Charge transfer complex

A charge transfer complex (CT complex) is defined as an electron donor–electron acceptor complex, characterized by electronic transition(s) to an excited state. In this excited state, there is a partial transfer of elementary charge from the donor to the acceptor. Almost all CT complexes have unique and intense absorption bands in the ultraviolet-visible (UV-Vis) region.

Apart from charge transfer interactions between donor and acceptor, electrostatic forces exist as well. The forces present are usually much weaker than hydrogen bonds or covalent bonds, but are useful for constructing crystal structures.

Metal to ligand charge transfer complexes
Metal to ligand charge transfer (MLCT) complexes experience a partial transfer of electrons from the metal to the ligand. This usually occurs for metals with well-filled d orbitals which can donate electrons into the antibonding orbitals of the ligand. Of course, the usual rules apply - the non-bonding d orbitals should match the antibonding orbitals in terms of size, shape, and symmetry.

Ligand to metal charge transfer complexes
Ligand to metal charge transfer (LMCT) complexes are the reverse of MLCT complexes - they experience a partial transfer of electrons from the ligand to the metal. This is common when the metal is at a high oxidation state; an example is permanganate (MnO4-), where the manganese atom has an oxidation state of +7.

Charge transfer complexes and color
Many metal complexes are colored due to d-d electronic transitions. Visible light of the correct wavelength is absorbed, promoting a lower d-electron to a higher excited state. This absorption of light causes color. These colors are usually quite faint, though. This is because of two selection rules:


 * 1) The spin rule: Δ S = 0 On promotion, the electron should not experience a change in spin. Electronic transitions which experience a change in spin are said to be spin forbidden.
 * 2) Laporte's rule: Δ l = ± 1 d-d transitions for complexes which have a center of symmetry are forbidden - symmetry forbidden or Laporte forbidden.

Charge transfer complexes do not experience d-d transitions. Thus, these rules do not apply and the absorptions are generally very intense.

For example, the classic example of a charge-transfer complex is that between iodine and starch to form an intense purple color. This has wide-spread use as a rough screen for counterfeit currency. Unlike most paper, the paper used in US currency is not sized with starch. Thus, formation of this purple color on application of an iodine solution indicates a counterfeit.

History
In 1954 researchers at Bell Labs and elsewhere reported charge-transfer complexes with resistivities as low as 8 ohms/cm. In 1962, the well-known acceptor, tetracyanoquinodimethane (TCNQ) was reported. Similarly, the classic donor, tetrathiafulvalene (TTF), was synthesized in 1970. A CT complex composed of the TTF and TCNQ was discovered in 1973. This was the first organic conductor to show almost metallic conductance. In a crystal of TTF-TCNQ, the TTF and TCNQ are stacked independently and an electron transfer from donor (TTF) to acceptor (TCNQ) occurs. Hence, electrons and holes can transfer in the TCNQ and TTF columns, respectively.

In 1980, the first organic molecule that was also a superconductor was discovered. Tetramethyl-tetraselenafulvalene-phosphorus hexafloride TMTSF2PF6 shows superconductivity at low temperature (critical temperature) and high pressure: 0.9 K and 12 kbar. Since 1980, many organic superconductors have been synthesized, and the critical temperature has been raised to over 100 K as of 2001. Unfortunately, critical current densities in these complexes are very small.

CT complexes have many useful applications and more properties are expected to be discovered.

Examples
Hexaphenylbenzenes like H (fig. 1) lend themselves extremely well to forming charge transfer complexes. Cyclic voltammetry for H displays 4 well separed maxima corresponding to H+ right up to H4+ with the first ionization at E1/2 of only 0.51 eV. oxidation of these arenes by for instance dodecamethylcarboranyl (B) to the blue crystal solid H+B- complex is therefore easy.

Fig. 1 Synthesis of H+B- complex: Alkyne trimerisation of bisubstituted alkyne with dicobalt octacarbonyl, delocalization is favored with activating groups such as a di(ethylamino) group The phenyl groups are all positioned in an angle of around 45° with respect to the central aromatic ring and the positive charge in the radical cation is therefore through space delocalised through the 6 benzene rings in the shape of a toroid. The complex has 5 absorption bands in the near infrared region which can be assigned to specific electronic transitions with the aid of deconvolution and the Mulliken-Hush theory.

Charge transfer complexes and disease
In humans, elevated systemic levels of transition-series metals, electron-donors, etc. are associated with specific disease symptoms. These include psychosis, movement disorders, pigmentary abnormalities, and deafness. This may involve charge-transfer complexes with the Melanin in the midbrain, skin, and the stria vascularis of the inner ear.