Maximizing Terbium-Europium Electronic Interaction: Insight from Variation of Excitation Energy

This work investigates the tuning of electronic interaction between excited Tb3+ (Tb3+*) and Eu3+ by varying the excitation wavelengths, which consist of different electronic origins spanning the entire range of excitation spectra. Direct excitation bands of trivalent lanthanide cations (Ln3+) are accessed in the Ln-doped calcium fluoride, Ca(Ln)F2, nanoparticles (NPs). The experimental outcomes from the NPs are compared to that in the bulk solvent with freely floating entities. Remarkable excitation wavelength-dependent Tb3+-Eu3+ electronic interaction is observed in the NPs, with 370 nm excitation of Tb3+ being found to provide the optimum energy to maximize the Eu3+ emission. This excitation energy dependence is found to be less prominent in the bulk medium. Different mechanisms for Tb3+-Eu3+ electronic interaction are argued to be operative in the confined NP and bulk environments. The energy transfer efficiency from Tb3+* to Eu3+ in NPs can be maximized by (i) sole excitation of Tb3+ and (ii) maintaining the energy difference between the excitation energy and the Eu2+ ground energy level in the range of 4500-7500 cm-1. Additionally, we suggest the necessity of concomitant consideration of the steady-state and time-resolved response of both Tb3+ and Eu3+ emissions to decipher the Tb3+* -> Eu3+ electronic interactions, instead of considering a single parameter to gauge such a process. These findings collectively provide important insights to design Tb-Eu-containing luminophores for their potential use in multiplex assays.