CMDITR Review of Undergraduate Research - Pluto - University of ...

Synthesis of C 60 -acetylenyl Based Molecules for Self-Assembly
Ashley Hamilton-Ross, Olympic College
Hong Ma, Mun-Sik Kang, Quigmin Xu, Neil Tucker, Joel Horwitz, and Alex K.-Y. Jen
Jen Lab, Dept. of Materials Science & Engineering, University of Washington
Self-assembly has gained much attention in
thin film development. Self-assembly is the
autonomous ordering of various components into
a structure or pattern without human
intervention. 1 This method of nanostructure
formation relies on two factors: 1) mobile
components and 2) equilibration. A liquid phase
facilitates the free movement of individual
molecules to balance attraction and repelling
associated with weak covalent or non-covalent
bonds, which enables molecules to align
themselves in an organized manner. The system
also must be at a local equilibrium, so if
individual molecules collide, they will not
aggregate to form a glass instead of a crystal.
Two forms of self-assembly exist: static and
dynamic. Static self-assembly, which entails no
dissipation of energy, is particularly appealing in
the field of nanotechnology.
Self-assembled monolayers (SAMs) have
proven to be superior to nanolithography in
forming both long-range order and short-range
order in molecular nanostructures. 2 Additionally,
self-assembled monolayers are inherently
manufacturable. 3 These SAMs are made by
immersing a noble metal substrate in a solution
of surface-active material. Over a set period of
time, the surface-active material aligns itself on
the surface of the metal substrate to form a
standing monolayer of individual molecules.
Self-assembled monolayers typically display
well-ordered and highly-packed two-dimensional
structures, providing a viable avenue to explore
the properties of functional molecules.
Self-assembled monolayers formed on noble
metal substrates also have applications in
photocurrent generation. The circuit setup
consists of a SAM on gold connected via a
copper wire to a platinum electrode; both are
submerged in a solution of methyl viologen.
Light (hv) excites an electron in the electrondonating
moiety of the SAM; this electron is
then transferred through the SAM to the
electron-affinitive portion of the molecule, and
then to the solution of methyl viologen, which is
reduced. The solution shuttles the electron to the
platinum electrode, and then back to the gold
substrate, producing a steady current.
Due to its electronic, photonic, and optical
characteristics, [60] fullerene has been valuable
in the advance of functional materials. C 60 can
store up to six electrons within its cage-like
structure at one time, which make it particularly
desirable in photocurrent generation. Previous
research on C 60 -(4-mercaptophenyl)
anthrylacetylene (C 60 -MPAA) suggests that
tailoring the alkyl functional groups which attach
the C 60 moiety to the noble metal substrate could
result in an optimal electronic response. 4
In this project, two molecules were
synthesized: C 60 -acetylene-anthracene methyl
thiol ester and C 60 -acetylene-benzene methyl
thiol ester. Each molecule is composed of three
basic elements: an electron-affinitive moiety, an
electron-donating moiety, and a “linker.” [60]
fullerene remained a desirable electron-affinitive
moiety for its electron storage. The thiol “linker”
group was also a common feature because of its
ability to effectively bind to the substrate. The
electron-donating moiety, however, was different
for both molecules. Anthracene and benzene
were selected based on previous C 60 -MPAA
research and the need for a conjugated structure
for electron transport. Research with C 60 -MPAA
yielded that anthracene’s strong intermolecular
π-π stacking and its larger size relative to that of
Figure 1. Synthetic scheme for C 60 -
acetylene-anthracene methyl thiol ester
32 CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004