DISTANCE DEPENDENCE OF PHOTOCHEMICAL ELECTRON-TRANSFER ACROSS PEPTIDE SPACERS

The distance dependence of the rate and activation parameters for photoinduced electron transfer (ET) across a series of L-proline peptide spacers has been determined in compounds 0-2. In 0-2, the dπ(Re) → π*(bpy) metal-to-ligand charge-transfer (MLCT) excited state is quenched by intramolecular ET from the aromatic amine. The rate for ET (ket) was determined by comparing the MLCT emission lifetimes of 0-2 with the lifetimes of appropriate model complexes. At 20°C ket is 9.8 (0.3) × 107, 5.3 (0.5) × 106, and 3.7 (2) × 105 s-1 for 0, 1, and 2, respectively. The activation parameters were derived from temperature dependence experiments over the range -7 to +40°C and for 0, 1, and 2 are respectively ΔH‡ = 3.3 (0.3), 4.0 (0.7), and 4.1 (1.8) kcal and ΔS‡ = -10.7 (1.5), -14 (2), and -19 (4) eu. The kinetic results are consistent with a nonadiabatic mechanism for ET in which the variation of ket with distance results mainly from a decrease in electronic coupling with increased donor-acceptor separation. © 1990 American Chemical Society.