Critical Role of Energy Transfer Between Terbium Ions for Suppression of Back Energy Transfer in Nonanuclear Terbium Clusters

Suppression of back energy transfer is crucial in realizing efficient luminescent lanthanide complexes. However, the only practical method reported to this day is to raise the energy of the triplet excited state so that back energy transfer is energetically unfavorable, which limits the application where the absorbing wavelength of the organic ligands is important. This chapter explores a radically new strategy that focus on the yield of the back energy transfer as opposed to the rate constant of the back energy transfer. By utilizing energy transfer between lanthanide ions, which is a competitive process to back energy transfer, the contribution of back energy transfer can be suppressed. This chapter describes the theoretical background of this concept using kinetic analysis, and then the experimental confirmation of the proposal using [TbnGd9−n(µ-OH)10(Bu)16]NO3 (n = 0, 1, 2, 5, 8, 9). It is revealed that indeed the contribution of the back energy transfer is suppressed in this cluster, implying the potential of lanthanide clusters as functionalized and efficient luminescent material.

Trivalent lanthanide complexes are an important class of luminescent material characterized by their strong absorption of light by the organic ligands and subsequent energy transfer to the lanthanide ion, realizing intense luminescence from the ion. With this mechanism of luminescence, the total quantum yield of a lanthanide complex is the product of the energy transfer efficiency from the ligand to the lanthanide ion and the "intrinsic" quantum yield of the lanthanide ion itself. The "absolute" method in measuring the quantum yield uses an integrating sphere, and this method can be used for measuring both the total and the intrinsic quantum yields. The presence of back energy transfer (the reverse process of energy transfer) adds complication to this by affecting both the dynamics of the excited state of the ligands and the lanthanide ion. Herein, we theoretically derive an equation that shows that in the presence of back energy transfer the intrinsic quantum yield may differ depending on whether it is determined from the measurement through excitation of the ligands or the lanthanide directly. The value measured by direct lanthanide excitation could decrease to 20% or less of the actual value when back energy transfer is prominent. Several previously reported Tb(iii) complexes are within the range to be cautious. This report shows that the "absolute" method for measuring the lanthanide ion-centered quantum yield may not be suitable in the presence of back energy transfer by principle. We also provide a possible workaround in the case that several approximations and assumptions can be made.

Effect of A-site cations on the broadband-sensitive upconversion of AZrO3:Er3+,Ni2+ (A = Ca, Sr, Ba) phosphors

We have analyzed broadband-sensitive upconversion from 1.1–1.6 μm to 0.98 μm in La(Ga,Sc,In)O3 doped with Er, Ni, and Nb, which could significantly boost the conversion efficiency of crystalline silicon solar cells, in particular, energy transfer from the Ni2+ sensitizers to the Er3+ emitters and back transfer from the Er3+ to the Ni2+. We have compared these processes and the resultant upconversion emission intensities depending on the host material compositions. With increasing the bond length between the Ni2+ and surrounding oxygen ions, the Ni2+ emission band located at around 1.2–1.6 μm red-shifts and hence overlaps more significantly with the Er3+ absorption band ranging from 1.45 μm to 1.6 μm, resulting in more rapid energy transfer from the Ni2+ to the Er3+. However, back energy transfer from the Er3+ to the Ni2+ deteriorates the performance more considerably, because of more significant overlap between the Er3+ emission band and Ni2+ absorption band. This trade-off relationship strongly affects the upconversion emission intensity. The key of the material design for further efficient upconversion is to achieve narrower bands and a larger Stokes shift of the Ni2+ absorption/emission to suppress the back energy transfer while maintaining the efficient energy transfer in the forward direction.

Enhanced upconversion of sub20 nm core/shell/shell nanophosphors for temperature and rhodamine B sensing

Kinetics of radiationless energy transfer from upper excited states

Dynamic exciton-to-dopant energy transfer tuning of Mn2+-Doped perovskite nanocrystals by PBG effect

Spectroscopic properties of Er3+/Yb3+ codoped barium gallogermanate glass demonstrate that an efficient energy transfer from Yb3+ to Er3+ ions occurs while the back energy transfer from Er3+ to Yb3+ ions cannot be ignored. Based on the rate equations of electron transitions, the forward energy transfer Yb3+(F25/2)+Er3+(I415/2)→Yb3+(F27/2)+Er3+(I411/2) and the back transfer Er3+(I411/2)+Yb3+(F27/2)→Er3+(I415/2)+Yb3+(F25/2) coefficients were calculated to be 9.2×10−17 cm3 s−1 and 1.1×10−17 cm3 s−1, respectively. Also the energy transfer upconversion coefficient Yb3+(F25/2)+Er3+(I411/2)→Yb3+(F27/2)+Er3+(F27/2) was determined to be 1.1×10−16 cm3 s−1. The calculated results show that a population inversion between the I413/2 and I415/2 levels can be attained at a low pumping flux of around 1.7×1022 cm−2 s−1, while the inversion threshold between the S43/2 and I415/2 levels is about 30 times higher when pumped by 980 nm laser diode. With increasing pump flux the back energy transfer probability becomes larger and when the pump flux is more than 2.0×1022 cm−2 s−1, the back energy transfer probability dominates the forward energy transfer probability.

