Supramolecular Charge Transfer Complexes

Supramolecular complexes as described herein are composed of an assembly of individual inorganic and organic components that act together to provide an overall function to the molecule. Such systems have been also proposed as photochemical molecular devices by Balzani. The overall function of the supramolecular complex can be modulated by careful selection of the individual components. Because the individual components within the supramolecular complex are chemically bonded, bimolecular reaction for energy or electron transfer is eliminated, increasing the efficiency of the photochemical processes. The covalent coupling of subunits provides significant perturbation of the component s basic properties. 

In the construction of polymetallic charge transfer complexes, bridging ligands (BLs) are used to chemically bond the individual components within the supramolecular assembly. A wide range of BLs have been used for this purpose. Some common BLs consist of simple ligands sueh as CN and polyazine-type BLs separated by rigid and nonrigid spacers (Fig. 8). Polyazine type BLs 

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Tetracyanoquinodimethane (TCNQ), which is famous for its supramolecular charge-transfer complexation, entered polymer science some time ago [109]. Here, main-chain polymers were introduced [110, 111], including T(2NQ polymers decorated with dendrons [112]. 

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

Supramolecular chirality can be introduced in columns of discotic molecules by using specific interactions, apart from the arene-arene interactions. To achieve this, a study with chiral dopants has been performed.73 Hexa-n-hexyloxytriphenylenes were mixed in dodecane solutions with a variety of chiral electron-deficient dopants and the resulting charge transfer complexes... 

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]. 

It should be noted that the word complex , often used in supramolecular chemistry, is not very specific. It is applied to charge-transfer complexes like the one formed by 21 with 22 [30] as well as to coordination complexes consisting of one or more atoms or ions with n ligands like K2[Pt(N02)4]. The same name complex also covers the Whitesides hydrogen bonded systems [10] shown in Figure 1.1 and inclusion complexes of 4 embedded in 5. Thus the term complex without any adjective has no specificity and can be applied to any type of molecular associates. 

In addition to ferrocene, the oxidative redox couple that has received the most attention in supramolecular chemistry is tetrathiofulvalene (TTF), 35. This compound undergoes two reversible one-electron oxidations, first to a radical cation and then to a dication (Eq. 1.21). TTF first came to prominence in the 1970s when it was discovered that the charge transfer complex between it and 7,7,8,8-tetracyanoquinonedimethane (TCNQ) shows metallic conductivity. As a result, a large variety of different TTF derivatives have been prepared and characterized. This rich synthetic chemistry, coupled with the electroactivity, has intrigued supramolecular chemists for some time, with the result that the TTF unit has been incorporated into a wide variety of... 

Wiliams R M and Verhoeven J W1992 Supramolecular encapsulation of Cgg in a water soluble calixarene a core-shell charge transfer complex Reel. Trav. Chim. Pays-Bas 111 531-2... 

Association of molecules changes their As as has been proven for metal complexes, charge transfer complexes, Lewis acids, etc. In any case, complex formation can be followed by the variation of the CD. Molecular association is the first step to an aggregation and, furthermore, is the presupposition for supramolecular structures. Compounds with inter-molecular exciton interaction, e.g., carotenoids or cyanine dyes, have been successfully analyzed. The spontaneous association to chiral associates from achiral compounds, e.g., cyanines, have been proven and analyzed with the help of ECD spectroscopy. In this context, the successful CD analyses with Langmuir-Blodgett films as well as the analyses of membranes have to be mentioned where suprastruc-tural chirality can also emerge from achiral monomers. 

Jeon YJ, Bharadwaj PK, Choi S-W, Lee JW, Kim K. Supramolecular amphiphiles spontaneous formation of vesicles triggered by formation of a charge-transfer complex in a host. Angew Chem Int Ed 2002 41 4474-6. 

Charge transfer complexes of aromatic donor—acceptor pairs are greatly stabilized within the hollow cavities of cucurbit[n]uril (CB[n]) in aqueous environ-ments. This host-stabilized charge transfer (HSCT) interaction has been recently applied to developing supramolecular polymers. - The supramolecular polymerization of multifunctional monomer 12 via HSCT interactions was demonstrated (Figure... 

Tetrathiafulvalene (TTF) is known in supramolecular chemistry for its many charge-transfer complexes [77, 78]. TTF-containing polymers [79] or... 

The conjugation in the molecular wire may be disrupted or modulated to create systems with different properties. For example, a porphyrin Ceo donor-acceptor system linked with a conjugated binaphthyl unit, has a preference for the atropi-somer where the fullerene unit is closer to the porphyrin system, thus increasing the through space interactions [82]. The charge transfer process on a dyad containing a crown ether in the linker structure can be modulated by complexation/ decomplexation of sodium cations [83] but even more interesting is the construction of supramolecular systems where the donor and acceptor moieties are... 

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