Light-driven electron transfer through a water-oil interface by a shuttle photosensitizer: Photoinduced electron transfer from tributylamine to Fe(CN)(6)(3-) using ethyl eosin as a mediator in a water-in-oil microemulsion system

Photoinduced electron transfer between water-oil phases is accomplished in a water-in-oil microemulsion system using a "shuttle photosensitizer" as an electron transporter. The system consists of a water-in-oil microemulsion in which ethyl eosin, (1)EtEo(-), acts as a photosensitizer, Fe(CN)(6)(3-) as an electron acceptor, and tributylamine, Bu3N, as an electron donor. The hydrophilic photosensitizer and electron acceptor are solubilized in the a queous microdroplets of the water-in-oil microemulsion, whereas the hydroph obic electron donor is present in the continuous oil phase. Photoinduced ox idative electron-transfer quenching in the water phase, k(q) = 1.7 x 10(5) s(-1), results in the oxidized photosensitizer, (2)EtEo(.), and Fe(CN)(6)(4 -). The hydrophobic oxidized photosensitizer is extracted to the continuous oil phase, resulting in the phase separation of the photogenerated redox s pecies, and the stabilization of the products against back electron transfe r, k(rec) = 7.1 x 10(2) s(-1). The stabilization of the redox species again st back electron transfer enables the reduction of the oxidized photosensit izer, (2)EtEo(.).by Bu3N in the oil phase, k(red) = 1.1 x 10(6) M-1.s(-1). The latter process regenerates the hydrophilic photosensitizer, (1)EoEt(-), that is transported back to the water microdroplets, a process leading to the electron transfer across the water-oil boundary by the shuttle photosen sitizer. The photosensitized reduction of Fe(CN)63- by Bu3N, in the water-i n-oil microemulsion system, proceeds with a quantum yield of phi = 0.04. Th e mechanism involved in the photoinduced electron transfer between the wate r-oil phases is elucidated by time-resolved laser flash photolysis experime nts and steady-state irradiation. A detailed mathematical model, assuming a Poisson distribution of the quencher in the water droplets, is formulated. This accounts for the different processes involved in the electron transfe r in the microheterogeneous system.