7i-radical cation

EPR data for the canthaxanthin cation radical (46), more recently generated in organic matrices, has been published [123-125]. A symmetrical unresolved EPR line at 9 GHz was assigned to a 7i-radical cation with electron density throughout the polyene chain [124],... 

In this light, a simple addition, rather than a redox reaction, was expected to dominate the reaction of [60]fullerene with the above mentioned radiolytically generated carbon- and heteroatomic-centered radicals (12-18,21,22). Spectrophotometric evidence for the postulated adduct formation stems from the broad 900 nm absorption band (Figure 1), substantially different from those of the singly reduced or oxidized fullerene, i.e. the 7i-radical anion (1080 nm) and 7i-radical cation (960 / 980 nm), respectively (12,13). 

The porphyrin 7i-radical cation and dication obtained after oxidation are species having a strong electrophilic character. So, the reaction of such electrophilic oxidized porphyrin species with nucleophilic compounds has been used to perform grafting onto porphyrin macrocycles. 

To illustrate the latter point, consider the butadiene radical cation (BD+ ). On the basis of Hiickel theory (or any single-determinant Hartree-Fock model) one would expect this cation to show two closely spaced absorption bands of very similar intensity, due to 7i i -> ji2 and ji2 —> JI3 excitation (denoted by subscripts a and v in Figure 28), which are associated with transition moments /xa and /xv of similar magnitude and orientation. Using the approximation fiwm) —3 eV288 the expected spacing amounts to about 0.7 eV. 

In the case of halocarbon matrices, the SOMO is of the a type and the cleavage is facilitated by the antibonding nature of the SOMO. In other ions, a or a orbitals of scissile bonds interact with a ti or 7i orbital, causing them to be weakened. Accordingly, radical anions of benzyl halides may generate benzylic radicals with loss of a halide ion. Conversely, benzylsilane radical cations may form benzylic radicals with loss of a silyl cation (Fig. 6.15). 

However, ab initio calculations [QCISD-(T)/6-31G //UMP2/6-31G ] on ethylene and its radical cation support an anti -7i-complex, in which the two components are joined by one long bond (190 pm), rather than the sandwich -type 71 complex. The complex is connected to two transition states leading to a (rhombic) cyclobutane radical cation (see above) or, by 1,3-H-shift, to 1-butene radical... 

In summary, we emphasize that homoaromatic stabilization appears to be more important for radical cations than for their neutral diamagnetic precursors. There are several reasons why radical cations may assume cyclic conjugated structures with 4n + 17i-electrons, one electron shy of the magic 4n + 2 7i-electrons which achieve aromaticity, when the parent molecules opt for a less delocalized nonaromatic structure. The principal cause might lie in the strength of the carbon-carbon single bond, which generally disfavors biradical or zwitterionic... 

For more than 30 years di-7i-methane rearrangements have stood as the paradigm reactions that occur in the excited state manifold exclusively. However, recent studies have shown that rearrangements of the di-7r-methane type can also occur in ground state of radical-cation and radical-anion... 

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. 

Figure 9. Schematic representation of (7- and 7i-SOMOs considered for isobutane radical cation.

Further investigation by the Adam group [140] showed electronic substituent effects on the diyl sites profoundly influences the regioselectivities of the 1,2 shift. The data obtained illustrate the distinct electronic character of the cationic intermediates involved in the electron transfer oxidation vs acid catalysis of the housanes. The radical anions and cations of dipleiadiene 64 and its 12b,12c-homo derivatives 65 are characterized by EPR and ENDOR spectroscopy [141]. The 7i-spin distribution over the perimeter is similar in the radical cations 64 +, 65, and an analogous statement holds for the corresponding radical anions. 

Generation of radicals by oxidation involves removing one electron from an anionic species or from a neutral electron-rich substance such as an enol. An electron is taken away from the 7i-orbital (the HOMO) to give a radical cation, which then loses a proton to give a free radical. As for producing radicals by reduction, addition of an electron to the antibonding orbital of R X gives a radical anion, which rapidly collapses into a free radical R and an anion X (Scheme 8.2). 

It is widely known that tetrathiafulvalene (15) - commonly known as TTF -can be easily oxidized to its radical cation and then on to its dicationic state. Previous research [36] from our laboratories had shown that reduction of the tetracationic cyclophane CBPQT4+ reduces enormously its binding affinity for 7i-electron-rich guests. In the light of this knowledge, it was considered possible to develop a pseudorotaxane-based switch in which the switching process is not only controlled and monitored electrochemically, but in which it can be accomplished [37] in one of two ways. 

---