Photoinduced electron transfer from Eu(II)-complexes to organic molecules: Rate and mechanistic investigation

Photoinduced electron transfer (PET) from europium(II)-complexes to variety of organic electron acceptors has been investigated in tetrahydrofuran (THF), 1,2-dimethoxyethane (DME) and acetonitrile (ACN). Laser flash photolysis (LFP) study indicates the formation of radical cation of the donor {Eu (II)} and radical/radical anion of corresponding acceptors, confirming the photoinduced electron transfer. Time resolved luminescence quenching experiments showed that rate constants of forward electron transfer are in the range of 10 ⁸-10⁹ M^-1 s^-1. Furthermore, experimentally obtained rate constants are in good agreement with calculated electron transfer rate constants using Marcus equation with a reorganization energy (λ) of 45 kcal/mol. Interestingly, the excited state decay from radical ion pair formed in Eu(II)-ketone systems revealed that back electron transfer (BET) rate constants are 4-5 order of magnitude less (k_bet = 10⁴-10⁵ s^-1) compared to that of forward electron transfer, under the present experimental conditions. The significant decrease in rate is attributed to the large energy barrier for the back electron transfer process, which involves transformation of an otherwise stable trivalent lanthanide to its thermodynamically less stable divalent oxidation state. This investigation reveals, for the first time, that divalent europium complexes are promising candidates for generating long lived charge separated states in the excited state in the presence of suitable acceptors.