7r stacking

This cell is veiy different from that of Ooct-OPV3 and contains four symmetry-related molecules. Due to the shorter substituents, the density is much higher. Despite this, there is no 7r-stacking (sec Fig. 16-21). The successive re-systems within one, non-centrosymmctric molecule are arranged in a helical fashion because the dihedral angles all have the same sign. 

Unlike solid state -stacks, however, double helical DNA is a molecular structure. Here CT processes are considered in terms of electron or hole transfer and transport, rather than in terms of material conductivity. Moreover, the 7r-stack of DNA is constructed of four distinct bases and is therefore heterogeneous and generally non-periodic. This establishes differences in redox energetics and electronic coupling along the w-stack. The intimate association of DNA with the water and counterions of its environment further defines its structure and contributes to inhomogeneity along the mole-... 

Redox participants are chosen to facilitate spectroscopic, biochemical and electrochemical probing of DNA CT. These include metallointercalators, organic intercalators, and modified bases that possess useful, well-described, and varied redox, photophysical and photochemical properties (Table 1). Our probes are readily incorporated into DNA assemblies where CT distances ranging from 3.4 to 200 A and driving forces spanning over two volts can be modulated with certainty. Most importantly, all redox probes which afford fast and/or efficient CT through DNA are well-coupled to the 7r-stack. 

Combined quantum mechanics/molecular mechanics studies have demonstrated that 7r-7r stacking interactions are important in determining the structural features of polyarene transition metal compounds such as the two ds Pd complexes.96... 

Figure 4 Chiral Pd complexes (a) PdBr(p-CNC6H4)( S -MeO-Biphep) (b) PdBr(C6F5)( S -MeO-Biphep). Potential aromatic 7r-7r stacking interactions are indicated by arrows.

Ag2L2](C2F5C02)2 have been prepared and they show 2D structures. In (190) the ligands are positioned face to face by the two Ag atoms and form a box structure the 7r-7r stacking between pyridine rings gives the 2D framework.1159... 

Metal complexes of pteridine have provided 3D H-bonded networks containing H bonding between pteridine ligands and water molecules, and 7r-stacking and H bonding between adjacent pteridine ligands. The complex [ZnL(H20)2].2H20 has been prepared.267... 

The strongly colored planar fused bicyclic system (25) is quinonoid with distinctly alternating bond lengths 1.33-1.35 A and 1.42-1.44 A. A strong 7r-stacking interaction in the molecule (28) between tolyl and phenanthroline planes is evident (interplane distance is 3.3 A), as well as between tolyl and cyclopentadienyl planes (the distance is 3.61 A) which results in columns of stacked molecules. 

Figure 7. Edge-to-edge sheets (A) and 7r-stacked columns (B) of [MeFT][4a].

The Merck system can be extended to the asymmetric synthesis of a-hydroxy ketones by using oxygen as electrophile (Scheme 18) (47). The hydroperoxide intermediates are reduced by triethyl phosphite. The sense of asymmetric induction is again consistent with the ion-paired, 7r-stacked assembly model given in Scheme 13. 

The cation radicals of terthiophene 18 reversibly dimerize even at low concentration (92JA2728). As a consequence, 71-dimers and 7r-stacks deserve attention as entities responsible for the properties of oxidized polythiophene and other conducting polymers. 

Apparently, the phenylenethienylene chromophores of 387 are present in the solid state as 7r-stacked H-aggregates. On the other hand, the red... 

This difference is a direct result of the close 7r-stacking interactions. Electrochemical reduction of the bis(bipyridinium) cyclophane occurs in three distinct stages ... 

Since intermolecular interactions at the interface play a key role, two representative interface structures were considered, which will be referred to as eclipsed (E) vs. staggered (S), see Fig. 3. These structures correspond to attractive vs. repulsive 7r-stacked interface configurations, which result from the following stacking patterns in the lowest-lying exciton state ... 

Fig. 3 TFB F8BT structures (slightly tilted forward) showing two different orientations of the polymer constituents. In each, the FBT (top chain) consists of 3 fluorene (F) units and 3 benzothiadiazole (BT) units while the TFB (bottom chain) consists of 3 F units and 2 triarylamine (TB) units. These co-monomer units are labeled with indices. The eclipsed orientation (panel a) has the middle F units (F12 and F22) of both chains 7r-stacked while the staggered orientation (panel b) has the middle BT of FBT (BT2) and the middle F of TFB (F22) 7r-stacked, see Eqs. (l)-(2). Reprinted with permission from Ref. [41]. Copyright 2007, American Institute of Physics.

A, while that between the pairs is 3.383(6) A. The 7r-stacked arrays of cations are linked by centrosymmetric pairs of Se(2)—N(l ) contacts, thereby generating ribbons running along the y-direction. These four-center Se—N supramolecular synthons, which are common in diselenadiazolylium salts and diselenadiazolyl radicals, are likely to dominate the solid-state structures of reduced diselenazolyl materials. 

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