Radical cations solution characteristics

ESR data obtained on y-irradiation of tetraalkylphosphonium salts in solution (96% sulphuric acid) at 77 K differ markedly from those for the trialkylphosphonium ions44. The latter irradiation produces the expected phosphabetaine radical cation and, for example, tetrabutylphosphonium iodide has features assigned previously to I2 45. On the other hand, PH was not detected44,46. Trialkylphosphonium salts, such as trimethyl and triethyl compounds, produce radicals whose ESR spectra are characteristic of the radical cations of R3P +. The central region of the ESR spectra is dominated by features from... 

In contrast to highly stable and prolific fullerene anionic species, fullerene cations are rare. The first fullerene cation was prepared in 1996 by Reed and co-workers500 by single-electron oxidation of C76 to form radical cation C76 + isolated in solid form as the CBnH6Br6 salt [Eq. (3.56)]. The cation was identified in solution by a characteristic visible-near-infrared absorption (Amax = 780 nm), FT-IR and EPR spectroscopy. C60 + was generated in an analogous way later.501 Reed et al.501 also succeded in... 

Immediately upon excitation of an IPCT band with a fs laser at 400 nm, transient absorption was observed for both salts in solutions with a peak at about 600 nm, characteristic of 4,4/-bipyridinium radical cations. Figure 20 shows the transient absorption spectra of PV2+(I )2 in methanol solution. A marked increase in the absorbance of the 4,4/-bipyridinium radical cations took place within 1 ps after excitation. 4,4/-Bipyridinium radical cations were thus formed in a fs time scale by the photoinduced electron transfer from a donor I- to an acceptor 4,4/-bipyridinium upon IPCT excitation [48], The time profiles of transient absorption at 600 nm are shown in Fig. 21 for (a) PV2+(I )2 in a film cast from DME and (b) PV2+(TFPB )2 in DME solutions. Both of them showed a very rapid rise in about 0.3 ps, which was almost the same as the time resolution of our fs Ti sapphire laser measurement system with a regenerative amplifier. Similar extremely rapid formation of 4,4/-bipyridinium radical cations was observed for PV2+(I )2 salts in methanol and dimethylsulfoxide solutions upon IPCT excitation, respectively. These results demonstrated that the charge separated 4,4/-bipyridinium radical cations were formed directly upon IPCT excitation because of the nature of IPCT absorption bands (that the electrons correlated with the IPCT band are transferred partially at the ground state and completely at the excited state). Such a situation is very different from usual photochromism which is caused by various changes of chemical bonds mainly via the excited singlet state. No transient absorption was observed for PV2+(I )2... 

Consistently, the PIA spectra of toluene solutions containing MP-Ceo and OPVn (n = 2, 3 or 4) in a 1 1 molar ratio, recorded using selective photoexcitation of MP C60 at 528 nm (Fig. 1.28b), invariably exhibit an absorption at 1.78 eV with an associated shoulder at 1.54 eV, characteristic of MP-C6o(7i) [103]. The monomolecular decay (—AT oc Ip, p = 0.89-0.96) with lifetime 150-260 ps associated with these PIA bands supports this assignment. Furthermore, weak fullerene fluorescence at 1.73 eV (715 nm) is observed under these conditions for all three mixtures. No characteristic PIA bands of OP Vw+ radical cations or MP-Cg0 radical anions are discernible under these conditions. From these observations we conclude that electron transfer from the ground state of the OPVn molecules to the singlet or triplet excited state of MP-Cgo does not occur in toluene solution. 

The blue-colored V 2+ -FeCl 4 complex decomposed in water or in methanol, and the solution became colorless, whereas a CT band appeared at 450nm in a mixture of ethanol and methanol (1 4), the absorption of which was not affected by near-UV irradiation. In contrast, a pale purple-colored aqueous solution of V2+ with K4 [Fe(CN) 6 ] 4 having a low redox potential turned an intense violet upon near-UV irradiation, and there was no absorption peak around 398nm, which is characteristic of a radical cation, as shown in Figure 9.8. 

