaces. Adsorbate–surface and adsorbate–adsor-
bate interactions determine adsorbate orientation and
monolayer morphology.5 Molecules that spontaneously
orient on surfaces in the absence of adsorbate–adsor-
bate contacts are of potential use in surface sensor and
molecular electronics applications. Tripodal6 molecules
constitute one class of self-orienting adsorbates. We are
investigating the ability of C2 symmetric triptycene
derivatives7 to spontaneously orient on noble metal and
graphite surfaces. A single adsorption orientation may
be realized using a C2 symmetric triptycene with a
‘surface adsorbing element’ (SAE) on two of the
molecule’s three aromatic rings. Simultaneous interac-
tion of the two SAEs with the surface should bring
their attached aryl rings into contact with the surface
and extend the third aryl ring (the ‘upspoke’) roughly
perpendicular to the surface. Extensive homo-conjuga-
tion among the three aryl rings should engender large
electronic coupling between the upspoke and the under-
lying surface.8 Ultimately, elaboration of the upspoke
will provide electronic function to these self-orienting
molecules.
Racemic mixtures of triptycenes with C2 symmetry may
be prepared by Diels–Alder reactions of benzynes with
C2h symmetric anthracene derivatives.9 A 1,5-substitu-
tion pattern for the triptycene SAEs (rather than 2,6-)
was chosen to avoid impeding electronic overlap of the
two aryl p systems with surface electronic states. Two
facile routes to 1,5-disubstituted anthracenes are nucle-
ophilic aromatic substitution reactions of 1,5-
dichloroanthraquinone10 and Friedel–Crafts bis-acyl-
ations of anthracene.11 The latter route was employed
for these syntheses.
van der Waals interactions between long alkyl chains of
adjacent adsorbates provide significant driving force for
SAM formation on highly oriented pyrolytic graphite
(HOPG).12 In a 1,5-bis-alkyl-triptycene, chain–chain
and chain–HOPG interactions should promote the
* Corresponding author. Tel.: 1-401-863-2909; fax: 1-401-863-9368;
Scheme 1. Reagents and conditions: (i) C15H31COCl, AlCl3, rt;
(ii) AlCl3, LiAlH4, THF, reflux; (iii) anthranilic acid, i-
C5H11ONO, reflux.
e-mail: matthew zimmt@brown.edu
–
† Undergraduate participant in Brown University Group Research
Project, 2001–2002.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.08.056

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A. J. Wolpaw et al. / Tetrahedron Letters 44 (2003) 7613–7615
of an individual triptycene molecule, provided the SAEs
are not too long. The 2%-thioethyl group was chosen as
the initial SAE target to provide a limited amount of
orientational flexibility upon adsorption.
desired adsorption orientation for each molecule in the
monolayer. Consequently, n-nexadecyl chains were
selected as the surface adsorbing elements for triptycene
studies on graphite electrodes. Friedel–Crafts acylation
of anthracene with palmitoyl chloride yielded an
approximately 1:1 mixture of 1,5- and 1,8-bis-palmi-
toyl-anthracene (Scheme 1).11 Recrystallization and
chromatography provided 35–45% yields of the 1,5-iso-
mer 1. Reduction with lithium aluminum hydride in the
presence of AlCl3 produced 1,5-bis-hexadecyl-anthra-
cene (2).13 Diels–Alder reaction with benzyne, formed
in situ by diazotization of anthranilic acid,9 generated a
racemic mixture of the desired 1,5-bis-hexadecyl-trip-
tycene (3).
Friedel–Crafts acylation of anthracene using acetyl
chloride/AlCl3 in CH2Cl2 followed by chromatography
afforded 1,5-bis-acetyl-anthracene (Scheme 2). Diels–
Alder reaction with benzyne produced 1,5-bis-acetyl-
triptycene (4) in 50–60% yield along with 30–40% of
unreacted anthracene. Diels–Alder reaction of 1,5-bis-
acetyl-anthracene with 2,3-dehydronaphthalene gave
1,15-bis-acetyl-benzotriptycene (5) in 40–45% yield. 2,3-
Dehydronaphthalene was formed in situ from 3-amino-
2-naphthalene carboxylic acid14,15 and iso-amyl nitrite
in dimethoxyethane. Thallium nitrate adsorbed to K-10
montmorillonite clay16 effected oxidative rearrangement
of the acetyl groups to methyl esters. Conversion of 4
to 6 was nearly quantitative whereas the rearrangement
of 5 to 7 afforded a 50% yield and formation of a single
unidentified side product.
