Color charge transfer systems

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

Homobenzvalene (HB) is an electron-rich donor (IP = 8.02 eV) owing to the presence of a strained ring system, and thus readily forms a charge-transfer complex with TCNE. Charge-transfer irradiation of the [HB, TCNE] complex leads to rapid bleaching of the yellow color, and the formation of a mixture of isomeric cycloadducts208 (equation 73). 

When located at opposite ends (or at conjugated positions) in a molecular system, a donor and an acceptor do more than simply add up their separate effects. A cooperative phenomenon shows up, involving the entire disubstituted molecule, known as charge transfer (C.T.). Such compounds are colored (from pale yellow to red, absorption from 3,000 to 5,000 A) and show high U.V. absorption oscillator strength. "Figure 2 helps understand the enhancement of optical nonlinearity in such a system. 

Very interesting method of template polymerization was proposed by Japanese scientists. The method is based on the charge transfer interaction between template and monomer. In the course of the studies on the interaction of poly(maleic anhydride) with organic amines, the authors found strong charge transfer interaction of pyridines with poly(maleic anhydride). The polymer with pyridine gives brown-colored system with the absorption maximum at 480 nm. 

For the spherically symmetric Cu d" ion, the common geometries are two-coordinate linear, three-coordinate trigonal planar, and four-coordinate tetrahedral. Some distortions from these ideal geometries are observed, particularly with chelating ligands a fairly small number of pentacoordinate Cu complexes have been isolated and characterized as well. Cu compounds are diamagnetic and colorless, except where the color results from charge-transfer bands or a counterion. Cu complexes are often fairly readily oxidized to Cu compounds the electron-transfer kinetics of several systems have been studied. ... 

In EDA complexes, there is always a donor and an acceptor molecule. The donor may donate an unshared pair (an n donor) or a pair of electrons in a ti orbital of a double bond or aromatic system (a n donor). One test for the presence of an EDA complex is the electronic spectrum. These complexes generally exhibit a spectrum (called a charge-transfer spectrum) that is not the same as the sum of the spectra of the two individual molecules. Because the first excited state of the complex is relatively close in energy to the ground state, there is usually a peak in the visible or near-uv region and EDA complexes are often colored. Many EDA complexes are unstable and exist only in solutions in equilibrium with their components, but others are stable solids. In most EDA complexes the donor and acceptor molecules are present in an integral ratio, most often 1 1, but complexes with nonintegral ratios are also known. There are several types of acceptor molecules we will discuss complexes formed by two of them. 

Nespurek et al.51 53 reported the mechanism of the photochromism of sydnone, a mesoionic compound. Since this system exhibits photochromism in the solid state, the origin of the colored form was assumed to be the formation of a colored center or intermolecular charge-transfer state. In contrast to this view, the combined evidence from experiments and MO calculations led Nespurek et al. to conclude that the nature of the photochromism was intramolecular. 

Perfluoronapthalene appears to form a charge transfer complex with cobaltocene but no further C-F activation is detected [74]. The crystal structure of a ruby-colored ferrocene-perfluorophenanthrene molecular complex has recently been reported [75]. The presence of only one tertiary C-F bond is required for reactivity in this system as illustrated by the synthesis of perfluorotoluene from perfluoromethylcyclohexane under similar conditions. 

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