Continuum resonance-Raman scattering can be observed under discrete resonance-Raman scattering conditions only if the resonance fluorescence is quenched, either with an inert gas, or (in the case of condensed phase studies) by the solvent or matrix. Thus, on excitation of a liquid, solution, or solid within the contour of an absorption band, the Raman spectrum observed has the characteristics of the continuum rather than the discrete case or, in other terminology, of resonance Raman, rather than resonance fluorescence spectra. Such spectra provide unique information on the spectroscopic properties of radical cations and ions, some of which species are unstable in air. Particularly noteworthy have been the studies by Andrews et al. (65) which have... 

UV-Vis spectroscopy of the [2]catenane 214+ revealed - by virtue of the presence of a characteristic broad charge-transfer band with a maximum absorbance at 835 nm and the absence of a peak at 515 nm - that the sole co-conformer in a room temperature acetonitrile solution is the one in which the TTF unit resides inside the cavity of the tetracationic cyclophane. Upon either chemical or electrochemical oxidation of the TTF unit to its radical cationic (or dicationic) state, the Coulombic repulsion between the cyclophane and the positively charged TTF unit leads to its expulsion from the cavity of the cyclophane. As the 1,5-dioxynaphthalene site is left untouched by the TTF oxidation and has an intermediate affinity for residing within the cyclophane,... 

Polymer processing can be of several types, including free radical, cationic, anionic, metal complex, or metal oxide catalyzed, as mentioned earlier [5], Polymers can be made by bulk polymerization, solution polymerization, suspension polymerization, or emulsion polymerization techniques [5], The automotive chemist or design engineer working for an OEM should be aware of these various manufacturing processes, which polymers are made by which process, and what characteristics can be expected from the type of process. 

Kricka and Ledwith146 have reviewed the photochemical synthesis of cyclobutanes by dimerization of mono-olefins. Routes which involve radical cations, species of recent interest, are discussed. Yamamoto et a/.147 have shown that p-methoxystyrene undergoes photodimerization in acetonitrile solution with jp-cyanobenzene as an electron acceptor. These conditions (Pyrex filter) yield the f/wis-head-to-head-dimer [(226a), 13%]. Dimerization does not occur in non-polar solvents. A study of the emission characteristics of the styrene-cyanobenzene system shows that the fluorescence of the styrene is quenched and a broad exciplex emission is observed at 440 nm. The likely path for dimerization involves electron transfer as shown in Scheme 22. p-7V7V-Dimethylaminostyrene... 

Although the S3N2+ radical cation (13) has been well characterized in solution by its characteristic five-line (1 2 3 2 1) ESR signal (g = 2.01 aw = 3.15G), this S-N heterocycle is usually found in the solid state as the dimeric cation SeN4 + (7), in which two 7 7r-electron S3N2+ rings are associated via weak intermolecular S-S interactions [d S-S) = 300-310 pm]. 

As noted above, most kinetic studies of alkene radical cations in solution have focused on aryl-substituted systems since they have convenient optical propenies and have been extensively studied by other techniques. Radical cations are frequently identified on the basis of their characteristic UV-visible absorptions and the comparison of their spectra to those obtained in matrices at low temperature." However, a number of other diagnostic tests are also commonly employed to identify these intermediates. For example, their kinetic behavior as a function of solvent nucleophilicity or added nucleophiles is analogous to that of other electrophilic species. Thus, reaction with nucleophiles such as azide and halide ions provides support for the assignment of a transient to a radical cation, although it will not serve to eliminate a carbocation intermediate. More useful in the latter respect is the method of generation of the transient since PET does not in general lead to the formation of carbocations. Quenching of the observed transient with a more easily oxidized... 

Pruett was the first to observe that the action of a halogen on a peraminoethylene (1) yielded a salt-like dihalide. Carrying out similar experiments with tetrakis(aralkylamino) ethylene (5), Kawano characterized the dication and identified the oxidation-state intermediate between ethylene and dication. When such oxidizing agents as iodine or silver salts were added to acetonitrile solutions of 5 the intense violet color characteristic of radical-cation 90 (A 517 m x. 

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