Photoinduced Electron Transfer Reactions by SmI_2 in THF:Luminescence Quenching Studies and Mechanistic Investigations

摘要

Photoluminescence quenching studies of SmI_2 in dry THF were carried out in the presence of five different classes of compounds:ketone,alkyl chloride,nitrile,alkene and imine.The free energy change(AGdeg)of the photoinduced electron transfer(PET)reactions was calculated from the redox potentials of the donor(SmI_2)and acceptors.The bimolecular quenching constants(k_q)derived from the Stern-Volmer experiments parallel the free energy changes of the PET processes.The observed quenching constants were compared with the theoretically derived electron transfer rate constants(k_a)from Marcus theory and found to be in good agreement when a value of lambda=167 kJmol~(-1)(40 kcalmoP1)was used for the reorganization energy of the system.A careful comparison of the excited state dynamics of Sm~II in the solid state to the results obtained in solution(THF)provides new insight in to the excited states of Sm" in THF.The activation parameters determined for the PET reactions in SmI_2/l-chloro-butane system are consistent with a less ordered transition state and high degree of bond reorganization in the activated complex compared to similar ground state reactions.Irradiation studies clearly show that SmI_2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions.