Kinetics of Photoinduced Electron Transfer in Alternately Stacked Eu3+:LaNb2O7- and W2O72- Nanosheets as Demonstrated by f-f Radiative Transition of Doped Eu3+

We successfully demonstrated that the kinetics of photoinduced electron transfer (PET) between alternately stacked Eu3+-doped LaNb2O7 (Eu3+:LaNb2O7-) nanosheets and W2O72- nanosheets can be investigated by the millisecond luminescence decay of an emissive relaxation between 4f orbitals of the doped Eu3+. The emission of the doped Eu3+ is triggered by energy transfer (EnT) from the band gap excitation energy of LaNb2O7-, and the EnT process 9 competes with PET from LaNb2O7- to W2O72-, meaning that the kinetics of the PET can be estimated from the time-resolved emission behavior of doped Eu3+. Alternately stacked nanosheets were synthesized by a thiol-ene click reaction of organic moieties immobilized on each nanosheet where the center-to-center distance of neighboring nanosheets was precisely controlled to be 1.94, 2.04, and 2.20 nm by changing the number of carbons in the covalent bridge generated by the thiol-ene click reaction. When the center-to-center distances of neighboring nanosheets were 1.94 and 2.04 nm, the decrement of the emission lifetime of the doped Eu3+ was observed in the luminescence decay curves, resulting in estimated rates of PET of 4.5 and 2.3 ms(-1), respectively. However, when the center-to center distance of neighboring nanosheets was 2.20 nm, electron transfer no longer occurred because EnT (0.80 ms(-1)) dominated the relaxation of the band gap excitation of LaNb2O7-. From these results, the tunneling decay constant of the PET was estimated to be 0.70 angstrom(-1),demonstrating that the mechanism of this PET is based on a through-space type of electron transfer reaction between semiconductors. This is the first demonstration of estimating the kinetics of electron transfer reactions at the interface of semiconductors using a doped rare-earth atom as a probe.