Radical-pair formation

Carbonera D, DiValentin M, Corva]a C, Agostini G, Giacometti G, Liddell P A, Kuciauskas D, Moore A L, Moore T A and Gust D 1998 EPR investigation of photoinduced radical pair formation and decay to a triplet state in a carotene-porphyrin-fullerene triad J. Am. Chem. Soc. 120 4398-405... 

Fig. 63. Molecular arrangement in (a, c) plane of a mixed ethylene-chlorine binary crystal illustrating (a) radical pair formation, (b) single chain growth and (c) chain growth in the vicinity of product line. Molecules labelled 1-4 are ethylene (C2H4), chlorine, chloroethyl radical (C2H4CI) and anti 1,2-dichloroethane (C2H4CI2), respectively.

In order to establish the generality of ion-radical pair formation by the charge-transfer activation of EDA complexes, let us focus on a few diverse... 

Some other covalently bound porphyrin-acceptor complexes such as por-phyrin-viologen [68-73] and pyromellitimide-bridged porphyrins [74, 75] have been synthesized and studied. As in the case of P-Q complexes, strong fluorescence quenching and ion radical pair formation were observed in these systems under irradiation of complexes in porphyrin absorption bands. 

Murata et al. described studies of 1 1 mokmol solution mixtures of Ur6IN with an aminoxyl-functionalized complement, DAPN.181 These did not show solution-phase ESR signals attributable to radical pair formation. Given the... 

In the PET reactions of ketone-amine pairs, proton transfer from the amine cation radicals to the ketyl radical anions is exceptionally efficient and radical pair formation (thus coupling or reduction) dominates other possible reaction modes (Eqs. 33, 34). In general coupling of the a-amino radical formed by proton transfer from initially formed amine radical cation ketyl radical anion terminates the reaction process. Early examples of ketone-amine photoreactions were reported by Cohen [170] and have been updated recently [10, 11]. Detailed mechanistic studies of these reactions have been discussed by Peters [171]. 

Free-radical activated complexes are formed by the creation of unpaired electrons during homolytic bond cleavage. Free-radical transition-state reactions with small or negligible solvent effects are found among radical-pair formation and atom-transfer reactions such as ... 

According to Kosower, a third category of reactions involving free-radical activated complexes may be defined. These complexes are formed either through the creation of unpaired electrons during radical pair formation or atom-transfer reactions [15, 468] cf. 

They were interpreted by assuming the formation of a or-complex only. On the basis of Colonna s work with 139 (X=SCH3) it is obvious that radical pair formation is observable only if the nucleophilic substrate has an extremely low oxidation potential. 

Chelate coordination-assisted SET reactivity as described in Eq. (12) may be invoked in some remarkable insertion reactions of alkylaluminium porphyrins [139] and are not restricted to the main group organometallics. Photoreactions of low-spin d systems with a filled fag sub-shell such as Co(III) [140] or Pt(IV) [141] with chelating 7r-acceptor ligands also involve CT absorptions and apparent radical pair formation (spin-trapping, CIDNP [141]) before typical followup reactions such as C-alkylation [38,95,140] occur. 

A normal emission/absorption/emission/absorption pattern that did not vary in the time interval 0.7-5 ps was observed for the radical pairs from 35 and 36, while the spectrum from 34 was initially totally absorptive (0.4-0.6ps), then rapidly changed to emissive/absorptive (1.9-2.1 ps), and eventually became totally emissive (4.6-5 ps).37 It is suggested that for ZP4V, due to the smaller spacer chain, the time elapsed between laser excitation and radical-pair formation was shorter than for ZP6V and ZP8V and the spin polarization in the porphyrin triplet was retained to a larger extent before electron transfer took place.37... 

The observed photooxidation yield for the mechanism outlined in Eq. (44) is given by the expression in Eq. (45), where >0 represents the primary quantum yield for radical pair formation/bond cleavage. The parameters 0 and k2 are expected to be independent of viscosity, whereas k4 is expected to decrease with increasing viscosity. 

Indirect evidence for radical pair formation in polymers has been obtained from the step-like character of radical decay at low temperature (section 4.2.1). 

RADICAL-PAIR FORMATION FROM HEXAARYLBIIMIDAZOLYL DERIVATIVES... 

Figure 5-5. Radical pair formation from 2,2 -di(o-chlorophenyl)-4,4 5,5 - tetraphenylbiimidazole...

The processes generating the spin polarizations are t)tpically completed within a few nanoseconds after the birth of the radical pairs. While this feature is lost when radical-pair generation is continuous, as in thermal reactions or photoreactions brought about by cw illumination, radical-pair formation can easily be made fast on that timescale when a pulsed laser is used as the light source. 

The present ESR studies using the selectively D-labeled methanes, CD4 and CH2D2, unambiguously revealed that the hydrogen atom and the methyl radical present in pairs originate from the same methane molecule, suggesting that the radical pair formation is caused by the homolytic C— H bond scission via the electronically excited methane molecule (CHp. The H-atoms initially formed may... 

A high probability for finding two radicals close together would also be expected in regions of high ionization density. Evidence for this effect being operative has been found from studies of the LET effect on radical pair formation in eicosane. ... 

EXPERIMENTS WITH REACTION cENTKk rnMPTFyRS. In the case of isolated reaction centers, the probability of radical pair formation is close to... 

A net Coulombic repulsion between and the radical pair could result in an upward energy shift of the radical pair state compared to open reaction centres. This would result in a lower yield of radical pair formation (3), If Q. is double reduced and protonated ("jammed" reaction centre), the free energy of the radical pair state would resemble that in open centres. Consequently, the yield of radical pair formation and triplet would increase upon double reducing Q. ... 

---