Cruciform π-systems for molecular electronics applications

Article abstract of DOI:10.1021/ja0350942

This study details a modular and general synthesis of a new class of molecules consisting of cruciform π-systems. The key to synthesizing these molecules was an unprecedented double Staudinger cyclization. Once formed, these rigid compounds assemble into

Full text of DOI:10.1021/ja0350942

Published on Web 04/23/2003
Cruciform π-Systems for Molecular Electronics Applications
Jennifer E. Klare, George S. Tulevski, Kenji Sugo, Anat de Picciotto, Kiley A. White, and
Colin Nuckolls*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
Received March 11, 2003; E-mail: cn37@columbia.edu
This study details a modular and general synthesis of a new class
of molecules (1) consisting of two perpendicularly disposed
π-systems. Once formed, these rigid compounds assemble into
ordered monolayer films that orient their bis-oxazole subunits
upright from the surface. This orientation is useful in molecular
electronics test structures that require a path of conjugation
perpendicular to the metallic electrode surface.1-3 Molecules
typically used in these devices are linear aromatics that terminate
in surface-active endgroups.4,5 The molecules shown in Figure 1
have orthogonal terphenyl and bis-phenyloxazole arms. The crowd-
ing in the central ring forces the external phenyl substituents out
of the ring plane, thereby preventing the prone conformation in
self-assembled monolayer (SAM6) films.
The 6,5-ring junction between the phenyl and benzoxazole rings
is utilized in these studies because polymers with this linkage have
Figure 1. (a) Cruciform π-systems: terphenyl (red) and bis-phenyloxazoles
been shown to be flat and conjugated.7 Also, these polymers have
been shown to be of high tensile strength,7 resistant to oxidation,8
and conductive.7-9 The typical reaction conditions used to make
phenyl oxazoles are harsh10 and do not allow the formation of
oligomers with sensitive groups such as acetate-protected thiols.
Shown in Scheme 1 is a new synthesis of a variety of substituted
bis-phenyloxazoles that affords these unusually shaped molecules
through an unprecedented11 double Staudinger cyclization12 of bis-
azide 3. The unpurified reaction mixtures do not show any
significant byproducts and can be easily purified by a simple
filtration if polymer-bound triphenylphosphine is used.13
(black); (b) CPK molecular model showing the out-of-plane twist of the
phenyl side chains blocking the prone position (carbon, gray; nitrogen, blue;
oxygen, red; sulfur, yellow).
Scheme 1 a
The azides are remarkably stable and easily introduced into this
crowded ring structure through a Michael addition/elimination on
the bis-bromoquinone 2. Sodium dithionite reduction of the bis-
azidoquinone 3 to its hydroquinone form, followed by an in situ
coupling with either carboxylic acids or the acid chlorides produces
the Staudinger precursors 4a-f. For 4e and 4f, differential
substitution of the hydroquinone was successful through sequential
introduction of two different activated benzoic acids. Moreover,
the monoaldehyde 1e can be cleanly oxidized through Lindgren
conditions14 to yield the monocarboxylic acid 1g. Some of the bis-
oxazoles in Scheme 1 were synthesized to carry surface-active
groups such as thioesters (1b, 1f),5b carboxylic acids (1g),15 and
nitriles (1d).16
To test for monolayer formation and structure, 1b and 1f were
deposited, by the method of Tour et al.,5b onto films of gold.
Likewise, 1g was chemisorbed onto a native aluminum oxide layer.
For 1f, monitoring the binding energy of the S2p core electron at
ca. 162 eV shows the characteristic transition and peakshape for a
thiolate bound to gold.17 For monolayers of alkane thiols, XPS has
given close estimates as to the thickness of the organic layer by
using a two-layer model.18 By this method, monolayers of 1f were
estimated to be 2.19 ( 0.05 nm thick. In an upright configuration
on gold, as shown in Figure 2a, molecular models predict the
distance between the terminal carbon, and the attached gold atom
is 2.26 nm, implying that the molecule has a ca. 15° tilt angle.19
a Reagents and conditions: (a) NaN3; (b) i, Na2S2O4, H2O, ii, see the
Supporting Information; (c) Ph3P‚polystyrene.
The XPS-derived film thickness is in good agreement with the 1.9
( 0.1 nm determined from ellipsometry from a monolayer of 1f
on gold.
Shown in Figure 2b are the IR spectra for 1f in a monolayer
and in bulk. Similar spectra were acquired for monolayers of 1b
on gold films. Loss of the 1693 cm-1 carbonyl stretch in the
monolayer indicates that the thioacetate is cleaved.5b The oxazole
VO-CdN stretch20 (at ca. 1613 cm-1), whose transition dipole is
parallel to the axis of conjugation, has a higher relative intensity
in the monolayersin agreement with the XPS and ellipsometry data
that the bis-oxazole is upright from the substrate.
On going from the bulk to the SAM, the 1495 cm-1 (VCdC
aromatic stretch10d) increases and the 1470 cm-1 (δCH2 scissoring)
decreases in relative intensity. This change is an indication that
the aliphatic side chains are anisotropic as in Figure 2a. Advancing
and receding water contact angle measurements of monolayers of
1f were found to be 86° and 76° ( 2°, respectively, similar to what
would be expected from a surface that is presenting a methylester.21
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C O M M U N I C A T I O N S
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(19) The length of the molecule from sulfur to carbon is calculated from energy
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Figure 2. (a) Molecular model of 1f on a Au (111) surface. (b) Infrared
spectrum (KBr) of bulk 1f (blue trace) and PEM-IRRAS spectrum from a
monolayer of 1f (red trace).
In aggregate, the results presented above indicate that monolayers
of 1f form with high coverage, orienting the bis-phenyloxazole
upright from the surface. Although the results are not as clear for
1b and 1g, the XPS data indicate that the molecules are also oriented
roughly upright in monolayers. A direct comparison with com-
pounds that lacked the phenyl side chains was not possible due to
their insolubility.
In summary, this study has put forth a new, modular method to
produce highly functionalized cross-shaped molecules. One arm
serves as the conjugated portion while the other provides structure
to thin films. In monolayer films the molecules orient upright from
the surface. This provides a means to conjugate a diversity of
functionality to a metallic underlayer and to test the electrical
properties of these films.3 Moreover, more complex functionaliza-
tion of the phenyl side chains is a promising route to molecular-
scale patterning of conductive substrates.
Acknowledgment. We thank Dr. Mike Steigerwald (Columbia
University) for useful discussions. This work is supported primarily
by the Nanoscale Science and Engineering Initiative of the National
Science Foundation under NSF Award Number CHE-0117752. This
work has used the shared experimental facilities that are supported
primarily by the MRSEC Program of the National Science
Foundation under Award Number DMR-0213574. C.N. thanks the
Beckman Young Investigator Program (2002), and the Dupont
Young Investigator Program (2002) for financial support. K.A.W.
acknowledges a summer fellowship through the NSF-REU program.
(20) (a) Hegedus, L. S.; Odle, R. R.; Winton, P. M.; Weider, P. R. J. Org.
Chem. 1982, 47, 2607-2613. (b) Maryanoff, C. A. Spectroscopy of
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(21) Advancing contact angle for ester-terminated SAMs are 67° and for
methyl-terminated SAMs are 114° (Bain, C. D.; Troughton, B. E.; Tao,
Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. N. J. Am. Chem. Soc. 1989,
111, 321-335). 1f has both a methyl-ester and an aliphatic component.
Supporting Information Available: Experimental details for the
synthesis and characterization of (1-4); XPS (1b, 1f, and 1g) and IR
(1b and 1f) for monolayer films (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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