Charge Transfer Complexes

Charge transfer complex is abbreviated as CTC, also known as charge transfer complex or donor-acceptor complex, which is a complex composed of electron donor and electron acceptor. There are two main mechanisms for the formation of charge transfer complexes. One is valence bond theory explanation, and the other is molecular orbital theory explanation. In general, charge transfer complexes have quasi-one-dimensional conductivity, superconductivity, and electrical bistable characteristics.

Figure 1. Schematic diagram of orbital energy and electron transition in charge transfer complex

Applications:

Due to the advantages of diverse synthesis methods, rich types, and excellent physical and chemical properties, the charge transfer complex has extensive applications in many fields such as photocatalysis, pharmaceutical analysis, and materials.

• Photocatalysis: TiO₂ photocatalysis technology has the advantages of low cost, non-toxicity, high catalytic activity, strong oxidation ability, simple operation, and mild operating conditions. However, the forbidden band width of TiO₂ is 3.2 eV, which determines that its photocatalytic properties are limited to the ultraviolet band, and ultraviolet light only accounts for 4% to 5% of sunlight, resulting in the low efficiency of TiO₂ if onlysunlight is used for photocatalysis. The use of TiO₂ with certain surface-adsorbed organic substances (such as polyethylene glycol, chlorophenol, etc.) can form charge transfer complexes, allowing TiO₂ to obtain visible light activity, and thus play a more extensive role in the field of photocatalysis.
• Drug analysis: After some substances form a charge transfer complex, their color, solubility, crystalline properties, especially the maximum absorption wavelength will change, showing the unique characteristics of the charge transfer complex. The charge transfer complex meets Lambert-Beer law at the maximum absorption wavelength, that is, the absorbance is proportional to the concentration of the charge transfer complex, so the maximum absorption wavelength of the charge transfer complex can be used for quantitative analysis. Many drugs contain electron-donating groups (amino, benzene ring, pyrimidinyl, etc.), which can be used as electron donors. The use of these drugs and electron acceptors to form charge transfer complexes to determine the content of drugs is simple, accurate, highly specific, and has high sensitivity. At the same time, it can also accurately analyze and determine the drug content in the blood, thus showing good application prospects. For example, pyrones are a class of antibiotic drugs, and common pyrone drugs are ofloxacin, norfloxacin, ciprofloxacin, etc. Their molecules all contain piperazinyl groups, and the nitrogen atom on them carries a lone pair of electrons, which can be used as electron donors to form charge transfer complexes with quinones and other electron acceptors. The charge transfer reaction between levofloxacin and tetrachlorop-benzoquinone can be used to determine the content of levofloxacin in drug inhibitors.

Figure 2. Charge transfer reaction between Levofloxacin and 2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione

• Materials: Charge transfer composites have excellent physical properties and are widely used in the field of materials. They can be used as conductive materials, luminescent materials and magnetic materials. Some charge-transfer complexes with 1,2,4,5-tetracyanobenzene (TCNB) as the acceptor can emit light, which leads to further in-depth research on their self-assembly and solid-state light-emitting properties and many progresses are made . For example, using naphthalene as the electron donor and TCNB as the electron acceptor, a charge transfer composite nanotube of naphthalene-TCNB is prepared by the precipitation method. Due to the aggregation-induced luminescence enhancement effect, the nanotube exhibits a strong blue light. By further using pyrene for doping, the luminescence of the three-component charge transfer nanotubes can be changed. After doping, the blue part of naphthalene-TCNB gradually quenches and the orange light of pyrene-TCNB is enhanced. When the concentration of doped pyrene is at 0.015%, the charge transfer nanotubes emit intense white light. The charge transfer compound formed by the nitrogen atom of N, N, N', N'-tetramethyl-p-phenylenediamine (TMPD) and the cyano group of TCNB through the n-π action has paramagnetic properties and can be used as a magnetic material. In addition, the charge transfer complex TTF-TCNQ formed by tetrathiofulvalene (TTF) and 7,7,8,8-tetracyano-p-benzodiquinodimethane (TCNQ) has metal-like electrical conductivity properties and can be used as a conductive material.

Preparation:

The preparation methods of the charge transfer complex include solvent evaporation method, precipitation method, grinding method and vapor deposition method, etc.

References:

1. Tomasz J. (2019). "Charge Transfer, Complexes Formation and Furan Fragmentation Induced by Collisions with Low-Energy Helium Cations." Int. J. Mol. Sci 20, 6022-6042.
2. Hill Tania N. (2018), "Binary charge-transfer complexes using pyromellitic acid dianhydride featuring C-H⋯O hydrogen bonds.." Acta crystallographica 86, 1772-1777..
3. Sugata Samanta. (2017), "Unusual solvent effect of molecular charge transfer complexes: Stacking/non-stacking interaction revealed by characterization of structure and photophysical aspects." Journal of Luminescence 190, 403-412.