Sensitized luminescence of Eu(III) complexes with Schiff-base and 1,10-phenanthroline: Role of Schiff-base as a sensitizer

• Two Eu(III) complex with Schiff, and mixed Schiff and phen were synthesized.
• Photophysical properties of the two Eu(III) complexes were investigated.
• The Schiff ligand playes a key role in the sensitized luminescence.
• Quantum mechanical calculations were performed for the photophysical properties.

The synthesis and characterization of two europium(III) complexes, [Eu(L)(H2O)]Cl and [Eu(L)(phen)(H2O)]Cl (L = N,N′-bis(salicylidene)-3,6-dioxa-1,8-diaminooctanato and phen = 1,10-phenanthroline) are reported. Exciting the Eu(III) complexes with near-UV light resulted in sensitized red luminescence by a transfer of energy from the triplet excited states of L to the Eu(III) ion. Introducing phen to the complex increased the quantum yield of the L-sensitized luminescence of [Eu(L)(phen)(H2O)]+ by more than 18 times relative to [Eu(L)(H2O)]+. The optimized structures and the configurational interaction singles (CIS) of the [Eu(L)(phen)(H2O)]+ and [Eu(L)(H2O)]+ molecules were theoretically studied using ab initio Hartree–Fock (HF). The theoretical calculations showed that the first nearly degenerate 1A and 2A excited states, more specifically the π → π* transitions of the two phenolate terminals, contributed significantly to the energy transfer process. Although the phen excitation route was forbidden in [Eu(L)(phen)(H2O)]+, the coordination of phen enhanced the absorbing ability of L markedly and caused the energy transfer from the 1A and 2A states to the 5D1 and 5D0 states of Eu(III) to predominate over any radiative and nonradiative processes occurring between the excited states and the ground states of the L moiety. Consequently, the quantum yield of the sensitized luminescence was enhanced significantly in [Eu(L)(phen)(H2O)]+.

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