No abstract

The binding constants with the host cyclobis(paraquat-p-phenylene), 1(4+), have been determined in CH(3)CN by UV-vis spectrophotometry for a series of p-phenylene guests, symmetrically substituted with side arms of varying length and functionality. Semiempirical molecular orbital theory was employed to provide a detailed structural and energetic interpretation of the experimental binding data. In particular, the length of the side arms and the type and position of the heteroatoms on the side arms were systematically varied in order to understand the effects of external interactions on the association constants of the guests with host 1(4+). A large chelate effect involving the ethyleneoxy side arm oxygen atoms and a cooperative effect between the guest aromatic core and the side arms are significant factors which determine the binding with this host. Sequential ethyleneoxy linkages along the side arms markedly increase the binding constant with respect to a compound in which the same number of oxygens along the side arms are separated by longer aliphatic linkages. In addition, a multiplicative rather than additive effect on the binding constant is observed which demonstrates that the oxygen atoms exhibit a strong chelate effect. It was also discovered that while the side arms of these guests contribute most of the driving force for complexation, an aromatic core is necessary for the guest to reside in the cavity of the host. The binding of these guests then is dependent upon cooperation between the arms and the aromatic core. Furthermore, elongation of the central aromatic core with aliphatic side arms containing no heteroatoms leaves the association constant relatively unchanged and replacement of the oxygen atoms with sulfur markedly decreases the observed binding. These effects have been used to rationalize several observations regarding this system in the literature and may serve to improve the design of new supramolecular systems and to better understand the host/guest interaction process.

The origin of differential binding affinity and structural recognition between the inclusion complexes of cyclobis(paraquat-p-phenylene), 1 4+ , and 1,4-substituted phenyl or 4,4′-substituted biphenyl derivatives has been jointly determined by spectrometric techniques and ab initio and semiempirical molecular orbital methods. The unusual boxed geometry and tetracationic charge distribution in 1 4+ are key molecular features which produce strong intermolecular interactions with guest and solvent molecules. Solvation was addressed by including up to 12 acetonitrile molecules in the theoretical model, which realigned the predicted gas-phase supramolecular structures and energies into excellent agreement with experiment. The computed complexation enthalpies, ∆H bind , from the semiempirical molecular orbital PM3 method are on average within 1 kcal/mol of the experimental free energy binding data collected from absorption spectroscopy in acetonitrile. In addition, the computed geometric penetration and positioning of 1 4+ /benzidine and 1 4+ /4,4′-biphenol complexes are consistent with that reported from NMR NOE data. The partitioning of self-consistent field complexation energies from both classical and quantum forces has been determined by using Morokuma's variational energy decomposition technique. It was determined that the primary basis for the molecular recognition between 1,4-substituted phenyl guests and 1 4+ is short-range stabilizing electrostatic forces complemented by small amounts of polarizability and charge-transfer. In contrast, the recognition force between 4,4′-substituted biphenyl guests and 1 4+ is dominated by polarizability with a small contribution from electrostatics. Therefore, the balance between molecular polarizability and electrostatics controls the differential binding affinity and structural recognition with 1 4+ . For the first time, we report that individual molecular properties of substituted guests correlate with the binding energies of corresponding 1 4+ inclusion complexes. Direct correlations between the 1 4+ binding energies and the computed molecular polarizability, maximum hardness, softness, and electronegativity of the guest have been identified. It is now plausible to consider the design and construction of new supramolecular assemblies based upon a few select molecular properties of the constituent molecules.

The adsorption of viologen-containing polyelectrolytes on the surface of carboxylate-terminated, selfassembled monolayers was investigated by voltammetric methods. The two polyelectrolytes used had either butyl (VC4) or undecyl (VC11) linkages between the repeating viologen (4,4′-bipyridinium) subunits. Both polyelectrolytes were found to adsorb at neutral pH on monolayers prepared by the self-assembly of either 3-mercaptopropionic (HSC2COOH) or 8-mercaptooctanoic (HSC7COOH) acids on gold. However, the adsorption was optimized by using the more hydrophobic polyelectrolyte (VC11) and the longer thiol (HSC7COOH). The gradual deprotonation of the HSC7COOH monolayer in the pH range 6-11 enhances the adsorption of the VC11 polyelectrolyte. From these data, an apparent pKa of ∼8 for the monolayer -COOH groups was obtained. The overall degree of packing of the viologen subunits at the monolayersolution interface was found to be rather poor. The combined monolayer-polyelectrolyte interfacial structures did not block effectively the reduction of dissolved Ru(NH 3)6 3+ ions on the underlying gold electrode.