STM images of SAMs formed from 3 exhibit a series of
nearly structureless, parallel ridges separated by 2 nm
(Fig. 1). The length and width of 3 (projection onto the
page of Scheme 1) are comparable to that of 2. SAMs
formed from 2 on HOPG display alternating lamellea
containing ordered alkyl chains or anthracenes, with a
2 nm spatial repeat between the anthracene lamellea.
Provided comparable alkyl chain interactions drive
SAM formation for both molecules, the monolayer
formed form 3 will also exhibit a 2 nm spatial repeat
and the triangular ridges in Figure 1 are due to the
triptycene groups. The poor spatial resolution in the
figure likely results from the inability of the STM tip to
resolve the highly corrugated monolayer. In addition,
tip-surface tunneling may be mediated by the upspokes
even when the tip lies directly above the alkyl chain
regions.
The rearranged esters were reduced to alcohols, acti-
vated as mesylates and reacted with potassium thioac-
etate to generate the bis-acetyl thioesters of the C2
symmetric triptycene dithiols, 8 and 9 (Scheme 3).17
The thioacetyl groups in 8 and 9 can be hydrolyzed18 to
thiols in conjunction with dosing of a gold surface and
collection of scanning tunneling microscope (STM)
images. STM studies to evaluate the self-orientation
proclivities of 3, 8 and 9 at monolayer and sub-mono-
layer coverages are currently in progress.
Isolated molecules of 3 on HOPG (i.e. at sub-mono-
layer coverages) will not experience intermolecular
chain–chain van der Waals interactions. The interaction
between the alkyl chains of a single molecule and
HOPG may be insufficient to ensure adsorption in the
desired orientation.
A
stronger, more directing
molecule–surface interaction is required to achieve ori-
ented adsorption at sub-monolayer coverages. Thiols
form strong bonds to gold and other noble metal
surfaces. Incorporating thiols at the SAE terminus
should enhance the propensity for oriented adsorption
Scheme 2. Reagents and conditions: (i) CH3COCl, AlCl3, rt;
(ii) anthranilic acid, i-C5H11ONO, reflux; (iii) 3-amino-2-
naphthalenecarboxylic acid, i-C5H11ONO, reflux; (iv)
Tl(NO3)3/K-10, CH2Cl2, rt, 6 days.
Figure 1. STM image (current mode, 0.5 V, 600 pA) of a
SAM formed from triptycene 3 on HOPG.

A. J. Wolpaw et al. / Tetrahedron Letters 44 (2003) 7613–7615
7615
9. Rogers, M. E.; Averill, B. A. J. Org. Chem. 1986, 51,
3308–3314.
10. Iwanaga, H.; Naito, K.; Sunohara, K.; Okajima, M. Bull.
Chem. Soc. Jpn. 1998, 71, 1719–1724.
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Wiley and Sons: New York, 1955; pp. 78–82.
15. A SmithCreator microwave reactor from Personal Chem-
istry was used in place of an autoclave to prepare 3-
amino-2-naphthalene
carboxylic
acid
from
Scheme 3. Reagents and conditions: (i) LiAlH4, 0°C; (ii)
3-hydroxy-2-naphthalene carboxylic acid. A 5 mL Smith
reaction vial containing concentrated NH4OH was slowly
charged with ZnCl2 and 3-hydroxy-2-naphthalenecar-
boxylic acid. After sealing, the tube was heated to 140°C
for 120 s. This dissolves all material and prevents exces-
sive pressure build up in the next step. The tube contents
were heated to 210°C for 16 h and worked up as in Ref.
14.
(MeSO2)O, pyridine, 2 h; (iii) KSC(O)CH3, acetone, 4 days.
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