Page 7 - Molecules for Charge-Based Information Storage

Molecules for Charge-Based Information Storage Lindsey and Bocian

FIGURE 6. (A) Voltammetric response upon repeated cycling of a
monolayer of Zn trimesitylporphyrinbenzyl alcohol attached to Si-
(100). (B) Integrity of charge storage over 10 10 cycles.

characteristics of the monolayer exhibit minimal variation
(few percent) over the course of the entire experiment. At
the time that cycling was arbitrarily stopped (>10 10 cycles;
∼27 days), the system showed no signs of degradation.
FIGURE 7. Orientations of molecules on a surface.
Similar results were obtained in a second series of experi-
ments where the system was cycled ∼10 12 times over the surface but instead are on average tilted with respect to the
course of 61 days. The robustness of the porphyrins to redox surface normal by angles ranging from 35° to 55°. 30,32,33,35
cycling is attributed to the fact that the positive charge on the A series of porphyrins bearing vibrational spectroscopic
molecules is delocalized over a number of carbon and labels enabled distinction of the two in-plane axes of the
nitrogen atoms in the macrocycle, thereby minimizing the porphyrin ring. 45 The spectroscopically labeled molecules
number of “hot spots” and diminishing susceptibility to allow evaluation of both the tilt angle (θ) with respect to the
attack from adventitious chemical agents. surface normal and the rotation angle (φ) about the molec-
Our studies of porphyrin monolayers on both metal and ular axis (Figure 7). Surface IR studies suggest that all of the
semiconductor surfaces showed that important physical porphyrins on both Au(111) and Si(100) exhibit a distribution
characteristics such as electron-transfer rates depend on of tilt and rotation angles. The distribution of φ angles is
factors such as surface coverage (packing density). 15,16 The (nearly) random about the molecular axis; the distribution of
surface coverage is in turn influenced by factors such as the tilt angles is less broad owing to steric interactions between
relative orientation of the porphyrins with respect to one the porphyrin substituents and the surface. The surface
another and with respect to the plane of the surface. Con- coverage affects the distribution of both the tilt and rotation
sequently, determining the surface orientation of porphyrin angles. At lower surface coverage, the molecules exhibit
molecules became a focal point of our studies of monolayers larger tilt angles and rotation angles, that is, the porphyrin is
on both metals and semiconductors. Our early studies more coplanar with the surface. The fact that all of the
showed that porphyrins tethered to either Au(111) or Si(100) porphyrins on both Au(111) and Si(100) exhibit qualitatively
assume neither an upright nor a prone orientation on the similar surface orientation characteristics suggests that the

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