Lanthanide (III) ions as multichannel acceptors for bimolecular photoinduced electron transfer reactions with coumarin dyes

Abstract

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions is of considerable research interest in photochemical sciences. In this study, interactions between trivalent lanthanide ions (Ln(III)) as electron acceptors and coumarin dyes as electron donors have been investigated in aqueous media using ground state absorption, steady-state (SS) emission, time-resolved (TR) fluorescence and laser flash photolysis (LFP) techniques. Among different Ln(III) ions, only Eu(III), Yb(III) and Sm(III) are found to display substantial interaction with the excited singlet (S1) state of the coumarin dyes, which correspond well with the favorable free energy changes (negative ΔG0) associated with the PET reactions in these coumarin-Ln(III) systems. For other Ln(III) ions, as ΔG0 is not favorable, they do not show any interaction with the excited coumarin dyes. Among the interacting coumarin-Ln(III) systems, bimolecular quenching rate constant (kq) increases consistently with the increasing exergonicity (−ΔG0), when each of the Ln(III) ions are considered independently, except for Eu(III), for which the kq values have already reached the diffusion controlled limit. However, considering all the Ln(III) ions together, the kq values do not show any common correlation with the changing ΔG0 values. The kq values are in general much lower for Yb(III) quencher as compared to Sm(III), even though the exergonicity values for coumarin-Yb(III) systems are more favorable than the coumarin-Sm(III) systems. Correlating the observed results based on the diffusion mediated bimolecular reaction model, we infer that depending on the ΔG0 values, some Ln(III) quenchers possibly also engage their upper electronic energy states as well to assist multichannel ET with the excited coumarin donors, and thereby modulating the kq values very substantially for different coumarin-Ln(III) systems. Apparently, the involvement of multichannel ET causes the total reorganization energy (λ) for ET reactions to differ largely for different Ln(III) ions, which would otherwise be very similar for all the coumarin-Ln(III) pairs. Observed results suggest that the intramolecular reorganization energy (λi) associated with lower electronic states of the hydrated Ln(III) ions is much larger than that associated with higher electronic states of these ions. While the involvement of multichannel ET is itself an intriguing inference, modulation in the ET kinetics and total λ values for different Ln(III) quenchers through the differential participation of the multichannel PET is also expected to find potential uses in the controlled utilization of such PET reactions in many practical areas.