Page 3 - Molecules for Charge-Based Information Storage

Molecules for Charge-Based Information Storage Lindsey and Bocian

FIGURE 2. Porphyrin-based memory element (left panel). Redox-based read/write process; P = porphyrin (right panel).

properties, which provide the basis for writing/reading the is to incorporate multiple redox states into a single molecule.
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memory cell. Accordingly, some type of an electrolyte Examples of the latter are provided in Figure 3.
material is an integral part of the memory cell. The important The ferroceneporphyrin dyad A provides three distinct
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redox characteristics of porphyrins include the following: cationic states. The porphyrin dyad B is designed so that the
(1) They form π-cation radicals that are relatively stable under two cationic states of the porphyrin proximal to the surface
ambient conditions, facilitating real-world applications. attachment group are at lower potential than those of the
(2) They exhibit multiple cationic states that are accessible distal porphyrin; accordingly, the dyad affords four distinct
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at relatively low potentials, affording multibit information cationic states. The potentials of the porphyrins are tuned
storage with low power consumption. (3) They are capable by the electronic properties of the nonlinking meso substit-
of storing charge for extended periods (up to minutes) in the uents (electron-releasing mesityl versus electron-withdraw-
absence of applied potential, further diminishing power ing pentafluorophenyl). Finally, the dyad C employs two
consumption and significantly attenuating the refresh rates porphyrins with essentially identical meso substituents, but
required in a memory device. only one carboncarbon single bond joining the two
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The read/write strategy for the porphyrin-based memory porphyrins. In contrast with dyad B, where the electronic
cell is also illustrated in Figure 2. The oxidation of the neutral interactions between the constituent porphyrins are weak,
porphyrin to the mono-π-cation radical constitutes writing of such interactions in dyad C are relatively strong. Conse-
a bit of information. Subsequent reduction of the mono-π- quently, dyad C affords four distinct cationic states.
cation radical to the neutral molecule constitutes reading out Other redox-active constituents examined for molecular
that bit of information. Accordingly, the read protocol is by information storage include the triple-decker lanthanide
its nature destructive. Monomeric Zn porphyrins exhibit two sandwich compounds. 810,15,20,23,25,43 A triple decker typi-
distinct cationic states. This is illustrated in Figure 2 by the cally exhibits four distinct cationic states, which can be tuned
oxidation/reduction (write/read) to/from the di-π-cationic by choice of (1) metals, (2) ligand composition (porphyrins,
state of the porphyrin. phthalocyanines), and (3) substituents about the perimeter
of a given ligand. One representative triple decker is shown
Tuning Electronic Properties of Molecules in Figure 4. We designed several dyads of triple deckers in an
Using Synthetic Design effort to construct an octal counter (i.e., 8 states = 3
One objective of our studies was to create molecular archi- bits). 12,19,36 The dyad shown in Figure 4 is composed of a
tectures for multibit information storage. For useful multibit Pc-Eu-Pc-Eu-Por triple decker and a Por-Eu-Pc-Ce-Por triple
storage at least three distinct cationic states are needed. decker where Pc and Por represent the ligands of the
Such multiple states, each at a distinct potential, are not phthalocyanine and porphyrin, respectively. While eight
individually addressable. The states resemble a Coulomb states could be achieved, the triple decker occupied a very
ladder and thereby constitute what is more appropriately large footprint, which decreased the charge density per state
called a molecular counter. One approach is to employ relative to that of a porphyrin monomer. The large footprint
mixtures of redox-active molecules. 26 A second approach stems from the size of the triple deckers, the placement of

640 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 638–650 ’ 2011 ’ Vol. 44, No. 8