Photochemically induced isotopic exchange between iodobenzene and molecular iodine

Abstract

The photochemical isotopic exchange between iodobenzene and radioactive molecular iodine (ImI) was studied in methylcyclohexane solutions. The limiting exchange yields, <#>EX°, measured at several excitation wavelengths in the uv range were found to be identical with the corresponding deiodination limiting yields, <£de°• This observation and a study of the effects of dissolved oxygen and iodine concentrations suggest that the exchange is initiated by photocleavage of iodobenzene to phenyl (Ph•) and iodine radicals, with 02 and I2 competing in the scavenging of Ph-(fci!+ph./fco,+ph.= 2.1 ± 0.5). The photocleavage of iodobenzene was found to be wavelength dependent in the near-uv range {e.g., 334 and 365 nm), where part of the absorption leads directly to the iodobenzene triplet. It is suggested that dissociation takes place from the lowest excited singlet, but practically not from the triplet state. It is shown that iodine atoms, produced by selective excitation in the visible range, do not exchange with iodobenzene. Neither does excitation of the charge-transfer (CT) complex between iodobenzene and iodine lead to exchange or deiodination.

Enhanced Intersystem Crossing in a Thiohelicene

“…Apparently, excitation of 6I into the S 1 $${\leftarrow }$$ S 0 absorption band leads to homolytic cleavage of the iodine atom at C 6 with high quantum yield [60–63] . The resulting radicals in solution are stabilized by hydrogen abstraction from the solvent, while the iodine atoms combine to molecular iodine.…”

“Click”‐Like η6‐Metalation/Demetalation of Aryl Iodides as a Means of Turning “ON/OFF” Halogen Bond Donor Functionality

“…We foresaw challenges, however, with photochemical demetalation of [CpRu] + ‐functionalized aryl iodides using the traditional broad‐band UV irradiation procedure [31] due to the photolabile C−I bond. For example, C 6 H 5 I absorbs at wavelengths below 365 nm ( ϵ 365 ≈1 M −1 cm −1 ), inducing C−I bond homolysis [32] . So, it was not surprising to find that irradiation of [ RuArI ][PF 6 ] in MeCN with a medium pressure Hg lamp resulted in significant decomposition in addition to the desired [CpRu(MeCN) 3 ][PF 6 ] and C 6 H 5 I (Supporting Information 1.7.6, S71).…”

“Click”‐Like η6‐Metalation/Demetalation of Aryl Iodides as a Means of Turning “ON/OFF” Halogen Bond Donor Functionality

“…For example, C 6 H 5 I absorbs at wavelengths below 365 nm (ɛ 365 � 1 M À 1 cm À 1 ), inducing CÀ I bond homolysis. [32] So, it was not surprising to find that irradiation of [RuArI][PF 6 ] in MeCN with a medium pressure Hg lamp resulted in significant decomposition in addition to the desired [CpRu(MeCN) 3 ][PF 6 ] and C 6 H 5 I (Supporting Information 1.7.6, S71). Fortunately, [CpRu II -(arene)] + compounds exhibit a low energy singlet to triplet absorption (λ max = 370-380 nm, ɛ � 10 M À 1 cm À 1 ) that is also active for release of the [CpRu] + moiety using the filtered 405 nm H-line Hg output.…”