Stacking charge transfer systems

A supramolecular device is a molecular-level system that acts or carries out a function of some kind in the same way as a larger-scale device, such as an electronic component (switch, rectifier) or mechanical object (motor, spring). The basis for the device s operation can be supramolecular in the sense that two or more components interact in a noncovalent manner. This is the case with rotaxane 13 (Fig. 7), in which the paraquat-derived macrocycle switches back and forth between the two biaryl "stations" in response to changes in pH. The interaction between the paraquat "train" and the two stations on the polyethyleneglycol "track" is entirely supramolecular and involves n-stacking. charge transfer, and CH-. O hydrogen bonds. 

Carrier generators in molecular conductors have been associated for a long time to a partial charge transfer between the HOMO (or LUMO) electronic band and other chemical species. These systems are known as two-component molecular conductors. Tetrathiofulvalene derivatives are versatile systems for the formation of molecular organic conductors due to their electron donor capacity by transferring one u-electron from the HOMO orbital, and to their planar shape that promotes their stacking as a consequence of the n-n orbital overlap. The electronic properties of these salts are essentially determined by the packing pattern of the donor molecules which, in turn, depends on the counter-ion. 

Bases stacked rather than hydrogen bonded have also been studied with quantum chemical methods [182, 244-247]. The nature of excited states in these systems has been debated and theoretical calculations are called to decide on the degree of excited state localization or delocalization, as well as the presence and energy of charge transfer states. The experimentally observed hypochromism of DNA compared to its individual bases has been known for decades [248], Accurate quantum chemical calculations are limited in these systems because of their increased size. Many of the reported studies have used TDDFT to calculate excited states of bases stacked with other bases [182, 244, 246, 247], However, one has to be cautious when us-... 

Kochi and co-workers engineered heteromolecular charge-transfer crystals of a tricyclic dithiin 34 stacked alternately with anthracene, which can undergo spontaneous Diels-Alder cycloaddition to give a novel artificial crystal system <2001JA87, 2001JA4951>. 

Induction of chirality in No phases was also shown to be possible using charge transfer interactions, via jt-jt stacking. The binary mixture of mesogen 8, which is electron rich, with chiral electron-deficient molecule 9 was shown to induce a twist in the mesophase.15 Furthermore, nonmesogenic 8 gave rise to a cholesteric mesophase, denoted as being of the columnar type (N ), when present in a ternary system with TNF (trinitrofluorenone, an electron acceptor)... 

Face-to-face aromatic stacking interactions are common in a wide range of organic charge-transfer compounds where there is a donor-acceptor interaction between an electron rich Ji-system and an electron... 

The Cambridge Crystallographic Database (CCDB, http //www.ccdc.cam.ac.uk/) holds approximately 1200 examples of structures containing the anthracene chro-mophore, although if charge-transfer complexes, benzannelated systems and duplicate spectra are discounted the total number of structures drops to only a few hundred. Of those, very few show anthracene in 3t-stacked arrangements. In gen-... 

A great number of chiral TTFs have been prepared [31,32], and some of their charge transfer salts have been crystallised. The 7r-donor 1 can be elec-trocrystallised to give a salt 12 PF6, which has a conductivity of 5 2 1 cm 1 at room temperature and metallic behaviour when cooled down [33]. While the structure of the crystals is chiral, the structure of this salt and other related ones [34] has an essentially achiral stack of donor molecules, which are pseudo-centrosymmetric [35]. The methyl groups at the periphery of the molecule are apparently not sufficient to cause a truly chiral stack of donors, which prefer to stack in parallel arrangements with partial overlap of the tr-systems in the solid state, a situation which is true for the majority of the efforts to prepare salts of this type (even if the salts are metallic). 

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