Analysis of Energy Level Structure and Excited-State Dynamics in a Sm3+ Complex with Soft-Donor Ligands: Sm(Et2Dtc)3(bipy)

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Abstract
Using both laser-excited fluorescence and optical absorption methods, we have determined 57 crystal-field (CF) energy levels of Sm3+ in crystals of Sm(Et2Dtc)3(bipy). The analysis of the energy levels is based on a model Hamiltonian consisting of both free-ion and CF terms. The CF modeling of the experimental energy levels yielded physically reasonable Hamiltonian parameters with a final rms deviation of 17.3 cm-1. In comparison with Sm3+ in other hosts, the CF splitting of Sm3+ in the lattice of Sm(Et2Dtc)3(bipy) is rather weak. The observed fluorescence decay of the 4G5/2 manifold is single-exponential, with a lifetime of 24.5 μs, indicating only one site of Sm3+ in the lattice. According to the Judd−Ofelt theory, values of three intensity parameters were obtained (Ω2,4,6 = 1.57, 2.65, and 3.65, in units of 10-20 cm-1). The calculated branching ratios for transitions from the 4G5/2 manifold are in agreement with experimental values. The calculated radiative lifetime of the 4G5/2 manifold is 3.24 ms, and the corresponding fluorescence quantum efficiency is only 0.75%. Efficient multiphonon relaxation processes induced by the localized high-frequency vibrational modes in the bipyridyl group may lead to the extremely low quantum efficiency observed. The thermal line broadening and shifts of the 4G5/2(1) → 6F1/2 transition were observed and fitted very well by the McCumber−Sturge equations with an assumption of Raman phonon scattering processes as the leading relaxation mechanism. The Debye temperature for this crystal is predicted to be 350 K.