Luminescence differences between two complexes of divalent europium

We report a computational study of the photophysical properties of two divalent europium cryptates. Our calculations provide an explanation for the bright yellow luminescence of the Eu-II-containing octaaza-cryptate compared to the less intense blue luminescence of the structurally similar EuII-containing 222-cryptate. Calculations using time-dependent density functional theory with the B3PW91 functional, the Stuttgardt-Dresden relativistic core potential basis for europium, and SMD implicit solvation are used to compute the excitation and emission spectra of both complexes. Emission is also calculated with state-specific solvation. The results are compared with experimental luminescence data acquired in methanol. Natural-transition orbitals reveal similar spin-allowed transitions between the 4f and 5d orbitals on the europium ion in both complexes. For the 222-cryptate, the emissive state is hidden underneath the broad ultraviolet absorption; therefore, the state is not experimentally differentiated in the spectra, despite being present in the calculated spectra. For the octaaza-cryptate, the emissive state is observed as a separate band, shifted to lower energy than the broad ultraviolet absorption. Using ligand-field arguments, sharp differences in luminescence and the bathochromic shift of the emissive state can be attributed to a greater splitting of the 5d orbitals of the octaaza-cryptate relative to the 222-cryptate. (C) 2017 Elsevier B.V. All rights reserved.