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The convenient unimolecular generation of phenoxyl radicals and phenoxy-phenoxy radical pairs: Studies of and applications to organic ferromagnets
Abstract
Known methods for production of phenoxyl radicals involve either bimolecular oxidations or photolysis of parent phenols. We have described the synthesis and characterization of three photochemical and thermal unimolecular leaving groups. These leaving groups are: aryloxy oxalyl tert-butylperoxides (AOB's), aryloxy oxalyl chlorides (AOC's), and diaryl oxalates (DAO's), of which the AOB and DAO are the most efficient and stable. The quantum efficiency of a typical AOB system is 0.42, while that of an analogous DAO system is 0.10. A photolysis time of $\sim$3.5 h yields $\sim$35% of radicals using the AOB method. Using these leaving groups, phenoxyl radicals can be obtained in good yield in solution and in the solid state. We have attempted to apply these leaving groups toward production of conjugated, non-Kekule phenoxy-based biradicals. Our results indicate production of a large amount of monoradical, which probably obscures any biradicals produced. In addition to producing radicals, photolysis of diaryl oxalates in the solid state yields up to three types of triplet phenoxy-phenoxy radical pairs, which we have termed the initial, wide, and narrow radical pairs. The initial radical pair has been observed only from photolysis of di(2,6-di-tert-butyl-4-methoxyphenyl) oxalate, and has $\vert$D/hc$\vert$ = 0.0108 cm$\sp{-1}$, with a corresponding interelectronic separation of 6.2A. We have observed the formation of the wide radical pair primarily through the use of short photolysis times; thermal annealing of the initial radical pair also leads to its observation in the case described above. Wide radical pairs are observed from photolysis of di(2,6-di-tert-butyl-4-methoxyphenyl) oxalate, 2,6-di-tert-butylphenyl phenyl oxalate, and di(2,4,6-tri-tert-butylphenyl) oxalate. These radical pairs have $\vert$D/hc$\vert$ = 0.0099 to 0.0124 cm$\sp{-1}$, corresponding to interelectronic separations of 6.0-6.4A. In the absence of oxygen, the wide radical pairs are stable at room temperature for from 2 h to $>$1 week. Narrow radical pairs are formed upon extended photolysis of all DAO's, and exhibit $\vert$D/hc$\vert$ = 0.0034 to 0.0070 cm$\sp{-1}$, corresponding to interelectronic distances of 7.2-9.2A.
Subject Area
Organic chemistry|Physical chemistry|Chemistry
Recommended Citation
Modarelli, David Anthony, "The convenient unimolecular generation of phenoxyl radicals and phenoxy-phenoxy radical pairs: Studies of and applications to organic ferromagnets" (1991). Doctoral Dissertations Available from Proquest. AAI9132886.
https://scholarworks.umass.edu/dissertations/AAI9132886