The 8-hydroxyquinoline (HQ) forms metal-ligand chelation bonds with almost all metals, albeit with various bonding energies and geometries, and at different pH. In particular, the monodentate complexes are good candidates for elementary molecular devices -to be integrated in larger molecular constructions- because of the energetic and geometric stability of the complex, because of the capacity of the ligand to form both ionic and chelation bonds with the metal, and because of the control of the nature of the bond via the pH of the solution.
This computational chemistry study regarding the feasibility of using Me-HQ monodentate complexes as data storage molecular devices was prompted by Bauschlicher, Ricca & Merkle's quantum mechanics assessment1 of the possibility of storing data using H and F on a polymer to signify 0 and 1 bits; and our previous work2 regarding the prediction of transition metal-ligand interaction via semi-empirical methods (ZINDO). Building on these studies we computed the "chemical map" sensed by an "AFM-like" HQ molecular tip moving atop of a metal surface.
The metal surface consisted of a "sea" of neutral atoms (i.e. metallic atoms which form weak or no chelating bond with HQ) with signaling atomic "islands" (i.e. metallic atoms which form strong bonds). Both the neutral and the signaling atoms were selected to have similar properties (i.e. atomic/ionic radius and propensity for the same crystallographic lattice -here the cubic lattice was preferred-). The "signal/noise" ratio was estimated by the difference in the bonding energies of the HQ-Me for the neutral and signaling atoms. The energy map was computed with ZINDO and scaled through a comparison of the ZINDO- and ab initio calculations (the latter only in selected positions).
It was found that the metallic pairs that give the best "signal/noise" ratio are Cu/Zn and Pd/Cd. Because some Me-HQ complexes are reversible vs. pH we also studied the feasibility of a molecular extractor that would pull out an atom, transport and then implant it on a different location. Finally, we studied the feasibility of the Me-HQ system to act as a steering mechanism for molecular "cars" travelling on Cu-tracks on Zn surfaces.
References
1. Bauschlicher Jr., C. W., Ricca, A. and Merkel R. Chemical storage of data. Nanotechnology, 8, 1-5, 1997.
2. Nicolau, D. V., Yoshikawa, S. A molecular modelling study of the Me2+(8-hydroxy-quinolin)ate complexes. Journal Molecular Graphics & Modelling, 16, 83-96, 1998.