Norrish Type I Reaction of Diphenyltetrahydrobenzocycloheptenones. Conformational Effects on Biradical Lifetime

Abstract

Isomeric 6,6-diphenyl-6,7,8,9-tetrahydro-5H-benzocyclohepten-7-one and 7,7-diphenyl-6,7,8,9-tetrahydro-5H-benzocyclohepten-6-one (2) were photolyzed in methanol to give rise to unsaturated aldehydes as well as diphenyltetrahydronaphthalene, a decarbonylation product (for 2). Lifetimes of the intermediate triplet biradicals were dependent on their structures, indicating that cyclic conformations play an important role in the intersystem crossing of biradicals.

Laser Flash Photolysis of [3,n]Paracyclophan-2-ones. Direct Observation and Chemical Behavior of 4,4‘-(1,n-Alkanediyl)bisbenzyl Biradicals

“…Biradicals have been postulated as reactive intermediates in numerous thermal and photochemical reactions. − Their direct detection by time-resolved techniques has allowed the study of their spectral properties and chemical behavior. The lifetimes of triplet biradicals are usually controlled by intersystem crossing (ISC), which is sensitive to the different biradical conformations. − While ISC can occur by two major pathways, spin−orbit coupling (SOC) and electron−nuclear hyperfine coupling (HFC), the latter is only expected to be important when the triplet−singlet energy gap is very small, as for example, when there is large separation between the two radical centers.…”

“…The lifetimes of triplet biradicals are usually controlled by intersystem crossing (ISC), which is sensitive to the different biradical conformations. [4][5][6][7][8][9] While ISC can occur by two major pathways, spin-orbit coupling (SOC) and electron-nuclear hyperfine coupling (HFC), the latter is only expected to be important when the triplet-singlet energy gap is very small, as for example, when there is large separation between the two radical centers.…”