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In an effort to explain the experimentally observed variation of the photocatalytic activity of <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mi>t</mi> </msup> </semantics> </math> </inline-formula>Bu<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P, <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mi>n</mi> </msup> </semantics> </math> </inline-formula>Bu<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P and (MeO)<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have explored the absorption characteristics of several phosphine– and phosphite–IC<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula>F<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>9</mn> </msub> </semantics> </math> </inline-formula> 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 <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mi>t</mi> </msup> </semantics> </math> </inline-formula>Bu<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P-IC<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula>F<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>9</mn> </msub> </semantics> </math> </inline-formula> and (MeO)<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P-IC<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula>F<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>9</mn> </msub> </semantics> </math> </inline-formula> into the visible-light region. Larger coordinate displacements of the ground and excited singlet potential energy wells in <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mi>n</mi> </msup> </semantics> </math> </inline-formula>Bu<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P-IC<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula>F<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>9</mn> </msub> </semantics> </math> </inline-formula>, in particular with regard to the P–I–C bending angle, reduce the Franck–Condon factors and thus the absorption probability compared to <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mi>t</mi> </msup> </semantics> </math> </inline-formula>Bu<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P-IC<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula>F<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>9</mn> </msub> </semantics> </math> </inline-formula>. Spectroscopic and computational evaluation of conformationally flexible and locked phosphites suggests that the reactivity of (MeO)<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>P 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 ← S₀ absorption of the adduct followed by intersystem crossing to the photochemically active T<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>1</mn> </msub> </semantics> </math> </inline-formula> state.