Visible Light-Induced Homolytic Cleavage of Perfluoroalkyl Iodides Mediated by Phosphines

In an effort to explain the experimentally observed variation of the photocatalytic activity of tBu3P, nBu3P and (MeO)3P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have explored the absorption characteristics of several phosphine- and phosphite-IC4F9 adducts by means of relativistic density functional theory and multireference configuration interaction methods. Based on the results of these computational and complementary experimental studies, we offer an explanation for the broad tailing of the absorption of tBu3P-IC4F9 and (MeO)3P-IC4F9 into the visible-light region. Larger coordinate displacements of the ground and excited singlet potential energy wells in nBu3P-IC4F9, in particular with regard to the P-I-C bending angle, reduce the Franck-Condon factors and thus the absorption probability compared to tBu3P-IC4F9. Spectroscopic and computational evaluation of conformationally flexible and locked phosphites suggests that the reactivity of (MeO)3P may be the result of oxygen lone-pair participation and concomitant broadening of absorption. The proposed mechanism for the phosphine-catalyzed homolytic C-I cleavage of perfluorobutane iodide involves S1 -So absorption of the adduct followed by intersystem crossing to the photochemically active T1 state.