Energy transfer from organic ligands to a lanthanide ion is the main concept of sensitized luminescence in lanthanide complexes. Back energy transfer is the reverse process, which occur when the energy of the triplet excited state is close to the emitting excited state of the lanthanide ion and can lead to decrease in sensitization efficiency and therefore the quantum yield as well. Generally, the back energy transfer rate constant is proportional to the energy gap between the triplet excited state and the emitting excited state of the lanthanide, but there are some compounds that deviate from this trend. This chapter focuses on [Tb9(μ-OH)10(L)16]NO3 where ligand L is a derivative of methyl salicylate ligands to change the triplet excited state energy without significantly changing the structure so that direct comparison of back energy transfer rate constant can be performed. The conclusion of this chapter is that back energy transfer rate constant is determined not only by the energy gap but also by the activation energy and the frequency factor (electronic correlation) of the process.

The enhanced phosphorescence from Alq3 fluorescent materials by phosphor sensitization

The pathways of electronic excitation back energy transfer processes (BET) in novel Eu3+ heterocyclic 1,3-diketonates bearing a perfluorinated moiety

A dyad, 1, built on an artificial special pair (bis(meso-nonyl)zinc(II)porphyrin), [Zn2], a spacer (biphenylene), a bridge (1,4-benzene), and an antenna (di-meso-(3,5-di(t-butyl)phenyl)porphyrin free base), FB, is prepared by Suzuki coupling and is analyzed by absorption and steady state, and time-resolved emission spectroscopy at 298 and 77 K. Using bases from the Förster theory, evidence for two pathways for S 1 energy transfer, FB* → [Zn2], and [Zn2]* → FB, along with their respective rates, k ET ( S 1)1 and k ET ( S 1)-1, are extracted from the comparison of the fluorescence decays monitored at the emission maximum. At 77 K, the unquenched (1.79 ([Zn2]) and 10.6 ns (FB)) and quenched components (<100 ps; i.e. k ET ( S 1) > 10 (ns)-1), are observed, hence, demonstrating the bidirectional paths with no back energy transfer. A 298 K, only two components are detected (0.44 ([Zn2]) and 2.64 ns (FB)) and the resulting reduced τFs indicates back energy transfer, therefore cycling and equilibrium. Their global rates are 0.31 and 1.8 (ns)-1 for k ET ( S 1)1 and k ET ( S 1)-1 at 298 K. This large temperature dependence on k ET ( S 1) is fully consistent with the participation of thermal activation. Finally, DFT calculations (B3LYP) were used to illustrate a clear correlation between the relative k ET ( S 1) s and the amplitude of the MO couplings between the artificial special pair and the antenna.

The energy transfer and back transfer processes of GaAs co-doped with Er and O (GaAs:Er,O) were experimentally distinguished by using a frequency response analysis of the AC photocurrent. The results were achieved by using the difference in the frequency dispersion between (1) the dispersion of the energy transfer, which is triggered by the trapping of free charges in the GaAs host and is represented with the Debye relaxation response and (2) the dispersion of the energy back transfer, which is induced by non-radiative transition of 4f bound electrons in the Er dopants and is described with a Lorentzian. The Debye relaxation response found in GaAs:Er,O provided a charge trapping time that was dependent on temperature, which was well correlated with the thermal quenching property of intense intra-4f-shell luminescence. The spectral shape of the Lorentzian dependence on the temperature was explained with the thermal excitation of Er 4f electrons and release of trapped charges in GaAs. The thermal excitation and release of charges consistently explained the characteristics of weak 4f luminescence in low- and high-temperature regions, respectively.

Photophysical properties of Lanthanide(III) 1,1,1-trifluoro-2,4-pentanedione complexes with 2,2′-Bipyridyl: An experimental and theoretical investigation

Photoluminescence of Terbium(III) complexes was investigated as a function of temperatures in the range of 80–280 K for [Tb(bfa)3(H2O)2] (bfa: 4,4,4-trifluoro-1-phenyl-1,3-butanedionato), [Tb(hfa)3(H2O)3] (hfa: hexafluoroacetylacetonato), [Tb(tfa)3(H2O)2] (tfa: trifluoroacetylacetonato), [Tb(acac)3(H2O)3] (acac: acetylacetonato), and [Tb(hfa)3(tppo)2] (tppo: triphenylphosphine oxide). These complexes were classified into the two groups with different temperature-dependences. The first group consisting of [Tb(bfa)3(H2O)2], [Tb(tfa)3(H2O)2], and [Tb(acac)3(H2O)3] showed a dependence determined by the energy gap between the excited triplet state of the ligand and the emitting level of terbium(III) ion. In contrast, for [Tb(hfa)3(H2O)3] and [Tb(hfa)3(tppo)2] containing hfa as a ligand, not only the energy gap but also the energy barriers of the “Forward energy transfer” from the ligand to terbium(III) ion and “Back energy transfer” from terbium(III) ion to the ligand were taken into account for understanding their dependences. These results are discussed based on the re-orientation of the complexes accompanied by the forward and back energy transfer processes using DFT calculations.