Aggregation charge transfer systems

However, careful kinetic measurements on related systems showed the invalidity of wire-type behavior [41]. Furthermore, Sen and coworkers [42] recently showed that the appearance of rapid, long-distance charge transfer for metallointercalators may be an artifact caused by the formation of aggregates. Currently, there are no data that clearly support the existence of a coherent transfer process in DNA over a distance greater than one or two base pairs [43, 44]. 

Both ion and electron transfer reactions entail the transfer of charge through the interface, which can be measured as the electric current. If only one charge transfer reaction takes place in the system, its rate is directly proportional to the current density, i.e. the current per unit area. This makes it possible to measure the rates of electrochemical reactions with greater ease and precision than the rates of chemical reactions occurring in the bulk of a phase. On the other hand, electrochemical reactions are usually quite sensitive to the state of the electrode surface. Impurities have an unfortunate tendency to aggregate at the interface. Therefore electrochemical studies require extremely pure system components. 

Related to these dimetallic systems, though not involving transition metals, are the boratastilbene complexes such as a " [ H 5 C 5 B-CH =CH-CH 4 -C H =C H P h ]" (isoelectronic with distyrylbenzene chromophores) that show aggregation-dependent photophysics. In nonpolar solvents, they form tightly bound ion pairs that are poorly luminescent, but in polar solvents, or when the counter ions are encapsulated in crown ethers, strong emission is observed as a result of intramolecular charge transfer.130... 

The prospective applications ofmolecular assemblies seem so wide that their limits are difficult to set. The sizes of electronic devices in the computer industry are close to their lower limits. One simply cannot fit many more electronic elements into a cell since the walls between the elements in the cell would become too thin to insulate them effectively. Thus further miniaturization of today s devices will soon be virtually impossible. Therefore, another approach from bottom up was proposed. It consists in the creation of electronic devices of the size of a single molecule or of a well-defined molecular aggregate. This is an enormous technological task and only the first steps in this direction have been taken. In the future, organic compounds and supramolecular complexes will serve as conductors, as well as semi- and superconductors, since they can be easily obtained with sufficient, controllable purity and their properties can be fine tuned by minor adjustments of their structures. For instance, the charge-transfer complex of tetrathiafulvalene 21 with tetramethylquinodimethane 22 exhibits room- temperature conductivity [30] close to that of metals. Therefore it could be called an organic metal. Several systems which could serve as molecular devices have been proposed. One example of such a system which can also act as a sensor consists of a basic solution of phenolophthalein dye 10b with P-cyciodextrin 11. The purple solution of the dye not only loses its colour upon the complexation but the colour comes back when the solution is heated [31]. 

When two or more reactants first approach one another, non-covalent recognition and attraction of the molecules or of parts of the molecules takes place and permits system aggregation, which may be of importance in obtaining the reaction products. For instance, the approach of a carbocation to a n system of olefins starts by forming a charge-transfer (CT) complex which precedes the formation of the C—C bond6. 

When considering structural aspects of polymeric systems, solutions wherein partial polymer association occurs, must also be taken into consideration. In concentrated or semi-dilute solutions, long polymer chains can form networks through the association of short segments randomly distributed along the chains the physical association may arise from charge transfer or from hydrophobic interactions networks may also result from the presence of chains which both enter in the formation of small aggregates and connect them to one another. 

The foregoing discussion plainly indicates that Sn... r interactions can and do exist in the structures described herein. At a minimum, molecules aggregate to form a dimer and more often than not, these form the supramolecular tecton that associates via other intermolecular interactions, sometimes of the type Sn... ITT, into one-, two- and three-dimensional supramolecular architectures. Without exception, Sn... tt interactions involve tin atoms present in the formal oxidation state -l-II. This leads to the conclusion that one possible explanation for Sn... itt interactions lies in a charge transfer from the tin-bound lone pair of electrons into the LUMO of the aromatic system. 

In numerous systems, a fundamental role is played by the van der Waals forces . Conformations of single molecules and intermolecular aggregations are strongly affected by van der Waals interactions which may be evaluated by quantum chemical methods. True van der Waals interactions are those when the dominant attractive contribution is the dispersion energythe term van der Waals complex may be considered better than charge-transfer complex when no charge is transferred during the complex formation. 

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