How minor structural changes generate major consequences in photophysical properties of RE coordination compounds; resonance effect, LMCT state

Abstract

Lanthanide coordination compounds of the formula Na[Ln(L)4] (1Ln), where Ln = La3+, Eu3+, Gd3+, Tb3+, L = [L]− and HL = dimethyl(4-methylphenylsulfonyl)amidophosphate, were synthesized. Their structural and spectroscopic properties were discussed in detail based on X-ray diffraction measurements, IR spectroscopy, absorption and emission spectroscopy at 293 and 77 K and theoretical calculations of the intramolecular energy transfer (IET) rates. DFT calculations were used to investigate the 1Ln electronic properties required to calculate the transition rates. 30 and 22 pathways of intramolecular nonradiative energy transfer were examined in the case of 1Eu and 1Tb, respectively. It is shown that the main pathway for sensitization of the lanthanide emission is either the triplet (1Eu) or singlet (1Tb) transfer, occurring mainly through the exchange mechanism. The energy rates for energy transfer from S1 and T1 equal WS=1.53×105s−1 (1Eu), WT=5.14×106s−1 (1Eu) and WS=4.09×107s−1 (1Tb), WT=6.88×105s−1 (1Tb). The crucial role of the 7F5 level in the energy transfer process of 1Tb and the participation of the LMCT state in the depopulation of the ligand singlet state of 1Eu were demonstrated. The influence of the resonance effect on the splitting of the 7F1 level in 1Eu was analyzed. By comparing the properties of 1Ln with the properties of 2Ln coordination compounds, sharing the same ligand and crystallizing in the same crystallographic system (monoclinic), but with a different space group, it is demonstrated how slight structural changes can affect the photophysical properties of Ln compounds.