Oligothiophene isocyanides for platinum-based molecular electronic applications

Article abstract of DOI:10.1021/ja045904p

Understanding molecular orientation on a metal surface is key to designing molecular electronic device junctions. Though platinum device electrodes are of particular interest as a more stable alternative to the often used gold electrodes, the chemisorptio

Full text of DOI:10.1021/ja045904p

Published on Web 09/02/2004
Oligothiophene Isocyanides for Platinum-Based Molecular Electronic
Applications
Dennis Bong,† Iris Tam, and Ronald Breslow*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
Received July 8, 2004; E-mail: rb33@columbia.edu
Oligo- and polythiophenes are well-known semiconducting
organic molecules and are of great interest as potential molecular
wire candidates.1,2 We report herein the synthesis of oligothiophenes
end-functionalized with isocyanides and characterization of their
chemisorption to platinum surfaces. While much work has described
the coupling of low band-gap organic molecules to gold surfaces
via gold-sulfur bonds,3,4 specific chemisorption of molecules of
Figure 1. Structure of oligothiophene isocyanides 1a-d and respective
this type to other metal surfaces has been less well-explored.2,3,5
Gold is a convenient electrode material with regard to patterning
and chemisorptive attachment of organic thiols, but it has significant
disadvantages in grain mobility and susceptibility to surface etching
by thiols;6 it is therefore useful to investigate conjugated molecule-
metal interfaces with more refractory metals such as platinum. This
study details the first synthesis of isocyanide-functionalized oligo-
thiophenes and characterization of their assembly on both platinum
surfaces and nanoparticle form.
end-to-end length. The structure of quaterthiophene stannane building block
2 is shown on the right.
Scheme 1a
Different device applications may require molecular coupling
to metal in both bulk and nanoparticular form;7 further, they will
have different length requirements for the molecular bridge between
metal source and drain electrodes. Thus, we have prepared a family
of oligothiophene monoisocyanides in which the members differ
only by the number of thiophene units, to investigate the dependence
of platinum surface self-assembly on oligothiophene length. Regio-
regularly alkylated oligothiophenes 1a-d, with calculated end-to-
end lengths of 2.6, 4.0, 5.5, and 7.0 nm, respectively (Figure 1),
were readily synthesized by cycles of thiophene terminal bromi-
nation and Stille couplings8 with stannane 2 as a quaterthiophene
building block through intermediates 3 and 4 (Scheme 1). Isobutyl
functionality in one â-position of each thiophene ring of 2 was
installed9 to impart organic solvent solubility by inhibiting effective
stacking of the flat ring systems. Isocyanide functionality was
introduced in the final step of oligothiophene synthesis by dehydra-
tion of a terminal phenyl formamide.10
Interestingly, this series displays only a moderate red shift as
the length of the conjugated system increases, as judged by ultra-
violet and fluorescence spectroscopy (Table 1). While the UV
maxima exhibit a small but steady shift to the red with increasing
molecule length, the red-shifting trend in emission appears to plateau
after 1c; furthermore, the sizable and relatively constant Stokes shift
of ∼130 nm is larger than that observed for thiophene systems of
similar length but different alkylation pattern.11 This is likely due
to the sensitivity of conjugation length in the ground state to steric
environment. The â-branching alkylation pattern may cause the
oligothiophene backbone to twist, breaking conjugation and leading
a Reagents and conditions: (a) Pd(PPh3)4, toluene reflux 2; (b) NBS,
CHCl3; (c) triflic anhydride, DIEA, CH2Cl2.
Table 1. Absorption and Emission Maxima, First Oxidation
Potentials of 1a-d in Solution and as a SAM on a Platinum
Electrode, and Film Thickness on a Platinum Surfacea
UV (nm)
Fl (nm)
E
pa (mV)
E
pa(mV)
film thickness
(nm)
λmax
λmax
(soln)
(SAM)
1a
1b
1c
1d
409
419
423
430
542
551
555
555
450
420
410
400
410
430
440
450
1.8
3.0
4.1
5.3
a Absorption and emission maxima were acquired in CH2Cl2. Cyclic
voltammetry was measured at a scan rate of 100 mV s-1 in CH2Cl2 with
0.1 M Bu4NPF6 using a Ag/AgCl standard electrode and Pt disk working
electrode. For details, see Supporting Information.
The ground-state similarity of this series of compounds is further
reflected in CV studies of the isocyanides, both in solution and in
the form of a self-assembled monolayer (SAM) on a Pt working
electrode. The first oxidation potentials in solution (which are
comparable to those previously reported for oligothiophenes12)
display a slight decrease (become more easily oxidized) as the
molecules become longer, but in the SAM they appear to be more
difficult to oxidize as molecule size increases. Despite this, the
absolute differences are small, possibly because ground state
twisting of the backbone renders these differently sized molecules
electronically similar. The critical observation, however, is that the
oligothiophene isocyanides not only bind to platinum but also can
be electrochemically charged,13 as would be required in a device
format.
to larger ground-state electronic energy separations (smaller λmax
)
than would be expected for conjugated systems of these lengths.
Emission spectra, however, are in line with literature values,
indicating a conjugatively stabilized system that stretches over
3.5-5 nm.
Infrared spectroscopic examination of 1a-d confirmed the
installation of the isocyanide functional group by the presence of
a characteristic CN stretch at 2118 cm-1. Unequivocal evidence
for platinum binding is provided by a CN band shift to higher
wavenumbers (2170 cm-1) upon treatment of isocyanide with
† Current address: Department of Chemistry, The Ohio State University,
Columbus, OH 43210.
9
11796
J. AM. CHEM. SOC. 2004, 126, 11796-11797
10.1021/ja045904p CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. This work was supported primarily by the
Nanoscale Science and Engineering Initiative of the National
Science Foundation (NSF) under NSF Award Number CHE-
0117752 and by the New York State Office of Science, Technology,
and Academic Research (NYSTAR). This work has used the shared
experimental facilities that are supported primarily by the MRSEC
Program of the NSF. We thank Prof. J. Norton for use of the CV
equipment. I.W.T. thanks the NSF for a predoctoral fellowship.
Supporting Information Available: Experimental details for the
synthesis of 1a-d; Pt film evaporation, XPS of Pt films, nanoparticle
IR, and solution and SAM CV experimental details. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Representative overlaid transmission infrared spectrum of 1b
on CaF2 before treatment with platinum nanoparticles (2118 cm-1) and after
platinum nanoparticles (2170 cm-1). The isocyanide region is shown on
the left, and the unperturbed thiophene region is shown on the right.
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