Stacked aromatic rings

Table 12.9 Interaction between parallel stacked aromatic rings for molecules shown in Fig. 12.10 inter-ring separation (A) and interaction energies (kJmol ). The two substituent groups or heteroatoms point in opposite direction (antiparallel arrangement)...

Some thiophenolate monolayers also have been investigated. Thiophenolate, C H S—, forms ordered monolayers on Ag(lll) with a (/7 X v, 88°)R40.9°, and benzene rings closely packed in face-to-face stacked columns (294). Benzylthiolate (295),/xpyridinethiolate (296), and (9-pyridinethiolate (296), also form ordered monolayers on Ag(lll), but with fewer close-packed aromatic rings. 

FIGURE 2.6 The procarcinogen benzo[a]pyrene oriented in the CYPlAl active site (stereo view) via n- n stacking between aromatic rings on the substrate and those of the complementary amino acid side chains, such that 7,8-epoxidation can occur. The substrate is shown with pale lines in the upper structures. The position of metabolism is indicated by an arrow in the lower structure (after Lewis 1996). 

We have now adjusted our molecular systems to provide a model in which both forces can operate simultaneously. The U-shaped relationship that exists between the imide function and amides of aryl amines creates a hydrogen bonding edge and a planar stacking surface that converge from perpendicular directions as in 44 to provide a microenvironment complementary to nucleic acid components. A large number of aromatic rings can be functionalized with this simple scaffold, and spacers (R) can also be incorporated. The imide function is a mimic of the thymine residues. 

Imagine a columnar stack of aromatic rings placed about 3.4 A apart in parallel planes (Figure 26). How well would such an array mediate ET and... 

Figure 26. Schematic of the mediation of ET by a jt-stack of aromatic rings, such as the base pairs of duplex DNA. The D and A groups associated with the DNA molecule may be covalently linked to the helix, they may be intercalators, or they may be DNA base pairs.

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