An approach to the storage of multiple bits of information at
the molecular level employs molecules with a large number of distinct oxidation
states. Europium triple-decker sandwich molecules composed of porphyrins and
phthalocyanines afford four cationic states and are very attractive for molecular
information-storage applications. A larger number of states can be achieved
by combinations of triple deckers that afford interleaved oxidation potentials.
In order to identify suitable candidates for effective interleaving of oxidation
potentials, a library of 19 new triple-decker complexes was prepared. Electron-donating
groups have been attached to the porphyrin and/or phthalocyanine moieties
in order to achieve oxidation states in the low potential regime. The triple
deckers are of three different types: (Pc)Eu(Pc)Eu(Por), (Pc)Eu(Por)Eu(Pc),
and (Por)Eu(Pc)Eu(Por). The solution electrochemistry
of each member of the library was examined. These studies revealed suitable
pairs of triple deckers that provide effective interleaving of oxidation potentials.
Six triple deckers of type (Pc)Eu(Pc)Eu(Por)
were derivatized with a thioacetyl or thiocyanate group on the porphyrin unit
for attachment to an electroactive surface. Each of the S-(acetylthio)-derivatized
triple deckers forms a self-assembled monolayer (SAM) on Au viain
situ cleavage of the thiol protecting group. The SAM of each triple decker
is electrochemically robust and exhibits four, well-resolved reversible oxidation
waves. Upon disconnection from the source of applied potential, the triple-decker
SAMs retain charge for tens to hundreds of seconds. The exact value of the
charge-retention time depends on the specific porphyrin/phthalocyanine
in the triple decker and the particular oxidation state of the molecules in
the SAM (e.g., mono- vs. di- vs. tri- vs.
tetracation). For all of the triple-decker SAMs, the charge-retention
time monotonically increases as the oxidation state of the molecules in the
SAM increases. Collectively, the studies suggest that the triple-decker complexes
are excellent candidates for multibit molecular information storage elements.
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Journal of Materials Chemistry
- Information Point