Charge-transfer complexes radicals

The absorptions at both 500 nm and 320 nm follow first order kinetics with a lifetime of 420 ns. This absorption species is neither the excimer of polystyrene nor free cationic species of polystyrene. Although the excimer of polystyrene has an absorption band around 500 nm, the lifetime is only 20 ns. Further the free cationic species of polystyrene should live for a longer time in this solution, and the absorption band should exist in a longer wavelength region (6). These considerations of lifetime and absorption spectrum lead us to conclude that the absorption spectrum shown in Figure 12 is due to the charge transfer-radical complex between polystyrene and Cl radical (2,4,17). A very similar... 

The transient absorption spectrum obtained in the pulse radiolysis of polystyrene solution in CC1 is shown in Figure 13. The spectrum is very similar to the charge transfer radical complex (PS4+C14-) species. The lifetime is about 200 ns. Consideration of the absorption spectrum and the lifetime suggest that this species is (PS4+C14-)-. The processes leading to formation of this species in liquid CC14 can be written as follows (4,7). 

The charge transfer radical complex is considered to be the main precursor of the polystyryl radical. 

Reaction Scheme of CMS Resists. The transient absorption spectrum shown in Figure 6 and observed for irradiated CMS films is mainly composed of two components as based on pulse radiolysis data of solid films of CMS and polystyrene, and CMS and polystyrene solutions in cyclohexane, chloroform, and carbon tetrachloride. An absorption with a maxima at 320 nm and 500 nm as due to the charge transfer radical-complex of the phenyl ring of CMS and chlorine atom (see Figure 14) and an absorption with maxima at 312 and 324 nm is due to benzyl type radicals (see Figure 11). 

Figure 14. The structure of the charge-transfer radical complex.

Radicals and Charge-Transfer Radical Complexes of Polystyrene... 

In carbon tetrachloride poly(a-methylstyrene) were degraded, even without oxygen, by the irradiation of ionizing radiation [58], The decay of the charge transfer radical complex, observed in this solvent, may be due to the reaction of a chlorine atom with p-site H of poly(a-methylstyrene) the reaction leads to the formation of P-position radicals of poly(a-methylstyrene). The produced polymer radicals were unstable and dissociated into neutral and radical species. 

Scheme 10.5 Tentative mechanism for cytochrome P450-cata-lyzed epoxidation of a double bond. The reactive iron-oxo species VII (see Scheme 10.4) reacts with the olefin to give a charge transfer (CT) complex. This complex then resolves into the epoxide either through a radical or through a cationic intermediate.

Photochemical ET reactions can be classified in at least three categories (which can co-exist), namely (i) simple homolysis of bonds of neutral molecules to give radicals of low redox reactivity (ii) excitation of a species D to produce an excited state D which initiates a second-order ET reaction involving another component of acceptor type, A, with formation of the radical pair D + A (iii) direct excitation of a charge transfer (CT) complex formed between two reaction components D and A to form the same radical pair D + A -. The first case is obviously an ideal situation if it can be realized, but this is seldom the case. The incursion or predominance of situations (ii) and/or (iii) in almost any system is possible, and precautions must be taken to avoid these complications. Much can be done by controlling the wavelength of the light source, but it is also possible to affect the chemistry in a predictable manner. 

It should also be mentioned that cases are possible when polarity of the solvent allows transforming charge-transfer intramolecular complexes into molecules containing the cation- and anion-radical... 

If the charge transfer (CT) complex is sufficiently strong, the ion-radical pair would dissociate to induce ionic and/or radical reactions. The mechanism of this photoexcitation is different from the n — n or n — n excitations. The later process is the excitation of isolated molecules whereas the CT excitation requires two molecules in contact. Surprisingly, rather limited attention has been directed to this field of photosensitized CT process from the viewpoint of organic reactions. 

The absorption IR spectra of the organic conductors, of both ion-radical salts and charge-transfer (CT) complexes, created by a given electron acceptor with various donors, share plenty of characteristic features. In addition to rather narrow and weak bands characteristic of the donor D and acceptor A molecules, a few novel absorption bands appear. They are polarized in the plane perpendicular to that of D and A molecules, broad and very intensive. The presence of such unusually polarized bands can be accounted for by the activation of totally symmetric donor or acceptor vibrations resulting from e-mv coupling. Typical polarized reflection spectra of the triethylammonium (TEA) (TCNQ)2 salt for three light polarizations [18] are shown in Fig. 1. It is fascinating that the reflectivity for... 

It is well known that cyano derivatives of anthracene form charge transfer (CT) complexes with certain aromatic compounds. It was reported [67] that the radical cations formed upon irradiation of these complexes played an important role in initiation of cationic polymerization of cyclic ethers. Pyridinium salts were also found [68] to form CT complexes with hexamethyl benzene and trimethoxy benzene which result in the formation of a new absorption band at longer wavelengths where both donor and acceptor molecules have no absorption. This way the light sensitivity of the pyridinium salts may be extended towards the visible range. According to the results obtained from the... 

In the above radical-cation salts, the crystal contains partially oxidized donors, while the electroneutrality is achieved by the presence of closed shell anions. The structural requirements necessary for electrical conductivity in solid salts can also be met upon mixing of donors and acceptors in the resulting charge-transfer (CT) complexes both the donor and acceptor exist in a partially oxidized and reduced state, respectively. Famous examples are the conducting CT complexes formed upon mixing of perylene (112) [323. 324] and iodine or of tetrathiafulvalene (TTF, 119) as donor and 7,7,8,8-tetracyanoquinodimethane (TCNQ, 120) as acceptor [325-327] the crucial structural finding for the... 

Nu, attacks the dication formed by the disproportionation of the cation radical A, + and the half-regeneration mechanism, equations 8 and 9, in which nucleophilic attack takes place directly on the radical cation. A third mechanism, termed the complexation mechanism, is closely related to the disproportionation mechanism but differs from it in that one of the reacting radical cation molecules of equation 6 is complexed to a molecule of the nucleophile in a donor-acceptor charge-transfer tt complex. The role of the nucleophile donor is to facilitate electron transfer to the second radical cation group. The disproportionation step of the complexation mechanism is indicated in equation 10. 

A long-term, comprehensive investigation of photochemical nitrations with tetranitromethane has produced several new papers in the period under review. The basic mechanism of the reactions involves electron transfer within a charge transfer (CT) complex between the aromatic substrate (ArH) and tetranitromethane, resulting in formation of a radical cation (ArH ), NO2 and the tiinitromethanide anion. 

Such a puzzling SET has been accounted for by assuming the formation of a transient p Tt eharge-transfer complex between the perchlorinated radical and the hydroxide ion (Ballester and Pascual, 1985). This is a reasonable assumption, since some perchloro aromatie compounds are known to act as acceptors, giving charge-transfer molecular complexes (p. 351). 

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