Spectroscopic aspects for the Yb³⁺ coordination compound with a large energy gap between the ligand and Yb³⁺ excited states

We present the experimental and theoretical results that made it possible to propose the energy transfer mechanism for a Yb complex with a large energy gap between the ligand and Yb excited states using a theoretical model and experimental data. Absorption and emission spectroscopy in the 300-4 K range is used for the study of the Yb3+ compound with N-phosphorylated sulfonamide (Na[YbL4]), which, despite the large energy gap, is characterized by high emission sensitization efficiency (eta(sens) = 40%) and relatively long Yb3+ emission lifetime (27 mu s). The crystal structure of Na[YbL4], radiative lifetime (930 mu s), refractive index (1.46), intrinsic (3.0%), and overall (1.3%) emission quantum yield were determined. To obtain the electronic properties of the Na[YbL4], a time-dependent density functional theory (TD-DFT) was performed. The intramolecular energy transfer (IET) rates from the excited states S-1 and T-1 to the Yb3+ ion as well as between the ligand and the ligand-to-metal charge transfer (LMCT) states were calculated. Once the intersystem crossing S-1 -> T-1 is not so effective due to a large energy gap between S-1 and T-1 (approximate to 10000 cm(-1)), it has been shown that the LMCT state acts as an additional channel to feed the T-1 state. Then, the T-1 can transfer energy to the Yb3+ 2F5/2 energy level (W-T), where W-T is dominated by the exchange mechanism. Based on IET and a rate equation model, the overall emission quantum yield Q(Ln)(L) was simulated with and without the LMCT, this also confirmed that the pathway S-1 -> LMCT -> T-1 -> Yb3+ is more likely than the S-1 -> T-1 -> Yb3+ one. (C) 2022 Published by Elsevier B.V.