Solid-state photochemical generation of stable radical pairs from bis(aryloxy)phosphine azide precursors

Two types of radical pairs, stable at room temperature, were found in monocrystals of 2,6-di-tert-butyl-4-methylphenol doped with bis(2,6-di-tert-butyl-4-methoxyphenoxy) phosphine azide after photoirradiation using a Xe are lamp. One pair consists of two 2,6-di-tert-butyl-4-methoxyphenoxyls (RP1), the other of 2,6-di-tert-butyl-4-methoxyphenoxyl and 2,6-di-tert-butyl-4-methylphenoxyl (RP2), as indicated by their EPR hyperfine splitting. In the temperature range of 120-200 K RP1 accumulates 7 times faster than at 77 K. Explanations for this photoaccumulation as well as a mechanism of RP formation are proposed.