Photoinduced electron transfer reactions by SmI2 in THF: luminescence quenching studies and mechanistic investigations

Photoluminescence quenching studies of SmI2 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 (DeltaG0) of the photoinduced electron transfer (PET) reactions was calculated from the redox potentials of the donor (SmI2) 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(et)) from Marcus theory and found to be in good agreement when a value of lambda = 167 kJ mol(-1) (40 kcal mol(-1)) was used for the reorganization energy of the system. A careful comparison of the excited state dynamics of SmII in the solid state to the results obtained in solution (THF) provides new insight in to the excited states of SmII in THF. The activation parameters determined for the PET reactions in SmI2/1-chlorobutane 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 SmI2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions.